  In this paper, we study an intelligent omni-surface (IOS)-assisted downlink
communication system, where the link quality of a mobile user (MU) can be
improved with a proper IOS phase shift design. Unlike the intelligent
reflecting surface (IRS) in most existing works that only forwards the signals
in a reflective way, the IOS is capable to forward the received signals to the
MU in either a reflective or a transmissive manner, thereby enhancing the
wireless coverage. We formulate an IOS phase shift optimization problem to
maximize the downlink spectral efficiency (SE) of the MU. The optimal phase
shift of the IOS is analysed, and a branch-and-bound based algorithm is
proposed to design the IOS phase shift in a finite set. Simulation results show
that the IOS-assisted system can extend the coverage significantly when
compared to the IRS-assisted system with only reflective signals.
