Intelligent reflecting surface (IRS) has emerged as a promising technology to realize smart radio environment for future wireless communication systems. Existing works in this line of research have mainly considered the conventional passive IRS that reflects wireless signals without power amplification, while in this article, we give an overview of a new type of IRS, called active IRS, which enables simultaneous signal reflection and amplification, thus significantly extending the signal coverage of passive IRS. We first present the fundamentals of active IRS, including its hardware architecture, signal and channel models, as well as practical constraints, in comparison with those of passive IRS. Then, we discuss new considerations and open issues in designing active-IRS-aided wireless communications, such as the reflection optimization, channel estimation, and deployment for active IRS, as well as its integrated design with passive IRS. Finally, numerical results are provided to show the potential performance gains of active IRS as compared to passive IRS and traditional active relay.

翻译：智能反射面（IRS）已成为实现未来无线通信系统智能无线电环境的一项有前景技术。该研究方向现有工作主要考虑的是传统无源IRS（信号反射无功率放大），而本文综述了一种称为有源IRS的新型智能反射面，其能够同时实现信号反射与放大，从而显著扩展无源IRS的信号覆盖范围。我们首先阐述有源IRS的基础理论，包括其硬件架构、信号与信道模型及实际约束条件，并与无源IRS进行对比分析。随后讨论有源IRS辅助无线通信设计中的新考量与开放问题，例如有源IRS的反射优化、信道估计、部署策略，以及其与无源IRS的集成设计。最后通过数值结果展示相比无源IRS和传统有源中继，有源IRS的潜在性能增益。