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combat fading in wireless

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How to combat fading in wireless channels?

How to combat fading in wireless channels?

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  • 1. J.U., 13th April, 2007 How to Combat Fading in Wireless Channels? Aniruddha Chandra ECE Department, NIT Durgapur aniruddha.chandra@ieee.org 1A. 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 2A. 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 3A. Chandra, NIT Durgapur – How to combat fading?
  • 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 4 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 5 A. Chandra, NIT Durgapur – How to combat fading?
  • 6. J.U., 13th April, 2007 Radio Wave Propagation Buildings Amplitude STOPTransmitter Fading Earth surface Receiver Envelope Line of Sight Reflection Diffraction Street Sign Scattering Distance•Multiple replica of signal combines with random phase resulting in randomamplitude attenuation and phase variation  This is Fading v•Relative velocity causes further change with time. f D = cos θ λ 6 A. Chandra, NIT Durgapur – How to combat fading?
  • 7. J.U., 13th April, 2007 Wireless Channel•Limited PowerSize, Weight, Battery Constraints•Limited BWSpectrum allocation•Path Lossup to 10 dB/km•Multipath Fading MultipathResolvable channel induced ISINon-resolvable  random amplitude variation t0•Time Variance of the Channel Time Variance t0+τ1Due to relative velocityIntroduces Doppler Effect t0+τ2 7 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. 8 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 9 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   10 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 11 A. Chandra, NIT Durgapur – How to combat fading?
  • 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  12A. 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 13A. 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 14A. 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 15A. Chandra, NIT Durgapur – How to combat fading?
  • 16. J.U., 13th April, 2007 Degradation Due to Fading Error Binary baseband digital data Demodulated signal0 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 16 A. Chandra, NIT Durgapur – How to combat fading?
  • 17. J.U., 13th April, 2007 Degradation Due to Fading (Contd.) Modulated signal + Noise Too Many Errors Binary baseband digital data Demodulated signal0 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 17 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. 18 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 19A. 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 20A. 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 21 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 22 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) 23 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) 24 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 25 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 OutSelection 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 averageA sub-optimal version of selection combining is switch-and-stay combining inwhich alternate antenna are chosen if signal falls below a certain threshold 26 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 27 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 28 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 29 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 demodulatorwill 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 TypesBlock Code  Hadamard, Golay, BCH, RSConvolutional Code  Viterbi Decoding Eb/N0 necessary for Pe=10-5 as aTurbo Code (Berrou ‘93)  Shannon limit function of code rate R J. Hagenauer et al., ’Forward ErrorTCM (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 30 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 Frequency Selective Fading Equalization OFDM Modulation FH/SS RAKE ReceiverMitigation of Fast Fading 31A. 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 32 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  removesISI from previous symbol•Much more sensitive to synchronization errors, High peak to averagepower ratio, Wastage of bandwidth in cyclic prefix 33 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 spreadSpread 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 34 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 coherentlyR. Price & P.E. Green, ’A Communication Technique for Multipath Channels’,Proc. IRE, vol. 46, 1958, pp. 555-570 35 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 36A. Chandra, NIT Durgapur – How to combat fading?
  • 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 FA1 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 6A1 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 FA1 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 37 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 850As the motion increases in velocity, so does MHz links to achieve a FER = 1% over a Rayleigh channel with two independent pathsthe benefit of a given interleaver to the errorperformance 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 38 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 channelappears 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 anothermeans 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. 39 A. Chandra, NIT Durgapur – How to combat fading?
  • 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. 40 A. Chandra, NIT Durgapur – How to combat fading?

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