An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VI. SPREAD SPECTRUM TECHNIQUES FOR PHYSICAL LAYER SECURITY <s> Historically developed for secure communication and military use, CDMA is now serving as one of the most widely used wireless airlink interface and has been identified as a major technique for 3G wireless communications. In addition to the wide bandwidth and low power spectrum density which make CDMA signals robust to narrow band jamming and easy to be concealed within the noise floor, the physical layer built-in information privacy of CDMA system is provided by pseudo-random scrambling. In this paper, first, the physical layer security weakness of the operational IS-95 CDMA airlink interface is analyzed. Secondly, based on the advanced encryption standard (AES), we propose to enhance the physical layer built-in security of CDMA systems through secure scrambling. Performance analysis demonstrates that while providing significantly improved information privacy, CDMA system with secure scrambling has comparable computational complexity and system performance with that of the IS-95 system. Moreover, it is shown that by scrambling the training sequence and the message sequence separately with two independent scrambling sequences, both information privacy and system performance can be further improved. The proposed scheme can readily be applied to 3G systems and IEEE 802.11b WLAN systems <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> VI. SPREAD SPECTRUM TECHNIQUES FOR PHYSICAL LAYER SECURITY <s> Originally adopted in military networks as a means of ensuring secure communication when confronted with the threats of jamming and interception, spread-spectrum systems are now the core of commercial applications such as mobile cellular and satellite communication. This book provides a concise but lucid explanation and derivation of the fundamentals of spread-spectrum communication systems. The level of presentation is suitable for graduate students with a prior graduate-level course in digital communication and for practicing engineers with a solid background in the theory of digital communication. As the title indicates, the author focuses on principles rather than specific current or planned systems. Although the exposition emphasizes theoretical principles, the choice of specific topics is tempered by their practical significance and interest to both researchers and system designers. Throughout the book, learning is facilitated by many new or streamlined derivations of the classical theory. Problems at the end of each chapter are intended to assist readers in consolidating their knowledge and to provide practice in analytical techniques. Principles of Spread-Spectrum Communication Systems is largely self-contained mathematically because of the four appendices, which give detailed derivations of mathematical results used in the main text. <s> BIB002
One of the potential methods for improving physical layer security is spread spectrum (SS) communications. In this method, a signal is spread over a frequency band such that the resulting bandwidth is much wider than that of the original signal. Historically, this method has been first used in military applications to achieve transmission schemes with low probability of intercept (LPI). Two main approaches in SS communications are direct sequence spread-spectrum (DSSS) and frequency-hopping spread-spectrum (FHSS). FHSS involves hopping the carrier frequency of the outgoing signal in a predetermined fashion, which is known to both the transmitter and the receiver BIB002 . In this way, it is extremely difficult for an eavesdropper to monitor the resulting spread spectrum signal. In DSSS, the outgoing symbol stream gets multiplied by a symbol sequence called spreading code which has a much smaller symbol duration, hence increasing the bandwidth of the resulting signal. For the case where multiple users share the same channel, they utilize different spreading codes to distinguish their signals. In order to avoid an eavesdropper to detect the signal, the spread signal can be scrambled using a pseudo-noise (PN) sequence  . One of the main differences between DSSS and traditional cryptography methods (where the key space is very large) is that the key space is limited by the available bandwidth. Physical layer security of code division multiple access (CDMA) systems is studied in BIB001 based on the use of relatively long PN scrambling sequences. In most cases, Walsh codes are used to generate channelization codes for this purpose as they are easy to generate. Li et al. BIB001 propose a method to generate the scrambling sequences by using advanced encryption standard (AES) operation. Specifically, they design an AES-CDMA scheme secured against exhaustive key-search attacks utilizing AES with three different key sizes up to 256. We close this discussion by noting that, while valid in practice, such spread spectrum based solutions do not provide information theoretically (provable) secure communications, unlike the physical layer security algorithms being reviewed.