  In a series of fundamental proof-of-principle experiments, comprising
numerical, controlled laboratory, and real-world experimentation, we have shown
that it is possible to use the angular momentum physical layer for radio
science and radio communication applications. Here we report a major, decisive
step toward the realization of the latter, in the form of the real-world
experimental demonstration that a radio beam carrying orbital angular momentum
(OAM) can readily be digitally phase shift modulated and that the information
thus encoded can be effectively transferred in free space to a remote receiver.
The experiment was carried out in an urban setting and showed that the
information transfer is robust against ground reflections and interfering radio
signals. The importance of our results lies in the fact that digital phase
shift keying (PSK) protocols are used in many present-day wireless
communication scenarios, allowing new angular momentum radio implementations to
use methods and protocols that are backward compatible with existing linear
momentum ones.
