regarded as one of the most promising and revolutionizing techniques for enhancing the
spectrum and/or energy efficiency of wireless systems. These devices are capable of
reconfiguring the wireless propagation environment by carefully tuning the phase shifts of a
large number of low-cost passive reflecting elements. In this article, we aim to answer four
fundmental questions: 1) Why do we need RISs? 2) What is an RIS? 3) What are RIS's …