Wireless Communication Fading Channel Overview


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EC744 Wireless Communication Fall 2008
Mohamed Essam Khedr
Department of Electronics and Communications
Wireless Communication Fading Channel Overview

Syllabus
• Tentatively

Week 1 Week 2 Week 3 Week 4
Week 5 Week 6 Week 7 Week 8 Week 9 Week 10 Week 11 Week 12 Week 13 Week 14 Week 15

Overview wireless communications, Probabilities Digital Communication fundamentals Channel characteristics (AWGN, fading) Modulation techniques Demodulation techniques (coherent and noncoherent) Source coding techniques Channel coding techniques Mid Term exam (take home), Diversity techniques Equalization techniques
Spread spectrum, MIMO and OFDM Wireless networking: 802.11, 802.16, UWB Hot topics Presentations Presentations Presentations Final Exam

Fading
Is due to multipath propagation.
With respect to a stationary base station, multipath propagation creates a stochastic standing wave pattern, through which the mobile station moves.
Caused by shadowing:
when the propagation environment is changing significantly, but this fading is typically much slower than the multipath fading.

Multipath Propagation - Fading

a
Diffracted wave

b
Reflected wave

No direct path

a b Antenna y = a + b
a & b are in phase

a b Antenna y = 0 a & b are out of phase by π

Complete fading when 2πd/λ = nπ, d is the path difference

Signal strength relative to 1uV (db)

• Slow (Long) Term Fading - Types
•Fast (Short) Term (Also known as Rayleigh fading) 30
Fast fading 20

10 0
0

Slow fading

5

10

15

20

25 Distance (λ)

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Fading - Slow (Long) Term
Slower variation in mean signal strength (distance 12 km)
Produced by movement over much longer distances Caused by:
Terrain configuration (hill, flat area etc.): Results in local mean (long term fading) attenuation and fluctuation.
The built environment (rural and urban areas etc.), between base station and the mobile unit: Results in local mean attenuation

Fading - Slow (Long) Term

Transmitter

τn,2
τn,1 LOS

τk,1

τk,2

Receiver

path n

τn,3

τ or d

τk,3 path k

θn

τk,4

one vmR(t)

subpath

d(t)
C. D. Charalambous et al

Fading- Fast (Short) Term
Describes the constant amplitude fluctuations in the received signal as the mobile moves. Caused by multipath reflection of transmitted signal by local scatters (houses, building etc.) Observed over distances = λ/2 Signal variation up to 30 dB. Is a frequency selective phenomenon. Can be described using Rayleigh statistics, (no line of sight). Can be described using Rician statistics, (line of sight). Causes random fluctuations in the received power, and also distorts the pulse carrying the information.

Fading- Fast (Short) Term - contd.
A received signal amplitude is given as the sum of delayed components. In terms of phasor notation it is given as:

N
Sr (t) = ai cos (2πfc + φi )
i =1

Or

N

N

Sr (t) = cos(2πfct) ai cos (φi ) − sin(2πfct) ai sin(φi )

i =1

i =1

In-phase

Quadrature

Fading- Fast (Short) Term - contd.
The phaseφi can be assumed to be uniformly distributed in the range (0, 2π), provided the locations of buildings etc. are completely random.
This for large N, the amplitude of the received signal is:

Sr (t) = X cos(2πfct) −Y sin(2πfct)

where

N

N

X = ai cos (φi ), Y = ai sin(φi )

i =1

i =1

X and Y are independent, identically distributed Gaussian random variables.

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Wireless Communication Fading Channel Overview