Reconfigurable intelligent surfaces (RISs) represent a radical new technology that can shape the radio wave propagation in wireless communication systems and offers a great variety of possible performance and implementation gains. Motivated by this, in this paper we study the achievable rate optimization for a multi-stream multiple-input multiple-output (MIMO) system equipped with an RIS, and formulate a joint optimization problem of the covariance matrix of the transmitted signal and the RIS elements. To solve this problem, we propose an iterative optimization algorithm that is based on the projected gradient method (PGM). We derive the step size that guarantees the convergence of the proposed algorithm and we define a backtracking line search to improve its convergence rate. Furthermore, we introduce the total free space path loss (FSPL) ratio of the indirect and direct links as a first-order measure of the applicability of an RIS in the considered communication system. Simulation results show that the proposed PGM achieves the same achievable rate as a state-of-the-art benchmark scheme, but with a significantly lower computational complexity. In addition, it is demonstrated that the RIS application is particularly suitable to increase the achievable rate in an indoor environment, as in this case even a small number of RIS elements is sufficient to provide a substantial achievable rate gain.
翻译:重新配置的智能表面(RIS)代表着一种能影响无线通信系统无线电波传播的激进的新技术,并提供了各种可能的性能和执行收益。为此,我们在本文件中研究多流多投入多输出产出(MIMO)系统可实现的速率优化,配有RIS的多投入多输出(MIMO)系统,对传输信号和RIS元素的共变矩阵提出一个联合优化问题。为了解决这一问题,我们提议了一个基于预测梯度法的迭代优化算法。我们得出了保证拟议算法趋同的步数,并定义了一条后行搜索,以提高其趋同率。此外,我们引入了间接和直接连接(FSPL)的完全自由空间路径损失比率,作为衡量TRIS在经过考虑的通信系统中适用性的第一个阶梯度衡量标准。模拟结果表明,拟议的PGM的可实现率与最新基准计划相同,但计算复杂度要低得多。此外,我们还表明,即使小的RIS应用率也足以提高室内可实现的率。