dept. of ee, ndhu 1 chapter five communication link analysis

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

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antenna receiving theof area effective is where4

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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

–

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ee A

AG

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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

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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

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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)(/

/

)(

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N

N

S

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N

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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(

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R

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source theof re temperatu theis where

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g

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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

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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