Research On Synchronization Technologies Based On Training Sequence For CO-OFDM System

It is the era of "Internet +", people drew higher demand in information transfer rate and bandwidth. Ultra-high speed and ultra-large capacity transmission has become the main target of communication network. It has been realized that optical fiber communication system is the only way to carry such ultra-speed and ultra-large capacity information. The emergence of new technologies promotes realization of optical communication with ultra-high speed, ultra-large capacity and high spectrum efficiency. Because of large capacity, high sensitivity, high spectral efficiency and good robustness against fiber dispersion and polarization mode dispersion,CO-OFDM system attracts widespread attention and becomes one of the most promising technologies in future optical communication.All digital communication systems require synchronization technology to support, timing synchronization and frequency synchronization are the most crucial synchronization technologies for CO-OFDM system. CO-OFDM system is the combination of coherent detection and OFDM, it absorbs the advantages of the two technologies but can’t avoid their shortcomings at the same time. Coherent detection requires the same frequency between local and sending lasers, the instability of device can arouse frequency offset. The core idea of OFDM technology is to maintain good orthogonality among subcarriers, frequency offset will destroy the orthogonality among subcarriers and will cause serious inter-carrier interference as a result, so CO-OFDM system is pretty sensitive to frequency offset. On the receive side, it is indispensable to determine the initial position of signals and estimate and compensate the frequency offset accurately for recovering the original signal exactly.Synchronization algorithm is divided into two categories: data-aided and non-data-aided. As data-aided synchronization algorithm has advantages of quick capture speed and high precision,which can meet the high demand of CO-OFDM system in synchronization algorithm. In this paper, we deeply study the synchronization algorithm of CO-OFDM system based on training sequence, and the main works are as follows:First, fully master the theory of CO-OFDM system through analyzing coherent optical communication and OFDM technology. As one of the most crucial technologies to implement CO-OFDM system, synchronous technology is the premise for accurately recovering signals at the receiving end of the system, accurate timing and frequency synchronization play an important role in the CO-OFDM system. Then, learn the impact of synchronization error on the system deeply and analyze the classic synchronization algorithm based on PN sequence.For timing synchronization algorithms based on PN sequence, the correlation of training sequence is not strong and they do not have the character of constant envelope. In this paper, the synchronization signal is constituted of CAZAC sequence, its good autocorrelation and cross-correlation characteristics can improve the performance of the synchronization algorithm,and the PAPR value of CAZAC sequence is desired, that can enhance the practicability of the algorithm. This paper proposes a timing synchronization algorithm based on ES-CAZAC sequence in order to obtain timing metric functions with sharp peaks at proper timing point and precise timing estimation without increasing the complexity of the algorithm.Through further analysis we can find that ES-CAZAC timing synchronization algorithm is sensitive to frequency offset, it has good performance only when the frequency offset is small in the system. In order to enhance the anti-offset feature of ES-CAZAC timing synchronization algorithm, we corrected the timing metric function and proposed an ES-CAZAC anti-offset timing synchronization algorithm. This modified algorithm can eliminate the interference of frequency offset to timing synchronization and the timing metric function still has sharp peak at right timing point. After obtaining the accurate timing, we proposed an algorithm for decimal frequency offset estimation and integer frequency offset estimation using ES-CAZAC sequence.Frequency synchronization algorithm based on ES-CAZAC sequence only uses one training sequence,the range of frequency offset estimation can achieve the whole OFDM symbols and the MSE of frequency offset estimation improved at least 1.6d B, it can meet the high requirement of CO-OFDM system for frequency synchronization.