High Security Transmission System Based on Bit Multidimensional Index Stream Encryption

This paper proposes a high-security transmission system based on bit multi-dimensional index stream encryption. Considering the sparsity and high efficiency of multi-user sparse code multiple access (SCMA) codebooks, where each user possesses a unique codebook, this work enhances the security performance of SCMA users by employing a seven-dimensional hyperchaotic system (7D Hyperchaotic System). The seven dimensional chaotic sequences are utilized to perform random multi-dimensional index stream encryption on SCMA users bit information. After data preprocessing, the multi-dimensional chaotic sequences are further used to encrypt SCMA codebooks, subcarrier modulation, and time slot allocation. Particularly, a multi-dimensional random indexing mechanism is implemented in subcarrier modulation to prevent eavesdropping by unauthorized users. Experimental validation is carried out on a 2 km seven-core fiber link, transmitting an encrypted orthogonal frequency division multiplexing-sparse code multiple access (SCMA-OFDM) signal carrying 34.9 Gb/s effective data rate. Results demonstrate that the proposed encryption scheme achieves a key space size of 10241, with the encryption cycle of user bit streams supporting up to 66key period combinations. Furthermore, to address the high computational complexity of SCMA codebook decoding, the receiver adopts a sphere decoding-based message passing algorithm (SD- MPA). Bit error rate performance achieves the forward error correction (FEC) threshold of 3.8 × 10-3at around −15 dBm, verifying the security and robustness of the proposed scheme in the transmission system. In addition, the detection performance of different multi-user detection algorithms at the receiver side of the SCMA-OFDM system over seven-core fiber is also investigated.