  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.
