How to combat fading in wireless channels?

1. J.U., 13th April, 2007
How to Combat Fading in
Wireless Channels?
Aniruddha Chandra
ECE Department, NIT Durgapur
aniruddha.chandra@ieee.org
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A. Chandra, NIT Durgapur – How to combat fading?

2. J.U., 13th April, 2007
Outline
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
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A. Chandra, NIT Durgapur – How to combat fading?

3. J.U., 13th April, 2007
Fading in Wireless Channels
What is Fading?
Fading Mechanisms
Degradation due to Fading
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
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4. J.U., 13th April, 2007
Why Wireless?
•Mobility Anytime, Anywhere connectivity Hi! I’m on
the prowl
Phone for people not for places
•Easy Installation Rapid deployment, reconfigurable, no cable,
easy maintenance
•Digital Companion Voice, message, internet, multimedia
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A. Chandra, NIT Durgapur – How to combat fading?

5. J.U., 13th April, 2007
Fading???
•Fading over time
•Fading over distance Lunch
break, at
Enemy last!
Oh No ..
Enemy ? Emily!
ahead Boss’s
wife
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A. Chandra, NIT Durgapur – How to combat fading?

6. J.U., 13th April, 2007
Radio Wave Propagation
Buildings
Amplitude
STOP
Transmitter Fading
Earth surface Receiver Envelope
Line of Sight
Reflection
Diffraction Street Sign
Scattering Distance
•Multiple replica of signal combines with random phase resulting in random
amplitude attenuation and phase variation  This is Fading
v
•Relative velocity causes further change with time. f D = cos θ
λ
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A. Chandra, NIT Durgapur – How to combat fading?

7. J.U., 13th April, 2007
Wireless Channel
•Limited Power
Size, Weight, Battery Constraints
•Limited BW
Spectrum allocation
•Path Loss
up to 10 dB/km
•Multipath Fading Multipath
Resolvable channel induced ISI
Non-resolvable  random amplitude variation
t0
•Time Variance of the Channel
Time Variance
t0+τ1
Due to relative velocity
Introduces Doppler Effect t0+τ2
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A. Chandra, NIT Durgapur – How to combat fading?

8. J.U., 13th April, 2007
Fading Mechanisms
Fading
Large Scale Small Scale
Fading Fading
Attenuation Variation
with distance about mean
Time delay Doppler shift /
spread /Multipath time variance of
channel
Frequency Fast Slow
Flat Fading
Selective Fading Fading Fading
•Empirical Model
•Okumura, Hata, COST 231
•Statistical Model
•Rayleigh, Rice, Nakagami-m, Hoyt, Log-Normal, Weibull, Gamma, K etc.
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A. Chandra, NIT Durgapur – How to combat fading?

9. J.U., 13th April, 2007
Large Scale & Small Scale Fading
Fading
Large Scale Small Scale
Fading Fading
•Large Scale Fading
•Due to general terrain, density and height of buildings, vegetation
•Variation occurs over very large distances (100m.-a few K.m.)
•Important for predicting the coverage and availability of a particular service
•Small Scale Fading
•Due to local environment, nearby trees, buildings
•Variation occurs over very short distances, on the order of the signal
wavelength (<1 m.)
•Important for design of modulation format and transmitter / receiver design
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A. Chandra, NIT Durgapur – How to combat fading?

10. J.U., 13th April, 2007
Large Scale Fading
Fading
(in dB)
Pr/Pt
Path Loss alone
Large Scale Shadowing and
Fading Path Loss
Slope 10n dB/ decade
Generally n>>2
(n=2 for free space)
Attenuation Variation
with distance about mean
log (d)
•Attenuation with Distance / Path Loss
2
Pr  λ 
•Friis equation for free space ∝ 
Pt  4πd 
•Empirical models (Okumura, Hata etc.) based on field measurements
•Variation about Mean / Shadowing
•Characterized by log-normal shadowing
•If ψ = Pt Pr the distribution of ψ is log-normal with parameters µ ψ and σ ψ
ξ  (10 log 10 ψ − µ ψ ) 2 
f ( ψ) = exp −  where ξ = 10 ln 10
ψ 2πσ ψ
2

 2σ ψ2


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A. Chandra, NIT Durgapur – How to combat fading?

11. J.U., 13th April, 2007
Small Scale Fading
Fading
Small Scale
Fading
Time delay Doppler shift /
spread /Multipath time variance of
channel
Frequency Fast Slow
Flat Fading
Selective Fading Fading Fading
•Slow Flat Fading
•Least severe fading type
•Multiplicative narrowband fading  Modeled with statistical distributions
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12. J.U., 13th April, 2007
Small Scale Fading (Contd.)
α  α2 
•Rayleigh f ( α ) = 2 exp − 2
 2σ 

σ  
α  α2 + s2   αs 
•Rician f ( α ) = 2 exp −
 I 0  2 
 K=
s2
σ  2σ 2  σ  2σ 2
m
2  m  2 m −1  m 
•Nakagami-m f ( α) =   α exp − α 2  Ω = α2
Γ( m )  Ω   Ω 
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A. Chandra, NIT Durgapur – How to combat fading?

13. J.U., 13th April, 2007
Small Scale Fading (Contd.)
Fading
Small Scale
Fading
Time delay Doppler shift /
spread /Multipath time variance of
channel
Frequency Slow
Selective Fading Fading
Ground to Ground: Highly frequency
selective, not very time selective
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A. Chandra, NIT Durgapur – How to combat fading?

14. J.U., 13th April, 2007
Small Scale Fading (Contd.)
Fading
Small Scale
Fading
Time delay Doppler shift /
spread /Multipath time variance of
channel
Flat Fading Fast
Fading
Air to Air: Almost frequency non-
selective, very time selective
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A. Chandra, NIT Durgapur – How to combat fading?

15. J.U., 13th April, 2007
Small Scale Fading (Contd.)
Fading
Small Scale
Fading
Time delay Doppler shift /
spread /Multipath time variance of
channel
Frequency Fast
Selective Fading Fading
Air to Ground: Frequency selective,
time selective
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16. J.U., 13th April, 2007
Degradation Due to Fading
Error
Binary baseband digital data Demodulated signal
0 1 1 1 1 0 0 1 0 0 0 0 1 1 1 0 0 1 0 0
AWGN
T
∫
0
dt
sin (ωCt) sin (ωCt)
Transmitter Channel Receiver
ASK modulated waveform Modulated signal + Noise
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17. J.U., 13th April, 2007
Degradation Due to Fading (Contd.)
Modulated signal + Noise
Too Many Errors
Binary baseband digital data Demodulated signal
0 1 1 1 1 0 0 1 0 0 Rayleigh 0 0 0 1 1 1 0 1 0 0
AWGN Fading
T
∫0
dt
sin (ωCt) sin (ωCt)
Transmitter Channel Receiver
ASK modulated waveform Rayleigh faded signal
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A. Chandra, NIT Durgapur – How to combat fading?

18. J.U., 13th April, 2007
The Good, the Bad & the Ugly!
•Good (AWGN channel)
Exponential decrease
•Bad (Slow, flat fading channel) ?
Linear decrease
Loss in SNR
•Ugly (Frequency selective/
fast fading channel)
Irreducible error floor
Loss
B. Sklar, ‘Rayleigh fading channels in digital
communication systems’, IEEE Communications
Magazine, July, 1997.
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A. Chandra, NIT Durgapur – How to combat fading?

19. J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
What is Diversity?
Diversity Types
Diversity Combining
Error Correction Coding
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
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A. Chandra, NIT Durgapur – How to combat fading?

20. J.U., 13th April, 2007
Improvement with Diversity
Rayleigh faded signal # path1
Lesser Errors
Rayleigh Demodulated signal
AWGN Fading
0 0 1 1 1 1 0 1 0 0
Rayleigh T
Diversity
AWGN Fading
Combiner ∫0
dt
sin (ωCt)
Channel Receiver
Rayleigh faded signal # path2
Output of Diversity Combiner
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A. Chandra, NIT Durgapur – How to combat fading?

21. J.U., 13th April, 2007
Why Diversity?
•Mitigate slow flat fading?
Increase the transmitted power
Not power efficient technique
•Alternative way 
Diversity path undergoes a deep
•If one signal
fade at a particular point of time,
another independent path may have a
strong signal
Gain
•If probability of a deep fade in one
channel is p, then the probability for L BER performance of BPSK with Rayleigh fading and
subsequent improvement with 2nd order diversity
channels is pL
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A. Chandra, NIT Durgapur – How to combat fading?

22. J.U., 13th April, 2007
Macroscopic & Microscopic Diversity
Base
•Macroscopic Diversity Station
A
Mitigate effects of large scale fading
Hill
or, shadowing
Reception
By selecting a base station which is Reception from only A
from only B
not shadowed when others are
Base Shadowed
Station Hill
Region
B
•Microscopic Diversity
Mitigate effects of small scale fading or, multipath
Require two or more uncorrelated received signals, with the same long-
term fading experienced in those signals
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A. Chandra, NIT Durgapur – How to combat fading?

23. J.U., 13th April, 2007
Diversity Types
•Space Diversity
•Spatial separation between antennas, so that the diversity branches
experience uncorrelated fading
•More hardware/ antennas
Receiver
•Receiver Diversity (SIMO)
Transmitter Combiner Antenna separation λ/2
Receiver
•Transmit Diversity (MISO)
Antenna separation 10λ
Transmitter
The total transmitted power is
Receiver
Transmitter
Combiner
split among the antennas
Open loop/ close loop (for 3G)
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A. Chandra, NIT Durgapur – How to combat fading?

24. J.U., 13th April, 2007
Diversity Types (Contd.)
•Frequency Diversity
•Modulate the signal through L different carriers
•The separation between the carriers should be at least the coherent bandwidth,
not effective over frequency-flat channel
•Only one antenna is needed
•The total transmitted power is split among the carriers, not BW efficient
Path 1 Path 1
Path 2 Path 2
t1 t2
freq
Transmitter f2 (path 2) Receiver Transmitter Receiver
(path 1) (path 2)
freq
f1 (path 1)
time time
•Time Diversity
•Each symbol is transmitted L times
•The interval between symbol repetitions should be at least the coherence time,
not effective over slow fading channel
•Only one antenna is needed
•Reduction in efficiency (effective data rate < real data rate)
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A. Chandra, NIT Durgapur – How to combat fading?

25. J.U., 13th April, 2007
Diversity Types (Contd.)
•Polarization Diversity
•Send signals with horizontal/ vertical polarization
•Compact co-located antennas
•Unequal branch powers, Less diversity gain, For fixed radio links
•Angle Diversity
•Field Component Diversity
•Antenna Pattern Diversity
S. Kozono et al., ‘Base Station Polarization Diversity
•Multipath Diversity
Reception for Mobile Radio’, IEEE Trans. on Veh. Tech.,
vol. VT-33, no. 4, Nov., 1984.
•RAKE receiver
•Space-Time-Frequency Diversity
•Space-Time/ Space-Frequency/ Space-Time-Frequency Diversity
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A. Chandra, NIT Durgapur – How to combat fading?

26. J.U., 13th April, 2007
Diversity Combining
Weights
Phase and Phase
Control estimation estimation
Unit
Weights
Phase
Phase
Selection
Out Out Out
Selection Combining Equal Gain Combining Maximal Ratio Combining
Y ; Y > Y2 Y1 + Y2
YC =  1 1 YC = YC = a1Y1 + a 2 Y2
Y2 ; Y2 > Y1 2
Choose the best Simple average Weighted average
A sub-optimal version of selection combining is switch-and-stay combining in
which alternate antenna are chosen if signal falls below a certain threshold
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A. Chandra, NIT Durgapur – How to combat fading?

27. J.U., 13th April, 2007
Diversity: Some Facts
•More is Less!
As number of diversity paths (L) increases
we have diminishing improvement
•Think Optimum
Performance of combiners
SWC < SC < EGC < MRC
•If it’s Worse, it’s Better!
More improvement for Rayleigh fading
channel than Rician
•Correlation
If correlation ρ is non-zero still we have
sufficient improvement up to ρ < 0.5
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A. Chandra, NIT Durgapur – How to combat fading?

28. J.U., 13th April, 2007
Diversity: Some Facts
•More is Less!
As number of diversity paths (L) increases
we have diminishing improvement
•Think Optimum
Performance of combiners
SWC < SC < EGC < MRC
•If it’s Worse, it’s Better!
More improvement for Rayleigh fading BER for BPSK system with diversity order L=4
channel than Rician
P. H. Phuong, ‘Analysis of Antenna Diversity Techniques
•Correlation Used in MIMO System’, In Proc. of International
Symposium on Electrical & Electronics Engineering, Oct.,
2005, HCM City, Vietnam, pp.18-22.
If correlation ρ is non-zero still we have
sufficient improvement up to ρ < 0.5
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A. Chandra, NIT Durgapur – How to combat fading?

29. J.U., 13th April, 2007
Diversity: Some Facts
•More is Less!
As number of diversity paths (L) increases
we have diminishing improvement
•Think Optimum
Performance of combiners
SWC < SC < EGC < MRC
•If it’s Worse, it’s Better!
Pe vs. SNR for selected values of ρ for 8PSK with L = 4
More improvement for Rayleigh fading
channel than Rician
E. Perahia & G. J. Pottie, ‘On Diversity Combining for
•Correlation Correlated Slowly Flat- Fading Rayleigh Channels’, In
Proc. of IEEE International Conference on Serving
Humanity Through Communications, SUPERCOMM/ICC,
If correlation ρ is non-zero still we have May 1994, pp.342-346
sufficient improvement up to ρ < 0.5
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30. J.U., 13th April, 2007
Error Correction Coding
•For a given Eb/N0, with coding present, the error floor out of the demodulator
will not be lowered, but a lower error rate out of the decoder can be achieved
•For a given error performance, a code reduces the required Eb/N0
•Effective data rate decreases
•Coding Types
Block Code  Hadamard, Golay, BCH, RS
Convolutional Code  Viterbi Decoding
Eb/N0 necessary for Pe=10-5 as a
Turbo Code (Berrou ‘93)  Shannon limit function of code rate R
J. Hagenauer et al., ’Forward Error
TCM (Ungerboeck ‘87)  Joint coding & modulation correction Coding for Fading
Compensation in Mobile Satellite
Channels’, IEEE JSAC, vol. 5, no. 2,
Feb 1987, pp. 215-225
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31. J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Equalization
OFDM
Modulation
FH/SS
RAKE Receiver
Mitigation of Fast Fading
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A. Chandra, NIT Durgapur – How to combat fading?

32. J.U., 13th April, 2007
Frequency Selective Fading Mitigation
•Equalization
•Frequency selective channel introduce different attenuation & phase shift
to different frequency components in transmitted signal
•Equalizer does the opposite
•Frequency selective channel appears as flat fading channel
•Decision Feedback Equalizer (DFE)
When a symbol is detected, the ISI it introduce on future symbols are
estimated and subtracted before the detection of subsequent symbols
•Maximum Likelihood Sequence Estimation
(MLSE) Equalizer
•All possible data sequences are tested  optimum case
•Viterbi Equalizer applied to GSM
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A. Chandra, NIT Durgapur – How to combat fading?

33. J.U., 13th April, 2007
Freq. Select. Fading Mitigation (Contd.)
•Orthogonal Frequency Division Multiplexing (OFDM)
•Available bandwidth is divided into several narrow band carriers
•Serial data stream is divided in N parallel data streams
W f
CP
ISI
1 2 3 N-1 N CP
W/N f
•Fast serial data stream is transformed into slow parallel data streams 
Longer symbol durations  A frequency selective channel appears as flat
•Cyclic Prefix is inserted between consecutive OFDM symbols  removes
ISI from previous symbol
•Much more sensitive to synchronization errors, High peak to average
power ratio, Wastage of bandwidth in cyclic prefix
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A. Chandra, NIT Durgapur – How to combat fading?

34. J.U., 13th April, 2007
Freq. Select. Fading Mitigation (Contd.)
•Choice of Modulation
•4-ary modulations (QPSK, OQPSK, MSK) are
more resistant to delay spread than BPSK for
constant information throughput
•4-ary keying is used widely in 2G & 3G
•Pilot signal assisted Modulation
•Facilitate coherent detection
•Freq domain  In-band tones
•Time domain  Digital sequences
•Frequency Hopping The irreducible BER performance for different
modulations plotted against rms delay spread
Spread Spectrum (FH/SS) normalized by bit period.
•Receiver frequency band is changed J. Chuang, ’The Effects of Time Delay Spread on Portable
before the arrival of the multiple diffused Radio Communications Channels with Digital Modulation’,
IEEE JSAC, vol. 5,no. 5, Jun’87, pp. 879-889
components
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A. Chandra, NIT Durgapur – How to combat fading?

35. J.U., 13th April, 2007
Freq. Select. Fading Mitigation (Contd.)
•RAKE Receiver RAKE
A receiver technique which uses several baseband ?
correlators to individually process several signal multipath
components. The correlator outputs are combined to
achieve improved communications reliability and
performance.
•IS-95
•Base station combines outputs
of its RAKE-receiver fingers (4 to
5) non-coherently
•Mobile receiver combines its
RAKE-receiver finger (generally
3) outputs coherently
R. Price & P.E. Green, ’A Communication Technique for Multipath Channels’,
Proc. IRE, vol. 46, 1958, pp. 555-570
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A. Chandra, NIT Durgapur – How to combat fading?

36. J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
Coding & Interleaving
Signal Redundancy
Robust Modulation
Doppler Diversity
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37. J.U., 13th April, 2007
Fast Fading Mitigation
•Coding & Interleaving
•Wireless multipath channels have memory  multiple copies of a symbol
arrive in delayed fashion and affect future symbol
•Even with fast fading several successive symbol transmissions are affected
 Burst Errors
•FEC schemes are designed for isolated errors, not for Burst Errors.
•Original code words
A B C D E F
A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 C1 C2 C3 C4 C5 C6 D1 D2 D3 D4 D5 D6 E1 E2 E3 E4 E5 E6 F1 F2 F3 F4 F5 F6
•Interleaved words Error burst
1 2 3 4 5 6
A1 B1 C1 D1 E1 F1 A2 B2 C2 D2 E2 F2 A3 B3 C3 D3 E3 F3 A4 B4 C4 D4 E4 F4 A5 B5 C5 D5 E5 F5 A6 B6 C6 D6 E6 F6
X X X X X X
•De-interleaved words
A B C D E F
A1 A2 A3 A4 A5 A6 B1 B2 B3 B4 B5 B6 C1 C2 C3 C4 C5 C6 D1 D2 D3 D4 D5 D6 E1 E2 E3 E4 E5 E6 F1 F2 F3 F4 F5 F6
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A. Chandra, NIT Durgapur – How to combat fading?

38. J.U., 13th April, 2007
Fast Fading Mitigation (Contd.)
•Coding & Interleaving (Contd.)
•Interleaving separate symbols in an error
burst and spread them over time
•If the time separation is more than coherence
time, errors are uncorrelated in time
•Channel can be viewed as memoryless
•Interleaving realizes time diversity
Typical Eb/N0,performance vs. vehicle speed for 850
As the motion increases in velocity, so does MHz links to achieve a FER = 1% over a Rayleigh
channel with two independent paths
the benefit of a given interleaver to the error
performance of any system
R. Padovani, Reverse Link Performance of 1S-95 Based
Cellular Systems’, IEEE Personal Communications, vol. 1,
no. 3, 3rd qtr. 1994, pp. 28 - 34
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A. Chandra, NIT Durgapur – How to combat fading?

39. J.U., 13th April, 2007
Fast Fading Mitigation (Contd.)
•Signal Redundancy
•Fast fading occurs in low data rate transmission
•If symbol duration is reduced compared to coherence time, the channel
appears as slow fading channel
•Robust Modulation
•Non-coherent or, differentially coherent modulation
•Phase tracking not required  Detector integration time reduces
•Doppler Diversity
•Doppler spread induced by temporal channel variations can provide another
means for diversity that can be exploited to combat fading
•Applicable to CDMA spread-spectrum RAKE receiver
A. M. Sayeed & B. Aazhang, ‘Joint Multipath-Doppler Diversity in Mobile Wireless
Communications’, IEEE Trans. on Commun., vol. 47, no. 1, 1999, pp 123-132.
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40. J.U., 13th April, 2007
Future???
•New/ Hybrid Technologies?
•Space Time Coding
•BLAST
•UWB
•MIMO-OFDM
•Cognitive Radio – Radio with Brain?
Cognitive radios will have the ability of devices to determine their location,
sense spectrum use by neighboring devices, change frequency, adjust
output power, and even alter transmission parameters and characteristics.
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A. Chandra, NIT Durgapur – How to combat fading?