VARIABLE-RATE VARIABLE-POWER MQAM FOR FADING CHANNELS

We propose a variable-rate and variable-power MQAM modulation scheme f or high-speed data transmission over fading channels. We first review results for the Shannon capacity of fading channels with channel side information, where capacity is achieved using adaptive transmission te chniques, We then derive the spectral efficiency of our proposed modul ation. We show that there is a constant power gap: between the spectra l efficiency of our proposed technique and the channel capacity, and t his gap is a simple function of the required bit-error rate (BER). In addition, using just five or six different signal constellations, we a chieve within 1-2 dB of the maximum efficiency using unrestricted cons tellation sets. We compute the rate at which the transmitter needs to update its power and rate as a function of the channel Doppler frequen cy for these constellation sets, We also obtain the exact efficiency l oss for smaller constellation sets, which may be required if the trans mitter adaptation rate is constrained by hardware limitations. Our mod ulation scheme exhibits a 5-10-dB power gain relative to variable-powe r fixed-rate transmission, and up to 20 dB of gain relative to nonadap tive transmission. We also determine the effect of channel estimation error and delay on the BER performance of our adaptive scheme. We conc lude with a discussion of coding techniques and the relationship betwe en our proposed modulation and Shannon capacity.