Chaotic direct-sequence spread-spectrum with variable symbol period: A technique for enhancing physical layer security

In chaotic direct-sequence spread-spectrum (DSSS) technique, chaotic sequences with typical properties such as aperiodic variation, wideband spectrum, good correlation, and initial condition sensitivity have been used as spreading codes in order to improve the security at physical layer. However, a number of recent studies have proved that an intruder can recover chaotic sequences by blind estimation methods and use the sequences to detect symbol period, which will result in the original data being exposed. To overcome this security weakness, this paper proposes a novel chaotic DSSS technique, where the symbol period is varied according to behavior of the chaotic spreading sequence in the communication process. The data with variable symbol period is multiplied with the chaotic sequence to perform the spread-spectrum process. Discrete-time models for the spreading scheme with variable symbol period and the despreading scheme with sequence synchronization are presented and analyzed. Multiple-access performance of the proposed technique in the presence of the additional white Gaussian noise (AWGN) is calculated by means of both theoretical derivation and numerical computation. Computer simulations are carried out and simulated performances are shown to verify the estimated ones. Obtained results point out that our proposed technique can protect the DSSS systems against the detection of symbol period from the intruder, even if he has full information on the used chaotic sequence.