  This paper exploits the networked integrated sensing and communications
(ISAC) to support low-altitude economy (LAE), in which a set of networked
ground base stations (GBSs) cooperatively transmit joint information and
sensing signals to communicate with multiple authorized unmanned aerial
vehicles (UAVs) and concurrently detect unauthorized objects over the
interested region in the three-dimensional (3D) space. We assume that each GBS
is equipped with uniform linear array (ULA) antennas, which are deployed either
horizontally or vertically to the ground. We also consider two types of UAV
receivers, which have and do not have the capability of canceling the
interference caused by dedicated sensing signals, respectively. Under each
setup, we jointly design the coordinated transmit beamforming at multiple GBSs
together with the authorized UAVs' trajectory control and their GBS
associations, for enhancing the authorized UAVs' communication performance
while ensuring the sensing requirements. In particular, we aim to maximize the
average sum rate of authorized UAVs over a given flight period, subject to the
minimum illumination power constraints toward the interested 3D sensing region,
the maximum transmit power constraints at individual GBSs, and the flight
constraints of UAVs. These problems are highly non-convex and challenging to
solve, due to the involvement of binary UAV-GBS association variables as well
as the coupling of beamforming and trajectory variables. To solve these
non-convex problems, we propose efficient algorithms by using the techniques of
alternating optimization, successive convex approximation, and semi-definite
relaxation. Numerical results show that the proposed joint coordinated transmit
beamforming and UAV trajectory designs efficiently balance the
sensing-communication performance tradeoffs and significantly outperform
various benchmarks.
