Research On Coding And Demodulation Techniques In Low SNR Satellite Communication

In recent years, with the rapid development of deep space exploration and communicationtechnologies all over the world, human has made remarkable progress in the exploration of theunknown space field. But the characteristics such as low SNR and serious Doppler frequency shiftwhich occurred in deep space communications have led traditional receivers failed to receiveeffective signals. Thus, it brings a great challenge for guaranteeing rather reliable receptions ofweak signals. In order to ensure the reliability of signal receiving in deep space communications,modern channel coding and digital modulation technologies are required to assist the constructionof a receiver system with excellent performance. According to these key problems, this dissertationdeeply carried out the researches on efficient low rate coding and demodulation of the weak signalsin deep space satellite communications. In addition, based on the software radio techniques and theabove theories and technologies researched of this work, the reception system of satellite signalswas modeled and simulated on a field programmable gate array (FPGA) hardware platform forfeasibility verification. The main contents of the dissertation are as follows:Firstly, the application backgrounds of channel coding, modulation&demodulation in digitalsatellite communication were briefly introduced. For the part of channel coding, LDPC codes weremainly discussed for their commonly used coding principles and schemes. And for the part ofmodulation&demodulation, the implementation of QPSK was mentioned, which involved in itsmain modules, such as shape forming filter, numerical controlled oscillator, matching filter andcarrier synchronization loop. And they were briefly analyzed as well.Secondly, according to the deep space satellite communication background, the requirementsof channel coding were analyzed. Also the effects of length and rate on the performance of thechannel codes were discussed. After that, a construction method of low rate quasi-cyclic structuredLDPC codes was proposed based on the protograph extension and QC-ACE optimization search.Then, the implementation of the proposed scheme was specifically analyzed as well. A typicalparameter set of the specific code meeting the requirements of the project was given, and thencompared and analyzed in the performance with that of Turbo codes and LDPC codes with shortlength. Simulation results showed that the proposed code had better performance than that of theseordinary codes, and it could satisfy the requirements in practices.Thirdly, the principle of carrier synchronization in coherent demodulation was brieflyintroduced. According to the analyses in the performance of the traditional phase locked loop, it couldn’t achieve high dynamic carrier synchronization. By analyzing the performance offrequency-locked-loops, Costas-loops and loop-filters, an order two frequency-locked-loop aidedorder three phase locked loop carrier synchronization was put forward, which was suited for severalkinds of phase shift keying modulations with good performance of capturing high dynamic carrier.Simulating results showed that: the suggested scheme could achieve high dynamic carrieracquisition with good performance under different kinds of phase shift keying modulations.Finally, with the help of software radio techniques, and also combined with above LDPC codeand carrier synchronization scheme, a complete FPGA hardware platform was set up for the LDPCcoding, QPSK modulating and demodulating reception system. At the same time, more detaileddepicts of the design process about the core modules such as coding, shape forming filter, digitalfrequency conversion, carrier synchronization as well as ambiguity of phase correction wereintroduced. Through the board test of the system in practice, the feasibility of the proposed designscheme was verified.The significance of this project lies as follows: through the construction of channel coding andthe improvement of carrier synchronization scheme, along with the FPGA verification platform ofcommunication system, a novel technical scheme was proposed, which ensured the efficiency andreliability of the satellite communication under deep space background. And the scheme couldcontribute to applied in practical applications of deep space explorations and communications in thefuture.