Research On Transmission Technology For End-to-end Encrypted Voice Over Public Mobile Networks

Mobile communication is already the most widely used means of communication. With constant developing of high speed space interface technology, the security of mobile communication becomes more important. One of the hot researches is to find ways of repairing the defect of current mobile communication network. Being urgent and hard, the topic of end-to-end encrypted voice communication is funded by the National High Technology and Development Program (called 863 program) of the Eleventh Five-year Plan of China.In the light of documents and information at China and abroad, the technology of end-to-end encrypted voice transmission developed from the aspects of speech production model, channel transmission model and data-wave mapping. The major innovation contents are as follows:(1) Based on the analysis of the mobile networks and working mode of voice, the paper abstracts general working flow for mobile voice service, and gets a universal transmission model. Thus makes the transmission problem of encrypted voice data come down to the design problem of MODEM algorithm which can efficiently resist the lossy compression caused by vocoder.(2) The relationship between speech synthesis and modulation, linear prediction analysis and demodulation is set up by taking speech production module, and thus generated algorithm of SA-SVQ (Simulated Algorithm-Split Vector Quantization) and SCAMDF (Simplified Circular Average Magnitude Difference Function). The MODEM algorithm for GSM FR vocoder works out on the basis of choosing reasonable parameters and codebook design, which could effective resist waveform distortion introduced by vocoder.(3) We proposed a black box channel model for GSM FR serial vocoders. Introducing the all-phase FFT (apFFT) algorithm to identify the channel model by frequency-response system identification method, it effectively solved the problem of shortcomings of classical DFT and FFT algorithm such as the modified accurate phase by complex frequency-offset correction. The success of equivalent filter model of GSM FR voice service channel proposed a way of adopting OFDM technology to realize modulation and demodulation. The simulation results verified the feasibility.(4)On this basis, we popularized the "model-identify-verify-design" pattern to other mobile serial vocoders channel in China. The simulation results show that transmission of classical modulated signal (such as modulated by OFDM, QPSK, FSK etc) on various kinds of vocoder channel depends on transmission performance of single-frequency cosine signal. The reason why it is difficult to transmit single-frequency cosine signal on GSM HR and CDMA2000 EVRC is also stated.(5) Put the OFDM symbol generation process into popularized application to map from data to waveform sets, introduced GA algorithm served as means of realization, used the GSM FR channel model above-mentioned and results of frequency-response identification, we proposed a modified method for waveform symbol generation on the basis of integrating effects on distortion of frequency, phase and amplitude of waveform brought by channel. The result improved the efficiency of algorithm and BER performance availably, and put into application on GSM FR voice service channel.Preliminary simulation experiments show that the basic conclusion of this paper is high valued for practice guide and helpful for prototype developing.