Spectral Efficiency of Adaptive MQAM/OFDM Systems With CFO Over Fading Channels

In this correspondence, we investigate the impact of power and continuous-rate adaptation using variable-power M-ary quadrature amplitude modulation (MQAM) on spectral efficiency of orthogonal frequency-division multiplexing (OFDM) systems subject to carrier frequency offset (CFO) in Rayleigh fading channels with perfect and imperfect channel information at the transmitter. Convex optimization-based adaptation schemes to maximize the spectral efficiency subject to average power and instantaneous bit error rate (BER) constraints are used in this correspondence. Adaptive MQAM/OFDM systems under CFO with perfect channel information at the transmitter are shown to have substantial gain in spectral efficiency over their nonadaptive counterparts. Theoretical and simulation results suggest that adaptive modulation can also improve the spectral efficiency of OFDM systems under CFO with imperfect channel estimates in comparison with nonadaptive modulation, provided the correlation between the true and estimated channels is above 0.5. Furthermore, we have shown that the outage power threshold in our adaptive scheme increases with increasing CFO values and/or decreasing correlation between the true and estimated channels.