Turbo equalization for OFDM modulated physical layer network coding

In this paper we consider a practical orthogonal frequency division multiplexing (OFDM) modulated and low-density parity-check (LDPC) channel coded two-way relay system employing physical-layer network coding (PLNC), where two terminals A and B desire to exchange information with each other with the help of a relay R which can be equipped with multiple receive antennas. Orthogonal frequency division multiplexing (OFDM) is adopted as the modulation scheme to resolve the synchronization problem in PLNC and doubly selective channels with intercarrier interference (ICI) are considered. The critical process in such a system is the calculation of the network-coded transmit codeword at the relay on basis of the superimposed channel-coded signals of the two terminals. Different from existing works on non-iterative receiver design, we here consider iterative receiver design. We propose two turbo equalization receivers, one is the conventional iterative separate detection and decoding (I-SDD), and the other one is based on a recently developed estimation scheme for PLNC. Gaussian message passing (GMP) and sum-product algorithm (SPA) are used for ICI-aware equalization and channel decoding respectively. We find through numerical simulations that the performance of the I-SDD receiver can catch up with that of the state-of-the-art PLNC-based receiver when more than one receive antennas are used. One promising feature about the ISDD receiver is that the channel decoding complexity is much lower than that of the PLNC-based receiver.