Near-Field Focusing and Beam Shaping Through Cooperative RIS for Communications and Sensing

This letter derives an electromagnetic model to account for the effect of multiple reconfigurable intelligent surfaces (RISs) within the same scenario. The model supports any number of RISs with arbitrary positions, orientations, and sizes, accounts for near-field (NF) interferences, and allows evaluation of field coverage at any point in space. After its formulation, the model is applied to evaluate focusing and beam shaping scenarios, the latter targeting wider coverage than a single spot, which is of interest in many communications and sensing applications. Remarkably, in the focusing example, multiple RISs working cooperatively can achieve performance comparable to that of a larger single RIS, whereas in the beam shaping case, the loss of coherent superposition in space has a more pronounced impact on performance in the collaborative scenario.