High-speed microwave photonic directional modulation for physical layer secure communication

Directional modulation (DM) has emerged as a promising physical-layer security technique for next-generation wireless communication systems. This work presents a breakthrough photonic-assisted DM architecture that simultaneously resolves the fundamental constraints in conventional implementations, including limited operating frequency range, low symbol rate and sluggish phase shifter switching rates. By carefully controlling the biases of the dual-parallel Mach-Zehnder modulator (DPMZM), as well as the amplitude of the encoded signals, high-quality physical layer security communication in a specific direction is achieved. The verification experiment demonstrates a wideband directional binary phase shift keying (BPSK) modulation with a widely tunable operating frequency from 1 to 40 GHz and a high symbol rate of 2 GS/s. The experiment in 40 GHz is conducted via numerical simulations, whereas the actual hardware implementation and measurements are conducted up to 10 GHz to validate performance under realistic channel conditions. The desired direction receiver can successfully recover the symbol information with a low bit error rate (BER) approaching 10^-7. In contrast, the undesired direction exhibits low power, serious phase distortion, and high BER, making it difficult to extract useful information. The innovative directional modulation technique holds substantial promise for advanced applications in various fields, including high-speed secure military communications and satellite communications.