A proposed combining technique for partially coherent optical signaling

In this paper, we propose a new least squares (LS) combining technique for coherent optical signals suffer from laser phase noise (partially coherent systems) and receiver shot noise. To assess the advantage of this method, we investigate the error rate performance of both homodyne and heterodyne partially coherent binary phase-shift keying (BPSK) systems. We also derive the gain penalties of the signal-to-noise ratio (SNR) due to the partial recovery of the incoming laser phase to verify the appropriateness of the new combining method in mitigating laser phase noise. For design purposes, we provide tabulated results for the number of required photoelectrons to achieve a certain error rate level for the systems under investigation. The evaluation results show that the proposed technique has the ability to improve the error rate performance, and hence, reduce the SNR gain penalties caused by the laser phase noise. Finally, we show that the error rate performance of ideal systems (no phase error) can be achieved through this method by increasing the number of bit segmentations up to a reasonable level.