Variable-rate variable-power MQAM for fading channels

We propose a variable power and rate adaptive MQAM modulation scheme for high-speed data transmission on fading channels. We show that using just five or six different signal constellations achieves within 1-2 dB of the maximum spectral efficiency. We also obtain the exact efficiency loss for a smaller constellation set. In addition, we compare our technique with a fixed-rate scheme which inverts the channel fading. Assuming perfect power control, the fixed-rate scheme exhibits a 5-10 dB loss in spectral efficiency relative to our adaptive technique. We conclude by discussing performance degradation caused by channel measurement errors.