Physical-Layer Network Coding with M-QAM Modulation

The denoise-and-forward (DNF) method of physical-layer network coding (PNC) is a promising approach for wireless relaying networks. Most related work in the literature focuses on low-level modulations, such as binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK). In this paper, we consider DNF-based PNC with high-level modulations, specifically the M-ary quadrature amplitude modulation (M-QAM). We propose a mapping scheme that maps the superposed M-QAM signal to coded symbols. The mapping scheme supports both square and non-square M-QAM modulations, with various original constellation mappings (e.g. binary-coded or Gray-coded). It also ensures that the expected symbol can be successfully decoded at the destination, while attempting to minimize the size of the coded symbol set. Subsequently, we evaluate the symbol error rate and bit error rate (BER) of M-QAM modulated PNC that uses the proposed mapping scheme, both analytically and with simulations. Afterwards, as an application, a rate adaptation scheme for the DNF method of PNC is proposed. Rate adaptation is achieved by selecting the appropriate modulation level, based on the channel status and the BER requirement. Simulation results show that rate-adaptive PNC is advantageous in various scenarios.