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EE359 – Lecture 10 Outline

EE359 – Lecture 10 Outline. Average P s ( P b ) MGF approach for average P s Combined average and outage P s P s due to Doppler ISI P s due to ISI. T s. Outage. P s. P s(target). t or d. Review of Last Lecture.

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EE359 – Lecture 10 Outline

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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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