Research On Applications Of Encryption And LDPC Technologies In PCM/FM Re-Entry Telemetry System

The re-entry telemetry system is a kind of variable and multipath fading system. There are many forms of fast and slow fading in the system, which have direct inference on telemetry data. The telemetry data contains a lot of important information, including the reconnaissance in electronic warfare, aircraft flying state information, etc. In order to reduce the inference of multipath fading in the telemetry data, and to prevent te data with information being intercepted or used by the third party, effective correction and encrption coding to the wireless transmission telemeter data are required. Therefore, encryption and encoding techniques become one of the key technologies in the re-entry telemetry system.This thesis mainly confocuses the application of encryption technology in the reentry telemetry system, the analysis of the performance in the PCM/PPK system, and the improvement of LDPC(Low-Density Parity-Check) decoding of the application in PCM/FM system. The main works and conclusions are described as follows.BER(Bit-Error-Rate) and error characteristics performance of the PCM/PPK system are analyzed in the detection of maximum value utilizing the feature that the transmitted codes are group code. The performance is compared with that in the decision of the fixation threshold. The result shows that the BER and error characteristics in the detection of maximum value are less than that in the decision of fixation threshold with small SNR(Signal-Noise-Ratio). But the characteristics in the detection of maximum value decision are better than that of the decision of fixation threshold with large SNR.To the issue of security that telemetry data is transmitted in the open space, a data encryption scheme in the PCM/PPK telemetry system is proposed. In this scheme, the PCM signal sources are encrypted by stream cipher where key streams are generated with clock controlled LSFR (Linear-Feedback-Shift-Register). The scheme can save system resource in the dual-modulation compound telemetry system. A backward key synchronization scheme is also proposed, which has the property of strong real-time and easy implementation. A simplified simulation model of PCM/PPK remote system is established based on the actual framework of a telemetry system, where the model is simple and easy to simulate. The error performance of the system is analyzed in theory and tested by computer simulations. And the security of the system is also analyzed. The simulation and analysis results show that the encrypted scheme can ensure the security of the telemetry data and does not cause error-diffusion. In the error frame of the short and middle length LDPC codes, the variation law of the check nodes' LLR (Logarithm-Likelihood-Ratio) and the checksum is discovered. Based on the law, an improved BP algorithm of LDPC decoding is proposed. In the algorithm, an early stopping criterion is developed to reduce the average number of decoding iterations in the region of low SNR (Signal-to-Noise-Ratio). In order to improve the decoding performance by weakening the oscillation of the LLR, the update algorithm of check node is modified by a low filter. Simulation results show that the algorithm reduces the number of the iterations while the performance is not reduced in the region of low SNR, the decoding performance is improved while the number of iteration is not changed in the middle and high region of the SNRs.An improved algorithm of LDPC decoding is proposed, based on the Offset Min-Sum algorithm and the difference-based decoding method. In this algorithm, the messages passed on the bipartite graph of LDPC are based on the probability differences, compared to Offset Min-Sum algorithm which can reduce performance loss from quantization because of high accuracy of quantization. For the update of the check nodes, the Offset Min-Sum algorithm is used, which can reduce the computational complexity compared with the original difference algorithm. The simulation results in a PCM/FM system show that the improved algorithm has fast convergence speed and no significant performance loss.5. The system simulation platform is builded programming by combining matlab with visual c++. The performance of the LDPC in PCM/FM system is simulated and analysed, including simulations of the LDPC codes with different length and the LDPC codes under different decoding algorithms.