Optimal Code Set Selection and Security Issues in Spectral Phase-Encoded Time Spreading (SPECTS) OCDMA Systems

• Published in: Journal of lightwave technology Vol. 30; no. 12; pp. 1882 - 1890
• Main Authors: Bertarini, P. L. L., Sanches, A. L., Borges, B-H V.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 15.06.2012; Institute of Electrical and Electronics Engineers
• Subjects: Applied sciences; Bit error rate; Coding, codes; Crosstalk; Decoding; Encoding; Exact sciences and technology; Information, signal and communications theory; multiple access interference (MAI); Noise; Optical code division multiple access; Optical fiber communications; Optical telecommunications; phase coding; Security; Signal and communications theory; Systems, networks and services of telecommunications; Telecommunications; Telecommunications and information theory; Transmission and modulation (techniques and equipments); Multiple access; optical fiber communications; Bit error rate; Optical telecommunication; Crosstalk; Wireless telecommunication; Time spreading; Single photon emission tomography; Algorithm; Optimization; Signal interference; multiple access interference (MAI); Code division multiple access; Phase coding; Optimal code; Optical fiber communication; Safety; Optical code division multiple access; Hadamard codes
• ISSN: 0733-8724; 1558-2213
• DOI: 10.1109/JLT.2012.2194986

In this paper, we perform a thorough analysis of a spectral phase-encoded time spreading optical code division multiple access (SPECTS-OCDMA) system based on Walsh-Hadamard (W-H) codes aiming not only at finding optimal code-set selections but also at assessing its loss of security due to crosstalk. We prove that an inadequate choice of codes can make the crosstalk between active users to become large enough so as to cause the data from the user of interest to be detected by other user. The proposed algorithm for code optimization targets code sets that produce minimum bit error rate (BER) among all codes for a specific number of simultaneous users. This methodology allows us to find optimal code sets for any OCDMA system, regardless the code family used and the number of active users. This procedure is crucial for circumventing the unexpected lack of security due to crosstalk. We also show that a SPECTS-OCDMA system based on W-H 32(64) fundamentally limits the number of simultaneous users to 4(8) with no security violation due to crosstalk. More importantly, we prove that only a small fraction of the available code sets is actually immune to crosstalk with acceptable BER (<;10 -9 ), i.e., approximately 0.5% for W-H 32 with four simultaneous users, and about 1 × 10 -4 % for W-H 64 with eight simultaneous users.