How many users should inform the BS about their channel information?

Abstract

Simultaneous transmission of multiple data streams from a multiple antenna base station (BS) to multiple single antenna users gives significant gain in spectral efficiency as compared to when a single such user is being served. This simultaneous transmission to multiple users is realizable only if BS knows the forward channels linking its transmitting antennas to these users which requires channel feedback from these users. This feedback overhead could be prohibitively large especially in large user systems, limiting the multi-user transmission gains. Exploiting the channel reciprocity in a time-division duplexed (TDD) broadcast channel, we give a simple transmission strategy, where users feedback independent of their channel realizations. We analyze the sum rate of this multi-user system when the channel acquisition load is completely accounted for. We derive a novel lower bound of the sum rate which allows us to optimize over how many users should inform the BS about their channel information, solving the intriguing trade-off of multiuser diversity, interference cancellation and feedback overhead.