Study On The Additional Spread Spectrum Modulation Technique In Low-frequency Time-code Timing Service System

The purpose of this paper is studying and discussing how to improve and advancethe performance of low-frequency time-code timing service system. The applicationof the low-frequency time-code timing service system is restricted by the presentcommunication system. In this paper, the additional spread spectrum modulationinformation based on ASK modulation is presented, furthermore a combinedmodulation scheme are given. The design of signal system, code synchronization, theability of anti-jamming, the transmitter control technique and the method of singlefrequency by multi-transmitter are discussed and studied .The results show that theadditional spread spectrum modulation is a feasible technique, moreover whoseprecision and anti-jamming ability can be improved, and whose performance, value inuse and accepted degree of the whole system can be advanced. The main work andcontributions can be summarized as follows:Based on the modern signal structure of low-frequency time-code, theadditional spread spectrum modulation information in idle carrier wave is presented,and it is an outspread and complementary to the low frequency time-code timingservice technique. The discussion and analysis to the additional spread spectrummodulation indicate that the original amplitude modulation system can not be affectby this additional method. The performance of SNR is perfect in the AWGN channel.The simulation and experiment testify the additional spread spectrum modulation isfeasible. The results above are the foundation to other researches. The preudo-random binary sequence is designed,and the coding signal format andthe frame construction are researched in low-frequency system additional spreadspectrum modulation. The various data signal constructions are discussed, and thetime-code format and frame construction are given. The choice of transmitter contentis associated with the technique implementation. The frame synchronization method isstudied according to the communication protocol and signal construction, and thereliability of this method is analyzed. Based on the characteristic of low-frequencycommunication channel, the corresponding RS code arithmetic is proposed, and thesimulation results show that the performance of error code can be improved by RScode arithmetic in the additional spread spectrum modulation system.The code capturing and tracking technique are studied in low-frequency systemadditional spread spectrum modulation, and the characteristic of capturing andtracking are analyzed through the typical circuit, and the code synchronization issimulated. The results indicate the additional spread spectrum modulation system canimprove the receiver timing precision largely.According to the noise and anti-jamming performance in the low-frequencysystem additional spread spectrum modulation, error code characteristic in thedifferent of interference conditions are analyzed and simulated. The results show thatadditional spread spectrum modulation system can work in the inferior signal-noiserate and have a better resistance to strength interference. Then this additional spreadspectrum modulation system can be used broadly.Combined with time-code signal characteristic, transmitter controlling is discussedand analyzed and a corresponding PSK control method is presented. A controllingsystem based on FPGA is designed and the experiment shows that this controllingmethod is feasible.Whereas there is limitation of single transmitter, the scheme of single frequencyby multi-transmitter is proposed. The influence of multi-transmitter to ASK isanalyzed, and the low-frequency timing service method additional spread spectrummodulation by multi-transmitter is studied. The simulation results indicate that thetiming service method of single frequency by multi-transmitter is feasible, and the code tracking precision in the system additional spread spectrum modulation by multitransmitteris lower than that in single transmitter, but better than that in the ASKsystem largely.