Blind scramble code detection period estimation in non-cooperative interception of a scrambled DSSS signal

摘要

Direct Sequence Spread Spectrum (DSSS) signals are well-known for their low probability of interception. Their statistics are similar to the statistics of a noise, and they are usually transmitted below the noise level. Modern transmitters employ certain techniques to make their transmission more secure and safe. One of them is scrambling which is performed on transmitter side to balance the spectrum around carrier frequency and to secure the data. Scrambling the signal after spreading perturbs its cyclostationarity and detection becomes a formidable challenge. In fact, it makes interceptor believe that the detected signal may not be DSSS. This creates the need to detect the scrambled signal, estimate scrambling code, descramble and then proceed further towards the detection of DSSS signal. A method based on correlation is proposed in this paper for detecting the presence of scrambled information in the captured stray signal. The proposed method can detect the scrambling code of scrambled DSSS signal even at low signal to noise ratios (SNRs), by observing the fluctuations in autocorrelation of the received signal. Though this method is proposed in the context of DSSS, it is worthy to know that this method can be employed to detect scrambling code of other non-spreaded signals.