Frequency Offset Compensation in Physical-Layer Network Coding Systems

Physical layer network coding (PNC) has the potential to improve the spectral efficiency of wireless relay communications by utilising the superposition of signal propagation. In the ideal case, PNC can double the throughput of a symmetric relay network when compared to conventional time division multiplexing. However, in practice imperfect channel conditions and hardware can lead to time and frequency offsets at the relay receiver that degrade PNC performance. In this paper we provide an introduction to PNC and describe an existing technique for frequency offset compensation that is based on using the mean of the independent uplink channel frequency offsets. For the BPSK signalling case we propose a modification to the method to remove residual frequency and phase offset. We then propose a novel threshold based conjugate compensation technique that is able to remove frequency offset while improving immunity to noise enhancement. Simulation results are presented that show significant performance improvement for the proposed method when compared to the mean compensation approach. We describe the PNC testbed that we have implemented using the Universal Software Radio Peripheral platform. We present hardware test results that verify the operation of the proposed compensation technique and provide some valuable insights into PNC synchronisation.