1 / 15

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

uyen
Download Presentation

Adaptive Modulation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related