Characterization of OFDM-Based Secure Data Transmission Over Voice Channels

This letter 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 letter 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 $2.45\cdot 10^{-2}$, $1.11\cdot 10^{-2}$ and $4.83\cdot 10^{-3}$ over Signal, Telegram and 3G cellular network voice channels, respectively.