Key embedding transmission scheme based on multi-layer index modulation

This paper proposes a key embedding transmission scheme based on multi-layer index modulation (ML-IM), utilizing a four-dimensional chaotic system for multi-layer encryption at the physical layer. We utilize ML-IM technology in an 8 × 16 subcarrier matrix. First, the key is masked by combining the positions of silent rows. Then, subcarrier index modulation (SIM) is performed in the silent rows, using the silent positions of subcarriers to achieve additional information modulation, compensating for the data occupied by the key during transmission. This results in a high-security, high-flexibility, and low-loss encryption scheme. The scheme is experimentally demonstrated by the transmission of a 16 Gb/s 16-quadrature amplitude modulation (QAM) signal over 25 km of single-mode fiber. The experimental results show that the proposed scheme exhibits high security and initial value sensitivity, does not impair system transmission performance during encryption, supports a key space of 10¹²⁰, and maintains a bit error rate (BER) of approximately 0.5 even when the single-layer indexing rule is compromised, sufficiently resisting attacks from unauthorized receivers. Additionally, compared to existing SIM-16QAM key-accompanying studies, the proposed scheme achieves a sensitivity gain of 0.51 dB at the limit of forward error correction (FEC) = 3.8 × 10^-3. This scheme reduces resource overhead for key-following while maintaining transmission security and improving signal performance, making it highly valuable for future optical access systems.