Reconfigurable intelligent surfaces (RISs) have attracted wide interest from industry and academia since they can shape the wireless environment into a desirable form at low cost. In practice, RISs have three types of implementations: 1) reflective, where signals can be reflected to the users on the same side of the base station (BS), 2) transmissive, where signals can penetrate the RIS to serve the users on the opposite side of the BS, and 3) hybrid, where the RISs have a dual function of reflection and transmission. Most existing works focus on the reflective type RISs, and the other two types of RISs have not been well investigated. In this letter, a downlink multi-user RIS-assisted communication network is considered, where the RIS can be one of these types. We derive the system sum-rate, and discuss which type can yield the best performance under a specific user distribution. Numerical results verify our analysis.

中文翻译：

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6G 中的可重构智能表面：反射式、透射式还是两者兼而有之？

可重构智能表面 (RIS) 已引起工业界和学术界的广泛兴趣，因为它们可以以低成本将无线环境塑造成理想的形式。在实践中，RIS 有三种实现方式：1) 反射式，即信号可以反射到基站 (BS) 同侧的用户，2) 透射式，即信号可以穿透 RIS 为基站上的用户提供服务。 BS 的另一侧，以及 3) 混合，其中 RIS 具有反射和传输的双重功能。大多数现有工作都集中在反射型 RIS 上，其他两种类型的 RIS 尚未得到很好的研究。在这封信中，考虑了下行链路多用户 RIS 辅助通信网络，其中 RIS 可以是这些类型之一。我们推导出系统总和率，并讨论哪种类型可以在特定用户分布下产生最佳性能。数值结果验证了我们的分析。