  In this paper, we investigate the joint resource allocation and trajectory
design for a multi-user, multi-target unmanned aerial vehicle (UAV)-enabled
integrated sensing and communication (ISAC) system, where the link capacity
between a ground base station (BS) and the UAV is limited. The UAV conducts
target sensing and information transmission in orthogonal time slots to prevent
interference. As is common in practical systems, sensing is performed while the
UAV hovers, allowing the UAV to acquire high-quality sensing data.
Subsequently, the acquired sensing data is offloaded to the ground BS for
further processing. We jointly optimize the UAV trajectory, UAV velocity,
beamforming for the communication users, power allocated to the sensing beam,
and time of hovering for sensing to minimize the power consumption of the UAV
while ensuring the communication quality of service (QoS) and successful
sensing. Due to the prohibitively high complexity of the resulting non-convex
mixed integer non-linear program (MINLP), we employ a series of transformations
and optimization techniques, including semidefinite relaxation, big-M method,
penalty approach, and successive convex approximation, to obtain a
low-complexity suboptimal solution. Our simulation results reveal that 1) the
proposed design achieves significant power savings compared to two baseline
schemes; 2) stricter sensing requirements lead to longer sensing times,
highlighting the challenge of efficiently managing both sensing accuracy and
sensing time; 3) the optimized trajectory design ensures precise hovering
directly above the targets during sensing, enhancing sensing quality and
enabling the application of energy-focused beams; and 4) the proposed
trajectory design balances the capacity of the backhaul link and the downlink
rate of the communication users.
