Chaotic index modulation and subcarrier intra-block-inter-block shuffling for secure CO-OFDM-PON based on a hybrid 4D chaos

In this paper, a four-dimensional (4-D) encryption scheme with chaotic index modulation (IM) and subcarrier intra-block-inter-block shuffling is proposed to enhance the physical layer security of coherent orthogonal frequency division multiplexing passive optical network (CO-OFDM-PON). The scheme utilizes a hybrid 4-D chaotic scheme consisting of 1-D Cubic chaotic mapping and 3-D T-S Fuzzy chaotic system to generate four dynamic scrambling factors to achieve 4-D ultra-high security encryption of index bits, data bits, intra-subcarrier block, and inter-subcarrier block. Firstly, the chaotic many-to-many mapping rule between index bits and subcarrier activation patterns (SAPs) is constructed based on the IM principle to realize the index bit encryption; secondly, the data bit encryption is realized by heterodyning with chaotic bits. Further, the chaotic shuffling algorithm is proposed to disrupt and rearrange the symbols within and between subcarrier blocks to realize OFDM-IM symbol secondary encryption. To validate the feasibility of the proposed encryption scheme, simulations in a CO-OFDM-PON system evaluate the performance of a 63.76 Gb/s encrypted 16 quadrature amplitude modulation (QAM) OFDM-IM signal over transmission distances of 25 km and 80 km through standard single-mode fiber (SSMF). The results demonstrate that legitimate users can successfully recover the encrypted signals, while eavesdroppers consistently experience a bit error rate (BER) close to 0.5, making it nearly impossible to extract useful information. The initial value sensitive tolerance precision of the key (x0, y0, z0, w0, a1, b1, c1, a2, b2, c2, a3, b3, c3, µ) is (10⁻¹⁵, 10⁻¹⁵, 10⁻¹⁴, 10⁻¹⁶, 10⁻¹⁵, 10⁻¹⁴, 10⁻¹⁵, 10⁻¹⁴, 10⁻¹⁶, 10⁻¹⁶, 10⁻¹⁵, 10⁻¹⁵, 10⁻¹⁵, 10⁻¹⁵), and the key space is as high as 10²¹⁰. The proposed scheme achieves the scrambling degree (SD) value of 100% under a different number of blocks, which demonstrates its exceptional security and suitability for secure communication applications.