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Probability bounds on the peak intensity of optical ultrashort light pulse CDMA

Decision Systems., Emmanouel Varvarigos, Jawad A. Salehi

Published Jan 1, 1991• 0 citations
OpenAlex

Abstract

Optical CDMA is being proposed as a suitable multiaccess technique to take advantage of the large bandwidth available in single-mode fiber-optic communications. In a CDMA system the communication link is shared by a number of users, each of which has a unique address code called signature. Recently an all-optical CDMA system based upon encoding and decoding coherent ultrashort light pulses was introduced. An important factor in the performance of the CDMA system is the peak intensity of the encoded signals. We consider using random signatures for the phase encoding of the light pulses in this system and prove that they result in a fairly low peak intensity of the signals. In particular, we obtain an upper bound on the probability that the peak intensity of the encoded signal is greater than some value. This bound is strictly better than previously reported bounds and decreases exponentially with the length of the signatures. We also find a lower bound on the maximum number of minimally interfering signatures (or, to be more accurate, A-separated signatures, which will be defined) of length No that can be designed. Assuming that the communication is reliable if and only if any pair of signatures in the system is A-separated, a notion of capacity is defined and a lower bound on it is found.

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CRC Press eBooks•Nov 15, 2018•1 citation
Optical CDMA is being proposed as a suitable multiaccess technique to take advantage of the large bandwidth available in single-mode fiber-optic communications. In a CDMA system the communication link is shared by a number of users, each of which has a unique address code called signature. Recently an all-optical CDMA system based upon encoding and decoding coherent ultrashort light pulses was introduced. An important factor in the performance of the CDMA system is the peak intensity of the encoded signals. We consider using random signatures for the phase encoding of the light pulses in this system and prove that they result in a fairly low peak intensity of the signals. In particular, we obtain an upper bound on the probability that the peak intensity of the encoded signal is greater than some value. This bound is strictly better than previously reported bounds and decreases exponentially with the length of the signatures. We also find a lower bound on the maximum number of minimally interfering signatures (or, to be more accurate, A-separated signatures, which will be defined) of length No that can be designed. Assuming that the communication is reliable if and only if any pair of signatures in the system is A-separated, a notion of capacity is defined and a lower bound on it is found.
Optical CDMA is being proposed as a suitable multiaccess technique to take advantage of the large bandwidth available in single-mode fiber-optic communications. In a CDMA system the communication link is shared by a number of users each of which has a unique address code, called signature. Recently an all-optical CDMA system based upon encoding and decoding coherent ultrashort pulses was introduced [l]. In the proposed optical CDMA system the transmitter consists of a bandhiited signal source, that generates a train of ultrashort light pulses. The output of the bandlimited source is multiplied by a data source that takes values 0 or 1 (ON-OFF Keying). If the data is 0, then no energy is transmitted. If the data is 1, then the ultrashort pulse is sent to the spectral-phase encoder which adds a determinate phase shift to each spectral component of the ultrashort pulse. When a pseudorandom set of spectral phase shifts is selected, spectral phase coding spreads the incident ultrashort pulse into a longer, lower intensity pseudonoise burst 111. The receiver is assumed to be a correlation receiver which is matched to the desired signal. The correlation process (optical decoder) is similar to that of the optical encoder, except that the decoder's spectral-phase code is the complex conjugate of the encoder's spectral-phase code. Thus a pulse is properly decoded when the encoding and decoding masks are a complex conjugate pair, in which case the spectral phase shifts are removed and the original pulse is recovered. On the other hand, when the encoding and decoding masks do not match, the spectral phase shifts are rearranged but not removed, and the output remains a low intensity pseudonoise burst. A threshold device is set to detect data corresponding to intense, properly decoded pulses and to reject low intensity, improperly decoded, pseudonoise bursts. An important factor in the performance of the proposed CDMA systrm is the peak intensity of the encoded signals. We consider using random signatures for the phase encoding of the phase pulses in the system and prove that they result in a fairly low peak intensity of the signals. We assume that the ultra-short light pulses are characterized by an ideal rectangular spectrum with bandwidth W. The ultrashort light pulse has peak intensity Po and duration t, $. Pulses of this form had been used in the experiments reported in [l]. The encodfng is taking place in the frequency domain. The signal
The authors obtain lower and upper bounds on the peak intensity of a spectrally phase encoded ultrashort light pulse as a function of the length of spectral phase encoding. It is shown that using random phase codes for spectral encoding of ultrashort light pulses in a coherent ultrashort light pulse code-division multiple-access (CDMA) system is almost optimal. Furthermore, simple upper bounds and an asymptotic approximation on the bit error rate (BER) for a wide class of ultrashort light pulse CDMA systems are obtained. It is shown that, in spite of being simple, these upper bounds can be used to evaluate the performance of a coherent ultrashort light pulse CDMA system.>
Optical CDMA transmission systems can not only increase system capacity, but also achieve physical- layer security against fiber tapping attacks. In this paper, we examine the information-theoretic security of optical CDMA systems by evaluating the tradeoff between the achieved information capacity and the confidentiality. Moreover, we also evaluate the impacts of key system parameters, such as the number of users M, the number of stages of shift-register r (or code lengths L=2r-1) and the input optical power P, on the secrecy capacity and the interception probability. Our results indicate that key system parameters play an important role in enhancing the secrecy capacities, thus choosing suitable values of these system parameters will be helpful to improve the secure performance of optical CDMA systems to some degree.
OCDMA) is an alternative method, which performs encoding and decoding through an optical signature code, in order to allow the selection of a desired signal so that different users can share the same bandwidth.In such a systems, data signal overlap both in time and wavelength.Tradeoffs between security and performance in optical CDMA are presented.Confidentiality against eavesdropper interception strategies for optical CDMA is considered.Coherent detection and combining signals shows better confidentiality than the incoherent one.Spectral amplitude optical CDMA based on Modified Double Weight (MDW) systems, with different code dimension, is investigated.Increasing the network system capacity or maximizing authorized specified SNRs will lead to eavesdropper to detect high SNRs.
(OCDMA) is an alternative method, which performs encoding and decoding through an optical signature code, in order to allow the selection of the desired signal so that different users can share the same bandwidth. In such a system, the data signal overlap both in time and wavelength. Tradeoffs between security and performance in optical CDMA are presented. Confidentiality against eavesdropper interception strategies for optical CDMA is considered. Coherent detection and combining signals shows better confidentiality than the incoherent one. Spectral amplitude optical CDMA based on Modified Double Weight (MDW) systems, with different code dimension, is investigated. Increasing the network system capacity or maximizing authorized specified SNRs will lead to eavesdropper to detect high SNRs.
Optical CDMA transmission systems can not only increase system capacity, but also achieve physical-layer security against fiber tapping attacks. In this paper, we examine the information-theoretic security of optical CDMA systems by evaluating the trade off between the achieved information capacity and the confidentiality. In particular, we established a wiretap channel model of optical CDMA systems, based on which we have evaluated the impacts of key system parameters, such as the type of code words, the number of users M , the code lengths N and the input optical power P , on the system security. Our results indicate that key system parameters play an important role and choosing suitable values of these system parameters will be helpful to improve the secure performance of optical CDMA systems to some degree. For example, choosing the code words whose cross-correlation λ values are smaller, increasing the code lengths N or the input optical power P and reducing the number of users M properly can all be used to improve the secure performance of optical CDMA systems.