Reconfigurable Intelligent Surface for Small Cell Network

Small cell network (SCN) is a promising solution to meet the demand for increasing data traffic for the sixth generation and beyond wireless networks. However, the power consumption and two-tier interference issues are two bottlenecks that hinder its further development. In this paper, we propose a novel intelligent reflecting communication (IRC) system in which a reconfigurable intelligent surface (RIS) is used to serve multiple micro users in an SCN while assisting the transmission from a macro base station (MBS) to a macro user. Compared to the conventional SCN, the RIS can achieve significant power reduction as it transmits the information by passively reflecting the incident signals. In addition, the reflected signal can be regarded as a multipath component instead of an interference to the macro user. We are interested in minimizing the total power consumption by jointly designing the phase shift matrix at the RIS and the beamforming vector at the MBS under the user rate constraints and the practical phase shift constraints. The solution is obtained by alternating optimization to iteratively solve two subproblems, one to optimize the phase shift matrix, and the other to optimize the beamforming vector. A TDMA transmission scheme is also proposed as an alternative to serve multiple users. Simulation results demonstrate that the total power consumption can be reduced significantly by deploying the RIS in the SCN when the number of reflecting elements is sufficiently large.