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Adaptive Modulation

Adaptive Modulation. Outline. Adaptive MQAM: optimal power and rate Finite Constellation Sets Update rate Estimation error Estimation delay Practical Considerations in Adaptive Modulation. Adaptive Modulation. Change modulation relative to fading Parameters to adapt: Constellation size

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Adaptive Modulation

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  1. Adaptive Modulation

  2. Outline Adaptive MQAM: optimal power and rate Finite Constellation Sets Update rate Estimation error Estimation delay Practical Considerations in Adaptive Modulation

  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. Adaptive Technologies • Rate Control • Fixing symbol rate，using multiple modulation schemes or constellation sizes • Fairly easy, used in current systems, such as GSM, IS-136 EDGE or 802.11a, et. al. • Fixing the modulation, changing the symbol rate • Varying signal bandwidth is impractical and complicates bandwidth sharing. • Power Control • Adapting the transmit power alone is generally used to compensate for SNR variation due to fading. • To maintain a fixed bit error probability, or a constant received SNR. • The power adaptation thus inverts the channel fading so that the channel appears as an AWGN channel to the modulator and demodulator.

  5. Adaptive Technologies (cont) • Error Control • Adapt the instantaneous BER subject to an average BER constraint. • Not an adaptive technique, since the transmitter does not adapt to SNR. • Error probability is typically adapted along with some other form of adaption such as constellation size or modulation type. • Hybrid techniques • Adapt multiple parameters of the transmission scheme, including rate, power, coding, and instantaneous error probability. • Joint optimization of the different techniques is used to meet a given performance requirement. • Rate adaption is often combined with power adaptation to maximize spectral efficiency.

  6. 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)] The rate and power of MQAM are varied to maximize spectral efficiency while meeting a given instantaneous Pb target.

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

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

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

  10. 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

  11. 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

  12. Efficiency in Rayleigh Fading

  13. 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

  14. Detailed Formulas • Error floor from estimation error () • Joint distribution p(,) depends on estimation: hard to obtain. For PSAM the envelope is bi-variate Rayleigh • Error floor from delay: let =g[i]/g[i-id]. • p(|) known for Nakagami fading ^ ^

  15. 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 and delay lead to irreducible error floors in adaptive MQAM

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