  We consider a hybrid active-passive radar system that employs a wireless
source as a passive illuminator of opportunity (IO) and a co-channel active
radar transmitter operating in the same frequency band to seek spectral
efficiency. The hybrid system can take advantage of the strengths of passive
radar (e.g., energy efficiency, bi-/multi-static configuration, and spatial
diversity) as well as those of active radar (dedicated transmitter, flexible
transmit beam steering, waveform optimized for sensing, etc.). To mitigate the
mutual interference and location-induced timing uncertainty between the radar
and communication signals, we propose two designs for the joint optimization of
the radar waveform and receive filters. The first is a max-min (MM) criterion
that optimizes a worst-case performance metric over a timing uncertainty
interval, and the other a weighted-sum (WS) criterion that forms a weighted sum
of the performance metric at each delay within the delay uncertainty interval.
Both design criteria result in nonconvex constrained optimization problems that
are solved by sequential convex programming methods. When timing uncertainty
vanishes, the two designs become identical and admit a simpler solution.
Numerical results are presented to demonstrate the performance of the proposed
hybrid schemes in comparison with conventional active-only and passive-only
radar systems.
