Two-Phase Multiuser Scheduling for Multiantenna Downlinks Exploiting Reduced Finite-Rate Feedback

This paper considers a multiantenna system with an M -antenna base station and K single antenna users, employing zero-forcing beamforming (ZFBF) and a finite-rate feedback strategy. We study scheduling to select KS users for transmit beamforming. To reduce the feedback requirement, we propose a two-phase scheduling scheme that exploits quantized channel direction information (CDI) from only a small subset of the overall users, as well as a 1-bit signal-to-interference-plus-noise ratio (SINR) indicator feedback per user. In the proposed scheme, we introduce a newly defined expression for evaluating SINR that extends the existing SINR approximation for KS = M to a more general case with arbitrary KS ¿ M. Asymptotic analysis reveals that the proposed two-phase scheduling is asymptotically optimal for large user numbers. Through numerical simulations, we also show that the proposed scheme requires less feedback than conventional schemes, whereas the resulting sum rate loss due to the reduced feedback is negligible. Moreover, we find that the choice of KS < M, instead of a full-multiplexing case with KS = M, for scheduling may achieve a better tradeoff between performance and feedback requirement in systems where fewer bits are utilized for feeding back CDI.