Increasing spectral efficiency through power control

The authors derive the spectral efficiency of narrow-band fading channels when channel estimation and transmitter feedback are used. They also determine an optimal policy for fading channels that achieves the maximum spectral efficiency by adapting the transmit power, data rate, and coding scheme. This is in contrast to a common policy which adapts only the transmit power to maintain a constant received SNR. Such constant power policies need to allocate most of their average power to compensate deep fades, thereby significantly reducing their spectral efficiency. The authors therefore propose a modified constant power policy which maintains a constant SNR at the receive only when the fade level is above an optimized threshold level. Although the modified policy is relatively simple, it performs very close to the optimal policy. The authors also analyze the spectral efficiency of uncoded M-QAM for adjustable and constant rate/power policies, and determine the maximum possible coding gain for M-QAM.>