An Algorithm for 2-User Downlink SDMA Beamforming with Limited Feedback for MIMO-OFDM Systems

Recent information theoretic results on MIMO broadcast channels have promoted research attention on devising practical transmit-receive strategies that achieve a reasonable fraction of the broadcast capacity. This paper proposes an algorithm for SDMA weight construction for two users. The weights are chosen to represent a subspace that lies between the dominant signal subspace of one user and the nullspace of the other. This proposal is unique due to its practical considerations - a broadband MIMO broadcast channel employing OFDM, imperfect CSI at the transmitter in the form of codebook based limited feedback, coded BER as a performance metric, and the ability to choose the users for SDMA arbitrarily. Simulation results in an IEEE 802.16e-style setup show that a coded BER within 1-2 dB of the case of perfect CSI can be achieved with a feedback of less than 1 bit per subcarrier. Additionally, maximum ratio combining receivers suffice and they may be oblivious to the multi-user nature of the SDMA transmission.