  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.
