  In dual-function radar-communication (DFRC) systems the probing signal
contains information intended for the communication users, which makes that
information vulnerable to eavesdropping by the targets. We propose a novel
design for enhancing the physical layer security (PLS) of DFRC systems, via the
help of intelligent reflecting surface (IRS) and artificial noise (AN),
transmitted along with the probing waveform. The radar waveform, the AN jamming
noise and the IRS parameters are designed to optimize the communication secrecy
rate while meeting radar signal-to-noise ratio (SNR) constrains. Key challenges
in the resulting optimization problem include the fractional form objective,
the SNR being a quartic function of the IRS parameters, and the unit-modulus
constraint of the IRS parameters. A fractional programming technique is used to
transform the fractional form objective of the optimization problem into more
tractable non-fractional polynomials. Numerical results are provided to
demonstrate the convergence of the proposed system design algorithm, and also
show the impact of the power assigned to the AN on the secrecy performance of
the designed system.
