Reconfigurable intelligent surface (RIS) is a promising technology to enhance the spectral efficiency of wireless communication systems. By optimizing the RIS elements, the performance of the overall system can be improved. Yet, in contrast to single-carrier systems, in multi-carrier systems, it is not possible to independently optimize RIS elements at each sub-carrier, which may reduce the benefits of RIS in multi-user orthogonal frequency division multiplexing (OFDM) systems. To this end, we investigate the effectiveness of RIS in multiple-input, multiple-output (MIMO) OFDM broadcast channels (BC). We formulate and solve a joint precoding and RIS optimization problem. We show that RIS can significantly improve the system performance even when the number of RIS elements per sub-band is very low.

翻译：可重构智能表面（RIS）是一种有望提升无线通信系统频谱效率的技术。通过优化RIS单元，可以改善系统整体性能。然而，与单载波系统不同，在多载波系统中，无法独立优化每个子载波上的RIS单元，这可能降低RIS在多用户正交频分复用（OFDM）系统中的优势。为此，我们研究了RIS在多输入多输出（MIMO）OFDM广播信道（BC）中的有效性。我们提出并求解了一个联合预编码与RIS优化问题。研究结果表明，即使每个子频带的RIS单元数量非常少，RIS也能显著提升系统性能。