  As the traditional resources (frequency, time, space, etc.) are efficiently
utilized, it becomes more and more challenging to satisfy the ever-lasting
capacity-growing and users-boosting demand in wireless networks. Recently, the
electromagnetic (EM) wave was found to possess not only linear momentum, but
also angular momentum. The orbital angular momentum (OAM) is a kind of
wavefront with helical phase. The OAM-based vortex wave has different
topological charges, which are orthogonal to each other, bridging a new way for
multiple access in wireless communications. In this article, we introduce the
fundamental theory of OAM and the OAM based wireless communications. The
research challenges regarding OAM signal generation, OAM beam converging, and
OAM signal reception are discussed. Further, we propose a new multiuser access
with different OAM-modes in wireless networks, where multiple OAM-modes are
used as a new orthogonal dimension for interference avoidance. Simulation
results reveal the inherent property of OAM waves and show that OAM based radio
transmission can significantly increase the spectrum efficiency in wireless
networks.
