The parameter of the stable non-Gaussian noise sequence is modulated by the binary message sequence to achieve a secure communication system. The characteristic exponent 'alpha' of a stable non-Gaussian noise sequence carries the binary information. The receiver of the proposed random communication system demodulates the received signal by estimating the parameters of the transmitted noise sequence to recover the binary message sequence.
A covert communication system using a random carrier having skewed α-stable distribution is introduced. The proposed method utilises the random signals with skewed α-stable distributions exhibiting antipodal characteristics to encode the binary information at the transmitter. The receiver recovers these positive and negative skewed α-stable distributed random signals by switching at different time instants, where only the receiver has the correct switching order. The third-order statistic, given as sample skewness, is used to determine the message inside the receiver structure. The main purpose of constructing such a communication system is to improve the security aspect by avoiding the self-repeating autocorrelation behaviour of the transmitted signal in the time domain. The improvement of security is shown by comparing the fractional lower-order covariance with the reference study. According to the bit error rate results, the proposed method also shows better performance when the impulsiveness of the random carrier having skewed stable distribution is increased.Introduction: Security in digital communication systems is a major requirement and has been a challenging topic for several decades. An artificially generated noise-like behaving signal called pseudo noise (PN) sequence has been the basic approach for conventional spreadspectrum communication systems, which was analysed in detail by Torrieri [1]. Although the PN sequence provides noise-like behaviour, it was stated in [2] that an intruder can estimate the transmitted message by performing statistical signal processing and monitoring the autocorrelation function since the noise-like autocorrelation repeats itself at a finite period determined by the length of the PN sequence. To overcome this problem, a noise modulation-based covert communication system was proposed in [2]. Alternatively, a secure communication system based on α-stable noise parameter modulation was introduced in [3] as another stochastic method. In a recent study [4], the non-coherent system used in chaotic communication systems described by Stavroulakis [5] and the method proposed in [3] were combined to describe a covert communication system using a 'symmetric α-stable (SαS)' distributed random signal instead of using a chaotic signal or the conventional PN sequence. However, the covariation method used in the receiver described in [4] involves correlation between samples within a symbol period; correspondingly, binary information can still be estimated by unwanted users. This Letter proposes a technique where random signals having skewed α-stable distribution are used to encode the binary message without depending on correlation to ensure security.
a b s t r a c tIn this paper, the continuous time wavelet entropy (CTWE) of auditory evoked potentials (AEP) has been characterized by evaluating the relative wavelet energies (RWE) in specified EEG frequency bands. Thus, the rapid variations of CTWE due to the auditory stimulation could be detected in post-stimulus time interval. This approach removes the probability of missing the information hidden in short time intervals. The discrete time and continuous time wavelet based wavelet entropy variations were compared on non-target and target AEP data. It was observed that CTWE can also be an alternative method to analyze entropy as a function of time.
In this paper, a spread-spectrum communication system based on a random carrier is proposed which transmits M-ary information. The random signal is considered as a single realization of a random process taken from prescribed symmetric α-stable (SαS) distribution that carries digital M-ary information to be transmitted. Considering the noise model in the channel as additive white Gaussian noise (AWGN), the transmitter sends the information carrying random signal from non-Gaussian density. Alpha-stable distribution is used to encode the M-ary message. Inspired by the chaos shift keying techniques, the proposed method is called M-ary symmetric alpha-stable differential shift keying (M-ary SαS-DSK).The main purpose of preferring non-Gaussian noise instead of conventional pseudonoise (PN) sequence is to overcome the drawback of self-repeating noise-like sequences which are detectable due to the periodic behavior of the autocorrelation function of PN sequences. Having infinite second order moment in α-stable random carrier offers secrecy of the information due to the non-constant autocorrelation behavior. The bit error rate (BER) performance of the proposed method is illustrated by Monte Carlo simulations with respect to various characteristic exponent values and different data length.
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