1Dept. of EE, NDHU

Chapter Five

Communication Link Analysis

2Dept. of EE, NDHU

Introduction

• Communication link encompasses the path from the information source to the

information sink

– Through all the encoding, modulation, the transmitter, the channel and the receiver

with all the signal processing

• Link budget

– The result of the communication link

– Describes the apportionment of transmission and reception resources, noise sources,

signal attenuation

– Help one learns if the system will meet many of its requirements

• Link analysis is to determine the actually system operating point in the BER

curve

3Dept. of EE, NDHU

The Channel

• The free space

– Free of all hindrances of RF propagation, such as absorption,

reflection, refraction or diffraction

– RF energy arriving at the receiver is assumed to be a function only of

distance from the transmitter

• Error performance degradation

– SNR degrades through the decrease of the desired signal power or

through the increase of noise power

– Not considered ISI in the link budget

4Dept. of EE, NDHU

Sources of Signal Loss and Noise

• Transmitting terminal

– Bandlimitting loss

– Modulation loss

– Antenna efficiency

• Channel

– Pointing loss

– Atmospheric loss and noise

• Receiving terminal

– Antenna efficiency

– Receiver loss

5Dept. of EE, NDHU

Satellite Transmitter-to-Receiver Link with Loss and Noise

6Dept. of EE, NDHU

Received Signal Power

• The range equation

– Relate power received to the distance between the transmitter and the receiver

– Transmitted power density

– Power extracted at a receiving antenna

• Effective radiated power to an isotropic radiator (EIRP)

– The product of the transmitted power and the gain of the transmitting antenna gain

–

22

s/ watt4

)( md

Pdp t

antenna receiving theof area effective is where4

)(2

er

erterr

Ad

APAdpP

ttGPEIRP

7Dept. of EE, NDHU

Isotropic Radiator

8Dept. of EE, NDHU

Antenna Gain

9Dept. of EE, NDHU

EIRP

10Dept. of EE, NDHU

EIRP with Range Equation

• The relationship between antenna gain and antenna effective area Ae

• Power extracted at a receiving antenna

• Received power with EIRP representation

• Received signal power is as a function of frequency

–

)(for 4 2

2

ee A

AG

24 d

AEIRPP er

r

s

rrr L

GEIRP

d

GEIRPP

2

2

)4(

2

2

2

2

)4(

)/(

)4( d

fcGGP

d

GGPP rttrtt

r

11Dept. of EE, NDHU

Received Power as a Function of Frequency

12Dept. of EE, NDHU

Example for Antenna Design for Measuring Path Loss

13Dept. of EE, NDHU

Thermal Noise Power

• Thermal noise is modeled as an AWGN process in communication systems

• Physical model for thermal noise maximum available thermal noise power

hertzBandwidth

kelvin e,temperatur

/1038.1constant sBoltzmann' where

)2/4(

isinput amplifier the todeliveredpower noise the

23

2

W

T

HzKWk

WkTR

WRkTNi

14Dept. of EE, NDHU

Link Budget Analysis

• The quantity of greatest interest is the SNR for the receiving-system

• SNR is sometimes called carrier-to-noise ratio (C/N)

• Pr/N representation

• Two Eb/N0 values of interest

– Required Eb/N0

– Received Eb/N0

– Link margin M

ndegradatio and lossesother allrepresent tois and

merit-of-figurereceiver thecalled sometimes is / where

/

/

0

000

L

TG

RN

E

LkL

TGEIRP

N

P

L

NGEIRP

N

P

r

b

s

rr

s

rr

0000 )/(

/)()()()(

LRkLNE

TGEIRPdB

N

EdB

N

EM

sreqdb

rreqd

br

b

15Dept. of EE, NDHU

Link Margin Design

• The margin needed depends on how much confidence one has in each of the link

budget entries

• Sometimes the link budget provides an allowance for fades due to weather direct

ly

• Examples of the link margin

– Satellite communication at C-band (uplink at 6 GHz, downlink at 4 GHz) 1 dB link

margin

– Satellite telephone system (INTELSAT system) 4 to 5 dB

– Designs using higher frequency (14/12 GHz) generally call for larger margins

• The margin will be positive if “the link can be closed”

16Dept. of EE, NDHU

Earth Coverage Versus Link Margin

17Dept. of EE, NDHU

Noise Figure

• Noise figure, F, denotes the degradation caused by the network

• Example for an amplifier

NetworkinSNR)( outSNR)(

out

inSNR

SNRF

)(

)(

18Dept. of EE, NDHU

Noise Treatment in Amplifiers

• The noise figure, F, can be rewritten as

gain.amplifier and

port,input the toreferred noiseamplifier

port,input amplifier at thepower noise

port,input amplifier at thepower signal

where

1)(/

/

)(

)(

G

N

N

S

N

N

NNGGS

NS

SNR

SNRF

ai

i

i

i

ai

aiii

ii

out

in

19Dept. of EE, NDHU

Noise Temperature

• The noise power is relative to the noise temperature

• The effective noise temperature of the receiver

• Output noise of an amplifier

WkTN

K 290)1(

K 290

)1(

)1(

FT

T

WkTFWkT

NFN

R

R

iai

source theof re temperatu theis where

)1( 0

g

g

aiiout

T

WGkTFWGkT

GNGNN

20Dept. of EE, NDHU

Line Loss

• SNR degradation due to the signal attenuation

gL

LgLi

LigLiggout

TG

GT

WkTWkTG

GN

GNWGkTGNNWkTN

1

1

00

21Dept. of EE, NDHU

Composite Noise Figure

• For the two-stages network

• For the n-stages network

1

21

1

G

FFFcomp

12121

3

1

21

111

n

ncomp GGG

F

GG

F

G

FFF

LFFLLL

FFF

FFLNS

L

NSF

comp

i

i

)1(/1

1

,/

1/

21

21

22Dept. of EE, NDHU

Composite Noise Temperature

• For the n-stages network

• The lossy line is in series with the amplifier

12121

3

1

21

111

n

ncomp GGG

T

GG

T

G

TTT

RLcomp

comp

comp

LTTFLLT

LFT

LFFLLF

K 290))1()1((

K 290)1(

)1(

23Dept. of EE, NDHU

System Effective Temperature

compAs TTT

24Dept. of EE, NDHU

Improve a Receiver Front-end

25Dept. of EE, NDHU

Key Parameters of a Link Analysis