Slide No.1

GSM SystemsRF Network Design - Introduction

Slide No.2

Frequency Bands

GSM-900The term GSM-900 is used for any GSM system which operates in any 900 MHz band.

P-GSM-900P-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-900In 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.3

Frequency Bands

R-GSM-900R-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-1800Frequency band: 2 x 75 MHz

1710 – 1785 MHz for MS to BTs (uplink)1805 – 1880 MHz for BTS to MS (downlink)

Slide No.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 < 124FL (n) = 890 + 0.2 x (n-1024) with 975 < n < 1024

FU (n) = FL (n) + 45

Slide No.5

Carrier Spacing and Channel Structure

R-GSM-900:FL (n) = 890 + 0.2 x n with 1 < n < 124FL (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 carrier

Slide No.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.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.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 carefulanalysis

Cell Range and Coverage AreaFor 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.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 planning

Slide No.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 performance

Slide No.11

RF Propagation

A radio wave transmitted to and from a moving mobile station is subject toseveral 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 ofthe mobile channels are :-

• The use of fade margins• Various types of diversity reception• Installation of supplementary BTSs

Slide No.12

Practical Attenuation

In practice, the mobile radio link is not set up in free space. The path loss ismore severe that the inverse square law would predict. The slope will besteeper, rather between –30 and –45 dB/decade, caused by :-

• Obstructions in the propagation path, particularly in the first Fresnelzone

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 inmost 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.13

Fade Margin

The concept of a fade margin is to reserve extra signal power to overcomepotential 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 enoughpower for a normal received signal level of (Pr + L(fade)) dBm

The fade margin is normally equal to the maximum expected fade or to asmaller value. The value is chosen in such a way that the threshold value isundershot in only a low percentage of time.

For this purpose, it is necessary to know the probability density function of thefading.

In RF planning, the impact of Rayleigh fading is taken into account byimplementing an extra fade margin of 8 dB.

Slide No.14

Multipath Propagation

The radio wave may be reflected, from a hill, a building, a truck, an aeroplaneor a discontinuity in the atmosphere. In some cases, the reflected signal issignificantly attenuated, while in others almost all the radio energy is reflectedand very little absorbed. The result is that not one but many different paths arefollowed between the transmitter and receiver. This is known as MultipathPropagation

Slide No.15

Multipath Propagation

Reflection and multipath propagation can cause positive and negative effects :-

• Coverage extensionMultipath propagation allows radio signal to reach behind hills and buildings and into tunnels

The latter effect is known as ducting

• Constructive and destructive interferenceThe 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 paths

Slide No.16

Ducting

Ducting may occurs in tunnels, valleys, building canyons, and in theatmosphere if the boundaries (steep hillsides, atmosphere layers) are goodreflectors 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 tunnelentrance radiating into the tunnel may help.

Slide No.17

Rayleigh Fading

The reflected radio wave will be altered in both phase and amplitude. Thesignal may effectively disappear if the reflected wave is 180 degrees out ofphase with the direct path signal. Partial out of phase relationships amongmultiple received signals produce smaller reductions in received signalstrength.

Rayleigh fading is dependent on :

• TimeTime dependent fading is applicable for moving mobiles only

The countermeasure against time dependent Rayleigh fading is the use of bit interleaving in burst building

Slide No.18

Rayleigh Fading

• LocationThe fading effect is a spatial effect. The depth and spacing of the fades isrelated to the wavelength. Maximum fades are very deep (down to –40 dB orless), a few inches apart. In between are many shallower fades. When amobile antenna moves through this field, the received signal strength will varyvery rapidly. Sometimes it is possible that a mobile is in a fade of the correctBTS but not in a fade of any “incorrect” BTS transmitting on the samefrequency.

The countermeasure against location dependent Rayleigh fading is diversityreception.

• FrequencyDue to the impact of the wavelength, the pattern of the fades is alsodependent on the radio frequency.

The countermeasure against frequency dependent Rayleigh fading isfrequency hopping reception

Slide No.19

Inter-symbol Interference

The sharp pulse that is transmitted arrives in the receiver as a delayed, smearedand flattened budge that lasts longer than the original pulse. This effect, calleddelay spread is caused by multipath propagation effects.

If the delay spread is large relative to the average symbol duration, theindividual symbols will overlap each other and ISI will occur.

Slide No.20

Doppler Shift

The movement of the MS relative to the BTS will cause a shift in frequency ofthe radio signal, known as doppler shift. This frequency shift variesconsiderably as the MS changes direction and/or speed.

Doppler shift introduces random frequency modulation in the radio signals.Thus Doppler frequency shift Δ f is :-

Δ f = Vr / λ

where Vr is the radial speed component pointing to/from the BTS or a reflectionpoint. Doppler shift affects all multiple propagation paths, some with positiveshift, others a negative shift at the same instant. The power spectrum of thereceived radio signal will be smeared.

Doppler shift effects can be limited by using a well-designed (adaptive)equaliser in the receiver.

Slide No.21

Equalisation

To some extent, the general countermeasure against distortion due to multipatheffects 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 ineach 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 thereceived signal.

Knowing the channel characteristics, the predicted distortion in thetransmitted pulses are subtracted from the received waveform and the mostlikely sequence of data for the distorted received signal is estimated.

The Viterbi algorithm is an example of an adaptive MLSE (maximumlikelihood sequence estimation) solution.

Slide No.22

Equalisation

The equaliser used for GSM is specified to equalise echos up to 15 μs after thefirst 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. Thesesignals should be considered as co-channel interference for which the requiredminimum C/I ratio of 9 dB must be met. This means that the sum of the echoswith delays of >= 15μs should remain >= 9 dB under the sum of the wantedcarrier signal plus the “useful” echos within the 15μs window. The echosresulting from reflections just outside the ellipse for Δt = 15 μs are mostly thestrongest and will cause most trouble.

Slide No.23

Fresnel Zone

A fresnel zone is a 3 dimensional body, bounded by ellipsoids that have theirfocal points at the transmitter and the receiver antennas. The sum of thedistances from a point (P) on the ellipsoid to the transmitter (T) and to thereceiver (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 ≡ 1. The radius of the first Fresnel zone is r(F1). Tokeep out of this zone, the distance r from the optical LOS should be :

SdSdFrr )1(1)1( −

=≥λ

The obstacles may be hills, buildings or vegetation.

Slide No.24

Diffraction

Shadowing does not always mean that no signal is received behind anobstacle. Radiowaves may bend around obstructions to a certain extent. Thiseffect is called diffraction. The diffraction effect depends on the wavelength inrelation to the size of obstacle, and is greater the longer the wavelength.

Slide No.25

Shadow Fading

The effect of shadowing by obstacles is fading of the received signal. Theproblems of shadowing are most severe in heavily built-up urban centres.Shadows as deep as 20dB may occur over very short distances, literally fromone 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. Thetopographical area is divided in a grid of pixels. Each pixel has an size in therange 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.26

Shadow Fading

The shadowing problem is approached in 2 ways, depending on the size of theobstruction :-

• Shadowing, diffraction and reflection by obstructions larger than the databaseresolution (e.g. hills) can be predicted by propagation models incomputerised planning tools. The distances between the fading dips are inthe magnitude of hundreds of meters.

• Shadowing by obstructions smaller than the database resolution (e.g.individual building) can be treated statistically. The distances between thefading dips are in the magnitude of tens of meters.

Slide No.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 fading

Slide No.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/decadeThis 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 area

Slide No.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 edgecoverage probability P(edge)

4. Find the fade margin for P(edge) in the CDF table for the standardnormal distribution table N(0,1)

Slide No.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 = 2Intercept 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 dB

Slide No.31

Propagation Modeling

• Statistical propagation models

− These calculate a median signal for each pixel. The level within this pixelvaries about the median in a way that can only be analysed statistically.

− Local mean signal levels are distributed around the pixel median with alog-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.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 theabsolute temperature T (Kelvin).

Ni = k T B WattWhere

k = Boltsmann’s constant = 1.38 x 10-23 J/K

Slide No.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 beworse than at the antenna because the amplifier has added some extra noiseby itself. The noise figure F is the ratio between :

• The total output noise level generated by both the external noise and theinternal 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 17degrees 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 dBm

Slide No.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 dBm

Slide No.35

Receiver Sensitivity

GSM 900 Receiver SensitivityThe 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 SensitivityThe 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.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 capacity

Slide No.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 criterion– The practical values for K range up from 3 to 21

Slide No.38

Cluster Size

Valid values for K are found by setting i and j to positive values in :-

K = i2 + 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 DistanceThe frequency reuse distance D can be derived from the K value:-

KRD 3=

Slide No.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 R2

Slide No.40

Base Station Antenna Problems

Problems that are encountered in the design and installation of cellular antennas :-

• Dead SpotsSlight unintentional tilts and minor lobes nulls in the radiation pattern may result in gain loss on some spots

• IsolationThe more spacing between transmitter and receiver antennas. Less the coupling

• Collinear antenna mountingOnly one antenna can be mounted at the top most point of the site tower. Several antennas cannot be mounted at the same point

Slide No.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 antenna

Slide No.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 isolators

Slide No.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 MHzA(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 MHzA(v) = 59.3 + 40 log d dB for 1800 MHz

Slide No.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.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 handovers

Slide No.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 same

Slide No.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 path

Slide No.48

Cell Structure Growth

Network growth can be required for the following reasons :-

• Extension of coverage areaA new coverage area needs to be added

• Capacity increaseThe traffic density in an existing cell has grown

• Coverage quality increaseFor 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 performance

Slide No.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 well

Slide No.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 Splitting

Slide No.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 congestion

Slide No.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 pattern

Slide No.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 beyond• Enhancers may impact the network of another operator

Slide No.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 hop

Slide No.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 location

Slide No.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 matrix

Slide No.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 importance