Research On Speech-Like Modulation And Demodeulation In End-to-End Secure Communication

As there are lots of security flaws in the public mobile communications network, there is a need of end-to-end voice secure communication which induces some problems. For that, this paper researches on.the voice service channel based scheme of end-to-end voice secure communication. To transmit encrypted data against vocoder's compression in the scheme, this paper analyses the existing methods, sums up the functions and characteristics of speech-like modulation and demodulation, and then lays its focus on the technology of speech-like modulation and demodulation from following two aspects.First of all,through researching vocoder's features on voice coding and decoding, The paper proposes a speech-like modulation and demodulation algorithm based on speech parameters model. The algorithm uses techniques such as sub-bits, mapping, vector quantization, quantization codebook design, speech-like signal synthesization and so on, to modulate data bit stream into speech-like signal, so that it can pass link vocoder's compression/decompression process, and restore the original data effectively when demodulated at the receiving terminal with low error rate. Simulation results show that this algorithm has higher modulation rate than the method based on formant model, when reaching the condition of bit error rate that can transmit speech.Second, by modeling analysis of vocoder, this paper mensurates the frequency response of "black box" channel system composed by GSM vocoders, researches and implements a speech-like modulation and demodulation algorithm based on channel model. This algorithm uses OFDM technology to carry data on the modulated signal, in which frequency band is limited to the approximate band-pass range of the "black box" system, so that the signal can pass through the vocoder channel with less distortion and be demodulated exactly. Then, data is transmited against vocoder's compression. Simulation results show that this algorithm has lower bit error rate and higher modulation rate, so it can better ensure real time communication.Finally, This paper designs and implements a simulation platform of end-to-end voice secure communication system. Then, applying these two algorithms of speech-like modulation and demodulation to the system, we successful carry on an experimentation of secure voice transmission, which verifies the effectiveness of these two algorithms and the feasibility of end-to-end voice secure communication.