This work discusses the optimal reconfigurable intelligent surface placement in highly-directional millimeter wave links. In particular, we present a novel system model that takes into account the relationship between the transmission beam footprint at the RIS plane and the RIS size. Subsequently, based on the model we derive the end-to-end expression of the received signal power and, furthermore, provide approximate closed-form expressions in the case that the RIS size is either much smaller or at least equal to the transmission beam footprint. Moreover, building upon the expressions, we derive the optimal RIS placement that maximizes the end-to-end signal-to-noise ratio. Finally, we substantiate the analytical findings by means of simulations, which reveal important trends regarding the optimal RIS placement according to the system parameters.

翻译：这项工作讨论了高度方向波段的最佳可重新配置智能表面位置。 特别是, 我们提出了一个考虑到RIS平面传输波束足迹和RIS大小之间关系的新型系统模型。 随后, 根据模型, 我们得出接收信号功率的端对端表达, 此外, 如果RIS大小小得多或至少等于传输波段足迹, 则提供近似封闭式表达方式。 此外, 在表达方式的基础上, 我们得出最佳的RIS位置, 最大限度地增加端对端信号对音的比率。 最后, 我们通过模拟来证实分析结果, 模拟揭示了根据系统参数优化RIS位置的重要趋势 。