  Reconfigurable intelligent surfaces (RISs) have been considered as a
promising technology for the sixth-generation (6G) wireless networks that can
control wireless channels in a desirable way and significantly enhance the
network performance. Simultaneous transmitting and reflecting-RISs (STAR-RISs)
can overcome limitation of reflecting-only RISs by leveraging the higher design
flexibility and full-space coverage. Despite the benefits, the modeling and
analysis of STAR-RISs are complicated because of various control parameters for
both transmission and reflection links. In this article, a general framework to
facilitate the design and implementation of STAR-RISs in 6G scenarios and
network topologies is presented. We provide a systematic introduction to the
STAR-RIS operating protocols for different communication modes and discuss
recent efforts to identify the research progress and combination solutions.
Finally, we provide the design concepts, research challenges, potential
solutions, and future directions related to the channel modeling, channel
estimation, hardware implementations, modeling and limitations, and
optimization.
