Beyond-line-of-sight physical-layer chaos communication based on troposcatter

We propose and demonstrate a scheme of beyond-line-of-sight (BLOS) physical-layer chaos communication based on troposcatter. In this scheme, a physical-layer chaos synchronization induced by a common digital signal between parameter-matched semiconductor lasers, after optoelectronic conversion and resampling, is utilized to generate synchronous radio chaos locally. The radio chaos-encrypted message at the transmitter is propagated through troposcatter to the receiver for decryption. Results reveal that the transmission distance, signal-to-noise ratio, and multipath number in the troposcatter channel have a significant influence on the synchronization of the chaos carrier, which in turn affects the bit error rate (BER) of message decryption. A typical demonstration shows that BLOS chaotic secure transmission of a quadrature phase-shift keying message over a distance of 50 km and at a rate of 200 Mb/s can be achieved, with a BER below the hard-decision forward error correction threshold of 3.8 × 10^-3. The distance and rate can be further extended, respectively, under troposcatter with superior path parameters and by employing higher-order message formats. Sensitivity analysis shows the system performances are sensitive to multipath delay spread while robust to Doppler shift frequency. This scheme is expected to pave the way for long-distance wireless chaos communication with physical-layer security.