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    • 1. RF Network Design - Introduction Tai Koon Sun GSM/UMTS RF Engineering - AP Level 31 Tower 2 Petronas Twin To wer, KLCC Kuala Lumpur, Ma laysiaSlide No.1 Lucent Technologies - Proprietary
    • 2. Frequency Bands GSM-900 The term GSM-900 is used for any GSM system which operates in any 900 MHz band. P-GSM-900 P-GSM-900 band is the primary band for GSM-900 Frequency band for primary GSM-900 (P-GSM-900) : 2 x 25 MHz 890 – 915 MHz for MS to BTS (uplink) 935 – 960 MHz for BTS to MS (downlink) E-GSM-900 In some countries, GSM-900 is allowed to operate in part or in all of the following extension band. E-GSM-900 (Extended GSM-900) band includes the primary band (P-GSM-900) and the extension band : 880 – 890 MHz for MS to BTS (uplink) 925 – 935 MHz for BTS to MS (downlink)Slide No.2 Lucent Technologies - Proprietary
    • 3. Frequency Bands R-GSM-900 R-GSM-900 (Railway GSM-900) band includes the primary band (P-GSM-900) and the following extension band: 876 – 890 MHz for MS to BTS (uplink) 921 – 935 MHz for BTS to MS (downlink) GSM-1800 Frequency band: 2 x 75 MHz 1710 – 1785 MHz for MS to BTs (uplink) 1805 – 1880 MHz for BTS to MS (downlink)Slide No.3 Lucent Technologies - Proprietary
    • 4. Carrier Spacing and Channel Structure Channel number – the carrier frequency is designated by the absolute radio frequency channel number (ARFCN). The frequency value of the carrier n in the lower band is called FL (n) while FU (n) is the corresponding frequency value in the upper band. Frequencies are in MHz P-GSM-900: FL (n) = 890 + 0.2 n with 1 < n < 124 FU (n) = FL (n) + 45 E-GSM-900: FL (n) = 890 + 0.2 x n with 1 < n < 124 FL (n) = 890 + 0.2 x (n-1024) with 975 < n < 1024 FU (n) = FL (n) + 45Slide No.4 Lucent Technologies - Proprietary
    • 5. Carrier Spacing and Channel Structure R-GSM-900: FL (n) = 890 + 0.2 x n with 1 < n < 124 FL (n) = 890 + 0.2 x (n-1024) with 955 < n < 1024 FU (n) = FL (n) + 45 GSM-1800: FL (n) = 1710.2 + 0.2 x (n-512) with 512 < n < 885 FU (n) = FL (n) + 95 • Carrier spacing is 200 kHz • 8 time slots per carrierSlide No.5 Lucent Technologies - Proprietary
    • 6. Coverage, Capacity, and Quality Providing coverage is usually considered as the first and most important activity of a new cellular operator. For a while, every network is indeed coverage driven. However, the coverage is not the only thing. It provides the means of service and should meet certain quality measures. The starting point is a set of coverage quality requirements. • To guarantee a good quality in both uplink and downlink direction, the power levels of BTS and MS should be in balance at the edge of a cell. Main output results of the power link budgets are: – Maximum path loss that can be tolerated between the MS and the BTS – Maximum output power level of the BTS transmitter. • These values are calculated as a function of design constraints: – BTS and MS receiver sensitivity levels – MS output power level – Antenna gain – Diversity reception – Losses in combiners, cables, etc.Slide No.6 Lucent Technologies - Proprietary
    • 7. Coverage, Capacity, and Quality The cell ranges are derived with propagation loss formulas such as Okumura- Hata, using inputs of maximum path loss, differences in the operating environments and the quality targets in different cell ranges. The traffic capacity requirements have to be combined with the coverage requirements, by allocating frequencies. This also may have impact on the cell range.Slide No.7 Lucent Technologies - Proprietary
    • 8. Coverage Planning Strategies The selection of site configurations, antennas and cables is the core of the coverage planning strategy. The right choice will provide cost savings and guarantees smooth network evolution. Some typical configurations are: • 3-sector sites for (sub)urban areas • 2-sector sites for road coverage • omni sites for rural areas These are not the ultimate solutions, decisions should be based on a careful analysis Cell Range and Coverage Area For any site configuration, the cell ranges can be determined given the equipment losses and gains. The site coverage areas can be calculated then and these will lead to the required number of sites for a given coverage region. This makes it possible to estimate the cost, e.g. per km2, to be used for strategic decisions.Slide No.8 Lucent Technologies - Proprietary
    • 9. Methodology Define design rules and parameters • Identify design rules to meet coverage and capacity targets efficiently • Acquire software tools and databases • Calibrate propagation models from measurements Set performance targets • Clear statement of coverage requirements (roll out and quality) • Forecast traffic demand and distribution • Test business plan for different roll out scenarios and quality levels Design nominal plan • Use computer tool to place sites to meet coverage and capacity targets • Verify feasibility of meeting service requirements • Ensure a frequency plan can be made for the design • Estimate equipment requirements and costs • Develop implementation and resource plans (including personnel requirements) • Radio plan will provide input to fixed network planningSlide No.9 Lucent Technologies - Proprietary
    • 10. Methodology Implement cell plan • Identify physical site locations near to nominal or theoretical locations, using search areas. • Modify nominal design as theoretical sites are replaced with physical sites • Modify search areas in accordance with evolving network. Produce frequency plan • Fixed cluster configuration, can be done manually. • Flexible, based on interference matrix using an automatic tool. Optimising the network Expand the network • In line with the roll out requirement • In line with the forecasted traffic level • Improve the coverage quality • Maintain the blocking performanceSlide No.10 Lucent Technologies - Proprietary
    • 11. RF Propagation A radio wave transmitted to and from a moving mobile station is subject to several effects. These effects will cause loss of signal strength and interference. The effects are:- • Distance attenuation • Shadowing − Diffraction • Rayleigh fading − Reflections − Inter-symbol interference − Doppler shift − Ducting The most important conventional countermeasures to deal with the problems of the mobile channels are :- • The use of fade margins • Various types of diversity reception • Installation of supplementary BTSsSlide No.11 Lucent Technologies - Proprietary
    • 12. Practical Attenuation In practice, the mobile radio link is not set up in free space. The path loss is more severe that the inverse square law would predict. The slope will be steeper, rather between –30 and –45 dB/decade, caused by :- • Obstructions in the propagation path, particularly in the first Fresnel zone This is frequently the case because of the low height of the mobile antenna. Even of line-of-sight conditions apply, the first Fresnel zone is obstructed in most cases. • Reflections from the ground and from objects Reflections combine different phases of the signal on the receiving antenna. This will cause multipath signal strength variations. The loss is depended upon the frequency, the antenna design and the terrain.Slide No.12 Lucent Technologies - Proprietary
    • 13. Fade Margin The concept of a fade margin is to reserve extra signal power to overcome potential fading. Assume : The mobile radio system needs an signal level of Pr dBm at the receiver • The maximum likely fade (loss) is calculated to be L(fade) dB The a received signal level of Pr dBm can be ensured by transmitting enough power for a normal received signal level of (Pr + L(fade)) dBm The fade margin is normally equal to the maximum expected fade or to a smaller value. The value is chosen in such a way that the threshold value is undershot in only a low percentage of time. For this purpose, it is necessary to know the probability density function of the fading. In RF planning, the impact of Rayleigh fading is taken into account by implementing an extra fade margin of 8 dB.Slide No.13 Lucent Technologies - Proprietary
    • 14. Multipath Propagation The radio wave may be reflected, from a hill, a building, a truck, an aeroplane or a discontinuity in the atmosphere. In some cases, the reflected signal is significantly attenuated, while in others almost all the radio energy is reflected and very little absorbed. The result is that not one but many different paths are followed between the transmitter and receiver. This is known as Multipath PropagationSlide No.14 Lucent Technologies - Proprietary
    • 15. Multipath Propagation Reflection and multipath propagation can cause positive and negative effects :- • Coverage extension Multipath propagation allows radio signal to reach behind hills and buildings and into tunnels The latter effect is known as ducting • Constructive and destructive interference The interference due to multipath propagation manifest itself in the following 3 most important ways:- – Random phase shift creates rapid fluctuations in the signal strength known as Rayleigh fading – A delay spread in the received signal causes each symbol to overlap with adjacent symbols : intersymbol interference – Random frequency modulation due to different doppler shifts on different pathsSlide No.15 Lucent Technologies - Proprietary
    • 16. Ducting Ducting may occurs in tunnels, valleys, building canyons, and in the atmosphere if the boundaries (steep hillsides, atmosphere layers) are good reflectors for radio waves. VHF frequencies do not propagate well in long tunnels, but higher frequencies (>800 MHz) follow the tunnel like a waveguide. If the coverage in a tunnel needs enhancement, repeater station at the tunnel entrance radiating into the tunnel may help.Slide No.16 Lucent Technologies - Proprietary
    • 17. Rayleigh Fading The reflected radio wave will be altered in both phase and amplitude. The signal may effectively disappear if the reflected wave is 180 degrees out of phase with the direct path signal. Partial out of phase relationships among multiple received signals produce smaller reductions in received signal strength. Rayleigh fading is dependent on : • Time Time dependent fading is applicable for moving mobiles only The countermeasure against time dependent Rayleigh fading is the use of bit interleaving in burst buildingSlide No.17 Lucent Technologies - Proprietary
    • 18. Rayleigh Fading • Location The fading effect is therefore a spatial effect. The depth and spacing of the fades is related to the wavelength. Maximum fades are very deep (down to – 40 dB or less), a few inches apart. In between are many shallower fades. When a mobile antenna moves through this field, the received signal strength will vary very rapidly. Sometimes it is possible that a mobile is in a fade of the correct BTS but not in a fade of any “incorrect” BTS transmitting on the same frequency. The countermeasure against location dependent Rayleigh fading is diversity reception. • Frequency Due to the impact of the wavelength, the pattern of the fades is also dependent on the radio frequency. The countermeasure against frequency dependent Rayleigh fading is frequency hopping receptionSlide No.18 Lucent Technologies - Proprietary
    • 19. Inter-symbol Interference The sharp pulse that is transmitted arrives in the receiver as a delayed, smeared and flattened budge that lasts longer than the original pulse. This effect, called delay spread is caused by multipath propagation effects. If the delay spread is large relative to the average symbol duration, the individual symbols will overlap each other and ISI will occur.Slide No.19 Lucent Technologies - Proprietary
    • 20. Doppler Shift The movement of the MS relative to the BTS will cause a shift in frequency of the radio signal, known as doppler shift. This frequency shift varies considerably as the MS changes direction and/or speed. Doppler shift introduces random frequency modulation in the radio signals. Thus Doppler frequency shift ∆ f is :- ∆ f = Vr / λ where Vr is the radial speed component pointing to/from the BTS or a reflection point. Doppler shift affects all multiple propagation paths, some with positive shift, others a negative shift at the same instant. The power spectrum of the received radio signal will be smeared. Doppler shift effects can be limited by using a well-designed (adaptive) equaliser in the receiver.Slide No.20 Lucent Technologies - Proprietary
    • 21. Equalisation To some extent, the general countermeasure against distortion due to multipath effects is adaptive equalisation :- • The distortion characteristics of the channel are measured continuously. It uses the well known 26 bits (or more) TSC training sequence transmitted in each timeslot burst (once per 0.5ms) to measure the channel characteristics. The TSC (training sequence codes) are specified in GSM Rec. 05.02. • The predicted distortions in the received signal are subtracted from the received signal. Knowing the channel characteristics, the predicted distortion in the transmitted pulses are subtracted from the received waveform and the most likely sequence of data for the distorted received signal is estimated. The Viterbi algorithm is an example of an adaptive MLSE (maximum likelihood sequence estimation) solution.Slide No.21 Lucent Technologies - Proprietary
    • 22. Equalisation The equaliser used for GSM is specified to equalise echos up to 15 µs after the first signal. This corresponds to 4.5 km in distance. One bit period is 3.69 µs. Hence, echos with about 4 bit lengths delay can be compensated. Echos with a delay of > 15 µs cannot be cancelled by the equaliser. These signals should be considered as co-channel interference for which the required minimum C/I ratio of 9 dB must be met. This means that the sum of the echos with delays of >= 15µs should remain >= 9 dB under the sum of the wanted carrier signal plus the “useful” echos within the 15µs window. The echos resulting from reflections just outside the ellipse for ∆t = 15 µs are mostly the strongest and will cause most trouble.Slide No.22 Lucent Technologies - Proprietary
    • 23. Fresnel Zone A fresnel zone is a 3 dimensional body, bounded by ellipsoids that have their focal points at the transmitter and the receiver antennas. The sum of the distances from a point (P) on the ellipsoid to the transmitter (T) and to the receiver (R) is n/2 wavelengths longer than the LOS path (S) : Distance (P-T) + Distance (P-R) = S + n (λ /2) For the first fresnel zone, n ≡ The radius of the first Fresnel zone is r(F1). To 1. keep out of this zone, the distance r from the optical LOS should be : λ1( S − 1) d d r ≥ ( F1) = r S The obstacles may be hills, buildings or vegetation.Slide No.23 Lucent Technologies - Proprietary
    • 24. Diffraction Shadowing does not always mean that no signal is received behind an obstacle. Radiowaves may bend around obstructions to a certain extent. This effect is called diffraction. The diffraction effect depends on the wavelength in relation to the size of obstacle, and is greater the longer the wavelength.VSlide No.24 Lucent Technologies - Proprietary
    • 25. Shadow Fading The effect of shadowing by obstacles is fading of the received signal. The problems of shadowing are most severe in heavily built-up urban centres. Shadows as deep as 20dB may occur over very short distances, literally from one street to another. The fading effects produced by shadowing are often referred to as slow fading The radio network planning tool uses a topographical database. The topographical area is divided in a grid of pixels. Each pixel has an size in the range of 50m x 50m to 500m x 500m. One pixel is characterised by :- • Terrain height • Clutter type : high/low building, forest, water etc.Slide No.25 Lucent Technologies - Proprietary
    • 26. Shadow Fading The shadowing problem is approached in 2 ways, depending on the size of the obstruction :- • Shadowing, diffraction and reflection by obstructions larger than the database resolution (e.g. hills) can be predicted by propagation models in computerised planning tools. The distances between the fading dips are in the magnitude of hundreds of meters. • Shadowing by obstructions smaller than the database resolution (e.g. individual building) can be treated statistically. The distances between the fading dips are in the magnitude of tens of meters.Slide No.26 Lucent Technologies - Proprietary
    • 27. Shadow Fade Margin Shadow fade margins must be added to the receiver sensitivities specified in GSM Rec 05.05, to give the probability of signal being greater than the receiver sensitivity. The fade margin depends on :- • The desired coverage probability • The propagation slope • The standard deviation of the log-normal fadingSlide No.27 Lucent Technologies - Proprietary
    • 28. Jakes Graphs A way to find an appropriate fade margin is the method according to Jakes. It can be used for a wide range of propagation slopes and standard deviations, by using a set of standard graphs. The inputs are :- • The propagation slope, e.g. 40 dB/decade This means that the signal will decay according to 1/rn where n = 4 • The shadow fading standard deviation σs, e.g. 7 dB • The required coverage probability, e.g. 90% coverage probability over the areaSlide No.28 Lucent Technologies - Proprietary
    • 29. Jakes Graphs The output will be the fade margin for a given required area coverage probability. This can be found as follows : 1. Find the abscissa value σs/n 2. Take required area coverage probability P(area) as the ordinate value 3. The intersection of the 2 values will provide a value for the cell edge coverage probability P(edge) 4. Find the fade margin for P(edge) in the CDF table for the standard normal distribution table N(0,1)Slide No.29 Lucent Technologies - Proprietary
    • 30. Slow Fade Margin – Example According to GSM 03.30, the normal case of urban propagation has a standard deviation of σs = 7 dB while the propagation path loss slope is –35dB/decade. In order to find the required fade margin to achieve 90% area coverage, the following steps are taken :- 1. Determine the σs/n abscissa value : The propagation slope is 35 dB/decade, then n = 35/10 = 3.5 Because the σs = 7 dB, the value for σs/n = 7/3.5 = 2 2. In the graph, the P(area) = 90% and σs/n = 2 Intercept at the curve for P(edge) ~ 0.73 = 73% 3. In the normal distribution N(0,1) table, 0.73 corresponds to 0.61 x σs. Hence, the fade margin = 0.61 x 7 = 4.3 dBSlide No.30 Lucent Technologies - Proprietary
    • 31. Propagation Modeling • Statistical propagation models − These calculate a median signal for each pixel. The level within this pixel varies about the median in a way that can only be analysed statistically. − Local mean signal levels are distributed around the pixel median with a log-normal probability distribution. − Formulas derived from measurements (e.g. Okumura-Hata). − No obstacles assumed to be close to the BTS antenna. • Deterministic propagation models − Take into account individual buildings and use ray tracing techniques. − Make use of high resolution map data (at least 10m).Slide No.31 Lucent Technologies - Proprietary
    • 32. Noise Levels There are 2 kinds of noise that play a role in mobile communication :- • Thermal noise • Man-made noise (e.g. spurious signals) The thermal noise depends on the receiver bandwidth B (in Hz) and the absolute temperature T (Kelvin). Ni = k T B Watt Where k = Boltsmann’s constant = 1.38 x 10-23 J/KSlide No.32 Lucent Technologies - Proprietary
    • 33. Noise Figure A mobile radio signal, received on the antenna, will be amplified by the front- end RF amplifier in the radio receiver. After amplification, the S/N ratio will be worse than at the antenna because the amplifier has added some extra noise by itself. The noise figure F is the ratio between : • The total output noise level generated by both the external noise and the internal noise of the amplifier • The output noise level due to external (thermal) noise only A typical noise figure for a GSM receiver is 6 dB. At a temperature of 17 degrees C and a receiver bandwidth of 200 kHz, the received thermal noise is :- 1.38 10-23 x (17 + 273) x 200 x 103 = 8 x 10-16 W = -120 dBmSlide No.33 Lucent Technologies - Proprietary
    • 34. Receiver Sensitivity With the thermal noise level of –120 dBm and a noise figure F = 6 dB, the noise floor will be at –114 dBm. The implementation margin being 2 dB and the fade margin for Rayleigh fading being 8 dB, the reference receiver sensitivity can be taken as :- For normal GSM 900 BTS -120 dBm + 6 dB + 2 dB + 8 dB = -104 dBmSlide No.34 Lucent Technologies - Proprietary
    • 35. Receiver Sensitivity GSM 900 Receiver Sensitivity The reference sensitivity levels specified in GSM Rec 05.05 are as follows:- • -104 dBm (Class 1, 2 and 3 mobile stations and normal BTS) • -120 dBm (Class 4 and 5 mobile stations) • -97 dBm (micro-BTS M1) • -92 dBm (micro-BTS M2) • -87 dBm (micro-BTS M3) This already take into account the effect of multipath fading on moving mobiles, Rayleigh Fading (time domain) and Doppler Effect (frequency domain) GSM 1800 Receiver Sensitivity The reference sensitivity levels specified in GSM Rec 05.05 are as follows :- • -102 dBm (class 3 mobile station or micro-BTS M1) • -100 dBm (GSM 1800 class 1 and 2 mobile stations) • -97 dBm (micro-BTS M2) • -92 dBm (micro-BTS M3)Slide No.35 Lucent Technologies - Proprietary
    • 36. Cellular Architecture The essential principles of the cellular architectures are :- • Low power transmitters with antenna heights between 20 – 50 m • Small coverage zones (cells), typical macro cell radius 3 – 30 km • Frequency reuse (factor n = 3, 4, 7 ... ) • Cell splitting to increase local capacity • Micro and pico cells act as patches for hot spots, tunnels and buildings Balance is to be found between conflicting requirements of : • Coverage • Traffic capacitySlide No.36 Lucent Technologies - Proprietary
    • 37. Cell Clustering Frequency reuse is the core concept of the cellular mobile radio system, given the fact that the number of allowed frequencies is fixed. A frequency can be reused simultaneously in different cells, provided that the cells using the same frequency set are far enough separated so that co-channel interference is kept at an acceptable level most of the time. The total frequency spectrum allocation can be divided into K frequency reuse patterns. • Theoretically, a large K is desired. In practice, the total number of allocated frequencies is fixed. When K is too large, the number of frequencies assigned to each of K cells becomes too small. Trunking inefficiency will be the result. • The challenge is to find the smallest K value which can still meet our system performance requirements. This involves :- – Estimation of the co-channel interference – Calculation of the minimum frequency reuse distance D to meet the co- channel interference criterionSlide No.37 – The practical values for K range up from 3 to 21 Lucent Technologies - Proprietary
    • 38. Cluster Size Valid values for K are found by setting i and j to positive values in :- K = i 2 + i j + j2 The smallest value for K is 3, found for i = j = 1. The K value can be found as follows :- • The starting direction of the i axis is arbitrary • j is rotated by one cell face (60 degrees) to the left from the i axis • After finding the first co-channel cell, go back to the starting cell • Rotate the i axis by one cell face • Repeat the procedure. Frequency Reuse Distance The frequency reuse distance D can be derived from the K value:- D = R 3KSlide No.38 Lucent Technologies - Proprietary
    • 39. Cell Types The 2 main cell types are :- • Omni cells : – Coverage is in principle a circle, but in reality a rough pattern • Sector cells : – 2 sectors (e.g. for highways) – 3 sectors Cell Coverage Area Omni cell (Hexagon) = 2.6 R2 Sector cell (Hexagon) = 1.96 R2Slide No.39 Lucent Technologies - Proprietary
    • 40. Base Station Antenna Problems Problems that are encountered in the design and installation of cellular antennas :- • Dead Spots Slight unintentional tilts and minor lobes nulls in the radiation pattern may result in gain loss on some spots • Isolation The more spacing between transmitter and receiver antennas. Less the coupling • Collinear antenna mounting Only one antenna can be mounted at the top most point of the site tower. Several antennas cannot be mounted at the same pointSlide No.40 Lucent Technologies - Proprietary
    • 41. Dead Spots A higher antenna gain is achieved by compressing the beamwidth in the elevation plane. Unfortunately, with compression, more minor lobes appear in the radiation pattern. In the desired coverage area, nearby dead spots may exist due to minor lobe nulls even though the distant coverage is good because of a high main lobe gain. Moving a dead spot away from a certain location can be done by :- • Tilting the antenna beam • Reduction of antenna height • Use of a lower gain antennaSlide No.41 Lucent Technologies - Proprietary
    • 42. Isolation Isolation between transmitter and receiver antennas is required to avoid receiver desensitisation, which is a reduction in receiver sensitivity. This is caused by :- • Receiver in-band noise caused by the co-site transmitter (spurious signals) • Gain reduction of the low-noise amplifier caused by an strong off-channel signal Techniques used for isolation are :- • Decoupling of the antennas by adequate spacing • Filtering the transmitter’s out of band channel noise by multicouplers, duplexers and isolatorsSlide No.42 Lucent Technologies - Proprietary
    • 43. Isolation Horizontal Spacing The isolation A(h) between 2 horizontally separated antennas is given by the empirical formula :- A(h) = 31.6 + 20 log d – (Gt + Gr) dB for 900 MHz A(h) = 37.6 + 20 log d – (Gt + Gr) dB for 1800 MHz Vertical Spacing The isolation A(v) in dB is given by :- A(v) = 47.3 + 40 log d dB for 900 MHz A(v) = 59.3 + 40 log d dB for 1800 MHzSlide No.43 Lucent Technologies - Proprietary
    • 44. Service Contour The propagation prediction model provides the signal level in terms of dBm. This is the median value, e.g. –88 dBm Given the standard deviation, there is a certain probability (e.g. 95%) that the signal in a given area will be at least a number of X dB below the median value of that area. Thus, with a 95% reliability, the signal level can only be guaranteed top be –102 dBm (or more) which is the receiver sensitivity of the mobile. The signal contour for a specified receiver sensitivity must be plotted around the cell site to define the coverage area. This contour is a statistical boundary. If the MS travels along the boundary, for 95% of all the locations it is expected to receive a signal that is above –102 dBm.Slide No.44 Lucent Technologies - Proprietary
    • 45. Cell Structure Planning A homogeneous cell structure is practically impossible. However it is desirable to design a cell structure as homogeneous as possible. This will lead to :- • Reliable coverage • Simple frequency planning • Easy calculation of traffic loads • Reliable handoversSlide No.45 Lucent Technologies - Proprietary
    • 46. Cell Structure Planning Good cell structures can be planned by keeping the following points in mind :- • Use as homogeneous a cell structure as possible (no abrupt changes in cell size, e.g. at the edge of towns) • Avoid random pointing of antenna direction. The front lobe at any BTS directional TX antenna should illuminate only the back lobe of its co-channel counterpart • Define cell boundaries firmly. Avoid areas with many equally good server, resulting in many handovers and many interferers • Sufficient overlapping zones • Avoid cell boundaries across traffic hot spots • Keep all antenna heights about the sameSlide No.46 Lucent Technologies - Proprietary
    • 47. Cell Structure Planning Once a BTS is located through site establishment, and good coverage can be achieved, there is no guarantee that the cell will maintain its original coverage. Cells are living because :- • New buildings may be erected within the coverage area • Existing building may be demolished • Trees are also a concern, when they grow across the LOS radio pathSlide No.47 Lucent Technologies - Proprietary
    • 48. Cell Structure Growth Network growth can be required for the following reasons :- • Extension of coverage area A new coverage area needs to be added • Capacity increase The traffic density in an existing cell has grown • Coverage quality increase For example, existing outdoor coverage needs to be upgraded to indoor coverage Integration of each new BTS or even each TRX has to be carefully planned into the greater system. In all cases, the existing cells adjacent to the growth area will be affected in the following aspects :- • Changes in cell size and shape • Changes in the BSS parameters • Updates in neighbour list • Frequency allocation • Interference performanceSlide No.48 Lucent Technologies - Proprietary
    • 49. Coverage Quality and Capacity Increase If the number of available channels is fixed, the basic cellular principle required that capacity increase is achieved by reusing frequencies more often over a certain coverage area. Hence more sites are needed within the existing area. This is accomplished by reducing the cell sizes in areas of high demand :- • This requires the creation of new small cells within the overall cluster pattern • Frequency reuse must not infringe on rules determining frequency allocation for the large pattern • Some coverage quality improvement can be expected as wellSlide No.49 Lucent Technologies - Proprietary
    • 50. Coverage Quality and Capacity Increase Increasing the cell density in a coverage area can be achieved by :- • Adding more sites in the coverage area • Cell splitting (sectorisation) The capacity increases while the number of sites remains the same – The size of the small cell is dependent on 2 factors:- • Radio aspect • Capacity of the system – Certain channels should be used as barriers • Cell SplittingSlide No.50 Lucent Technologies - Proprietary
    • 51. Coverage Limited System In a noise limited cell, there is a limitation due to SNR limitations only. This is also called coverage limitation • No interference (C/I is good) − Co-channel interference − Adjacent channel interference • No traffic congestionSlide No.51 Lucent Technologies - Proprietary
    • 52. Coverage Extension The coverage can be increased by one or a combination of the following actions :- • Increase transmitted power. Doubling the power gives a gain of +3dB • Increase BTS antenna height. Doubling the height may give +6 dB gain • Use a high gain or a directional antenna at BTS • Lower the threshold level of a received signal • Install a masthead amplifier • Decrease the front-end noise figure F (low noise receiver) • Use a diversity receiver • Select proper BTS site locations • Use enhancers or micro/pico cells to enlarge coverage or to fill in holes • Engineer the antenna patternSlide No.52 Lucent Technologies - Proprietary
    • 53. Filling Coverage Holes In areas where the traffic intensity is low, its is not cost effective to install a BTS. An enhancer can be use to fill these coverage holes at low investments. Savings are installation and operational costs. Two types of enhancers are distinguished : • Wideband • Channelised The enhancer can be considered as a relay, that receives at a low height and transmit to a higher height and vice versa. Aspects:- • The antenna pointing to the cell site BTS is directional • The lower antenna is omni or directional • Enhancers do not improve the SNR, they have only a relay function • Repeater gain 10 – 85 dB adjustable • Typical repeater range 0.5 – 3 km • Interference aspects may make implementation difficult • Ring oscillation shall be avoided • Distance to serving BTS site as small as possible to avoid spread of power into a large area in the vicinity of BTS and beyondSlide No.53 • Enhancers may impact the network of another operator Lucent Technologies - Proprietary
    • 54. Interference The C/I ratio can be increase in a number of different ways :- • Good frequency management chart − Grouping the channels into subsets • Intelligent frequency assignment − Allocation of specific channels to cell sites and MS, avoiding problems from co-channel and adjacent channel interference • Selection of a proper channel − Among a set of assigned channels to a particular MS − If the quality of the signal is poor, an intracell handover to another frequency or time slot should occur • Frequency hopping − Effective on uplink and downlink path − Choose different hopping sequences for co-channel cells, resulting in a different interferer from hop to hopSlide No.54 Lucent Technologies - Proprietary
    • 55. Interference • Antenna pattern design − In some directions a strong signal is required, in other directions no signal may be needed • Tilting of antenna patterns − To confine energy within a small area − Downward tilt of directional antenna • Reduction of antenna height − Reducing interference is as important as radio coverage • Power reduction of interfering transmitter − RF power control, adaptive power control to keep transmission power as low as possible, on a per time slot basis − DTX, interrupted transmission during gaps in speech • Choosing cell site locationSlide No.55 Lucent Technologies - Proprietary
    • 56. Planning the frequencies The frequency plan can be made in different ways :- • Fixed cluster configuration – For example, cluster of K = 21 cells will use 21 frequencies (at least). This fixed frequency planning can be done manually. It is simple but not particularly efficient • Flexible assignment – Based on the interference matrix using an automatic tool. In general, this method can lead to a more efficient frequency use, e.g. 18 frequencies doing the job instead of the fixed K = 21 frequency cluster size for the same level of coverage quality. • A mix of these methods is also possible – Control channels are always transmitted at maximum power. The basic idea is to protect the BCCH frequency – It is a good solution to use first e.g. f1, ..., f21 for the control channels on a safe K = 21 cell cluster, and then let the other frequencies be at a closer range, determined by the interference matrixSlide No.56 Lucent Technologies - Proprietary
    • 57. Extension and Frequency Changes When a network is to be extended, e.g. by increasing the cell density in order to improve the traffic capacity and the coverage quality, a revised frequency plan is necessary • To minimise the re-tuning, the already operational base station should be left unchanged as much as possible • The pre-assigned frequencies of the cell cliques that will change significantly should be abandoned in favour of new frequencies It is convenient to define a set of frequency group • Initially, each cell starts with a layer of a particular frequency group • In a later stage, new frequencies from other layers of the same frequency group can be added in that cell. • No interference analysis is required • It is possible however that frequencies from adjacent layers in differnt groups can be adjacent channels. This needs to be verified • If the frequency planning is performed by a computer tool, the frequency group are of less importanceSlide No.57 Lucent Technologies - Proprietary