Chaotic encryption scheme for physical layer security in twin-SSB system via phase ambiguity

We propose what we believe to be a novel physical layer encryption method based on digital chaos via phase ambiguity in an optical twin-single-sideband (twin-SSB) signal single-photodiode (PD) detection system without optical bandpass filters (OBPFs). The proposed scheme achieves dual-security protection for both the left sideband (LSB) and the right sideband (RSB) by utilizing chaotic sequence generation, bit-level XOR encryption, and phase ambiguity to alter the distribution of constellation points. To the best of our knowledge, this work presents the first integration of digital chaotic encryption into a twin-SSB system, while also demonstrating the synthesis of a multiple-sideband-superposed 64-ary quadrature amplitude modulation (MSBS-64QAM) signal from a geometric-shaping quadrature phase shift keying (GS-QPSK) and a geometric-shaping 16QAM (GS-16QAM) using an in-phase-quadrature modulator. Simulation results show that the encrypted MSBS-64QAM signal achieves stable transmission over 10-km standard single-mode fiber (SSMF). The bit error rate (BER) of the LSB GS-16QAM and RSB GS-QPSK signals can reach hard-decision forward error correction (HD-FEC) of 3.8 × 10^-3 when the received optical power (ROP) was >-15 and >-19 dBm, respectively. Furthermore, the scheme exhibits a key space magnitude of 10⁹⁰ to resist brute-force attacks. Owing to its prominent capacity advantages and superior security performance, this scheme demonstrates great potential for future short-reach high-capacity secure communication systems.