Reconfigurable intelligent surface (RIS) has emerged as a cost-effective and energy-efficient technique for 6G. By adjusting the phase shifts of passive reflecting elements, RIS is capable of suppressing the interference and combining the desired signals constructively at receivers, thereby significantly enhancing the performance of communication system. In this paper, we consider a green multi-user multi-antenna cellular network, where multiple RISs are deployed to provide energy-efficient communication service to end users. We jointly optimize the phase shifts of RISs, beamforming of the base stations, and the active RIS set with the aim of minimizing the power consumption of the base station (BS) and RISs subject to the quality of service (QoS) constraints of users and the transmit power constraint of the BS. However, the problem is mixed combinatorial and non-convex, and there is a potential infeasibility issue when the QoS constraints cannot be guaranteed by all users. To deal with the infeasibility issue, we further investigate a user admission control problem to jointly optimize the transmit beamforming, RIS phase shifts, and the admitted user set. Specifically, we first decompose the original non-convex problem into several rank-one constrained optimization subproblems via matrix lifting. A difference-of-convex (DC) algorithm is then developed to solve each decomposed subproblem. The proposed AO framework efficiently minimizes the power consumption of wireless networks as well as user admission control when the QoS constraints cannot be guaranteed by all users. To further address the complexity-sensitive issue for practical implementation, we propose an alternative low-complexity beamforming and RISs phase shifts design algorithm based on zero-forcing (ZF) to enable the green cellular networks.

翻译：可重构智能表面（RIS）作为一种经济高效的6G技术应运而生。通过调整无源反射单元的相位偏移，RIS能够抑制干扰并在接收端建设性地合并期望信号，从而显著提升通信系统性能。本文考虑一个绿色多用户多天线蜂窝网络，其中部署多个RIS为终端用户提供节能通信服务。我们联合优化RIS的相位偏移、基站的波束成形以及有源RIS集合，目标是在满足用户服务质量（QoS）约束和基站发射功率约束的前提下，最小化基站（BS）和RIS的总功耗。然而，该问题为混合组合非凸问题，且当所有用户无法满足QoS约束时存在潜在不可行性问题。为处理不可行性问题，我们进一步研究用户接入控制问题，联合优化发射波束成形、RIS相位偏移以及已接入用户集合。具体而言，我们首先通过矩阵松弛将原始非凸问题分解为多个秩一约束优化子问题，随后开发了一种凸差（DC）算法求解每个分解子问题。所提出的交替优化（AO）框架在用户QoS约束无法全部满足时，能有效最小化无线网络功耗并实现用户接入控制。为应对实际部署中的复杂度敏感问题，我们进一步提出一种基于迫零（ZF）的低复杂度波束成形与RIS相位偏移联合设计算法，以赋能绿色蜂窝网络。