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ECE637 : Fundamentals of Wireless Communications

ECE637 : Fundamentals of Wireless Communications. Lecture 7,8: Diversity Aliazam Abbasfar. Outline. Diversity types Diversity combining. Diversity. Relying on a single channel Deep fade disrupts communication All modulations perform very badly in fading P e Decays linearly with SNR

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ECE637 : Fundamentals of Wireless Communications

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  1. ECE637 : Fundamentals of Wireless Communications Lecture 7,8: Diversity Aliazam Abbasfar

  2. Outline • Diversity types • Diversity combining

  3. Diversity • Relying on a single channel • Deep fade disrupts communication • All modulations perform very badly in fading • Pe Decays linearly with SNR • Solution : diversity • Multiple independent channels for communication • Micro-diversity : multipath fading • Time, frequency, spatial, polarization, … • Macro-diversity : shadowing • Network management

  4. Time diversity • Sending bits again in different times • Same bits experience different fades • Repetition code + Interleaving • Interleaver spread > Coherence time (Tc) • L = # of repetition • Diversity branches • Less likely to be in deep fade all the time • Rate reduction penalty • Coded systems • Diversity, and • Coding gain

  5. Frequency diversity • Sending data over different frequency bands at the same time • Bandwidth separation > Coherence bandwidth (Wc) • The same baseband signal, different carrier frequencies • Multiple up-converter/down-converter circuits • Less likely to be in deep fade in all bands • Bandwidth penalty • Spectrum is very expensive

  6. Case study : GSM

  7. Spatial (antenna) diversity • Sending/receiving bits to/from multiple antennas • Antennas are separated enough to ensure un-correlated channels • Transmit/Receive diversity • 10l / l/2 antenna separation • No rate reduction or bandwidth penalty • Polarization diversity • Multipath channel have uncorrelated polarizations • No spatial separation

  8. Diversity combining • Selection(switch) diversity • Simplest type • Pick one branch to detect data • Maximal Ratio Combining (MRC) • Optimum combining of the received signals in all branches • Equal Gain Combining (EGC) • Combining of the received signals in all branches with equal gain

  9. Switch diversity • Best branch selection • Need L receivers to monitor SNRs • Switch to another branch when SNR is less than a threshold (SNR0) • Needs only one receiver • Switch and examine • Switch and stay • Deep fade is less likely • when all branches are in deep fade • Switching criterion • Narrowband : total received signals power • Wideband : Signal power + ISI measure

  10. Best branch selection • Outage probability • Fading margin for 1% outage • L= 1 : 20 dB • L=2 : 10 dB • L=3 : 6 dB • Probability of error

  11. Switch and stay • Switch to another branch when SNR falls bellow a threshold • Less sensitive to noise • Outage probability can be as good as best branch selection

  12. Maximal Ratio Combining (MRC) • Combine branches linearly • Find linear coefficients to maximize SNR • Optimum coefficients : hi* • Co-phase the branches • Branches with high SNR have higher weights • SNR = S SNRi =‖h‖2 SNRavg • E[ SNR] = L SNRavg • ‖h‖2 Chi-square distribution • BER

  13. Maximal Ratio Combining (MRC) • PeUpperbound • At high SNR • Achieves full diversity order

  14. Equal Gain Combining (EGC) • Co-phase the branches and add them • Simpler receiver (No variable gain) • Small performance degradation • 1 dB of power penalty

  15. Diversity gain – Array/Coding gain

  16. Reading • Ch. 7 Goldsmith • Ch. 3 Tse

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