Doubled spreading efficiency for an ultra-wideband microwave covert communication system using dual optical frequency...

Spread spectrum is a leading solution for covert communications, but is currently constrained by the limited bandwidth and sampling rates of pure electronic hardware. To address this issue, an ultra-wideband microwave covert communication system using dual optical frequency combs (OFCs) with doubled spreading efficiency is proposed in this paper. A joint optimization design that combines the electro-optic integrated spread spectrum processing and the spectrum stitching technology doubles the spectrum spreading efficiency. It significantly simplifies the system hardware complexity with a dual-OFCs architecture comprising only 37 channels, and successfully generates a spreading signal with an instantaneous bandwidth of 11.68 GHz, a frequency-domain flatness within 10 dB, and a spreading factor of up to 1825. Moreover, the center frequency of the spreading signal is tunable from 13.63 to 34.16 GHz, greatly enhancing the flexibility and the scalability of the system. At the receiver, a two-stage processing architecture consisting of an optical pre-despreading followed by a digital post-despreading is designed. A substantial processing gain of 29.5 dB is confirmed, which enables high-fidelity and reliable signal reception with a signal-to-noise ratio as low as -21 dB. Overall, the proposed system constitutes a promising solution for the advancement of covert communication technologies.