| Signal System is one of the most important technical part for Global Navigation Satellite System (GNSS), and reasonable signal system can improve the navigation performance in the aspects of positioning and ranging. As a result, it is of great theoretical value and realistic significance to research new and efficient GNSS signal system.The Coding and modulation are significant parts for the GNSS signal system design, in the traditional system they are designed independently,however they have some defects.People expect to realize bandwidth efficiency coding which is an efficient coding and modulation scheme without decreasing the information effective transmission rate, and the joint coded modulation technology is a significant way to realize the bandwidth efficiency coding. Low density parity check (LDPC) codes has been widely applied with its excellent error correcting capability. Continuous phase modulation (CPM) is an efficient digital modulation with high spectrum efficiency and power efficiency, besides it has constant envelope and it is especially suitable for satellite navigation system that use nonlinear power amplifier. Therefore, it is necessary to discuss the application feasibility of LDPC-CPM joint coded modulation technology for the future satellite navigation system.This paper aims at designing new and efficient satellite navigation signal transmission mechanism, and combining LDPC codes and CPM modulation with Turbo detection principle to enhance the navigation signal anti-jamming capability and improve the band efficiency and reliability, meanwhile, the reference basis for Chinese new generation satellite navigation signal waveform design is provided. Based on above targets, the main contents of this paper are as follows:1. The implementation scheme of LDPC codes which is suitable for GNSS system is studied. The coding gain of LDPC is close related to the structure of the check matrix, coding and decoding algorithm. But high coding gain can bring high implementation complexity, so it is necessary to study the construction method of LDPC codes with both high performance and low complexity, which can provide the reference basis of the channel coding choice in GNSS system.2. The application feasibility for new type of modulation scheme is explored. Firstly,the MSK-BOC modulation which combines the minimum shift keying (MSK) with binary offset carrier (BOC) modulation is introduced and the performance in the aspects of the spectral characteristic and the mathematical model is discussed. Secondly, the mathematical model,signal state and power spectral density of CPM modulation is studied. Finally, this paper compares and analyses the spectral characteristic of the above two kinds of modulation with BOC modulation,and gives the condition that SS-CPM can effectively substitute BOC.Meanwhile, this paper designs a new substitute modulation for multiplexed binary offset carrier (MBOC) modulation based on the speciality of SS-CPM signal that is called CPM-BOC modulation, and discusses its mechanism and performance.3. The signal performance evaluation method for satellite navigation is studied. The evaluation method comprehensively considered the performance of precision,anti-interference, anti-multipath and compatibility, meanwhile, the various performance indexes of MSK-BOC modulation and SS-CPM modulation is tested, which can provide theoretical support for GNSS signal waveform design, performance analysis and parameters optimization.4. The scheme of joint coded modulation based on spreading spectrum system is explored. The system model of LDPC-MSK-BOC and LDPC-CPM based on C/A code are established, besides, the iterative detection mechanism based on Turbo theory is designed,then the effect of main parameters on the performance of the system is studied, which can provide the reference basis for the parameters selection of practical application. |
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