Adaptive demodulate-and-forward relaying for asymmetric two-way relay channels

This paper presents an adaptive relaying scheme for an asymmetric two-way relaying system in which physical-layer network coding (PNC) is applied. In particular, we study a demodulate-and-forward (DmF) relay and indicate that PNC may yield to other schemes in some specific channel states, such as a case when the received powers of the two sources differ much in the uplink channel. To tackle the problem of PNC in such asymmetric two-way relay channels, we propose the adaptive DmF relaying scheme by optimizing system sum rate of various channel states. Specifically, we set a power ratio threshold to separate two different relay modes: if the ratio of the received powers of the two sources is smaller than the threshold, PNC is used at the relay to map the uplink overlapped signals into network-coded signal (namely PNC mode); if the ratio is larger than the threshold, the relay demodulates the stronger signal only, while treating the other one as interference, and then it forwards the demodulated signal in the downlink phase (namely single-user mode). That is, the proposed scheme allows the relay to operate in either PNC mode or single-user mode, based on the instantaneous uplink channel conditions. Finally, simulation results are provided to validate that the proposed adaptive DmF relaying scheme achieves higher data rate compared with the traditional PNC scheme in asymmetric channel scenarios.