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Understanding Fading Effects in Wireless Communications

Explore fading phenomena in wireless communication systems, including Doppler and ISI effects. Learn MGF and outage probability approaches for Ps calculation. Understand the impact on modulation schemes and error probabilities.

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Understanding Fading Effects in Wireless Communications

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  1. EE359 – Lecture 10 Outline • Average Ps (Pb) • MGF approach for average Ps • Combined average and outage Ps • Ps due to Doppler ISI • Ps due to ISI

  2. Ts Outage Ps Ps(target) t or d Review of Last Lecture • Linear modulation more spectrally efficient but less robust than nonlinear modulation • Psapproximation in AWGN: • Probability of error in fading is random • Characterized by outage, average Ps, combination • Outage probability • Probability Ps is above target; Probability gs below target • Fading severely degrades performance • Used when Tc>>Ts

  3. Average Ps • Expected value of random variable Ps • Used when Tc~Ts • Error probability much higher than in AWGN alone • Alternate Q function approach: • Simplifies calculations (Get a Laplace Xfm) Ts Ps Ps t or d

  4. 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

  5. Doppler Effects • High doppler causes channel phase to decorrelate between symbols • Leads to an irreducible error floor for differential modulation • Increasing power does not reduce error • Error floor depends on BdTs

  6. Tm ISI Effects • Delay spread exceeding a symbol time causes ISI (self interference). • ISI leads to irreducible error floor • Increasing signal power increases ISI power • ISI requires that Ts>>Tm (Rs<<Bc) 1 2 3 4 5 0 Ts

  7. Main Points • In fading Psis a random variable, characterized by average value, outage, or combined outage/average • Fading greatly increases average Ps . • Alternate Q function approach simplifies Pscalculation, especially its average value in fading (Laplace Xfm). • In fast/slow fading, outage is due to shadowing, probability of error then averaged over fast fading pdf • Doppler spread only impacts differential modulation causing an irreducible error floor at low data rates • Delay spread causes irreducible error floor or imposes rate limits • Need to combat flat and frequency-selective fading • Focus of the remainder of the course

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