Secure voice communication over radio systems

In this thesis, alternative solutions to the NATO SCIP-214.6 protocol, which has been design for secure voice communication over radio systems, are proposed to enhance voice quality. The late net entry (LNE) mechanism periodically transmits crypto synchronization information by replacing audio frames with the LNE frames in the SCIP narrowband mode supporting 2.4 Kbps data rate. To minimize the voice quality degradation, the receiver replaces the dropped frames with the generated new frames. In the first study conducted in the scope of this thesis, the cross correlation information of the frequency responses of the voice frames is transmitted to the receiver to decide whether the previous or next frame will be used instead of the dropped frame. In the wideband mode of the SCIP protocol supporting 16 Kbps data rate, Extended MELPe frames are used to improve the voice quality, where extra 120 bits are transmitted in addition to the 54 bits MELPe core. In the second study conducted in the scope of this thesis, two methods are proposed to generate the payload frames for the wideband mode of the protocol. In the first method, we add forward error correction (FEC) bits to the 54 bits MELPe core and the encoded bits are repeated three times for majority voting. In the second method, the ILBC vocoder containing corrective mechanisms against packet losses is used. All the methods proposed in this thesis together with the SCIP protocol are implemented in the simulation environment and the performance analysis under different channel conditions is reported using the PESQ scores. The method proposed for the narrowband mode provides higher PESQ scores at high SNR levels while yielding similar performance results at low SNR levels compared to the standard mechanism. In the wideband mode, the best voice quality is provided by the Triple MELPe and ILBC at low and high SNR levels, respectively.