Performance analysis of discrete-rate adaptive modulation for OFDM systems with imperfect CSI in Rayleigh fading channel

Performance of orthogonal frequency-division multiplexing (OFDM) systems with discrete-rate adaptive M-ary quadrature amplitude modulation (M-QAM) in Rayleigh fading channel with imperfect channel state information (CSI) is investigated in this paper. The adaptive scheme used in this paper is to maximize spectral efficiency (SE) subject to a target bit error rate (BER) and fixed power constrain. The fading gain value of each subchannel is partitioned into a number of regions by which the modulation of each subchannel is adapted in terms of the region the fading gain falls in. Based on this adaptive scheme, closed-form expressions of the average SE(ASE) and average BER with perfect and imperfect channel estimation are respectively obtained and confirmed by Monte Carlo simulations. Furthermore, the effect of channel estimation error on the performance of adaptive modulation OFDM system is analyzed.