  In this paper, we derive the performance bounds for joint communication data
rate and estimation error of radar target parameters from first principles,
where the targets are assumed to be passive. Specifically, we let the targets
to have control over their passive reflectors in order to transmit their own
information back to the radar via reflection-based beamforming or
backscattering. Such a setup avoids active radio frequency transmission from
battery operated devices such as friendly (or reconnaissance) drones. The
concept of target ambiguity function arises naturally from these derivations,
which not only poses challenge to waveform designers, but also provides an
opportunity for a joint design of waveform and array geometries to achieve an
optimal performance. We derive the Cram\'er-Rao lower bounds for the mean
squared error in the estimation of target parameters, and derive lower bounds
on the data rates with both radar-only and joint radar and communications
scenarios. The challenge of transmit waveform design for joint radar-data
communication is illustrated via numerical examples.
