Coherent Detection of Physical Network Coding in the Presence of Phase Offsets

Physical-Layer Network Coding (PNC) has the potential to greatly increase the throughput of wireless networks. When BPSK signals are transmitted by the sources, employing coherent detection at the relay poses no difficulty. However, when QPSK is used, the performance strongly depends on the phase difference between the carrier phases of the two sources. For a range of phase offset values, the performance of QPSK can be unacceptably poor. To overcome this phase misalignment problem, we propose to use Offset-QPSK at the sources and maximum-likelihood sequence detection by the receivers. The resulting spread in bit error rate performance for a range of phase offset PNC signals is less than 2dB at moderate SNR for transmissions at a rate of 2 bits/symbol. Our results confirm that the proposed solution is able to resolve the problems incurred by phase asynchrony.