Special issue on reconfigurable intelligent surface for B5G & 6G

Reconfigurable intelligent surface (RIS) or intelligent reflecting surface (IRS) is recently recognized as one of the most promising technologies to effectively increase coverage and improve communication performance for future wireless communications beyond 5G and 6G (B5G & 6G). Specifically, the RIS is a planar surface consisting of an array of passive reflecting elements, each of which can independently induce a controllable reflection coefficient on the incident signal. By installing an RIS on the facade of buildings, an additional virtual line of sight (LoS) link can be established between the base station (BS) and the user. By judiciously adjusting the phase shifts of the reflecting elements, the reflected signal can be constructively superimposed with the signal from the direct path to enhance the desired signal power or destructively mitigate unfavourable signals such as multi-user interference or signal leakage to eavesdroppers. The RIS is less expensive to deploy than conventional active transmitters since its reflecting elements only passively reflect the incoming signal without any signal processing operations and does not require costly and power-hungry radio frequency (RF) chains. Moreover, it is lightweight and has limited layer thickness, so it can be easily integrated into the environment. Despite the above-mentioned appealing advantages of RIS, it also brings some new signal processing challenges such as channel estimation, robust transmission design, angle/position estimation, distributed algorithm design, etc.