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Point-to-Point Wireless Communication ISI & Equalization, Diversity

Point-to-Point Wireless Communication ISI & Equalization, Diversity 43
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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Point-to-Point Wireless Communication (II): ISI & Equalization, Diversity (Time/Space/Frequency) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 1Multi-dimensional Fading  Time, Frequency, Space Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 2Plan  First, compare 1-tap (i.e. flat) Rayleigh-fading channel vs AWGN.  i.e. y = hx + w vs y = x + w  Note: all multipaths with random attenuation/phases are aggregated into 1-tap  Next consider frequency selectivity, i.e. multi-tap, broadband channel, with multi-paths  Effect: ISI  Equalization techniques for ISI & complexities  Generalize Consider diversity in time, space, frequency, and develop efficient schemes to achieve diversity gains and coding gains Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 3Single-tap, Flat Fading (Rayleigh) vs AWGN Why do we have this huge degradation in performance/reliability? Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 4Rayleigh Flat Fading Channel BPSK: Coherent detection. Looks like Conditional on h, AWGN, but… p needs to be e “unconditioned” To get a much Averaged over h, poorer scaling at high SNR. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 5BER vs. SNR (cont.) Frequency-selective channel BER (equalization or Rake receiver) () P e Frequency-selective channel (no equalization) “BER floor” AWGN channel Flat fading channel (no fading) () SNR 0 P14 means a straight line in log/log scale e 0 Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 6Typical Error Event Conditional on h, When the error probability is very small. When the error probability is large: Typical error event is due to: channel (h) being in deep fade … rather than (additive) noise being large. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 7Preview: Diversity Gain: Intuition  Typical error (deep fade) event probability:  In other words, h w/x  i.e. hx w  (i.e. signal x is attenuated to be of the order of noise w) Chi-Squared pdf of Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 8Recall: BPSK, QPSK and 4-PAM  BPSK uses only the I-phase.The Q-phase is wasted.  QPSK delivers 2 bits per complex symbol.  To deliver the same 2 bits, 4-PAM requires 4 dB more transmit power.  QPSK exploits the available degrees of freedom in the channel better. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 9MQAM doesn’t change the asymptotics…  QPSK does use degrees of freedom better than equivalent 4-PAM  (Read textbook, chap 3, section 3.1) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 10Frequency Selectivity: Multipath fading & ISI Mitigation: Equalization & Challenges Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 11ISI Mitigation: Outline  Inter-symbol interference (ISI): review  Nyquist theorem  Pulse shaping (last slide set)  1. Equalization receivers  2. Introduction to the diversity approach  Rake Receiver in CDMA  OFDM: decompose a wideband multi-tap channel into narrowband single tap channels Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 12Recall: Attenuation, Dispersion Effects: ISI Inter-symbol interference (ISI) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute Source: Prof. Raj Jain, WUSTL : “shiv rpi” 13Recall: Multipaths: Power-Delay Profile path-1 path-2 path-3 multi-path propagation path-2 Path Delay path-1 path-3 Mobile Station (MS) Base Station (BS) Channel Impulse Response: Channel amplitude h correlated at delays . Each “tap” value kTs Rayleigh distributed (actually the sum of several sub-paths) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 14 PowerInter-Symbol-Interference (ISI) due to Multi- Path Fading Transmitted signal: Received Signals: Line-of-sight: Reflected: The symbols add up on the Delays channel  Distortion Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 15Multipath: Time-Dispersion = Frequency Selectivity  The impulse response of the channel is correlated in the time-domain (sum of “echoes”)  Manifests as a power-delay profile, dispersion in channel autocorrelation function A()  Equivalent to “selectivity” or “deep fades” in the frequency domain  Delay spread: 50ns (indoor) – 1s (outdoor/cellular).  Coherence Bandwidth: Bc = 500kHz (outdoor/cellular) – 20MHz (indoor)  Implications: High data rate: symbol smears onto the adjacent ones (ISI). Multipath effects O(1s) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 16BER vs. S/N performance: AWGN In a Gaussian channel (no fading) BER = Q(S/N) erfc(S/N) Typical BER vs. S/N curves BER Frequency-selective channel (no equalization) Gaussian channel Flat fading channel (no fading) S/N Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 17BER vs. S/N performance: Flat Fading Flat fading: BER BER S N z p z dz   z = signal power level Typical BER vs. S/N curves BER Frequency-selective channel (no equalization) Gaussian channel Flat fading channel (no fading) S/N Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 18BER vs. S/N performance: ISI/Freq. Selective Channel Frequency selective fading = irreducible BER floor Typical BER vs. S/N curves BER Frequency-selective channel (no equalization) Gaussian channel Flat fading channel (no fading) S/N Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 19BER vs. S/N performance: w/ Equalization Diversity (e.g. multipath diversity) = improved performance Typical BER vs. S/N curves BER Gaussian Frequency-selective channel channel (with equalization) Flat fading channel (no fading) S/N Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 20