Physical Layer Security Enhancement Scheme Based on Joint Encryption of Three-Dimensional Unequal-Length Arnold and Constellation Scrambling

This paper proposes a physical layer encryption scheme to enhance the physical layer security and transmission performance of coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The scheme first employs a three-dimensional unequal-length Arnold transformation to scramble OFDM data in the subcarrier domain, symbol domain, and complex domain. Subsequently, the data undergoes three-dimensional constellation mapping, combined with a chaotic system to perform rotational scrambling in three-dimensional space. Through the two steps of three-dimensional chaotic scrambling, the correlation between the three domains is greatly disrupted, and the data distribution becomes more complex. At the same time, the key space is increased, significantly raising the difficulty of cracking, thus enhancing the system's security. The proposed scheme has been simulated in a 16 QAM CO-OFDM system with a 30 Gbaud rate over 80 kilometers of standard single mode fiber (SSMF). The results show that the encryption scheme enables authorized users to successfully decrypt the encrypted data, while unauthorized users receive useless information with a bit error rate (BER) of approximately 0.5. Moreover, the encryption scheme achieves a key space of 10²³⁶, providing robust resistance against brute-force attacks. In addition, it improves the system's transmission performance and reduces the BER.