  In the past decade, more and more researchers have concentrated on
orbital-angular-momentum (OAM) based radio vortex wireless communications,
which is expected to provide orthogonality among different OAM-modes. The
uniform circular array (UCA) is considered as one promising antenna structure
for OAM based radio vortex wireless communications. However, most studies
regarding UCA focus on the scenario where the transmit and receive UCAs are
aligned with each other. In this paper, we investigate the radio vortex
wireless communications with non-coaxial UCA, i.e., the UCA transceivers are
parallel but non-coaxial. We study the channel model and develop the
mode-decomposition scheme to decompose the OAM-modes. Then, we discuss the
impact of included angles on the channel model under non-coaxial scenario.
Numerical results are presented to evaluate our developed scheme and show that
the spectrum efficiency of the non-coaxial UCA transceiver in some cases is
larger than that of the aligned UCA transceiver based radio vortex wireless
communications.
