mobile networks 7

7/30/2019 Mobile Networks 7

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Link Budget

Prof. Johnson I Agbinya

University of the Western Cape

http://www.cs.uwc.ac.za/~jagbinya


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Link Budget

DefinitionThe aggregation of the signal losses from a TX

to its RX is called its link budget Is a product of gains and losses expressed in decibels or

decoupled logarithmically Sets limit for power levels at various points in the network

Preferred for calculating propagation losses instead of

prediction expressions

Link budgets required for different terrains

Should consider all sources of power losses in the link


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Product ParametersProduct parameters is a list of all power loss and

gain sources in the link resulting from

hardware.

Transmit power(Pm

)- the power one base

station transmits 30 to 35 dBm for base stations

30 dBm for handsets

Receiver sensitivityis the lowest signal level

a receiver needs to be able to recover the

voice signal with acceptable quality usually in the range of -102 to -110 dBm

Base station sensitivity is Sb

Mobile station sensitivity is Sm


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Product Parameters (1)

Antenna Gain Gis a measure of the ability of the

antenna to amplify the signal it receives fortransmission. It will do so by

directing the beam

Narrowing the beam (focusing)

usually in the range of 18dBi Base station antenna gain is Gb

Mobile station antenna gain is Gm

Feeder lossoccurs in the cable connecting the

antenna to the electronics in the base station Is usually about 3 dB


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Diversity techniquesare used to tolerate weak

signals in the network

Types of diversity

Space diversity

Time diversity

Frequency diversity

Diversity gain is usually in the range of 3 to 5 dB

Diversity gain of a base station is Gd

Combiner Lossoccurs when a single antenna is

used to combine signals at different frequencies

and from many sources


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Filter lossaccounts for losses in the

filters used in the base station

Is usually 2 to 3 dB

Duplexer loss Ldis used to account

for the device which duplexes the up

and down links


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Loss Margins In link budget

Margins need to be provided for propagation losses in

the links (i.e, environmental loss effects)

Interference marginaccounts for interference during the

busy hour Due to traffic, frequency reuse etc

is usually about 1 dB

Losses in Buildingsoccurs due penetration into

buildings and is a function of the type of building material Is around 5 to 20 dB (quite heavy or severe)

Vehicle Penetration:Losses due to RF penetrating intocar body and structure

Is usually around 6 dB


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Power Balance Equations

Power balance equation Is a summary (sum) of the gains and losses in the

link Gains are positive, losses are negative

Use of decibels makes summing easy

Two power balance equations are used for uplink and downlink

Uplink Power Balance Equation

where

Lpu is the maximum allowable uplink path loss in dB

Lj

is the jumper loss in dB

bjddbmmpu SLLGGGPL


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Power Balance Equations (1)

Downlink Power balance equation

where

Lpd is the maximum allowable downlink path loss in dB Ltf is the BTS transmitter filter loss in dB P

b

is the setting of the power of the BTS in decibels

bbmtfjdbpd SGGLLLPL


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Summary of ProductParameters

Parameter Symbol Units Uplink Downlink

Frequency F MHz 1900 1900

MS TX Power Pm dBm 30

MS RX Power Sm dBm -101

MS antenna gain Gm dBi 20 2

MS feeder loss Lm dB 0 0BS Transmit power Pb dBm 47.0

BS RX sensitivity Sb dBm -107.0

BS Antenna gain Gb dBi 20 20

BS Diversity gain Gd dB 3.5

BS Duplexer loss Ld

dB 0.6 0.6

BS Jumper/connectorloss

Lj dB 0.9 0.9

BS TX filter loss Ltf dB 2.3

Product Path Loss Lp dB 161.0 166.2


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Example of Link Budget

Includes urban, suburban and rural

Only major losses of concern are included hereArea Classification

Urban Suburban Rural

Symbol Units

UL DL UL DL UL DL

Product path loss Lp dB 161.0 166.2 161.0 166.2 161.0 166.2BS antennaheight

Hb M 30 30 45 45 60 60

Feeder loss per m Lf/f dB/m

0.0646

0.0646

0.0646

0.0646

0.0477

0.0477

Total feeder loss Lf dB 1.938 1.938 2.907 2.907 2.862 2.862

Fade margin Mf dB 5.60 5.60 5.60 5.60 5.60 5.60

Body Attenuation Ab dB 2 2 2 2 2 2

Vehicleattenuation

Av dB 0 0 0 0 6 6

Buildingattenuation

Abld dB 15 15 12 12 0 0

Total Path Loss Lpt dB 136.5 141.7 138.5 143.7 144.5 149.8


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Receiver SensitivityNoise in a receiver and the networks on which it

operates affect how sensitive it can be.

Thermal noise is the major culprit.The sensitivity of the receiver is:

where C/N is carrier to noise ratio In sub-Saharan Africa, thermal noise is a major concern

Neither thermal nor carrier to noise ratio are within thecontrol of the engineer

The noise figure is however within her control and isnormally reduced to improve the sensitivity of the

receiver The receiver contributes a noise figure into the system

and can be improved with good system design

Initial stages of the receiver contribute most to its noisefigure

NCNNFS tRX /


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Noise Figure

The noise figure of the receiver is:

G1 and G2 are the gains of the early stages of

the receiver

For a cellular base station they are the low

noise amplifier (LNA) on the mast and the

first stages of the receiver electronics

LNA is usually deployed at the top of the mast

to provide a low noise figure first in the chain

of noise figures

The gains of LNAs are usually very low to

start the chain

211

2

1

131

GG

NF

G

NFNFNFtotal


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Noise Figure (1) MountingLNA

Where should the LNA be mounted to reduce noise figure?

Cabinet Mounting:

Place the LNA at the edge of the BTS cabinet.

Assume the lengths of the feeder is 50 metres

of type 7/8-in air dielectric coaxial cable

with 6.06 dB/100 metre loss.

Gain of the LNA is 12 dB and the

noise figure (NF) is 2 dB


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Cabinet / Tower Mounting

NF1 = 3.03 dB = 2 NF2 = 2 dB = 1.58 G1 = -3.03 dB = 0.5 The total noise figure: NFTotal = 10xlog[2 + ((1.58 - 1)/0.5)] =

4.99 dBTower Mounted LNA:

NF1 = 2 dB = 1.58

NF2 = 3.03 dB = 2

G1 = 12 dB = 15.85 The total noise figure is NFTotal = 10xlog[1.58 + ((2 - 1)/15.85)]

= 2.15 dB.

The tower mounted LNA has reduced the total noise figure

by 2.84 dB in this case.


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Tower Mounted LNA

The LNA provides the first noise figure in the chain

Exercise

If the antenna mast height becomes 80metres, by how much is

the total noise figure reduced by mounting the LNA at the

top of the mast?

BTS

LNA

NF2

NF1,

G1


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Diversity

Diversity techniques are used to tolerate weak

signals and reduce the effects of interference

Types of Diversity

time space

polarisation

frequency

Frequency diversity Frequency hoping

In GSM a user hops from one frequency to another every time

slot to avoid interference

Hops through a group of frequencies


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Frequency Diversity

Probability of fading occurring in all the sets

of frequencies is low

Is used to reduce Rayleigh fading

Clyclic hoping with a small set of frequenciesRandom hoping is preferred with a large set of

frequencies Is used to reduce co-channel interference

All traffic channels can hop but control channels cannot

Frequency hoping improves frequency reuse by an orderof 7 to 4 thereby increasing capacity


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Time Diversity

Is used by Rake receivers to provide gain

Signals separated in time to ensure they do not correlate

Separation should be more than coherence time

Polarisation Diversity

Orthogonal signals have very low correlation

Horizontal and vertically polarised signals are used in a two-branch

diversity


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Space or Receive Diversity

Popular in antenna systems for decorrelation of signals in

the antenna

Separation of 10 to 20 times the signal wavelength is used

in suburban areas

In receive diversity two or more antennas are used for

reception

Most cellular receivers use 2 antennas and onetransmission antennas (3 per sector)


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Diversity Processing

Diversity processing is mostly exploitation ofthe spatial dimension more effectively (eg.)

(i) Smart Antenna Technology

(ii) Transmit Diversity Schemes (iii) Spatial Multiplexing and

(iv) Space-Time Coding.


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Smart Antennas

Are used for beam focusing from basestations (or beam forming), for example:

adaptive phased array antennasSmart antennas can provide up to 4 times

improved system capacity


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mobile networks 7

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