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1 Wireless fundamentals_04022010

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



Short-term fading due to mobility

Slow Fading

Fast Fading

velocity

velocity

d

cB

t1

)(

Coherence Time Doppler Spread



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)



Fading channel measurement



Types of received power variations

Wireless Communication, Andreas F. Molisch, IEEE Press, John Wiley



Example of received power measurement



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



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




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




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.htm


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22 Wirelessfundamentals 04022010

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

Documents

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