FBMC/OQAM-Based Secure Voice Communications Over Voice Channels

The core objective is to securely transmit digital data over widely accessible voice channels like VoIP or GSM networks. Encryption at the highest security level is applied digitally, but a challenge lies in selecting a modulation technique that effectively converts encrypted digital data into analogue signals capable of navigating the obstacles inherent in voice channels. These channels are lossy and optimized for speech transmission, employing sophisticated signal processing techniques such as Voice Activity Detection (VAD) to maintain speech quality and suppress noise. This paper introduces a novel system design and approach for transmitting Data over Voice (DoV) channels using Offset Quadrature Amplitude Modulation (OQAM) combined with Filter Bank Multicarrier (FBMC) modulation. FBMC/OQAM generates speech-like waveforms that can evade detection by non-speech signal detection mechanisms inherent in voice channels, marking its debut in this domain. In order to demonstrate the feasibility of the proposed system, we have simulated the distortion of FBMC/OQAM signals when processed by an AMR-NB codec using MATLAB®. Subsequently, the performance of the proposed modulation technique is evaluated across various real voice channels, considering actual time and phase deformations, real-time voice transmission bit-rate constraints and possible bit errors, which all demand specific waveform structures for synchronization and algorithms for correct demodulation and decoding at the receiver. Evaluation is done for waveforms that were sent through GSM and VoIP channels. The obtained results demonstrate a favorable bit error rate (BER) on the 3G channel compared to existing methods, such as conventional Orthogonal Frequency Division Modulation (OFDM), thereby proving that FBMC/OQAM is suitable for secure DoV transmission. Finally, the decoded speech from all channels is assessed using Perceptual Evaluation of Speech Quality (PESQ) metric.