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SYSC 4607 –Slide Set 13 Outline

SYSC 4607 –Slide Set 13 Outline. Review of Previous Lecture Outage Probability and Average P s Doppler Effects on Differential Modulations ISI Effects for Frequency-Selective Fading Modulations for Major Wireless Standards. Review of Previous Lecture.

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SYSC 4607 –Slide Set 13 Outline

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  1. SYSC 4607 –Slide Set 13 Outline • Review of Previous Lecture • Outage Probability and Average Ps • Doppler Effects on Differential Modulations • ISI Effects for Frequency-Selective Fading • Modulations for Major Wireless Standards

  2. Review of Previous Lecture • Error Probability Analysis – AWGN Channel • Performance of Digital Modulations on Flat-Fading Channels (Outage Probability and Average Probability of Error)

  3. Linear Modulations in Fading • For fading channels, SNR γs, and thus probability of error Ps are random. • Performance metrics: - Outage Probability Pout=p(Ps > Ptarget)=p(γs < γtarget) - Average Ps : - Combined outage and average Ps

  4. Ts Outage Ps Ps(target) t or d Outage Probability • Probability that Ps is above target • Equivalently, probability that gs is below target • Used when Tc>>Ts

  5. Average Ps • Expected value of random variable Ps • Used when Tc~Ts • Error probability much higher than in AWGN alone • Alternate Q function approach greatly simplifies calculations Ts Ps Ps t or d

  6. Average BER in AWGN and Rayleigh Fading Channels BER=10-3: SNR=8dB for AWGN SNR=24dB for Rayleigh

  7. Combined Outage and Average Ps : random SNR for fixed path loss and shadowing, but random fast fading. Probability of Error = Ps(γs) : random SNR for fixed path loss and random shadowing, but averaged over fast fading, i.e., Average Probability of Error = : average SNR for a fixed path loss; averaging over fast fading and shadowing, i.e.,

  8. Ps(gs) Ps(gs) Combined outage and average Ps Outage • Used in combined shadowing and flat-fading • Ps varies slowly, locally determined by flat fading • Declare outage when Ps above target value Ps(gs) Pstarget

  9. Doppler Effects • High Doppler causes channel phase to decorrelate between symbols • Leads to an irreducible error floor for differential modulation schemes such as DPSK, i.e., increasing power does not reduce error • Error floor depends on BDTs (equivalently, Ts/Tc)

  10. DPSK – Rician Fading • ρc is channel correlation coefficient after a bit time Tb, and K is the ratio of LOS power to multipath power

  11. Irreducible BER due to Doppler • For fixed fD, decreasing Tb (increasing the bit rate) decreases the error floor DPSK in fast Rician fading ( )

  12. Tm ISI Effects (Frequency-Selective Fading) • Delay spread exceeding a symbol time causes ISI (self interference). • ISI leads to irreducible error floor • Increasing signal power increases ISI power • For ISI-free transmission: Ts>>Tm (Rs<<Bc) 1 2 3 4 5 0 Ts

  13. ISI Effects (Frequency-Selective Fading) • Irreducible error rate is difficult to analyze. It depends on the specific channel model and choice of modulation. Closed form solution is often not possible. Simulation-based studies available. • An approximation: Pr and I are the signal power and ISI power, respectively.

  14. Irreducible BER due to ISI • Simulation results (Ave. Irreducible BER vs. d = σTm / Ts )

  15. Modulations for Major Standards • Second Generation: - GSM: GMSK - IS136: π/4DQPSK - IS-95: BPSK/QPSK - PDC: π/4DQPSK • Third Generation: - CDMA2000: QPSK (DL), BPSK (UL) Phase I MPSK (DL), QPSK (UL) Phase II - W-CDMA: QPSK (DL), BPSK (UL)

  16. Modulations for Major Standards • Wireless LAN - 802.11: BPSK, QPSK - 802.11a: BPSK, QPSK, MQAM - 802.11b: BPSK, QPSK - 802.11g: BPSK, QPSK, MQAM • Short Range Wireless Network - ZigBee (802.15.4): BPSK, OQPSK - Bluetooth(802.15.1): GFSK - UWB (802.15.3): BPSK, QPSK (proposal)

  17. Main Points • Fading greatly increases average Ps - Alternate Q function approach simplifies Ps calculation, especially its average value in fading - Moment Generating Function approach can be used effectively for average error probability calculations in fading • Doppler spread only impacts differential modulation, causing an irreducible error floor at low data rates • Delay spread causes ISI and irreducible error floor or imposes limits on transmission rates • Need to combat flat and frequency-selective fading - Focus of the rest of the course

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