An OFDM speech scrambler without residual intelligibility

Since an FFT-based speech scrambler retains a considerable residual intelligibility in the scrambled speech, this paper proposes a new speech-scrambling technique to remove the residual intelligibility from the scrambled speech. The proposed new technique, based on the combination of an appropriate QAM mapping method and an orthogonal frequency division multiplexing (OFDM) scheme, can scramble speech without residual intelligibility by permuting several frequency components. Without residual intelligibility in the scrambled speech, the proposed OFDM speech scrambler is secure from frequency domain attacks by eavesdroppers. In addition, the OFDM speech scrambler needs only two FFT operations instead of the four required by the FFT-based speech scrambler in system structure. Simulation results are also presented to show the effectiveness of this proposed technique.