| Due to the lack of safety precautions of voice communication, the security of end-to-end mobile voice communications has attracted more and more attention in recent years. The speech signal encryption is one of the most direct method to ensure the information security of end-to-end mobile voice communications, and the technology of digital voice encryption is used as the main voice encryption technology because of its advantages, such as easy operation, fast encryption speed, high encryption quality and so on. It needs to transform the analog voice signal to the digital signal by the way of voice compressed coding at the time of doing digital voice encryption. And because that the data channel in the communications systems cannot ensure the fast and reliable transmission of digital signal, and it can easily appear intercommunicating problem when it needs to be connected with international networks.The voice channel is a typical analog channel which is used to transfer the voice signal, the analog speech signal and the digital signal have many differences in the terms of composition, the range of frequencies and other aspects, especially, it doesn’t contain any liking speech characters which the speech signal does. The key part of the voice channel is vocoder, and signals can pass through the vocoder well only when they own liking speech characters. So after the digital signal which didn’t contain liking speech characters has passed through the vocoder, some parts of it could be thought as noise and thrown away. Therefore, after recovered by the receiving end, this situation can cause serious error bits and distortion of the synthesized speech. So, the study of the reliable and secure transmission method which can be used to transfer modulated digital signal over the speech channel is one of the key technologies which can decide the possibility of realizing the end-to-end secure speech communications.For the problems the transmission of the digital signal through the voice channel had met, the paper has done the following work:1.It has put forward the program of coding the speech signal by the way of MELP and using the SM2 algorithm to encrypt the signal, so that it can ensure the security of transmission.2.For the existing problems of liking speech modulation, the paper has proposed a transmission method which is based on existing achievements. The method which process the digital signal by the way of channel coding, OFDM modulating and FFT interpolation these three procedures at the receiving end to realize the liking speech modulation and optimization of digital signal, so that the digital signal will contain liking voice characters and could reliably passed through the GSM voice channel. In particular, the paper has discussed and chosen the parameters setting method of OFDM liking speech modulating and FFT interpolation.3.Based on this, the paper has proposed a designing program of system which could be used to realize the security and anti voicoder compression transmission of the digital signal over the GSM voice channel, and discussed the theoretical feasibility of application of the system in 3G and 4G communications system.4.At the same time, the system has been simulated in the GSM voice channel and the 3G voice channel by the use of MATLAB simulating software, then the paper has analyzed the results of simulation.According to the results of simulation, it can be considered that after converting the analog signal to the digital signal by the method of MELP voice compressed coding and then processing the digital signal by the way of SM2 encryption and liking speech modulation, the digital signal can pass through the GSM voice channel in the shape of liking speech signal, and the signal error bits is less at the receiving end. For the 3G voice channel, the digital signal processed by the same way also can pass the 3G voice channel in the shape of liking speech signal, and the performance of error bits is good but not good enough. At the same time, it can ensure the security of information because the high encryption strength of SM2 algorithm. Therefore, the paper provide a feasible algorithm for the secure and reliable transmission of the digital signal over the GSM voice channel, and the algorithm is applicable for the secure and reliable transmission of the digital signal over the 3G voice channel. |
