  Integrated sensing and communication (ISAC) qualifies mobile radio systems
for detecting and localizing of passive objects by means of radar sensing.
Advanced ISAC networks rely on meshed mobile radio access nodes (infrastructure
and/or user equipment, resp.) establishing a distributed, multistatic MIMO
radar system in which each target reveals itself by its bistatic
backscattering. Therefore, characterization of the bistatic reflectivity of
targets along their trajectories of movement is of highest importance for ISAC
performance prediction. We summarize several challenges in bistatic modeling
and measurement of extended, potentially time-variant radar targets. We
emphasize the specific challenges arising for distributed (hence multi-link)
ISAC networks and compare to the state of the art in propagation modeling for
mobile communication.
