Research On Synchronization Technology Of Non-binary LDPC-CPM System For VLF Communication

Nowadays,very low frequency(VLF)communication is limited by the transmission conditions,which has the disadvantages of poor reliability and low communication rate,so it is necessary to rely on high power efficiency channel coding and high bandwidth efficiency modulation technology to complete the design of the new VLF communication system.By combining low-density parity-check(LDPC)code with continuous phase modulation(CPM),the transmission capability of data link in VLF communication can be further improved while ensuring the reliability,and the bottleneck problem of VLF communication can be effectively solved.As the key module of the receiver,the performance of synchronization is also one of the factors that affect the practical process of non-binary LDPC-CPM system.Therefore,it has important research value and application prospect to explore the synchronization technology of non-binary LDPC-CPM system for VLF communication.In view of the above research objectives,the main contents of this paper are as follows:1.According to the characteristics of narrow-band and low signal-to-noise ratio(SNR)in VLF communication,the specific realization scheme of non-binary LDPC-CPM system is designed.Using the CPM decomposition model of Rimoldi,combined with the external non-binary LDPC code,interleaving and weighting technology,a serial concatenated non-binary LDPC-CPM system is established.At the same time,Turbo iterative detection mechanism is introduced to design the receiver structure based on soft-input soft-output(SISO)mode.By analyzing the influence of various parameters on the system performance,considering the effectiveness,reliability and complexity,the optimization of signal transmission scheme in the target system is completed.2.On the basis of Turbo iterative detection mechanism,the idea of code-aided(CA)synchronization is introduced,and the CA carrier phase synchronization(or timing synchronization)algorithms based on expectation maximization(EM)criterion and maximum-mean likelihood ratio(MMLR)criterion are proposed.Both the EM synchronization algorithms and MMLR synchronization algorithms take advantage of the check and error correction characteristics of non-binary LDPC code.In the process of synchronization iteration,the soft information is first decoded to aid the offset estimation,and then the compensation is completed by matched filter.In addition,the performance simulation is carried out around the effective synchronization range,synchronization reliability and synchronization accuracy of the proposed algorithms,and the performance limits of carrier phase synchronization and timing synchronization in the system are deduced to complete the synchronization precision evaluation of the proposed algorithms.3.In order to realize the joint carrier phase and timing synchronization,a CA joint synchronization algorithm based on MMLR criterion is proposed.The proposed MMLR joint synchronization algorithm adopts the parallel synchronization method,and can complete the joint estimation and compensation of carrier phase offset and timing offset in one synchronization process through two-dimensional iterative search.In addition,the performance simulation of MMLR joint synchronization algorithm about reliability is carried out after combining the effective carrier phase and timing synchronization range,and the synchronization effect of the proposed algorithm for joint carrier phase and timing synchronization is verified.