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Reconfigurable intelligent surfaces (RISs) are a promising solution to improve the coverage of cellular networks, thanks to their ability to steer impinging signals in desired directions. However, they introduce an overhead in the communication process since the optimal configuration of a RIS depends on the channels to and from the RIS, which must be estimated. In this paper, we propose a novel fast iterative configuration (FIC) protocol to determine the optimal RIS configuration that exploits the small number of paths of millimetre-wave (mmWave) channels and an adaptive choice of the explored RIS configurations. In particular, we split the elements of the RIS into a number of subsets equal to the number of channel taps. For each subset, then an iterative procedure finds at each iteration the optimal RIS configuration in a codebook exploring a two-dimensional grid of possible angles of arrival and departure of the path at the RIS. Over the iterations, the grid is made finer around the point identified in previous iterations. Numerical results obtained using an urban channel model confirm that the proposed solution is fast and provides a configuration close to the optimal in a shorter time than other existing approaches.
Guglielmi et al. (Tue,) studied this question.