  We explore the use of orbital-angular-momentum (OAM)-multiplexing to increase
the capacity of free-space data transmission to moving platforms, with an added
potential benefit of decreasing the probability of data intercept.
Specifically, we experimentally demonstrate and characterize the performance of
an OAM-multiplexed, free-space optical (FSO) communications link between a
ground station and a moving unmanned-aerial-vehicle (UAV). We achieve a total
capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams,
each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal.
Moreover, we investigate for static, hovering, and moving conditions the
effects of channel impairments, including: tracking errors, propeller-induced
airflows, power loss, intermodal crosstalk, and system bit error rate (BER). We
find the following: (a) when the UAV hovers in the air, the power on the
desired mode fluctuates by 2.1 dB, while the crosstalk to the other mode is -19
dB below the power on the desired mode; and (b) when the UAV moves in the air,
the power fluctuation on the desired mode increases to 4.3 dB and the crosstalk
to the other mode increases to -10 dB. Furthermore, the channel crosstalk
decreases with an increase in OAM mode spacing.
