Joint Cooperative Diversity and Scheduling with Distributed Transmit Beamforming in OFDMA Relay System

Cooperative relay diversity and adaptive scheduling for active source/destination pairs in OFDMA-based relay network can significantly improve the reliability of data transmission over wireless fading channels as well as enhance the aggregate and per-link throughputs. However, in slow-varying fading channel, this technique suffers from low per-link throughput since some users with highest SNR at the destination access the channel for a long period while others will have to wait until their channel condition improves. In this paper we propose joint transmit beamforming and fixed cyclic delay diversity (CDD) at the relay nodes to enhance the transmit signals from relays to the destination link and to improve the perlink throughput performance. Combining transmit beamforming with CDD approach creates more fluctuation among subcarriers resulting in time-varying SNR at each destination. Therefore, the scheduler can now successfully provide more chance to users to have access to the channel by allocating subcarriers to users whose SNR are highest and thus the per-link throughput is enhanced. Simulation results show that the system performance using the proposed method increases significantly compared with the other previously proposed cooperative diversity techniques.