An Iterative Noncoherent Relay Receiver for the Two-way Relay Channel

Physical-layer network coding improves the throughput of the two-way relay channel by allowing multiple source terminals to transmit simultaneously to the relay. However, it is generally not feasible to align the phases of the multiple received signals at the relay, which motivates the exploration of noncoherent solutions. In this paper, turbo-coded orthogonal multi-tone frequency-shift keying (FSK) is considered for the two-way relay channel. In contrast with analog network coding, the system considered is an instance of digital network coding; i.e., the relay decodes the network codeword and forwards a re-encoded version. Crucial to noncoherent digital network coding is the implementation of the relay receiver, which is the primary focus of the paper. The relay receiver derived in this paper supports any modulation order that is a power of two, and features the iterative feedback of a priori information from the turbo channel decoder to the demodulator; i.e., it uses bit interleaved coded modulation with iterative decoding (BICM-ID). The performance of the receiver is investigated in Rayeligh fading channels through error-rate simulations and a capacity analysis. Results show that the BICM-ID receiver improves energy efficiency by 0.5-0.9 dB compared to a non-iterative receiver implementation.