Characterization of OFDM-Based Secure Data Transmission Over Voice Channels

This paper proposes a novel approach to speech-like signal design for reliable real-time transmission of encrypted data over voice channels. The structure of the digitally encrypted information-carrying segment relies on the known concept of the OFDM/QPSK modulation. The segment is extended with time-synchronization sequences, amplitude variations and silence intervals, all necessary for successful passing through realistic channels. Phase distortion has been identified as a significant source of error and inspired the proposal of a phase-shift compensation and a time-and-phase fine-tuning algorithms. Finally, this paper outlines a complete secure real-time communication system. Transmission quality of the resulting speech-like scheme is first evaluated on the AMR codec at various compression rates, followed by the analysis over realistic voice channels. The proposed secure DoV system achieves a mean BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2.45\cdot 10^{-2}$</tex-math></inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.11\cdot 10^{-2}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4.83\cdot 10^{-3}$</tex-math></inline-formula> over Signal, Telegram and 3G cellular network voice channels, respectively.