Highly Secure Hybrid FSO-mmWave System Based on Key Accompanying Transmission in Pilot Subcarriers

To improve the capacity, reliability, and adaptability of data transmission in x-haul networks, hybrid systems that integrate free-space optical (FSO) and millimeter wave (mmWave) technologies have been proposed as promising solutions. This paper proposes a novel key accompanying scheme for hybrid FSO-mmWave systems, utilizing a six-dimensional (6D) hyperchaotic model to enhance system security. Meanwhile, the chaotic key is embedded in the pilot subcarriers, enabling the simultaneous transmission of key and signals. In the proposed scheme, the 6D hyperchaotic system is utilized to obtain six distinct sequences exhibiting chaotic behavior, subsequently employed for multidimensional encryption of orthogonal frequency division multiplexing (OFDM) signals. The key is embedded into the pilot subcarriers of the OFDM signal to enable key accompanying transmission. In the experimental demonstration, 8 Gbaud 16-quadrature amplitude modulation (16QAM) signals are successfully transmitted over a 40 m FSO link and a 2 m wireless link operating at 30 GHz, achieving a data rate of 32 Gb/s. Furthermore, the key space is up to 10207, which significantly enhances the system's security. Experimental results verify that the scheme effectively enhances system security while facilitating seamless key accompanying transmission. The proposed scheme exhibits significant potential for practical deployment in future Hybrid FSO-mmWave secure communication systems.