1-Bit Time-Modulated Array-Based UAV-Assisted Secure SDMA Communication

Unmanned aerial vehicle (UAV)-assisted communication systems, characterized by low cost, high mobility, and on-demand deployment, are suited for establishing communication links in adversarial environments. However, the broadcast of wireless channels makes these signals susceptible to eavesdropping. Considering the limited payload of UAV and the need to improve system throughput while ensuring security, this paper proposes UAV-assisted physical layer security communication scheme based on a 1-bit time-modulated array (TMA), where the signal transmitted from the UAV-mounted transmitter undergoes space-time modulation through the TMA, forming both fundamental and harmonic components and achieving harmonic beamforming. When the UAV transmitter sends multi-user signals at intervals defined by the 1-bit TMA modulation timing frequency, the signals of different users on various harmonics are remapped to the same frequency, enabling the system to achieve space-division multiple access (SDMA) using only a single RF chain. While multiple users communicate using various harmonic beams, aliasing is introduced in the direction of eavesdroppers, reducing their achievable rate and thereby enhancing system security. This paper analyzes the secrecy rate of the proposed system and employs particle swarm optimization (PSO) to jointly optimize the UAV deployment position, power allocation, and TMA modulation sequence, with the aim of maximizing the secrecy rate. Simulation results demonstrate that the proposed scheme maintains communication performance for legitimate users while significantly reducing the achievable rate for eavesdroppers, thereby improving physical layer security.