Lecturing Notes 7

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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: [email protected]
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

Additive noise, like thermal background noise

Multiplicative noise

Distortion due to time dispersion

Corruptive elements are in the forms of:

Multipath delay spread

Doppler spread due to motion

Signal fading of frequency-selective and nonfrequency-selective variety
Prof. Xi Zhang
Multipath propagation environment

Wireless propagation channel contains objects
(particles) randomly scattering the energy of
transmitted signals

Scattered signals arrive at receiver out of step

These objects (particles) are called scatterers

Scatterers introduce:

Fading

Multipath delay spread

Doppler spread

Attenuation
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Multipath delay spread

Scattering by randomly located scatterers gives rise to different
paths with different path-lengths/propagation-delays, resulting in
multipath delay spread

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

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

The effect of multipath propagation on digital transmission can
be characterized by time dispersion and fading
Prof. Xi Zhang
Wireless channel time dispersion

The transmitted point source will be received as a smeared wave
due to multipath delay spread

Non-overlapping scatterers give rise to distinct multi paths –
characterized by their locations in scattering medium.

All scatterers are located on ellipses with transmitter (Tx) and
receiver (Rx) as the foci. One ellipse is associated with one path
length/delay

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

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
Prof. Xi Zhang
Flat fading vs. frequency-selective fading and ISI

If max difference in delay spread is small compared with symbol
duration of transmitted signal, channel is said to exhibit flat fading

If difference in delay spread is large compared with the symbol duration
of transmitted signal, the channel exhibits frequency-selective fading

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)

ISI is a signal-dependent distortion

The severity of ISI increase with the width of delay spread.

ISI distortion in time domain can also be examined in frequency domain

ISI degrades transmission performance, which can be overcome by the
channel equalization techniques
Prof. Xi Zhang
Background noise and AWGN

Inherent background noise can be approximated
as thermal noise and treated as Additive White
Gaussian (AWGN)

Digital transmission over practical wireless
channels is mainly limited by interference or
distortion other than AWGN
Prof. Xi Zhang
Wireless channel fading

The multipath components can affect the received signal
strength constructively or destructively, depending on
carrier frequency and delay differences among the multi
paths

As a mobile station moves, the position of each scatterer
w.r.t. transmitter and receiver may change

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
Prof. Xi Zhang
Line-of-Sight (LOS) vs. non-line-of-sight (NLOS)

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.
Prof. Xi Zhang
Multipath propagation & LOS
Prof. Xi Zhang
An example of two-path channel

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

When mobile station moves, alpha_1, alpha_2, and tau change with time
and thus received signal amplitude and phase also change with time.

Assuming alpha_1 = 2 and alpha_2 = 1

Assuming alpha_1 = 1.1 and alpha_2 = 1.0, resulting in deeper fading
Prof. Xi Zhang
Effects of channel fading

When signal components from two paths add destructively, transmitted signal
experiences deep fading with a small value of the amplitude alpha

During each deep fading, the instantaneously received signal power is very low, resulting
in poor transmission quality (high transmission error rate)

Diversity and error-correction coding are effective to combat channel fading for better
transmission accuracy

Channel fading is classified long-term fading or short-term fading:

Short-term fading is rapid fluctuations caused by the local multipath (e.g., Rayleigh
fading)

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

Multipath propagation in wireless mobile environment yields fading dispersive channel

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
Prof. Xi Zhang
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
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