Physical layer security for CO-OFDM based on chaotic real-imaginary component shuffling and dynamic Latin square permutation.

We propose a hierarchical complex symbol encryption scheme to enhance physical layer security in CO-OFDM systems. Leveraging a 6-D chaotic system, the scheme operates after QAM mapping (including 16QAM and 64QAM) and consists of two scrambling layers: intra-block scrambling on 2 × 2 micro-blocks, where the real and imaginary components are mapped to the vertices of a 3D cube and permuted using 24 distinct 3D Hilbert curve patterns driven by chaotic keys, combined with dynamic phase rotation; and inter-block scrambling on L × L macro-blocks using dynamic Latin squares with chaotic-controlled bidirectional row/column permutations. The scheme introduces no BER penalty and reduces PAPR by about 0.5 dB compared to the original OFDM signals. Simulations are conducted over transmission distances up to 240 km of SSMF under different launch powers to account for nonlinear effects. Authorized users recover data with nearly the same BER as the unencrypted baseline, whereas eavesdroppers experience a BER approaching 0.5 regardless of SNR. With a key space exceeding 10101 and a high scrambling degree, the proposed scheme provides a highly secure and efficient solution for next-generation coherent optical communications.