Cell planning principles

2,927 views

Published on

Published in: Technology, Business
0 Comments
2 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
2,927
On SlideShare
0
From Embeds
0
Number of Embeds
6
Actions
Shares
0
Downloads
262
Comments
0
Likes
2
Embeds 0
No embeds

No notes for slide

Cell planning principles

  1. 1. Cell Planning Principles © Copyright 2003 AIRCOM International Ltd All rights reserved AIRCOM Training is committed to providing our customers with quality instructor led Telecommunications Training. This documentation is protected by copyright. No part of the contents of this documentation may be reproduced in any form, or by any means, without the prior written consent of AIRCOM International. Document Number: P/TR/003/P013/1 This manual prepared by: Grosvenor House 65-71 London Road Redhill, Surrey RH1 1LQ ENGLAND Telephone: Support Hotline: Fax: Web: AIRCOM International +44 (0) 1737 775700 +44 (0) 1737 775777 +44 (0) 1737 775770 http://www.aircom.co.uk CELL PLANNING PRINCIPLES © AIRCOM International 2003 1
  2. 2. Cell Planning Principles 2 © AIRCOM International 2003
  3. 3. Cell Planning Principles Contents 1 Cell Planning Principles: a Brief Introduction..................5 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Introduction..........................................................................................................................5 Initial Scenario .....................................................................................................................6 Evolution of the Requirement: Coverage Enhancement......................................................7 Evolution of the Requirement: Capacity Enhancement ....................................................11 Evolution of Requirement: Coverage and Capacity .........................................................13 The Need for Frequency Planning. ....................................................................................14 Different Approaches for different environments..............................................................15 2 Link Budget for GSM...................................................19 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 Introduction........................................................................................................................19 Free Space Propagation Model ..........................................................................................20 Plane Earth Propagation Model .........................................................................................22 Cellular Radio Propagation Models ...................................................................................24 Okumura’s Measurements .................................................................................................25 The Hata Propagation Model.............................................................................................27 Diffraction Attenuation Models .........................................................................................30 CW Drive Testing ..............................................................................................................40 Propagation Model Tuning ................................................................................................46 Power Budgets ...................................................................................................................53 3 Frequency Planning....................................................73 3.1 3.2 3.3 3.4 3.5 3.6 Introduction........................................................................................................................73 Cellular Structures and Frequency Reuse Patterns ............................................................73 Interference Calculations ...................................................................................................76 Cell Splitting Techniques...................................................................................................77 GSM Frequency Patterns ...................................................................................................80 Self- Assessment Exercises.................................................................................................82 4 Traffic Analysis ...........................................................85 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Introduction........................................................................................................................85 Traffic Measurements – Erlangs and Blocking..................................................................85 The Traffic Analysis Process .............................................................................................88 Using Demographic Data...................................................................................................89 Market Projections and Traffic Maps ................................................................................90 Roll-Out Strategy ...............................................................................................................93 Capturing Traffic and Assessing Resource Requirements.................................................95 Traffic Capturing Demonstration.......................................................................................97 5 Grid References and Bearings ....................................99 5.1 5.2 5.3 Grid References..................................................................................................................99 Bearings ...........................................................................................................................104 Self- Assessment Questions ..............................................................................................114 6 Base Station Positioning ........................................... 115 6.1 6.2 6.3 6.4 Introduc tion......................................................................................................................115 BTS Positioning for Different Environments ..................................................................115 The Site Acquisition Process ...........................................................................................118 Multilayer Cell Design.....................................................................................................122 © AIRCOM International 2003 3
  4. 4. Cell Planning Principles 6.5 6.6 6.7 6.8 6.9 6.10 Microcell Positioning.......................................................................................................123 Picocell Arrangements .....................................................................................................125 Resource Sharing .............................................................................................................125 Cell and Handover Admission Strategies ........................................................................127 Use of Repeaters ..............................................................................................................129 Self- Assessment Exercises...............................................................................................134 7 Base Station Engineering.......................................... 137 7.2 7.3 7.4 7.5 Estimation of Fresnel Zone Radius:.................................................................................143 Antenna Configurations ...................................................................................................143 Base Station Equipment ...................................................................................................150 Self- Assessment Exercises...............................................................................................163 Appendix A Model Tuning Demonstration Procedure..... 169 Appendix B Erlang B Tables ......................................... 175 Appendix C: Solutions to Questions............................... 179 4 © AIRCOM International 2003
  5. 5. Cell Planning Principles 1 Cell Planning Principles: a Brief Introduction _____________________________________________________________________ 1.1 Introduction Imagine that we are set the task of establishing a GSM mobile communications network in a particular area. Our first task will be to establish a system that will provide coverage over a small geographic area (20 km2 perhaps). We are given the job of ensuring that the air interface operates adequately. This means that we are responsible for providing coverage and capacity to the customer. In order to design the network, we must make an initial estimate of the likely demand for the service that we are to provide. We will start by assuming a simple initial situation and use that as a platform on which to build our understanding of the essential tools that we need in order to design a progressively more demanding system. The more complicated systems will be designed in response to foreseen customer demand that will lead to the existing configuration becoming unsuitable. Examining these situations will lead to an assessment of the issues that need to be understood in order for performance to be improved in a logical, exact, scientific manner. Section 1 – Introduction to Cell Planning Cell-Planning for GSM Networks • Major Issues: •Coverage •Capacity •Interference •Cell Planning •Environmental Aspects © AIRCOM International 2003 5
  6. 6. Cell Planning Principles 1.2 Initial Scenario An area of 20 km2 is defined. It is almost in a perfect circle with a radius of approximately 2.5 km. We are told it is estimated that 120 subscribers will be located within this area. The subscriber behaviour is such that the offered traffic is estimated to be 25 mE per subscriber during the busy hour. In designing a solution that will satisfy the demands of the user, we are very lucky that this is the sort of demand that GSM was almost designed to accommodate. The obvious solution would be to establish a single omni-directional site and allocate a single carrier. A range of 2.5 km is usually achievable from an omni-directional macro cell positioned above roof height. The carrier allocated would be capable of serving 7 simultaneous voice connections. From our Erlang B tables we can see that 7 simultaneous connections are sufficient to accommodate 3 Erlangs of traffic with a 2% blocking probability. Our estimated 120 subscribers will generate exactly 3 Erlangs of traffic in the busy hour if each one offers 25 mE (one fortieth of an Erlang) to the network. In this way we have what could be considered a perfect solution to the problem posed. Section 1 – Introduction to Cell Planning An Initial Challenge • GSM Services are to be provided over a small area: 20 km2. • Number of subscribers estimated to be 120. • Average subscriber generates 25 mE of traffic in the busy hour 6 © AIRCOM International 2003
  7. 7. Cell Planning Principles Section 1 – Introduction to Cell Planning An Initial Challenge • Calculations: •20 km 2: Circle approximately 2.5 km in radius •120 x 25 mE = 3 Erlangs (average activity is 3 users: “peak” of 7 users). 2.5 km •Single GSM carrier sufficient. •Omni-directional (single cell) site would be used. 1.3 Evolution of the Requirement: Coverage Enhancement The traffic offered to a GSM network does not stay relatively constant for long. Indeed the initial scenario described is so simple to accommodate as to be remarkably convenient and unlikely to be encountered in practice. Suppose, for example, the coverage requirement increased beyond that normally possible from the omni-directional antenna used to date. What options are open to the planner to improve the coverage? We can go through a number of possible solutions: © AIRCOM International 2003 7
  8. 8. Cell Planning Principles Section 1 – Introduction to Cell Planning Increasing the Coverage Range • Possible Methods: •Increase Antenna Height •Increase Antenna Gain •Increase Cell Power •Sectorise the Site •Add Additional Sites Increasing the mast height. As a general rule, increasing the mast height will reduce the path loss at a particular distance. The decrease in path loss is not, however, dramatic (a typical decrease would be 4 dB for doubling the height of the mast) but can lead to significant increases in the range (4 dB would represent a range increase from 2.5 km to about 3.2 km). Section 1 – Introduction to Cell Planning Predicting Effects: Increasing Antenna Height • Increasing Antenna Height reduces path loss. • Not dramatic: e.g. doubling mast height can give 4 dB reduction in path loss. • 4 dB reduction in path loss can lead to a 25% increase in range (approx 50% increase in coverage area). Increasing the gain of the antenna. Not all omni-directional antennas have the same gain. Examination of catalogues reveals that a variation from 10 dBi to 14 dBi is possible. If a 14 dBi antenna is used instead of a 10 dBi antenna, an increase in range is possible. However, this would not be achieved without a cost. A 14 dBi antenna would have a smaller vertical 8 © AIRCOM International 2003
  9. 9. Cell Planning Principles beamwidth that could lead to problems closer to the mast. Careful, accurate consideration would have to be given to down-tilting the antenna beam. Section 1 – Introduction to Cell Planning Predicting Effects: Increasing Antenna Gain • Omni- directional antennas can have 14 dBi antenna different gains (10 dBi to 14 dBi being the commonly available range). 10 dBi antenna • Lower gain antennas will have a larger vertical beamwidth. No downtilt • Down-tilting the antenna is a technique used to ensure that there are no coverage gaps and also to 12° downtilt restrict interference. Increasing the transmitter power. This will help coverage in the downlink only. Two-way communication is required. If the coverage problem is due to uplink constraints (and coverage problems usually are due to uplink constraints) then increasing the downlink power will do no good whatever. It is vital that the link is balanced (a topic dealt with later). Increasing the downlink power is often accompanied by introducing an extra (diversity antenna) at the base station. Section 1 – Introduction to Cell Planning Predicting Effects: Increasing Cell Tx Power • Communication must be twoway. Downlink Coverage Uplink Coverage limit limit • Increasing the Cell Transmit power increases downlink coverage but does not affect uplink coverage. • The link will become “unbalanced”. • Balance can be restored by implementing diversity. • 5 dB increase typical. © AIRCOM International 2003 A site implementing Space Diversity 9
  10. 10. Cell Planning Principles Sectorising the site. Instead of having an omni-directional antenna at the base station, it is possible to divide the coverage area up into (usually) three sectors. Because it has a narrower beamwidth, the sectored antenna will have a higher gain (about 6 dB higher). This would lead to the range of the cell increasing from its original value of 2.5 km to about 3.7 km. Sectorising will also raise the problem of a single carrier (often called a “single TRX”) no longer being sufficient. If 3 sectors are used, each would require a separate TRX. Coverage would be improved as a result, so would capacity – a factor considered in the next sub-section. Section 1 – Introduction to Cell Planning Predicting Effects: Sectorising the Site • Instead of a single omnidirectional antenna use 3 sectored antennas. • Gains up to 18 dBi • Requirement is for 3 TRXs as a minimum. • Improvement in both coverage and capacity. A base station employing sectored antennas. Adding sites. This solution will almost certainly be effective – although sometime it leads to the result that the original site location is not appropriate. Adding a further site will almost certainly be the most expensive solution by far and is often seen as the “solution of last resort” by planners. As in the case of sectorisation, it will lead to the need for a different TRX frequency to be allocated so as to avoid interference problems. In general, this leads to the need for Frequency Planning to be done on the network. Frequency Planning is a key skill for the GSM Network Planner. 10 © AIRCOM International 2003
  11. 11. Cell Planning Principles Section 1 – Introduction to Cell Planning Predicting Effects: Adding an Extra Site • A single extra site will increase coverage in a particular direction. • Very expensive. • Separate carriers must be allocated to the new site (frequency planning) to avoid interference. Interference Region 1.4 Evolution of the Requirement: Capacity Enhancement It may be that, rather than coverage becoming a problem, the demand for voice connections becomes greater and the network cannot deliver a satisfactory quality of service (that is the blocking probability is seen to exceed 2% on a regular basis). To this end, a number of enhancements are considered: Section 1 – Introduction to Cell Planning Increasing the Capacity Increasing the Capacity • Possible Methods: •Adding more carriers •Sectorising the site © AIRCOM International 2003 11
  12. 12. Cell Planning Principles Adding additional carriers to the site. Two TRXs can be connected to the same omnidirectional antenna. This will increase the number of simultaneous connections from 7 to 14 or 15. The number of Erlangs that this will accommodate will in fact more than double, due to an increase in trunking efficiency. The offered traffic that can be serviced will increase from 3 Erlangs to about 9 Erlangs, sufficient to accommodate the demand from approximately 360 “typical” subscribers. Section 1 – Introduction to Cell Planning Predicting Effects: Increasing TRXs • 1 Carrier: 7 timeslots: 3 Erlangs: 120 subscribers • 2 Carriers: 15 timeslots: 9 Erlangs 360 Subscribers • Maximum for one cell is typically 6 carriers (45 timeslots: 36 Erlangs 1440 subscribers). • Maximum is influenced by network allocation (e.g. 60 carriers occupies 12 MHz) and frequency re-use strategy. Sectorising the site. As described when this method was put forward as a solution to the coverage problem, this entails replacing the omni-directional antenna by three sectored antennas. Each antenna would require its own TRX and therefore three carrier frequencies would have to be allocated to this site. In this way, each sector would be able to service 3 Erlangs of offered traffic resulting in a total of 9 Erlangs for the site. Note that this is the same as could be serviced by only 2 TRXs on an omni-directional antenna. However, the three-sectored solution will have the added benefit of increasing the coverage from the site as described in the previous sub-section. 12 © AIRCOM International 2003
  13. 13. Cell Planning Principles Section 1 – Introduction to Cell Planning Predicting Effects: Increasing TRXs • Sectorising can lead to 3 separate single-TRX cells each serving 3 Erlangs. • This provides 9 Erlangs (the same as with 2 TRXs on an omni site). • Coverage will also be improved. Approximate radiation patterns from sectored antennas. 1.5 Evolution of Requirement: Coverage and Capacity Typically, the requirement or demand from the user will not vary by just increasing either traffic or coverage requirements but, more typically, both. In practice the above techniques will be refined and implemented in combination so as to achieve the desired effect. Throughout the process it is vital that quantitative analyses are made (that is, instead of simply saying that the performance will be improved we can say by how much (20%?... 50%?). We must be able to “quantify” any improvement so that alternative solutions can be compared and the best solution implemented. The following sections deal with specific issues that will provide us with techniques and tools to permit the analysis of situations in sufficient depth so that a high degree of confidence can be placed in the resulting predicted performance. © AIRCOM International 2003 13
  14. 14. Cell Planning Principles Section 1 – Introduction to Cell Planning Network Evolution: Increasing Coverage and Capacity • All previously-analysed methods may be considered. • Quantitative analysis required. • e.g. coverage area increased by 100%; Capacity increased by 200%. 1.6 The Need for Frequency Planning. As mentioned when we placed an extra site as a possible solution to a coverage problem, if the same frequency is allocated to both the sites, there is an unusable region where the interference is too great. A typical requirement is that the wanted signal should be 12 dB above the interfering signal. Physically, this leads to a requirement that any interferer should be at least three times as far away as the source of the wanted signal (if all transmitters are transmitting at the same power through similar antennas). A network operator will be allocated many carriers (60 being a typical number). It is therefore possible to make sure that adjacent cells are not given the same frequency. However, there will be many more than 60 cells in a network and some cells will require more than one carrier. It is therefore necessary to re-use carriers within the same network. Frequency Planning aims to ensure that continuous coverage is provided over the required area without unacceptable interference occurring. This will entail adopting an appropriate frequency re-use strategy. Typically, a re-use factor of 12 has to be adopted so that, if 60 carriers are allocated, no more than 5 can be placed on any one cell. It should be noted that frequency planning is a highly-skilled job and that the figures quoted above are only approximate examples of what the outcome may be. 14 © AIRCOM International 2003
  15. 15. Cell Planning Principles Section 1 – Introduction to Cell Planning The Need for Frequency Planning • GSM network capacity is eventually limited by inteference. • Frequencies can only be re-used at a far enough distance so as not to exceed inteference limits. • C/I of 12 dB typically required. • Distance to interfering cell must be 3 times that to serving cell. 1.7 Different Approaches for different environments. Both cell configuration and density will depend upon the environment. Open rural areas will have a different requirement to dense, urban areas. Individual approaches need to be adopted for roads where the coverage pattern may need to be different from those provided by standard antennas. Additionally, the propagation model will be different in different environments such that the signal may attenuate more quickly in a dense urban environment than it does in a rural environ ment. This will lead to the need for different reuse factors in the two environments. The coverage versus capacity discussion will have different outcomes in different environments. This will impact on antenna heights and configuration. In suburban areas, building planning constraints are likely to prevent antennas from exceeding a certain height. This will impact on the number of cells required to cover a particular area. © AIRCOM International 2003 15
  16. 16. Cell Planning Principles Section 1 – Introduction to Cell Planning Environmental Aspects • Different Environments pose different challenges. • Rural environments have coverage as a priority over capacity • Capacity and Inteference issues take priority in Dense Urban environments. • Path Loss Prediction difficult in a Dense Urban environment. Section 1 – Introduction to Cell Planning Environmental Aspects • Planning constraints may limit antenna heights, particularly in suburban areas. • Roads are sometimes covered by antennas with a narrow beam with special arrangements made for intersections. 16 © AIRCOM International 2003
  17. 17. Cell Planning Principles Section 1 – Introduction to Cell Planning Environmental Aspects • Small areas of high subscriber density can be best served by a low level antenna forming a micro-cell. • Office buildings are sometimes served by indoor antennas that form a pico-cell. © AIRCOM International 2003 17
  18. 18. Cell Planning Principles 18 © AIRCOM International 2003
  19. 19. Cell Planning Principles 2 Link Budget for GSM 2.1 Introduction Predicting the strength of a signal received by a mobile is not straightforward. Often the mobile receiver cannot see the base station and the signal strength is determined largely by reflection (also called “scattering”) and diffraction. The variety of possible environments encountered by the radio wave means that the distance between the base station and the mobile is not the only parameter that affects the path loss. A variety of possible methods exist that allow the engineer to predict the path loss with sufficient speed and accuracy. It is vital that a prediction method that engineers are confident in is adopted before the network is designed. Section 2 – Link Budget for GSM Why we need a Propagation Model • Mobile communication is made possible using multipath propagation • The radio wave undergoes scattering, diffraction and attenuation • Propagation model calculates the path loss between transmitter and receiver • Required for calculating power budgets and system balance requirements • Model used for setting up a network ? and subsequent optimisation © AIRCOM International 2003 19
  20. 20. Cell Planning Principles ________________________________________________________________________________ 2.2 Free Space Propagation Model 2.2.1 FREE SPACE RADIATED POWER Ideal propagation implies radiating equally in all directions from the radiating source and propagating to an infinite distance with no degradation. Section 2 – Link Budget for GSM Free Space Radiated Power • In free space the wave is not reflected or absorbed • Attenuation is caused by spreading the power flux over greater areas • Power Pt is transmitted equally in all directions • Surface area of sphere = 4πd2 Power Pt (W) d Power flux Pd at distance d from antenna given by: Pd Pd = P t / 4πd2 (W / m2) However, if the radiating element is generating a fixed power flux and this power flux is spread over an ever-expanding sphere, the energy will be spread more thinly as the sphere expands. Therefore in any given direction the energy will diminish with distance, even in an ideal propagation environment. The measurable power at any point on this sphere is known as the power (or power flux density (Pd)) measured in Watts per square metre. 2.2.2 FREE SPACE RECEIVED POWER Having identified the power flux density at any point of a given distance from the radiator, if a receive antenna is placed at this point, the power received by the antenna can be calculated. The amount of power ‘captured’ by the antenna at the required distance (d), is dependant upon the ‘effective aperture’ of the antenna and the power flux density at the receiving element. The effective aperture is dependant upon the wavelength of the received signal (i.e. the longer the wavelength, the greater the effective aperture required to capture the same power. 20 © AIRCOM International 2003
  21. 21. Cell Planning Principles Section 2 – Link Budget for GSM Free Space Received Power • Actual power received by antenna depends on: • Aperture of receiving antenna (Ae) • Wavelength of received signal (λ) • Power flux density at receiving antenna (Pd ) Isotropic antenna • Effective area of an isotropic antenna is: A e = λ2 / 4π • Power received: Pr = Pd x Ae = (Pt / 4πd2) Effective area Ae x (λ 2 / 4π) = Pt x (λ / 4πd) 2 The formulas for calculating the effective antenna aperture and received power are shown above. 2.2.3 FREE SPACE PATH LOSS Section 2 – Link Budget for GSM Free Space Path Loss • Receive Power is given by the formula: Pr = Pt x ( λ / 4πd) 2 • Expressing this formula in terms of dBs gives: Pr = Pt − 20log10(4π) − 20log 10(d) + 20log10( λ) dBm • If path loss (LP) = P t – Pr then: LP = 20log10(4π) + 20log10(d) − 20log10(λ) dBm • Substituting (λ = 0.3/f) and rationalising the equation produces the generic free space path loss formula: LP = 32.5 + 20 log10(d) + 20 log10(f) © AIRCOM International 2003 dBm 21
  22. 22. Cell Planning Principles Having determined formulas for the power at any point on the sphere of an isotropic radiator at a given distance and the received power for as given antenna aperture, basic path loss is simply the difference between the two. i.e. PL = PT - PR ________________________________________________________________________________ 2.3 Plane Earth Propagation Model The free-space propagation model does not consider the effects of propagating over ground. When a radio wave propagates over ground, some of the radio wave power will be directed into the ground. Some of this power will be reflected back up to the receive antenna. To determine the effects of the reflected power, the free-space propagation model is modified and referred to as the ‘Plain-Earth’ propagation model. This model better represents the true characteristics of radio wave propagation over ground. The Plane Earth model computes the received signal to be the sum of a direct signal and that reflected from a flat, smooth earth. The relevant input parameters include the antenna heights, the length of the path, the operating frequency and the reflection coefficient of the earth. This coefficient will vary according to the terrain type (e.g. water, desert, wet ground etc) Section 2 – Link Budget for GSM Plane Earth Model Plane Earth Model Rx Tx Reflection at Earth’s surface Signals at Rx may interfere constructively or destructively to different degrees Image Tx This depends on: Antenna heights (h1, h 2) Link distance d Wavelength Reflection coefficient of Earth 22 © AIRCOM International 2003
  23. 23. Cell Planning Principles Section 2 – Link Budget for GSM Plane Earth Model Equation • Calculations on the plane earth model lead to the following equation for path loss: LPEL = 20 log (d2 / h1 h2 ) dB LPEL = 40 log (d) - 20 log (h 1) - 20 log (h 2) • d = path length in meters • h1 and h2 are antenna heights • Problems with using plane earth model in GSM: • Does not deal with multipath reflections • Mobile height is constantly changing For a perfectly reflecting earth, the path loss L is expressed by: L = 40 log (d ) − 20 log (h1 ) − 20 log (h2 ) where d is the path length in metres and h1 and h2 are the antenna heights at the base station and the mobile. Section 2 – Link Budget for GSM Free Space vs Plain Earth Propagation = Free Space Loss = Plain Earth Loss © AIRCOM International 2003 23
  24. 24. Cell Planning Principles The plane earth model is not appropriate for mobile systems as it does not consider reflections from buildings, multiple propagation or diffraction effects. Further, if the mobile height changes (as it will in practice) then the predicted path loss will also change. ________________________________________________________________________________ 2.4 Cellular Radio Propagation Models The models described above are referred to as ‘theoretical’ as they are based on mathematical calculations only. Empirical models differ from theoretical models in that the formulas are determined from the analysis of actual measured practical data rather than theoretical mathematical models. The cellular radio propagation models described below all fall into the ‘empirical’ model type. 2.4.1 EMPIRICAL CELLULAR PROPAGATION MODELS Empirical models are mathematical equations that are based on the result of measurements made in typical, realistic situations. In general, such models are “tuneable”, that is, they contain coefficients that can be altered to make the model agree with measurements made in the location in which the network is being planned. Section 2 – Link Budget for GSM Cellular Propagation Models • • Models based on published data Main models available are: • Okumura - Hata • COST 231 - Hata • COST 231 Walfisch - Ikegami • Sakagami - Kuboi • Different models are more appropriate depending on: • Location • Frequency range • Clutter type Typically, more than one model will be used in any given network in order to predict the changes in topography (land use; also known as “clutter category”). A few models are now widely used as they have been shown to produce reliable predictions in a wide variety of circumstances. Furthermore they produce predictions very rapidly. Remember that a prediction will be made over a grid that will have a resolution of, say, 20 24 © AIRCOM International 2003
  25. 25. Cell Planning Principles metres. That means 2500 predictions must be made for every base station for every square kilometre. Speed of calculation of the propagation model is vital. Some “traditional” models (such as the Okumura-Hata) have been modified to extend their range of applicability. One very active group in this field has been the COST 231 project; a collaborative affair involving engineers and scientists from universities and industry throughout Europe. 2.5 Okumura’s Measurements A lot of current mobile propagation models have at their heart the measurements made by a Japanese engineer named Okumura.. Section 2 – Link Budget for GSM Okumura’s Measurements • Okumura, a Japanese engineer, carried out extensive drive test measurements, with a range of: • clutter type • frequency • transmitter height • transmitter power • Main conclusion from Okumura field tests: • Signal strength decreases at a much greater rate with distance than that predicted by free space loss © AIRCOM International 2003 25
  26. 26. Cell Planning Principles Section 2 – Link Budget for GSM Field Strength (dB) Okumura’s Results Free space loss Increasing base station antenna height Log Distance (km) •Typical curves for different base station antenna heights with fixed mobile height (1.5m) and frequency (900 MHz) •Field strength drops off more rapidly than free space line, particularly closer to the base station From the graphs of lines of best fit for a number of situations it was possible to quantify the effect of link length and base station height on the received signal strength. 26 © AIRCOM International 2003
  27. 27. Cell Planning Principles 2.6 The Hata Propagation Model Section 2 – Link Budget for GSM Hata’s Propagation Model • Hata based the model on Okumura’s field test results • Predicted various equations for path loss with different types of clutter • Limitations on Hata model due to range of test results: • Carrier Frequency: 150 MHz to 1500 MHz • Distance from the base station (d): 1km to 20 km • Height of base station antenna (hb ): 30m to 200m • Height of mobile antenna (hm): 1m to 10m Using Okumura’s results, the Japanese engineer Hata created a number of representative mathematical models for each of the urban, suburban and open country environments. Section 2 – Link Budget for GSM Hata’s Equations • Path loss for urban clutter: Lp(urban) = 69.55 + 26.16 log(f) - 13.82 log(hb) - a(h m ) + (44.9 - 6.55 log(h b)) log(d) • Path loss for suburban clutter: Lp(suburban ) = Lp(urban) - 2{log(f / 28)}2 - 5.4 • Path loss for open country: Lp(open country) = Lp(urban ) - 4.78 {log(f)}2 + 18.33 log(f) - 40.94 The coefficients can generally be changed as part of a “tuning” process. It must be remembered that the model should be regarded as applicable only over the ranges for which measurements were made. © AIRCOM International 2003 27
  28. 28. Cell Planning Principles Section 2 – Link Budget for GSM Limitations on Hata Model for GSM • Maximum carrier frequency = 1500 MHz • Not valid for 1800 MHz or 1900 MHz systems • Assumes base station antenna is above surrounding clutter • Not suitable for microcell planning where antenna is below roof height d hb hm microcell The upper frequency limit of 1500 MHz with regard to the Okumura-Hata model posed a serious problem when GSM systems commenced operation in the 1800 MHz band. One of the major objectives of the COST 231 project was to establish an appropriate macrocell model, or models, for frequencies up to 2000 MHz. 2.6.1 THE COST 231-HATA MODEL Section 2 – Link Budget for GSM Cost 231 - Hata Model • COST : European Co-operation in the Field of Scientific and Technical Research • COST 231 - Hata extends the Okumura-Hata model for medium to small cities to cover the 1500 to 2000 MHz band • Path loss equation is: L p= 46.3 + 33.9 log(f) - 13.82 log(hb ) - a(hm) + {44.9 -6.55 log(hb )}log(d) + Cm Cm = 3 dB for metropolitan centers Cm = 0 dB for medium sized cities and suburban areas • The model is not valid for hb <= hroof (i.e. base station below roof height) so it is not suitable for microcell planning 28 © AIRCOM International 2003
  29. 29. Cell Planning Principles 2.6.2 THE COST 231-WALFISCH-IKEGAMI PROPAGATION MODEL Section 2 – Link Budget for GSM COST 231 Walfisch-Ikegami Model • Combination of theoretical models (Walfisch / Ikegami) • Said to be a microcell model (but more realistically a small Macrocell model • Model parameters include: • Building separation (m) • Average Building Height (m) • Road width (m) • Road angle of orientation (degress) 2.6.3 THE SAKEGAMI-KUBOI MODEL Section 2 – Link Budget for GSM Sakagami Kuboi Model • General model based on detailed analysis of Okumura’s results • • Valid over wide frequency range: 450 to 2200 MHz • Claims validity for base antennas below roof top making it useful in planning Processes a large number of parameters relating to the urban environment microcells Ø Tx W © AIRCOM International 2003 29
  30. 30. Cell Planning Principles Sakagami Kuboi Formula The Sakagami -Kuboi formula is a level of sophistication above that of the Okumura-Hata or COST 231-Hata models. Its claim to be valid for base station heights below the building height makes it suitable for microcell and well as macrocells. However, it requires information regarding street width and orientation as well as building heights. This sort of information may not be readily obtainable for the mapping data to hand. Even if such data were available, the time taken to run the model may be prohibitive. ________________________________________________________________________________ 2.7 Diffraction Attenuation Models Terrain obstacles will sometime obstruct the line of sight (LOS) path between the base station and the mobile. The effect of an obstruction depends on: The amount by which it obstructs the line of sight path The distance from the obstruction to each end of the radio path The frequency of operation. These four parameters can be reduced to a single parameter by expressing the clearance in terms of the ‘Fresnel zone’ Radius. 2.7.1 FRESNEL ZONES The Fresnel zone describes an ellipsoid in three dimensional space. If you add the distance to each end of the link from any point on the surface of this ellipsoid, the sum is half a wavelength more than the straight-line distance between the two ends. 30 © AIRCOM International 2003
  31. 31. Cell Planning Principles The radius of this ellipsoid is referred to as the radius of the First Fresnel Zone. Section 2 – Link Budget for GSM Fresnel Zones Fresnel Zones r1 0.6 r 1 Tx Rx First Fresnel Zone Second Fresnel Zone First Fresnel Zone Third Fresnel Zone Forbidden Region (0.6 r1) Section 2 – Link Budget for GSM Fresnel Zone Dimensioning • Path length of any wave reflected from a Fresnel zone surface is nλ/2 more than direct path: a + b = d + n(λ/2) a b R1 Tx d1 R1 Rx d2 d • Radius of ellipsoid at d 1 from Tx is given by: R1 = n © AIRCOM International 2003 d1 d2 λ d 31
  32. 32. Cell Planning Principles Section 2 – Link Budget for GSM First Fresnel Zone Radius Calculation • Normally only first Fresnel zone considered. Hence: • d1d 2λ d For macrocell planning d2 >> d 1 and therefore d2 ˜ d. Hence: a + b = d + λ/2 and R1 ≈ R1 = d1dλ = d d1λ • The site of the base station and antenna height should allow complete clearance of at least the first 100 metres of the first Fresnel zone • For the 900 MHz band , λ ˜ 0.33m. Hence: R 1 ≈ 100 × 0.33 = 33 = 5.7 m 2.7.2 FIRST FRESNEL ZONE CLEARANCE If the first Fresnel zone is clear of obstructions then the link can be regarded as if no obstructions existed. It is important to remember that the curvature of the earth must be considered when evaluating the degree of obstruction Section 2 – Link Budget for GSM First Fresnel Zone Clearance First Fresnel Zone Clearance • At 100 m from the base station, the first Fresnel zone radius is about 5.7 m for a distant receiver • Using this figure, the required height of the antenna can be estimated • Some compromise may be necessary in the clearance allowed First Fresnel Zone 100 m 5.7m BTS MS 32 © AIRCOM International 2003
  33. 33. Cell Planning Principles Section 2 – Link Budget for GSM First Fresnel Zone Clearance Firs t clea Fresne r of l obs zone tacle s BTS MS Atmospheric effects can lead to the curvature being either exaggerated or diminished. This is accounted for by applying a “k-factor” to the actual radius of 6373 km when determining the curvature effect. Typically, the whole of the first Fresnel zone would be expected to be obstruction -free for a k-factor of 1.33 and 60% free when the k-factor is 0.67. 2.7.3 SINGLE OBSTRUCTION DIFFRACTION FORMULA If the clearance requirements are not met, then the amount of “diffraction loss” must be calculated. For a single obstruction (or ‘knife edge’) protruding into the first Fresnel zone, the diffraction loss can be calculated from the following formula: Section 1 - Propagation Models Single Knife-Edge Diffraction Formula h d2 d1 Fresnel diffraction parameter = v=h 2( d1 + d 2 ) λd1d 2 where λ = wavelength © AIRCOM International 2003 33
  34. 34. Cell Planning Principles Section 2 – Link Budget for GSM Diffraction Effects Section 2 – Link Budget for GSM Single Knife-Edge Diffraction Formula The diffraction loss (L) is given by: L = -20 log(x) Where x=1 for v < -0.8 x = 0.5 - 0.62v for -0.8 <= v < 0 x = 0.5exp(-0.95v) for 0 <= v < 1 x = 0.4 – (0.1184-(0.38-0.1v) 2 )1/2 x = 0.255/v 34 for 1 <= v <= 2.4 for 2.4 < = v © AIRCOM International 2003
  35. 35. Cell Planning Principles 2.7.4 MULTIPLE KNIFE EDGE DIFFRACTION MODELS However, if there is a series of obstacles in the radio path, the computation becomes extremely complex. A number of methods have been developed for approximating the total diffraction loss for multiple obstructions to the radio path. Each has advantages and disadvantages in certain circumstances and all those described here use the approach summing a number of individual diffraction losses. Section 2 – Link Budget for GSM Multiple Knife Edge Diffraction Models • Objects protruding into the first Fresnel zone will cause significant diffraction effects • A variety of models are available to calculate diffraction loss or gain at the receiver due to a series of knife edges • The commonly used knife edge models are: • Bullington • Epstein-Peterson • Deygout • Japanese Atlas © AIRCOM International 2003 35
  36. 36. Cell Planning Principles 2.7.5 THE BULLINGTON DIFFRACTION MODEL Section 2 – Link Budget for GSM Bullington Model • Defines a new effective knife edge obstacle at the point where the line-of-sight from the two antennas cross • Advantage: • Simple method • Disadvantage: • Significant obstacles may be ignored leading to an optimistic es timate of field strength Tx 1 1 22 Rx Equivalent knife edge Burlington developed a diffraction model which was presented in 1947. It involves taking two diffracting knife edges and replacing them with a single equivalent knife edge. This process involves extending a line from one terminal through the peak of the first knifeedge. The process is then repeated from the second terminal through the peak of the second knife edge. Where these two extended lines intersect, an equivalent knife-edge is created. The diffraction effect is then calculated for the single equivalent obstacle rather than for the two separately. The Bullington model affords reasonable accuracy when the two obstacles are reasonably close together and a reasonable distance from the terminals. Otherwise significant inaccuracies can be introduced leading to underestimation of overall diffraction loss. 2.7.6 THE EPSTEIN-PETERSON DIFFRACTION MODEL The Epstein-Petersen Model was introduced in 1953. This model estimates the total diffraction loss for a multiple-obstacle path by calculating individual losses fore each obstacle and adding these together to estimate the overall diffraction loss. 36 © AIRCOM International 2003
  37. 37. Cell Planning Principles Section 2 – Link Budget for GSM Epstein - Peterson Model • • This finds the total loss as the sum of the diffraction losses at each obstacle For obstacle 1, consider Rx to be at obstacle 2. Then for obstacle 2, consider Tx to be at 1 and Rx to be at 3 and so on. Tx 2 Rx 1 Rx2 Tx1 Tx 1 2 2 3 Rx • Advantage: • does not ignore important obstacles as Bullington method may • Disadvantages: • may overestimate path loss when the obstructions are close together Referring to the above diagram, the process for calculating a single knife edge diffraction loss is as follows: Assume the Rx terminal is at the peak of the obstacle 2. Draw a new line of Sight (LOS) between the Tx terminal and the new Rx terminal. The diffraction loss of the obstacle 1 intrusion into this new LOS in then calculated. The next step is to shift the terminal positions one obstruction to the right. The Tx terminal is now deemed to be at the peak of obstacle 1 and the Rx terminal at the peak of obstacle 3. A new LOS is drawn between the two terminals. The diffraction loss of the obstacle 2 intrusion into this new LOS in then calculated. The next step is to shift the terminal positions one obstruction to the right again. The Tx terminal is now deemed to be at the peak of obstacle 2 and the Rx terminal at its actual position. A new LOS is drawn between the two terminals. The diffraction loss of the obstacle 3 intrusion into this new LOS in then calculated. The individual diffraction losses at each stage of calculation are then added together to produce the total estimated diffraction loss. This process can be repeated for any number of obstacles. The Epstein-Peterson Model provides a reasonable approximation provided the obstacles are well separated. In urban areas where intrusions are commonly close together, the accuracy of this model diminishes and tends to overestimate losses. © AIRCOM International 2003 37
  38. 38. Cell Planning Principles 2.7.7 THE JAPANESE-ATLAS DIFFRACTION MODEL This model was introduced by the Japanese Postal Service in 1957. Section 2 – Link Budget for GSM Japanese Atlas Model • Effective Tx position for each obstacle is found by extending the line to the previous obstacle back to meet the vertical line through the actual Tx Loss for obstacle 1: Line from 2 to Tx – LOS Tx to 2 Loss for obstacle 2: Line from 2 through 1 to Tx1 – LOS Tx1 to 3 Loss for obstacle 3: Line from 3 through 2 to Tx2 – LOS Tx2 to Rx Tx2 Tx 1 Tx • 1 2 Rx 3 Advantage: • gives improved results when the obstructions are closely spaced • Disadvantage: • still suffers from underestimating the path loss • Non-reciprocal The Japanese Atlas method is a variation on the Epstein-Petersen model whereby the contribution of obstacle 2 is determined not by assuming obstacle 1 to be the Tx terminal but as follows: The diffraction loss for the first obstacle is calculated as normal by extending a line from the peak of obstacle 2 back to the actual position of the Tx terminal and calculating the diffraction loss introduced by obstacle 1. However, to calculate the diffraction loss of obstacle 2, a line joining the peaks of obstacles 1 and 2 is extended back towards the Tx terminal to a point where it crosses an imaginary line extending above the actual Tx terminal height. Where this imaginary line is crossed, a new virtual Tx source is established (T above). A new LOS is then drawn to the next obstacle (3 in this case) and the diffraction loss caused by obstacle 2 into this LOS is calculated. This second step is then repeated for each successive obstruction until all have been included. The individual diffraction losses are the added together to determine the overall loss. This method can produce more accurate results than the Epstein-Peterson model but must be treated with care as the results are non-reciprocal. This means that if the calculations are initiated from the Rx terminal, the results will be different. 38 © AIRCOM International 2003
  39. 39. Cell Planning Principles 2.7.8 THE DEYGOUT DIFFRACTION MODEL Section 2 – Link Budget for GSM Deygout Model • • • • This calculates a V parameter for each obstacle Obstacle with highest value of V is the ‘main obstacle’ Loss for this edge is calculated Loss over next most significant obstacle is then found and added to loss and so on main edge Tx 1 1 Rx 2 3 • Advantages: • 2.7.9 • Accurate when there is a clearly dominant edge • For three or four obstructions, Deygout gives the best results of any of the approximate methods Disadvantages: • Where there is no dominant edge, Deygout tends to overestimate the loss • Does not work well when there are two similar height edges such as ends of a building SELECTING A SUITABLE MODEL Section 2 – Link Budget for GSM Which Model to Use? • Cell planners using various planning tools have their favourite models • No one model is accurate in every situation • Before using a model to predict coverage it must be verified by drive testing • Model must be tuned or calibrated according to the local situation Asset’s model is based on COST 231Hata and is tuned by varying a combination of parameters k1 - k7 © AIRCOM International 2003 39
  40. 40. Cell Planning Principles Section 2 – Link Budget for GSM Summary • • Need for propagation models: predictions, power budgets, optimisation Theoretical Propagation Models: • Free Space Model, Plain Earth Model • Empirical Propagation Models: • Okumura’s results, Hata model, COST 231 Hata, Sakagami - Kuboi, • Diffraction Models • Fresnel zones • Diffraction Formula • Diffraction Models: • Epstein - Peterson, Bullington, Japanese Atlas, Deygout 2.8 CW Drive Testing ________________________________________________________________________________ The propagation models described in the previous section are ‘generic’ for a particular notional environment. That is to say, only one model is defined for each environment type (e.g. urban, suburban, rural etc). Clearly, each physical urban environment will be different, i.e. different countries, different cities, different urban densities and building types etc. Therefore, it is necessary to carry out measurements of the physical environment in order to adjust the ‘generic’ propagation model to be more representative of the local physical environment. This is the purpose of ‘CW drive testing’. CW drive testing involves covering an extensive part of the proposed coverage area measuring the signal strength received from individual BTSs. For this purpose a narrow-band unmodulated carrier wave (CW) signal is used. Hence the term ‘CW’ drive testing. ________________________________________________________________________________ 2.8.1 THE TRANSMITTER It is usually necessary to establish a temporary transmitter in order to conduct a drive-test. The transmitter could utilise its own mobile mast or, alternatively, make use of existing buildings. It is, of course, vital that either the transmitter is battery powered or there is ready access to a mains power supply. 40 © AIRCOM International 2003
  41. 41. Cell Planning Principles Section 2 – Link Budget for GSM Drive Test - Transmitter • Temporary Transmitter Arrangement: • Roof top or crane mounted • Access to AC power source • Low gain omni-directional antenna RADIO TRANSMITTER BATTERY Temporary mast ________________________________________________________________________________ 2.8.2 THE RECEIVER The equipment must be capable of recording both the signal strength and the location of the mobile at the measurement instant. This is so that the measurement can be compared with the prediction for the signal strength at that position. The measurement equipment must include a sensitive, narrow band, receiver; usually including a spectrum analyser. The location information is provided by a GPS receiver. As a considerable distance may be travelled between one GPS reading and the next, the position information may be supported by speed and direction information provided by in-car equipment. © AIRCOM International 2003 41
  42. 42. Cell Planning Principles Section 2 – Link Budget for GSM C W Analysis - Receiver GPS Receiver Positional Information PC Radio Receiver Disk Storage Speed Information Gyro Directional Information Wheel pulser ________________________________________________________________________________ 2.8.3 CW MEASUREMENT PROCESS Section 2 – Link Budget for GSM CW Test Process • Drive testing should be performed on radial and circumferential routes • Radial routes show variation in signal strength with distance from base station • Circumferential routes provide predictions for signal strength in different directions from the base station 42 © AIRCOM International 2003
  43. 43. Cell Planning Principles It is important that as much variety as possible is incorporated into the measurements. It is not very useful if the mobile is just moved in a circle at a constant distance from the base station as no data would be gathered regarding the variation of signal strength with distance from the mobile. Similarly moving the mobile along one radial line from the base station would render the results likely to be unrepresentative due to some particular characteristic of the terrain in the direction chosen. Section 2 – Link Budget for GSM CW Analysis - Transmitter Siting • • • Select sites which are good representatives of your network • Do not place the test transmitter on a building having long roof Use a range of antenna heights Ensure there are no immediate obstructions near the test transmitter Long roof obstructs transmission in this direction Section 2 – Link Budget for GSM CW Analysis - Drive Testing • Test drive in the main vertical lobe of the omni antenna - low gain antenna large main lobe - consistent good coverage over wide area • Do not test drive in the shadow regions Omni antenna X • Take panoramic photographs around the test site to relate measurements to actual ground features © AIRCOM International 2003 43
  44. 44. Cell Planning Principles Section 2 – Link Budget for GSM Clutter and Man -made Features • Cover all clutter types equally throughout the test • Be aware of man-made features such as bridges or tunnels Most models assume a mobile height of 1.5m, bridges can affect the actual height above ground: Actual position of mobile Bridge Ground level - given by map data Model assumes mobile is here When comparing results with the model it is important that the validity of any assumptions made by the model is maintained. One of these assumptions will concern the gain of the antenna. It is therefore very important that the mobile remains within the main lobe of the base station antenna. This entails ensuring that the mobile does not get too close to the antenna such that it is “under” the main lobe. Further, it is essential that, when the positional information gathered is translated onto a map of the test route, the mobile is calculated to be where it actually is. This may sound obvious but there are situations where discrepancies can occur. One such situation is when the mobile is on a bridge over a valley. The model may well assume that the mobile is on the valley floor when, in fact, it would be many metres above this point. An additional case to be avoided is placing the mobile in a tunnel when the mapping data would lead to the assumption that it was on the land surface vertically above. 44 © AIRCOM International 2003
  45. 45. Cell Planning Principles Section 2 – Link Budget for GSM CW Measurement Equipment • Measurements should be distance based • • Take readings no closer than 0.38 wavelengths apart. Equipment can be: • • Distance triggered • Time triggered GPS outputs position every second. • • Position interpolation is required for each measurement Test mobile measurements are NOT suitable • Fluctuating power levels (TCH carriers) • Minimum 2dB steps • 200kHz channel bandwidth Measurements should be taken at regular intervals. If the equipment is distance-triggered, care must be taken to ensure that th e speed of the drive test vehicle is not so large that the equipment cannot complete a measurement before the next trigger pulse arrives. If the equipment is time triggered, a slow moving vehicle will produce the desired small distances between measuremen ts. In this situation, the speed of the vehicle should be maintained constant. It is tempting to economize on the measurement equipment by using a test mobile instead of a professional measuring receiver. Test mobile receivers are inadequate for a number of reasons: They record signal levels to the nearest 2 dB. They are wideband (200 kHz) devices and hence not sufficiently sensitive. Power levels fluctuate on a live traffic carrier © AIRCOM International 2003 45
  46. 46. Cell Planning Principles Section 2 – Link Budget for GSM Summary – CW Drive Testing • Purpose of CW Drive Testing • CW Drive Test Equipment • The Drive Testing Process • The Practicalities of CW Drive Testing GPS Receiver Positional Information Radio Receiver Disk Storage PC Speed Information Gyro Directional Information Wheel pulser 2.9 Propagation Model Tuning ________________________________________________________________________________ The measurements obtained from the CW drive testing are compared with those predicted by the theoretical model. Where discrepancies exist, the parameters of the theoretical model are modified until the mathematical model becomes the closest approximation to the measured data. This modified mathematical propagation model is then generally used for all planning purposes in a particular environment (urban, rural etc). This modification process is known as ‘model tuning’ or ‘model calibration’. This allows the coefficients of the various terms contained in the model to be altered so as to produce a “best-fit” curve to the measured data. The equation for this curve is then used as the propagation model. 46 © AIRCOM International 2003
  47. 47. Cell Planning Principles Section 2 – Link Budget for GSM Purpose of Model Tuning • The theoretical propagation model is ‘generic’ • The physical environment will vary • The generic model is compared to CW measurements • Where differences exist, the theoretical model is modified to reflect the physical environment. • The modification process is known as model ‘tuning’ or ‘calibration’ ________________________________________________________________________________ 2.9.1 ANALYSIS OF A THEORETICAL PROPAGATION MODEL In order to describe how a theoretical mathematical propagation model can be ‘tuned’ is if first necessary to examine the components of the formula than describes the propagation model. Section 2 – Link Budget for GSM A Mathematical Propagation Model • A ‘modified’ path loss model based on the COST-Hata 321 model: Ploss = k1 + k2(log(d)) + k3(HMS ) + k4 log(HMS) + k5 log(Heff) + k6 log (Heff) + k7 + Closs Where d = distance from MS to BTS HMS = MS antenna height Heff = BTS effective antenna height © AIRCOM International 2003 47
  48. 48. Cell Planning Principles The above equation is an adaptation of the COST 231-Hata propagation model. It can be seen that the equation comprises the following components: The effective heights of the MS and BTS antennas The distance between the MS and BTS A number of variables known as ‘k’ factors. The factors can be modified in order to ‘tune’ the model to become a more accurate representation of the chosen environment. Each time the value of a factor is changed, the model has to be re-analysed to determine whether or not the mathematical model is more closely aligned with the measured CW data. The model is deemed to be tuned when the mathematical model is most closely aligned with the measured data. Section 2 – Link Budget for GSM Propagation Model ‘K’ Factors • k1/k2 – attenuation intercept and slope • k3 – mobile antenna height correction factor • k4 – mobile antenna height multiplying factor • k5 – BTS antenna height multiplying factor • k6 – Hata multiplying factor • k7 - diffraction loss (model- dependant) • Closs – Clutter attenuation adjustment ___ 48 © AIRCOM International 2003
  49. 49. Cell Planning Principles _____________________________________________________________________________ 2.9.2 THE MODEL TUNING PROCESS 2.9.2.1 MODEL TUNING PROCESS OVERVIEW Section 2 – Link Budget for GSM Model Tuning Process • Different approaches possible • Trial and error process • Some planning tools have automated process • Optimise ‘k’ values in sequence to: • minimise mean error value • minimise standard deviation value Because all the factors are inter-related, a number of different approaches can be taken to the tuning process. However, there are essentially two criteria by which the effectiveness of the tuning process can be gauged: Minimised mean error value Minimised standard deviation (SD) value © AIRCOM International 2003 49
  50. 50. Cell Planning Principles 2.9.2.2 EXAMPLE TUNING PROCESS Section 2 – Link Budget for GSM Example Model Tuning Process (1) Start CW Data Analyse Apply Filters Prediction Model Slope/Intercept Correction Final Slope Correction? YES Clutter Offset Corrections Finish NO Diffraction Correction Eff Antenna Height Correction The above diagram shows one approach to the model tuning process. This outline process includes the following steps: STEP 1 Import the captured CW drive test data. STEP 2 Analyse the CW data. Filter out any erroneous or unwanted measurements. STEP 3 Apply the filtered data to the mathematical propagation algorithm by: Selecting the propagation model to be used Setting the default K parameter values Analysing the data STEP 4 Optimise K parameters STEP 5 Make any clutter offset corrections. Clutter offsets allow the user to tune the model to particular types of clutter. In the Standard Macrocell model positive clutter offsets add additional path loss to areas of clutter. Offsets can be set individually on each clutter type. A negative offset would reduce the path loss over the chosen clutter type. 50 © AIRCOM International 2003
  51. 51. Cell Planning Principles The following flow chart illustrates the steps that take place in optimising the K parameters (Step 4 above): Section 2 – Link Budget for GSM Example Model Tuning Process (2) Analyse Select Diffraction Algorithm Adjust Eff Heights (k3-k6) Adjust Slope (k1) Analyse Analyse Start Analyse NO NO SD minimum? NO YES Mean Error minimum? Adjust diffraction (k7) SD minimum? EFFECTIVE ANTENNA HEIGHT CORRECTION YES Adjust Slope (k1) YES Analyse Adjust Intercept (k2) Analyse NO Analyse SD minimum? YES NO NO SD minimum? YES DIFFRACTION CORRECTION Mean Error minimum? FINAL SLOPE CORRECTION YES SLOPE/INTERCEPT CORRECTION Finish The K value tuning process can be divided into four parts: Slope/Intercept Correction Diffraction Correction Effective Antenna Height Correction Final Slope Correction 2.9.2.3 Slope/Intercept Correction Initial analysis of the data will indicate a mean error. The K1 parameter should be adjusted by a value equal to the current mean value to reduce it to zero. For example, if the mean value is -8.5, the K1 value should be set to 8.5 to compensate. This should reduce the mean value to 0. Further adjustment may be necessary on a trial-and-error basis to achieve the goal. The K2 value should now be adjusted to minimise the initial SD value. This is an interactive process and it will be found that if an increase in K2 reduces the SD, further increases will eventually result in the SD beginning to increase again at some point. Hence, adjustment of the K2 value will follow the characteristic shown in the diagram below and should be adjusted to achieve the minimum SD value. © AIRCOM International 2003 51
  52. 52. Cell Planning Principles Section 2 – Link Budget for GSM Mean Deviation Value Standard Deviation (SD) ‘k’ Value Optimisation ‘k’ Value Optimum value 2.9.2.4 Diffraction Correction This process is initiated by selecting a diffraction model and analyzing the data to assess the effect on the SD. The model that produces the lowest SD should be retained. The K7 parameter should now be adjusted using the same interactive process described in 3.3.2.1 above to minimise the SD. 2.9.2.5 Effective Antenna Height Correction This process involves using the same iterative process described above but using parameters K3-K6. Note that id the antenna height of the MS is deemed to be fixed, K3 and K4 can be ignored. 2.9.2.6 Final Slope Correction The final stage is to return to the K1 parameter and make any further adjustments to minimise the mean error value. 2.9.3 PRACTICAL MODEL TUNING DEMONSTRATION This is a practical demonstration of tuning a propagation model using the AIRCOM ASSET tool. The procedures used are described in Appendix A to these course notes 52 © AIRCOM International 2003
  53. 53. Cell Planning Principles 2.10 Power Budgets ________________________________________________________________________________ One essential part of the system planning procedure is to establish the signal strength that a mobile will receive in a particular area and, further, to ensure that wherever coverage is provided on the “downlink” (from the base station to the mobile) then system balance is maintained by engineering the sites so that there is sufficient signal power received on the uplink. Section 2 – Link Budget for GSM Power Budgets • Calculations to allow for losses and gains in signal strength to ensure level is acceptable throughout service area • Minimum level must be greater than sensitivity of mobile, with a margin for fading and penetration loss • Decibel calculations allow simple tracking of losses and gains • Power input to mobile = Tx output - Losses + Gains Feeder Radio Path TRX When assessing downlink coverage, it is important that due attention is paid to the threshold level (or “sensitivity”) of the mobile. This is the minimum signal level for which a service of acceptable quality will be provided. In practice, the signal strength will suffer fades due to multipath propagation. A margin should be allowed for this fading. Further, the propagation model will predict the signal strength in the street rather than inside buildings. If coverage is to be provided within buildings, an additional “penetration loss” must be accounted for. These margins will be added to the path loss and other losses to give a total loss. The antennas will provide some gain to offset against this. A net loss can then be calculated as the difference between the total losses and the total gains. This loss is subtracted from the transmit power to give an estimate of the power level received at any point. © AIRCOM International 2003 53
  54. 54. Cell Planning Principles ________________________________________________________________________________ 2.10.1 REVIEW OF DECIBEL SCALE Section 2 – Link Budget for GSM Review of Decibel Scale • • • • Logarithmic scale for comparing power levels Gain = Pout / Pin as a ratio In decibels: Gain = 10 log (Pout / Pin ) Using logarithms allows a sequence of gains and losses to be found by adding and subtracting decibel values rather than multiplying and dividing • Decibel scale can be used to measure an actual power level by using a reference level • Power in dBm is compared to a power of 1 milliwatt (1 mW = 1/1000 W) • Example, convert 2 watts to dBm : P = 10 log ( 2 W / 1 mW) = 10 log (2000 / 1 ) = 10 x 3.3 = 33 dBm • Note: 1 mW = 0 dBm (since log(1) = 0) The use of the decibel scale allows the result of combining losses and gains within a system to be determined by means of addition and subtraction rather than multiplication and division. A gain or a loss is expressed in decibels (dB) in accordance with the equation given in the above slide. It is possible to express an absolute power, rather than a gain, using the decibel scale by adopting 1 milliwatt as a reference power. The absolute power is then quoted in dBm, the “m” referring to a milliwatt. Certain commonly-encountered gains and losses are tabulated below. Gain as a ratio 2 4 10 50 100 1000 0.5 (a loss of a factor of 2) 0.1 (a loss of a factor of 10) 54 Gain in dB (= 10 log10[gain as ratio]) 3 dB 6 dB 10 dB 17 dB 20 dB 30 dB -3 dB -10 dB © AIRCOM International 2003
  55. 55. Cell Planning Principles Power in milliwatts Power in dBm (= 10 log10[power in mW]) 3 dBm 10 dBm 20 dBm 30 dBm 43 dBm -30 dBm -60 dBm -90 dBm 2 10 mW 100 mW 1000 mW 20 W = 20000 mW 1 µW 1 nW 1 pW ________________________________________________________________________________ 2.10.2 ANTENNA GAIN Section 2 – Link Budget for GSM Antenna Gain • • • Antenna gain is quoted relative to an isotropic radiator Units : dBi Gain is achieved because the output power is concentrated into a smaller region Isotropic pattern Typical antenna gains Omni-directional dipole pattern Omni: 8 to 12 dBi Sector: 10 to 18 dBi Antenna gains are also quoted using a decibel scale. The gain of an antenna refers to its ability to concentrate the radiated energy into a narrow beam rather than spread it equally in all directions. A theoretical antenna (known as an “isotropic radiator”), that radiates power equally in all directions, is usually chosen as a reference against the field strength produced by a practical antenna. In order to make it clear that an isotropic antenna has been adopted as a reference, the gain is quoted using the unit dBi (‘i’ being for isotropic). ________________________________________________________________________________ © AIRCOM International 2003 55
  56. 56. Cell Planning Principles 2.10.3 THE DOWNLINK POWER BUDGET Section 2 – Link Budget for GSM Downlink Power Budget Gab Antenna Gain Lp Ld MS Antenna Gain G Path Loss Duplex Filter Loss am L fb Feeder Loss Combiner Loss BTS Tx Output Power Feeder Loss L fm Lc Input to mobile PinMS PoBS PinMS = PoBS - Lc - Ld - L fb + Gab - L p + Gam - Lfm The above diagram highlights the contributing factors to gains and losses as the signal progresses from the Base Transceiver Station (BTS) to the mobile. A base station will probably transmit more than one carrier. These will share a common antenna. The combiner involved will introduce some loss. Additionally, in order to be able to use the same antenna for transmitting and receiving, it is necessary to use a device known as a duplexer. The duplexer will provide isolation between the transmitter and the receiver to prevent interference between the transmit and receive signals. In providing this isolation, it will inevitably insert a small loss into the wanted signal path. In calculating the received signal power it is simply necessary to insert the appropriate numbers into the equation. This is demonstrated in the following slides. 56 © AIRCOM International 2003
  57. 57. Cell Planning Principles Section 2 – Link Budget for GSM Downlink Power Budget Analysis • Power input to the mobile (dBm): PinMS = PoBS - Lc - Ld - Lfb + Gab - Lp + G am - Lfm PoBS = Power output from BTS TRX dBm Lc Ld = BTS combiner loss = BTS duplex filter loss dB Lfb = BTS Feeder loss dB Gab Lp = BTS antenna gain = Path loss dBi dB Gam = Mobile antenna gain Lfm = Mobile station feeder loss dB dBi dB Section 2 – Link Budget for GSM Downlink Power Budget - Example • A class 4 mobile has a sensitivity of -102 dBm. Allowing a margin for fading, we take the minimum signal strength at the cell boundary as - 90 dBm. This is to be 10 km from the base station. • Find the BTS output power, PoBS , required given the following data: BTS combiner loss L c = 6 dB BTS duplex filter loss L d = 1 dB BTS feeder loss Omni antenna gain Hata path loss for 10 km Mobile antenna gain Mobile station feeder loss © AIRCOM International 2003 L fb = 7 dB G ab = 12 dB L p = 132 dB G am = 0 dBi L fm = 0 dBi 57
  58. 58. Cell Planning Principles Section 2 – Link Budget for GSM Downlink Power Budget - Solution Downlink power budget equation: PinMS = PoBS - Lc - Ld - Lfb + Gab - Lp + Gam - Lfm -90 = PoBS - 6 - 1 -7 + 12 -132 + 0 - 0 -90 = PoBS - 134 PoBS = 44 dBm ________________________________________________________________________________ 2.10.4 THE UPLINK POWER BUDGET Section 2 – Link Budget for GSM Uplink Power Budget Antenna Gain Gab G dBS Diversity Gain Ld Path Loss Duplex Filter Loss L fb Feeder Loss Lp PinBS 58 am Feeder Loss L fm Input to BTS Rx BTS Rx MS Antenna Gain G Output from mobile PoMS PinBS = PoMS - Lfm + Gam - Lp + GdBS + Gab - Lfb - Ld © AIRCOM International 2003
  59. 59. Cell Planning Principles The uplink power budget has two distinct differences when compared with the budget for the downlink. There is no combiner in the base station on the downlink path, hence no combiner loss Two receive antennas are often used at the base station (this is not feasible on the downlink as it is not possible to provide more than one antenna at the mobile). This provides, what is known as a “diversity gain” due to the fact that, if the signal received from one antenna suffers a severe fade, the signal at the second antenna is unlikely to suffer from a fade. Thus the uplink has the advantage of no combiner loss and diversity gain. Additionally, the receiver in the base station is more sensitive than the receiver in the mobile. These three factors help compensate for the lower transmit power available on the uplink. Section 2 – Link Budget for GSM Uplink Power Budget - Example • Using the data given earlier for downlink, find the input power to the base station if: • • Output power of mobile PoMS= 33 dBm (2 W class 4 mobile) • Diversity reception gain at base station GdBS= 5 dB Solution: PinBS = 33 - 0 + 0 - 132 + 5 + 12 - 7 - 1 = -90 dBm © AIRCOM International 2003 59
  60. 60. Cell Planning Principles ________________________________________________________________________________ 2.10.5 OTHER POWER BUDGET FACTORS Section 2 – Link Budget for GSM Other Power Budget Factors • Building/Vehicle Penetration Loss • Body Loss • Antenna Orientation Loss • Polarisation Loss • Additional Fast Fade Loss • Interference Degradation Loss 2.10.5.1 BUILDING PENETRATION LOSS Section 2 – Link Budget for GSM Building Penetration Loss • Defined as the difference between measurements taken on a specific building floor and measurements taken externally at street level. • Losses dependant upon: • Type of outside wall construction (brick, glass, thickness etc) • Floor elevation • Propagation environment near the building • Orientation of building with respect to BTS antenna 60 © AIRCOM International 2003
  61. 61. Cell Planning Principles Section 2 – Link Budget for GSM Building Penetration Loss • ETSI GSM 03.03 recommends : BPL (dB) Floor 42 22 2nd 39 - 37 20 4th 27 - 24 20 15 - - 19 - - 9th Varies with frequency used - 1st 8th • 31 7th • 9-11dB 22 Ground 6th • Standard deviation values of: 40 5th • Urban: 15-18dB • Rural 10dB Building 2 Basement 3rd • mean values of Building 1 - 17 2.10.5.2 VEHICLE PENETRATION LOSS Section 2 – Link Budget for GSM Vehicle Penetration Loss • Defined as the difference between measurements taken inside and outside of a stationary vehicle • Losses dependant upon: • Materials used for the vehicle construction • Thickness of vehicle skin • Typical values 10-15dB © AIRCOM International 2003 61
  62. 62. Cell Planning Principles 2.10.5.3 BODY ABSORPTION LOSS Section 2 – Link Budget for GSM Body Absorption Loss • Two Types: • Body loss: • Body proximity loss • Losses dependant upon: • Proximity of body • Direction of LoS • Antenna type • Frequency in use • Typical values 2-6dB ________________________________________________________________________________ 2.10.6 COVERAGE LOCATION PROBABILITIES Section 2 – Link Budget for GSM Coverage Location Probabilities • Defined as the probability of an acceptable signal being received by an MS at any point in the cell coverage area • This probability is determined from the characteristics of slowfade margin applied in the power budget • Two probabilities used: • Point Location Probability (cell boundary) • Area Location Probability (within cell coverage area) 62 © AIRCOM International 2003
  63. 63. Cell Planning Principles Section 2 – Link Budget for GSM Point Point vs Area Location Probabilities Point location probability Area location probability Section 2 – Link Budget for GSM Coverage Location Probabilities Slow Fade Margin vs Location Probability 15 Slow Fade Margin (dB) 10 5 0 0 10 20 30 40 50 60 70 80 90 100 -5 -10 -15 Location Probability (%) © AIRCOM International 2003 63
  64. 64. Cell Planning Principles Section 2 – Link Budget for GSM Point Location Probability • • • • Defined as the reception probability at a location on a cell boundary Higher slow fade margin improves probability Higher slow fade margin reduces cell coverage for the same EIRP. Calculated from the mean (µ) and standard deviation (s) values of the received signal strength: Px = where: 1 2π σ 2 e ( x−µ )2 2σ 2 P x = location probability x = signal level s = standard deviation of x µ = mean value of x Section 2 – Link Budget for GSM Point Location Probability location probability of 50% (0dB fade margin) location probability of 75% (5dB fade margin) location probability of 95% (12dB fade margin) r Increasing the fade margin allowance will decrease the available power budget and therefore decrease the cell radius (r) 64 © AIRCOM International 2003
  65. 65. Cell Planning Principles Section 2 – Link Budget for GSM Area Location Probability • Defined as the probability of receiving a call at any location within the cell coverage • • Accounts for degradation of signal strength as distance from BTS increases • Calculated from the mean (µ) and standard deviation (s) values of the received signal strength: Measured as the ‘useful area’ within cell coverage for a specific probability of reception  Auseful =  2 ab + 1  1     1+ erf (a) +  e   2 b2        where:        ab + 1         b         1 − erf   Auseful = useful service area within a circle of radius R erf = error function a = point probability value b = propagation slope characteristic Section 2 – Link Budget for GSM Point / Area Slow Fade Margins Standard Deviation (s ) dB Location Probability (%) Point Location Probability Area Location Probability 6 7 8 6 7 8 10 -7.69 -8.97 - 10.25 -19.34 -20.19 -21.12 20 -5.05 -5.89 -6.73 -14.14 -14.85 -15.59 30 -3.15 -3.67 -4.2 -10.92 -11.46 -11.99 40 -1.52 -1.77 -2.03 -8.43 -8.77 -9.09 50 0 0 0 -6.25 -6.39 -6.51 60 1.52 1.77 2.03 -4.18 -4.11 -4.01 70 3.15 3.67 4.2 -2.08 -1.76 -1.41 80 5.05 5.89 6.73 0.28 0.9 1.55 90 7.69 8.97 10.25 3.43 4.48 5.56 95 9.87 11.51 13.16 5.95 7.36 8.8 96 10.5 12.25 14.01 6.67 8.18 9.73 97 11.28 13.17 15.05 7.55 9.19 10.87 98 12.32 14.38 16.43 8.72 10.53 12.38 99 13.96 16.28 18.61 10.54 12.62 14.74 ________________________________________________________________________________ © AIRCOM International 2003 65
  66. 66. Cell Planning Principles 2.10.7 LINK BUDGET TYPES AND THRESHOLDS Section 2 – Link Budget for GSM Link Budget Types / Thresholds Link Budget Types / Thresholds • Link Budget Types: • • • • • Outdoor Indoor In-building In-car Thresholds: • • • • • Outdoor (-92 dBm) Good In-building (-70 dBm) Average in-building (-78 dBm) Good in-car (-85 dBm) Marginal in-car (-88dBm) ________________________________________________________________________________ 2.10.8 SYSTEM BALANCE Section 2 – Link Budget for GSM System Balance System Balance • Power budget calculations show the maximum distance of the mobile from the base station at which uplink and downlink can be maintained • In a balanced system, the boundary for uplink and downlink must be the same Downlink limit Uplink limit Unbalanced system • 66 Downlink limit Uplink limit Balanced system An unbalanced system would drop many calls in the fringe region © AIRCOM International 2003
  67. 67. Cell Planning Principles There is little point in being able to communicate in one direction only on a mobile telephone system. The coverage area, and hence the maximum path loss tolerated, should be the same for the uplink and the downlink. If this is the case the system is said to be “balanced”. By considering the asymmetries between the uplink and the downlink it is possible to derive an equation that will allow the required base station transmit power to be calculated in terms of the mobile transmit power, the combiner loss, the diversity gain and the threshold levels of the base station and mobile receivers. Section 2 – Link Budget for GSM Conditions for System Balance • The conditions for system balance depend on the asymmetries between the uplink and downlink power budgets • The asymmetries are: • Maximum output power from MS and BTS are not the same • MS has less sensitive receiver than BTS • Diversity reception can be used at the BTS but not at the MS • Combiner loss occurs at the BTS on the downlink only When making modifications to a network design, it is important to consider the impact on the system balance of any changes. For example, if it is required to increase coverage, simply increasing the BTS transmit power will not help as it will upset the system balance. However, utilising a higher gain antenna will provide increased coverage whilst maintaining balance. Similarly, adjusting the tilt of the antenna will affect coverage without upsetting the system balance. © AIRCOM International 2003 67
  68. 68. Cell Planning Principles Section 2 – Link Budget for GSM System Balance Equation • Power budget equations: Downlink: P inMS = PoBS - Lc - Ld - L fb + Gab - L p + Ga m - L fm Uplink: P inBS = P oMS - Lfm + Ga m - L p + GdBS + Gab - L fb - Ld • When the mobile is at the extreme boundary of the cell: PinMS = P refMS P inBS = PrefBS These are the reference sensitivities of the MS and BTS The output levels PoBS and P oMS are the maximum allowed values • If the boundaries for uplink and downlink are the same, the path loss Lp will be the same in each direction • Subtracting the uplink equation from the downlink gives the system balance equation: P oBS = PoMS + Lc + GdBS + ( PrefMS - PrefBS ) Section 2 – Link Budget for GSM Field Implications of System Balance • When changing cell size to alter coverage, consider whether the change will affect the system balance, for example: • Increasing BTS Tx power (P oBS ) to increase coverage would upset the balance • Ways of altering coverage without affecting balance include: • Decreasing BTS Tx power - the BSS can force the MS to use dynamic output power control (adjusting PoMS to maintain balance) • Altering the gain of the base station antenna - G ab is a symmetrical term in the power budgets • Antenna down tilting changes coverage area without affecting balance 68 © AIRCOM International 2003
  69. 69. Cell Planning Principles Section 2 – Link Budget for GSM System Balance Example • A downlink power budget calculation leads to a requirement that: PoBS + Gab = 56 dBm • Find PoBS using the system balance equation, with the following data: GdBS = 3dB , Lc = 6 dB , PrefBS = -105 dBm , PrefMS = -102 dBm PoMS = 33 dBm ( GSM Class 4 mobile) • Solution: PoBS = 33 + 6 + 3 + ( -102 - (-105 )) = 45 dBm • This is the maximum value for P oBS as balance can be maintained with reduced power • The downlink power budget now gives the antenna gain as 11 dBi • If the antenna gain is greater than 11 dBi, it can be down tilted to adjust coverage. As an example consider the case where a downlink power budget is conducted using the following figures Minimum mobile receive power Maximum Path loss Feeder Loss Combiner Loss Duplexer Loss Mobile antenna gain Mobile feeder loss -90 dBm 131 dB 8 dB 6 dB 1 dB 0 dB 0 dB Using the equation PinMS = PoBS − Lc − Ld − L fb + G ab − Lp + G am − Lfm PoBS + G ab = PinMS + Lc + L d + Lfb + L p + Lfm − G am = −90 + 6 + 1 + 8 + 131 + 0 − 0 = 56 dBm Notice that, as no information is given regarding the base station antenna gain, it is possible only to state the sum of the transmit power and the gain. We are now required to determine conditions for system balance using the following additional information. © AIRCOM International 2003 69
  70. 70. Cell Planning Principles Diversity Gain Receive threshold for mobile Receive threshold for base station Mobile transmit power 3 dB -90 dBm -93 dBm 33 dBm Now we can determine a required figure for the base station transmit power. PoBS = PoMS + L c + G dBS + (PrefMS − PrefBS) = 33 + 6 + 3 − 90 + 93 = 45 dBm This value is the maximum possible value for base station transmit power at which system balance can be maintained. It is possible to maintain balance at lower power levels by relying on the power control features within the mobile station. However, if this value was adopted, it is simple to calculate the required antenna gain to be 11 dBi. If a transmit power of 45 dBm is adopted and an antenna of higher gain is used, coverage can be restricted to the nominal value of 131 dB path loss by means of controlling the tilt of the antenna. Section 2 – Link Budget for GSM Summary • • Uplink and downlink power budget calculations: equations tracking gains and losses in up or downlink directions, final results for power inputs in dBm • • • Other power budget factors including: • Calculations for system balance: equations relating asymmetric terms in power budgets • 70 Review of decibel scale: dB (gain or loss), dBm (absolute measure of power input or output), dBi (antenna gain) Practical implications: ways of altering coverage without upsetting system balance Slow fade margin, building penetration, body losses etc Concept of system balance: cell boundary for uplink and downlink power budgets should be the same © AIRCOM International 2003
  71. 71. Cell Planning Principles 2.10.9 SELF-ASSESSMENT EXERCISES A particular radio link has a path loss of 146 dB. In one direction the sensitivity of the receiver is –102 dBm. The receiving antenna has a gain of 2 dBi and the transmitting antenna has a gain of 17 dBi. Miscellaneous feeder, combiner and filter losses amount to 7 dB. Determine the required transmitter power. Answer: 2. In a GSM system, the mobile terminal receiver has a sensitivity of –102 dBm and the Base Station receiver has a sensitivity of –105 dBm. The uplink generates an additional space diversity gain of 2 dB. The downlink suffers from a combiner loss of 4 dB. If the maximum mobile terminal transmit power is 33 dBm, what must the downlink transmit power be to maintain balance? Answer © AIRCOM International 2003 71
  72. 72. Cell Planning Principles 72 © AIRCOM International 2003
  73. 73. Cell Planning Principles 3 Frequency Planning ____________________________________________________________________ 3.1 Introduction In this section the following aspects of frequency planning will be covered: • • • • • Cellular structures and frequency reuse patterns Interference calculations Cell splitting Practical frequency planning Multiple reuse patterns ____________________________________________________________________ 3.2 Cellular Structures and Frequency Reuse Patterns Section 3 – Frequency Planning Cellular Structure • • • Cellular radio systems divided into small cells • Real pattern rather different - use planning tools Each cell surrounds a fixed radio site (BTS) Hexagon shape used for convenience - tessellates to cover large area Hexagons for planning © AIRCOM International 2003 Ideal Coverage Reality 73
  74. 74. Cell Planning Principles Section 3 – Frequency Planning Frequency Re -use • Cellular structure allows carrier frequencies to be re-used • High frequency re-use: • Short distance between same carriers • High traffic capacity • Low C/I ratio (i.e. worse interference) • Frequency planning involves a compromise between requirements for capacity and interference • Digital systems like GSM can cope with lower values of C/I than analog systems (20dB analogue compared to 12dB digital) Simple frequency plans assume a homogeneous distribution of carriers and equal sized cells. We can use this to give an estimate of the interference that is likely. Section 3 – Frequency Planning 7 Cell Cluster • • • Simple pattern - interlock 7 cell cluster to cover area Same number of carriers in each cell Re-use same carriers in corresponding cells, A, B etc. C F A E G C D A B G B E C D B G E G C D C A F F B E D C D A B E F F G B E D F D A C A A C G C G E D E D F E F G B G A B F B F C D C D 74 G E G A B F B E C A D A A F B G © AIRCOM International 2003
  75. 75. Cell Planning Principles Section 3 – Frequency Planning Frequency Re -use Distance • Around each cell, there are 6 cells in adjacent clusters using the same carriers • These cells will cause mutual co-channel interference • The C/I due to these cells can be found from the re-use distance, D • D can be calculated from the geometry of the clusters as: C A F G C D A B G B E R = radius of cell to a corner C C D B E E G F B E G E C D C A D C D A F G B F E B F D A C D A A C D G C D F E F G G G A B E B F C D C D B D= 3 7R G E G E B F B F A R A D A F A F B G Finding the re-use distance is the first step towards estimating the interference in the plan. Section 3 – Frequency Planning General Re -use Patterns • For a frequency re-use pattern based on clusters of N sites, each of cell radius R, the re-use distance, D is: D =R 3N • Typical cluster sizes are: 3, 4, 7, 12, 21 • Larger cluster sizes give better C/I ratios • However, smaller cluster sizes give higher traffic capacity per cell - more carriers available in each cell © AIRCOM International 2003 75
  76. 76. Cell Planning Principles ____________________________________________________________________ 3.3 Interference Calculations Section 3 – Frequency Planning Estimating C/I for Re Estimating C/I for Re -use Patterns • To estimate C/I we assume: • Each base station radiating the same power • Homogeneous propagation throughout the service area • Propagation follows a 1/R x law (x is the propagation co-efficient) • • the edge of the serving cell: On Re-use distance, D, is large compared with cell radius, R 1  1 I = 6 x  C= x R D   1  D x  C / I = 10 log      6R      C / I = 10 log    If C = ( D =R ) D 3N 3N   6   x Serving cell 6 nearest interfering cells 1 6 and I = x Rx D 1 x C R x =  1  D x  = 1  D  Then =   6  6 R  6  R x   I   x D However, D = R 3 N x C 1  R 3N   = Therefore, =  I 6 R    ( 3N 6 )x Note the assumption that D >> R. This is more appropriate for large clusters. The value of x will depend on the local radio propagation properties, but 3.5 is generally a good estimate. The higher the value of x the better the C/I. 76 © AIRCOM International 2003
  77. 77. Cell Planning Principles Section 3 – Frequency Planning C/I for Typical Cluster Sizes •  C / I = 10 log    Estimates of C/I in dB, using the equation: ( ) x 3N   6   Propagation Coefficient x Cluster Size N 2 3 3.5 4 3 1.76 6.53 8.92 11.3 4 3.01 8.41 11.1 13.8 7 5.44 12.05 15.36 18.66 6.53 13.68 17.27 20.85 7.78 15.56 19.45 23.34 21 • • 9 12 10.21 19.21 23.71 28.21 Analog systems require a minimum C/I of about 20 dB Digital systems can cope with C/I as low as 9 dB ____________________________________________________________________ 3.4 Cell Splitting Techniques Section 3 – Frequency Planning Cell Splitting • • Initial network based on omni-directional antenna sites To increase capacity, split each cell into 3 using sectored antennas Original omni site New tri-sectored site Our estimates of interference have assumed a simple pattern of omni sites. Realistic plans will have sites spilt into sectors. Different methods of achieving a split are shown here. © AIRCOM International 2003 77
  78. 78. Cell Planning Principles Section 3 – Frequency Planning Further Splitting New cell Rotate original antennas through 30o Add new sites as shown New site Old site rotated Section 3 – Frequency Planning 1:4 Cell Split • • • Alternative way of further splitting the cells No re-alignment of antennas needed Increases traffic capacity, frequency re-use and number of sites by a factor of 4 By reducing mutual interference effects, sectoring cells reduces the overall interference in the network. 78 © AIRCOM International 2003
  79. 79. Cell Planning Principles Section 3 – Frequency Planning Effect of Cell Splitting on Interference • Directional pattern of sectored antennas reduces response to interference • • Increases C/I significantly • If cells A and B use the same carrier: Allows greater frequency re-use, i.e. smaller cells A B • B will cause co-channel interference in A • A will cause very little co-channel interference in B • Interference is no longer mutual Section 3 – Frequency Planning Transition Zones Large rural cells • Problems may occur at the boundaries between high and low traffic areas • Large cells in rural areas will use higher power - can cause interference with smaller urban cells nearby • Requires careful frequency planning possibly reserve carriers for use in such transition zones • Alternatively, hierarchy of cells (e.g. overlay / underlay) may be used Transition Zone Small urban cells © AIRCOM International 2003 79
  80. 80. Cell Planning Principles 3.5 GSM Frequency Patterns Simple frequency plans for sectored networks are the 3/9 and 4/12 patterns. Again these assume a regular distribution of carriers and equal sized cells. Section 3 – Frequency Planning GSM Frequency Patterns • Two common re-use patterns in GSM are 3/9 and 4/12 • 3/9 consists of 3 sites, each of which has been tri-sectored giving a cluster of 9 cells Frequencies are assigned in sequence to the cells A1 - C3 A1 C1 C2 C3 A1 A3 A2 B3 B1 B3 B2 C2 A3 B3 4 5 6 7 8 C3 9 11 12 13 14 15 16 17 18 20 21 22 23 24 25 26 27 B1 A2 A3 A2 3 19 C2 C1 2 10 C3 B1 1 C1 A1 B2 C1 C2 C3 B2 A1 A3 B1 B3 3/9 Frequency Group in ASSET B2 A2 Section 3 – Frequency Planning Interference in the 3/9 Pattern • • 3/9 pattern allows frequencies to be allocated so no physically adjacent cells use the same frequency C1 C/I is about 9 dB, which is the minimum specified for GSM with frequency hopping Cells A1 and C3 are physically adjacent and are allocated adjacent carriers • • 80 On the boundary of A1 and C3: C/A = 0 dB C3 A2 A3 C2 A1 B1 B3 B1 B3 A1 C1 • C2 C3 B2 C1 C3 B2 C2 A1 B1 B3 B2 GSM specifies a minimum C/A of -9 dB A3 A2 A3 A2 © AIRCOM International 2003
  81. 81. Cell Planning Principles Section 3 – Frequency Planning 4/12 Re-use Pattern • 4 sites, each tri-sectored to give a 12 cell cluster • Numbering of D cells allows carriers to be allocated so that no adjacent carriers are used in physically adjacent cells D3 D2 D1 C1 C3 A1 B1 A2 A3 D3 B2 D2 B1 A2 A3 C2 B3 C2 B2 B1 A2 A3 C1 C3 A1 C1 C3 A1 D2 D1 B3 D1 D3 C2 B2 B3 Frequencies are assigned in sequence to the cells A1 - D3 A1 C1 D1 A2 B2 C2 D2 A3 B3 C3 D3 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 4/12 Frequency Group in ASSET B1 1 26 27 28 29 30 31 32 33 34 35 36 Section 3 – Frequency Planning Interference in the 4/12 Pattern • • • • • 4/12 pattern has no physically adjacent cells with co-channel or adjacent channel carriers D3 D1 C/I is about 12 dB This is adequate in GSM without frequency hopping C/A is higher than in 3/9 pattern D2 A1 D3 D1 A3 C1 C3 C3 A1 A3 C2 C1 C2 B1 A2 B3 B2 B1 A2 B3 © AIRCOM International 2003 D2 D1 B2 B3 D2 D3 C2 B1 A2 A3 A1 Traffic capacity is lower than 3/9 as there are fewer carriers per cell C1 C3 B2 81
  82. 82. Cell Planning Principles 3.6 Self-Assessment Exercises 3.6.1 FREQUENCY RE-USE CLUSTER SIZES The number of hexagon cells in a cluster, which can be repeated to form a frequency re-use pattern can be found from the formula: i2 + ij + j2 where i and j and integers. The table gives the numbers produced by this formula for small values of i and j. i/j 0 1 2 3 4 5 0 0 1 4 9 16 25 1 1 3 7 13 21 31 2 4 7 12 19 28 39 3 9 13 19 27 37 49 4 16 21 28 37 48 61 5 25 31 39 49 61 75 Which cluster sizes are typically used in GSM group frequency planning? For: An analogue mobile system A digital mobile system Choose a suitable cluster size (giving a reasonable compromise between frequency re-use and interference) and calculate the corresponding re-use distance. Take the radius of a cell to be 10 km and the propagation coefficient as 3.5. 3.6.2 FREQUENCY PLANNING ADJUSTMENTS In part of a network using a 4/12 frequency re-use pattern as shown below, one B1 cell is particularly heavily loaded with traffic. The cells adjacent to it, B2 and B3 are lightly loaded, each with a spare carrier. 82 © AIRCOM International 2003

×