  The dual-functional radar and communication (DFRC) technique constitutes a
promising next-generation wireless solution, due to its benefits in terms of
power consumption, physical hardware, and spectrum exploitation. In this paper,
we propose sophisticated beamforming designs for multi-user DFRC systems by
additionally taking the physical layer security (PLS) into account. We show
that appropriately designed radar waveforms can also act as the traditional
artificial noise conceived for drowning out the eavesdropping channel and for
attaining increased design degrees of freedom (DoF). The joint beamforming
design is formulated as a non-convex optimization problem for striking a
compelling trade-off amongst the conflicting design objectives of radar
transmit beampattern, communication quality of service (QoS), and the PLS
level. Then, we propose a semidefinite relaxation (SDR)-based algorithm and a
reduced-complexity version to tackle the non-convexity, where the globally
optimal solutions are found. Moreover, a robust beamforming method is also
developed for considering realistic imperfect channel state information (CSI)
knowledge. Finally, simulation results are provided for corroborating our
theoretical results and show the proposed methods' superiority.
