Analysis of Opportunistic Contention-Based Feedback Protocol for Downlink OFDMA

Abstract

In this paper, a new opportunistic feedback protocol for orthogonal frequency division multiple access (OFDMA) systems with mixed realtime (RT) and non-realtime (NRT) traffic is proposed and analysed. Based on the proposed feedback mechanism, a dedicated feedback channel is allocated for each RT user, while shared feedback channels are available for all NRT users. NRT users have to compete to send feedback via a contention-based random access feedback channel. The proposed feedback protocol requires a fixed amount of resource to carry feedback from all RT users and some selected NRT users. Simulation results show that the achievable throughput can be improved significantly by allocating a dedicated feedback channel for RT user compared with allocating a dedicated traffic channel. For NRT users, the achievable sum-rate of the proposed protocol when the number of users is moderate (20-50 users) approaches the ideal case when the channel quality of all users is known at the base station. The sum-rate also is close to the ideal case when the number contention minislots per subcarrier is as low as 2 minislot per subcarrier.