GSM LINK BUDGET

Naveen Jakhar, ITS

Content

What is a link budget?

Objectives

Commonly used terms

Calculations

Link Budget

It is a way of quantifying the link performance

It includes analysis of all losses and gains in a

transmission system ( from the transmitter

through the medium to the receiver)

why link budget ?

Link budget is calculated in design of mobile

communication system for the proper call

establishment.

Quality of call establishment is based on received

signal strength. The received signal strength

depends on the path loss and the parameters of the

transmitter and receiver. Signal strength also varies

based on the environment and the intermediate

losses

OBJECTIVES

to estimate the maximum allowable path loss

to compute the required EIRP for a balanced path

to estimate the coverage design threshold

to evaluate technology performance

Parameters

The link budget includes the following key parameters.

Transmitted power

Antenna gains (both transmitter antenna gain and

receiver antenna gain)

Antenna feeder losses

Path loss

Receiver sensitivity

ERP vs EiRP

ERP (Effective Radiated Power)

the Radiated power (transmit power times antenna

gain) with respect to a dipole antenna

EiRP (Effective isotropic Radiated Power)

the radiated power from an isotropic antenna

EiRP = ERP + 2.15 (dB)

Antenna Gain

A relative measure of an antenna's ability to direct or concentrate radio frequency energy in a particular direction or pattern.

describes how much power is transmitted in the direction of peak radiation to that of an isotropic source

A transmitting antenna with a gain of 3 dB means that the power received far from the antenna will be 3 dB higher (twice as much) than what would be received from a lossless isotropic antenna with the same input power.

Units for Antenna Gain

dB - decibels, ratio of power levels. 10 dB means 10 times the energy

relative to an isotropic antenna in the peak direction of radiation

dBi - "decibels relative to an isotropic antenna". This is the same as dB . 3

dBi means twice (2x) the power relative to an isotropic antenna in the peak

direction

dBd - "decibels relative to a dipole antenna.

A half-wavelength dipole antenna has a gain of 2.15 dBi. Hence, 7.85 dBd

means the peak gain is 7.85 dB higher than a dipole antenna; this is 10 dB

higher than an isotropic antenna.

gain in dBd = gain in dBi − 2.15 d

Diversity Gain

Diversity gain is the increase in signal-to-interference ratio due to some diversity scheme

A diversity scheme refers to a method for improving the reliability of a message signal by using two or more communication channels with different characteristics. Diversity is mainly used in radio communication and is a common technique for combating fading and co-channel interference and avoiding error bursts. It is based on the fact that individual channels experience different levels of fading and interference

Rx Sensitivity

Receiver sensitivity is a measure of how well the

receiver performs and is defined as the power of

the weakest signal the receiver can detect

Path Loss

The path loss is the unwanted reduction in the signal strength during the propagation of the transmitted signal from the transmitter to the receiver.

It may be due to many effects, such as free-space loss, refraction, diffraction, reflection,aperture-medium coupling loss, and absorption.

It is also influenced by terrain contours, environment (urban or rural, vegetation and foliage), propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas

HATA model is the most widely used radio frequency propagation model for predicting the behaviour of cellular transmission.

Feeder loss

ALL feeders have loss

the longer the feeder the greater the loss

Body Loss

Body loss indicates the loss generated due to signal

blocking and absorption when a terminal antenna is

close to the body. This affects handsets in particular.

An Node B antenna is mounted at a height of tens of

meters, in which case body loss can be ignored as the

body loss value is 0 dB. For mobile terminals, body

loss must be considered and in this instance, the body

loss is about 3 dB.

Penetration loss

an extra signal attenuation associated to building

penetration

Fade margin

Fade Margin is an expression for how much margin - in dB - there is between the received signal strength level and the receiver sensitivity of the radio.

A design allowance that provides for sufficient system gain or sensitivity to accommodate expected fading, for the purpose of ensuring that the required quality of service is maintained.

The amount by which a received signal level may be reduced without causing system performance to fall below a specified threshold value.

Link Budget Element of a GSM Network

BTS Antenna GainMax. Path Loss

(PLmax)Fade Margin

LNA

(optional)

Feeder Loss

Diversity

Gain

BTS Receiver

Sensitivity

ACE

Loss

BTS Transmit

Power

Penetration Loss

MS Antenna Gain,

Body and Cable

Loss

Mobile

Transmit Power

Mobile Receiver

Sensitivity

Link Budget Equation

In order to formulate a link budget equation, it is required to look into all the

areas where gains and losses may occur between the transmitter and the

receiver.

PRX = PTX + GTX + GRX - LTX - LFS - LFM – LRX

PRX= received power (dBm)

PTX= transmitter output power (dBm)

GTX = transmitter antenna gain (dBi)

GRX= receiver antenna gain (dBi)

LTX = transmitter feeder and connector losses (dB)

LFS = free space loss or path loss (dB)

LFM = many-sided signal propagation losses (dB)

LRX = receiver feeder connector losses (dB)

Link Budget calculation

Link budget calculation involves both uplink and

downlink calculation.

In uplink, the signal transmission is from MS to the

BTS. Here MS acts as a transmitter and the BTS acts

as receiver.

In downlink, the signal transmission is from the BTS

to the MS. Here BTS acts as a transmitter and the

MS acts as a receiver.

z

RF LINK BUDGET UL DL TRANSMITTING END MS BTS

Tx RF Output 33 dBm 43 dBm

Body Loss -2.0dB 0dB

Combiner Loss 0dB 0dB

Feeder Loss (@2dB/100m) 0dB 1.5dB

Connector Losses 0dB 2dB

Tx Antenna Gain 0dB 17.5dB

EIRP 31dBm (A)

57dBm ( C)

RECEIVING END BTS MS

Rx sensitivity -107 dBm

-102 dBm

Rx. Antenna gain 17.5dB 0dB

Diversity Gain 3dB 0dB

Connector Loss 2dB 0dB

Feeder Loss 1.5dB 0dB

Interference Degradation Margin 3dB 3dB

Body Loss 0dB 3dB

Duplexer Loss 0dB 0dB

Rx Power -121dBm -96dBm

Fade Margin 4dB 4dB

Required Isotropic Rx .Power -117dBm (B)

-92dBm ( D)

Maximum Permissible Path 148dB 149dB

RF LINK BUDGET

21

Types of Coverage

Indoor Coverage : RSS ≥ -65 dBm

In-Car Coverage : RSS ≥ -75 dBm

Outdoor Coverage: RSS ≥ -85 dBm

Radio coverage requirements

Indoor Coverage: Signal level measured at street level shall be better than–65dBm.

In car coverage: Signal level measured at street level shall be better than–75 dBm.

Outdoor Coverage: At least 95% of the remaining coverage area. Signallevel measured at street level shall be better than –85 dBm.

The indoor coverage shall be provided in all the commercial areas, Multi-storied housing complexes, star hotels, industrial areas, software/Hardwaretechnology parks, airports, railway stations etc.

The coverage shall be provided in all the residential areas, tourist spots,roads, lanes, high ways, bypasses and rail routes.

The remaining areas shall be provided with out-door coverage.

THANK YOU