A reconfigurable intelligent surface (RIS) can shape the wireless propagation channel by inducing controlled phase shift variations to the impinging signals. Multiple works have considered the use of RIS by time-varying configurations of reflection coefficients. In this work we use the RIS to control the channel coherence time and introduce a generalized discrete-time-varying channel model for RIS-aided systems. We characterize the temporal variation of channel correlation by assuming that a configuration of RIS' elements changes at every time step. The analysis converges to a randomized framework to control the channel coherence time by setting the number of RIS' elements and their phase shifts. The main result is a framework for a flexible block-fading model, where the number of samples within a coherence block can be dynamically adapted.

翻译：可重构智能表面（RIS）能够通过引入受控的相移变化来塑造无线传播信道中的入射信号。多项研究已考虑通过时变反射系数配置来使用RIS。本文利用RIS控制信道相干时间，并提出一种用于RIS辅助系统的广义离散时变信道模型。我们通过假设RIS元件配置在每个时间步发生变化，来表征信道相关性的时间变化。该分析收敛为一个随机框架，通过设置RIS元件的数量及其相移来控制信道相干时间。主要成果是一个灵活的块衰落模型框架，其中相干块内的样本数量可动态调整。