Abstract
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<sup>-9</sup>), i.e., approximately 0.5% for W-H 32 with four simultaneous users, and
about 1 × 10<sup>-4</sup>% for W-H 64 with eight simultaneous users.
© 2012 IEEE
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