  Intelligent reflecting surfaces (IRSs) tune wireless environments to increase
spectrum and energy efficiencies. In view of much recent attention to the IRS
concept as a promising technology for 6G wireless communications, we present a
survey of IRSs in this paper. Specifically, we categorize recent research
studies of IRSs as follows. For IRS-aided communications, the summary includes
capacity/data rate analyses, power/spectral optimizations, channel estimation,
deep learning-based design, and reliability analysis. Then we review IRSs
implementations as well as the use of IRSs in secure communications,
terminal-positioning, and other novel applications. We further identify future
research directions for IRSs, with an envision of the IRS technology playing a
critical role in 6G communication networks similar to that of massive MIMO in
5G networks. As a timely summary of IRSs, our work will be of interest to both
researchers and practitioners working on IRSs for 6G networks.
