Iterative Power Control for Wireless Networks with Distributed Reconfigurable Intelligent Surfaces

Reconfigurable Intelligent Surfaces (RIS) are a new paradigm which, with judicious deployment and alignment, can enable more favorable propagation environments and better wireless network design. As such, they can offer a number of potential benefits for next generation wireless systems including improved coverage, better interference management and even security. In this paper, we consider an uplink next generation wireless system where each user is assisted with an RIS. We study the uplink power control problem in this distributed RIS-assisted wireless network. Specifically, we aim to minimize total uplink transmit power of all the users subject to each user's reliable communication requirements at the base station by a joint design of power, receiver filter and RIS phase matrices. We propose an iterative power control algorithm, combined with a successive convex approximation technique to solve the problem with non-convex phase constraints. Numerical results illustrate that distributed RIS assistance leads to uplink power savings when direct links are weak.

I. Introduction

Future wireless networks continue to demand more than the current generation (5G) can deliver. In particular, emerging applications include those that require ultra high data rates, high reliability and spectral efficiency simultaneously [1]. To that end, new physical layer innovations are needed for 6G. Reconfigurable intelligent surfaces (RIS) - also known as intelligent reflecting surfaces (IRS) [2]–[4] - can facilitate such innovations. An RIS is a planar meta-surface that contains a large number of passive reflecting elements, each of which is capable to independently adjust the reflecting phase and (possibly) amplitude of the incident signal [2]. By judicious deployment of RISs, propagation environments of wireless signals can be steered towards desired channel realizations and/or distributions. A main advantage of RIS is its low-cost implementation. Specifically, compared to massive MIMO, RIS deployment results in remarkably smaller number of the transmitter antennas when achieving a target beamforming [5]. In addition, an RIS operates (effectively) in full-duplex (FD) mode without requiring active radio-frequency (RF) chains and generating any antenna noise amplification or self-interference, and is naturally advantageous over relay assisted communications [6].