Adaptive scheduling for OFDM bidirectional transmission with a buffered relay

Most existing works about scheduling and resource allocation for orthogonal frequency division multiplexing (OFDM) based two-way relay networks have focused on immediate relay forwarding. In this paper, we consider relay buffering in delay-tolerant networks. The relay node is aided by two buffers and one for each user, so that it can adaptively decide when to buffer the received packets or to forward them according to the instantaneous channel and queue conditions. We formulate the joint optimization of subcarrier assignment, transmission mode selection (direct or relay mode), and relay strategy selection (buffering or forwarding), for maximizing the long-term average throughput. An efficient dual-based algorithm is proposed to characterize the optimal policy. Simulation results show that relay buffering can significantly enhance the long-term throughput in OFDM bidirectional transmission systems.