  In this paper, we investigate the average achievable data rate (AADR) of the
control information delivery from the ground control station (GCS) to
unmanned-aerial-vehicle (UAV) under a 3-D channel, which requires
ultra-reliable and low-latency communications (URLLC) to avoid collision. The
value of AADR can give insights on the packet size design. Achievable data rate
under short channel blocklength is adopted to characterize the system
performance. The UAV is assumed to be uniformly distributed within a restricted
space. We first adopt the Gaussian-Chebyshev quadrature (GCQ) to approximate
the exact AADR. The tight lower bound of AADR is derived in a closed form.
Numerical results verify the correctness and tightness of our derived results.
