Performance Analysis of SAC-OCDMA and Physical Layer Security Utilizing Block Shifted Circulant Matrix Code

In this study, we propose a zero correlation code block shifted circulant matrix (BSCM) using the circular shifting of identity matrix mapping technique for spectral amplitude coding optical code division multiple access. The design addresses cardinality limitations of zero cross-correlation (ZCC) codes by enabling flexible configurations between the number of users and code weight without sacrificing ZCC properties. We employ a direct detection technique using an avalanche photodiode (APD). Our simulation result, using OptiSystem 15, achieved a transmission distance of 130 km within a bit error rate (BER) $\le 10^{-9}$ for three users, closely aligning with the theoretical 133 km limit. The theoretical analysis shows that increasing transmission distance also increases the BER while degrading the signal-to-noise ratio (SNR). Our BSCM model outperformed random diagonal and diagonal eigenvalue unity codes in terms of transmission distance, SNR, and channel capacity. Furthermore, the BSCM model achieved an information-theoretic channel capacity of $\sim 10^{13}$ bps, indicating its potential for establishing secure and high-bandwidth optical communication. Our study further explores the physical layer security using the wiretap channel model. Our analysis shows that a low extraction ratio reduces the security leakage. Also, increasing code length improves the robustness of the system channel against eavesdropping by ensuring the eavesdropper’s channel remains in a noisy state.