GSM Coverage v4-0 Notes

7/25/2019 GSM Coverage v4-0 Notes

1/192 Network Consultants / TNC Ltd 2005

Network Consultants / TNC Ltd 2005 1Coverage Planning v 4.0

The Cellular Academy

GSM Radio Network Planning and Optimisation

Capacity and Frequency Planning

Network Growth and Network Optimisation

Advanced Topics

Coverage and Cell Structure Planning

Version 4.0


2/192

Network Consultants / TNC Ltd 2005


Contents2 Introduction

Coverage

Capacity

QualityCost

10 Radio Engineering Basics

Units

Antennas

Path loss

Sensitivity

24 GSM Air Interface

Frequency allocations

TDMA structure

Transmitter specifications

Receiver specifications

36 Propagation Mechanisms

Distance attenuation

Reflection

Diffraction

Refraction

Multipath fading

Intersymbol interference

Ducting

Doppler

60 Propagation modellingStatistical models

Shadow fading

Deterministic modelsGeographical data

Model calibration

89 Improvement measures

Antenna diversity

Time diversity

Frequency diversityEqualisation

Repeaters

Mast-head pre-amplifiers

123 Link Budgets

Examples

Power output

Sensitivity

Fade margin

Antenna gain

Cable loss

Other components

Site RF configuration

145 Network dimensioning

Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process

Site selection

Site acquisition

Antenna configurationNeighbour list planning

Introduction


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Introduction

Design aims:Maximise coverage

Best coverage for minimum expenditure

Minimise interference

Giving customers acceptable quality

Maximise capacity

Provide customers with service at acceptablequality

Limited by frequency allocation


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Introduction

CoverageBreadth of coverage

Cities?

Suburbs?

Roads?

Rural areas?

Depth of coverage Outdoors?

In car?

In building?

Excellent / good / fringe?


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Introduction

CapacityOperator allocated a number of channels

eg 2 x 10MHz

Cellular principle is frequency re-use

Close re-use

Many channels per cell

Less chance of blocking or congestion

More chance of interference

Loose re-use

Fewer channels per cell

More chance of blocking / congestion

Less chance of interference


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Introduction

Quality Is a function of

Coverage probability

Interference probability

Blocking probability

If the mobile cannot hear the base station or the base station cannot hearthe mobile, there is not enough coverage. If other mobiles are received atthe base station, or the mobile can receive signals from other base stationson the same or the adjacent radio channel, then there is interference. If toomany users want to use the channels available on the base station at the

same time then there is blocking.


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Introduction

CostsBase Station Hardware

Network Planning

Site Acquisition and Contract

Power

Communication Links

Rent

Construction

Maintenance

Base Station Hardware

The equipment in the base station is not the only cost associated with thecell site, but it is the one that is visible on the financial quotation fromsuppliers. However, it is often difficult to price a base station from these

quotations. Normally the price includes many other services and products,and will inevitably depend on the number of base stations being ordered.Nowadays there is also a tendency for the equipment vendors to include afinance deal, whereby the operator pays no money at the start, buteffectively borrows the equipment from the vendor and pays the money overa period of time.


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Introduction

Balancing quality and costCost & revenue vs coverage quality

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

50 60 70 80 90 100

% probability of coverage

$100,0

00/km

2

Cost related to quality

Revenue vs quality

Max?


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Introduction

Capacity

Cost

Quality

The compromise between cost, quality and capacity is a little bit likesqueezing a balloon. The volume of air inside the balloon is constant so ifone side is squeezed, the volume of air must push out somewhere else.

In the diagram, squeezing the balloon from the capacity side (improvingcapacity is squeezing towards the red dot), will make the quality worse and/or the cost more expensive. Improving the quality will either make capacityless or cost higher, and trying to reduce the cost can only be done at theexpense of either a reduction in capacity or quality or, more likely both.

The network planning and optimisation engineer aims to improve on all thevariables at once, or to continue the analogy, he tries to reduce the volumeof air in the balloon.

It is important that the same person is responsible for the capacity and thequality planning, otherwise a reasonable compromise cannot be madebetween the two aims.


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Coverage

Capacity

QualityCost


Units

Antennas

Path loss

Sensitivity



TDMA structure





Reflection

Diffraction

Refraction

Multipath fading


Ducting

Doppler


Shadow fading


Model calibration


Antenna diversity

Time diversity


Repeaters


123 Link Budgets

Link balance

Examples

Power output / EIRP

Sensitivity / planning thresholds

Fade margin

Building attenuation

Antenna gain

Cable loss

Other components



Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process

Site selectionSite acquisition

Antenna configuration

Neighbour list planning

Radio Engineering Basics


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UnitsExponential:

y = 10x

Inverse:

x = log10 (y)

dB:

[x] dB = 10 log [y/10]

y = 10[x/10]

Signal amplitude vary enormously, often by a ratio of billions to one.Therefore exponential notation is useful in engineering, to avoid the need towrite down many zeros.

The inverse of the exponent function is the logarithm function. The inverseor y=ax is x=log

a

y. (Pronounced x = log to the base a of y)

The decibel is one tenth of a Bel, and is the normal expression of a ratio inengineering.


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Units

10log(x y) = [X]dB + [Y]dB

10log (x/y) = [X]dB [Y]dB

Multiplying in linear units is equivalent to adding in logarithmic or dB units.Dividing is equivalent to subtracting.


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Units10log(x 1mW) = [X]dB + [0]dBm = [X]dBm

1nW = -30 dBm

1mW = 0 dBm

1W = 1000mW = 30 dBm

2W = 2000mW = 33 dBm

4W = 4000mW = 36 dBm

5W = 5000mW = 37 dBm10W = 10000mW = 40 dBm

The dB unit is a simple ration. If we want to express an absolute level wehave to define a base unit. In radio signal levels the base unit is usually themilliwatt, so levels are expressed as a ratio relative to one milliwatt, or dBm.

Doubling the signal is the same as adding 3dB. Multiplying by 10 is thesame as adding 10dB.


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Antennas Isotropic radiation

An Isotropic Radiatoris a theoretical point source of radio waves whichradiates in all directions in three dimensions. No real isotropic radiator existsbecause it is impossible to make an antenna which is only a single point.

Real antennas must have electric current flowing in them to generate theelectromagnetic waves, which means that they must have some length.

An isotrope is defined to have a gain of 1, since the energy is not focussed inany particular direction. Its gain is normally referred to as 0 dBi. The iindicates 0 dB relative to an isotropic radiator.


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Antennas Directional radiation

GainIsotropic pattern

Directional pattern

Real antennas focus the radiated power so that it does not transmit equallyin all directions. The power which is not transmitted in one direction isinstead transmitted in another direction. The ratio of the power transmittedin any direction, to the power which would result if all the energy had beentransmitted equally in all directions is known as the Directivityof the antenna

in that direction. If losses in the antenna are taken into account, this is knownas the Gain. Each direction has its own Gain value, but the Maximum Gain isnormally known as the Gain.

The Gain is a ratio, so can be expressed in dB. However, the gain is relativeto an Isotropic Radiator, so the units are dBi.

Sometimes the gain is expressed relative to a dipole radiator in dBd. Adipole has a gain of 2.2dBi.


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Antennas Aperture

Small aperture = wide beam = low gain

Large aperture = narrow beam = high gain

Half power beamwidth 3dB

The gain is related to the aperture by the equation:

Where Ae is the effective aperture area

and is the wavelength

and D is the directivity

2

4

e

AD=


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Antenna downtilt

Typical verticalbeam pattern

14: -18dB

Possible loss

of coverage

0

4: -3dB

Coverage

Interference:

-18dB?

In practice, reflections fill up nulls and sidelobes


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Antenna array: how it works

Equal distanceto each element(waves in phase)

Waves notin phase

Near field Far field

Elementsfed in phase

2

2D


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Antenna array: how it works

Wavesin phase

Feed elementsout of phase


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Path lossAs the signal gets further from the source it

dissipates and its Power Densitybecomesless

The ratio of the transmitted power to thepower which would be received by anisotropic antenna is the Path Loss

The Path Loss ratio is normally very large(1080 to 10100), so it is normally expressed in

dB


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Path lossPtPr

= 10 log Pt 10 log Pr


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SensitivityThe lowest power that can be received and yet

the message can still be heard

Signal

Quality

Required

quality

Sensitivity


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SensitivityDefined by

Thermal noise

kTB

k = Boltzmans Constant = 1.38 x 1023 W/Hz/K

T = Absolute Temperature in Kelvin (0 K = -273 C)

B = Bandwidth in Hz

Minimum required Signal/Noise ratio

Normally about 3 9 dB for digital

About 12dB for analogue FM


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Coverage

Capacity

QualityCost


Units

Antennas

Path loss

Sensitivity



TDMA structure





Reflection

Diffraction

Refraction

Multipath fading


Ducting

Doppler


Shadow fading


Model calibration


Antenna diversity

Time diversity


Repeaters


123 Link Budgets

Link balance

Examples

Power output / EIRP


Fade margin


Antenna gain

Cable loss

Other components



Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process




GSM Air Interface


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GSM Air Interface

GSM standardsAir interface, physical layer:

05.01: Physical layer on the radio path: general description

05.02: Multiplexing and multiple access on the radio path

05.03: Channel coding

05.04: Modulation

05.05: Radio transmission and reception

05.08: Radio subsystem link control

05.10: Radio subsystem synchronisation

The GSM standards are available from the Internet. If they were printed outthey would use and enormous a mount of paper. Many are to do with fixednetworking and with detailed protocols. The ones mainly relevant to theradio interface are the 05 and 08 series. It is thoroughly recommended thatradio planners and optimisation engineers read the 05 and 08 series

specifications entirely.


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GSM Air Interface

GSM Standard 05.05Reference sensitivity performance

Channel Propagation Conditions

Static TU50 TU50 RA250 HT100

no FH ideal FH no FH ideal FH

SDCCH FER 0.1% 9% 9% 8% 13%

RACH FER 0.5% 13% 13% 12% 13%

SCH FER 1% 19% 19% 15% 25%

TCH/F9.6&H4.8 BER 10-5 0.4% 0.4% 0.1% 0.7%

TCH/F4.8 BER 10-4 10-4 10-4 10-4

TCH/F2.4 BER 10-4 10-5 10-5 10-5

TCH/H2.4 FER 10-4 10-4 10-4 10-4

TCH/FS FER 0.1% 4% 3% 2% 7%

class I b RBER (0.4/))))% (0.3/))))% (0.3/))))% (0.2/))))% (0.5/))))%

class II RBER 2% 8% 8.1% 7% 9%

For the purpose of type approval, the specifications define a referencesensitivity level, and define the performance in terms of Bit Error Rate andFrame Erasure Rate for various channel types (e.g. speech, control, dataetc) under various defined propagation environments. (TU = Typical Urban,RA = Rural Area, HT = Hilly Terrain, numbers show mobile speed in km/h).


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GSM Air Interface

SensitivityGSM value:

B = 200 000 Hz

T = 290 K

k = 1.38 x 1023

kTB = 8.28 x 10-16 W = 8.0 x 10-13 mW

10 log (8.28 x 10-13 mW) = -121 dBm

The GSM reference sensitivity comes about from thermal noise, the noisefigure of the receiver, and the minimum required Signal to Noise ratio.


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GSM Air Interface

Sensitivity Official GSM value:

Thermal noise = -121 dBm

Additional noise due to receiver amplifier = 8 dB

Required Eb/No (signal/noise) = 9 dB

Minimum required signal = -121 + 8 + 9 = -104 dBm

This is the Reference Sensitivityof a GSM BS

-121 dBm

-113 dBm

-104 dBm

9 dB

8 dB


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GSM Air Interface

Sensitivity TypicalGSM value:

Thermal noise = -121 dBm

Additional noise due to receiver amplifier = 4 dB

Required Eb/No (signal/noise) = 7 dB

Minimum required signal =

-121 + 4 + 7 = -110 dBm

This is the sensitivity of a typicalGSM BS

The reference sensitivity assumes a relatively poor system noise figure of8dB, while 4dB is easily achievable. It also includes a 2dB implementationmargin. If realistic parameters are used it is found that a sensitivity of 110dBm is easily achievable.


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GSM Air Interface


890

915935

960

1710

1785

1805

1880

1850

1910

60 MHz 60 MHz

75 MHz75 MHz2525

1930

1990

925

880

GSM 900

GSM PCS 1900

E-GSMGSM 1800 / DCS1800

GSM 400

GSM was originally allocated 2 x 25MHz in the 900MHz region. In 1992 theUK government introduced PCN or Personal Communications Networks.These were licensed in the 1800MHZ band and eventually it was decided tobase them on the GSM standard, so they became DCS1800 orGSM1800. The Americas already have this band used for other things, so

it was necessary to use the 1900MHz band there. This was called PCS orPCS1900, although now all systems are simply called GSM900 or GSM1800or GSM1900.

An attempt was made to define GSM400, so that ex NMT analoguefrequencies could be migrated to GSM. However, this was not successful.


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GSM Air Interface

Multiple access methodGSM uses an FDMA /TDMA method of

allowing many channels at once

FDMA: Frequency Division Multiple Access

200 kHz radio channels

124 in 900 MHz band

374 in 1800 MHz band

TDMA: Time Division Multiple Access

8 time slots per radio channel

Each time slot 577 ms long

8 time slots 4.615 ms long


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GSM Air Interface

Multiple access methodFDMA /TDMA

890.2 890.4 890.6 891.8 892.0 892.2 892.4 892.6 892.8 893.0890 .6 890 .8 891 .0 891 .2 891 .4 891 .60

0.577

1.154

1.731

2.308

2.885

3.461

4.038

4.615

Time, ms

Frequency, MHz

TS0

TS1

TS2

TS3

TS4

TS5

TS6

TS7

TS0

NB:Prin

twithout

theshad

inganim

ation!


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GSM Air Interface

Receiver characteristicsReceiver sensitivities (nominal)

-104 dBm (all BS and GSM900 class 1 to 3 MS)

-102 dBm (GSM900 class 4 / 5 MS & GSM1800 MS from 1/2001)

-100 dBm (all GSM1800 MS to 12/2000)

Carrier/Interference ratio (co- and adjacent channel)

C/Ic = 9 dB, C/Ia1 = -9 dB

Equaliser performance

Maximum time dispersion: 16s


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GSM Air Interface

Transmitter characteristicsMS 900 MHz

Class 1: 20 Watts, 43 dBm




MS 1800 MHz

Class 1: 1 Watt, 30 dBm

Class 2: 0.25 Watts, 24 dBm


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GSM Air interface

Manufacturer specificsActual BTS and MS transmit powers

Tolerance +0 dB/ -2 dB

Actual receiver sensitivities

BS sensitivity from 107 dBm to 110 dBm

Hardware configuration

TRX per cell/site, antenna / combiner types, pre-amplifiers, cable losses

Feature availability

frequency hopping, power control, handoveralgorithms, underlay/overlay


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Coverage

Capacity

QualityCost


Units

Antennas

Path loss

Sensitivity



TDMA structure





Reflection

Diffraction

Refraction

Multipath fading


Ducting

Doppler


Shadow fading


Model calibration


Antenna diversity

Time diversity


Repeaters


123 Link Budgets

Link balance

Examples

Power output / EIRP


Fade margin


Antenna gain

Cable loss

Other components



Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process




Propagation Mechanisms


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0

100

200

300

400

500

600

700

800

900

1000

0 0.2 0.4 0.6 0.8 1

Distance

Signalpower

(Nominal

units on thegraph axes)


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Distance attenuation(Nominal

units on thegraph axes)

0

5

10

15

20

25

30

35

40

-2 -1.5 -1 -0.5 0

log distance

10log(sigpwr)


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Distance attenuationFree space propagation

Pr : Received power, dBunit

Pt : Transmitted power, dBunit

Gt : Transmitter Antenna gain, dB

Gr : Receiver Antenna Gain, dB

r : Range between the antennas

: Wavelength

Free space propagation rarely occurs in mobile radio environments

( ) rGGPPrttr

log20log20 4 +++=


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Distance attenuationFree space propagation

Pr : Received power = -104 dBm

Pt : Transmitted power = 33 dBm

Gr : BS antenna gain = 16 dBi

Gt : MS antenna gain = 0 dBi

F : Frequency = 900 MHz

C : Speed of light = 3 x 108 m/s

r : What is the maximum rangebetween the antennas?


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Reflection

P P

is the reflectioncoefficient: a complexvalue, containing bothamplitude and phase


42/192




ReflectionPlane earth propagation

rhhPPmbtr

log40log20 +=

r

hbhm


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DiffractionBending around obstacles


44/192




DiffractionKnife edge diffraction model

h

d1d2

d1, d2 >> h >> 1

+=

21

112dd

hv

v is the effectiveobstruction heightexpressed in thenumber of fresnel

zones obstructed


45/192




Diffraction Fresnel zones

Fresnel Loss

-25

-20

-15

-10

-5

0

5

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

v

Loss(dB)

v

Fresnel Loss

-25

-20

-15

-10

-5

0

5

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3

v

Loss(dB)


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Diffraction Calculation of diffraction loss

Ld (dB) = 0 -1

Ld (dB) = 20log(0.5 - 0.62) -1 0

Ld (dB) = 20log[0.5exp(- 0.95)] 0 0.8

Ld (dB) = 20log[0.4 -(0.1184-(0.38-0.1)2)] 0.8 2.4

Ld (dB) = 20log(0.225/) > 2.4


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Diffraction Fresnel zones

the space bounded by an ellipsoid, which has the fociiat the transmitter and receiver

nth Fresnel zone: The path from transmitter to receiver via any point on the

ellipse is n/2 longer than the direct transmitter to receiverdistance d

Shadowing occurs if an obstruction lies within the first

Fresnel zone

dd+n/2


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RefractionWaves bend when they meet the boundary

between two materials (e.g. atmosphericlayers)

This is the same principle by which light bends when it passes through aprism. It occurs when radio waves pass through the atmosphere and throughbuildings.


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Multi-path (Rayleigh) fading Many rays take different paths to the final

destination

Path lengths are different, so phases are different

In-phase waves add up, out of phase cancel out

In an urban area a typical mobile receives waves from the base station fromall 360 degrees.


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Distance, metres

Relativesignal,d

B

Multi-path (Rayleigh) fading

-20

-15

-10

-5

0

5

50 55 60 65 70 75 80 85 90 95 100

The phasor combination of a number of waves results in dramatic variationsin amplitude. It is emphasised by the dB scale. Zero in linear terms is -dB!


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Multi-path (Rayleigh) fadingStationary and slow moving mobiles

particularly affected

Problems when trying to measure arepresentative signal level

Improvement measures: Antenna diversity

Motion of the mobile

Frequency hopping

Wideband channels

If the signal is a bit weak, a good way to maintain a call is to pace up anddown the room very fast.


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Shadow (log-normal) fading Shadowing behind objects that are too small to appear

in the terrain database (e.g. buildings)

Shadowing and diffraction that is not calculateddeterministically

Diffraction or shadowing occurs over the tops of buildings and aroundobstacles. Often this local diffraction effect is considered statistically ratherthan deterministically and referred to as shadow fading, or log-normal fadingbecause of the nature of the probability density function.


53/192




Intersymbol Interference (Time dispersion) Multipath propagation with long path delays

Impulse response:

Late echoes overlap with following bits Increased bit error rate

Countered by equaliser in the GSM receiver

Waves which bounce off distant obstacles arrive some time after those thattake a direct path or which bounce off nearer structures.

By the time the wave which has travelled the longer distance has arrived, thenext bit has also arrived via the shorter path, and the two bits will interferewith each other, causing Intersymbol Interference.


54/192




Intersymbol InterferenceCritical are strong

echoes with a delayof more than 4bit-lengths (15s) Exercise:Calculate

critical length ofpath difference

Have to be avoided by means of

Site location

Antenna azimuth and tilt Cell splitting

The GSM equaliser will counteract the effects of intersymbol interference upto 15 microseconds, but delays beyond this will cause problems. This isparticularly noticeable in mountainous regions, e.g. Switzerland, Austria etc.


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Inter-symbol interferenceTest Propagation conditions (GSM 05.05)

R

eceivedsignals(rel.dB)

-15

-10

-5

0

0 5

Typical Urban

TU50 (50 km/h)

-20

-15

-10

-5

0

0 5

Rural Area

RA250 (250 km/h)

-15

-10

-5

0

0 5 10 15 20

Hilly Terrain

HT100 (100 km/h)

Propagation delays (s)

s s

The GSM specifications define various delay profiles which are meant tosimulate the multipath propagation in various environments. The speed oftravel must also be defined, since each signal will also be experiencingRayleigh fading.


56/192




Intersymbol Interference Can be measured with a channel sounder

Using test transmitter with a known bit sequence orthe training sequence from a live GSM base station

Drive

nrou

te

t

P

A channel sounder is effectively a radar system used to measure echoes.Many of them transmit a psuedo random binary sequence correlated withthe same sequence at the receiver to extract the delay profile. A similartechnique actually uses a known sequence transmitted from each cell, calledthe training sequence. Thus it is possible to measure delay profiles from

standard GSM base stations.


57/192




Doppler EffectFrequency shift due to speed v: f=v/

ca. 200 Hz at 900 MHz and 250 km/h

ff-f f+ff55 Network Consultants 55

Doppler shift is exactly the same effect experienced when a car passes athigh speed. The frequency of the noise is higher as the car is travellingtowards you, and drops to a lower frequency as it drives away.

You experience the same thing travelling at speeds close to the speed oflight. Stars you are moving towards appear to turn blue, while ones you aremoving away from appear to turn red.


58/192




DuctingWaveguide effects, e.g.:

Tunnels

Narrow valleys/gorges

Different layers in the atmosphere

Street canyons

If the radio waves do not spread out, but are instead confined within aguide, the free space loss equation does not apply. If there is no loss inthe walls of the guide, then the signal can propagate long distances (e.g.optical fibres).


59/192




DuctingExtended propagation

often causes moreproblems than it solves: Frequency re-use

Neighbour cell definitions

Interference with neighbouring countriesacross water

Scotland Norway

Good propagation along street canyons results in very unevenly shaped cellsand undesired interference.

Very long distance propagation is possible over the sea in certain weatherconditions. TV interference sometimes exists between France and Britain,while Cellnet experienced interference from Norwegian networks.


60/192




Coverage

Capacity

QualityCost


Units

Antennas

Path loss

Sensitivity



TDMA structure





Reflection

Diffraction

Refraction

Multipath fading


Ducting

Doppler


Shadow fading


Model calibration


Antenna diversity

Time diversity


Repeaters


123 Link Budgets

Link balance

Examples

Power output / EIRP


Fade margin


Antenna gain

Cable loss

Other components



Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process




Propagation Modelling


61/192




Statistical models

Diffraction over many building tops -

multi-screen diffraction, slope 38

No reflections or obstructions - free spaceslope = 20

Ground reflection and line of sight - plane earth

slope = 40

Tx

Rx

Real life propagation is usually a combination of free space, reflections anddiffraction, depending on the environment between the transmitter and thereceiver.

It is virtually impossible to model these effects deterministically since data ofsufficient accuracy and resolution is not available. Instead propagationmodels normally attempt to describe the situation statistically.


62/192




Clutter lossShadowing behind objects that are too small

to appear in the terrain database (e.g.buildings and trees)

Shadowing and diffraction that is notcalculated deterministically

Many researchers have established that the path loss is heavily influencedby the density of the buildings or clutter in the immediate vicinity of themobile. Certain clutter loss figures in dB can be defined, which model theaverage additional loss due to this clutter. For example, a dense urban areamight exert 30 dB more loss than open plains.


63/192




Statistical models

Most propagation effects are contained in theclutter correction factor Lc for the target pixel,and are therefore characterised by the vicinityof the MS only

This assumption is valid if the BS antenna is wellabove the surrounding clutter

Lc

Propagation models in a computer tool are used to calculate the path lossfrom each base station to all pixels on a grid within a certain radius. Eachpixel is allocated a terrain height, so that diffraction loss can be calculated,and a clutter type, so that clutter loss can be allocated.


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Statistical model: Hata formulaBased on measurements by Okumura in 1968

L=69.55+26.16 log(f)-13.82 log(hb)-a(hm)+(44.9-6.55 log(hb))log(d)-Lc

Clutter correction factors Lc (COST231):

U2: (0dB) Dense UrbanU1: (3dB) Low Density Urban

S3: (8dB) Dense SuburbanS2: (5dB) Leafy Suburban

S1: (11dB) Low Density Suburban

F2: (9dB) High, dense forestO2: (19dB) Open, few obstructions

W: (29dB) Water

L Path loss (dB)

f Frequency (MHz)

hb BS antenna height (m)

hm MS antenna height (m)

a(hm) = (1.1log(f)-0.7)hm-(1.56log(f)-0.8),

MS antenna height correction

d Distance BS - MS (km)

Lc Clutter correction factors (dB)

COST 231: L=46.3+33.9 log(f)-13.82 log(hb)-a(hm)+(44.9-6.55 log(hb))log(d)-Lc

In 1968 Okumura did a very large measurement campaign in Japan. Hederived various propagation graphs to model these results. It was not until1980 when computers became more generally available at least toacademics, that Hata developed these graphs into a path loss formula.

He also derived various formulae for different environments, e.g. small city,large city, rural, etc, but these have generally been superseded by clutterloss values derived by the COST231 working group funded by the EuropeanCommission.

COST231 also compared the formula with measurements made at 1800MHzand found that it was necessary to modify the coefficients slightly to extendthe validity of the equation to 2GHz.


65/192




Statistical model: Hata formula Important limitations

f: 200 - 2000 MHz for COST231(Original Okumura/ Hata : - 1500 MHz)

hb: 30 - 200 m, effective antenna height(above the target area),

BS antenna above surrounding clutter

hm: 1 - 10 m

d: 1 - 20 km

Any model is only valid over the range of values for which input data wasused to derive it. In the case of Hata the most significant limitation is the cellrange (1km, while many cells are less than this today) and the base antennaheight (many are less than 30 m today). This emphasises the need forplanners to do their own measurements and to calibrate their own

propagation models for the cities in which they will be used.


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Hata propagation formula: Exercise!Pr : Received power = -104 dBm

Pt :Transmitted power = 33 dBm

Gr :BS antenna gain = 16 dBi

Gt :MS antenna gain = 0 dBi

f: frequency = 900 MHz

hm: Mobile height = 1.5 m

hb: Base antenna height = 25 m

Lc: Clutter correction factor = 0 dB

d : What is the maximum range

between the antennas?


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Critical cases for statistical modelsClutter type boundaries, e.g.:

Sea-Land

Forest-Open

Some propagation modelsalso take into account severalpixels before the target pixel

Path clutterswitched on

Microcells: BS antenna below rooftop level

Microcell prediction model required

Although the path loss is mostly affected by clutter in the immediate vicinityof the mobile, there are some situations where adjacent pixels also have asignificant effect. If a mobile is in an open pixel which is shadowed from thebase station by an urban or forest pixel, then the path loss will not becorrectly predicted if only the open pixel is taken into account.

It is possible to switch on the path clutter function in the propagation model,so that the clutter value used is a weighted average of the clutter values ofthe first few pixels from the mobile in the direction of the base station.

Alternatively there is a clutter heights model. This models each clutterclass as a set of fences of a given height and separation. Thus the effectsof a shadow cast by an adjacent pixel are modelled.


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Slow (Lognormal-, Shadow-) Fading Small scale lognormal fading

Caused by objects that are too small to appear in theterrain database (e.g. buildings)

Target pixel One terrain height level

One clutter classificationbut no information about objects within the pixel

A single clutter loss factor is used in the propagation prediction, but clearlythe signal level / path loss will vary considerably. The factor is the medianvalue for a whole pixel, while individual buildings will cause fluctuations aboutthis value.


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Lognormal / shadow fadingPlanning tool predicts only a median signal

strength value for the entire pixel

This level should be sufficiently above thesensitivity threshold

Receiv

edlevel(dBm)

Driven route

Median

Sensitivity

Margin

Locations with insufficient signal

Within the pixel, field strength will vary around the median value as themobile moves in and out behind buildings and other obstructions. Themedian value should be sufficiently high that the amount of time or locationsthat have insufficient signal are very few.


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Small scale lognormal fading Various measurements

(e.g. Okumura) have shownthat the instantaneoussignal levels in a shadowingenvironment take alognormal distribution

(normal distribution in dB)

with a standard deviation

of 5 to 7 dB in urban and suburban environments

( )

N

xmi

2

=

The deviation of the path loss from the median value has a Normal orGaussian probability density function. The probability that the signal levelis between two points on the x-axis is the integral under the curve betweenthose two values. Therefore the probability that it is above a certain value isthe integral between that value and infinity.

Calculation of these integrals is not possible deterministically and numericalmethods must be used. Fortunately the results are well documented andavailable in any book on statistics. They are nowadays commonly availablein spreadsheet functions.

The standard deviation is an indication of the spread of values.Mathematically this is a root-mean-square value, but normalised to a zeromedian.


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Normal cumulative distribution table(mean=0, standard deviation =1)

-1.00 15.87% 0.10 53.98% 1.20 88.49%

-0.95 17.11% 0.15 55.96% 1 .2 8 9 0 .0 0 %

-0.90 18.41% 0.20 57.93% 1.30 90.35%

-0.85 19.77% 0.25 59.87% 1.32 90.68%

-0.80 21.19% 0.30 61.79% 1.34 91.01%

-0.75 22.66% 0.35 63.68% 1.36 91.33%

-0.70 24.20% 0.40 65.54% 1.38 91.64%

-0.65 25.78% 0.45 67.36% 1.40 91.95%

-0.60 27.43% 0.50 69.15% 1.42 92.24%

-0.55 29.12% 0.55 70.88% 1.44 92.53%

-0.50 30.85% 0.60 72.57% 1.46 92.81%

-0.45 32.64% 0.65 74.22% 1.48 93.08%

-0.40 34.46% 0.70 75.80% 1.50 93.34%

-0.35 36.32% 0.75 77.34% 1.52 93.59%

-0.30 38.21% 0.80 78.81% 1.54 93.84%

-0.25 40.13% 0.85 80.23% 1.56 94.08%

-0.20 42.07% 0.90 81.59% 1.58 94.31%

-0.15 44.04% 0.95 82.89% 1.60 94.54%

-0.10 46.02% 1.00 84.13% 1.62 94.75%

-0.05 48.01% 1.05 85.31% 1.64 94.97%

0 .0 0 5 0 .0 0 % 1.10 86.43% 1.66 95.17%

0.05 51.99% 1.15 87.49% 1.68 95.37%

-1.85 3.2% 0.35 63.7% 1.01 84.4% 1.67 95.3% 2.33 99.0%

-1.90 2.9% 0.36 64.1% 1.02 84.6% 1.68 95.4% 2.34 99.0%

-1.85 3.2% 0.37 64.4% 1.03 84.8% 1.69 95.4% 2.35 99.1%

-1.80 3.6% 0.38 64.8% 1.04 85.1% 1.70 95.5% 2.36 99.1%

-1.75 4.0% 0.39 65.2% 1.05 85.3% 1.71 95.6% 2.37 99.1%

-1.70 4.5% 0.40 65.5% 1.06 85.5% 1.72 95.7% 2.38 99.1%

-1.65 4.9% 0.41 65.9% 1.07 85.8% 1.73 95.8% 2.39 99.2%

-1.60 5.5% 0.42 66.3% 1.08 86.0% 1.74 95.9% 2.40 99.2%

-1.55 6.1% 0.43 66.6% 1.09 86.2% 1.75 96.0% 2.41 99.2%

-1.50 6.7% 0.44 67.0% 1.10 86.4% 1.76 96.1% 2.42 99.2%

-1.45 7.4% 0.45 67.4% 1.11 86.7% 1.77 96.2% 2.43 99.2%

-1.40 8.1% 0.46 67.7% 1.12 86.9% 1.78 96.2% 2.44 99.3%

-1.35 8.9% 0.47 68.1% 1.13 87.1% 1.79 96.3% 2.45 99.3%

-1.30 9.7% 0.48 68.4% 1.14 87.3% 1.80 96.4% 2.46 99.3%

-1.25 10.6% 0.49 68.8% 1.15 87.5% 1.81 96.5% 2.47 99.3%-1.20 11.5% 0.50 69.1% 1.16 87.7% 1.82 96.6% 2.48 99.3%

-1.15 12.5% 0.51 69.5% 1.17 87.9% 1.83 96.6% 2.49 99.4%

-1.10 13.6% 0.52 69.8% 1.18 88.1% 1.84 96.7% 2.50 99.4%

-1.05 14.7% 0.53 70.2% 1.19 88.3% 1.85 96.8% 2.51 99.4%

-1.00 15.9% 0.54 70.5% 1.20 88.5% 1.86 96.9% 2.52 99.4%

-0.95 17.1% 0.55 70.9% 1.21 88.7% 1.87 96.9% 2.53 99.4%

-0.90 18.4% 0.56 71.2% 1.22 88.9% 1.88 97.0% 2.54 99.4%

-0.85 19.8% 0.57 71.6% 1.23 89.1% 1.89 97.1% 2.55 99.5%

-0.80 21.2% 0.58 71.9% 1.24 89.3% 1.90 97.1% 2.56 99.5%

-0.75 22.7% 0.59 72.2% 1.25 89.4% 1.91 97.2% 2.57 99.5%

-0.70 24.2% 0.60 72.6% 1.26 89.6% 1.92 97.3% 2.58 99.5%

-0.65 25.8% 0.61 72.9% 1.27 89.8% 1.93 97.3% 2.59 99.5%

-0.60 27.4% 0.62 73.2% 1.28 90.0% 1.94 97.4% 2.60 99.5%

-0.55 29.1% 0.63 73.6% 1.29 90.1% 1.95 97.4% 2.61 99.5%

-0.50 30.9% 0.64 73.9% 1.30 90.3% 1.96 97.5% 2.62 99.6%

-0.45 32.6% 0.65 74.2% 1.31 90.5% 1.97 97.6% 2.63 99.6%

-0.40 34.5% 0.66 74.5% 1.32 90.7% 1.98 97.6% 2.64 99.6%

-0.35 36.3% 0.67 74.9% 1.33 90.8% 1.99 97.7% 2.65 99.6%

-0.30 38.2% 0.68 75.2% 1.34 91.0% 2.00 97.7% 2.66 99.6%

-0.28 39.0% 0.69 75.5% 1.35 91.1% 2.01 97.8% 2.67 99.6%

-0.26 39.7% 0.70 75.8% 1.36 91.3% 2.02 97.8% 2.68 99.6%

-0.24 40.5% 0.71 76.1% 1.37 91.5% 2.03 97.9% 2.69 99.6%

-0.22 41.3% 0.72 76.4% 1.38 91.6% 2.04 97.9% 2.70 99.7%

-0.20 42.1% 0.73 76.7% 1.39 91.8% 2.05 98.0% 2.71 99.7%

-0.18 42.9% 0.74 77.0% 1.40 91.9% 2.06 98.0% 2.72 99.7%

-0.16 43.6% 0.75 77.3% 1.41 92.1% 2.07 98.1% 2.73 99.7%

-0.14 44.4% 0.76 77.6% 1.42 92.2% 2.08 98.1% 2.74 99.7%

-0.12 45.2% 0.77 77.9% 1.43 92.4% 2.09 98.2% 2.75 99.7%

-0.10 46.0% 0.78 78.2% 1.44 92.5% 2.10 98.2% 2.76 99.7%

-0.08 46.8% 0.79 78.5% 1.45 92.6% 2.11 98.3% 2.77 99.7%

-0.06 47.6% 0.80 78.8% 1.46 92.8% 2.12 98.3% 2.78 99.7%

-0.04 48.4% 0.81 79.1% 1.47 92.9% 2.13 98.3% 2.79 99.7%

-0.02 49.2% 0.82 79.4% 1.48 93.1% 2.14 98.4% 2.80 99.7%

0.00 50.0% 0.83 79.7% 1.49 93.2% 2.15 98.4% 2.81 99.8%

0.02 50.8% 0.84 80.0% 1.50 93.3% 2.16 98.5% 2.82 99.8%

0.04 51.6% 0.85 80.2% 1.51 93.4% 2.17 98.5% 2.83 99.8%

0.06 52.4% 0.86 80.5% 1.52 93.6% 2.18 98.5% 2.84 99.8%

0.08 53.2% 0.87 80.8% 1.53 93.7% 2.19 98.6% 2.85 99.8%

0.10 54.0% 0.88 81.1% 1.54 93.8% 2.20 98.6% 2.86 99.8%

0.12 54.8% 0.89 81.3% 1.55 93.9% 2.21 98.6% 2.87 99.8%

0.14 55.6% 0.90 81.6% 1.56 94.1% 2.22 98.7% 2.88 99.8%

0.16 56.4% 0.91 81.9% 1.57 94.2% 2.23 98.7% 2.89 99.8%

0.18 57.1% 0.92 82.1% 1.58 94.3% 2.24 98.7% 2.90 99.8%

0.20 57.9% 0.93 82.4% 1.59 94.4% 2.25 98.8% 2.91 99.8%

0.22 58.7% 0.94 82.6% 1.60 94.5% 2.26 98.8% 2.92 99.8%

0.24 59.5% 0.95 82.9% 1.61 94.6% 2.27 98.8% 2.93 99.8%

0.26 60.3% 0.96 83.1% 1.62 94.7% 2.28 98.9% 2.94 99.8%

0.28 61.0% 0.97 83.4% 1.63 94.8% 2.29 98.9% 2.95 99.8%

0.30 61.8% 0.98 83.6% 1.64 94.9% 2.30 98.9% 2.96 99.8%

0.32 62.6% 0.99 83.9% 1.65 95.1% 2.31 99.0% 2.97 99.9%

0.34 63.3% 1.00 84.1% 1.66 95.2% 2.32 99.0% 2.98 99.9%


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Shadow fading marginsto take the effects of small scale lognormal

fading into account

Increase value of to take into accountprediction model inaccuracy

Cell edge

probability

50 %

75 %

90 %

95 %

Cell area

probability

77 %

91 %

97 %

99 %

Margin for

= 7dB

0

5 dB

9 dB

12 dB

Exercise:

Margins for

= 9dB

The standard deviation of the signal level relative to the actual (measured)median for a pixel is found to be about 5 dB to 7 dB.

However, we are interested in the standard deviation relative to the predictedmedian. Since the propagation model will not be 100% accurate, thepredicted median will differ from the true median. The standard deviation ofthe signal level relative to this predicted median will be greater than thatrelative to the true median.

More accurate propagation models will have a lower standard deviation. Theaccuracy of the model can normally be determined by comparing measuredvalues against predicted values in the relevant module in the planning tool.

NB. The error spread is sometimes expressed as Root Mean Square, orRMS error, rather than standard deviation.


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Model accuracy

~6 - 7 dBSeparate model for each sector

~7 - 8 dBIncluding path clutter

~8 - 9 dBHata, all coefficients calibrated

~10 dBHata, clutter calibrated

> 13 dBHata uncalibrated

Degree of calibration

If more effort can be put into calibrating a model it can be made moreaccurate. However, diminishing returns may apply. It is relatively easy toimprove a model to give 8 to 9 dB standard deviation, or under 8 dB if pathclutter is included, To improve on this would require a separate model foreach cell and preferably very detailed building data.


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Deterministic propagation modelse.g. Walfisch-Ikegami

Line-of-Sight (LOS) case: Free spacepropagation

Non-Line-of-Sight (NLOS) case:

Multi-screen diffraction loss (several buildings)

Roof-to-Street diffraction loss and scatter

hbhm

bw

The Walfisch Ikegami model was an attempt to enable radio planners tobegin predictions without the need for a campaign of calibrationmeasurements. The intention was that by looking at the building heights,separations and street widths, it should be possible to have a reasonablyaccurate model without any RF measurements.


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Deterministic propagation modelse.g. Microcell models

Ray tracing

Very time consuming - but most accurate

Vector following

Quicker and quite accurate

2- dimensional or 3- dimensional

Infinitely high buildings or

Rays pass round andover


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Propagation Modelling Microcell prediction models:

Field strength

In this diagram the signal is shown as field strength (dBV/m). This isrelated to the signal power (dBm) in the same way that volts is related topower in an electrical circuit.

Notice that the signal level inside the buildings is lower than on the streets,since the propagation model explicitly adds the loss through each wall.Statistical macrocell models do not do this, since they predict a mediansignal for a whole 50m pixel, and the signal threshold is adjusted to allow forbuilding loss.


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Geographical dataPlanning tool requires in

sufficient resolution: Terrain height

Morphology(Clutter, land use)

Vectors(Roads, rivers, railways)

Sources: Paper maps (Restrictions

apply in many countries)

Satellite imagery (Requirespost processing - available off the shelf)

Aerial photographs

TerrainDatabase

(DTM)

Planning tools need sufficient disk space for the terrain and clutter data. Acountry of 250 000 km2 requires 100 megapixels, with each pixel taking upabout 8 bytes, at least 1Gbyte of disk space is needed. Not a problem now,but 10 years ago it was a different story

Most of the value in clutter data is in the post processing. Satellite and aerialphotographs are interesting to look at. However, for use in a planning tool itis necessary to convert collections of buildings interspersed with open spaceinto areas of particular clutter classes. The best way to do this is still by eye,although image processing techniques are improving.


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Propagation Modelling Geographical data

Microcell prediction models: Clutter data

The buildings in this example are indicated using pixels in the same way asconventional clutter data, as raster data. Two disadvantages arise from thistechnique:

First the amount of data becomes huge. Instead of a country requiring 1Gbyte of data at 50m resolution it would now require 400 Gbytes. It wouldonly be possible to store maps of city centres at this resolution.

Second: The edges of the buildings are all ragged to to the pixellation. Thiswould lead to incorrect diffraction calculations around the buildings.

The solution is to use vector data instead, so that a building could berepresented by the co-ordinates of its 4 corners. This saves space and alsoresults in the correct modelling of diffraction around its walls.


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Model calibrationTypical terrain and clutter features can differ

from city to city and influence radiopropagation

Planning tool must be calibrated for availabletopographical data for best prediction results


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Model calibration

A + B log f + C log Hb + (D - E log Hb) log d + Lc

Calibrate coefficients for zero mean error andminimum standard deviation of error

Log distance

Path loss, dB Error

The coefficients A to E should be calibrated, as well as each different valueof Lc, in order to achieve the minimum standard deviation of error. A canalways be adjusted to give a zero mean error.

The graph shows how A and (D-E log Hb) can be calibrated by plotting pathloss against log D. Similar plots are needed for log f to find B, and for log Hbto find C. However, the coefficients are not orthogonal, so the process isvery long winded and iterative.


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Model calibrationSufficient measurements must be collected:

for different antenna heights and frequencies for all clutter types at different distances from the transmitter in various street widths and orientations with and without diffraction

Features like tunnels, bridges are excluded

Log distance

Path loss, dB

?

Rural

Urban

A common problem is that although vast amounts of data are collected,there are insufficient measurements available to calibrate the particularcoefficients. For example if data is only collected between 1km and 4 kmfrom the site, this will not enable a good estimate of the slope to be made.Measurements should be collected from about 250m to about 10km from a

site for best results.

Each clutter class ought to be analysed separately, otherwise a false slopewill be found as illustrated in the diagram.


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CW measurement proceduresVarious street widths and directions in built

up areas

Accurate conduct, especially with regard toantenna installation and measurementdocumentation

Regular calibration of measurementequipment

Murphys Law:

If anything can go wrong with a measurement exercise it will.

If nothing can go wrong with a measurement exercise it will still go wrong

There is a tendency for drivers to follow the wide streets which are radial tothe site, because it is easier. Proper representative measurements need tocover all street directions and narrower streets as well as wide ones.


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CW measurement proceduresChoice of antenna: Omni or Directional?

Measurement antenna should be the same asplanned BS antenna

Directional has higher gain: more dynamic range

Omni measurement antenna allows 360 survey

Sector antenna patterns may be unreliable

outside the 3 dB beamwidth

An omni antenna can give different propagation characteristics to a sectorbecause it may lead to reflections which would not be generated by thesector configuration. Therefore it is best to use the same antennaconfiguration as is planned for the actual BS.

Directional antennas also have more gain, so more dynamic range will bepossible, which means that measurements can be made out to greaterdistances.

On the other hand omni antennas allow a continuous 360 degree survey, sologistics are easier. Also, measurements outside the 3dB beam-width of asector antenna must be excluded from the analysis.


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Model calibration

A time consuming task, but some automaticalgorithms can help, for example:

Newton-Raphson Gauss-Seidel Jacobi

(ref: Advanced Engineering Mathematics: E.Kreyszig)


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Types of RF measurementsSignal level (CW) measurements

Verification of critical and borderline coverage areas

Calibration of the prediction model

Microcell planning without suitable prediction model

GSM test mobile (TMS) measurements Analysis of system parameters and handover

behaviour (during network optimisation)

Reflection (Channel Sounder) measurements Analysis of multipath propagation and delay spread


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CW measurements In a fading environment, a suitable averaging

procedure is crucial

Aim to eliminate the Rayleigh-Fading, but not theLognormal Fading

t

Level

before

after averaging

A combination of Rayleigh fading and Log-normal fading is being measured,making individual measurements unrepresentative. A system of averaging isrequired that will remove the short term fluctuations and leave us withsamples that can be realistically compared with the predicted values.

The predicted values are medians for 50 x 50m or 100m x 100m pixels. Wewould like to know the standard deviation around these values, so it isdesirable not to remove the log-normal fading component. However, theRayleigh fading is un-predictable so is of no interest to us when calibrating aprediction model. The Rayleigh component needs to be averaged out.


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CW measurement equipment

Navigation

Trigger

Antenna

Receiver /

Processor Storage

Transmitter

Amplifier

Antenna

Dynamic range:20 dB more than GSM

GSM:155dB,Measurements: 175dB

A measurement system consists of a transmitter, receiver, navigation deviceand storage device. The trigger, possibly driven from the wheels, or possiblyon a time-base, marks when a measurement is to be taken.

Sufficient power must be transmitted, and the receiver must have sufficientsensitivity to enable measurements to be taken well beyond the normal cellradius, so that the model is valid for the calculation of interference as well ascoverage. Therefore a dynamic range of at least 20dB more than that ofGSM is desired.

GSM test mobiles are not suitable for calibration measurements for thisreason.


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Averaging Lee-Criterion:

Minimum sampling rates for allowable RMS error

Averaging intervals

< 40 outdoor (~13.5m at 900 MHz) < 20 indoor (~ 6.5m at 900 MHz)

Number of Maximum

RMS error averaged sampling interval

(dB) samples ()

0.50 144 0.28

1.00 36 1.11

1.50 16 2.50

2.00 9 4.44

A much quoted averaging technique was defined by W C Y Lee. He took aheavily over sampled data set of a few metres of measurements in aRayleigh fading environment and calculated the TRUE mean of those. Hethen took various sub-samples of these and determined how much variationthere was when he used various numbers of samples. He found that 144

samples were necessary to ensure that the RMS (Standard deviation) of theerror was less than 0.5dB.

In order not to average out the log-normal fading, the averaging windowshould be less than the size of a building, or less than about 15m. Thiscorresponds to about 40 wavelengths at 900 MHz, and 80 at 1800 MHz.Thus if 144 measurements are taken every 15 metres, the distance betweenmeasurements will be about 0.25 wavelengths at 900MHz or 0.5wavelengths at 1800MHz. This is satisfactory considering that Rayleighfades can be as little as half a wavelength apart.


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Coverage

Capacity

QualityCost


Units

Antennas

Path loss

Sensitivity



TDMA structure





Reflection

Diffraction

Refraction

Multipath fading


Ducting

Doppler


Shadow fading


Model calibration


Antenna diversity

Time diversity


Repeaters


123 Link Budgets

Link balance

Examples

Power output / EIRP


Fade margin


Antenna gain

Cable loss

Other components



Cell range

Cell area

Cell structures

167 Detailed design

Planning tools

Planning process




Improvement measures


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Antenna (Space-) DiversityCombination of received

signals from separatedantennas

Normally only used on uplink

Fading in the two receiver pathsmust be independent (uncorrelated)

Correlation factor k: Function of antenna

separation

RX

The hope is, that statistically, if one antenna is in a fade, the other one wontbe.


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Antenna DiversityDifferent combining methods

Switched (Selection) Diversity

The currently strongest signal is selected

Equal Gain Combining

Mix the two signals in equal proportions

Maximum Ratio Combining(nowadays commonly used)

The individual signals are weightedaccording to their S/N ratios, co-phased,amplified, and finally combined.

Combining may be done at RF or at baseband. Baseband combininginvolves selection of the path which gives the lowest bit error rate determinedon a burst by burst basis.


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Antenna Diversity Switched Combining

Signal A

Signal B

Result

RX A RX B

Switch

Switched combining simply results in the better of the two signals beingused.


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Antenna Diversity Maximum Ratio Combining

Signal A

Signal B

Result

A

+

B

Maximum ratio combining uses energy from both paths so the resultingsignal is higher than either individual signal.


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Antenna Diversity Performance of different combining methods, k=0.7

0

2

4

6

8

10

12

1 2 3 4 5 6 7 8 9 10Number of branches N

Gain(dB)

Selection

Equal GainMaximum Ratio

Maximum ratio combining gives the best results and is most commonly used.Normally the number of branches used is 2, so the expected gain is 3 dB.


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Antenna DiversityDiversity gain vs. correlation factor

0

1

2

3

4

5

6

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Correlation coefficient k

Gain(dB)

Maximum Ratio combining, 50 km/h

As the correlation between the antennas increases, the expected gaindecreases. If the two signals are completely correlated then there is no gain.


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Vertical antenna diversity Achievable decorrelation vs. antenna spacing

0

0,10,20,30,40,50,60,70,80,9

1

0 5 10 15 20 25 30

Antenna separation (wavelength)

Correlationcoefficientk

TX

RX

RX-Div

Vertically spaced diversity receive antennas are not common, since at 900MHz the two antennas would need to be about 6 m apart. This only makessense on relatively tall masts. This configuration has generally only beenused in rural areas, but the tendency is to replace them with horizontal spacediversity.


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Horizontal antenna diversityBetter decorrelation

Directional effect

TXRX RX-Div.

Correlationcoefficientk

0

0,2

0,4

0,6

0,8

1

0 10 20 30 40 50

Antenna separation (wavelengths)

= 5 = 45 = 90

TXRX RX-Div.

Effectivecell area

Omni cell

Decorrelation is better with horizontal space diversity, but the problem is thecorrelation varies with direction.This is not too bad for a sectored cell, but itgives a non-circular shape to an omni cell.

For sufficient decorrelation at an angle of 45 degrees from the beamdirection, 900MHz antennas need to be about 2.3m apart. For 1800MHz theseparation reduces to 1.15m.


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Antenna diversityThe diversity gain also depends on

clutter density

difference between the average signal levels inthe 2 branches

direction of movement of the MS

BS antenna height

Other diversity techniques

Antenna Polarisation diversity

Frequency diversity

Time diversity

The gain figure assumed is an average. The actual figure at any instant intime might vary from this average.

Space receive diversity has been commonly used for many years, butincreasingly there are new techniques available, notable polarisationdiversity.


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Antenna diversity polarisationSignals in orthogonal polarisations are

uncorrelated.

Phase changes at reflection surface:

An electromagnetic wave is effectively an oscillating vector. Like any vector itcan be resolved into two components at 90 degrees to each other. Thesemight be horizontal and vertical, or they may be +45and 45.

Each polarisation behaves differently when it is reflected. If the polarisation isperpendicular to the reflection surface the phase changes by 180. If it isparallel to the surface there is no phase change. After many reflections(often at somewhere between perpendicular and parallel) the twopolarisations become decorrelated in their phase.

Rayleigh fades in the two polarisations are therefore also decorrelated and across-polar antenna can be used for diversity reception.


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Polarisation diversityCross-polarised (X-polarised) antenna:

Diversity gains comparable to horizontalspace diversity (a bit more in urban areas)

RX RX-DRX RX-D

It is possible to encase an array of cross polar antennas into one random(antenna casing), rather than having to have two casings which is therequirement with space diversity.

This dramatically improves the environmental impact of a site.


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Polarisation diversityA 3-sector site with

cross polar antennas.

Lighter mast structurethan with 2 antennasper sector


With cross polarised antennas it is possible to have both receive pathscontained in the same ray dome (antenna cover). With duplexors, both ofthese can also double up as transmitting antennas.

It is also possible to purchase products which have all three sectorscontained within one tube of about 30cm radius. Hence it is possible todisguise the mast and antenna structure as a telegraph pole or chimney.


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Time diversitySend message again if not received

= pardon for voice

= packet resend in data networks

Spread message out over time

Only part of message may be lost in a fade

Recover lost bits using channel coding (ForwardError Correction)

Interleaving in GSM spreads a speech frameover 8 TDMA frames


The GPRS packet radio service for GSM resends data when necessary. Thecircuit switched data and voice channels organise the bits for transmissionsuch that a 20ms frame is transmitted over a 40ms period. This preventsmany bits from the same frame being lost in one fade.


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Frequency diversityFading envelope has a Frequency

dependency as well as distance dependency

Different frequency waves arrive with differentphases

ReceivedLevel(rel.dB)

0

-10

-20

-30

Frequency

At different frequencies the wavelength is of course different, so the phase ofwaves arriving at a particular point is going to be different for differentfrequencies. At one frequency this may result in a fade while at anotherfrequency there may be no fade.

The potential therefore exists to transmit on more than one frequency andchoose the one giving the best signal. However, this would be wasteful ofcapacity. Instead transmission takes place on a number of frequencies inturn, that is the transmissions hops across different frequencies .

Error correction coding and interleaving enable the data lost to be retrieved.


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Frequency Hopping

Call in progress changes carrier frequency every TDMA

frame Limitation: Control channel carrying time slots

0 1 2 3 4 5 6 7 ...0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 ...0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 ...0 1 2 3 4 5 6 7

TDMA frame

(8 time slots)

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

f1

f2

f3

All the timeslots in one TDMA frame on one BS transceiver use the samefrequency. The frequency is changed between timeslot 7 and 0 every 4.615ms.

The BCCH channel is carried on timeslot 0 of the first transceiver. Thiscannot hop, since mobiles in idle mode decode information from thischannel. They also measure the signal level received from this transceiverand that must be a fixed frequency.

It is nevertheless possible to hop traffic channels onto and off the BCCHcarrier, provided that when there is no traffic burst on the BCCH carrier,there is a dummy burst transmitted instead.


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Frequency hopping Required fast fading margins

-2

-1

0

1

2

3

4

5

1 10 100 1000Mobile speed (km/h)

4 freqs.Margin(dB)re.

50km/h

2 freqs.

No hopping

Hopping improves the bit error rate in a Rayleigh fading environment. Athigher speeds, the movement of the mobile tends to give some improvementanyway, so the additional improvement due to the hopping is less.

A 3-4dB margin for fast fading is included in the calculation of the GSMreceiver sensitivity, so it does not need to be included additionally whencalculating the link budget.


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Frequency HoppingThe fading processes on the different

frequencies in the hopping sequence must bedecorrelated

function of the frequency spacing

1 MHz for k < 0.5 (corellation bandwidth)

If the channel hops to a frequency which is too close, then there is apossibility that the mobile will be experiencing the same fade at bothfrequencies. Just as for antenna diversity, the two channels must be de-correlated. Experiments have shown that a frequency spacing of about1MHz gives sufficient de-correlation. This is the correlation bandwidth and

corresponds to 5 GSM frequency channels of 200kHz.


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Frequency HoppingThe hopping gain increases with the number

of frequencies involved

Hopping with more than 8 frequencies (IdealHopping) does not deliver additional gains

Interleaving depth is 8

More frequencies gives improved gain, until the interleaving depth isreached. Each speech or data frame is transmitted over 8 bursts / timeslots.The maximum improvement is achieved when each burst is sent on 8different frequencies. If the hopping sequence has more than 8 frequencies,then the data frame still only uses 8 of them.


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Frequency hoppingGSM speech frame: 260 bits

456 bits after coding

50bits Class Ia 132bits class Ib 78 bits class II

Error Correction coding

A GSM speech frame consists of 260 bits and is generated once every 20ms. In a speech frame some of the bits are more important than others, andhave more coding, but overall the coding increases the number of bits to 456per frame.


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456 bits 456 bits456 bits456 bits

Re-ordering

TDMA bursts F1 F3 F2F4 F5F8F6 F7

The coded bits are jumbled up or re-ordered because the error correctioncoding can correct errors that are randomly distributed, but not bunches orbursts of errors.

Each 456 frame is split into 8 blocks of 57 bits. Each block of 57 istransmitted on a different burst in a different timeslot. Each burst carries atotal of 114 traffic bits, so it carries bits from two frames.

If a burst is lost in a frame it means that 12.5% of the bits are lost.However, these are randomly distributed because of the re-ordering, so theerror correction coding manages to recover the original data after the bits areun-re-ordered.


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Frequency Hopping Baseband Hopping

Hopping over as many frequencies as TRXsare installed in the cell

minus 1 (BCCH carrier) TX Ant.

TRX1f1

TRX2f2

TRX3f3

TRX4f4

f1

f2

f3

f4

Frequency

HoppingUnit

Baseband

(TDMA

Frames)

FilterCombiner

Between timeslots is a 28s guard period. The base station must changefrequencies du

GSM Coverage v4-0 Notes

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