Amplitude variation phenomenon in optical Physical-layer Network Coding with direct detection

Abstract

This paper studies the amplitude variation phenomenon in optical physical-layer network coding (OPNC) with intensity modulation and direct detection (IM/DD). Although PNC has been studied thoroughly in wireless communications, the application of PNC to optical communication networks is still challenging. In OPNC, two optical signals are transmitted simultaneously, and the received signals are overlapped in the optical field and then detected by a photo-detector (PD). Previous works assumed the OPNC signal and its electrical signal converted by a PD are linearly related. We find that the relationship is not the case. To fill this gap, we theoretically analyze OPNC and discover that an interference signal is generated when an OPNC signal is converted to its corresponding electrical PNC signal in most conditions. Besides, we implement the simulation and experiment to validate the theoretical analysis. The results coincide with our analysis and indicate that the PNC system does not work when the signal is severely damaged by the interference signal. However, we can recover the PNC signal by filtering the interference signal when their frequency spectrums are not aliasing. We further evaluate the BER performance in the simulation and experiment and found that when the frequency spectrum of the PNC signal is not aliased with the interference signal, using a low-pass filter can improve the BER performance by approximately 12 dB.