An Adaptive Transmission Scheme for Two-Way Relaying with Asymmetric Data Rates

In this paper, we address the problem of asymmetric data rates in two-way communication systems. In particular, we propose an adaptive transmission scheme that combines network coding (NC) and opportunistic user selection (OUS) with a threshold that determines which transmission mode to use. The underlying system model comprises two source nodes communicating with each other through a relay node. The source nodes are assumed to have different data rate requirements; therefore, they employ different modulation schemes. As per the proposed scheme, if the end-to-end (E2E) signal-to-noise ratio (SNR) of both users are above a specified threshold, both sources transmit over orthogonal channels, and the relay node uses hierarchical modulation and NC to relay the combined signals to both sources in the third time slot. Otherwise, the user with the better E2E SNR transmits, whereas the other user remains silent. The advantage of the proposed scheme is that it compromises between throughput and reliability. That is, when both users transmit, the throughput improves, whereas when the better user transmits, multiuser diversity is achieved. Assuming asymmetric channels, we derive exact closed-form expressions for the E2E bit error rate (BER), access probability, and throughput for this scheme and compare its performance with existing schemes. We also investigate the asymptotic performance of the proposed scheme at high SNRs where we derive the achievable diversity order for both users. We show through analytical and simulation results that the proposed scheme improves the overall system throughput, the fairness between the two users, and the transmission reliability. This all comes while achieving full spatial diversity for both users.