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Wireless Modulation Schemes

Wireless Modulation Schemes. Lecture 5. Announcements . Mid term test on Wednesday April 24. Project proposals. Wireless Modulation Tradeoffs. Want high rate, low power, robust to channel variations, low cost. Amplitude/Phase Modulation (MPSK,MQAM)

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Wireless Modulation Schemes

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  1. Wireless ModulationSchemes Lecture 5

  2. Announcements • Mid term test on Wednesday April 24. • Project proposals

  3. Wireless Modulation Tradeoffs • Want high rate, low power, robust to channel variations, low cost. • Amplitude/Phase Modulation (MPSK,MQAM) • Linear: Information encoded in amplitude/phase • High spectrum efficiency • Major issue: sensitive to channel variations • Frequency Modulation (FSK) • Nonlinear: Information encoded in frequency • More robust to channel variations • Major issue: Low spectrum efficiency

  4. Amplitude/Phase Modulation • Signal over ith symbol period: • Signal constellation defined by (si1,si2) pairs • M possible sets for (si1,si2): log2 M bits per symbol • Probability of symbol error (Ps ) depends on: • Minimum distance dmin (depends on gs) • Number of nearest neighbors aM • Approximate expression:

  5. Alternate Q Function Representation • Traditional Q function representation • Infinite integrand • Argument in integral limits • New representation (Craig’93) • Leads to closed form solution for Ps in PSK • Very useful in fading and diversity analysis

  6. Receiver Structure in AWGN Channel Si1(t) MAX SiN(t)

  7. Performance Comparison Goldsmith, Table 6.1 • Notice that for higher order constellation become higher the modulation scheme become less efficient

  8. Linear Modulation in Fading • In fading gsand therefore Psrandom • Performance metrics: • Outage probability: Prob(Ps>Ptarget)=Prob(g<gtarget) • Average Ps , Ps:

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

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

  11. Average BER for Common Schemeswith Rayleigh fading In general:

  12. Loss of Fading (BPSK)

  13. Fading Performance of MQAM

  14. Main Points • Linear modulation more spectrally efficient but less robust than nonlinear modulation • Psapproximation in AWGN: • In fading Psis a random variable, characterized by average value • Fading greatly increases average Ps .

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