  This letter addresses the synthesis of reflective cells approaching a given
desired Floquet's scattering matrix. This work is motivated by the need to
obtain much finer control of reflective metasurfaces by controlling not only
their co-polarized reflection but also their cross-coupling behavior. The
demonstrated capability will enable more powerful design approaches -involving
all field components in phase and magnitude- and consequently better
performance in applications involving reflective metasurfaces. We first expose
some fundamental theoretical constraints on the cell scattering parameters.
Then, a successful procedure for controlling all four scattering parameters by
applying parallelogram and trapezoid transformations to square patches is
presented, considering both normal and oblique incidence.
