5010 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 22, NOVEMBER 15, 2009 Jamming Resistance Capabilities of Spectrally Phase Encoded OCDMA Communication Systems With Optimum and Suboptimum (Nonlinear Two-Photon-Absorption) Receiver Structures Mohammad J. Emadi and Jawad A. Salehi, Senior Member, IEEE Abstract—In this paper, we study three types of jam- mers, namely, pulse-jammer, partial-band jammer, and fol- lower-jammer, in a typical fiber-optic-based spectrally phase-en- coded optical code division multiple-access (SPE-OCDMA) system. We analyze, mathematically, the effects of the aforemen- tioned jammers on the performance of an SPE-OCDMA system for two scenarios, namely, ideal noiseless channel with an ideal optimum receiver and an ultrahigh-speed nonlinear receiver based on two-photon-absorption (TPA) in a noisy channel. Also, for each of the above cases, two types of modulation, namely, ON–OFF keying (OOK) and two-code keying are investigated and their system performances are compared. It is shown that under certain conditions, the system performance can be dramatically degraded due to the jamming signals; also, systems using modulation show a better resistance and performance when compared to systems using OOK modulation. Index Terms—Jammer, on–off keying modulation, optical code division multiple access, resistance capabilities, spectrally phase encoded, two-code keying modulation, two-photon-absorption. I. INTRODUCTION E NHANCED security is often mentioned as one of the key benefits of spectrally phase-encoded optical code divi- sion multiple-access (SPE-OCDMA) systems [1]. However, the margin and the degree of various security enhancement have not been considered deeply in the literature. One of the cri- teria in determining the degree of security of a system is to as- sess its resistance capabilities against various jammers and at- tacks. In SPE-OCDMA system, coding is applied to the phase of frequency spectrum of the ultrashort light pulses. Thus, at the receiver front end the desired user’s data signal shape is a noise-like Gaussian with low intensity and can be simply sepa- rated from multiple-access interference (MAI) by simple match filtering (decoding). In this scheme, the properly decoded sig- nals convert in to their original pulse shapes, and improperly Manuscript received March 18, 2009; revised June 20, 2009. First published July 07, 2009; current version published September 17, 2009. This work was supported in part by Iran National Science Foundation (INSF). The authors are with the Optical Networks Research Laboratory (ONRL), Electrical Engineering Department, Sharif University of Technology, Tehran, Iran (e-mail: emadi@ee.sharif.ir; jasalehi@sharif.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2009.2027030 decoded signals remain as noise-like Gaussian with low inten- sity, equivalent to temporarily broadened signals [1]–[3]. Thus, as a consequence of this noise-like Gaussian behavior, the security of the system is enhanced, and it is considered as one of the advantages of the SPE-OCDMA systems. At first glance, this conclusion, i.e., security enhancement, seems rea- sonable. In general, the adversary tries to impose his control over the communication of the information and its objectives can be categorized as follows: 1) intercepting the information furtively and 2) transmitting jamming signals to prevent suc- cessful communication for the target user [4]. The first objective is introduced and analyzed in [5]–[8]. It is demonstrated that the SPE-OCDMA systems with OOK modulation do not have se- curity at all. The interceptor can easily break the system secu- rity nonwith simple energy detector without requiring knowl- edge about the spreading code. This vulnerability of the system against eavesdropper can be somehow evaded by sending equal energy for transmitting data bits of “1” and “0” (by using dif- ferent encoders); this modulation is called . So, using this type of modulation and utilizing balanced detection not only the system security improves but also the optimum threshold due to the symmetry of transmission is zero, and it is also independent of the number of users and the amount of power used by the users [5]–[9]. The technology advancement in optical communication makes it possible that even in fiber environment, the adversary can tap in or by other means jam the target users. Although various applications of SPE-OCDMA have found their place in research and industry [10], performance of such schemes against various jammers have not been investigated in literature previously. In this paper, we cover jamming issues and we introduce three types of jamming signals and evaluate their effects on a typical SPE-OCDMA system. In order to investigate jamming resis- tance and capabilities of SPE-OCDMA system, we have con- sidered two scenarios. In the first scenario, we assume optimum and ideal receiver structure and neglect all source of noise ex- cept multiple access. In general, the bandwidth of photodetec- tors is less than the bandwidth of transmitted optical signals; hence, as a result, the performance of SPE-OCDMA systems decreases [11]. Among a few solutions to improve the system performance is to utilize ultrahigh-speed nonlinear TPA receiver structure. Thus, in second scenario, the resistance capabilities of SPE-OCDMA system with TPA receiver structure is ana- 0733-8724/$26.00 © 2009 IEEE