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EECS 142
Lecture 12: Noise in Communication Systems
Prof. Ali M. Niknejad
University of California, Berkeley
Copyrightc 2005 by Ali M. Niknejad
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 1/31 – p. 1/31Degradation of Link Quality
As we have seen, noise is an ever present part of all
systems. Any receiver must contend with noise.
In analog systems, noise deteriorates the quality of the
received signal, e.g. the appearance of “snow” on the
TV screen, or “static” sounds during an audio
transmission.
In digital communication systems, noise degrades the
throughput because it requires retransmission of data
packets or extra coding to recover the data in the
presence of errors.
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 2/31 – p. 2/31BER Plot
Bit Error Rate
1
0. 1
0.01
0.001
0.0001
0 10 20 30 40 50
SNR (dB)
It’s typical to plot the Bit-Error-Rate (BER) in a digital
communication system.
This shows the average rate of errors for a given
signal-to-noise-ratio (SNR)
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 3/31 – p. 3/31SNR
In general, then, we strive to maximize the signal to
noise ratio in a communication system. If we receive a
signal with average powerP , and the average noise
sig
power level isP , then theSNR is simply
noise
S
SNR =
N
P
sig
SNR(dB)=10·log
P
noise
We distinguish between random noise and “noise” due
to interferers or distortion generated by the amplifier
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 4/31 – p. 4/31Spurious Free Dynamic Range
S(ω)
fund
SFDR
SNR
spur
distortion
ω
The spurious free dynamic rangeSFDR measures the
available dynamic range of a signal at a particular point
in a system. For instance, in an amplifier the largest
signal determines the distortion “noise” floor and the
noise properties of the amplifier determine the “noise
floor”
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 5/31 – p. 5/31Noise Figure
The Noise Figure (NF) of an amplifier is a block (e.g.
an amplifier) is a measure of the degradation of the
SNR
SNR
i
F =
SNR
o
NF =10·log(F) (dB)
The noise figure is measured (or calculated) by
specifying a standard input noise level through the
source resistanceR and the temperature
s
For RF communication systems, this is usually specified
◦
asR =50Ω andT =293 K.
s
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 6/31 – p. 6/31Noise Figure of an Amplifier
Suppose an amplifier has a gainG and apply the
definition ofNF
P
sig
SNR =
i
N
s
GP
sig
SNR =
o
GN +N
s amp,o
The termN is the total output noise due to the
amp,o
amplifier in absence of any input noise.
P
sig
SNR =
o
N
amp,o
N +
s
G
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 7/31 – p. 7/31Input Referred Noise (I)
LetN denote the total input referred noise of the
amp,i
amplifier
P
sig
SNR =
o
N +N
s amp,i
The noise figure is therefore
SNR Pր N +N
sig s amp,i
i
F = = ×
SNR N Pր
o s sig
N
amp,i
F =1+ ≥1
N
s
All amplifiers have a noise figure≥1. Any real system
degrades theSNR since all circuit blocks add additional
noise.
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 8/31 – p. 8/31Input Referred Noise (II)
P +N
in s G
+
N
amp,i
The amount of noise added by the amplifier is
normalized to the incoming noiseN in the calculation
s
ofF. For RF systems, this is the noise of a 50Ω source
◦
at 293 K.
Since any amplification degrades theSNR, why do any
amplification at all? Because often the incoming signal
is too weak to be detected without amplification.
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 9/31 – p. 9/31Noise Figure of Cascaded Blocks
G G
2
1
P +N
in s
F F
1 2
G G
1 2
F
If two blocks are cascaded, we would like to derive the
noise figure of the total system.
Assume the blocks are impedance matched properly to
result in a gainG=G G . For each amplifier in
1 2
cascade, we have
N
amp,i
F =1+
i
N
s
A. M. Niknejad University of California, Berkeley EECS 142 Lecture 12 p. 10/31 – p
