Channel coding and decoding in a relay system operated with physical-layer network coding

Abstract

This paper investigates link-by-link channel-coded PNC (physical layer network coding), in which a critical process at the relay is to transform the superimposed channel-coded packets received from the two end nodes (plus noise), Y3= X1+ X2+W3, to the network-coded combination of the source packets, S1oplus S2. This is in contrast to the traditional multiple-access problem, in which the goal is to obtain both S1and S2explicitly at the relay node. Trying to obtain S1and S2explicitly is an overkill if we are only interested in S1oplusS2. In this paper, we refer to the transformation Y3rarr S1oplus S2as the channel-decoding- network-coding process (CNC) in that it involves both channel decoding and network coding operations. This paper shows that if we adopt the repeat accumulate (RA) channel code at the two end nodes, then there is a compatible decoder at the relay that can perform the transformation Y3rarr S1oplusS2efficiently. Specifically, we redesign the belief propagation decoding algorithm of the RA code for traditional point-to-point channel to suit the need of the PNC multiple-access channel. Simulation results show that our new scheme outperforms the previously proposed schemes significantly in terms of BER without added complexity.