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02 gsmp&o b-en-gsm radio network planning principle-word--201009
 

02 gsmp&o b-en-gsm radio network planning principle-word--201009

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    02 gsmp&o b-en-gsm radio network planning principle-word--201009 02 gsmp&o b-en-gsm radio network planning principle-word--201009 Document Transcript

    • GSM Radio network planning principle Course Objectives: ·Understand capacity planning and calculation methods ·Understand principles for SDCCH and LAC planning ·Grasp link balance calculation ·Grasp various frequency multiplex methods and common anti-interference technologies ·Understand dual-band networking and parameter settings
    • i Contents 1 Capacity Planning...................................................................................................................................... 1 1.1 Basic Concepts.................................................................................................................................. 1 1.1.1 Traffic Volume and BHCA..................................................................................................... 1 1.1.2 Call Loss Probability and Erlang B........................................................................................ 2 1.2 Capacity Predication ......................................................................................................................... 5 1.2.1 Overview................................................................................................................................ 5 1.2.2 Methods.................................................................................................................................. 5 1.2.3 Traffic Distribution Prediction ............................................................................................. 14 1.3 Capacity Planning Process.............................................................................................................. 15 1.3.1 Work Flow............................................................................................................................ 15 1.3.2 Prerequisites of Capacity Planning ...................................................................................... 16 1.3.3 Calculation of Capacity Planning......................................................................................... 16 1.4 Channel Capacity Planning............................................................................................................. 17 1.4.1 SDCCH Capacity Planning.................................................................................................. 17 1.4.2 CCCH Configuration Principle............................................................................................ 20 1.4.3 Recommended CCCH and TCH Allocation......................................................................... 20 1.5 Optimization of Capacity Planning................................................................................................. 21 1.6 Improvement of Network Capacity................................................................................................. 22 1.6.1 Methods for Improving Network Capacity .......................................................................... 22 1.6.2 Analysis of Methods for Network Capacity Improvement................................................... 23 1.7 Location Area Planning................................................................................................................... 24 1.7.1 Determining LA Edges......................................................................................................... 25 1.7.2 LA Paging Capacity ............................................................................................................. 26
    • ii 1.7.3 LA Capacity Calculation.......................................................................................................29 1.7.4 Affect of SMS on LA Paging Capacity ................................................................................31 2 Link Budget and Coverage Planning......................................................................................................33 2.1 Purposes of Link Budget .................................................................................................................33 2.2 Calculation of Uplink and Downlink Balance.................................................................................33 2.2.1 Analysis of Parameters in Uplink Budget.............................................................................34 2.2.2 Analysis of Parameters in Downlink Budget........................................................................43 2.3 Coverage Planning...........................................................................................................................46 3 Frequency Planning..................................................................................................................................51 3.1 Cellular Structure Creation Rule......................................................................................................51 3.2 Interference Models.........................................................................................................................52 3.3 Frequency Reuse Technology and Interference Analysis ................................................................57 3.4 Frequency Reuse in Groups.............................................................................................................57 3.4.1 4 X 3 Frequency Reuse.........................................................................................................57 3.4.2 3 x 3 Frequency Reuse..........................................................................................................61 3.4.3 1 x 3 Frequency Reuse..........................................................................................................62 3.4.4 2 x 6 Frequency Reuse..........................................................................................................63 3.4.5 Multiple Reuse Pattern (MRP) .............................................................................................64 3.4.6 Concentric Cell Technology .................................................................................................68 3.5 Cell Splitting....................................................................................................................................72 3.6 Common Anti-Interference Technologies........................................................................................73 3.6.1 Discontinuous Transmission (DTX) .....................................................................................74 3.6.2 Frequency Hopping (FH)......................................................................................................74 3.6.3 Dynamic Power Control (DPC)............................................................................................78 3.6.4 1 x 3 Reuse + RF Frequency Hopping + DTX + DPC .........................................................78 3.7 Summary of GSM Frequency Allocation ........................................................................................79
    • iii 3.8 Neighbor Cell Planning................................................................................................................... 81 3.8.1 Planning Principles............................................................................................................... 81 3.8.2 Case Analysis....................................................................................................................... 84 3.8.3 BSIC Planning...................................................................................................................... 86 4 Dual Band Technology............................................................................................................................. 91 4.1 Structure of Dual Band Networks................................................................................................... 91 4.1.1 Shared HLR/AUC, EIR, OMC and SC................................................................................ 91 4.1.2 Shared Switching Subsystem ............................................................................................... 91 4.1.3 Shared Switching Subsystem and BSC................................................................................ 92 4.1.4 Shared Network Subsystem ................................................................................................. 92 4.2 Dual Band Network Planning ......................................................................................................... 94 4.2.1 Requirement Analysis.......................................................................................................... 94 4.2.2 Coverage Planning ............................................................................................................... 95
    • 1 Capacity Planning 1.1 Basic Concepts 1.1.1 Traffic Volume and BHCA The time when a user initiates a call and the duration are random but comply with a rule in some degree. To reflect the frequencies of giving calls and call duration, the term traffic is introduced. Traffic usually refers to the volume of calls in the given period, often measured in Erlang (one Erlang means the traffic load where one call circuit is occupied completely for one hour, or the traffic load where two call circuits are occupied completely for half an hour). The average traffic ρ of a user is calculated by the following formula: 3600 1 ÷×= μ λρ where, λ , also called call reach rate, stands for the number of calls orginated by each user within the unit time. μ 1 stands for the average call duration of each user and the unit is second. μ stands for the call completion rate. The traffic of each cell, that is, A, is expressed in the following equation: dSA ρ= where, ρ stands for the average traffic per user (Erlang / user), stands for user density (number of users / km2 ), and d S stands for cell area (km2 ). In actual situations, traffic changes over time. Even if long time changes are ignored, it can still change over a short period, for example, by day or week. The hour when the traffic is the largest is called busy hour. The call amount in the busy hour is known as Busy Hour Call Attempt or Busy Hour Calling Amount (BHCA). Busy-hour traffic is calculated by the following formula: 1 3600 1 ÷×= μ ρ BHCABH
    • GSM Radio network planning principle In network planning, busy-hour traffic is always used as a design index. If a GSM network can deal with the busy hour traffic, it is sure to deal with common traffic. Busy-hour traffic per user is also used in network planning, which is expressed in the following formula: 3600 1 0 ÷××= μ βαρ where, 0ρ stands for busy-hour traffic per user, α stands for the number of calls in a day per user, and β stands for the busy hour factor (ratio of busy-hour traffic to day traffic). Thus, busy-hour traffic can be: NBH 2 ×= 0ρρ where N stands for the number of users. This formula is quite important in capacity planning. Obviously, the planned capacity should be greater than the expected BHρ . The average busy hour traffic per user of a system can be obtained from the statistical data. The previous formula shows that total busy hour traffic divided by the number of subscribers in busy hour on the VLR is the average busy hour traffic per user of the system. In network planning, leave margins for the average busy hour traffic per user of the system. In China, the experience value of average busy hour traffic per user is 0.025~0.03erl / user. In other words, 6 calls (incoming and outgoing) are allowed for each user and each call lasts for 2 minutes in average. 1.1.2 Call Loss Probability and Erlang B Call loss, or congested call, refers to call lost due to disconnection after all the channels occupied in mobile telecommunications system. Call loss probability indicates the probability of call congestions. Grade of service (GOS), expressing the congestion level, is used to define the congestion probability. In GSM network planning, the TCH GOS is 2% or 5%. According to Public Mobile Telephony Network Technology Mechanism, the radio channel loss probability should be less than or equal to 5% (in areas with high traffic
    • 1 0BCapacity Planning density, 5%). Generally, public mobile telephone network is a system with call losses. Through a cell (or sector) is designed with the assumption that idle a call attempt cannot get an idle channel at the first time and keep on originating call attempts, the sector sharing or directed retry function guides the congested call to another sector for idle channels and thus leave the original sector to be accessed. Therefore, for each sector, a call is discarded once no idle channel is available. As a result, the total congestion characteristic is close to the requirement in Erlang-B call rule. According to the Erlang call loss formula and calculation table, a call must have the following features: 1. Any two calls are independent of each other (The calls are random). 2. Each call has the same probability in time. 3. When a call cannot obtain idle channels, this is taken as call loss, instead of waiting for some time for idle channels. The Erlang-B formula is as follows: This formula shows the relationship between call loss probability (B), traffic (A), and number of channels (n). Traffics with different loss probability and different channels can be calculated according to the Erlang formula and summarized into an Erlang-B table. Then, when any two items are known, the third item can be calculated. The following table is an Erlang-B table calculated by the Erlang formula: N 1.0% 1.2% 1.5% 2% 3% 5% 1 0.0101 0.0121 0.0152 0.0204 0.0309 0.0526 2 0.153 0.168 0.19 0.223 0.282 0.381 3 0.455 0.489 0.535 0.602 0.715 0.899 4 0.869 0.922 0.992 1.09 1.26 1.52 5 1.36 1.43 1.52 1.66 1.88 2.22 6 1.91 2 2.11 2.28 2.54 2.96 7 2.5 2.6 2.74 2.94 3.25 3.74 8 3.13 3.25 3.4 3.63 3.99 4.54 9 3.78 3.92 4.09 4.34 4.75 5.37 10 4.46 4.61 4.81 5.08 5.53 6.22 11 5.16 5.32 5.54 5.84 6.33 7.08 12 5.88 6.05 6.29 6.61 7.14 7.95 3
    • GSM Radio network planning principle 4 N 1.0% 1.2% 1.5% 2% 3% 5% 13 6.61 6.8 7.05 7.4 7.97 8.83 14 7.35 7.56 7.82 8.2 8.8 9.73 15 8.11 8.33 8.61 9.01 9.65 10.6 16 8.88 9.11 9.41 9.83 10.5 11.5 17 9.65 9.89 10.2 10.7 11.4 12.5 18 10.4 10.7 11 11.5 12.2 13.4 19 11.2 11.5 11.8 12.3 13.1 14.3 20 12 12.3 12.7 13.2 14 15.2 21 12.8 13.1 13.5 14 14.9 16.2 22 13.7 14 14.3 14.9 15.8 17.1 23 14.5 14.8 1 15.2 15.8 16.7 8.1 24 15.3 15.6 16 16.6 17.6 19 25 16.1 16.5 16.9 17.5 18.5 20 26 17 17.3 17.8 18.4 19.4 20.9 27 17.8 18.2 18.6 19.3 20.3 21.9 28 18.6 19 19.5 20.2 21.2 22.9 29 19.5 19.9 20.4 21 22.1 23.8 30 20.3 20.7 21.2 21.9 23.1 24.8 31 21.2 21.6 22.1 22.8 24 25.8 32 22 22.5 23 23.7 24.9 26.7 33 22.9 23.3 23.9 24.6 25.8 27.7 34 23.8 24.2 24.8 25.5 26.8 28.7 35 24.6 25.1 25.6 26.4 27.7 29.7 36 25.5 26 26.5 27.3 28.6 30.7 37 26.4 26.8 27.4 28.3 29.6 31.6 38 27.3 27.7 28.3 29.2 30.5 32.6 39 28.1 28.6 29.2 30.1 31.5 33.6 40 29 29.5 30.1 31 32.4 34.6 41 29.9 30.4 31 31.9 33.4 35.6 42 30.8 31.3 31.9 32.8 34.3 36.6 43 31.7 32.2 32.8 33.8 35.3 37.6 44 32.5 33.1 33.7 34.7 36.2 38.6 45 33.4 34 34.6 35.6 37.2 39.6 46 34.3 34.9 35.6 36.5 38.1 40.5 47 35.2 35.8 36.5 37.5 39.1 41.5 48 36.1 36.7 37.4 38.4 40 42.5 49 37 37.6 38.3 39.3 41 43.5 50 37.9 38.5 39.2 40.3 41.9 44.5 51 38.8 39.4 40.1 41.2 42.9 45.5 52 39.7 40.3 41 42.1 43.9 46.5 53 40.6 41.2 42 43.1 44.8 47.5 54 41.5 42.1 42.9 44 45.8 48.5 55 42.4 43 43.8 44.9 46.7 49.5 56 43.3 43.9 44.7 45.9 47.7 50.5 57 44.2 44.8 45.7 46.8 48.7 51.5 58 45.1 45.8 46.6 47.8 49.6 52.6 59 46 46.7 47.5 48.7 50.6 53.6 60 46.9 47.6 48.4 49.6 51.6 54.6
    • 1 0BCapacity Planning 5 N 1.0% 1.2% 1.5% 2% 3% 5% 61 47.9 48.5 49.4 50.6 52.5 55.6 62 48.8 49.4 50.3 51.5 53.5 56.6 63 49.7 50.4 51.2 52.5 54.5 57.6 64 50.6 51.3 52.2 53.4 55.4 58.6 1.2 Capacity Predication 1.2.1 Overview ented on the basis of initial and future traffic distribution obtained in various ways. The following factors should be considered in capacity prediction: 1. Income situation 2. Distribution of users with different ages and incomes 3. Economic development level of the area 4. Service competition 5. Special offers or reduction of mobile service charges 6. Advertising and vision of the operator 1.2.2 Methods 1. Short-term prediction (1 – 2 years) and long-term prediction (3 – 5 years) 2. Popularization rate 3. Growth trend prediction 4. Growth curve 5. Quadratic curve 1.2.2.1 Growth Trend Prediction In cellular network planning, the capacity requirement must be determined at first, as it is the basic of the whole engineering design. Capacity requirement involves the number of users in the system and the corresponding traffic. The purpose of capacity prediction is to give the actual and future capacity requirement to help estimate the required channels. Network planning is implem Mobile communications technology, especially GSM and other 2nd generation mobile
    • GSM Radio network planning principle 6 communications, is developing rapidly in the whole world. In some developed European countries, mobile phones still grows even through the popularization rate reaches a certain level, for example, above 50% in Finland. In 1997, some countries, such as Sweden, Norway, and Denmark, the rate was close to 30% and the annual growth rate reached 70%-80%. In China, the number of mobile phone users doubles the growth rate for continuous 10 years. In 1996, mobile phone numbers occupy 18% of the allocated numbers in the whole country, 29% in 1997, and 37% in 1998. This proves that the mobile phone market is prosperous. The following table shows the situation of mobile users in China: Ta wth obi scr C N rsble 1.2-1 Gro of M le Sub ibers in hina in ine Yea Year 1990 1991 1992 1993 1994 1995 1996 1997 1998 Subscribers (Unit: 10,000) 1.53 4.75 17.69 63.82 156.8 362.9 684.8 1364 2496 Annual growth (%) 160 272 261 146 132 89 99 83 According to the prediction from related experts, the annual growth of mobile subscribers in China will reach over 40% after 2000, and the net growth rate of subscribers in each year will go down stably. The average growth rate of mobile subscriber between 1998 and 2010 will be 29.28%. The prediction data are shown as diction of the growth of mobile subscribers in China in the near and medium terms. Table 1.2-2 wt ob scr n C n T rs below. Table 1.2-2 shows the pre Gro h of M ile Sub ibers i hina i en Yea Year 1998 1999 2000 2001 2002 2003 2004 2005 Subscribers (Unit: 2254 3432 5053 7016 9219 11861 14582 17680 10,000) Growth quantity (Unit: 10,000) 931 1178 1621 1963 2203 2642 2721 3098 Growth rate (%) 70.33 52.26 47.23 38.85 31.40 28.66 22.94 21.25 According to the comparison and analysis of the subscriber growth in 1998 (24.96 million) and August 1999 (33.05 million), we find that the data in Table 1.2-2 are somewhat conservative, but the prediction results are in good line with the development objectives specified in the Post & Telecom 1998-2002 Rolling Plan. The
    • 1 0BCapacity Planning 7 growth is appropriate and has some values of reference. Table 1.2-3 show 1.2-3 Growth of Subscribers of Previo rs in a Place s the growth of mobile subscribers in an area. Table us Yea 1992 1993 1994 1995 Place Number of Subscribers Rate Growth Number of Subscribers Growth Rate Number of Subscribers Growth Rate Number of Subscribers Growth Rate HX 1085 3033 180 7367 143 14539 97 1996 1997 1998 1999 31180 114 49761 60 93922 89 177659 89 In the area, the subscriber growth rate did not fall year by year during 1992 ~ 1999, but fluctuated, without any particular reason. Therefore, we take the growth rate for 2000 is taken as the average of 1995-1999, that is 70%, and 40% for 2001, slightly lower than the national average of 38.85%. This matches its status as a medium-sized city. We take mobile subscriber number for the area according to the trend pre Ta -4 Prediction of Mobile for the Area the growth rate of 30% for 2002. Table 1.2-4 shows the prediction of the diction method. ble 1.2 Subscribers 2000 2001 2002 Place Number of Subscribers Growth Rate (%) Number of Subscribers Growth Rate (%) Number of Subscribers Growth Rate (%) HX 302021 70 422829 40 549678 30 1.2.2.2 Populatio tries in the world ing years e area n Penetration Rate Method When focusing on penetration rate, consider the following factors: 1. Mobile phone penetration rate of the medium developed coun 2. Expected penetration rate in the country for upcom 3. Current penetration rate of the carrier in th 4. Economic development status in the area 5. Potential factors that affect the purchasing ability During the mobile communication development in China, an important event is the establishment of China Unicom. As the second telecom operator in China, its growth in
    • GSM Radio network planning principle 8 recent years leaves much to be desired, with still a low market share. However, it is because of the appearance of China Unicom, China Telecom made greater efforts in building the GSM network by greatly reducing the access charges. The building of the GSM systems, the introduction of competition, and the decrease of the charges contribute to the prosperity of the mobile communication industry in China. In terms of technology, the mobile communication will evolve to the 3G mobile communication system. In terms of service types available, it will also provide data services of various rates in addition to global all-rounded voice services for individual subscribers. We can see that as the charges decrease and new technologies are introduced, the future mobile communication will be based on integrated networks bearing multiple services oriented to individual subscribers. The expansion of services will attract more subscribers to the f 26.61% for mobile phone, which is still growing at 43.24%, as shown in Table 1.2-5. Table 1 Growth a f Mo Hong 98 Year Mobile Subscribers Growth Rate (%) enetration Rate (%) wireless network. Currently, the penetration of mobile phone has reached over 50% in Finland, but it is still growing. In 1997, some developed counties such as Sweden, Norway, and Denmark have penetration up to 30%, but they still grow at a high rate, with an annual growth of 70% ~ 80%. In 1998, Hong Kong SAR of China has a penetration rate o .2-5 nd Penetration Rates o bile Subscribers in Kong During 1992-19 P 1992 189664 3.3 1993 233324 23.02 4.02 1994 290843 24.65 4.93 1995 484823 66.7 8.03 1996 798373 64.67 12.97 1997 1143566 43.24 18.58 1998 1638010 43.24 26.61 Table 1.2-6 shows the penetration rates of mobile phone of China from 1995 to 2002. ble 1.2-6 etra ates of fro toTa Pen tion R of Mobile Phone China m 1995 2002 Year 1995 1996 1997 1998 1999 2000 2001 2002 Subscribers (Unit: 684.8 1364 2496 3432 5053 7016 9219 10,000) 362.9 Penetration Rate (%) 0.302 0.57 1.14 2.08 2.64 3.89 5.40 7.69
    • 1 0BCapacity Planning 9 According to the prediction of experts, the penetration rate of mobile phone in China will reach 10 per 100 persons in two or three years. Lanzhou is an important city in the northwest of China, and its economic development level is among the rank of the medium-developed cities in China. Its penetration rate of mobile phone has now reached 6% and the penetration rate of 10 handsets/100 persons is to be achieved two or three years earlier. In other words, the penetration rate will reach 10% in 2000. Table 1.2-7 shows the penetration rates of Gansu and China as a whole in recent years. Table 1.2-7 Penetration Rates of Mobile Phone in an Area 1996 1997 1998 1999 Place Number of Subscribers Penetration Rate (%) Number of Subscribers Penetration Rate (%) Number of Subscribers Penetration rate (%) Number of Subscribers Penetration Rate GL 31180 1.13 49761 1.77 93922 3.25 177659 6.12 The penetration rate of the area has reached and exceeded the national average ever since 1998. This matches the status of the city as a provincial capital city. In 1999, the penetration rate of the area is nearly three times higher than the national average. According to this percentage, the penetration rates of Lanzhou in 2000-2002 will reach 10%, 15%, and 20% respectively. The penetration rates of Shanghai and Beijing have exceeded 15%. As a medium-sized city, it is possible for its penetration rate to lag behind Beijing for two years (15% for 2001). Therefore, the penetration rates for the future three years for the area can be taken as 10%, 15% and 20%. Currently, China Unicom has a market share of about 10% throughout the country, and its objective is 20-30%. After the splitting of China Telecom, China Mobile will take on a new look in meeting the competition from China Unicom. It is estimated that the growth in market share of China Unicom will slow down. Therefore, we predict that the market share of China Unicom in the area will be 10%, 15% and 20%. In addition, as the market develops, the government will gradually remove its support for China Unicom, and the market will become normalized. In future, what attract subscribers are service quality and new services available. Table 1.2-8 shows the population development of the area in 2000-2002.
    • GSM Radio network planning principle Table 1.2-8 Population Development of Lanzhou in 2000-2002 1996 1997 1998 2000 2001 2002 No. Place Population (10,000 persons) Population (10,000 persons) Population (10,000 persons) Natural Growth (‰) Population (10,000 persons) Population (10,000 persons) Population (10,000 persons) 1 Lanzhou 276.09 280.46 288.56 5.90 291.98 293.70 295.43 According to the population estimation data and the market share of China Mobile in the area, the population penetration method is used to predict the number of mobile subscribers for three prefectures/cities in the province for 2000-2002, as shown in Table 1.2-9. Table 1.2-9 Development of Mobile Subscribers of Three Prefectures/Cities of Lanzhou for 2000-2002 2000 2001 2002 Place Population(10,000 persons) PenetrationRate(%) MarketShare(%) NumberofSubscribers Population(10,000 persons) PenetrationRate(%) MarketShare(%) NumberofSubscribers Population(10,000 persons) PenetrationRate(%) MarketShare(%) NumberofSubscribers GL 291.98 10.0 90 262778 293.70 15.0 85 374465 295.43 20.0 80 472689 GW 192.42 0.8 98 14448 194.50 1.1 92 20564 196.59 1.5 88 26510 DX 293.63 0.5 100 13236 296.40 1.0 98 29459 299.19 1.8 90 47336 1.2.2.3 Growth Curve Method The research on prediction methods finds that the development of equipment and growth of market demands has similarities in some degree. For example, during the development of the local calls, when the penetration reaches a certain level, it gradually reaches saturation, instead of increasing simply in the exponential or linear trend. For such saturation curve, common equations are Gompertz curve equation and Logistic curve equation. For this prediction, the Gompertz curve equation is used. kt be t LeY − − = 10
    • 1 0BCapacity Planning The shape of the Gompertz curve is as follows: t Yt L t Yt L The parameter can be determined in the following ways: 1. Determine the saturation peak L. 2. Perform logarithm operations to both sides of the equation, which is then changed to lnln (L/Yt) =lnb-kt. Take A=lnb, yt’= lnln(L/yt), then the equation becomes a linear expression: yt’=A+Bt. With the least square method, user can resolve A and B of the linear equation. Step 1: Determine the saturation peak L. Mobile phones have ready mobility. Population mobility is related to ages. The population in the 15-64-age range may have higher mobility, and is more likely to use mobile phones. On the other hand, the population within 0-14 ages and above 65 ages is less mobile, and fixed telephone is nearly enough to meet their communication needs. Therefore, when user predict the number of subscribers, user should focus userr attention to the needs of the persons in this age range. According to the trend in the change of the population age structure in China (错误!未找到引用源。), the population of 15-64 ages accounts for 67.7% of the total in 2000. We can take this percentage as the saturation value (L) of mobile phone penetration.. The following table shows the trend of age structure change in China from 1995 to 2020: Total Population 0-14 Ages 15-64 Ages 65 Ages and Above Year Population Percentage Population (10,000 persons) Population percentage Population (10,000 persons) Population percentage Population (10,000 persons) Population percentage Population (10,000 persons) 1995 100 121121 26.7 32303 66.6 80727 6.7 8091 1996 100 122248 26.4 32273 66.8 81662 6.8 8313 1997 100 123385 26.1 32203 67 82668 6.9 8514 1998 100 124532 25.8 32129 67.2 83686 7 8717 11
    • GSM Radio network planning principle Total Population 0-14 Ages 15-64 Ages 65 Ages and Above Year Population Percentage Population (10,000 persons) Population percentage Population (10,000 persons) Population percentage Population (10,000 persons) Population percentage Population (10,000 persons) 2000 100 126859 25.3 32105 67.7 85841 7 8913 2005 100 131438 22.9 30099 69.5 91350 7.6 9989 2010 100 136183 20.7 28248 71.1 96799 8.2 11136 Step 2: Calculate the values of A and B. Take the city GL as an example: Year SN (t) Number of Users (Yt) Yt'=lnln(L/Yt) Ytt'=t*Yt' t2 1996 1 31180 1.422966 1.422966042 1 1997 2 49761 1.303444 2.60688764 4 1998 3 93922 1.114066 3.342198523 9 1999 4 177659 0.879344 3.517377014 16 Sum ∑ 10 352522 4.719820 10.88942922 30 Use the least square method to get the values of A and B and set up a mathematical model: 5.2 4 10 ,4 === tn t e 182024.0 129540. − 12954.5== A e t ey 5 1976705 − = ,182024.0=−= bBK 63502.1*' =−= tByA t 182024.0 * *** 22 '' −= − − = ∑ ∑ tnt ytnyt B tt 4 71982.4 ' =ty Estimate the user quantity in GL in 2000. Introduce t=5 into the above formula to obtain y2000=250804. Introduce t=6 to the formula to obtain y2001=353629. Introduce t=7 to the formula to obtain y2002=470899. Use the growth curve method to predict the number of mobile subscribers for the GL area in 2000-2002. 12
    • 1 0BCapacity Planning Number of UsersNo. City Year 2000 Year 2001 Year 2002 1 GL 250804 353629 470899 Any telecom service involves four stages: initiation, growth, saturation and decline. For the services with this feature, user can use the growth curve method for prediction. This method is very suitable for medium-term prediction. 1.2.2.4 Curve of Second Order Method Many engineering problems need several groups of experimental data of two variables to find their relation represented by an approximate expression, called as empirical formula. After an empirical formula is set up, users can combine their experiences in production process or experiments with the theory during analysis. When forecasting mobile users, users can set an empirical formula based on the situations in the past years and use the formula to forecast the future trend. For the growth of mobile users in a city, the empirical formula can be: y ax bx c= + +2 where, x- is the year and y- is the number of mobile communications users. Input the number of mobile users in the past years. Use the least square method to select the constants a, b, and c. σ 2 2 2 1 = − + + = ∑ [ ( )]y ax bx c i N Obtain the values of a, b and c according to the following formulas: ∂σ ∂2 2 2 1 0a y ax bx c xi i i i i N N = − + + = = ∑[ ( )] ∂σ ∂ ∂σ ∂ 2 2 1 2 2 1 0 0 b y ax bx c x c y ax bx c i i i i i i i i i N = − + + = = − + + = = = ∑ ∑ [ ( )] [ ( )] Introduce the number of mobile subscribers of GL in 1996-1999 to the formula and obtain the values of a, b and c. Then, user can predict the number of the mobile subscribers of the city in the coming three years. Use the curve of second order method to predict the number of mobile subscribers of 13
    • GSM Radio network planning principle 14 GL for 2000-2002, as shown in the following table: Number of UserSN City Year 2000 Year 2001 Year 2002 1 GL 287371 430790 606184 1.2.3 Traffic Distribution Prediction The traffic distribution of the mobile cellular services in China has the following characteristics: The traffic is mainly concentrated in medium and big cities, and there are traffic dense areas in the downtown areas of a city. In such areas, there are usually local higher traffic hot areas. In addition, the traffic volumes in the suburbs are low. If these factors are not considered in network building, with sites distributed evenly, the resources in the low-traffic areas are wasted, while the capacity in the heavy-traffic areas is insufficient, affecting the Return On Investment (ROI) of the network and the service quality. To solve this problem, user must predict the traffic density distribution, and deploy the sites and configure the channels according to the prediction results. In earlier phases, use the statistical data of population distribution, income situation, vehicle usage, and telephone usage to forecast the geographical distribution of traffic demand. After the network is constructed, obtain more accurate traffic distribution data of the serving area based on the traffic statistics by the OMC, which can serve as reference in network optimization and expansion. Three methods for forecasting traffic density are available: 1. Percentage distribution method 2. Linear forecast 3. Linear forecast in conjunction with manual adjustment Percentage distribution method: This method divides the serving area into high density area, medium density area, and low density area (for example, high density district, common district, and suburbs), and allocate the forecasted percentage of mobile users to each density area. Then, get the number of users in the density area by multiplying the forecasted number by the percentage, and get the user density by equally dividing the area. Linear forecast: Use the planning software and electronic map to distribute the existing busy hour traffic to each cell. Input the total traffic of the target year in the system.
    • 1 0BCapacity Planning 15 Then, the planning software generates the traffic distribution graph of the target year based on the existing traffic distribution. 1.3 Capacity Planning Process From the previous prediction on the capacity and traffic distribution, the total traffic demand and the traffic distribution and area of each special district in the serving area can be obtained. Correctly predict the user development in the planned area. Select an appropriate frequency multiplexing method based on the available band resources. In combination of the configuration capacity of the wireless products and features of wireless environment/user distribution in the planned area, determine the site types of different areas. At last, get the number of sites that meet the capacity requirement. The result of capacity planning is as follows: 1. Number of base stations that meet the traffic requirement in the planned area 2. Site configuration of each base station 3. Number of TCHs provided by each sector, traffic, and user quantity 4. Number of TCHs provided by each base station, traffic, and user quantity 5. Number of TCHs provided by the entire network, traffic, and user quantity The previous procedure is an initial planning. In later radio coverage planning and analysis, some sites may be added or reduced. Then, the number of bases stations and site locations can be finally determined. 1.3.1 Work Flow Work flow is as follows: Predict capacity → Analyze traffic distribution → Determine site configuration → Determine site quantity → Determine site lausert 1. In initial phase of network development, less capacity is required. Consider the basic coverage. The site is generally small and the network structure is simple.
    • GSM Radio network planning principle 16 2. In middle phase of network development, comparatively more capacity is required and higher coverage requirement must be met. This can be realized through site expansion and cell splitting. The network structure is comparatively complex. 3. In the advanced phase of network development, large capacity is required and no blind spots are allowed in the coverage. This can be realized through adding micro cells and deploying dual band networks. The network structure is complex. 1.3.2 Prerequisites of Capacity Planning 1. Total traffic of the planning area and traffic distribution prediction 2. GOS, that is, congestion rate or loss probability 3. Available band resource and sequence multiplexing mode 1.3.3 Calculation of Capacity Planning 1. Estimate the number of base stations, site types, and capacity in the capacity limited districts. Estimate the largest site types of various districts based on the frequency resources and frequency multiplexing mode Get the capacity of each base station based on the traffic model Get the number of base stations required by dividing the total traffic by the maximum capacity (sum of each cell) 2. Estimate the number of base stations, site types, and capacity in the coverage limited districts. Based on the district type, divide the area by the corresponding coverage area (estimated) to get the total number of base stations that meet the coverage requirement. Multiply the cell coverage area (estimated) by the corresponding traffic density to get the traffic that meets the cell requirement. Estimate the number of required voice channels and control channels by referring to the Erlang-B table. Divide the sum of the two numbers by 8 to get the frequency required by the cell.
    • 1 0BCapacity Planning 17 3. Output the result: number of base stations, site type. 1.4 Channel Capacity Planning 1.4.1 SDCCH Capacity Planning 1. SDCCH structure and bearing service type The SDCCH has two types of structure: SDCCH/4 (used in mixed control channel structure) and SDCCH/8 (used in separate control channel structure). In a GSM network, the cell broadcast service can be activated to broadcast the short messages in the SMS center to all users registered in the location area. After the broadcast service is activated, each cell broadcast cell (CBCH) must seize one SDCCH. Combined channel: BCCH + CCCH + SDCCH/4 (TS0) Non-combined (separate) channel: BCCH+CCCH (TS0) + X x SDCCH/8 (1 – 7 timeslots of BCCH frequency or timeslots of any other frequencies) In network planning, configure X according to the number of frequencies (that is, number of TCHs), and the ratio of TCH traffic to SDCCH traffic. The SDCCH mainly bears the following services: 1. Location update, periodic location update 2. IMSI attach/detach 3. Call setup 4. SMS 5. Fax and supplementary services The seizure time of above events vary depending on the network structure and traffic model. 2. SDCCH GOS and capacity ratio of SDCCH to TCH When defining the number of SDCCHs, the congestion rate of both SDCCHs and
    • GSM Radio network planning principle TCHs must be considered. The reason is that during a conversation, the SDCCH is required to transfer call connection signaling and the TCH is required to transfer voice and data messages. For the setup of a conversation, the SDCCH and TCH are of the same importance. However, the SDCCH can use the physical channel more effectively. In such a case, the congestion rate of SDCCH should be lower than that of TCH. The principle for determining SDCCH congestion rate is that the congestion rate of SDCCH should be 25% of that of TCH. If the actual ratio is greater than 25%, more SDCCHs should be defined. For the SDCCH/4 configuration, the congestion rate of SDCCH should be 50% of that of TCH. The SDCCH/8 GOS is 1/4 of that of TCH. The SDCCH/4 GOS is 1/2 of that of TCH. For example, if the TCH GOS is designed to be 2%, then: SDCCH/4 GOS = 1% SDCCH/8 GOS = 0.5% According to the BSC channel assignment algorithm, signals can be transferred on TCH, and messages of early assignment and dynamic SDCCH allocation can also be transferred on TCH. This function of immediately assigning TCH to transfer call connection signals during the call setup process can reduce call losses and improve the GOS. 3. SDCCH capacity prediction SDCCH traffic is predicted on the basis of common traffic model. The SDCCH traffic model varies from network to network and from traffic model to traffic model. During SDCCH planning, use the traffic model provided by the operator. In case that the traffic model is known, the traffic per user of different mobile phone acts can be calculated. The calculation formula is as follows: 18 (mE/user) 3.6 durationuserrateesectuiontimbusytimeex Traffic × × = The calculation process (considering only location update, SMS, and call setup) is as follows:
    • 1 0BCapacity Planning 19 Known conditions: Location update factor: L Ratio of SMS to calling amount: S Average call duration: T Cell traffic: Acell Location update duration: TLU Call setup duration: TC SMS duration: TSMS SDCCH clear-down protection duration: TG Then: Number of cell calls in busy hour: λCALL =Acell x 3600/T Number of location updates in busy hour: λLU =L x Acell x 3600/T Number of SMSs in busy hour: λSMS =S x Acell x 3600/T=6Acell Then, the traffic that the SDCCH needs to bear is as follows: ASDCCH=[λCALL x TC +λLU x (TLU + TG)+ λSMS x (TSMS +TG)]/3600 After the traffic that the SDCCH needs to bear is obtained, refer to the Erlang-B table according to the GOS and acquire the number of SDCCHs. 4. Recommended SDCCH configuration TRX Quantity Channel Quantity SDCCH Structure SDCCH Quantity TCH Quantity TCH Traffic (GOS=2%) 1 8 SDCCH/8 1 6 2.28 2 16 SDCCH/8 8 14 8.2 3 24 2*SDCCH/8 16 21 14.9 4 32 2*SDCCH/8 16 29 21 5 40 2*SDCCH/8 16 37 28.3 6 48 2*SDCCH/8 16 45 35.6 7 56 3*SDCCH/8 24 52 43.1 8 64 3*SDCCH/8 24 60 49.6 9 72 3*SDCCH/8 24 68 57.2 10 80 4*SDCCH/8 32 75 64.9
    • GSM Radio network planning principle 20 1.4.2 CCCH Configuration Principle 1. CCCH structure a. CCCH consists of Access Grant Channel (AGCH) and Paging Channel (PCH), Random Access Channel (RACH). b. Uplink channel sends channel request message. The downlink channel sends access granted (that is, immediate assignment) message and paging message. c. All the TCHs of each cell share the CCCH. 2. CCCH configuration CCCH-CONF Meaning Number of CCCH Message Blocks of a BCCH Multiframe 1 basic physical channel used for CCCH0 Not combined with SDCCH 9 1 basic physical channel used for CCCH1 Combined with SDCCH 3 2 basic physical channel used for CCCH2 Not combined with SDCCH 18 3 basic physical channel used for CCCH4 Not combined with SDCCH 27 4 basic physical channel used for CCCH6 Not combined with SDCCH 36 1.4.3 Recommended CCCH and TCH Allocation In combination with SDCCH recommended configuration and CCCH channel structure, suppose that the average busy hour traffic is 0.025Erl/user and call loss GOS of radio channel is 2%, refer to the Erlang-B table to acquire the maximum traffic provided by each cell configured with different number of frequencies. Use this value to calculate the maximum number of users supported by each cell, and get the maximum number of users supported by the system. CCCH Capacity (Erlang) Frequency Quantity Channel Quantity Channel Structure CCH Quantity (SDCCH) TCH Quantity GOS=2% 1 8 (1BCCH+9CCCH)+SDCCH/8 1 6 2.28 2 16 (1BCCH+9CCCH)+SDCCH/8 1 14 8.2 3 24 (1BCCH+9CCCH)+2*SDCCH/8 2 21 14.9 4 32 (1BCCH+9CCCH)+2*SDCCH/8 2 29 22
    • 1 0BCapacity Planning CCCH Capacity (Erlang) Frequency Quantity Channel Quantity Channel Structure CCH Quantity (SDCCH) TCH Quantity GOS=2% 5 40 (1BCCH+9CCCH)+2*SDCCH/8 2 37 28 6 48 (1BCCH+9CCCH)+2*SDCCH/8 2 45 35.5 7 56 (1BCCH+9CCCH)+3*SDCCH/8 3 52 42.12 8 64 (1BCCH+9CCCH)+3*SDCCH/8 3 60 49.64 9 72 (1BCCH+9CCCH)+3*SDCCH/8 3 68 57.2 10 80 (1BCCH+9CCCH)+4*SDCCH/8 4 75 64.9 Based on the traffic model and GOS, obtain the channel configuration and traffic by referring to the configuration recommended in the above table. 1.5 Optimization of Capacity Planning The initial capacity planning is performed on the basis of prediction and assumptions. With the implementation of the plan and network construction, the traffic model may change, which has a direct effect on the capacity planning. Therefore, the initial plan should be adjusted and optimized according to the actual situation. This can help future network optimization and reduce investment cost. The recommended calculation method of traffic model is as follows: Adjust and optimize the capacity planning in any of the following situations: 1. User behavior change: It refers to the user capacity deviation because of user movement in the local network. User behaviors can be classified into user traffic behaviors and user movement behaviors, which respectively result in the deviation of user traffic from macroscopical and microcosmic aspects. The fluctuation factor (generally 1.05 - 1.1) is used to measure the degree of affect of traffic deviation on the network. The network margin resulted from the deviation cannot be saved in network construction. 2. Channel configuration nonlinear effect: Channel configuration is calculated by frequency amount rather than linear configuration based on requirements, which leads to channel wastage. For example, if only 9 TCHs are required for a cell after the calculation, 2 TRX (14 TCHs) must be configured. According to the 21
    • GSM Radio network planning principle 22 research result, the nonlinear configuration reduces the network utilization by 20% - 25%, with 20% for provincial capitals, 25% for common cities, and 30% for backward districts with many single carrier cells. 3. At initial phase of network construction, if large numbers of traffics are congested, the traffic added for solving congestion should be taken into consideration when predicting the traffic model. 4. For different phases of network construction, periodically analyze and predict the traffic model. 5. Take the user activation ratio (make sure the value is reasonable) into consideration. This can reduce the traffic module value of each user, reduce cost, and remedy the effects resulted from uncontrollable factors. 6. Other factors 1.6 Improvement of Network Capacity In the initial phase of GSM network construction, configuring little number of base stations and small site type can meet the requirement of limited users. The prediction focus is coverage. With the increase of user quantity and promotion of new services, the cell congestion is more and more serious. To improve network quality, the network capacity must be improved. The sequence of capacity design priority is as follows: Compact base station → base station expansion&cell splitting → micro cells for hot areas → dual band network → half rate 1.6.1 Methods for Improving Network Capacity Split cells Use aggressive frequency reuse pattern Add micro cell devices Expand frequency band Half rate
    • 1 0BCapacity Planning 23 1.6.2 Analysis of Methods for Network Capacity Improvement 1.6.2.1 Cell Splitting 1. During the initial phase of GSM network construction, the coverage is the main problem, with large antenna mount height, spaces between base stations, and cell coverage radius. 2. With the development users, the original cells can be split into smaller cells with less coverage area. 3. Cell coverage radius can be reduced by minimizing the space between base stations and lowering antenna height or enlarging antenna downtilt. 4. After cells are split, the numbers of bases stations, frequencies, and channels are increased and thus the traffic capacity and user quantity are increased. The methods to split cells are as follows: 1. Add a new base station in the middle of the existing two base stations. 2. Split the cell with the radius being1/2 of the existing cell. The antenna directions are not changed. 3. A cell cannot be split infinitely. The space between macro-cellular base stations should be at least 400 m. 4. Antenna height cannot be excessive. In medium cities, the height is recommended to be around 25 m. 5. For base stations with high antenna height, better reduce the height when splitting cells. 1.6.2.2 Aggressive Frequency Reuse Pattern If network capacity cannot be improved through cell splitting, use aggressive frequency reuse (AFR) pattern can be used to improve band utilization. Add the site types supported by the network, and thus improve the entire network capacity. Common AFR patterns are as follows: 1. Multiple Reuse Pattern (MRP) 2. 1 x 3 (or 1 x 1) multiplexing technology 3. Concentric Cell
    • GSM Radio network planning principle 24 1.6.2.3 Adding Micro Cell Devices 1. Micro cells are used to reduce coverage holes, and solve the problem of traffic overflow in traffic hot spots. 2. The comparison between macro cells and micro cells are as follows: Macro cells are in the lower layer and covers larger areas. They absorb the main traffic. Micro cells are in the higher layer and supplement the coverage of macro cells by mainly improving indoor coverage. They absorb hot spot traffic and improve network quality. 1.6.2.4 Expanding Band 1. Add the number of carriers by expanding the band. This can add system capacity. 2. After 900M resources are assigned to GSM, introduce 1800M band to build dual band network which can absorb traffic to improve network capacity. 3. The dual band network of the same manufacturer can use the co-bcch technology to save one CCCH. 1.6.2.5 Half Rate 1. The frequency resources are limited. The expansion method by adding frequencies is not applicable. Adding frequency points also increases the cost. 2. Traffic Channel/Half-rate Speech (TCH/HS) enables the channel that can bear one TCH/FS or TCH/EFS to bear two TCH/HSs. That is, the channel capacity is doubled. 3. TCH/HS uses the VSELP coding. To adapt to the half rate bandwidth, the coding rate is reduced to 5.6 kbps. Compared with the 13 kbps of full rate speech coding, the speech quality is a little reduced. 1.7 Location Area Planning Location area (LA) is an important concept in GSM. As defined by the GSM protocol, the entire mobile telecommunications network is divided into different service areas according to location area identity. LA is the basic unit of paging range in GSM network. That is, messages are paged based on LA. The paging message of a mobile user is sent to all cells in the LA. One LA may contain one or more BSCs, but it belongs to only one MSC. One BSC or MSC can contain multiple LAs.
    • 1 0BCapacity Planning 25 LA size (that is, the coverage of a location area code (LAC)) is a key factor in the system. From the perspective of reducing frequent location update and saving channel resources, large LA size is recommended. The reason is that more location updates enlarges the SDCCH load, reduce channel resources, and add the load of MSC and HLR. In addition, a cell update takes about 10 s and during the update, an MS cannot make or receive calls. However, if the LA is so large that paging capacity cannot match, the paging signaling load is excessive, which causes missing paging messages and lowers the paging success rate. Also, low paging success rate make users call again and thus the paging load is increased and the success rate is further reduced. Therefore, the LA cannot be set to a too large value. In this case, when performing network planning, consider the balance between LA capacity, channel resources, and paging capacity. Try to reduce frequent location update to the minimum on prerequisite that the paging load is not excessive. 1.7.1 Determining LA Edges During the initial phase of GSM network construction, the base stations under multiple BSCs can be set as an LA. With the increase of traffic and frequency capacity, the traffic that a BSC bears is enlarged. In this situation, the definition of LAs gradually evolves to division by BSC. That is, a BSC is set as an LA or multiple LAs in some cases. If the LAs are too small, new problem may occur, such as too frequent location updated across LAs. This increases the exchange load. During location updates cross LAs, an MS cannot make or receive calls. However, in high traffic areas, the MS may have frequent activities in overlapped areas. This poses higher requirement on the determination of edges between two or several LAs. With the network development and increase of user density, the location updates across LAs has more and more effect on the system load. Therefore, the determination of LA edges is more and more important. The principles of determining LA edges are as follows: 1. Determine the edges away from high traffic districts. Try to set them in low traffic districts, such as suburbs or factories. These districts has small density and MS locate update range is small and therefore the location update across LAs has comparatively smaller effect on the network load. If the dense downtown cannot be bypassed, try to set the edge in the district having users with low mobility, such as inhabited communities. 2. Set the LA edge to be athwart to the road. Avoid setting the overlapped area n the
    • GSM Radio network planning principle 26 high mobility area. This can avoid large numbers of ping-pang location updates when crossing LAs. If the setting does not obey this rule, the system may be greatly affected. 3. Avoid setting the edges of several location areas at the same small area. This reduces constant location updates of MSs in a small area. 4. When determining LA edges, consider the growth trend of traffic. Based on the design of paging capacity and traffic capacity, the expansion margin should also be considered to avoid frequent LA division and splitting. 1.7.2 LA Paging Capacity 1.7.2.1 Paging Principles When an MS under an LAC is paged, the MSC sends a paging request to all cells of the LAC through the BSC. Currently, two paging modes are available: TMSI paging and IMSI paging. In a GSM system, each user is assigned a unique IMSI, which is written in the SIM card. Each IMSI is 8 bytes long and used to identify user ID. TMSI is temporary number assigned by the VLR after the mobile is successfully authenticated. It is only used to replace the IMSI on the air interface in the VLR managing scope. A TMSI is 4 bytes long and corresponds to the IMSI. Therefore, when the PCH of the air interface uses the IMSI paging mode, the paging request message can contain only two IMSI numbers. When the TMSI paging mode is used, four TMSI numbers can be contained. In this case, the paging load of using IMSI mode doubles that of TMSI mode. When the MSC obtains the location area identity (LAI) of the MS from the VLR, it sends paging messages to all the BSCs in the LA. After receiving the paging message, the BSC sends the paging command message to all its cells of the LA. After receiving the paging command, the base station sends a paging request message on the paging sub-channel to which the paging group belongs. This message carries the IMSI or TMSI of the paged user. After receiving the paging request message, the MS request for SDCCH assignment through the RACH. After the BSC determines that the base station activates the required SDCCH, it assigns the SDCCH to the MS on the AGCH through the immediate assignment command message. The MS uses the SDCCH to send the Paging Resp message to the BSC. The BSC forwards the Paging Resp message to the MSC. Then, a radio paging is complete.
    • 1 0BCapacity Planning 27 1.7.2.2 Paging Policy If the location area of the MS is known in the VLR, the first paging message is broadcasted only within the LA registered by the MS, that is, local paging. If the MS does not respond to the first paging, the MSC originates a second paging. The second paging is always broadcasted within the original LA, but the message can be paged to all cells within the MSC, that is, global paging. Global paging improves the paging success rate. The MSs can be distinguished through TMSI or IMSI during a paging. 1.7.2.3 Paging Parameter Configuration According to the GSM specification, the CCCH configuration has two modes: 1. Combining CCCH and SDCCH, also called combined BCCH. Each multiframe transfers three paging groups. 2. Not combining CCCH and SDCCH, also called non-combined BCCH. Each multiframe transfers nine paging groups. Paging groups can broadcast paging requests as a paging channel (PCH) and respond to the MS access requests as an AGCH. In operation, several multiframes can be combined to form a paging period to add the number of paging groups in a cell. The MS periodically listens to its paging groups. When the MS is called, it will detect the paging request sent by the base station and make responses. If many paging groups are set, the MS needs to wait for a long time before detecting the correct paging group. This adds the paging time. If less paging groups are set, the MS will frequently listen to the paging group and reduce the call setup duration. In this case, the phone battery consumes fast. The number of paging groups of a cell can be adjusted through the following two parameters: 1. Number of access grant blocks BS-AG-BLK-RES This parameter defines the number of AGCH dedicated paging groups within each multiframe. For cells with combined CCCH and SDCCH, the value range is 0-2. For cells with non-combined CCCH and SDCCH, the value range is 0-7. If the cell broadcast channel (CBCH) is used, the value range is 1-7. If the value is set to 0, it indicates that the dedicated AGCH is not available and all the paging groups are shared by PCH and AGCH. If the value is set to more than or equal to 1, it indicates that the paging group is kept as the AGCH dedicated channel. The specific value depends on the cell traffic.
    • GSM Radio network planning principle 28 The following table shows the number of CCCH message blocks contained in each BCCH multiframe (containing 51 frames) under different CCCH configurations. CCCH _CONF BS_AG_BLK_RES Number of Blocks for SGCH Within Each BCCH Multiframe Number of Blocks for PCH Within Each BCCH Multiframe 0 0 3 1 1 2 2 2 1 1 Other (invalid) - - 0 0 9 1 1 8 2 2 7 3 3 6 4 4 5 5 5 4 6 6 3 Other 7 7 2 2. Number of multiframes of PCHs (BS_PA_MFRAMS) This parameter defines the number of multiframes for the 51 TDMA frames of the MSs that transmit paging messages to the same paging group. According to the GSM specification, each mobile user (or each IMSI) belongs to a paging group. Each paging group in each cell corresponds to a paging sub-channel. The MS calculates its paging group according to its IMSI and then gets the paging sub-channel location of the paging group. In actual network, the MS only listens to the paging sub-channel it belongs to and ignores the contents of other paging sub-channels. Power off some hardware of the MS so as to save the power overhead (that is, the DRX origin). The number of multiframes of PCH BsPaMframs indicates the number of multiframes to form a cycle of paging sub-channel. In other words, this parameter defines the number of paging sub-channels that are allocated by the PCH in a cell. The MS uses this parameter to calculate the paging group that it belongs to so that it can listed to the corresponding paging sub-channel. The following table shows the relation between AGCH, MFRMS, number of paging groups, and interval. Number of Paging Groups in Combined Situation Number of Paging Groups in Non-Combined Situation BS_PA_M FRAMS Interval Between Paging Groups (second) AGCH=0 AGCH=1 AGCH=0 AGCH=1
    • 1 0BCapacity Planning 29 2 0.47 6 4 18 16 3 0.71 9 6 27 24 4 0.94 12 8 36 32 5 1.18 15 10 45 40 6 1.41 18 12 54 48 7 1.65 21 14 63 56 8 1.89 24 16 72 64 9 2.12 27 18 81 72 1.7.3 LA Capacity Calculation The method for calculating LA capacity is as follows: Number of paging blocks/s x paging messages/paging block = Max paging times per second → paging times per hour → allowable traffic of each LA → frequencies of each LA 1. Number of paging blocks per second One frame equals 4.615ms. One multiframe equals 51 frames or 0.2354s. Suppose that the number of access allowed blocks is AGB. The number of paging blocks per second is calculated in the following way: For non-combined BCCH: Number of paging blocks per second = (9-AGB) / 0.2354 (paging blocks/s) For combined BCCH: Number of paging blocks per second = (3-AGB) / 0.2354(paging blocks/s) For non-combined BCCH, the ZTE configuration is AGB=2. Then, the number of paging blocks per second is 29.7 paging blocks/s. For combined BCCH, the AGB equals 1, and then the number of paging blocks per second is 8.5 paging blocks/s. In the same LA, it is not recommended to deploy both combined BCCH and non-combined BCCH. The number of access grant blocks should be consistent at the same LA. Otherwise, the paging capacity may be reduced to the smallest in the LA. If the LA does not have great capacity and the location area coding resource is insufficient, the combined BCCH and non-combined cells can be deploying the same LA, thus increasing the number of TCHs in 10 and S111 base stations.
    • GSM Radio network planning principle 30 2. Paging times per paging block (X) When a BTS broadcasts paging requests from paging groups, the following configurations may be available: a) 2 IMSIs, or b) 2 TMSI and 1 IMSI, or c) 4 TMSIs Then, the average allowed paging times per paging block (X) is as follows: X = 2 paging times/paging block If IMSI paging mechanism is used X = 4 paging times/paging block If TMSI paging mechanism is used 3. Maximum paging times per second (P) The calculation formula is as follows: For non-combined BCCH: P = (9-AGB) /0.2354 (paging blocks/s) x X (paging times/paging block) For combined BCCH: P = (3-AGB) /0.2354 (paging blocks/s) x X (paging times/paging block) IMSI paging mechanism: for non-combined BCCH, when AGB=2, P=59.47 paging times / s; for combined BCCH, when AGB=1, P=16.99 paging times / s. TMSI paging mechanism: for non-combined BCCH, when AGB=2, P=118.95 paging times / s; for combined BCCH, when AGB=1, P=33.98 paging times / s. 4. Allowed traffic of each LA (T) One principle of designing LA capacity is that the LA size must not exceed the allowed maximum paging capacity. For an operating network, the times of message paging (C11621) sent by the BSC can be collected from the daemon and converted into paging messages per second. The value of T must not exceed the result. When no traffic statistical data can be referred, for example, a new network, calculate the value by supposing a traffic model: Average call duration: 60 s, that is, 1/60 Erl Ratio of Times of MS terminated calls (causing paging and generating TCH traffic) to total call times: 30% Suppose 75% of MSs respond at the first paging, 25% of MSs respond at the second paging. Ignore the MSs that respond at the third paging. Then, each successful paging of an MS needs 1.25 paging.
    • 1 0BCapacity Planning 31 Suppose after 50% of theoretical maximum paging capacity is exceeded, the PCH is congested. That is, in the case that 50% of the maximum paging load is not exceeded, the original paging messages will not discarded because the BTS paging queue is full. In this case, the paging volume in one second is P*50%. For IMSI paging mechanism, when AGB=2 and non-combined BCCH is used, the calculation of the traffic volume allowed in an LA is as follows: T*30%/(1/60)*1.25 = P*50% = 59.47*3600*50% T = 4757.6 Erl (AGB=2, non-combined BCCH) Similarly: T= 1359.39 Erl (AGB=1, combined BCCH) For TMSI paging mechanism, the traffic allowed by an LA is as follows: T= 9515.72 Erl (AGB=2, non-combined BCCH) T= 2718.78 Erl (AGB=1, combined BCCH) 1.7.4 Affect of SMS on LA Paging Capacity Short messages can be sent through SDCCH or SACCH. According to the difference of SMS sending and receiving, the process can be divided into SMS originating procedure and SMS terminating procedure. The affect of SMS on LA paging capacity mainly goes to the SMS receipt on the MS. When an MS receives SMSs, just like the case when the MS serves as the termination, the system needs to page the MS. Therefore, the effect of MS receiving an SMS on the network is the same as that the MS is called. The following part calculates and analyzes the effect on the network based on a certain SMS traffic model. The SMS service is to receive 3 SMSs / user / day. The retransmission rate is 30%. The busy hour factor is 0.12. Suppose that there are 100,000 users in an LA, then the busy hour SMS paging times are as follows: 100000 x 3 x 0.12 x (1+30%) = 46800 (times/h) This shows that that the large amount of paging comes from SMS, which affects the system performance.
    • GSM Radio network planning principle 32 In addition, SMS have bursty traffic. In peak period, for example, festivals, the burst factor can reach 3-8. That is, the SMS quantity in festivals is 3 to 8 times of normal situation. The paging coming from SMS can reach: 100000 x 3 x 0.12 x 8 x (1+30%) = 374400 (times/h) This data is amazing. Also, the SMS peak accompanies traffic peak. The two peaks lead to a large paging volume, which has great impact on the system. In this case, flow control measures need to be taken to ensure that the network survive the SMS and traffic peak. The measures can be setting SMS to retransmission prohibit, delaying data processing in peak, and reducing maximum paging times.
    • 2 1BLink Budget and Coverage Planning 2 Link Budget and Coverage Planning This chapter describes the following contents: System parameters and design parameters used in link budget Parameter definitions and recommend values in link budget 2.1 Purposes of Link Budget When performing tasks related to coverage during network planning and optimization, link budge is an important step. Through link budget, the maximum UL/ DL path loss is obtained, which is useful in future tasks. Link budget must be performed during the planning phase to make the uplink signals and downlink signals balanced in the coverage area. If uplink signal coverage is greater than downlink signal coverage, the signals at the cell edge are weaker and thus may be “submerged” by stronger signals in other cells. If downlink signal coverage is greater than uplink signal coverage, the MS is forced to “perch” under the strong signals. However, the uplink signals are weak and thus the voice quality is poor. The balance mentioned here is only a relative word. Deviation within a certain range is allowed. 2.2 Calculation of Uplink and Downlink Balance Figure 2.2-1 Power budget model 33
    • GSM Radio network planning principle Figure 2.2-1 shows that the purpose of link budget is to analyze the power balance between downlink and uplink through the given system parameters and design parameters. 2.2.1 Analysis of Uplink Budget Parameters The formula of uplink budget is as follows: Maximum allowable path loss = MS transmission power (dBm) + MS antenna gain (dB)–body loss (dB)–base station feeder loss (dB) + base station receiving antenna gain (dBi)–building or vehicle penetration loss(dB)–slow fading margin (dB)– fast fading margin(dB)–interference margin(dB)–base station receiver sensitivity (dBm) The parameters for uplink budget can be classified into four types: system parameter, MS transmitter parameters, base station receiver parameters, and margin reservation. 2.2.1.1 System Parameters 1. Carrier frequency Carrier frequency affects the transmission loss. Radio waves of different frequencies have different propagation models and different losses. 2. System bandwidth In a GSM system, the receiver bandwidth is 200 kHz (that is, 53 dBHz) 3. Data rate The full rate of GSM voice service is 9.6 kbit/s and the corresponding half rate is 4.8 kbit/s. Table 2.2-1 shows the CS1–CS4 rate in GPRS data service. Table 2.2-1 GPRS data rate 4. Background noise 34
    • 2 1BLink Budget and Coverage Planning 35 Background noise, also called thermal noise, is produced by the thermal movement of electrons. The formula is as follows: N=kTB where, k is Boltzmann constant, which equals 1.38 x 10-23 J/K, T is absolute temperature (K), and B is system bandwidth. Spectral density of thermal noise is kT. It is -174dBm/Hz in room temperature (300K). 2.2.1.2 MS Transmitter Parameters 1. Max TCH transmitter power At MS side, according to GSM protocol, the max MS transmission power is 2W (33 dBm). 2. Adapter loss It refers to the signal attenuation on various components on the route from transmitter to antenna. This value is generally ignored for MSs, that is, 0dB. 3. Transmitting antenna gain For MSs, electronically small antennas are always used. In addition, MS antennas must receive and transmit reliably in any direction. For GSM MSs, unipole antenna and planar inverted-F antennas (PIFA) are always used and the gain is 0 dBi. 2.2.1.3 Base Station Receiver Parameters 1. Antenna gain Select the antenna gain based on the area to be covered. Table 2.2-2 shows the values of base station antenna gains in different areas. Table 2.2-2 Base station antenna gains in different areas Area Antenna Gain (dBi) City 15.5 Suburb 15.5 - 17 Village 17 – 18 Highway or narrow valley 18 – 21 Mountain, hill 17 - 18
    • GSM Radio network planning principle 36 2. Feeder, adaptor, and combiner loss It refers to the signal attenuation on various components on the route from antenna to transmitter. The value is generally 3 dB in link budget. In actual situation, the value should be calculated based on the loss of cables with different lengths and types and various connectors. a) Adapter loss Generally, for one path feeder, 6 adapters are needed from transceiver to antenna input. Each adapter loss is 0.05 dB. Then, the total loss is 0.3 dB. If antenna amplifier is required, one more adaptor is required. The loss of lightening arrester is generally 0.2 dB. b) Feeder loss At the base station side, antennas and radio frequency front ends are connected through feeders. The feeder loss varies depending on feeder type and manufacturer. The loss of ZTE feeders is shown in Table 2.2-3. Table 2.2-3 Feeder loss Loss (dB/100m)Feeder Type 900 M 1800 M/1900 M 1/2 soft jumper 7.22 11.3 7/8 main feeder 3.89 6.15 15/8 main feeder 2.34 3.84 When selecting main feeders, observe the following principle: Feeder loss should be lower than 3 dB. If the loss exceeds 3 dB, use thicker feeders. Therefore, for main feeders used in DCS1800 cells in open land with more than 40 m and GSM sites with more than 70 m, 15/8 cables are recommended. Otherwise, the main feeder loss exceeds 3 dB. In normal situations, each route feeder needs 5 m 1/2 soft jumper and the loss is 0.35 dB. 3. Base station divider loss As combiners are used in the downlink, the uplink must use divider. Thus, loss is introduced. The loss varies depending on the dividing device. Determine the divider
    • 2 1BLink Budget and Coverage Planning 37 type according to the number of frequencies of the cell. Table 2.2-4 Divider / Combiner loss Divider/Combiner Type 900 M Insertion Loss (dB) 1800 M Insertion Loss (dB) CDUG 4.4 4.6 CEUG 3.5 3.6 CENG 5.3 5.5 CENG/2 5.3 5.5 ECDU 0.9~1.0 0.9~1.0 4. C/I required by TCH Carrier-to-Interference ratio (C/I) is the SNR requirement on the air interface. The target value varies depending on the propagation environment, mobility speed, and coding rate. According to the GSM protocol, C/I should be greater than or equal to 9dB. In actual situations, the C/I is greater than or equal to 12 dB with 3 dB margin added. 5. Noise figure Noise figure is generally used to measure the following issues: (a) Added value of environment noise received by the antenna compared with the thermal noise (b) Reduction in SNR after the signal passes the receiver (c) Added value of antenna noise temperature compared with receiver noise temperature to antenna caused by noise source from the antenna end (generally satellite antenna) In link budget of mobile telecommunications, noise figure includes the noise figure of base station receiver and the noise figure of MS receiver. When signals pass a receiver, noise is added to the signal and thus the noise figure is a method to measure the noise addition. The numerical value is the ratio of input signal to noise ratio (SNR) to output SNR. When signals and noises are input to an ideal receiver with no noise, they are equally attenuated or amplified. Thus, the SNR is changed, that is, F = 1 or 0 dB. In actual situations, a receiver has noise and the output noise power is greater than signal power. Thus, the SNR is worse and F > 1. Noise figure is an attribute of a receiver. As defined in GSM protocol, the noise figure
    • GSM Radio network planning principle 38 of a base station receiver is 8 dB. 6. Receiver sensitivity It refers to the minimum signal power which ensures that the receiver input can successfully discern and decode (or retain the required FER) signals. In telecommunications system, receiver sensitivity is given by: Receiver sensitivity = noise spectral density (dBm / Hz) + bandwidth (dBHz) + noise figure (dB) + C/I (dB) Noise floor = noise spectral density + bandwidth + noise figure C/I is the SNR requirement on the air interface. In narrowband system, C/I is the requirement of receiver base band demodulation performance. It is generally negative. Receiver sensitivity refers to sensitivity of MS receiver and BS receiver. Uplink is used for BS to receive signals. Therefore, BS receiver sensitivity should be considered. For example, for voice service, BS receiver sensitivity = noise spectral density (dBm / Hz) + bandwidth (dBHz) + noise figure (dB) + C/I (dB) = -174 + 53 + 8 + 9 = -104dBm. This formula can calculate the theoretical reference value for BS receivers in different situations. In actual situations, affected by various factors, the receiving sensitivity is better than the theoretical value. 7. TMA Effect Tower mounted amplifier (TMA) is actually a RF low noise amplifier (LNA). After a TMA is installed, the receiver noise figure is reduced, as proved by the relation between noise figures in cascaded system. Therefore, the receiver sensitivity is improved by around 3 dB. 2.2.1.4 Margin Reservation 1. Shadow fade margin Shadow fade is also named slow attenuation. It follows a lognormal distribution in the calculation of radio coverage. To reach the specified coverage probability, during network planning, certain power margin must be reserved for BS or MS receivers to reduce the attenuation effect. Reserved power is called shadow fade margin, whose value is associated with sector edge communication probability and shadow fading standard deviation.
    • 2 1BLink Budget and Coverage Planning a. Shadow fading standard deviation Shadow fading standard deviation is related to electromagnetic wave propagation environment. In urban areas, the shadow fading standard deviation is about 8 – 10 dB. In rural areas or villages, the value range is 6 – 8 dB. b. Edge coverage probability To evaluate the reliability of communication links in shadow fading environment, edge coverage probability is used to express the coverage quality. Coverage probability refers to the probability that the quality of communication between terminals in radio coverage edge (or inside coverage) and the base station meet the requirement (eg. BER). Coverage probability can be classified into location probability and time probability. For terrestrial radio communications system, changes in time have little effect on the communications probability. Therefore, location probability is the main factor to be considered during network planning. Coverage probability can also be classified into area coverage probability and edge coverage probability. The requirement defined by the former one is direct whereas using the latter one is convenient. Edge coverage probability is an index determining the coverage quality. It is defined as the time percentage of edge receiving signals exceeding the receipt threshold. In radio propagation, for a given distance, the path loss changes quickly and can be regarded as a random variable in lognormal distribution. If the network is designed based on the average path loss, the loss value on cell edge will be greater than the path loss median in one 50% of the time and smaller in the other 50% of the time. That is, the edge coverage probability is 50%. In this case, users at the cell edge will receive unsatisfactory service with a half chance. To improve cell coverage, fade margin should be deserved in link budget. Generally, the link budget is based on 75% edge coverage, and 90% for cities and 75% or villages. The following part explains by taking the 75% edge coverage probability as an example: Suppose the propagation loss random variable is ζ , thenζ is Gauss distribution in dB. Let the average be , standard deviation bem δ , and the corresponding probability distribution function be function. Set a loss thresholdQ 1ζ . When the propagation loss exceeds the threshold, signals fails to meet the demodulation requirement of expected services. Then, at the cell edge, satisfying 75% edge 39
    • GSM Radio network planning principle coverage probability can be translated into: ∫∞− − − =<= 1 2 )( cov 2 2 2 1 )1( ζ δ ζ ζ δπ ζζ dePP m rerage For outdoor environment, the standard deviation of propagation loss random variable is always 8 dB. Then, the margin of 75% edge coverage probability is as follows: dBm 4.58675.0675.01− = = × =δζ Figure 2.2-2 and Figure 2.2-3 show the graph: Figure 2.2-2 Attenuation margin Figure 2.2-3 Attenuation margin—normal distribution The previous two graphs show that during network planning and design, the 5.4 40
    • 2 1BLink Budget and Coverage Planning 41 dB margin must be reserved to ensure a 75% edge coverage probability. If 90% edge coverage probability is required, the 5.4 dB margin must be reserved. c. Area coverage probability In actual situation, area coverage probability is often important. Area coverage probability is the percentage of area of the location where receiving signal strength is greater than receiving threshold to the total area in a round region with the radius R. This parameter corresponds to the edge coverage area. When μ=3 and σ= 8dB, the 90% edge coverage probability corresponds to 96% area coverage probability and 75% edge coverage probability corresponds to 89% area coverage probability. d. Shadow fading margin In the same radio propagation environment, the shadow fading margin is mainly determined by coverage probability. Higher requirement on coverage probability leads to greater reserved margin and less maximum coverage area. Thus, the number of base stations is affected. In actual network, base station lausert is not regular because of building blockage. The effect of coverage probability on base station quantity is smaller than the theoretical calculation. However, it is sure that the base station quantity grows with the increase of coverage probability. Table 2.2-5 Common edge coverage probability and shadow fading margin Edge Coverage Probability (%) 70 75 80 85 90 95 98 Shadow fading margin/dB 0.53σ 0.68σ 0.85σ 1.04σ 1.29σ 1.65σ 2.06σ Note: σ is the standard deviation of shadow fading. The value is 6, 8, or 10 Table 2.2-6 Common area coverage probability and shadow fading margin μ=3 μ=4 σ=8dB σ=10dB σ=8dB σ=10dB Area Coverage Probability. (%) Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB 98 95 13.2 96 17.6 93 11.8 94 15.6 95 87 9 89 12.3 85 8.3 87 11.3
    • GSM Radio network planning principle 42 μ=3 μ=4 σ=8dB σ=10dB σ=8dB σ=10dB Area Coverage Probability. (%) Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB Edge Coverage Probability (%) Shadow fading margin/dB 90 77 6 80 8.5 73 5 76 7.1 75 52 0.5 56 1.6 47 0 51 0.3 Note: σ is the standard deviation of shadow fading. μ is path loss index. Generally, in urban areas, when σ= 8dB and edge coverage probability is 90%, shadow fading margin is 10.3dB, and in rural areas, when σ=8dB and edge coverage probability is 75%, shadow fading margin is 5.4dB. (2) Fast fading margin (Rayleigh fading margin) Fast fading is a type of multipath wave interference generated because the propagation is reflected by scattering objects (mainly buildings) or natural obstacles (mainly forest) around the MS (within 50-100 wave length). Fast fading always produce standing wave field. When an MS passes the standing wave field, the receiving signal is attenuated in a short period. Deterioration refers to the addition of receiving level in order to realize that the voice quality in multipath propagation effect and man-made noise (mainly vehicle spark interference) is the same as the condition when only receiver interval noise exists In GSM system, the deterioration of voice and data are both 3 dB. (3) Antenna density gain Antenna density gain refers to the gain brought by the usage of density technology on the base station. Generally, density gain can be considered in receiver sensitivity or separately considered. The BTS uses two-way density. The density gain is 3 dB. (4) Body loss Body loss refers to the loss produced by the signal blockage and absorption when hand-held mobile phones are near to the human body. The body loss is determined by the distance of mobile phone relative to the human body. When the hand-held mobile phone is near the waist or shoulder, compared with the case that the antenna is several wave lengths away, the signal field strength is reduced by 4-7 dB and 1-2 dB
    • 2 1BLink Budget and Coverage Planning 43 respectively. In the link budget of voice service, the value is 3 dB. In the link budget of data service using data card, the value is 0 dB. In the link budget of data service using mobile phones, the value is small and can be regarded as 0 dB. ZTE link budget always set the body loss to 3 dB. (5) Penetration loss Building loss is associated with building style and structure, for example, concrete structure, brick structure, window size, and style. Determine the building penetration loss according to the type of actual coverage area. Table 2.2-7 Values of penetration losses in normal situations Area Type 900 M Loss (dB) 1800 M Loss (dB) Dense urban 18-22 23-27 Common urban 15-20 20-25 Suburb and village 10-15 15-20 (6) Interference margin In a GSM system, the internal interference can be co-channel interference, adjacent channel interference, cross-modulation interference, and near-end-to-remote-end interference. Based on these factors, the interference margin is generally set to 3 dB. 2.2.2 Analysis of Downlink Budget Parameters The formula of downlink budget is as follows: Maximum allowable path loss = BS transmission power (dBm) + BS antenna gain (dB)– base station feeder loss (dB) – base station combiner loss + MS receiving antenna gain (dBi)–body loss (dB) – building or vehicle penetration loss(dB)–slow fading margin (dB)– fast fading margin(dB)–interference margin(dB)–MS receiver sensitivity (dBm) Similar to uplink budget, the parameters for downlink budget can be classified into four types: system parameter, MS transmitter parameters, base station receiver parameters, and margin reservation.
    • GSM Radio network planning principle 44 2.2.2.1 System Parameters Except carrier frequency, other parameters are the same as uplink. 2.2.2.2 BS Transmitter Parameters (1) BS transmit power The following table lists the transmit power. Table 2.2-8 BS transmit power Mode Transmit Power GSMK 40W 46dBm GSMK 60W 47.7dBm GSMK 80W 49dBm 8–PSK 30W 44.7dBm (2) BS divider/combiner loss For further details, refer to the corresponding content of uplink. (3) BS feeder and adaptor loss For further details, refer to the corresponding content of uplink. (4) BS antenna gain For further details, refer to the corresponding content of uplink. (5) Additional gain brought by new technology In ZTE V3 equipment, new technologies are added to the RF module, which brings additional gain to transmit signals. The DPCT technology can provide the gain of 2.5 dB. 2.2.2.3 MS Receiver Parameters (1) MS antenna gain For further details, refer to the corresponding content of uplink. (2) MS C/I ratio For further details, refer to the corresponding content of uplink. The value is determined by the protocol. (3) Noise figure
    • 2 1BLink Budget and Coverage Planning 45 For further details, refer to the corresponding content of uplink. The value is set to 10 dB as defined in the protocol. (4) MS receiver sensitivity MS receiver sensitivity is similar to the communication receiver sensitivity. The difference is that the C/I value and noise figure are different from the uplink. The noise figure is set to 10 dB. According to the C/I value defined in the protocol, the theoretical MS receiver sensitivity can be obtained, as shown in the following table: Table 2.2-9 MS receiver sensitivity using different GPRS coding modes GSM 900 and GSM 850 Propagation conditionsType of channel static TU50 (no FH) TU50 (ideal FH) RA250 (no FH) HT100 (no FH) PDTCH/CS-1 dBm -102 -101 -102 -102 PDTCH/CS-2 dBm -98 -97 -98 -98 PDTCH/CS-3 dBm -96 -95 -96 -95 PDTCH/CS-4 dBm -88 -87 -87 * DCS 1800 and PCS 1900 Propagation conditionsType of channel static TU50 (no FH) TU50 (ideal FH) RA130 (no FH) HT100 (no FH) PDTCH/CS-1 dBm -102 -102 -102 -102 PDTCH/CS-2 dBm -98 -98 -98 -98 PDTCH/CS-3 dBm -96 -95 -95 -95 PDTCH/CS-4 dBm -88 -84 -84 * Table 2.2-10 Receiver sensitivity using different GPRS coding modes (actual value) Coding Mode Static Model TU50 Model (no FH) Voice -110 -108 CS1 -113 -107.5 CS2 -111 -104.5 CS3 -109 -103 CS4 -105 -95
    • GSM Radio network planning principle 46 2.2.2.4 Margin Reservation For further details, refer to the corresponding content of uplink. 2.3 Coverage Planning 2.3.1.1 Coverage Simulation Coverage simulation is to use the planning software to plan the sites based on the user distribution in a certain area. It aims to ensure the area coverage and capacity and avoid interference. ZTE uses the AIRCOM network planning software. The initial simulation involves coverage prediction and BS engineering data determination. 1. Select the design indicators: The indicators include minimum receiving power and edge connectivity rate. 2. Select design parameters. The parameters include antenna height (from floor), antenna azimuth and gain, antenna downtilt, BS height, BS type, feeder type, feeder antenna loss, combiner/divider mode, transmitter output power, receiver sensitivity, BS diversity receiving type, and diversity gain. 3. Predict the coverage of each BS cell according to the propagation model of different districts. Give suggestions on BS site, antenna direction, downtilt, and height based on the possible blind spots and weak signal spots to get the actual engineering data of the BS. 2.3.1.2 Layered Coverage Networking has the following formats: macro cell, cell, highway, multi-layer, dual-band, as shown in the following figure:
    • 2 1BLink Budget and Coverage Planning GSM900 macroGSM1800 macro 900 micro1800 micro P-cell P-cell GSM900/1800 伞形小区 macro Figure 2.3-1 Layered coverage 2.3.1.3 Coverage Methods in Special Districts Tunnel: The following combinations may be used in actual situations: Micro BTS + single antenna scheme Micro BTS + distributed antenna system Micro BTS + leaky coaxial cables Repeater + single antenna scheme Repeater + distributed antenna system Repeater + leaky coaxial cables Railway 450 M leaky coaxial cables can be used. But these cables have greater radial loss (11dB/100 m). The coverage distance is relative small. When determining whether to use micro cell or repeater as the GSM signal source of the tunnel, consider the following factors: (1) Whether strong GSM signals are available around the tunnel entrance (2) Whether transmission lines are available around the tunnel Generally, if existing signal levels around the tunnel entrance (including high space such as hill) are lower than -80 dBm. Micro BTS is recommended. If the levels are higher than -80 dBm, repeater or micro BTS is recommended. If the transmission problem is difficult to solve, it is recommended to use repeaters. When using the repeater solution, take full consideration on repeater isolation. Otherwise, use the micro 47
    • GSM Radio network planning principle 48 BTS solution and improve system capacity. Offshore: Towers of offshore base stations are always installed on the seaside mountain top, with the height from 50 to 200 m. Because oversea propagation has little loss, signals can be transmitted to faraway seas. In this case, the ground should be regarded as a spherical surface. That is, the earth curvature has effect on the signal propagation. In addition, islands, hill, and large ships also have shadowing effect on signal propagation. Literature considers that oversea propagation model can be regarded as a free space propagation model. But according to the result of offshore testing, free space propagation model is not suitable for testing offshore coverage. Also, the Okumura-Hata model and amended parameters are not applicable to oversea propagation environment. It requires further study. Offshore large distance overage should be measured in combination with the technologies of cell expansion and dual-timeslot cell. These technologies are applicable to situations with the required coverage more than 35 km. Indoor coverage: With the development of social economy, skyscrapters and underway architecture such as subway, and underway carbarns emerge. In this situation, mobile phones are more and more frequently used indoors. Users require not only excellent outdoor mobile service but also satisfactory indoor mobile service. The problems always exist for indoor mobile communications: 1. Coverage: Due to complex indoor structure and building protective shielding and absorbing function, great transmission attenuation is lost in radio wave propagation. Weak field-strength area even blind area is formed. This cause the basement, storey 1 and storey 2 of a building to have weak field strength, even blind spots. Because of poor indoor coverage, the problems such as call drops, no response to paging, and user not available in the serving area are common. 2. Quality: Higher stories of a building always have radio frequency interference. The serving cell signals are unstable. Pingpang effect always occurs. The speech quality is poor and call drops occur. 3. Capacity: In buildings such as super market and conference center, because the
    • 2 1BLink Budget and Coverage Planning 49 usage density of mobile phones is large, the LAN capacity cannot meet user requirement. Thus, radio channels may be congested. 4. Currently, indoor coverage mainly depends on the extension mode of the existing network coverage, for example, using repeater, outdoor large power BTS, or mounting antenna high. However, these methods bring the following problems: 5. Due to great penetration loss, the indoor coverage effect is bad. Large amount of blind spots are generated. Conversation fails. 6. The repeater method has high requirement on the level of the source signals. In addition, the cross-modulation interference and co-channel and adjacent-channel interference are all severe. The conversation quality cannot be ensured. Even the network quality may be degraded. 7. The repeater and outdoor BTS methods does not basically solve the capacity problem. The network capacity is limited and the connection success rate is low. 8. Mounting antenna high may cause across-boundary coverage and affects the entire network quality. 9. When outdoor cells add frequencies, it is difficult to plan the frequencies and expand network capacity.
    • 51 3 Frequency Planning This chapter describes the following contents: Analysis of frequency reuse and interference models based on ideal cellular structure Common anti-interference technologies 3.1 Cellular Structure Creation Rule In ideal conditions, the basic unit (BTS area) of the cellular structure is a hexagon (handover edge). Several hexagons form a radio area cluster, and every two adjacent radio area clusters constitute the coverage area of the entire mobile network. The radio area cluster is the basic unit of frequency reuse. Inside one radio area cluster, all the available channels are evenly distributed to each BTS area or sector cell. Two same radio area clusters can be adjacent to each other to ensure the one-to-one relationship between various BTS areas or sector cells. Since the channel group allocated to each BTS area or sector cell is fixed, the corresponding BTS cells or sector cells in any adjacent radio area clusters are co-frequency cells, thus forming a complete co-frequency reuse pattern. The radio area cluster must meet the following conditions: 1. The radio area clusters should be adjacent to each other. 2. The central distance between any two co-frequency reuse areas in adjacent radio area clusters should be the same.
    • GSM Radio network planning principle 52 A B C D E F G A B C D E F G R 60o j i D Figure 3.1-1 Composition of a radio area cluster As shown in Figure 3.1-1, “i" and “j” are two parameters. Starting from one cell, user can take different values for these two parameters (cannot be 0) to reach one cell. According to the triangle relationship in the diagram, we can obtain the distance (D) between two co-frequency reuse areas: 22 jijiD ++= The radio area cluster complying with this distribution includes clusters for a number of N : 22 jijiN ++= Suppose the central distance between two adjacent BTS areas is 1 and the BTS area radius is R, then: 3/1=R Define the Dq R/= as the co-frequency reuse distance protection coefficient or co-frequency interference attenuation factor: N R D q 3== 3.2 Interference Models 1. Co-frequency interference protection ratio ( B )
    • 3 2BFrequency Planning It is defined as the minimum ratio of the useful RF signals measured on the input end of the receiver to the un-useful RF signal, usually measured in dB, when the useful signals of the output end of the receiver reach the specified quality. 2. Estimating the C/I Ratio in the N-Reuse Radio Area Cluster 53 A B C D E F G A B C D E F G A B C D E F G A B C D E F G A B C D E F G A B C D E F G A B C D E F G A B C D E F G A B C G A B C D E F G A B C D E F G A C D E F G A B C D E F G A B C D E F G B C G A B E F G A B C D E F G A D F E A B C D E Figure 3.2-1 Interference source For the radio transmission feature, user can obtain the model mentioned above for description: DiffkkkHdkHkdkkPL effeff 765loglog4log3log21 ++++++= For the radio transmission feature, user can obtain the model mentioned above for description: DiffkkkHdkHkdkkPL effeff 765loglog4log3log21 ++++ ++= Since the ideal cellular system is studied, each cell has the same transmitted power and also the same height of antenna, without diffraction loss. Thus, we can obtain the C/I ratio as below:
    • GSM Radio network planning principle ∑∑ ∑∑∑ = +− +− = − − = − − = − − = == === M k dkk dkk M k PL PL M k PL PL M k PLP PLP M k k kHeff Heff k kkt t I C I C 1 10/log)log42( 10/log)log42( 1 10/ 10/ 1 10/ 10/ 1 10/)( 10/)( 1 10 10 10 10 10 10 10 10 Let effHkkk log42'2 += , be the cell radius (d R ), and be the transmission distance between each interference resource to the cell ( ). kd D 10/'210/'2 10'2 12 1 10/2log'2 6 1 10/log'2 10/log'2 )2(126 1010 10 kk lk k Dk k Dk Rk DD R I C −− − = − = − − + = + = ∑∑ (4-5) Let '2k=γ /10 (the propagation path loss slope determined according to the actual geographical environment): γ γ γγ γ 2 12 6)2(126 + = + = − −− − q DD R I C Perform logarithm operation to them to obtain: )6log(log'2)( += kdB I 2γ 40'2 12C 10+q (4-6) To be general, take =k , 4=γ . Now dB5.06log10) 2 12 6log(10 4 ≈−+ . We can see that the next most powerful interference sources on the second circle can be ignored since they contribute to the interference far less than the most powerful interference sources on the first circle. Now, we have created the interference model in the ideal cellular environment. We will use this model to examine the interference when we describe the common reuse modes later in this document. 54
    • 3 2BFrequency Planning 3. Co-frequency interference probability )/( BICP ≤ In fact, because of non-ideal BTS location and the landform fluctuation, the signals received by a MS in motion are affected by Rayleigh fast fading and Gauss slow fading. Whether it is signal or interference, when it reaches the MS, the transient value and mean value of the filed strength are random variables. Even if a MS is still, the transient value and mean value of its field strength are still random variables due to the presence of various interferences, including the objects moving around it. Therefore, the C I/ value on the input end of the receiver is not constant, but a random variable. Only when C I B>/ , there is no interference. Co-frequency interference occurs at a certain frequency. According to the CCIR740-2 report, France in 1979 put forward that multi-path fading complies with Rayleigh distribution, and shadow fading complies with Gauss distribution. The probability of co-frequency interference is: ∫− ∞+ ∞ −−− + −u2 }exp{1 = duBCP u 10/)2 )/( σ π ≤I BIC( 101 Where, is the integral interference, andu σ is the standard deviation between signal and interference, IC σσσ −= . BICZ p −−= 100 10-1 10-2 10-3 10-4 20 40 60 dB σ = 12 σ = 0 σ = 6 σ = 8 Figure 3.2-2 Co-frequency interference probability Figure 3.2-2 provides the typical co-frequency interference probability. 55
    • GSM Radio network planning principle To be general, take σ = 6 and interference probability )/( BICP ≤ =0.1, and find from the table. The GSM requires that the co-frequency interference protection ratio (B) must be less than 9 dB. However, in engineering B=12 dB is often taken. Therefore, the dBZ p 12= IC / calculated in the ideal interference model is greater than: 9(12)+12=21 dB (24 dB). William C.Y. Lee believes that the margin of 6 dB is sufficient, hence the IC / calculated in the ideal interference model must be greater than: 9(12)+6=15dB (18dB). 3. Near-remote interference A C D B d2 d1 Cell 1 d2 d1 Cell 2 Figure 3.2-3 Near-Remote Interference According to the interference model, let the C/I of MS B against MS A be dB d d kdB I C 9log'2)( 2 1 −== , hence 69.1 1 2 = d d . If the frequency used by MS B is adjacent to that used by MS A, when 69.1 1 2 > d d , the adjacent interference protection ratio is not met and call drop will occur. The same case may occur in the adjacent cell. Let us see an extreme case: Suppose the output power of the antenna of cell 2 is 34 dBm, and the reception level at D is -85 dBm, and the BTS sensitivity is -110 dBm. Suppose the uplink/downlink powers are balanced, the transmitted power of MS D is -110+(34-(-85))=9dBm. 56
    • 3 2BFrequency Planning 57 Now, MS C in a very close distance is turned on and works at its maximum transmitted power, supposedly at 30 dBm (1W). Suppose the path loss for the signals arriving at cell 2 is the same as MS D, then, cell 2BTS receives the interference signals of 30-(34-(-85) = -89 > -110 + 9, so call drop occurs. 3.3 Frequency Reuse Technology and Interference Analysis Frequency reuse is a commonly used technique in GSM networks to use the same frequency to cover different areas. There must be an appropriate distance between these areas that use the same frequency. This distance is known as the co-frequency reuse distance. If omni directional antennas are used, the 4 x 3 frequency reuse pattern is recommended. In areas with large traffic, other frequency reuse patterns such as 3 x 3 and 2 x 6 can be adopted according to the capability of the equipment. In whatever mode, the basic principle is to consider different propagation conditions, different reuse modes, and multiple interference factors to meet the requirements of the interference-protection ratio, that is: Co-frequency interference-protection ratio: C/I ≥ 9 dB Adjacent-frequency interference-protection ratio: C/I -9 dB. 400KHz adjacent-frequency interference-protection ratio: C/I -41 dB 3.4 Frequency Reuse in Groups 3.4.1 4 X 3 Frequency Reuse The GSM adopts many frequency reuse patterns, including 4 x 3, 3 x 3, and 2 x 6. All frequency reuse patterns divide the limited frequencies into several groups to form a cluster of frequencies for allocating to adjacent cells, as shown in Fig4.2-1 According to the GSM specification, the 4 x 3 frequency reuse pattern is commonly used in various GSM systems. This pattern divides frequencies into 12 groups and allocates them to four sites in turn. That is, three frequency groups are available to each site. This frequency reuse pattern can reliably meet the requirement of co-frequency interference-protection ratio and adjacent frequency interference-protection ratio of GSM network to have high quality and secured service, as shown in Figure 3.4-1:
    • GSM Radio network planning principle A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 58 A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 A3 B1 B3B2 A1 A2 A3 B1 A1 A2A3 D2B1 D1 D3 A1 A2 A1 A1 A3 D2 A2 D1 B1 D3 B2 B3 C1 C2 C3 Figure 3.4-1 4 x 3 frequency reuse Let the side length of a cellular hexagon be 1. According to Figure 3.4-1 and the interference model described above, we can obtain: dBdB I 18 )2.7(28 log10)( 52.352.3 = + = −− C 2 52.3− The result deducted by the 6 dB margin as recommended by William C.Y. Lee is exactly 12 dB. The following part discusses the 4 x 3 frequency reuse pattern in engineering application: As the name implies, 4 x 3 reuse divides the available frequency into 4 x 3 = 12 groups, which are tagged as A1, B1, C1, D1, A2, B2, C2, D2, A3, B3, C3, and D3, as shown in the following table: A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 Assign A1, A2, and A3 as a group to three sectors of a base station. Assign B1, B2, and B3 as a group, C1, C2, and C3 as a group, and D1, D2, and D3 as a group to three sectors of an adjacent base station. Obviously, the following frequency reuse patterns are available:
    • 3 2BFrequency Planning Using the above grouping method, co-channel phenomenon does not appear among adjacent cells. However, opposite cells may have adjacent channels (as shown by the red arrows in the figure above): Pattern 1: D1---A2; Pattern 2: D2---A3; Pattern 3 D1---A2; Pattern 4: D2---A3; Pattern 5: D3---A1; Pattern 6: D3---A1. Change a frequency grouping pattern, as shown in the following table: A1 B1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3 1 2 4 3 5 8 7 6 9 11 10 12 13 14 16 15 17 20 19 18 21 23 22 24 25 26 28 27 29 32 31 30 33 35 34 36 Six frequency reuse patterns: Pattern 1 and 4 do not have adjacent channel. Pattern 2: C1---A2; Pattern 3: B2---A3; Pattern 5: C1---A2, B2---A3, D3---A1; Pattern 6: D3---A1 59
    • GSM Radio network planning principle Therefore, it is recommended to use the reuse pattern 1 and 3. As the base stations of each system are not exactly located in the grid, the former grouping pattern can also be used, but pay attention to the adjacent channel of opposite cells. The example table shows that the maximum configuration of the BTS is 3/3/3. The frequency utilization is very low and cannot meet the requirement of network capacity expansion in areas with high traffic. In some medium and large cities, the population is dense, and site distance is not more than 1 km and some of the coverage radius is only several hundred of meters, or even 300 m for some sites. Obviously, it is not realistic to improve the network capacity by using massive cell splitting. To meet the requirement of ever-increasing network capacity, there are two solutions available. One solution is to develop the GSM900/1800 dual band network, and the other solution is to adopt the aggressive frequency reuse pattern. 60
    • 3 2BFrequency Planning 3.4.2 3 x 3 Frequency Reuse A3 C2B1 C1 C3 B3B2 61 A1 A2 A3 C2B1 C1 C3 B3B2 A1 A2A3 C2B1 C1 C3 B3B2 A1 A2 A3 C1 A1 A2 A1 A3 C2 A2 C1 B1 C B3B2 A3 C1 A1 A2 A1 AA2 B1 B2 B Figure 3.4-2 3 x 3 frequency reuse Let the side length of a cellular hexagon be 1. According to Figure 3.4-2 and the interference model described above, we can obtain: dBdB C 3.13 2 log10)( 4 == I )57.5(2)7(2 44 + −− − The following part discusses the 3 x 3 frequency reuse pattern in engineering application: 3 x 3 reuse pattern generally uses baseband frequency-hopping. It divides the available frequency into 9 groups, which are tagged as A1, B1, C1, A2, B2, C2, A3, B3, and C3, as shown in the following table: A1 B1 C1 A2 B2 C2 A3 B3 C3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 The two reuse patterns are:
    • GSM Radio network planning principle A1 A2 A3 B1 B2 B3 C1 C2 C3 C1 C2 C3 B1 B2 B3 A1 A2 A3 A1 A2 A3 C1 C2 C3 B1 B2 B3 A1 A2 A3 C1 C2 C3 B1 B2 B3 B1 B2 B3 C1 C2 C3 A1 A2 A3 A1 A2 A3 B1 B2 B3 C1 C2 C3 Pattern 1: No adjacent frequencies for opposite cells. Pattern 2: C1---A2, C2---A3, C3---A1 Obviously, pattern 1 is better. 3.4.3 1 x 3 Frequency Reuse A 3 62 A 1 A 2 A 3 A 1 A 2 A 3 A 1 A 2 A 3 A 1 A 2A 3 A 1 A 2 A 3 A 1 A 2 A 1 A 2 A 3 Figure 3.4-3 1×3 Reuse Let the side length of a cellular hexagon be 1. According to Figure 3.4-3 and the interference model described above, we can obtain: dBdB I C 43.9 )36.4(25 2 log10)( 44 4 = + = −− − The following part discusses the 1 x 3 frequency reuse pattern in engineering application: 3 x 3 reuse pattern is the most closed pattern in frequency reuse. It is generally used in synthesizer hopping frequency system. In addition, anti-interference technologies such as DTX, power control, and antenna diversity also need to be used so as to rectify the interference deterioration resulted from the reduction of reuse distance. This technology divides the non_bcch frequency into A1, A2, and A3 groups, which individually serve as the MA of three sectors of each base station, as shown in the
    • 3 2BFrequency Planning following table: A1 1 4 7 10 13 16 19 22 25 28 31 34 A2 2 5 8 11 14 17 20 23 26 29 32 35 A3 3 6 9 12 15 18 21 24 27 30 33 36 When the hopping frequency load (cell frequencies/MA length) is less than 50%, ensure that the MAIOs of the three cells of the same base station do not have adjacent frequencies, MAIOs of the cells with the same direction in each base station are consistent, HSNs of the three cells of the same base station are the same, HSNs of adjacent base stations are different, and base stations with the same HSN should be as far as possible. 3.4.4 2 x 6 Frequency Reuse 63 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 A1 A2 A3 A4 A1 A2 A3 A4 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 A1 A2 A1 A2A6 A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 Figure 3.4-4 2×6 Reuse The 2x6 reuse model is not a symmetrical model. Cell A1 and cell A4 have different reuse distances from other cells. Let the side length of a cellular hexagon be 1. According to Figure 3.4-4 and the interference model described above, user can obtain the C/I ratios of cell A1 and cell A4:
    • GSM Radio network planning principle dBdB I C 86.16 )64.2( 1 log10)( 4 4 == − − C/I ratio of other cells: dBdB I C 04.12 )2( 1 log10)( 4 4 == − − 3.4.5 Multiple Reuse Pattern (MRP) MRP technology divides the whole frequency band into BCCH and TCH bands that are mutually orthogonal by using different reuse patterns. Each segment of carrier serves as an independent layer. Frequencies at different layers adopt different reuse patterns and the frequency reuse becomes increasingly closer layer by layer. One way of improving system capacity is to use closer reuse patterns. Because the BCCH plays an important role during the access and handover of the MS, the use of the frequency orthogonal to the TCH band to ensure BCCH quality brings about the following benefits: 1. BCCH can use the 4 x 3 or higher reuse coefficient to ensure quality, while the TCH can use the relatively intensive frequency reuse pattern. 2. The decoding of BSIC is independent of the load of speech channels. 3. Because the BCCH band and the TCH band are mutually orthogonal, the increase of TCH load hardly affects the BCCH, and thus the decoding of BSIC is not affected. In this way, the handover performance can be improved. 4. The configuration of the adjacent cell table is simplified. According to related literature, the adjacent cell table, if too long, may reduce the handover performance. 5. Because BCCH uses a separate frequency (12 frequencies in the 4×3 pattern), the adjacent cell table (composed of the BCCH frequencies) length can be reduced significantly. 6. The anti-interference technologies such as power control and DTX are brought into play. The BCCH cannot dynamically use the technologies such as power control and DTX. The BCCH always transmits signals at the maximum transmission power. Therefore, if the BCCH and TCH use the same frequency band, the effect of these anti-interference technologies will be affected. 64
    • 3 2BFrequency Planning 65 7. The BCCH and TCH are independent of each other. This makes it easy to maintain and expand each layer individually. The addition or deletion of TRXs of sites or cells will not affect the existing BCCH planning, which makes easier network maintenance. 6MHz MRP Segmentation Carrier No. 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 BCCH(12) 1 2 3 4 5 6 7 8 9 11 12 TCH1(8) 13 14 15 16 17 18 19 20 TCH2(6) 21 22 23 24 25 26 TCH3(4) 27 28 29 30 The MRP is one of the important technologies in the development of frequency planning in recent years. As mentioned in the related literature, when the MRP is used with anti-interference technologies such as frequency hopping, DTX, and power control, the average frequency reuse coefficient can be reduced to around 7.5, without affecting the network quality. Example: Table 3.4-1 Carrier Distribution of Different BTSs Number of Cell TRXs 2 3 4 Percentage of Such Cell 20% 30% 50% MRP Segment 12/8 12/8/6 12/8/6/4 Average Frequency Reuse Coefficient (12+8)/2=10 (12+8+6)/3=8.7 (12+8+6+4)/4=7.5 Frequency Hopping Diversity Gains Small Medium Large In Table 3.4-1, the number of cells with 2 TRX accounts for 20% of the total carriers, with 3 TRX for 30%, and that with 4 TRX for 50%. Suppose that these cells are “distributed evenly”, the average frequency reuse coefficient must be lower than the actual reuse coefficient. For cells with 3 TRX: The cells with three or more TRXs actually account for 80% of the total, and they are distributed evenly, so the actual reuse coefficient of L3 TRX is 6/0.8=7.5. Expanded MRP is the expansion of the MRP concept. After segmentation, each layer can include the frequencies of each subsequent layer: The TCH0 layer includes various frequencies of layers TCH1- TCHn, the TCH1 layer includes those of layers TCH2- TCHn, and so on. First, allocate the frequencies of layer TCHn, and then those of layer
    • GSM Radio network planning principle TCHn-1, and so on. However, this affects the structured feature of the MRP planning. Table 3.4-2 6MHz Band Expansion MRP Segmentation Carrier No. 1 2 3 4 5 6 7 8 9 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 BCCH(12) 1 2 3 4 5 6 7 8 9 11 12 TCH1(8) 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 TCH2(6) 21 22 23 24 25 26 27 28 29 30 TCH3(4) 27 28 29 30 Example: Take the 7.2 MHz frequency bandwidth as an example. Use the MRP to divide the 36 pairs of carriers into groups according to 12/9/8/7, as shown in Table 3.4-3:. Table 3.4-3 Carrier Allocation Table Channel Type Logical Channel TCH1 TCH2 TC3 Frequency Band Number 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 The BCCH uses the 4 x 3 reuse (as shown in Figure 3.4-5 A), the TCH1 uses the 3 x 3 reuse (as shown in Figure 3.4-5 B), and TCH2 and TCH3 use 2 x 3 reuse (as shown in Figure 3.4-6 A and Figure 3.4-6 B), in four groups: 60 64 62 66 70 63 67 68 71 61 65 69 72 75 73 76 79 72 75 78 7874 77 80 Figure 3.4-5 (A) BCCH uses 4 x 3 reuse mode (B) TCH1 uses 3 x 3 reuse mode 66
    • 3 2BFrequency Planning Figure 3.4-6 (A) TCH uses 2 x 3 reuse mode (B) TCH3 uses 2 x 3 reuse mode 60 72 81 89 64 75 83 91 85 93 68 78 62 73 82 90 66 76 84 92 70 80 85 94 63 72 82 90 67 75 92 84 71 86 78 94 65 77 83 91 61 74 81 89 85 93 69 80 Figure 3.4-7 Schematic Diagram for 7.2 MHz Band MRP Carrier Configuration Comparison of system capacity between group reuse and MRP technology According to the analysis and description of various reuse patterns described above, now we compare the increase in capacity between these four reuse patterns. Table 3.4-4 shows the BTS configuration that can be implemented of various patterns in different bandwidths, the average capacity of each BTS, and capacity ratio (with the 4*3 pattern as reference). Table 3.4-4 Comparison of system capacity between various reuse patterns Reuse Pattern BTS Configuration Average Site Capacity (Subscribers) Capacity Ratio 4 x 3 3/2/2 or 3/3/2 1440 1 3 x 3 3/3/3 1788 1.24 1 x 3 4/4/4 2640 1.83 6 MHZ MRP (12, 9, 6) 3/3/3 1788 1.24 67
    • GSM Radio network planning principle 68 ** 2 x 6 2/2/2/2/2/2 2160 1.5 4 x 3 4/4/4 2628 1 3 x 3 5/5/5 3384 1.29 1 x 3 6/6/6 4272 1.63 MRP (12, 9, 6) ** 6/6/6 4272 1.63 9.6 MHZ 2 x 6 3/3/3/4/4/4 4416 1.68 Note: GOS=0.02, 0.025Erl/subscriber **( ) means the reuse pattern of each carrier. 3.4.6 Concentric Cell Technology 1. Basic Principles By concentric cell, it means that a common cell is divided into two areas: outer layer and inner layer, also known as overlay and underlay. The coverage of the outer layer is the traditional cell, while the coverage of the inner layer is around the BTS. In addition to the coverage, the frequency reuse coefficients of the outer layer and the inner layer are also different. The outer layer usually adopts the traditional 4 x 3 frequency reuse pattern, while the inner layer adopts the closer frequency reuse patterns such as 3 x 3, 2 x 3, or 1 x 3. Therefore, all TRXs are divided into two groups: one group is used for the outer layer and the other for the inner layer. It is because the outer layer and the inner layer share the same site, the same antenna system, and the same BCCH, such a structure is called concentric cell. However, the common control channel must belong to the channel group of the outer layer. In other words, a conversation must be established on an outer layer channel. Figure 3.4-8 shows the structure of a concentric cell.
    • 3 2BFrequency Planning Figure 3.4-8 Schematic diagram for the concentric structure According to the implementation mode, the concentric cell is divided into common concentric cell and intelligent underlay overlay (IUO). The major difference between these two types of concentric cell lies in the transmission power of the inner layer and the handover algorithm between the inner and outer layers. The transmission power of the inner layer of a common concentric cell is usually lower than that of the outer layer to reduce the coverage, increase the distance ratio, and meet the requirement of co-frequency interference. The handover between the inner layer and outer layer of a common concentric cell is usually based on the power and the distance. The transmission power of the inner layer (because the frequency adopts the closer reuse pattern, this layer is called super layer) of the IUO is completely the same as that of the outer layer (usually called conventional layer) because the transmission power is related to the handover algorithm. The IUO handover algorithm is a handover algorithm based upon C/I. The actual implementation process is briefly described as below: First, the call is established on the conventional layer, and then the BSC continuously monitors the C/I ratio of the downlink super group channel of the call. When the C/I ratio of one super channel reaches the available threshold (known as GoodC/Ithreshold in the IUO), the call channel is switched over to the super channel. At the same time, 69
    • GSM Radio network planning principle 70 continues to monitor the C/I of the channel. If it degrades to a certain threshold (BadC/Ithreshhold), the channel is switched to the common channel. Therefore, the following functions must be added in order to use IUO: A. Estimation of the downlink co-frequency C/I ratio B. Handover algorithm related to IUO C. Intra-cell handover from conventional layer to super layer (the measured C/I ratio must be greater than GoodC/Ithreshold) D. Intra-cell handover from super layer to conventional layer (the measured C/I ratio is smaller than BadC/Ithreshhold) 2. Capacity Because the inner layer adopts the closer frequency reuse pattern, more TRXs can be allocated to each cell to increase the frequency utilization and the network capacity. It should be noted that the coverage radius of the inner layer of a concentric cell is less than that of a common cell. The traffic absorption of the inner layer is restricted by the traffic distribution and the coverage. Table 3.4-5 shows the comparison between a concentric cell and the traditional 4 x 3 pattern of different traffic distribution and different coverage ranges, Where Si means the inner layer coverage, Sout means the outer layer coverage area, and Erlang the unit of capacity: Table 3.4-5 Comparison in Traffic Volume Between Various Reuse Patterns Coverage Ratio (Si / Sout) 3TRX 2TRXout+2TRXin 4TRX 3TRXout+2TRXin 0.3 14.04 10.57 21.04 20.05 0.7 14.04 20.55 21.04 28.25 Even distribution of traffic 0.9 14.04 21.04 21.04 28.25 0.3 14.04 15.09 21.04 21.92Linear distribution of traffic 0.7 14.04 21.04 21.04 28.25 It should be noted that the coverage ratio is related to the frequency reuse type: the closer the frequency reuse pattern, the higher the co-frequency interference, and the smaller the inner coverage percentage. In addition, it is also related to the settings of the handover parameters and the surrounding environment. Therefore, user should not set the coverage radius randomly but user must consider the network quality, which can
    • 3 2BFrequency Planning 71 seldom exceed 50%. According to the above analysis, the concentric technology can barely increase or even decrease the capacity when the traffic is distributed evenly. The better the effect in increasing the capacity, the more the traffic is concentrated around the BTS. Generally, the capacity increase is very limited. For a common concentric cell, the transmitted power of the inner layer is low and cannot easily absorb the traffic indoors, so the efficiency of the frequency is not high. The actual capacity increases about 10-30%. For IUO, the transmitted power of the inner layer is not changed, it can absorb the indoor traffic, and it can absorb the capacity flexibly for handover based on quality. Therefore, the actual capacity increases greatly by about 20-40%. 3. Characteristics and applications a. Common concentric cell The characteristics of a common concentric cell include: 1. It is unnecessary to change the network structure. 2. It is necessary to add some special handover algorithms. However, the implementation is simple as a whole. 3. There is no special requirement for the handsets. 4. The capacity increase is limited, usually within 10% to 30%, and is also related to the traffic distribution. The power of the outer layer is low so that it is difficult for the outer layer to absorb indoor traffic. 5. The common concentric cell is applicable to the case where the traffic is concentrated around the BTS and distributed outdoors. Pay attention to the following in application: 1. Perform good network planning. On one hand, it should be used in the areas with concentrated traffic. On the other hand, user should well plan the coverage area of the inner layer. In other words, do not allow the quality to be affected by the interference resulting from close reuse, while adequate traffic should be absorbed. If the planning is not good, not only the capacity is not increased, but also the network quality may be reduced. 2. It is preferable to combine the concentric cell with the anti-interference technologies such as power control and DTX
    • GSM Radio network planning principle 72 b. Intellignet underlay overlay (IUO) The IUO has the following characteristics: 1. As a type of concentric cell, the IUO can utilize the existing site. The network change is small and there is no special requirement for the handsets. 2. The system function requires the measurement and estimation of C/I ratio as well as special handover algorithm additionally. 3. The capacity can be increased by 20% to 40%. The capacity increase is related to the traffic distribution and the traffic absorbed by the super layer. The quality can be ensured when the capacity is increased. 4. The super layer can adopt the closer frequency reuse pattern. When the frequency band is wide enough, part of the frequencies can be reserved for micro cells. 5. The IUO is applicable to cells where the traffic is highly concentrated around the BTS. Pay attention to the following when user use the IUO: 1. Perform network planning. Perform cell planning according to the traffic distribution and take steps to reduce interference. 2. When user configure the cell channels, pay attention to the reasonable configuration of the super group frequency and common group frequency. Enable the bottom layer to absorb adequate capacity to reduce call drop. Set the cell parameters appropriately. 3. To reduce the interference, technologies such as power control and discontinuous transmission (DTX) can be combined with the IUO. 4. Preferably C/I-based handover should also be used on the conventional layer. 3.5 Cell Splitting At the early stage of the GSM network construction, the number of subscribers is not large, and therefore there are usually idle channels. As the number of subscribers increases, the channels originally assigned to each BTS cell will be congested. In this case, new channels can be added and allocated within the original BTS. If the number of subscribers continues to increase and all available channels are already assigned, only the cell splitting technology is applicable to increase BTS quantity and co-frequency reuse to meet the requirement. Usually, the radius of a split cell is only a
    • 3 2BFrequency Planning 73 half of that of the original cell. Radius of new cell = Radius of old cell/2 (1-5-1) Based on formula (1-5-1), the following formula is derived: Coverage area of new cell = coverage area of old cell/4 (1-5-2) Let each new cell have the same maximum traffic load as the old cell, we can theoretically obtain: New traffic volume/unit area = 4 old traffic volume/unit area (4-3-3) Therefore, the relationship in capacity between cell splitting and subscriber addition can be expressed as the following formula: Tn = 4n T (1-5-4) Where, Tn is network capacity after "n” times of cell splitting T0 is network capacity before cell splitting Formula (4-3-4) is applicable to the case where a cell is split into four smaller cells according to 1:4. In simple words, after one time of splitting, the number of subscribers increases to four times the original, but the actual capacity is smaller than four times. 3.6 Common Anti-Interference Technologies The GSM itself has many anti-interference technologies such as frequency hopping, power control, and DTX based on voice activity detection. The effective application of these technologies can improve the C/I ratio to form the closer frequency reuse pattern and increase the frequency reuse coefficient and frequency utilization. These parameters are described one by one as below, with purely mathematical and simulated models to study their gains.
    • GSM Radio network planning principle 3.6.1 Discontinuous Transmission (DTX) DTX codes voices at a rate of 13 kbit/s during voice activation period, and codes comfort noises at the rate of 500 kbit/s during silent period. DTX contributes little to interference during silent period, and it can be believed that its power is zero (non-activated state). Suppose the activation factor of DTX is 480 ms Voice frame Comfort noise frame BTS MS TRAU BTS p , then the gain is: p CC dBIC log10log10log10)(/ −=−=Δ IpI 3.6.2 Frequency Hopping (FH) As a type of spectrum spread communication mode, frequency hopping, when used in the cellular mobile communication system, can enhance the immunity of the system against multi-path fading, and can suppress co-frequency interference that may affect the communication quality, so it is very valuable. Particularly at a time when the frequency spectrum resource becomes increasingly insufficient, frequency hopping will become one of the most effective means for improving the spectrum utilization. In the GSM, the data of each logical frame are interleaved in a distributed way in eight TDMA frames for transmission, while all these data have undergone convolutional coding. If the code blocks of these eight bursts are partly interfered or damaged, the good convolutional decoder can be used to restore the data transmitted. However, if too many code blocks are damaged, it is very difficult to restore the old data. Through frequency hopping, the bursts of one channel will not stay in the deep fading area continuously for a long time (this is very likely occur to one MS still or in slow motion that works on a fixed carrier), and will also not always be interfered by the same 74
    • 3 2BFrequency Planning 75 powerful co-frequency signal. This way, the channel coding/decoding can be used to obtain good transmission effect. This is the simple principle for using the frequency hopping technology to improve the communication quality. The frequency hopping sequence used by the GSM system is a Poisson pseudo random variable sequence, which can provide up to 64 frequency hopping sequences, with the length same as that of an ultra-high frame (lasting for 3 hours, 28 minutes, 53 seconds, 760 milliseconds), to ensure various sequences to be orthogonal between each other as far as possible, for good frequency hopping effect. The frequency hopping sequence in the GSM is described by two parameters: HSN (hopping sequence number) and MAIO (mobile allocation index offset). Usually, different cells are allocated to different HSNs, while different MAIO values are allocated to different channels in cells. It can be seen that various channels in one cell use the same HSN, only with different MAIOs. This ensures that the channels in one cell will not occupy the same frequency at the same time. Different cells have different HSNs, using different types of frequency hopping sequences. This way, the HSNs of various cells are independent of each other as far as possible, and the powerful interference source signals are allocated to multiple channels, ensuring the coding effect. When HSN=0, the MAIO repeats in cycle from low to high, known as Cyclic Hopping. Because this method has a very low gain, it is usually not used in GSM. GSM supports baseband frequency hopping and RF frequency hopping (also known as Synthesized Frequency Hopping). For baseband frequency hopping, multiple transmitters work at their respective frequencies, while the signals of different channels on the baseband are switched over to different transmitters for transmission according to the HSN, for frequency hopping. On the other hand, RF frequency hopping means that the transmission frequency of the transmitter hops according to the HSN. Baseband frequency hopping is easy to implement, but it has few hopping frequencies due to the restriction of the number of TRXs. The frequency hopping simulation system established by ZTE CORPORATION mainly supports the RF frequency hopping, while the baseband frequency hopping is taken as a special example of RF frequency hopping (that is, when the number of frequencies is equal to the number of TRXs). The main benefit of frequency hopping is the so-called frequency diversity and interference diversity. The former increases the coverage area of the network, while the
    • GSM Radio network planning principle 76 later increases the capacity of the network. Since baseband frequency hopping has its number of available frequencies equal to that of the TRXs, it brings about only frequency diversity gains, without interference diversity gains. However, now the GSM operator is more concerned with capacity, since coverage is no longer a problem in most cities. To solve the capacity problem, the RF frequency hopping is a very effective means. RF frequency hopping represents the application trend in network planning. (1) Frequency diversity gain Frequency diversity means the immunity against Rayleigh fading. Due to the independency between the Rayleigh fading on different carriers (the larger the frequency difference, the smaller the dependency), the bursts distributed over different carriers will not be affected by the same Rayleigh fading. This is very valuable for the Static MSs or in slow motion. It is said that this can provide gains about 6.5 dB. While for MSs in fast motion, the time and location difference between two consecutive bursts of one channel is adequate to make them independent of Rayleigh changes. In other words, they are nearly not affected by the same fading. In this case, slow frequency hopping can provide very small frequency diversity gain. When an MS is moving rapidly, the number of frequencies configured for a cell has little impact on the frequency hopping performance. On the other hand, for the case without frequency hopping, there is about 1~2 dB frequency diversity gain. When a MS is moving slowly (TU3), due to the frequency diversity effect, the number of frequencies configured has a significant impact on the system performance. Once the number of frequencies doubles, a gain of about 0.2 ~ 1 dB can be obtained, and the load ratio increases for about 10%. (2) Interference diversity gain Interference diversity means the ability to suppress the interference signals from other co-frequency reuse cells, by providing frequency hopping on the transmission path, to improve the interference in harsh conditions so that all subscribers can evenly obtain good communication quality. This is very important for the mobile communication system with a great number of subscribers, particularly for increasing the communication capacity by increasing the frequency reuse ratio. Usually, to provide the interference diversity effect, the number of hopping frequencies should be no less
    • 3 2BFrequency Planning than 3. Frequency hopping set MA {ƒ1, ƒ2, ƒ3 … ƒn} Number of TRXs: m (m≤n) Interference cell Figure 3.6-1 Frequency Hopping Allocation Schematic Diagram Suppose the MS use fk for a call at the moment “t”. Now, the probability that the interference cell (fk) is activated is: nmCCp m n m n //1 1 == − − m n I C pI C dBIC log10log10log10)(/Gain =−=Δ (3) Frequency hopping planning and capacity analysis Suppose there are 10 MHz frequencies. When frequency hopping is not used, the frequency planning and capacity analysis are described as below: The reuse pattern of BCCH is 4X3, and that of the traffic channel is 3X3. The 10 MHz offers 50 frequency points. In addition to one protection frequency point and 12 BCCH points, there are 37 left. Therefore, each cell can be allocated with four traffic frequency points ((37-1)/9). Totally, there is one frequency point left. In other words, the maximum configuration is 5+5+5. Each cell can provide 37 channels (1BCCH+2SDCCH+37TCH). When the RF frequency hopping technology is used, the traffic channels can adopt 1X3 reuse. When the load is 50%, each cell can provide six traffic logical frequency points, which are so called because they use the same 12 frequency hopping sets ((37-1)/3), only with different HSNs and MAIOs. Similarly, there is one frequency point left, while the maximum configuration becomes 7+7+7, providing 53 traffic channels 77
    • GSM Radio network planning principle (1BCCH+2SDCCH+53TCH), increasing the capacity by 43%. At the same time, more than 90% areas can still reach the C/I ratio of 9dB. When the DTX and the ZTE unique fast power control algorithm are used at the same time, the capacity of the system can increase dramatically. If the intelligent traffic control technology is used, GSM can also obtain flexible capacity, where larger system capacity is obtained in the hot areas at the cost of certain voice quality. 3.6.3 Dynamic Power Control (DPC) A3 78 A1 A2 A3 A1 A2 A3 A1 A2 A3 A1 A2A3 A1 A2 A3 A1 A2 A1 A2 A3 Figure 3.6-2 Schematic Diagram for Dynamic Power Control As shown in Figure 3.6-2, only when the interference MS under the dynamic power control is on the edge of the cell, can the BTS work at the maximum transmitted power. Obviously, the location of the interference MS is a probability. This case is particularly obvious in frequency hopping. Suppose the DPC factor is p: p I C pI C dBIC log10log10log10)(/ −=−=ΔGain 3.6.4 1 x 3 Reuse + RF Frequency Hopping + DTX + DPC Let us examine the “1 x 3” reuse interference to see how much the anti-interference technology contributes to the reduction of interference and the increase of system capacity.
    • 3 2BFrequency Planning 79 The “1 x 3” reuse, compared with “4 x 3” reuse, brings about the following interference degrade: CIR 4 x 3- CIR 1 x 3 =18 - 9.43 ≅ 8.57 dB “1 x 3” frequency hopping and 50% load can bring about the following interference diversity gain: 10log10(2/1) = 3dB Suppose the frequency hopping length is 12 frequency points. Then, the frequency diversity gain brought is about: 2dB Suppose the activation factor of DTX is 0.5, then the gain is: -10log10(0.5) = 3dB Suppose the activation factor of DTX is 0.9, then the gain is: -10log10(0.9) =0.5dB The total gain is 3+2+3+0.5=8.5dB. According to the above analysis, we can see that the anti-interference technology can basically compensate the interference degrade brought about by closer reuse pattern. 3.7 Summary of GSM Frequency Allocation Signals of the same base station cannot be co-channel and adjacent-channel. Adjacent base stations should avoid co-channel (even if the antenna main lobe directions are different, the side lobe and back lobe cause interference) Opposite cells cannot be co-channel. Adjacent channel should be avoided, especially the BCCH and SDCCH frequencies (generally the first and second frequencies of a cell). When frequency hopping is used, the starting point of an adjacent base station can be the same, but the frequency hopping algorithm must be different. For synthesizer frequency hopping, the algorithm (HSN) of each cell in the same base station is consistent, but the Mobile Allocation Index Offset (MAIO) can not be adjacent. Note that the CCB combiner does not support frequency hopping. BSIC = 8 x NCC + BCC and BCC ranges from 0 to 7. Therefore, the BSIC of close co-channel and adjacent-channel cells should be inconsistent. Avoid
    • GSM Radio network planning principle 80 co-channel (especially BCCH) and co-BSIC in a short distance. If a mountain exists at the cell edge, do not set the base station as the neighbor station. If a water area exists, consider to set the base station as the neighbor station. Before frequency hopping, the usage range of BCCH is not limited and the BCCH can be staggered. During frequency hopping, a band should be segmented from the BCCH for 4 x 3 frequency reuse. If the frequencies are sufficient, the 5 x 3 reuse mode even 6 x 3 reuse mode can be considered so that the interference between BCCHs can be reduced. In large and medium-sized cities, use different close frequency reuse modes based on the functions supported by the equipment, for example, MRP, 1*3, 1*1 hopping frequencies. In addition, reserve some frequency points for micro cells to construct layered network. The frequency reuse coefficient is comparatively small. In small and medium-sized cities, use different frequency reuse modes based on the functions supported by the equipment. Determine whether to construct layered network based on the actual situations. The frequency reuse coefficient is a little greater than that of large and medium-sized cities. In towns and villages, the frequency resource is rich. Use the 4 x 3 frequency reuse mode. For mountain sites, allocate separate frequency points. In addition to the principles mentioned above, there is another important principle, that is, the frequency planning must agree with the actual situations. The terrain and base stations of each system are different and radio propagation environments are different. This requires that the frequency planning should correspond to the actual situation without being limited by the accumulated experience. If possible, use special planning tools and electronic maps to predict the field strength. First observe whether the coverage area of each cell is reasonable. Then, modify the coverage or frequency planning for areas whose interference does not meet the requirement (the co-channel interference is set to around 12 dB with 3 dB margin in frequency planning). After the base station is commissioned, determine whether the coverage planning is reasonable by using drive test and other statistical data. For severely interfered areas, adjust the
    • 3 2BFrequency Planning 81 coverage and modify the frequency planning. During frequency planning, the geographical area is always virtually sliced. Note that edges between slices must reserve some frequency points (when frequencies are sufficient) or have band plan. The determined edges should be far from hot areas or complicated networking areas. Generally, plan the frequency from the densest districts, for example, central business district, to suburbs with low frequency configuration (generally O1 or S1/1/1 type). Pay special attention to urban areas with a river or large lake and avoid interference resulted from strong transmission of water surface. Because base stations are irregularly distributed in actual situations, the frequencies in the same layer are not sure to be planned as 4*3 or 3*3. Adjust the frequency planning according to the actual situations. 3.8 Neighbor Cell Planning 3.8.1 Planning Principles The planning of neighbor cells determines the continuous coverage of a GSM network and the network performance. The principles for planning neighbor cells are as follows: 1. Main cell and neighbor cell must not be co-channel. 2. The number of neighbor cells cannot exceed 32. OMCR can be configured with a maximum of 32 neighbor cells. To obtain the optimal planning effect, take the following factors into account: one is quality of service and the other is system load. More neighbor cell relations lead to more system load resulted from handover. Appropriate neighbor cell planning can effectively reduce call drops due to handover failure. When defining neighbor cells, pay attention to the following issues: 1. Excessive neighbor cells result in excessive handovers, and moreover, overload signals. 2. Insufficient neighbor cells result in call drops due to handover failure, and degrade the quality of service.
    • GSM Radio network planning principle For neighbor cell planning, cells are considered to arrange regularly in a cellular structure. Therefore, pay attention to the following issues: a. For macro cells in urban areas, configure two-level adjacency relationship, as shown in Figure 3.8-1: A3 D2B1 D1 D3 C1 A1 A2 82 B3 C2 B2 C3 A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 A1 A3 D2 A2 D1 B1 D3 B2 B3 C1 C2 C3 Figure 3.8-1 Neighbor cell planning for urban areas b. For suburbs or villages, each cell has a large coverage and thus the distance between the first and second levels is comparatively large. Therefore, only one-level cells need to be configured when configuring adjacency relationship, as show in Figure 3.8-2: A3 D2B1 D1 D3 C1B3 A1 A2 C2 B2 C3 A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 A3 D2B1 D1 D3 C1B3 C2 B2 C3 A1 A2 A1 A3 D2 A2 D1 B1 D3 B2 B3 C1 C2 C3 Figure 3.8-2 Neighbor cell planning for rural areas
    • 3 2BFrequency Planning In a dual band network, cooperation of the two systems and predefined rule are important for configuring neighbor cells. Therefore, configure the adjacency relationship based on different network sharing schemes. Usually, we believe that cells are arranged in good order like a beehive, but the fact is that they are not in perfect order due to the various factors that affect site selection. The solution to this can only be the configuration according to the data simulated in network planning. In addition, if the BTS has a large transmitted power, the edge of the coverage area accounts for a large portion of the total area. In this case, the adjacency relationship cannot be obtained from the geographical location alone. Instead, user should make site measurement or configure more adjacency relationships, as shown in Figure 3.8-3. Figure 3.8-3 Neighbor cell configuration As shown in the diagram, when an MS is moving in the curve on the edge of the coverage area, the MS theoretically chooses the service cells in the following order: A to B to C. However, due to influence of some complicated radio propagation environment, the signals of site B may never dominate in this moving direction. In this case, if site C cell is not configured to be the adjacent cell of sector 1 of site A, the MS will stay in sector 1 of site A all the time until call drop or cell reselection. The solution is to configure sector 1 and 2 of site A and sector 1 of site C as the adjacent cells (monitoring frequency band). However, user should not expand without limit. For example, user cannot set all the cells such that any two are adjacent cells to each other, as this cause many undesired cell reselections and handovers. 83
    • GSM Radio network planning principle 84 Unreasonable Neighbor Cell Planning: 1. Unidirectional neighbor cells 2. Excessive neighbor cells 3. Insufficient neighbor cells Problems Resulted from Unreasonable Neighbor Cell Planning 1. Call drop 2. Handover failure 3. Frequent handovers 4. Isolated cell 5. Exceptional inter-cell handover 6. Unbalanced traffic 7. Reduced handover precision 3.8.2 Case Analysis Case 1 [Problem Description] The situation of the problem base station is as follows: S333 base station configuration GSM900 network 1*3 RF frequency hopping A sector of this base station has continuous high handover failure rate. The original cell A accounts for 80% of the incoming handover failure. Other indicators such as call drop rate and voice channel allocation failure are normal. [Reason Analysis] The problem does not occur due to hardware faults and interference. Though the handover failure rate is high, TCH allocation never fails. This proves that the MS can successfully seize the TCH allocated by the BSC. Additionally, call drops never occur and the voice quality is good. This proves that no interference exists. As the original
    • 3 2BFrequency Planning 85 cell A, which has high incoming handover rate, is far from the cell, less handover requests should have been originated. Therefore, the cause of the problem is isolated cell effect. [Fault Location] Check the cells around the original cell A for cells co-channel and co-color with the problem cell C and find cell B. Further research finds that a large square is established between cell B and cell A. The open square improves the radio propagation condition between cell B and cell A. Thus, the MS actually listens to the signals from cell B whereas the BSC identifies that MS sends Handover Command to cell C. Actually, the level of cell C may be low and as a result, the handover fails. [Solution] Modify the frequency point of cell C and add the isolated cell B to the neighbor cell list of cell A. [Summary] Pay attention to environment changes when handling network problems. If the change in environment affects or improves radio signal propagation, timely adjust the cell parameters (for example, add, delete, or modify neighbor cells or frequencies) and engineering parameters (for example, antenna mount height, downtilt, and azimuth). Because GSM network frequencies are limited, the isolated cell effect is more probable to occur with the expansion of network dimension. In addition, severe co-channel interference may greatly affect handover success rate. Case 2 [Problem Description] A user living at the border of a province complains that his mobile phone does not have roaming problems in his residence, but after receiving roaming signals from another province, it cannot disengage from the roaming signals. The two princes have no neighbor cell relationship. [Reason Analysis] The drive test finds that the network structure is as shown in Figure 3.8-4:
    • GSM Radio network planning principle Figure 3.8-4 Network structure The user location is spot P which is located in cell A. Cell A and cell B are mutually neighbor cells and they are subscriber homing network. Cell C and cell D are mutually neighbor cells and they are roaming network. Cell A does not have neighbor cell relationship with cells C and D. Because cell D has the same BCCH frequency point as the neighbor cell B of cell A, the MS in spot P may select cell C and then select cell C through cell D. Cell A frequency point is not defined in the neighbor cell lists of cell C and cell D. Therefore, the MS is resides in the network of the roaming location. After the user powers on the MS again in cell C, the MS still keeps the frequency point of the cell when it is powered off. As a result, the MS first seeks the frequency points of cell C and its neighbor cells and the roaming problem occurs. [Solution] Define the neighbor cells between provinces. If this method is not applicable, modify cell B frequency point. 3.8.3 BSIC Planning 3.8.3.1 Definition In the GSM system, each BTS is allocated with one local color code, known as base station identify code (BSIC). If MS receives the BCCH carriers of two cells at a location with the same channel number, the MS distinguishes them according to the BSIC. In network planning, the BCCH carriers of the adjacent cells should use different frequencies in order to reduce co-frequency interference. On the other hand, 86
    • 3 2BFrequency Planning the characteristics of the cellular communication system determine that the BCCH carriers are certain to have the possibility of reuse. For these cells using the same BCCH carrier frequencies, user must ensure that they have different BSICs, as shown in Figure 3.8-5: Figure 3.8-5 Schematic Diagram for Selection of BSIC In the diagram, the BCCH carriers of cells A, B, C, D, E and F have the same absolute channel numbers, and other cells use different channel numbers as the BCCH carriers. Usually, cells A, B, C, D, E and F must use the same BSIC. When the BSIC resource is insufficient, different BSICs should be first ensured for the close cells. For cell E, if there are not sufficient BSIC resources, different BSICs should be first used for cells D and E, B and E, and F and E, while cells A and E, and C and E can have the same BSICs. Its major functions are: 1. After a MS receives the SCH, it believes that it has been synchronized with the cell. However, to correctly decode the information on the downlink public signaling channel, the MS also needs to know the Training Sequence Code (TSC) used by the signaling channel. According to the specification of GSM, the TSC is available in eight fixed formats, which are represented with sequence numbers 0 ~ 7 respectively. The common signaling channel of each cell uses the TSC determined by the BCC of the cell. Therefore, one of the functions of the BSIC is to notify the MS of the TSC used by the common signaling channel of the current cell. 2. Since the BSIC has participated in the decoding process of the Random Access Channel (RACH), it can be used to prevent the BTS from sending the MS to the RACH of the adjacent cell, for misinterpretation as the access channel of the current cell. 87
    • GSM Radio network planning principle 3. When the MS is in the busy mode (during calls), it measures the BCCH carrier of the adjacent cell and reports the results to the BTS, according to the specification of the adjacent cell table on the BCCH. At the same time, the MS must give the BSIC of the carrier it has measured for each frequency point in the uplink measurement report. When in a special environment where the adjacent cells of one cell have two or more cells use the same BCCH carriers, the BTS can distinguish these cells based on the BSIC, to avoid incorrect handover, or even handover failure. 4. The MS must measure the signals of the adjacent cells in the busy mode, and report the measurement results to the network. Since each measurement report sent by the MS only includes the contents of six adjacent cells, the MS must be controlled to report only the cells actually with handover relationships with the current cells. The higher three bits in the BSIC (that is, NCC) are used for the above purpose. The network operator can use the broadcast parameter “allowed NCC” to control the MS to report the adjacent cells with the NCCs in the allowed range. 3.8.3.2 Format The BSIC consists of the Network Color Code (NCC) and Base Station Color Code (BCC), as shown in Figure 3.8-6. The BSIC is transmitted on the Synchronization Channel (SCH) of each cell. Figure 3.8-6 Composition of the BSIC Format of the BSIC: NCC-BCC Value range of NCC: 0 ~ 7 Value range of BCC: 0 ~ 7 3.8.3.3 Setting and Influence In many cases, different GSMPLMNs use the same frequency resources, which, 88
    • 3 2BFrequency Planning 89 however, are somewhat independent in network planning. To ensure that the adjacent BTSs with the same frequency points have different BSICs in this case, it is usually specified that the adjacent GSMPLMNs should select different NCCs. It is special in China. Strictly speaking, the GSM network provided by China Telecom is a complete and independent GSM network. Although China Telecom has numerous local mobile offices under it, they all belong to one operator – China Telecom. However, as China is a large country with a vast territory, it is very difficult to implement complete unified management. Therefore, the entire GSM network is divided into parts managed by the mobile offices (or local organizations) in various provinces and cities. On the other hand, the mobile offices in various places are independent of each other in network planning. To ensure that the BTSs with the same BCCH frequencies on the borders between various provinces and cities have different BSICs, the NCCs of various provinces and cities in China should be managed by China Telecom in a unified manner. As part of the BSIC, the BCC is used to identify different BTSs with the same BCCH carrier number in one GSMPLMN. Its value should meet the above requirement as far as possible. In addition, according to the GSM specification, the TSC of the BCCH carrier in a cell should be the same as the BCC of the cell. Usually, the manufacturer should maintain such consistency. 3.8.3.4 Precautions It must be ensured that the adjacent or nearby cells with the same BCCH carrier have different BSICs. Particularly, when one cell has two more adjacent cells having the same BCCH carriers, it must be ensured that these two cells have different BSICs. User must pay special attention to the configuration of the cells on the borders of various provinces and cities to avoid inter-cell handover failure.
    • 4 Dual Band Technology This chapter describes the following contents: Networking modes of dual band networks Coverage mode of dual-band networks Traffic sharing parameters settings Differences between uplink and downlink transmit power 4.1 Structure of Dual Band Networks The GSM900/1800 dual-band network may be in one of the following four structures: 4.1.1 Shared HLR/AUC, EIR, OMC and SC This structure is also called separate MSC networking structure. GSM1800 and GSM900 are connected to their respective BSCs and MSCs and share the HLR, AUC, OMC, SC, and EIR. The structure of shared HLR/AUC, EIR, OMC and SMC is shown in Figure 4.1-1: Figure 4.1-1 Network structure of separate MSC 4.1.2 Shared Switching Subsystem This structure is also called shared MSC networking structure. GSM1800 and GSM900 are connected to their respective BSCs and share the MSC/VLR, HLR/AUC, OMC, SC, 91
    • GSM Radio network planning principle and EIR. The BSSs of the DCS1800 and GSM900 are connected to the MSC/VLR respectively via the A interface, as shown in Figure 4.1-2: Figure 4.1-2 Network structure of shared MSC 4.1.3 Shared Switching Subsystem and BSC This structure is also called shared BSC networking structure. DCS1800 and GSM900 share the BSC, as shown in Figure 4.1-3. As the Abis interface is not standardized yet, the BSSs of the DSC1800 and GSM900 should come from the same equipment manufacturer, and the BSCs shared should have functions added accordingly. Figure 4.1-3 Hybrid network structure of shared BSC 4.1.4 Shared Network Subsystem This structure is also called shared BTS networking mode. DCS1800 and GSM900 share the entire network subsystem. In other words, the GSM900 and DCS1800 reception/transmission functions should be implemented in one BTS, as shown in 92
    • 4 3BDual Band Technology Figure 4.1-4: Figure 4.1-4 Hybrid network structure of shared BTS It is recommended to use the shared BSC mode and shared MSC mode. Use the separate MSC mode with caution. The reasons are as follows: 1. Dual band network requires the incorporate planning of the radio part. The separate MSC mode cannot ensure the planning integrity. 2. The purpose of building dual-band network is to provide a high quality network. Therefore, the shared BSC mode is the most simple and effective mode for the operation of a dual-band network. 3. The signals to be processed are the smallest if the shared BSC mode is used. The handover traffic is extremely large due to mobility of users. Even if the DCS1800 has a good coverage, large numbers of handover cannot be avoided. The shared BSC mode enables a large portion of handovers to be complete within the BSC. In this way, the network signaling loads are greatly reduced and thus the investment is saved. 4. The co-sited mode is commonly used in BSS. If the shared BSC mode is used, the base stations with low configurations can use daisy chain to solve the transmission problem. This saves investment and simplifies the construction. 5. The shared BSC mode brings limited equipment selection, complicated planning, and difficult network maintenance and management. However, the preliminary task is to guarantee and improve network quality as well as reducing investment. It is worthwhile to use the shared BSC mode. If the equipment selection is a big 93
    • GSM Radio network planning principle 94 problem, the shared MSC mode is an alternative. The separate MSC mode is the last choice. 6. When using the shared BSC mode, use BSCs with as large capacity as possible. This helps network planning, engineering construction, and O&M management simpler. 4.2 Dual Band Network Planning 4.2.1 Requirement Analysis 1. Available Frequency Scope The planned available bandwidth is XMHz. The detailed data is as follows: Uplink: X∼X MHz Downlink: X∼X MHz Corresponding channel No.:X∼X Whether the frequency for micro cells is reserved. If yes, get the frequency segmentation. Whether the frequency band is used by other operators. If yes, get the frequency segmentation. 2. Traffic sharing requirement Determine the ratio of traffic sharing based on the requirement of the operator. During the early phase of dual band network construction, the main task is to make use of new DCS1800 network to share the traffic of the GSM900 network. In terms of traffic control, observe the following principles based on the purpose of the DCS1800: a. During the early phase of dual band network construction, use the DCS1800 cell to absorb dual band users to reduce the load of GSM900 system. b. When the number of dual band users reaches a certain degree, share the traffic between each frequency band so as to reduce dual band switching. In actual situation, the traffic control policies can be different by adjusting parameter settings:
    • 4 3BDual Band Technology 95 First, in idle mode, when an MS selects the cell after being powered on or reselects the cell in standby status, define the system message parameters CBQ (Cell Bar Qualify), CBA(Cell Bar Access), CRO, TO, and PT to upgrade the priority level of the DCS1800 cell or give better neighbor cell measurement comparison values to the CSC1800. In this way, users are more likely to reside in the DCS1800 cell and the calls are established in the DCS1800 cell. Second, if traffic congestion occurs to the serving cell during the process of call establishment, use the retry function to assign the MS to the idle TCH of the neighbor cell and adjust the traffic allocation. Last, in the conversation status, arrange cells by layer and levels to make the enable the traffic flow to the lower-layered lower-leveled DCS1800 cell. Additionally, set handover parameters to make the traffic load reasonable. 3. Coverage Requirement on DCS1800 a. Outdoor coverage The outdoor coverage is easy to realize in case of short distance. If necessary, add sites in some places in addition to establishing DCS1800 at the original GSM900 site. b. Indoor coverage To ensure a good indoor coverage of the DCS1800, the distance between base stations in urban area should be less than 1000 m. As the buildings mainly use reinforced concrete structure and the penetration loss is great, it is recommended that the site spacing be around 500 m to 800 m. c. Other requirements Determine that the operator has no other requirements on the network planning. 4.2.2 Coverage Planning 4.2.2.1 DCS1800 Coverage Modes 1. Sparse coverage mode in hot areas The DCS1800 absorbs the traffic of hot areas. Sparsely distribute DCS1800 base stations. In the initial phase, the coverage planning is simple. When the DCS1800 base station has low configuration, the SDCCH and TCH congestion
    • GSM Radio network planning principle and frequency handover between the two bands should be solved in network planning. Figure 4.2-1 is the schematic diagram of GSM1800 coverage mode. Figure 4.2-1 Sparse coverage mode in hot areas This coverage mode is based on the original GSM900 network. As the DCS1800 base stations are established in hot areas, the continuous coverage is not realized. If a dual-band mobile phone is in conversation in area covered by DCS1800, then it switches to the GSM900 cell after it leaves the DCS1800 area. This handover is caused by coverage. Similarly, when the dual-band mobile phone is in conversation in the area covered by GSM900, it switches to the area covered by DCS1800 because GSM900 area has large traffic and is busy. This handover is caused by traffic. The disadvantage of this type of coverage mode is that it only relieves the traffic pressure in a short time. In addition, frequent handover in frequency bands increases the signal loads and thus result in the loss of system coverage. 2. Continuous coverage in hot areas This mode increases the shared traffic and improves network capacity, thus solving the problem of the sparse coverage in hot areas mode. 3. Continuous coverage The continuous coverage in the whole planning area can share the traffic of GSM900 at the maximum, increase network capacity, reduce switching between network layers, and improve operation quality. The following figure is the schematic diagram of continuous coverage: 96
    • 4 3BDual Band Technology Figure 4.2-2 Continuous coverage mode This coverage mode features easy expandability and can meets the medium and long period of coverage requirement. Compared with the sparse coverage in hot areas mode, this mode realizes high density and large area of coverage. Therefore, the handovers between frequency bands are greatly reduced. This coverage mode is comparatively ideal. The difference to the sparse coverage in hot areas is that DCS1800 is independent rather than attached to GSM900. For the engineering design of DCS1800, take the following points into account for the purpose of sharing as many traffic as possible: DCS1800 antennas should be 2-3 m higher than GSM900 antennas. DCS1800 antennas and GSM900 antennas should be in the same direction to facilitate traffic control. The V-plane and H-plane half-power angles should be small and close. The DCS1800 main lobe gain should be equal to or 2-5 dBi greater than that of GSM9002. Radio paramete From the perspective of traffic sharing, in addition to engineering parameters, radio parameters such as cell selection, cell reselection, and handover parameters also need consideration. By adjusting related parameters, make the mobile phone select the DCS1800 network on prerequisite that the conversation quality can be ensured. This can share the GSM900 network load. 97
    • GSM Radio network planning principle 98 (1). Cell reselection parameter Because signal attenuation of DCS1800 is greater than GSM900, the signals of DCS1800 cells are weaker than GSM900. To make a dual-band mobile phone primarily access the DCS1800 system, set the values of CBQ and CBA to differentiate the priories of DCS1800 and GSM900. The priority of cells does not affect cell reselection. The relationship between CBQ, CBA and cell selection priority and reselection is as follows: CellBarQualify CellBarAccess Cell Selection Priority Cell Reselection Status 0 0 Normal Normal 0 1 Barred Barred 1 0 Low Normal 1 1 Low Normal Adjust the minimum receiving level allowed by DCS1800, expand the coverage range of DCS1800 site, share the traffic in edge areas, and consider cell reselection. For GSM900 network, maintain the parameters of existing network, thus ensuring that the GSM900 coverage is consistent before and after the activation of dual-band network. This guarantees the network service of peripheral users. Based on the previous relationship, the planning of access parameters of co-sited and co-addressed dual band cells are as follows: Cell Name Max Repeat times Timeslots of transmitting distribution Cell Bar Qualify Cell Access Bar TCH Max Power Level Allowable Min Receiving Level USERLA BEL MAXRE TRANS TXINTEGER CELLBARQ UALIFY CELLBARA CCESS MSTXPWRM AXCCH RXLEVACCESS MIN 1800-1 2 14 0 0 0 12~15 1800-2 2 14 0 0 0 12~15 1800-3 2 14 0 0 0 12~15 900-1 2 14 1 0 5 10 900-2 2 14 1 0 5 10 900-3 2 14 1 0 5 10 (2). Cell reselection parameters Cell reselection has no priority. In proper situations, the MS reselects a cell with a greater C2 value. According to C2 algorithm, set the parameters such as CRO,
    • 4 3BDual Band Technology 99 TO, and PT to make the C2 value of DCS1800 be greater than GSM900. In case that DCS1800 cell signals are weaker than those of GSM900, the dual-band MS can also be enabled to reselect DCS1800 cell through parameter settings. Table 4.2-1 Cell reselection parameters Cell Name Cell Reselection Delay Additional Cell Reselection Parameter Indicator Cell Reselection Parameter Indicator Cell Reselection Offset Temporary Offset Penalty Time USERL ABEL RESELHYST ERESIS ADDITIONR ESELPI CELLRESELPI RESELOFFS ET TEMPORA RYOFFSET PENAL TYTIM E 1800-1 4 0 1 6~8 1 0 1800-2 4 0 1 6~8 1 0 1800-3 4 0 1 6~8 1 0 900-1 4 0 1 0 1 0 900-2 4 0 1 0 1 0 900-3 4 0 1 0 1 0 For DCS1800 cell, C2 = C1 + CRO (12~16dB) –TO (10db) *H (PT (10s) –T) whereas for GSM900, C2 = C1 + CRO (0db) –TO (10db) *H (PT (10s) –T) . In this case, once an MS successfully resides in the DCS1800 network, the high C2 value enables it to stay in the DCS1800 network for a long time. Similarly, an MS is easy to reselect the DCS1800 network and implement the traffic sharing task. Through reselection parameter setting, in idle mode, an MS can stay in the DCS1800 network providing effective coverage. (3) Cell handover parameters Dual-band networking currently is based on the existing GSM network. Building the DCS network is to expand the capacity of existing digital mobile network to provide better and colorful communications service. Because DCS frequency resources are more colorful than GSM, its capacity is more than twice of the GSM network. Therefore, ignoring the speed factor, the traffic of dual-band MSs should be absorbed and processed by the DCS network on condition that the conversation quality is ensured. This is the start point of dual band handover.
    • GSM Radio network planning principle 100 Table 4.2-2 Parameters of DCS1800 cell handover selection Cell Name Handover Control Whether related cell Static priority of cell handover Min interval of inter cell handover Min receiving strength level Min threshold of PBGT handover Min strength handover Min threshold of quality handover USERL ABEL HoControl IsRelated Cell HOPRIORITY HOMININTE RVAL RXLEVMI N HOMARGI NPBGT HOMAR GINRXL EV HOMARGI NRXQUAL 1800-1 22 0 3 10 12 36 28 28 1800-2 22 0 3 10 12 36 28 28 1800-3 22 0 3 10 12 36 28 28 900-1 22 0 3 10 15 14 28 28 900-2 22 0 3 10 15 14 28 28 900-3 22 0 3 10 15 14 28 28 These parameters are set for co-sited co-directional cells. Set the parameters normally for non-co-sited cells. (4) Other parameter settings Set early classmark sending control (ECSC) to Y The ECSC parameter indicates whether a multi-band MS can send early classmark modification message to the BSC through the BTS. This function enables the MSC to send the message to the target BSC once receiving messages about multiple bands. This facilitates the call setup and allows timely handover when necessary. The ECSC value can be Y or N. Y indicates that an MS needs to report its CLASSMARK3 to the network immediately after the link is set up. N indicates that an MS does not allow actively reporting its CLASSMARK3 to the network. Because CLASSMARK3 is mainly about dual band application, set ECSC to N for single-band GSM application area and set ECSC to Y for dual-band GSM application area. Set MULTIBAND_REPORT to 3. In a single-band GSM network, when an MS reports neighbor cell measurement results to the network, the contents of only six cells with the strongest signals are needed. When a network is composed of multiple bands, the operator always has a band with the highest priority when an MS performs inter-cell handover.
    • 4 3BDual Band Technology 101 Therefore, the MS is expected to report measurement results based on not only signal strength but also signal band. The multiband-reporting (MBR) parameter is used to inform the MS that neighbor cell contents of multiple bands are required. 0: The MS needs to report the measurement results of six neighbors with strongest signals and that are allowable and known by NCC, irrespective of the bands of the neighbor cells. 1: The MS needs to report the measurement result of a neighbor cell with the strongest signal and this is allowable and known by NCC on each band (excluding the band being used by the current service area) in the neighbor cell list, and report the neighbor cell of the current service area in all bands in the remaining space. If there is still remaining space, report the situations of other neighbor cells, irrespective of the band. The value range of MBR is 0-3. In multi-band application situations, the value is related to the traffic volume of each band. Generally, refer to the following principles during the setting: Set MBR to 0 when each band has equal traffic. Set MBR to 3 when the traffic volume of each band is obviously different and the operator has a preferable band. Set MBR to 1 or 2 when the situation is between the previous two cases. Activate traffic handover The parameters of traffic handover are as follows: Traffic handover layer control value (TrafficHoLayrCtl) = 1 (same layer) Traffic handover frequency control value (TrafficHoFreqCtl) = 0 (or2) (0: GSM900; 2:DCS1800) Traffic handover threshold (TrafficThs) = actual traffic load