UAV Altitude, Relay Selection, and User Association Optimization for Cooperative Relay-Transmission in UAV-IRS-Based THz Networks

Unmanned aerial vehicles (UAVs) are widely adopted as aerial relays assisting seamless coverage due to flexible deployment and high maneuverability, but their service time is considered as the bottleneck due to the constrained energy. The energy-efficient intelligent reflecting surface (IRS) is adopted in the UAV system for long-term service. In this paper, the cooperative relay-transmission in UAV-IRS based terahertz (THz) networks is studied, where UAVs with IRSs as relays facilitate long-term seamless coverage through THz transmission, and the maximum transmission capacity based on sum rate and energy consumption is formulated as a maximization problem to optimize UAV altitude, relay selection and user association. In the single-user-single-UAV scenario, how relay selection is affected by the IRS element number and UAV transmitting power is analyzed. In the multiple-user-multiple-UAV scenario, the maximization problem is decomposed into three sub-problems and solved by an alternating optimization method: optimizing UAV height using gradient descent and interior point algorithms, solving relay selection as a linear programming problem by continuous variable relaxation, and optimizing user association as a knapsack problem through association matrix transformation. Simulation results demonstrate the effectiveness of the proposed method and indicate that the cooperative relay-transmission scheme achieves quasi-optimal performance when compared with existing schemes.