  This paper studies an unmanned aerial vehicle (UAV)-enabled wireless sensor
network, in which one UAV flies in the sky to collect the data transmitted from
a set of sensors via distributed beamforming. We consider the delay-sensitive
application scenario, in which the sensors transmit the common/shared messages
by using fixed data rates and adaptive transmit powers. Under this setup, we
jointly optimize the UAV's trajectory design and the sensors' transmit power
allocation, in order to minimize the transmission outage probability, subject
to the UAV's flight speed constraints and the sensors' individual average power
constraints. However, the formulated outage probability minimization problem is
non-convex and thus difficult to be optimally solved in general. To tackle this
issue, we first consider the special problem in the ideal case with the UAV's
flight speed constraints ignored, for which the well-structured optimal
solution is obtained to reveal the fundamental performance upper bound. Next,
for the general problem with the UAV's flight speed constraints considered, we
propose an efficient algorithm to solve it sub-optimally by using the
techniques of convex optimization and approximation. Finally, numerical results
show that our proposed design achieves significantly reduced outage probability
than other benchmark schemes.
