  We study the design parameters for an orbital angular momentum (OAM)
multiplexed free-space data link. Power loss, channel crosstalk and power
penalty of the link are analyzed in the case of misalignment between the
transmitter and receiver (lateral displacement, receiver angular error, or
transmitter pointing error). The relationship among the system power loss and
link distance, transmitted beam size and receiver aperture size are discussed
based on the beam divergence due to free space propagation. We also describe
the trade-offs for different receiver aperture sizes and mode spacing of the
transmitted OAM beams under given lateral displacements or receiver angular
errors. Through simulations and some experiments, we show that (1) a system
with a larger transmitted beam size and a larger receiver aperture is more
tolerant to the lateral displacement but less tolerant to the receiver angular
error; (2) a system with a larger mode spacing, which uses larger OAM charges,
suffers more system power loss but less channel crosstalk; thus, a system with
a small mode spacing shows lower system power penalty when system power loss
dominates (e.g., small lateral displacement or receiver angular error) while
that with a larger mode spacing shows lower power penalty when channel
crosstalk dominates (e.g., larger lateral displacement or receiver angular
error); (3) the effects of lateral displacement and receiver angular error are
not necessarily independent; as an example of them combined, the effects of the
transmitter pointing error on the system are also investigated.
