Transmission Strategies and Sum Rate Maximization in Multi-User TDD Systems

In a system comprising of a multiple antenna enabled base station (BS) and multiple single antenna users, very high data rates can be obtained if BS transmits to multiple users simultaneously. This simultaneous transmission to multiple users over the same bandwidth is realizable only if BS knows the forward channels linking its transmit antennas to these users. We study a time-division duplexed (TDD) broadcast channel with initial assumption of channel information neither at the BS nor at the users' side. We propose two simple transmission strategies, one where users feedback independent of their channel realizations (earlier proposed in [1]) and the other where users feedback based upon their channel realizations. We derive a lower bound of the sum rate which reflects the rate loss compared to a system with perfect channel knowledge and the corresponding approximate sum rate expressions are developed for both schemes. These expressions capture the benefits of channel feedback, multi-user diversity and inter-user interference cancellation, and the cost of exchange of information required, and hence can be optimized for the sum rate maximization.