Abstract

A crucial issue in chaotic communications is the provided security that is mainly related to the degree of complexity of the chaotic carrier, since it is known that high-dimensional chaos may prohibit the interception of the message in contrast to low-dimensional attractors [1]. The degree of complexity of a chaotic signal can be assessed by means of the Lyapunov exponents from which the Kaplan-Yorke dimension (d_KY), and the Kolmogorov-Sinai entropy are calculated [2]. In this paper, the encryption efficiency of the chaotic carrier, which is defined as the signal-to-noise ratio (SNR) of the encrypted message after filtering the combined signal of chaos and signal, is considered as the basic measure of the offered security. Hence, the lower the SNR of the filtered signal is, the higher the encryption efficiency of the system that is achieved. For the numerical calculations of this work we have considered a typical semiconductor laser using the typical parameters given in [2]. The semiconductor laser is operating at 1550nm and is subject to optical feedback with feedback delay time equal to 200ps. The message is encoded utilizing chaos-shift keying (CSK) encryption technique with modulation amplitude equal to 5% of the bias current. The combined signal is filtered with a fifth-order butterworth filter with cut-off frequency equal to bit rate. In fig. la, the calculated SNR is depicted in terms of the message bit rate for three different pairs of current and feedback strength values (K_F) along with d_KY.

© 2007 IEEE