Research On Key Technology Of Differential Encoding And Modulation

With the growing development of longer distance, larger capacity and higher speed of the optical communication system, high-order modulation and coherent detection become mainstream technologies. In order to avoid phase ambiguity, differential encoding techniques are introduced with the cost of remarkable performance degradation. This thesis focuses on the differential encoding and decoding algorithms. And then the channel encoding is joined into the differential modulation system, which can improve the system reliability. In this thesis, through the design of system structure, the system can not only overcome phase ambiguity, but also can fully compensate for the loss of adding differential modulation.The first chapter will show some background of the encoding and modulation, which will lead the direction of the following content.The second chapter introduces the effect of channel coding firstly, and then focus on the process of encoding and decoding of the convolutional code and Turbo code, including their algorithm formula. The analysis and comparison of different decoding algorithm performance will be shown at last.The third chapter is one of the key chapters of this paper. Differential modulation can be seen as a combination of differential encoding and ordinary modulation, and the key point of the technology’s realization lies in differential encoding part. In this paper, differential encoding algorithms of three kinds of differential modulation and demodulation are given in detail, including DPSK、DQPSK and differential16QAM modulation. In the chapter four, the system of channel coding combined with differential modulation will be introduced. First, the chapter gives the system model, and then describes the key technology of every module in the system model. At last, the simulation results of the given system will be given, and the results will be compared and analyzed.The fifth chapter will introduce the theory and method of the extrinsic information transfer (EXIT) chart. Using the EXIT chart can analyze and verify the system performance about the fourth chapter. As the main part of this chapter, this paper provides the algorithm of optimization about generator polynomials. After optimization, the simulation results show that the performance of the whole system can be improved obviously by using the optimization algorithm.The last chapter of this paper will give the conclusion about the recent results, and also propose the future work.