Secure chaotic spread-spectrum communication systems

Summary form only given. In a hostile environment, a commercial or military interceptor may try to detect the presence of radio frequency (RF) energy and then to determine the location of its transmitter. Although direct-sequence (DS) spreading signals have generally good low probability of intercept (LPI) performance, there still exist some intercept detectors, such as those with likelihood ratio test technique, which are able to determine the presence of direct-sequence spread-spectrum (SS) signals. To improve the security of the DS SS systems, non-binary and non-periodic chaotic sequences are desirable for covert communications because their pseudorandom waveforms can be very useful in disguising signals as noise. Another advantage of using chaotic sequences is the large number of available spreading sequences for multiple access applications. Several intercept receivers, including energy detectors, synchronous and asynchronous, coherent and non-coherent structures, which are typically used to detect binary DS SS signals, are examined here to detect the presence of chaotic DS SS signals. A simple detection approach using a binary correlating function to detect non-binary chaotic sequences is proposed. Comparisons between systems using chaotic and binary sequences are given in terms of the LPI performance and the performance improvement with chaotic spreading sequences is observed. Another detection scheme employing dual antennas is also explored to detect the presence of chaotic or multi-level spreading signals. The detection performance using dual antennas is compared to that using the simple binary detection method with a single antenna. Detection probability improvement is observed with the use of dual antennas due to the reduction of waveform mismatch. Detection probabilities are also examined when the effect of mutual coupling is considered due to the small antenna spacing between the dual antennas and noticeable performance degradation is observed.