Abstract
The recent development of metasurfaces, which may enable several use
cases by modifying the propagation environment, is anticipated to have a
substantial effect on the performance of 6G wireless communications.
Metasurface elements can produce essentially passive sub-wavelength
scattering to enable a smart radio environment. STAR-RIS, which refers
to reconfigurable intelligent surfaces (RIS) that can transmit and
reflect concurrently (STAR), is gaining popularity. In contrast to the
widely studied RIS, which can only reflect the wireless signal and serve
users on the same side as the transmitter, the STAR-RIS can both reflect
and refract (transmit), enabling 360-degree wireless coverage, thus
serving users on both sides of the transmitter. This paper presents a
comprehensive review of the STAR-RIS, with a focus on the most recent
schemes for diverse use cases in 6G networks, resource allocation, and
performance evaluation.
We begin by laying the foundation for RIS (passive, active, STAR-RIS),
and then discuss the STAR-RIS protocols, advantages, and applications.
In addition, we categorize the approaches within the domain of use
scenarios, which includes increasing coverage, enhancing physical layer
security (PLS), maximizing sum rate, improving energy efficiency (EE),
and reducing interference. Next, we will discuss the various strategies
for resource allocation and measures for performance evaluation. We
aimed to elaborate, compare, and evaluate the literature in terms of
setup, channel characteristics, methodology, and objectives. In
conclusion, we examine the open research problems and potential future
prospects in this field.