Block relay for physical-layer network coding

In this paper, we study the transmission strategy in a two-way relay channel. In particular, we propose a new class of log_M1 (M₂)-MA (multi-access) block relay protocols where an M1-ary modulation is used in the MA phase, and an M₂-ary in the broadcast phase. Lower-order (e.g., BPSK) and higher-order (e.g., 16-QAM, 32-PSK) modulations are used respectively for the multi-access phase and the broadcast phase, in contrast to the conventional method of using the same modulation type (of either higher or lower order) throughout. To achieve this, the relay buffers the received symbols from consecutive multi-access phases and performs signal combining in conjunction with the decode-and-forward protocol. We derive the theoretical performance bounds for the Rayleigh fading channels and closed forms for the AWGN channels. Numerical results confirm the advantages of the proposed scheme with an improved throughput in lower signal-to-noise ratio range.