ECEN 621-600
“Mobile Wireless Networking”
Course Materials: Papers, Reference Texts: Bertsekas/Gallager, Stuber, Stallings, etc
Grading (Tentative): HW: 20%, Projects: 40%, Exam-1:20%, Exam-II:20%
Lecture notes and Paper Reading Lists: available on-line
Class Website: http://ece.tamu.edu/~xizhang/ECEN621/start.php
Research Interests and Projects: URL:http://ece.tamu.edu/~xizhang
Instructor: Professor Xi Zhang
E-mail: xizhang@ece.tamu.edu
Office: WERC 331
ECEN 621 , Prof. Xi Zhang
Characterizations and Modeling
of the Wireless Channel
Lecture Notes 7.
ECEN 621 , Prof. Xi Zhang
Wireless channel disturbances
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Additive noise, like thermal background noise
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Multiplicative noise
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Distortion due to time dispersion
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Corruptive elements are in the forms of:
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Multipath delay spread
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Doppler spread due to motion
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Signal fading of frequency-selective and nonfrequency-selective variety
Prof. Xi Zhang
Multipath propagation environment
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Wireless propagation channel contains objects
(particles) randomly scattering the energy of
transmitted signals
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Scattered signals arrive at receiver out of step
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These objects (particles) are called scatterers
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Scatterers introduce:
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Fading
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Multipath delay spread
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Doppler spread
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Attenuation
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Multipath delay spread
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Scattering by randomly located scatterers gives rise to different
paths with different path-lengths/propagation-delays, resulting in
multipath delay spread
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If the propagation channel doesn’t exhibit multipath delay
spread, a point source (a single tone sinusoid) appears at front
end of receiver as another point source
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A multipath situation arises when a transmitted point source is
received as multipoint source, with each of individually received
points experiencing a different transmission delay
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The effect of multipath propagation on digital transmission can
be characterized by time dispersion and fading
Prof. Xi Zhang
Wireless channel time dispersion
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The transmitted point source will be received as a smeared wave
due to multipath delay spread
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Non-overlapping scatterers give rise to distinct multi paths –
characterized by their locations in scattering medium.
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All scatterers are located on ellipses with transmitter (Tx) and
receiver (Rx) as the foci. One ellipse is associated with one path
length/delay
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Signals reflected by scatterers located on the same ellipse experience
the same propagation delay and thus signal components from these
multi-paths are indistinguishable at the receiver
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Signals that are reflected by scatterers located on different ellipses
arrive a the receiver with different delays
Prof. Xi Zhang
Ellipsoidal portrayal of scatterer location
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Flat fading vs. frequency-selective fading and ISI
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If max difference in delay spread is small compared with symbol
duration of transmitted signal, channel is said to exhibit flat fading
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If difference in delay spread is large compared with the symbol duration
of transmitted signal, the channel exhibits frequency-selective fading
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In time domain, received signals corresponding to successive transmitted
symbols through frequency-selective fading channel will overlap, giving
rise to a phenomenon called inter-symbol interference (ISI)
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ISI is a signal-dependent distortion
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The severity of ISI increase with the width of delay spread.
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ISI distortion in time domain can also be examined in frequency domain
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ISI degrades transmission performance, which can be overcome by the
channel equalization techniques
Prof. Xi Zhang
Background noise and AWGN
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Inherent background noise can be approximated
as thermal noise and treated as Additive White
Gaussian (AWGN)
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Digital transmission over practical wireless
channels is mainly limited by interference or
distortion other than AWGN
Prof. Xi Zhang
Wireless channel fading
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The multipath components can affect the received signal
strength constructively or destructively, depending on
carrier frequency and delay differences among the multi
paths
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As a mobile station moves, the position of each scatterer
w.r.t. transmitter and receiver may change
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The overall effect caused by multipath delay spread,
Doppler spread, attenuation, thermal noise, etc. is that the
received signal level fluctuates with time, which is the
phenomenon called fading
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Line-of-Sight (LOS) vs. non-line-of-sight (NLOS)
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The delay of Line-of-Sight (LOS) or direct path is
the shortest path among the multi paths, having
smallest propagation delay (often assumed to be
zero); the delay of non-line-of-sight (NLOS) or
reflected path has longer propagation delay.
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Multipath propagation & LOS
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An example of two-path channel
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Consider transmitting a single-tone sinusoid signal:
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A wireless channel model with two propagation paths
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Amplitude fluctuation of the two-path channel
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Wireless channel fading analysis
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When mobile station moves, alpha_1, alpha_2, and tau change with time
and thus received signal amplitude and phase also change with time.
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Assuming alpha_1 = 2 and alpha_2 = 1
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Assuming alpha_1 = 1.1 and alpha_2 = 1.0, resulting in deeper fading
Prof. Xi Zhang
Effects of channel fading
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When signal components from two paths add destructively, transmitted signal
experiences deep fading with a small value of the amplitude alpha
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During each deep fading, the instantaneously received signal power is very low, resulting
in poor transmission quality (high transmission error rate)
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Diversity and error-correction coding are effective to combat channel fading for better
transmission accuracy
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Channel fading is classified long-term fading or short-term fading:
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Short-term fading is rapid fluctuations caused by the local multipath (e.g., Rayleigh
fading)
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Long-term fading is long-term slow variation in the mean level of received signal
strength (e.g., Lognormal fading) caused by movement over large enough distance
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Multipath propagation in wireless mobile environment yields fading dispersive channel
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Signal propagation environment changes as the mobile station moves and /or as any
surrounding scatterers move  the wireless channel is time-varying and can be
modeled as a linear time-variant (LTV) system
Prof. Xi Zhang
Linear time-variant (LTV) channel model
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Input/output model of Wireless Channel
Linear time-invariant (LTI) channel model —Review
“Channel impulse response”
Prof. Xi Zhang
Input/output model of Wireless Channel
Linear time-variant (LTV) channel
Prof. Xi Zhang