  The development of metasurfaces has unlocked various use cases in wireless
communication networks to improve performance by manipulating the propagation
environment. Intelligent omni-surface (IOS), an innovative technique in this
category, is proposed for coverage extension. In contrast to the widely studied
reflective metasurfaces, i.e., intelligent reflecting surfaces (IRSs), which
can only serve receivers located on the same side of the transmitter, the IOS
can achieve full-dimensional wireless communications by enabling the
simultaneous reflection and refraction of the surface, and thus users on both
sides can be served. In this paper, we provide a comprehensive overview of the
state-of-the-art in IOS from the perspective of wireless communications, with
the emphasis on their design principles, channel modeling, beamforming design,
experimental implementation and measurements, as well as possible applications
in future cellular networks. We first describe the basic concepts of
metasurfaces, and introduce the corresponding design principles for different
types of metasurfaces. Moreover, we elaborate on the reflective-refractive
model for each IOS element and the channel model for IOS-aided wireless
communication systems. Furthermore, we show how to achieve full-dimensional
wireless communications with the IOS for three different scenarios. In
particular, we present the implementation of an IOS-aided wireless
communication prototype and report its experimental measurement results.
Finally, we outline some potential future directions and challenges in this
area.
