Spectral-amplitude-coding optical code-division multiple-access (OCDMA) systems are limited by interference between incoherent sources. A detailed analysis of this limit for a system with a balanced receiver is presented. Additional pulse-position modulation (PPM) coding is proposed as a method to improve the system performance beyond this limit. A simple and robust PPM decoding structure is proposed, and the performance analysis of the whole PPM-OCDMA system is presented. The interference-limited performance of the PPM-OCDMA system is found to be superior to that of the original system when the number of simultaneous users is of the order of the PPM word length or larger. In particular, for a PPM word length of two, an increase in spectral efficiency of up to 100% is possible with no change in the signaling rate, data rate, or bit-error rate (BER).

Performance enhancement of spectral-amplitude-coding optical CDMA using pulse-position modulation

Enhanced security has often been cited as an important benefit of optical CDMA (O-CDMA) signaling. However, the quality and degree of security enhancement has not been closely examined in the literature. This paper examines the degree and types of security that may be provided by O-CDMA encoding. A quantitative analysis of data confidentiality is presented for O-CDMA encoding techniques that use both time spreading and wavelength hopping. The probability of successful data interception is calculated as a function of several parameters, including signal-to-noise ratio and fraction of total available system capacity. For reasonable choices of system and encoding parameters, it is shown that increasing code complexity can increase the signal-to-noise ratio (SNR) required for an eavesdropper to "break" the encoding by only a few dB, while the processing of fewer than 100 bits by an eavesdropper can reduce the SNR required to break the encoding by up to 12 dB. The overall degree of confidentiality obtainable through O-CDMA encoding is also compared with that obtainable through standard cryptography. time-spreading/wavelength-hopping in particular, and O-CDMA in general, are found to provide considerably less data confidentiality than cryptography, and the confidentiality provided is found to be highly dependent on system design and implementation parameters.

Security performance of optical CDMA Against eavesdropping

The authors present a review of all-optical signal-processing technologies based on /spl chi//sup (2)/ nonlinear interactions in guided-wave devices and their applications for telecommunication. In this study, the main focus is on three-wave interactions in annealed proton-exchanged periodically poled lithium niobate waveguides due to their suitable properties with respect to nonlinear mixing efficiency, propagation loss, and ease of fabrication. These devices allow the implementation of advanced all-optical signal-processing functions for next-generation networks with signal bandwidths beyond 1 THz. In this paper, integrated structures that will allow for improvements of current signal-processing functions as well as the implementation of novel device concepts are also presented.

All-optical signal processing using /spl chi//sup (2)/ nonlinearities in guided-wave devices

Analysis of hybrid wavelength hopping/time spreading systems and their classification into symmetric and asymmetric systems (undercolored and overcolored asymmetric systems) is given. The characteristics of a hybrid system using extended quadratic congruence sequences for spreading and prime sequences and their cyclic shifts for hopping are examined. It is shown that the sequences exhibit excellent cross-correlation properties even in the case of a general asymmetric overcolored system. Analysis of the security properties of hybrid systems is presented both from the viewpoint of exhaustive search and from the viewpoint of chosen attack. Initial investigations indicate that the system can be so configured to allow high level of protection against determined attack, with this being especially true for an asymmetric system. Because of the massive number of users the system can support, the proposed scheme is suitable for large optical LAN's with increased security demands.

Hybrid wavelength hopping/time spreading schemes for use in massive optical networks with increased security

Image authentication techniques have recently gained great attention due to its importance for a large number of multimedia applications. Digital images are increasingly transmitted over non-secure channels such as the Internet. Therefore, military, medical and quality control images must be protected against attempts to manipulate them; such manipulations could tamper the decisions based on these images. To protect the authenticity of multimedia images, several approaches have been proposed. These approaches include conventional cryptography, fragile and semi-fragile watermarking and digital signatures that are based on the image content. The aim of this paper is to present a survey and a comparison of emerging techniques for image authentication. Methods are classified according to the service they provide, that is strict or selective authentication, tamper detection, localization and reconstruction capabilities and robustness against different desired image processing operations. Furthermore, we introduce the concept of image content and discuss the most important requirements for an effective image authentication system design. Different algorithms are described and we focus on their comparison according to the properties cited above.

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Methods for image authentication: a survey

The capacity of local area networks (LAN) can be increased substantially by amalgamating different access techniques. Wavelength division multiplexing (WDM) and code division multiple access (CDMA) can both offer an asynchronous mode of communication and can be integrated into a hybrid system. Here, the characteristics of one particular approach to achieving a hybrid system, analogous to hybrid systems used in electrical CDMA are examined. An optical CDMA concept using wavelength hopping and time spreading is described employing prime sequences for both the hopping and the spreading pattern (prime-hop sequences), and proofs are given that the autocorrelation function has zero side lobes and that the cross-correlation function is at most 1. In such systems, the number of possible stations is greatly increased as is the number of simultaneous users. Code generation and correlation are described in detail and networks for efficient code generation are analysed. It is shown that the system offers increased security levels leading to a very secure transmission. The system is suitable for truly asynchronous highly secure LAN applications.

Hybrid wavelength hopping/time spreading code division multiple access systems

A hybrid wavelength hopping/time spreading scheme is proposed for use in secure optical network applications. Featuring a perfect autocorrelation function and optical orthogonality, hybrid systems can offer truly asynchronous communications with an increased system throughput. The security aspects of such a network are investigated and it is shown that a properly configured network can offer unprecedented levels of security. Calculation of the number of code combinations reveals that many years will be needed to investigate all in order to penetrate the network. >

Secure optical network architectures utilizing wavelength hopping/time spreading codes

A hybrid wavelength hopping/time spreading all-optical code division multiple access (CDMA) system using extended quadratic congruence codes for spreading and prime codes and their cyclic shifts for hopping is examined. The codes feature excellent correlation properties even in the case of an asymmetric system with random wavelength selection and the number of codes in the family is substantially increased making them attractive for massive, asynchronous LAN's with increased security demands.

Massive optical LANs using wavelength hopping/time spreading with increased security

An approach is described for obtaining a correlation output corresponding to the difference between agreements and disagreements in incoherent optics, through the use of additional preceding and balanced detection. Calculations of the correlation spectra and of the probability of BER reveals that the proposed technique, in tandem with gold codes, can accommodate more users than previously reported systems. >

Incoherent asynchronous optical CDMA using gold codes

In order to minimise the harmful influence of the ground reflections on antenna-pattern measurements, diffraction fences are frequently used. This letter treats the diffraction of the direct wave radiated by a directional antenna. The analysis is based on the Kirchhoff?Fresnel approximation of the diffracted field.

J. Budin

Papers

Hybrid wavelength hopping/time spreading code division multiple access systems

Secure optical network architectures utilizing wavelength hopping/time spreading codes

Massive optical LANs using wavelength hopping/time spreading with increased security

Incoherent asynchronous optical CDMA using gold codes

Effect of the illumination taper on the knife-edge diffraction