MIMO-OFDMA rate allocation and beamformer design using a multi-access channel framework

This paper tackles the downlink user scheduling and transmit beamforming problems in MIMO-OFDMA by extending a recent algorithm that maximizes the weighted sum rate (WSR) to all users in MIMO flat fading channels. The proposed method has a complexity that is proportional to the number of OFDMA subcarriers, which makes it practically attractive. Having assigned users to each subcarrier and designed beamformers for each user in each subcarrier, it remains to find the best, in terms of rate maximization, adaptive modulation and coding (AMC) mode to use for each data stream. The latter problem is solved in the second half of the paper through viewing the channel from the base station to the k-th receiver as a multiple access channel (MAC) with N_k “users”, where N_k is the number of antennas at receiver k. The proposed method maps the available AMC modes to the space of allowed theoretical rates, using the signal-to-noise ratio (SNR) gap to capacity concept, and selects the operating point with the largest sum-rate.