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EE359 – Lecture 13 Outline

EE359 – Lecture 13 Outline. Adaptive MQAM: optimal power and rate Finite Constellation Sets Practical Constraints Update rate Estimation error Estimation delay. Review of Last Lecture. Maximal Ratio Combining MGF Approach for Performance of MRC EGC

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EE359 – Lecture 13 Outline

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  1. EE359 – Lecture 13 Outline • Adaptive MQAM: optimal power and rate • Finite Constellation Sets • Practical Constraints • Update rate • Estimation error • Estimation delay

  2. Review of Last Lecture • Maximal Ratio Combining • MGF Approach for Performance of MRC • EGC • Harder to analyze than MRC; lose ~1 dB • Transmit diversity • With channel knowledge, similar to receiver diversity, same array/diversity gain • Without channel knowledge, can obtain diversity gain through Alamouti scheme over 2 consecutive symbols

  3. Adaptive Modulation • Change modulation relative to fading • Parameters to adapt: • Constellation size • Transmit power • Instantaneous BER • Symbol time • Coding rate/scheme • Optimization criterion: • Maximize throughput • Minimize average power • Minimize average BER Only 1-2 degrees of freedom needed for good performance

  4. One of the M(g) Points log2 M(g) Bits To Channel M(g)-QAM Modulator Power: P(g) Point Selector Uncoded Data Bits Delay g(t) g(t) 16-QAM 4-QAM BSPK Variable-Rate Variable-Power MQAM Goal: Optimize P(g) and M(g) to maximize R=Elog[M(g)]

  5. Optimization Formulation • Adaptive MQAM: Rate for fixed BER • Rate and Power Optimization Same maximization as for capacity, except for K=-1.5/ln(5BER).

  6. gk g Optimal Adaptive Scheme • Power Adaptation • Spectral Efficiency g Equals capacity with effective power loss K=-1.5/ln(5BER).

  7. K2 K1 K=-1.5/ln(5BER) Spectral Efficiency Can reduce gap by superimposing a trellis code

  8. Constellation Restriction • Restrict MD(g) to {M0=0,…,MN}. • Let M(g)=g/gK*, where gK* is later optimized. • Set MD(g) to maxj Mj: Mj M(g). • Region boundaries are gj=MjgK*, j=0,…,N • Power control maintains target BER M3 M(g)=g/gK* MD(g) M3 M2 M2 M1 M1 Outage 0 g0 g1=M1gK* g2 g3 g

  9. Power Adaptation and Average Rate • Power adaptation: • Fixed BER within each region • Es/N0=(Mj-1)/K • Channel inversion within a region • Requires power increase when increasing M(g) • Average Rate

  10. Efficiency in Rayleigh Fading Spectral Efficiency (bps/Hz) Average SNR (dB)

  11. Practical Constraints • Constellation updates: fade region duration • Error floor from estimation error • Estimation error at RX can cause error in absence of noise (e.g. for MQAM) • Estimation error at TX causes mismatch of adaptive power and rate to actual channel • Error floor from delay: let r(t,t)=g(t-t)/g(t). • Feedback delay causes mismatch of adaptive power and rate to actual channel

  12. Main Points • Adaptive modulation leverages fast fading to improve performance (throughput, BER, etc.) • Adaptive MQAM uses capacity-achieving power and rate adaptation, with power penalty K. • Comes within 5-6 dB of capacity • Discretizing the constellation size results in negligible performance loss. • Constellations cannot be updated faster than 10s to 100s of symbol times: OK for most dopplers. • Estimation error/delay causes error floor

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