High-security key accompaniment transmission scheme based on constellation flat coding over seven-core fiber

This paper proposes a seven-core fiber key synchronization transmission scheme based on constellation flat coding (CFC). This scheme maps binary keys to the positions of silent subcarriers to achieve synchronous transmission of keys and data. A four-dimensional (4D) chaotic model is used to perform XOR, CFC rules, and symbol and subcarrier masking encryption on the data. After completing XOR and three-dimensional (3D) constellation mapping, flatten the encoding of high-dimensional constellations, reduce the order of 3D constellations to two-dimensions (2D), and mask keys and data from the constellation dimension, thereby achieving chaotic encryption and enhancing system security. We conducted experiments on 56 Gb/s 3D index modulation (IM) CFC signals on a 2-kilometer-long seven-core fiber. The results show that the difference between each core of the seven-core fiber does not exceed 0.1 dB, and the bit error rate (BER) performance of 2D-CFC demodulation and key mismatch is around 0.5. This scheme deeply couples index selection, constellation mapping, and chaotic driving to embed encryption into modulation, achieving inherent physical-layer security with an effective parameter space of approximately 10¹⁰⁹ for the chaotic seed, rendering brute-force attacks infeasible.