Research On Methods Of Weak Signal Reception And Detection For Deep Space Communications

The TT&C system of deep space exploration functions as telemetry, tracking and command for the faraway detector in space, which keeps a reliable commmunication and data transmission with the object so as to ensure the orbit accuracy and acquire the scientic information. Based on the analysis of the characteristic of a representative TT&C system, several new solution for its key technique such as high gain channel coding and weak signal detection is presented.A practical Turbo coding scheme using quadratic permutation polynomial (QPP) interleaver and linear Log-Map algorithm is presented. The adoption of linear Log-Map algorithm reduces the computation complexity sharply, and the application of QPP interleaver avoids the access collision problem in parallel decoding effectively. The algorithm has some advantages such as good performance, simple algorithm, the same structure for both interleaver and deinterleaver, few storage space and easy hardware implementation.Based on EXIT chart and APSO algorithm, a new method to optimize the degree distributions of irregular LDPC codes is proposed. An overall cost function is first designed to measure the matching extent of EXIT curves. Then the degree distributions are optimized iteratively by using the adaptive particle swam optimizer (APSO) algorithm. Such a procedure needs not to fasten the CND curve. Therefore, some new degree distributions with higher noise threshold are obtained. Simulation results show that the algorithm has a good performance to achieve optimal code structures, more effective than Gaussian Approximation in computation.To enable the practicability of the low density parity check codes, a QCE-PEG algorithm is presented, followed by the description of the implementation approach and a design case in detail. The algorithm divides the construction into two process, combining the quasi-cyclic extension method and progressive edge-growth technique. It has several advantages such as good degree distribution, long average girth, rapid encoding process and good performance. The irregular LDPC codes the algorithm constructed has good performance, simple and practicable.To improve the correcting error performance of LDPC code, the LDPC-SPC technique is introduced which based on the check matrix construction of QCE-PEG. It uses LDPC code as horizontal code and single parity code as vertical code. Thus it can not only fall the decoding threshold and error floor effectively, but also reaches BER less than 10-5, while SNR is 0.7dB with shorter frame length.To overcome the capturing and tracking problem of weak signal under the low SNR environment of deep space and to obtain the precise measurement for the speed and distance, a practical receiver scheme is designed. In the scheme, A 2D capture method was applied to capture both the Doppler frequency and Doppler variety rate of downlink signal in parallel mood. A 2-rank FLL associate with 3-rank PLL mode to track carrier is presented. Simulation and test results show that the system performance of our design can satisfy the requirement of deep space exploration.Finally, a summary of the whole paper is given and a prospect of the issues concerned in this field is also made from the author's research perspective.