EE445S Real-Time Digital Signal Processing Lab
Spring 2014
Channel Impairments
Prof. Brian L. Evans
Dept. of Electrical and Computer Engineering
The University of Texas at Austin
Lecture 12
http://courses.utexas.edu/
Outline
• Analog communication systems
• Channel impairments
• Hybrid communication systems
• Analog pulse amplitude modulation
12 - 2
Communication Systems
Communication System Structure
• Information sources
Voice, music, images, video, and data (baseband signals)
• Transmitter
Signal processing block lowpass filters message signal
Carrier circuits block upconverts baseband signal and bandpass
filters to enforce transmission band
baseband
m(t)
baseband
Signal
Processing
bandpass
Carrier
Circuits
TRANSMITTER
bandpass
Transmission
Medium
s(t)
CHANNEL
baseband
Carrier
Circuits
r(t)
baseband
Signal
Processing
RECEIVER
12 - 3
mˆ (t )
Review
Communication Channel
• Transmission medium
Wireline (twisted pair, coaxial, fiber optics)
Wireless (indoor/air, outdoor/air, underwater, space)
• Propagating signals degrade over distance
• Repeaters can strengthen signal and reduce noise
baseband
m(t)
baseband
Signal
Processing
bandpass
Carrier
Circuits
TRANSMITTER
bandpass
Transmission
Medium
s(t)
CHANNEL
baseband
Carrier
Circuits
r(t)
baseband
Signal
Processing
RECEIVER
12 - 4
mˆ (t )
Wireline Channel Impairments
• Linear time-invariant effects
Attenuation: dependent on channel frequency response
Spreading: finite extent of each transmitted pulse increases
y0 (t )
x0 (t )
input
Tb
t
x(t)
-A
x1 (t )
A
Tb
t
Bit of ‘0’ or ‘1’
output
Communication
Channel
Model channel as
LTI system with
impulse response
h(t)
Th
y(t)
Assume that Th < Tb
t
-A Th
y1 (t )
h(t )
A Th
1
Th
Th+Tb
t
Th
Th+Tb
12 - 5
t
Wireline Channel Impairments
• Linear time-varying effects
Phase jitter: sinusoid at same fixed frequency experiences
different phase shifts when passing through channel
Visualize phase jitter in periodic waveform by plotting it over
one period, superimposing second period on the first, etc.
• Nonlinear effects
Harmonics: due to quantization, voltage rectifiers, squaring
devices, power amplifiers, etc.
Additive noise: arises from many sources in transmitter,
channel, and receiver (e.g. thermal noise)
Additive interference: arises from other systems operating in
transmission band (e.g. microwave oven in 2.4 GHz band)
12 - 6
Home Power Line Noise/Interference
Power Spectral Density Estimate
-75
Power/frequency (dB/Hz)
-80
-85
-90
-95
-100
-105
-110
-115
-120
-125
0
10
20
30
40
50
60
70
80
90
Frequency (kHz)
Measurement taken on a wall power plug in an
apartment in Austin, Texas, on March 20, 2011
12 - 7
Home Power Line Noise/Interference
Power Spectral Density Estimate
-75
Power/frequency (dB/Hz)
-80
-85
-90
-95
-100
-105
-110
-115
Spectrally-Shaped
Background Noise
-120
-125
0
10
20
30
40
50
60
70
80
90
Frequency (kHz)
Measurement taken on a wall power plug in an
apartment in Austin, Texas, on March 20, 2011
12 - 8
Home Power Line Noise/Interference
Power Spectral Density Estimate
-75
Power/frequency (dB/Hz)
-80
Narrowband Interference
-85
-90
-95
-100
-105
-110
-115
Spectrally-Shaped
Background Noise
-120
-125
0
10
20
30
40
50
60
70
80
90
Frequency (kHz)
Measurement taken on a wall power plug in an
apartment in Austin, Texas, on March 20, 2011
12 - 9
Home Power Line Noise/Interference
Power Spectral Density Estimate
Periodic and
Asynchronous
Interference
-75
Power/frequency (dB/Hz)
-80
Narrowband Interference
-85
-90
-95
-100
-105
-110
-115
Spectrally-Shaped
Background Noise
-120
-125
0
10
20
30
40
50
60
70
80
90
Frequency (kHz)
Measurement taken on a wall power plug in an
apartment in Austin, Texas, on March 20, 2011
12 - 10
Wireless Channel Impairments
• Same as wireline channel impairments plus others
• Fading: multiplicative noise
Talking on a mobile phone and reception fades in and out
Represented as time-varying gain that follows a particular
probability distribution
• Simplified channel model for fading, LTI effects
and additive noise
a0
FIR
+
noise
12 - 11
Hybrid Communication Systems
• Mixed analog and digital signal processing in the
transmitter and receiver
Example: message signal is digital but broadcast over an
analog channel (compressed speech in digital cell phones)
• Signal processing in the transmitter
m(t)
Error
Correcting
Codes
A/D
Converter
Digital
Signaling
• Signal processing in the receiver
A/D
Equalizer
Detection
digital
sequence
baseband signal
Decoder
digital
sequence
D/A
Converter
Waveform
Generator
D/A
code
12 - 12
Optional
Pulse Amplitude Modulation (PAM)
• Amplitude of periodic pulse train is varied with a
sampled message signal m(t)
Digital PAM: coded pulses of the sampled and quantized
message signal are transmitted (lectures 13 and 14)
Analog PAM: periodic pulse train with period Ts is the carrier
(below)
m(t)
s(t) = p(t) m(t)
p(t)
t
T
Ts T+Ts 2Ts
12 - 13
Optional
Analog PAM
• Pulse amplitude varied
with amplitude of
sampled message
• Transmitted
signal

s (t ) 
n  
m(t)
m(Ts)
t
t
T
Ts T+Ts
hold
for 0  t  T
 1

h(t )  1 / 2 for t  0, t  T
 0
otherwise

1
T
n) h(t  Ts n)
h(t) is a rectangular pulse
of duration T units
s(t)
m(0)
s
sample
Sample message every Ts
Hold sample for T seconds
(T < Ts)
Bandwidth  1/T
h(t)
 m(T
2Ts
As T  0,
1
h(t )   (t )
T
12 - 14
Optional
Analog PAM
• Transmitted signal
s (t ) 
 m(T n) h(t  T n)



s
n  
 m(T
s
n  

s
n)  (t  Ts n) * h(t ) 


   m(Ts n)  (t  Ts n) * h(t )
n  

msampled(t)
• Fourier transform
S( f ) 

M sampled ( f ) H ( f )

fs
M( f  f
s
k) H ( f )
• Equalization of sample
and hold distortion
added in transmitter
H(f) causes amplitude
distortion and delay of T/2
Equalize amplitude
distortion by post-filtering
with magnitude response
1
1
 f


H ( f ) T sinc ( f T ) sin ( f T )
k  
H ( f )  T sinc ( f T ) e j 2 
 T sinc ( f T ) e j 
f T /2
fT
Negligible distortion
(less than 0.5%) if
T
 0 .1
Ts
12 - 15
Optional
Analog PAM
• Requires transmitted pulses to
Not be significantly corrupted in amplitude
Experience roughly uniform delay
• Useful in time-division multiplexing
public switched telephone network T1 (E1) line
time-division multiplexes 24 (32) voice channels
Bit rate of 1.544 (2.048) Mbps for duty cycle < 10%
• Other analog pulse modulation methods
Pulse-duration modulation (PDM),
a.k.a. pulse width modulation (PWM)
Pulse-position modulation (PPM): used
in some optical pulse modulation systems.
12 - 16