User Selection for SDMA Beamforming with Imperfect CSI in MIMO-OFDM Systems

Leveraging multi-user diversity by selecting users from a user pool has been a critical strategy for realizing a fraction of the MIMO broadcast channel capacity. This paper considers exploiting multi-user diversity in a unique and practical setting that includes - significant transmit correlation, a broadband channel using OFDM, and imperfect CSI at the transmitter derived from channel reciprocity techniques. These imply a higher correlation amongst user channels and an increase in the volume of CSI per user. Under these assumptions, algorithms are presented for selecting G out of Nu users for applying SDMA beamforming and also for switching off SDMA when the G selected user channels may not support simultaneous transmission. In a EUTRA-type simulation setting, the algorithms lead to gains on the order of 5-7 dB in coded FER with reasonable costs in terms of CSI, sum-rate and computational complexity.