Techniques for detecting & characterising covert communication signals

Conventional linear and non-linear receivers are generally ineffective in detecting direct-sequence spread spectrum (DS/SS) signals if the spreading sequences are unavailable. State-of-the-art techniques based on short-time Fourier analysis and its modem variants are reviewed. An investigation into using correlation-based processing is also reported showing that the cyclostationary property of DS/SS provides detection capability. Finally we describe with results an emerging technique based on higher-order statistics where bispectral analysis is used for detection together with triple correlation for identification leading to an estimation of the DS/SS modulated chip code sequence(s) operating in noisy channels.