A co-operation method of phase estimation and channel equalization for coherent optical communications

With the requirement of high communication rate, coherent optical communication with multi-level phase modulations has become one of the main technologies for optical fiber communication systems at 100-Gb/s and above. The application of coherent detection and multi-level modulation format makes the laser phase noise to be one of the major factors that would influence the system performance, and a carrier phase estimator is necessary to track the time-varying phase changes caused by the phase noise in such systems. On the other hand, electronic channel equalization can be used to compensate for fiber dispersion, but the interaction of the carrier phase estimator and the channel equalizer can not be ignored any more. This paper proposes a method to combine the electronic channel equalization module with carrier phase estimation module in electric field at the receiver side of the coherent optical fiber communication systems. The phase estimation is achieved after the electric equalization and the results from the phase estimation are feed-backed to the electric equalization for the equalizer's tap coefficients updating. When both phase estimation and electronic channel equalization are used in the system, smaller step-size for electric equalization can be used for lower bit error rate, more stable error performance and longer transmission distance.