  The future mobile communication systems will support intelligent applications
such as Internet of Vehicles (IoV) and Extended Reality (XR). Integrated
Sensing and Communication (ISAC) is regarded as one of the key technologies
satisfying the high data rate communication and highly accurate sensing for
these intelligent applications in future mobile communication systems. With the
explosive growth of wireless devices and services, the shortage of spectrum
resources leads to the fragmentation of available frequency bands for ISAC
systems, which degrades sensing performance. Facing the above challenges, this
paper proposes a Carrier Aggregation (CA)-based ISAC signal aggregating high
and low-frequency bands to improve the sensing performance, where the CA-based
ISAC signal can use four different aggregated pilot structures for sensing.
Then, an ISAC signal processing algorithm with Compressed Sensing (CS) is
proposed and the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA) is
used to solve the reconfiguration convex optimization problem. Finally, the
Cram'er-Rao Lower Bounds (CRLBs) are derived for the CA-based ISAC signal.
Simulation results show that CA efficiently improves the accuracy of range and
velocity estimation.
