  This paper considers the secrecy transmission in a large-scale unmanned
aerial vehicle (UAV)-enabled wireless network, in which a set of UAVs in the
sky transmit confidential information to their respective legitimate receivers
on the ground, in the presence of another set of randomly distributed
suspicious ground eavesdroppers. We assume that the horizontal locations of
legitimate receivers and eavesdroppers are distributed as two independent
homogeneous Possion point processes (PPPs), and each of the UAVs is positioned
exactly above its corresponding legitimate receiver for efficient secrecy
communication. Furthermore, we consider an elevation-angle-dependent
line-of-sight (LoS)/non-LoS (NLoS) path-loss model for air-to-ground (A2G)
wireless channels and employ the wiretap code for secrecy transmission. Under
such setups, we first characterize the secrecy communication performance (in
terms of the connection probability, secrecy outage probability, and secrecy
transmission capacity) in mathematically tractable forms, and accordingly
optimize the system configurations (i.e., the wiretap code rates and UAV
positioning altitude) to maximize the secrecy transmission capacity, subject to
a maximum secrecy outage probability constraint. Next, we propose to use the
secrecy guard zone technique for further secrecy protection, and analyze the
correspondingly achieved secrecy communication performance. Finally, we present
numerical results to validate the theoretical analysis. It is shown that the
employment of secrecy guard zone significantly improves the secrecy
transmission capacity of this network, and the desirable guard zone radius
generally decreases monotonically as the UAVs' and/or the eavesdroppers'
densities increase.
