Highly secure non-orthogonal multiple access with key accompanying transmission based on subcarrier-indexed modulation

This paper proposes a key-accompanying transmission scheme based on subcarrier indexed modulation (SIM). The key is used to control the activation state of subcarriers, with key masking achieved through the position information of silent subcarriers, which enables the cooperative transmission of both key and primary messages. Meanwhile, a four-dimensional hyperchaotic model is adopted to ensure system security. By utilizing power multiplexing, the scheme realizes the parallel transmission of two signals and disrupts the carrier frequency and symbol of the original signals. The scheme is experimentally demonstrated with a 54.25 Gb/s SIM-chaotic power division multiplexing (CPDM) signal transmission over 2 km of 7-core fiber.. The results indicate that the proposed scheme does not degrade system transmission performance at either high or low power levels. At the limit of forward error correction (FEC)=3.8×10^-3, the impact of our scheme on receiver sensitivity is no greater than 0.1 dB. The accuracy and sensitivity of the key are maintained, with the transmission performance of the key remaining excellent. The bit error rate (BER) for the main signal is consistently kept at 0, while the BER for the key rises to around 0.5 if the key is misaligned by one bit. Moreover, the key space can reach 10¹³⁵, effectively verifying the system's high security.