  We propose a novel space division multiplexing (SDM) technique based on
spatial phase encoding, which may provide a new perspective beyond the
conventional SDM in free space optical communications, such as multiplexing of
orbital angular momentum (OAM). In our scheme, Gaussian beams are multiplexed
with a set of spatial phase masks, and then transmitted in the Fourier domain
via spherical lens. And another phase masks with matched patterns sandwiched by
two lenses are used to de-multiplex the corresponding channels. Three different
phase masks, namely, planar linear encoding, radial linear encoding and hybrid
of radial and azimuthal linear encoding, are proposed and analyzed. The
multiplexing and de-multiplexing of Gaussian beams are successfully implemented
using these phase encoding approaches. We prove that the OAM multiplexing and
radial phase encoding can be combined to further increase the communications
capacity in free space
