Reconfigurable Intelligent Surface Assisted UAV in Heterogeneous Networks with NOMA

Recently, unmanned aerial vehicles (UAVs) and reconfigurable intelligent surface (RIS) have been used to enhance the performance of wireless communications. Signal transmission supported by the UAV can have a direct link with mobile users due to its flexible deployment and controllable trajectory. Non-orthogonal multiple access (NOMA) achieves high spectral efficiency by utilizing power domain multiplexing in a multiuser NOMA system. In this paper, a heterogeneous network (HetNet) is considered with a UAV acting as a small base station (BS) that serves terrestrial users in a microcell. The BS in the macro cell serves multi-users using NOMA technology with the aid of RIS. A joint optimization problem is formulated to optimize the allocated powers of all users and the phase shifts of the RIS to maximize the sum rate. The allocated power is optimized and closed form expressions are obtained using lagrangian dual transform, while the RIS phase shifts are optimized using the full search method. Numerical results show that the joint optimization of RIS phase shifts and power has a higher sum rate than optimizing the allocated power only or optimizing phase shifts only.