The adoption of a Reconfigurable Intelligent Surface (RIS) for downlink
multi-user communication from a multi-antenna base station is investigated in
this paper. We develop energy-efficient designs for both the transmit power
allocation and the phase shifts of the surface reflecting elements, subject to
individual link budget guarantees for the mobile users. This leads to
non-convex design optimization problems for which to tackle we propose two
computationally affordable approaches, capitalizing on alternating
maximization, gradient descent search, and sequential fractional programming.
Specifically, one algorithm employs gradient descent for obtaining the RIS
phase coefficients, and fractional programming for optimal transmit power
allocation. Instead, the second algorithm employs sequential fractional
programming for the optimization of the RIS phase shifts. In addition, a
realistic power consumption model for RIS-based systems is presented, and the
performance of the proposed methods is analyzed in a realistic outdoor
environment. In particular, our results show that the proposed RIS-based
resource allocation methods are able to provide up to $300\%$ higher energy
efficiency, in comparison with the use of regular multi-antenna
amplify-and-forward relaying.

研究了采用可重构智能表面 (RIS) 用于来自多天线基站的下行多用户通信。我们针对移动用户的个体链路预算保证，开发了能源有效的传输功率分配和表面反射元素的相位移位的设计。我们提出了两种算法来解决这些非凸设计优化问题。此外，我们提出了一种逼真的基于 RIS 的系统功耗模型，并在现实的户外环境中分析了所提出方法的性能。