  The existing phase shifter models adopted for reconfigurable intelligent
surfaces (RISs) have ignored the electromagnetic (EM) waves propagation
behavior, thus cannot reveal practical effects of RIS on wireless communication
systems. Based on the equivalent circuit, this paper introduces an
angle-dependent phase shifter model for varactor-based RISs. To the best of our
knowledge, this is the first phase shifter model which reveals that the
incident angle of EM waves has influence on the reflection coefficient of RIS.
In addition, the angle-reciprocity on RIS is investigated and further proved to
be tenable when the reflection phase difference of adjacent RIS unit cells is
invariant for an impinging EM wave and its reverse incident one. The
angle-dependent characteristic of RIS is verified through full-wave simulation.
According to our analysis and the simulation results, we find that the
angle-reciprocity of varactor-based RIS only holds under small incident angles
of both forward and reverse incident EM waves, thus limits the channel
reciprocity in RIS-assisted TDD systems.
