Adaptive M-qam Modulation over Nakagami Fading Channels

We study the performance of constant-power variable-rate M-QAM schemes over Nakagami fading channels. We obtain closed-form expressions for the outage probability, spectral eeciency and average bit-error-rate (BER) assuming perfect channel estimation and negligible time delay. For a target BER of 10 ?3 , the spectral eeciency of adaptive continuous rate M-QAM comes within 5 dB of the Shannon capacity limit, and adaptive discrete rate M-QAM comes within 6.2 dB of this limit. Nonadaptive BPSK suuers a large spectral eeciency penalty relative to these adaptive techniques. We also analyze the impact of time delay on the BER of adaptive M-QAM. Results show that systems with low BER requirements will be more sensitive to time delay, but will still operate satisfactorily if the delay is below a critical value.