Resource Allocation for RIS-Assisted Mixed Near-and Far-Field Communication with Directional Antenna

This paper studies resource allocation for a reconfigurable intelligent surface (RIS)-assisted mixed near- and far-field communication utilizing a directional antenna. The resource allocation algorithm is designed as a non-convex optimization problem to maximize the data rate in the communication system. To circumvent the problem intractability, the non-convex problem is transformed into a standard semidefinite relaxation (SDR) programming problem by exploiting radiation field electromagnetic theory. This allows us to characterize the solution structure of the joint height and axis of the directional antenna and the RIS phase shift matrix that facilitates the design of an efficient iterative algorithm for obtaining the solution. We reveal that the maximal data rate of RIS-assisted mixed near- and far-field communication is achieved if the axis of the directional antenna aligns the plane that includes both the center of the RIS and the UE's location. Simulation results demonstrate a significant data rate improvement with the proposed resource allocation compared to the two baseline schemes.