Aniruddha Chandra, profile picture
Uploaded byAniruddha Chandra
PPT, PDF13,449 views

combat fading in wireless

The document discusses different techniques to mitigate fading in wireless channels. It describes slow flat fading, frequency selective fading, and fast fading. To mitigate slow flat fading, the document discusses diversity which uses multiple independent fading paths. It explains different types of diversity including space, frequency, and time diversity. It also discusses diversity combining techniques such as selection, equal gain, and maximal ratio combining which combine signals from multiple diversity paths. The document aims to explain how to combat different types of fading in wireless channels.

Embed presentation

Downloaded 1,004 times
J.U., 13th April, 2007 How to Combat Fading in Wireless Channels? Aniruddha Chandra ECE Department, NIT Durgapur aniruddha.chandra@ieee.org 1 A. Chandra, NIT Durgapur – How to combat fading?
J.U., 13th April, 2007 Outline Fading in Wireless Channels Mitigation of Slow Flat Fading Mitigation of Frequency Selective Fading Mitigation of Fast Fading 2 A. Chandra, NIT Durgapur – How to combat fading?
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 3 A. Chandra, NIT Durgapur – How to combat fading?
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?
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?
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 6 A. Chandra, NIT Durgapur – How to combat fading?
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 7 A. Chandra, NIT Durgapur – How to combat fading?
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?
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?
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?
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?
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  12 A. Chandra, NIT Durgapur – How to combat fading?
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 13 A. Chandra, NIT Durgapur – How to combat fading?
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 14 A. Chandra, NIT Durgapur – How to combat fading?
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 15 A. Chandra, NIT Durgapur – How to combat fading?
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 16 A. Chandra, NIT Durgapur – How to combat fading?
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 17 A. Chandra, NIT Durgapur – How to combat fading?
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?
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 19 A. Chandra, NIT Durgapur – How to combat fading?
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 20 A. Chandra, NIT Durgapur – How to combat fading?
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?
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?
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?
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?
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?
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 26 A. Chandra, NIT Durgapur – How to combat fading?
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?
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?
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?
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 30 A. Chandra, NIT Durgapur – How to combat fading?
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 31 A. Chandra, NIT Durgapur – How to combat fading?
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?
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 33 A. Chandra, NIT Durgapur – How to combat fading?
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 34 A. Chandra, NIT Durgapur – How to combat fading?
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 35 A. Chandra, NIT Durgapur – How to combat fading?
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 36 A. Chandra, NIT Durgapur – How to combat fading?
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 37 A. Chandra, NIT Durgapur – How to combat fading?
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 38 A. Chandra, NIT Durgapur – How to combat fading?
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. 39 A. Chandra, NIT Durgapur – How to combat fading?
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?

More Related Content

PPTX
OFDM
byShubham Dhingra
 
PPT
Precoding
byKhalid Hussain
 
PPTX
Concept of Diversity & Fading (wireless communication)
byOmkar Rane
 
PPTX
Wireless Channel Impairment Mitigation Techniques
bymohammedalimahdi
 
PPT
Fading Seminar
byRajesh Kumar
 
PDF
Diversity techniques presentation material
byNini Lashari
 
PDF
IS95 CDMA Technology
byAditya Sharat
 
PPT
MIMO OFDM
byDarshan Patil
 
OFDM
byShubham Dhingra
 
Precoding
byKhalid Hussain
 
Concept of Diversity & Fading (wireless communication)
byOmkar Rane
 
Wireless Channel Impairment Mitigation Techniques
bymohammedalimahdi
 
Fading Seminar
byRajesh Kumar
 
Diversity techniques presentation material
byNini Lashari
 
IS95 CDMA Technology
byAditya Sharat
 
MIMO OFDM
byDarshan Patil
 

What's hot

PPTX
OKUMURA, HATA and COST231 Propagation Models
byMohammed Abuibaid
 
PPTX
Basic cellular system
byShubhamMishra485
 
PPTX
MIMO.ppt (2) 2
byDeepak Kumar Dhal
 
PPT
Radio propagation
byNguyen Minh Thu
 
PDF
4.5 equalizers and its types
byJAIGANESH SEKAR
 
PPT
Multiple access techniques for wireless communications
byMETHODIST COLLEGE OF ENGG & TECH
 
PPTX
Frequency Division Multiple Access (FDMA)
byMiles Kevin Galario
 
PDF
Diversity Techniques in Wireless Communication
bySahar Foroughi
 
PPT
GSM channels
byMohd Nazir Shakeel
 
PPTX
OFDM
bySuman Yadav
 
PPT
SPREAD SPECTRUM
byMETHODIST COLLEGE OF ENGG & TECH
 
PPT
Small scale fading
byAJAL A J
 
PPTX
Chap 5 (small scale fading)
byasadkhan1327
 
PPT
Interference and system capacity
byAJAL A J
 
PPT
Ee424 fading
byShraddha Bansal
 
PDF
Multiple Access Techniques
byInternational Islamic University Chittagong
 
PPTX
WCDMA
byHarshal Tiwari
 
PPT
Introduction To Wireless Fading Channels
byNitin Jain
 
PPT
Small Scale Multi path measurements
bySiva Ganesan
 
PPTX
IS-95 Cdma
byyogesh singh
 
OKUMURA, HATA and COST231 Propagation Models
byMohammed Abuibaid
 
Basic cellular system
byShubhamMishra485
 
MIMO.ppt (2) 2
byDeepak Kumar Dhal
 
Radio propagation
byNguyen Minh Thu
 
4.5 equalizers and its types
byJAIGANESH SEKAR
 
Multiple access techniques for wireless communications
byMETHODIST COLLEGE OF ENGG & TECH
 
Frequency Division Multiple Access (FDMA)
byMiles Kevin Galario
 
Diversity Techniques in Wireless Communication
bySahar Foroughi
 
GSM channels
byMohd Nazir Shakeel
 
OFDM
bySuman Yadav
 
SPREAD SPECTRUM
byMETHODIST COLLEGE OF ENGG & TECH
 
Small scale fading
byAJAL A J
 
Chap 5 (small scale fading)
byasadkhan1327
 
Interference and system capacity
byAJAL A J
 
Ee424 fading
byShraddha Bansal
 
Multiple Access Techniques
byInternational Islamic University Chittagong
 
WCDMA
byHarshal Tiwari
 
Introduction To Wireless Fading Channels
byNitin Jain
 
Small Scale Multi path measurements
bySiva Ganesan
 
IS-95 Cdma
byyogesh singh
 

Viewers also liked

PDF
wireless communications
byfaisalsaad18
 
PPTX
Multichannel fading
byShree Krupa
 
PDF
Co-channel and adjacent-channel interference evaluation of an outdoor telecom...
byKonstantinos Stamatakis
 
PPT
Cellular network
byMr SMAK
 
PPT
Cellular communication
byPrashant Gajendra
 
PDF
Gsm channel calculations
byAlthaf Hussain
 
PPTX
CCI
byNimisha Chauhan
 
DOCX
Hata model + youngs model + okumara model summarizad
byKhalid Ali
 
PPTX
Cellular network
byshreb
 
PPTX
cellular concepts in wireless communication
byasadkhan1327
 
PPT
Geometry plane figures
bynbratteli
 
DOCX
Improving coverage and capacity in cellular systems
byTarek Nader
 
PPTX
CELLULAR COMMUNICATION SYSTEM
bySuryani Ilias
 
PPT
The cellular concept
byZunAib Ali
 
wireless communications
byfaisalsaad18
 
Multichannel fading
byShree Krupa
 
Co-channel and adjacent-channel interference evaluation of an outdoor telecom...
byKonstantinos Stamatakis
 
Cellular network
byMr SMAK
 
Cellular communication
byPrashant Gajendra
 
Gsm channel calculations
byAlthaf Hussain
 
CCI
byNimisha Chauhan
 
Hata model + youngs model + okumara model summarizad
byKhalid Ali
 
Cellular network
byshreb
 
cellular concepts in wireless communication
byasadkhan1327
 
Geometry plane figures
bynbratteli
 
Improving coverage and capacity in cellular systems
byTarek Nader
 
CELLULAR COMMUNICATION SYSTEM
bySuryani Ilias
 
The cellular concept
byZunAib Ali
 

Similar to combat fading in wireless

PDF
Fading Techniques in Wireless Communication and their Appropriate Solution A ...
byijtsrd
 
PDF
Fading in wireless propagation channel
bySunandita Debnath
 
PPT
Characteristics of radio waves
byKundan Kumar
 
PDF
Fading models text
bySagar Ahir
 
PPT
Fading in wireless communication.ppt
bypravin patil
 
PPTX
Power delay profile,delay spread and doppler spread
byManish Srivastava
 
PDF
Rf Signal Loss And Its Minimization
bymscs12
 
PPTX
wireless networks
byKanwal Khan
 
PPT
Lecture2
byManwinder Singh
 
PDF
Spatial Interference and It’s Effect Towards the Performance of 4G Network
byRosdiadee Nordin
 
PPTX
Introduction to wireless fading channels
byManwinder Singh
 
PDF
Bit Error Rate Assessment of Digital Modulation Schemes on Additive White Gau...
byinventionjournals
 
PDF
1propagation
byCanzie
 
PPT
Fadingseminar 100713035600-phpapp02
byAUT
 
PPT
Multipl access and satellite Communication
byZunAib Ali
 
PPT
wireless networks wireless communication lecture 3.ppt
byhodcsituoj
 
PPT
EEE440 Lecture Slide Part 2ver2011-2012student.ppt
bytripoli University
 
PDF
Survey of analysis and performance of ofdm signals in time and frequency disp...
byeSAT Publishing House
 
PPT
Combating fading channels (1) (3)
byliril sharma
 
PPTX
Implementation of Wireless Channel Model in MATLAB: Simplified
byRosdiadee Nordin
 
Fading Techniques in Wireless Communication and their Appropriate Solution A ...
byijtsrd
 
Fading in wireless propagation channel
bySunandita Debnath
 
Characteristics of radio waves
byKundan Kumar
 
Fading models text
bySagar Ahir
 
Fading in wireless communication.ppt
bypravin patil
 
Power delay profile,delay spread and doppler spread
byManish Srivastava
 
Rf Signal Loss And Its Minimization
bymscs12
 
wireless networks
byKanwal Khan
 
Lecture2
byManwinder Singh
 
Spatial Interference and It’s Effect Towards the Performance of 4G Network
byRosdiadee Nordin
 
Introduction to wireless fading channels
byManwinder Singh
 
Bit Error Rate Assessment of Digital Modulation Schemes on Additive White Gau...
byinventionjournals
 
1propagation
byCanzie
 
Fadingseminar 100713035600-phpapp02
byAUT
 
Multipl access and satellite Communication
byZunAib Ali
 
wireless networks wireless communication lecture 3.ppt
byhodcsituoj
 
EEE440 Lecture Slide Part 2ver2011-2012student.ppt
bytripoli University
 
Survey of analysis and performance of ofdm signals in time and frequency disp...
byeSAT Publishing House
 
Combating fading channels (1) (3)
byliril sharma
 
Implementation of Wireless Channel Model in MATLAB: Simplified
byRosdiadee Nordin
 

More from Aniruddha Chandra

PDF
Undergraduate research and IEEE
byAniruddha Chandra
 
PPT
Green wireless communication with relays
byAniruddha Chandra
 
PPT
Walkie Talkie
byAniruddha Chandra
 
PPT
wireless overview
byAniruddha Chandra
 
PPT
Information & Communication Technology for disabled
byAniruddha Chandra
 
PPT
Phase locked loop
byAniruddha Chandra
 
PPT
Simulating communication systems with MATLAB: An introduction
byAniruddha Chandra
 
PPT
Energy conservation in wireless communication systems with relays
byAniruddha Chandra
 
Undergraduate research and IEEE
byAniruddha Chandra
 
Green wireless communication with relays
byAniruddha Chandra
 
Walkie Talkie
byAniruddha Chandra
 
wireless overview
byAniruddha Chandra
 
Information & Communication Technology for disabled
byAniruddha Chandra
 
Phase locked loop
byAniruddha Chandra
 
Simulating communication systems with MATLAB: An introduction
byAniruddha Chandra
 
Energy conservation in wireless communication systems with relays
byAniruddha Chandra
 

Recently uploaded

PDF
TravelTech Paris 2025 | The EU Digital Identity Wallet and it’s impact on Tra...
byapidays
 
PPTX
Tech Trends 2026: AI Agents, Quantum Computing, Robotics & Cybersecurity
byridwansassman
 
PPTX
Workflow and decision Automation with Flowable
bychakib ch
 
PDF
Logical Optimal Actions – Towards Knowledge-based Reinforcement Learning with...
byMichiaki Tatsubori
 
PDF
GDG Cloud Southlake #49: Pradeep R Kumar: Implications of Agentic AI for Iden...
byJames Anderson
 
PDF
Explaining the flow of purpose-specific BOM
byakipii ogaoga
 
PDF
How does MES(Manufacturing Execution System) work?
byakipii ogaoga
 
PPTX
Do You Control the AI, or Does the AI Control You?
bymedhateltoukhy
 
PDF
final~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.pdf
byomarbishtawi04
 
PPTX
TravelTech Paris 2025 | Beyond the pipes: Why Channel Management isn’t really...
byapidays
 
PDF
Spec-Driven Development with Kiro: Elevating Software Quality, Traceability, ...
byHaim Michael
 
PDF
February 2026 Patch Tuesday hosted by Chris Goettl and Todd Schell
byIvanti
 
PDF
Session 1/5: Enhancing Automation with Screenplay & Business Rules
bysuhanisingh58689
 
PDF
Escape from the Forbidden Zone: Smuggling green and inclusive tech past the g...
byBookNet Canada
 
PDF
Antminer S23Hyd-580_Manual-V1.1-oneminers.pdf
byFranzhLawrenceBataan1
 
PDF
Rustici Software: eLearning standards in the age of AI
byRustici Software
 
PDF
The new book “Marketing in the Metaverse” spotlights Scott M. Graffius’ research
byScott M. Graffius
 
PDF
Automated Governance for FME Flow: Smarter Admin at Scale
bySafe Software
 
PDF
GTM-and-Sales-Plan for a cyber security product
byAshish Jangir
 
PPTX
Critique Prompting: The Secret to High-Quality AI Outputs
bySemantic SEO BD
 
TravelTech Paris 2025 | The EU Digital Identity Wallet and it’s impact on Tra...
byapidays
 
Tech Trends 2026: AI Agents, Quantum Computing, Robotics & Cybersecurity
byridwansassman
 
Workflow and decision Automation with Flowable
bychakib ch
 
Logical Optimal Actions – Towards Knowledge-based Reinforcement Learning with...
byMichiaki Tatsubori
 
GDG Cloud Southlake #49: Pradeep R Kumar: Implications of Agentic AI for Iden...
byJames Anderson
 
Explaining the flow of purpose-specific BOM
byakipii ogaoga
 
How does MES(Manufacturing Execution System) work?
byakipii ogaoga
 
Do You Control the AI, or Does the AI Control You?
bymedhateltoukhy
 
final~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.pdf
byomarbishtawi04
 
TravelTech Paris 2025 | Beyond the pipes: Why Channel Management isn’t really...
byapidays
 
Spec-Driven Development with Kiro: Elevating Software Quality, Traceability, ...
byHaim Michael
 
February 2026 Patch Tuesday hosted by Chris Goettl and Todd Schell
byIvanti
 
Session 1/5: Enhancing Automation with Screenplay & Business Rules
bysuhanisingh58689
 
Escape from the Forbidden Zone: Smuggling green and inclusive tech past the g...
byBookNet Canada
 
Antminer S23Hyd-580_Manual-V1.1-oneminers.pdf
byFranzhLawrenceBataan1
 
Rustici Software: eLearning standards in the age of AI
byRustici Software
 
The new book “Marketing in the Metaverse” spotlights Scott M. Graffius’ research
byScott M. Graffius
 
Automated Governance for FME Flow: Smarter Admin at Scale
bySafe Software
 
GTM-and-Sales-Plan for a cyber security product
byAshish Jangir
 
Critique Prompting: The Secret to High-Quality AI Outputs
bySemantic SEO BD
 

combat fading in wireless

  • 1.
    J.U., 13th April,2007 How to Combat Fading in Wireless Channels? Aniruddha Chandra ECE Department, NIT Durgapur aniruddha.chandra@ieee.org 1 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 2 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 3 A. 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 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 6 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 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  12 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 13 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 14 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 15 A. Chandra, NIT Durgapur – How to combat fading?
  • 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 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 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 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 19 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 20 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 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 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 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 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 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 31 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 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  removes ISI from previous symbol •Much more sensitive to synchronization errors, High peak to average power 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 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 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 coherently R. 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 36 A. 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 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 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 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 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 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. 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?