Angle-Insensitive Spherical T-RIS-Enabled Base Station

The powerful phase modulation capability of reconfigurable intelligent surfaces (RIS) endows them with the potential to replace future base station (BS) antenna arrays. However, current research on planar transmissive RIS-enabled BSs (PT-RIS-BSs) suffers from angle-sensitive limitations, failing to meet the demands of dynamic low-altitude communications. This paper proposes a spherical transmissive RIS-enabled BS (ST-RIS-BS) architecture, where traditional array antennas in BS are replaced by a combination of an omnidirectional antenna and a ST-RIS, achieving stable gain across all spatial directions. Specifically, considering angle-sensitive gain characteristics, three-dimensional (3D) signal incidence/departure angles, and the effective responsive elements of ST-RIS, we establish a ST-RIS-BS model tailored for dynamic low-altitude communications. Furthermore, through spatial integration methods, we derive the spatial average gain for both PT-RIS and ST-RIS, theoretically demonstrating a performance enhancement of up to 36.6% for the spherical configuration. Additionally, addressing a typical application scenario in the future low-altitude economy where uplink user communications coexist with unmanned aerial vehicle (UAV) data collection, we verify the performance of proposed ST-RIS-BS by the joint optimization of RIS phase shifts, transmit power, user scheduling, and UAV 3D trajectory, which is solved by using block coordinate descent and successive convex approximation techniques. This approach maximizes data collection while guaranteeing uplink user rates. Simulation results validate the effectiveness of the proposed scheme, showing that the ST-RIS solution achieves a 45% increase in data collection compared to the PT-RIS configuration.