Research On Code-aided Carrier Synchronization In Deep Space Communication

Deep space communication refers to communication between ground station and deep space probe off the orbit of earth, which is a guarantee of successful deep space exploration missions. Due to a long transmitting distance, signals are received under low SNR after severe attenuation. To make communication more reliable, channel codes with high coding gain, such as Turbo code and LDPC code, are employed widely in deep space communication. However, these codes are sensitive to carrier synchronization errors, and only by accurate carrier synchronization can they bring extraordinary coding gain into full play.While traditional synchronization approaches fail to secure a satisfactory performance of carrier synchronization under low SNR conditions, code-aided carrier synchronization, a combination of iterative decoding and carrier synchronization taking advantage of soft information output by Turbo decoder, can improve estimation performance of carrier offset parameters. Consequently, code-aided carrier synchronization algorithms for deep space communication under low SNR are studied in this thesis, including Turbo encoding and decoding, code-aided carrier synchronization and code-aided carrier synchronization with large estimation range.Firstly, theories of parallel-concatenated convolutional coding Turbo and Maximum A Posteriori decoding are studied. A series of simulations of Turbo code recommended by Consultative Committee for Space Data Systems are performed from the aspects of code rate, block length and number of iteration. The sensitivity of Turbo code to carrier synchronization errors is also simulated.Secondly, code-aided carrier synchronization is researched. After an analysis of carrier synchronization based on maximum likelihood criteria, Expectation Maximization algorithm, an iterative and searching solution to maximum likelihood equation, is focused. Subsequently, it is applied to carrier synchronization system. In this thesis, the parameter estimation of EM algorithm-based carrier synchronization is simplified, and the searching range is narrowed down, which decrease the computation.Finally, two kinds of code-aided carrier synchronization algorithm with large estimation range are investigated. One is pilot-based and code-aided carrier synchronization, whose coarse synchronization relies on pilot estimation and fine synchronization on EM algorithm. It can enhance the synchronization range as well as keeps synchronization accuracy. The other one is based on evaluation function and carrier offset searching. In this thesis, a simplified evaluation function is adopted, and meanwhile the searching complexity is reduced, cutting the computation greatly.