dept. of ee, ndhu 1 chapter five communication link analysis
TRANSCRIPT
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
–
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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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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
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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
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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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SNR
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ai
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i
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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
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20Dept. of EE, NDHU
Line Loss
• SNR degradation due to the signal attenuation
gL
LgLi
LigLiggout
TG
GT
WkTWkTG
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GNWGkTGNNWkTN
1
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00
21Dept. of EE, NDHU
Composite Noise Figure
• For the two-stages network
• For the n-stages network
1
21
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FFFcomp
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3
1
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ncomp GGG
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21
21
22Dept. of EE, NDHU
Composite Noise Temperature
• For the n-stages network
• The lossy line is in series with the amplifier
12121
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1
21
111
n
ncomp GGG
T
GG
T
G
TTT
RLcomp
comp
comp
LTTFLLT
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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