  Orbital angular momentum (OAM) based radio vortex wireless communications
have received much attention recently because it can significantly increase the
spectrum efficiency. The uniform circular array (UCA) is a simple antenna
structure for high spectrum efficiency radio vortex wireless communications.
However, the OAM based electromagnetic waves are vortically hollow and
divergent, which may result in the signal loss. Moreover, the divergence of
corresponding OAM based electromagnetic wave increases as the order of OAM-mode
and radius of UCA increases. Therefore, it is difficult to use high-order
OAM-mode, because the corresponding received signal-to-noise ratio (SNR) is
very small. To overcome the difficulty of high-order OAM modes transmission, in
this paper we propose a lens antenna based electromagnetic waves converging
scheme, which maintains the angular identification of multiple OAM-modes for
radio vortex wireless communications. We further develop a bifocal lens antenna
to not only converge the electromagnetic wave, but also compensate the SNR loss
on traditional electromagnetic waves. Simulation results show that the proposed
bifocal lens can converge the OAM waves into cylinder-like beams, providing an
efficient way to increase the spectrum efficiency of wireless communications.
