Orthogonal physical layer network coding

Abstract

Physical-layer network coding (PNC) has been used as a high throughput technique in a relay when two users have no direct connections. However, it becomes difficult to separate the two different signals since they reach the relay simultaneously with the same modulation, e.g., binary phase shift keying (BPSK). The relay in a conventional PNC system demodulates the combined signal without separation, and sends the modulo-sum bit back to the users for their detection of the other user information bit. In this work, two orthogonal signals from two different users are transmitted and hence separated at the relay. This is abbreviated as OPNC. For example, user signals are modulated with sine and cosine waveforms at user 1 and 2, respectively. At the relay, the received signal is demodulated, separately, using two different correlation receivers; one is matched to the sine waveform and the other to the cosine waveform. Then, hard decision is made using correlator outputs to estimate the information bits from each user. Following that, a modulo-sum combined bit is modulated with BPSK and transmitted back to the users. Each user node is able to detect the other node bit information as the conventional PNC. Simulation and analytical results show that the proposed coherent OPNC is 3 dB better than the conventional coherent PNC, and 8 dB better than the conventional non-coherent PNC with continuous phase frequency shift keying.