A new speech signal scrambling method for mobile radio applications

Secure communications systems have acquired a particular importance today, especially as concerns mobile communications. At present, however, highly secure signal scrambling systems are limited in that they require frame synchronizations; this not only limits the system's reliability but complicates its use. This paper presents a two-dimensional signal scrambling method based on digital signal processing techniques which eliminate the need for frame synchronization, but maintain an extremely high level of communications security. Such techniques include short-time Fourier analysis and the filter bank concept. The paper also discusses the use of special FIR filters which make it possible to completely implement the system algorithm via commercial processor software. As a result, the system can be configured with very little hardware. Finally, the paper presents an investigation of valid keys and available key space.