Research On Digital Signal Processing Algorithm In High-speed Optical Fiber Transmission

Nowadays, with the high development of multimedia service and data trafficssuch as P2P and IPTV, the requirements for bandwidth have increased dramatically.The ultra bandwidth service is promoting the development of the next generationoptical transmission system. The simple deployment of intensity modulation anddirect detection in traditional optical fiber communication systems can’t meet therequirements of longer span and higher capacity of communication systems. Coherentoptical communication system with high spectral efficiency has raised researcher’sinterests. With the continuous improvement of digital signal processing algorithmsand the realization of high-speed analog to digital converter, many transmissionimpairments can be compensated in electric domain, as a result simplification incommunication link and reduction in cost are reached. Therefore, the combineddeployment of high order modulation，coherent detection, digital signal processingand polarization multiplexing will be a promising candidate for next generationoptical transmission system. Under these backgrounds, this thesis mainly describesthe core digital signal processing algorithms used in100Gbps PM-QPSK and100Gbps16-QAM coherent optical receivers. The main contents are as follows:1. The basic structure of coherent optical system is introduced in this thesis, andthen the core DSP algorithms used in coherent optical system are described. The DSPalgorithms discussed in this thesis mainly include polarization demultiplexing, carrierfrequency offset estimation and carrier phase estimation.2. To solve the frequency ambiguity caused byM thpower algorithm used intraditional frequency offset estimation, we propose a non-data-aided wide-rangecarrier frequency offset estimator for optical coherent receivers, which can increasethe theoretical estimated range from [RS/2M,RS/2M]to [RS/2,RS/2](RSis symbol rage, M is modulation order). The simulation results demonstrate that thisimproved algorithm can achieve precise estimation and cover the possible frequencyoffsets of most commercially available lasers, which are as large as±5GHz.3. Blind phase search (BPS) technique is known for its best phase noisetolerance but at the expense of high computational complexity. In this thesis, we propose a linewidth-tolerant and low-complexity two-stage feed-forward carrierphase estimation algorithm by employing a modified QPSK partition in the first stageand a BPS in the second stage. Simulation results for a100Gbps16-QAM opticalcoherent system demonstrates similar linewidth tolerance and a computationalcomplexity reduction by a factor of at least three compared with single-stage BPS. Inaddition, more tolerant towards phase noise is achieved when symbol duration timeslinewidth is larger than2×104.4. We build a simulation platform of100Gbps PM-QPSK and100Gbps16-QAM optical coherent transmission systems by using VPItransmissionMaker8.3and Matlab. All the algorithms in this thesis have been tested experimentally underthis platform.