Cell-free systems can effectively eliminate the inter-cell interference by enabling multiple base stations (BSs) to cooperatively serve users without cell boundaries at the expense of high costs of hardware and power sources due to the large-scale deployment of BSs. To tackle this issue, the low-cost reconfigurable intelligent surface (RIS) can serve as a promising technique to improve the energy efficiency of cell-free systems. In this article, we consider an RIS aided cell-free MIMO system where multiple RISs are deployed around BSs and users to create favorable propagation conditions via reconfigurable reflections in a low-cost way, thereby enhancing cell-free MIMO communications. To maximize the energy efficiency, a hybrid beamforming (HBF) scheme consisting of the digital beamforming at BSs and the RIS-based analog beamforming is proposed. The energy efficiency maximization problem is formulated and an iterative algorithm is designed to solve this problem. The impact of the transmit power, the number of RIS, and the RIS size on energy efficiency are investigated. Both theoretical analysis and simulation results reveal that the optimal energy efficiency depends on the numbers of RISs and the RIS size. Numerical evaluations also show that the proposed system can achieve a higher energy efficiency than conventional ones.

中文翻译：

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超越无细胞 MIMO：高能效可重构智能表面辅助无细胞 MIMO 通信

无小区系统可以有效地消除小区间干扰，使多个基站（BS）能够在没有小区边界的情况下协同服务用户，但由于BS的大规模部署而导致硬件和电源成本高昂。为了解决这个问题，低成本的可重构智能表面（RIS）可以作为一种有前途的技术来提高无细胞系统的能源效率。在本文中，我们考虑了 RIS 辅助的无蜂窝 MIMO 系统，其中在基站和用户周围部署多个 RIS，以低成本方式通过可重构反射创造有利的传播条件，从而增强无蜂窝 MIMO 通信。为了最大限度地提高能效，提出了一种混合波束成形（HBF）方案，该方案由基站的数字波束成形和基于 RIS 的模拟波束成形组成。制定了能效最大化问题，并设计了迭代算法来解决该问题。研究了发射功率、RIS 数量和 RIS 大小对能效的影响。理论分析和仿真结果都表明，最佳能效取决于 RIS 的数量和 RIS 的大小。数值评估还表明，所提出的系统可以实现比传统系统更高的能源效率。