Research On Demodulation Algorithm Of PCM/FM With Large Dynamic Range Doppler Frequency Shift

In the area of aerospace telemetry, Pulse Code Modulation/Frequency Modulation (PCM/FM) system is one of the main plans in strategic weapons and rockets because of its high noise resistance, high flexibility. In the procedure of telemetry, displacement between the target being tracked and the receiver is inevitable, which leads to the Doppler effect. It will cause frequency shift to the carrier of signal being received. Doppler frequency shift has a negative impact on signal demodulation and needs to be compensated. Up to now, there is a lack of appropriate algorithm for Doppler frequency shift compensation in the situation of low base-band transmission rate, large dynamic range of Doppler frequency shift and implement in the universal hardware platform. So in this dissertation, a demodulation algorithm aimed to compensate the large dynamic range of Doppler frequency shift in low base-band transmission rate is being researched.Firstly, this dissertation analyzes the characteristic of PCM/FM signal in time domain and frequency domain. The effect of high dynamic range Doppler frequency shift on the signal received is emphasized. Based on the summary of algorithms used in compensating Doppler frequency shift, this thesis compares the advantages and disadvantages of them and draws conclusion that the methods using automatic frequency control have inadequate speed of response, the methods using carrier frequency estimation are too complex to implement.Secondly, this dissertation researches the demodulation algorithm for signals with high dynamic range Doppler frequency shift. The algorithm includes two parts. On the one hand, estimation of carrier frequency by the signal frequency characteristic to obtain the rough value of frequency shift can reduce the large frequency shift to a small range. On the other hand, differential coding offsets the remaining frequency shift as the characteristic that the frequency shift between adjacent codes is nearly constant. The dissertation combines the algorithm and traditional coherent demodulation method of Frequency Modulation system, designs the demodulation system offsetting high dynamic range Doppler frequency shift.Simulations of system performance are made and the results certify that the frequency shift is offset and the system bit error rate meets the demand of index. Finally, based on the simulations, the dissertation designs the demodulation system with hardware language. For the need of low resource consumption and the easiness of implement, the methods of modules implement are compared and optimized. Hardware simulations are made and the results verify that the demodulation system meets requires of function and timing. The program of demodulation system designed is downloaded and run in a FPGA chip. The result testifies that the demodulation system designed has the possibility of implementation.