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Wireless Fundamentals
Peter Wang
January 16,2011
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OUTLINE
Wireless channels
Far-field average power
Long-term fading channel
Short-term fading channel
Signal transmission through a system
Nonlinear system
Linear system
Linear time invariant Linear time variant
Fading mitigation techniques
Six ways to mitigate fading
Diversity combining methods
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Wireless channels
Wireless -> Mobility -> Channel complexity
The radiating electromagnetic field is reflected, diffracted,and scattered by various obstacles
The received signal has a spatially and temporally varyingenergy pattern (fading) due to the combined interference
Three phenomena together effects the received power
Far-field average power (i.e., the area-mean power)
Long-term fading due to the shadowing
Short-term fading due to multipath and mobility
The statistically varying received signal power can bemodeled as:
RTTX
R GGPdgP )(
10 10/2
fadingshadowisX 10/10
fadingmultipathis2 pathlosisdg )(
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Long-term & short-term fading regions
Long-term fading region Short-term fading region
Scatterer
Diffrector
Reflector
Ground
Semi-transmissionBuilding
RTTX
R GGPdgP )(
10 10/2
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Long-term fading
Local mean power
Path Loss
For Free-space propagation n = 2 For two ray propagation n=4
Long-term fading (i.e., Lognormal shadowing) Slowly varying fading (~20 wavelength in distance)
Lognormal distribution
RTTX
meanlocalR GGPdgP )(
10 10/_
ndkdg )()(
th
rh 1d 2d
r
d
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Short-term fading due to multipath (1/2)
Flat Fading (e.g., Rayleigh or Ricean distribution)
Frequency-Selected Fading
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Short-term fading due to multipath (2/2)
Flat Fading Example
Frequency Selective Fading Example
Transmitted Signal Received Signal
Transmitted Signal Received Signal
m
cT
f1
)(
Coherence Bandwidth Delay Spread
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Short-term fading due to mobility
Slow Fading
Fast Fading
velocity
velocity
d
cB
t1
)(
Coherence Time Doppler Spread
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Power density
Frequency (Hz)
Time delay (us)
1
4
3
5
2
0
5us
4us
3 us
1 us
2 us
20 40-20 60
80 Hz-80Hz
Example of short-term fading channel
Fast channel varying (~0.5 wavelength in distance)
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Fading channel measurement
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Types of received power variations
Wireless Communication, Andreas F. Molisch, IEEE Press, John Wiley
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Example of received power measurement
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Summary of propagation channels
Propagation effects
Large-scale/Long-term fading Small-scale/Short-term fading
Shadowing
variation (~20 )
Multipath delay spread Doppler spread
Rayleigh/Ricean
distribution
Log-normaldistribution
Suzukidistribution
Flat fading Frequencyselective
fading
Slow fading
Pathloss
Fast fading
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Signal transmission through a system (1/3)
Nonlinear Systems
A memoryless nonlinear case:
If g(t) bandwidth is B(Hz), then gk(t) bandwidth is kB(Hz)
If a signal is transmitted over a nonlinear channel (system), the
nonlinearity
Distorts the signal
Causes interference with other signals in the channel because of its
spectral dispersion (spreading)
)()()()( 2210 tgatgatgaatyk
k
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Signal transmission through a system (2/3)
Linear Systems
Distortionless System The input and the output have identical waveshapes within a multiplicative constant. A
delay output that retains the input waveform is also considered distortionless
For distortionless transmission condition
is linear with respect to
systemLTIttxkty d)()(
dtfjefXkfY2
)()(
dh tfffH 2)()(
kfH )(
dh tfdf
d2)(
)(fH
f
)(fh
k
)(fh f
th
rh
r
d
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Signal transmission through a system (3/3)
Linear Distortion Systems
Signal distortion caused by non-ideal channel characteristics ofmagnitude distortion, phase distortion, or both
Linear channel distortion (time dispersion) introduce theintersymbol interference (ISI)
Linear time-invariant distortion caused by multipath effect
Linear time-variant distortion caused by mobility + multipath
th
rh 1d 2d
r
d
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Fading mitigation techniques (1/2)
Various techniques to overcome the combined effects of
fading, noise, and signal interference Interleaving: reduce the effect of fast fading and possible bursts
and noise
OFDM: transmit sequences of digital signals in parallel reducing
their BW requirements. Frequency-selective-fading becomes flat-fading for each signal BW reducing ISI
Channel coding: detect or correct digital-signal errors
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Fading mitigation techniques (2/2)
Another three techniques to mitigate the effect of fading on
signal Equalizer: Overcome ISI (used in GSM system)
Diversity techniques: Overcome short-term fading
Space, Frequency, Time, Angle, Polarization, Multipath diversities
RAKE receiver: overcome multipath fading (used in CDMA system)
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Diversity Combining Methods
Receive diversity (Linear combining)
Selection combining (simple implementation)
Equal-gain combining
Maximum-ratio combining (optimum combining)
Transmit diversity Space-time block code
MIMO reception
Trade off between diversity and multiplexing schemes
Diversity scheme improves SINR
Multiplexing scheme improves transmission bit rate
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Summary Wireless channels
Far-field average power(A general equation) Long-term fading channel (Large-scale fading, Log-normal shadowing)
Short-term fading channel (Caused by Delay spread & Doppler pread)
Signal transmission through a system
Nonlinear system (Spectral Dispersion)
Linear system (Time Dispersion)
Time invariant (Due to multipath)
Time variant (Due to multipath & mobility)
Fading mitigation techniques
Six methods to mitigate fading
Diversity combining methods (e.g., Maximum Ratio Combining, Space-
Time Block Code)
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Reference
Reference website:
http://www-ee.uta.edu/Online/PWang/ee5368/index.htm )
Reference books:
(1) Mobile Wireless Communications, by M. Schwartz.
(2) Modern Digital & Analog Communication Systems, by B. P. Lathi and Zhi Ding.
(3) Wireless Communications, by A. F. Molisch.
http://www-ee.uta.edu/Online/PWang/ee5368/index.htmhttp://www-ee.uta.edu/Online/PWang/ee5368/index.htmhttp://www-ee.uta.edu/Online/PWang/ee5368/index.htmhttp://www-ee.uta.edu/Online/PWang/ee5368/index.htm7/28/2019 Copie de wireless.pdf
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LTE GSM IS-95 IS-136 AMPS
Analog or
Digital
System
Digital Digital Digital Digital Analog
Multiple
Access
Method
OFDMA/
SC-FDMA
FD/TDMA FD/CDMA FD/TDMA FDMA
FDD or TDDMode
FDDTDD
FDD FDD FDD FDD
BW/Radio
Channel
Flexible
(Up to
40 MHz)
200 KHz 1.25 MHz 30 KHz 30 KHz
Voice
annels/Radio
Channel
Flexible 8 Users (~20
Users)
3 Users 1 Users
Modulation
Seme
PSK, QAM MSK PSK FM
Hard or Soft
Handoff
Hard Hard Soft Hard Hard
DQPSK4/
Cellular System Evolutions