Adaptive Modulation Based on Finite-Rate Feedback in Multiuser Diversity Systems

In a single-input single-output (SISO) downlink with multiple users, to obtain the multiuser diversity gain, the full channel state information (CSI) for all users is required for selecting the ´best´ user and transmission mode. However, feedback channels are often capable of carrying only a limited number of bits. With such rate-limited feedback links, we intend to maximize the sum-rate of systems with average power and bit-error rate (BER) constraints by varying the transmission mode according to feedback information. Althrough in multiuser systems with finite-rate feedback, the scheduled user is not necessarily the ´best´ user, this work turns out to be a design of transmission modes based on CSI of the ´best´ user. A nested iterative algorithm is proposed to obtain optimal thresholds and discrete transmission modes. Simulation results demonstrate the throughput with finite-rate feedback approaches that with perfect CSI when the feedback rate increases. As the number of users increases, the multiuser diversity gain is achieved and the average feedback load is reduced without the decrease of throughput.