Degrees of Freedom in Adaptive Modulation: A Uni ed View

We examine adaptive modulation schemes for at-fading channels where the data rate, transmit power, and instantaneous BER are varied to maximize spectral e ciency, subject to an average power and BER constraint. Both continuous-rate and discrete-rate adaptation are considered, as well as average and instantaneous BER constraints. We nd that for a large class of modulation techniques and fading distributions, adapting only the power or only the BER in addition to the data rate yields near-optimal performance. The optimal adaptation of these parameters is a waterlling: increasing the power or decreasing the BER as the channel quality improves. Surprisingly, if the rate is adapted continuously, setting the power and BER to constant values is close to optimal, especially at high SNRs. Hence, the spectral e ciency of adaptive modulation is relatively insensitive to which degrees of freedom are adapted. Therefore, the design of adaptive modulation should be based on practical constraints like implementation complexity and tolerance to uctuations in BER and transmit power. SEONG TAEK CHUNG, DEGREES OF FREEDOM IN ADAPTIVE MODULATION: A UNIFIED VIEW 1