Multiuser diversity with outdated channel feedback

Inspired by the information theoretic results concerning multiuser diversity, we address practical issues in implementing multiuser diversity in a realistic multiple access wireless setting. Using the optimal channel-assigning policy that assigns the channel to only one user with the best channel gain [1], our emphasis is on the effects of channel feedback delay in downlink transmissions. A finite set of M-QAM constellations is adopted and a constant transmit power is assumed in this practical scenario. Based on the closed-form expressions for exact and approximate average BER and sum throughput we derive under the outdated channel feedback model, we illustrate the impact of channel feedback delay on the achievable multiuser diversity gain with the number of users. Simple and accurate asymptotic approximations are also provided in the limit of large number of users. A novel robust constant power, variable rate M-QAM scheme that is less sensitive to feedback delay is proposed using the optimal set of switching thresholds, which is derived to maximize the average sum throughput subject to an average BER constraint.