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

A. Chandra, NIT Durgapur – How to combat fading?1

J.U., 13th April, 2007

How to Combat Fading in How to Combat Fading in Wireless Channels?Wireless Channels?

Aniruddha ChandraECE Department, NIT Durgapur

[email protected]



Fading in Wireless Channels

Mitigation of Slow Flat Fading

Mitigation of Frequency Selective Fading

Mitigation of Fast Fading

Outline







What is Fading?

Fading Mechanisms

Degradation due to Fading



Why Wireless?

•Mobility Anytime, Anywhere connectivity

Phone for people not for places

•Easy Installation Rapid deployment, reconfigurable, no cable, easy maintenance

•Digital Companion Voice, message, internet, multimedia

Hi! I’m on the prowl



Fading???

•Fading over time

•Fading over distance

Enemy ahead

Enemy?

Oh No .. Emily! Boss’s

wife

Lunch break, at

last!



Radio Wave Propagation

Street Sign

STOP

Line of Sight Reflection DiffractionScattering

TransmitterReceiver

Buildings

Earth surface

•Multiple replica of signal combines with random phase resulting in random amplitude attenuation and phase variation This is Fading

•Relative velocity causes further change with time.

Fading Envelope

Distance

Am

plitu

de

cosv

f D



Multipath

t0+τ1

t0

t0+τ2

Time Variance•Limited Power Size, Weight, Battery Constraints

•Limited BW Spectrum allocation

•Path Loss up to 10 dB/km

•Multipath Fading Resolvable channel induced ISI Non-resolvable random amplitude variation

•Time Variance of the Channel Due to relative velocity Introduces Doppler Effect

Wireless Channel



FadingFading

Large Scale Fading

Large Scale Fading

Small Scale Fading

Small Scale Fading

Attenuation with distanceAttenuation

with distanceVariation

about meanVariation

about mean

Time delay spread /Multipath


Doppler shift / time variance of

channel


channel

Flat Fading

Flat Fading

Frequency Selective Fading


Fast FadingFast

FadingSlow

FadingSlow

Fading

Fading Mechanisms

•Empirical Model•Okumura, Hata, COST 231

•Statistical Model•Rayleigh, Rice, Nakagami-m, Hoyt, Log-Normal, Weibull, Gamma, K etc.



FadingFading

Large Scale Fading

Large Scale Fading

Small Scale Fading

Small Scale Fading

Large Scale & Small Scale Fading

•Large Scale Fading

•Small 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

•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



FadingFading

Large Scale Fading

Large Scale Fading

Attenuation with distanceAttenuation

with distanceVariation

about meanVariation

about mean

Large Scale Fading

•Attenuation with Distance / Path Loss

•Variation about Mean / Shadowing

•Friis equation for free space

•Empirical models (Okumura, Hata etc.) based on field measurements

•Characterized by log-normal shadowing

•If the distribution of is log-normal with parameters and

2

4

dP

P

t

r

Path Loss aloneShadowing and Path Loss

log (d)

Pr/P

t (i

n

dB

)

Slope 10n dB/ decadeGenerally n>>2(n=2 for free space)

2

210

2 2

log10exp

2f

rt PP

10ln10where



FadingFading

Small Scale Fading

Small Scale Fading




channel


channel

Flat Fading

Flat Fading



Fast FadingFast

FadingSlow

FadingSlow

Fading

Small Scale Fading

•Slow Flat Fading•Least severe fading type•Multiplicative narrowband fading Modeled with statistical distributions



2

2

2 2expf

202

22

2 2exp

sI

sf

2

2

2s

K

212 exp2 mm

mf m

m

2

•Rayleigh

•Rician

•Nakagami-m

Small Scale Fading (Contd.)



FadingFading

Small Scale Fading

Small Scale Fading




channel


channel



Slow FadingSlow

Fading

Ground to Ground: Highly frequency selective, not very time selective




FadingFading

Small Scale Fading

Small Scale Fading




channel


channel

Flat Fading

Flat Fading

Fast FadingFast

Fading

Air to Air: Almost frequency non-selective, very time selective




FadingFading

Small Scale Fading

Small Scale Fading




channel


channel



Fast FadingFast

Fading

Air to Ground: Frequency selective, time selective




Degradation Due to Fading

AWGN

T

0dt

sin (ωCt) sin (ωCt)

Channel ReceiverTransmitter

Binary baseband digital data

ASK modulated waveform Modulated signal + Noise

0 1 1 1 1 0 0 1 0 0

Demodulated signal

Error

0 0 1 1 1 0 0 1 0 0



Degradation Due to Fading (Contd.)

AWGN

T

0dt

sin (ωCt) sin (ωCt)

Rayleigh Fading

Channel ReceiverTransmitter

Binary baseband digital data

ASK modulated waveform

Modulated signal + Noise

Rayleigh faded signal

0 1 1 1 1 0 0 1 0 0 0 0 0 1 1 1 0 1 0 0

Demodulated signal

Too Many Errors



The Good, the Bad & the Ugly!

B. Sklar, ‘Rayleigh fading channels in digital communication systems’, IEEE Communications Magazine, July, 1997.

•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







What is Diversity?

Diversity Types

Diversity Combining

Error Correction Coding



Improvement with Diversity

T

0dt

sin (ωCt)

Diversity Combiner

AWGN

Rayleigh Fading

AWGN

Rayleigh Fading

Channel Receiver

Rayleigh faded signal # path1

Rayleigh faded signal # path2Output of Diversity Combiner

0 0 1 1 1 1 0 1 0 0

Lesser Errors

Demodulated signal



Why Diversity?

BER performance of BPSK with Rayleigh fading and subsequent improvement with 2nd order diversity

Gain

•Mitigate slow flat fading? Increase the transmitted power Not power efficient technique

•Alternative way Diversity•If one signal path undergoes a deep

fade at a particular point of time, another independent path may have a strong signal

•If probability of a deep fade in one channel is p, then the probability for L channels is pL



Macroscopic & Microscopic Diversity

Base Station

A

Base Station

BHill

Hill

Reception

from only AReception

from only B

Shadowed Region

•Macroscopic Diversity Mitigate effects of large scale fading or, shadowing

By selecting a base station which is not shadowed when others are

•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



Diversity Types

Receiver

CombinerTransmitter

Receiver •Receiver Diversity (SIMO) Antenna separation λ/2

•Transmit Diversity (MISO) Antenna separation 10λ

The total transmitted power is split among the antennas

Open loop/ close loop (for 3G)

Transmitter

TransmitterReceiver Combiner

•Space Diversity•Spatial separation between antennas, so that the diversity branches experience uncorrelated fading•More hardware/ antennas



Diversity Types (Contd.)

ReceiverTransmitter

Path 2

Path 1

freq

time

f1 (path 1)

f2 (path 2)

•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

ReceiverTransmitter

Path 2

Path 1

freq

time

t1 (path 1)

t2 (path 2)

•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)



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

•Space-Time-Frequency Diversity

•Angle Diversity

•Field Component Diversity •Antenna Pattern Diversity

•Multipath Diversity •RAKE receiver

•Space-Time/ Space-Frequency/ Space-Time-Frequency Diversity

S. Kozono et al., ‘Base Station Polarization Diversity Reception for Mobile Radio’, IEEE Trans. on Veh. Tech., vol. VT-33, no. 4, Nov., 1984.



Control Unit

Selection

Out

Selection Combining Equal Gain Combining

Out

Weights and Phase estimation

Weights

Phase

Maximal Ratio Combining

Out

Phase estimation

Phase

Diversity Combining

122

211

;

;

YYY

YYYYC 2

21 YYYC

2211 YaYaYC

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

Choose the best Simple average Weighted average



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








P. H. Phuong, ‘Analysis of Antenna Diversity Techniques Used in MIMO System’, In Proc. of International Symposium on Electrical & Electronics Engineering, Oct., 2005, HCM City, Vietnam, pp.18-22.

BER for BPSK system with diversity order L=4








Pe vs. SNR for selected values of ρ for 8PSK with L = 4

E. Perahia & G. J. Pottie, ‘On Diversity Combining for Correlated Slowly Flat- Fading Rayleigh Channels’, In Proc. of IEEE International Conference on Serving Humanity Through Communications, SUPERCOMM/ICC, May 1994, pp.342-346



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

Turbo Code (Berrou ‘93) Shannon limit

TCM (Ungerboeck ‘87) Joint coding & modulation

Eb/N0 necessary for Pe=10-5 as a function of code rate R

J. Hagenauer et al., ’Forward Error correction Coding for Fading Compensation in Mobile Satellite Channels’, IEEE JSAC, vol. 5, no. 2, Feb 1987, pp. 215-225






Equalization

OFDM

Modulation

FH/SS

RAKE Receiver




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)

•Maximum Likelihood Sequence Estimation (MLSE) Equalizer

When a symbol is detected, the ISI it introduce on future symbols are estimated and subtracted before the detection of subsequent symbols

•All possible data sequences are tested optimum case•Viterbi Equalizer applied to GSM



•Orthogonal Frequency Division Multiplexing (OFDM)

Freq. Select. Fading Mitigation (Contd.)

•Available bandwidth is divided into several narrow band carriers•Serial data stream is divided in N parallel data streams

•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

1 2 3 N-1 N

fW

W/N f

CP

CP

ISI



J. Chuang, ’The Effects of Time Delay Spread on Portable Radio Communications Channels with Digital Modulation’, IEEE JSAC, vol. 5,no. 5, Jun’87, pp. 879-889

The irreducible BER performance for different modulations plotted against rms delay spread normalized by bit period.

•Choice of Modulation

•Pilot signal assisted 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

•Frequency Hopping Spread Spectrum (FH/SS)

•Receiver frequency band is changed before the arrival of the multiple diffused components

•Facilitate coherent detection•Freq domain In-band tones•Time domain Digital sequences




•RAKE ReceiverA 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.

RAKE ?

R. Price & P.E. Green, ’A Communication Technique for Multipath Channels’, Proc. IRE, vol. 46, 1958, pp. 555-570

•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






Coding & Interleaving

Signal Redundancy

Robust Modulation

Doppler Diversity




Fast Fading Mitigation

X X X X X X

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

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

A B C D E F

A1 B1 C1 D1 E1 F1 A2 B2 C2 D2 E2 F2 A6 B6 C6 D6 E6 F6A3 B3 C3 D3 E3 F3 A4 B4 C4 D4 E4 F4 A5 B5 C5 D5 E5 F5

1 2 3 4 5 6Error burst

•Original code words

•De-interleaved words

•Interleaved words

•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.



R. Padovani, Reverse Link Performance of 1S-95 Based Cellular Systems’, IEEE Personal Communications, vol. 1, no. 3, 3rd qtr. 1994, pp. 28 - 34

Typical Eb/N0,performance vs. vehicle speed for 850 MHz links to achieve a FER = 1% over a Rayleigh channel with two independent paths

•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

Fast Fading Mitigation (Contd.)

As the motion increases in velocity, so does the benefit of a given interleaver to the error performance of any system



•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

Fast Fading Mitigation (Contd.)

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.



•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.

Future???



Questions???



Thank You!

Documents

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