  In the existing cellular networks, it remains a challenging problem to
communicate with and control an unmanned aerial vehicle (UAV) swarm with both
high reliability and low latency. Due to the UAV swarm's high working altitude
and strong ground-to-air channels, it is generally exposed to multiple ground
base stations (GBSs), while the GBSs that are serving ground users (occupied
GBSs) can generate strong interference to the UAV swarm. To tackle this issue,
we propose a novel two-phase transmission protocol by exploiting cellular plus
device-to-device (D2D) communication for the UAV swarm. In Phase I, one swarm
head is chosen for ground-to-air channel estimation, and all the GBSs that are
not serving ground users (available GBSs) transmit a common control message to
the UAV swarm simultaneously, using the same cellular frequency band, to combat
the strong interference from occupied GBSs. In Phase II, all the UAVs that have
decoded the common control message in Phase I further relay it to the other
UAVs in the swarm via D2D communication, by exploiting the less interfered D2D
frequency band and the proximity among UAVs. In this paper, we aim to
characterize the reliability performance of the above two-phase protocol, i.e.,
the expected percentage of UAVs in the swarm that can decode the common control
message, which is a non-trivial problem due to the complex system setup and the
intricate coupling between the two phases. Nevertheless, we manage to obtain an
approximated expression of the reliability performance of interest, under
reasonable assumptions and with the aid of the Pearson distributions. Numerical
results validate the accuracy of our analytical results and show the
effectiveness of our protocol over other benchmark protocols. We also study the
effect of key system parameters on the reliability performance, to reveal
useful insights on the practical system design.
