Practical User Pairing and Selective Network Coding for Multi-User Bidirectional Relaying

A low-complexity user pairing algorithm for the broadcast phase of the XOR-based network coding is presented. The proposed scheme achieves the same spectral efficiency as an exhaustive search approach based on maximizing the sum-rate when network coding is utilized for all transmissions. The proposed scheme is further complemented with an adaptive switch, per user pair, between network coding and conventional relaying to maximize the achievable sum-rate based on the instantaneous link conditions. The proposed user pairing and its extension are based on measurements readily available in today's networks, thus not requiring any signaling overhead. System-level simulations based on adaptive coding and modulation are utilized to quantify the gains from the proposed user pairing and adaptive transmission scheme switch. The obtained results show significant throughput gains especially in the low signal-to-noise ratio region. The gains are due to alleviating the link asymmetry problem in bidirectional relaying with network coding by exploiting multi-user diversity. This improved performance is obtained without any notable increase in complexity, thus rendering the proposed approach a good candidate for practical implementation purposes.