combat fading in wireless
DESCRIPTION
How to combat fading in wireless channels?TRANSCRIPT
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 [email protected]
A. Chandra, NIT Durgapur – How to combat fading?2
J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
Outline
A. Chandra, NIT Durgapur – How to combat fading?3
J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
What is Fading?
Fading Mechanisms
Degradation due to Fading
A. Chandra, NIT Durgapur – How to combat fading?4
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?5
J.U., 13th April, 2007
Fading???
•Fading over time
•Fading over distance
Enemy ahead
Enemy?
Oh No .. Emily! Boss’s
wife
Lunch break, at
last!
A. Chandra, NIT Durgapur – How to combat fading?6
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?7
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?8
J.U., 13th April, 2007
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
Time delay spread /Multipath
Doppler shift / time variance of
channel
Doppler shift / time variance of
channel
Flat Fading
Flat Fading
Frequency Selective 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.
A. Chandra, NIT Durgapur – How to combat fading?9
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?10
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?11
J.U., 13th April, 2007
FadingFading
Small Scale Fading
Small Scale Fading
Time delay spread /Multipath
Time delay spread /Multipath
Doppler shift / time variance of
channel
Doppler shift / time variance of
channel
Flat Fading
Flat Fading
Frequency Selective Fading
Frequency Selective Fading
Fast FadingFast
FadingSlow
FadingSlow
Fading
Small Scale Fading
•Slow Flat Fading•Least severe fading type•Multiplicative narrowband fading Modeled with statistical distributions
A. Chandra, NIT Durgapur – How to combat fading?12
J.U., 13th April, 2007
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.)
A. Chandra, NIT Durgapur – How to combat fading?13
J.U., 13th April, 2007
FadingFading
Small Scale Fading
Small Scale Fading
Time delay spread /Multipath
Time delay spread /Multipath
Doppler shift / time variance of
channel
Doppler shift / time variance of
channel
Frequency Selective Fading
Frequency Selective Fading
Slow FadingSlow
Fading
Ground to Ground: Highly frequency selective, not very time selective
Small Scale Fading (Contd.)
A. Chandra, NIT Durgapur – How to combat fading?14
J.U., 13th April, 2007
FadingFading
Small Scale Fading
Small Scale Fading
Time delay spread /Multipath
Time delay spread /Multipath
Doppler shift / time variance of
channel
Doppler shift / time variance of
channel
Flat Fading
Flat Fading
Fast FadingFast
Fading
Air to Air: Almost frequency non-selective, very time selective
Small Scale Fading (Contd.)
A. Chandra, NIT Durgapur – How to combat fading?15
J.U., 13th April, 2007
FadingFading
Small Scale Fading
Small Scale Fading
Time delay spread /Multipath
Time delay spread /Multipath
Doppler shift / time variance of
channel
Doppler shift / time variance of
channel
Frequency Selective Fading
Frequency Selective Fading
Fast FadingFast
Fading
Air to Ground: Frequency selective, time selective
Small Scale Fading (Contd.)
A. Chandra, NIT Durgapur – How to combat fading?16
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?17
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?18
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?19
J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Frequency Selective Fading
Mitigation of Fast Fading
What is Diversity?
Diversity Types
Diversity Combining
Error Correction Coding
A. Chandra, NIT Durgapur – How to combat fading?20
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?21
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?22
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?23
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?24
J.U., 13th April, 2007
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)
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
•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.
A. Chandra, NIT Durgapur – How to combat fading?26
J.U., 13th April, 2007
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
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
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 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
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! 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
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
A. Chandra, NIT Durgapur – How to combat fading?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
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
A. Chandra, NIT Durgapur – How to combat fading?31
J.U., 13th April, 2007
Fading in Wireless Channels
Mitigation of Slow Flat Fading
Mitigation of Fast Fading
Equalization
OFDM
Modulation
FH/SS
RAKE Receiver
Mitigation of Frequency Selective Fading
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)
•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
A. Chandra, NIT Durgapur – How to combat fading?33
J.U., 13th April, 2007
•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
A. Chandra, NIT Durgapur – How to combat fading?34
J.U., 13th April, 2007
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
Freq. Select. Fading Mitigation (Contd.)
•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
A. Chandra, NIT Durgapur – How to combat fading?35
J.U., 13th April, 2007
Freq. Select. Fading Mitigation (Contd.)
•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
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
Coding & Interleaving
Signal Redundancy
Robust Modulation
Doppler Diversity
Mitigation of Fast Fading
A. Chandra, NIT Durgapur – How to combat fading?37
J.U., 13th April, 2007
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.
A. Chandra, NIT Durgapur – How to combat fading?38
J.U., 13th April, 2007
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
A. Chandra, NIT Durgapur – How to combat fading?39
J.U., 13th April, 2007
•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.
A. Chandra, NIT Durgapur – How to combat fading?40
J.U., 13th April, 2007
•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???