  Dual-function radar-communication (DFRC) based on frequency hopping (FH) MIMO
radar (FH-MIMO DFRC) achieves symbol rate much higher than radar pulse
repetition frequency. Such DFRC, however, is prone to eavesdropping due to the
spatially uniform illumination of FH-MIMO radar. How to enhance the physical
layer security of FH-MIMO DFRC is vital yet unsolved. In this paper, we reveal
the potential of using permutations of hopping frequencies to achieve secure
and high-speed FH-MIMO DFRC. Detecting permutations at a communication user is
challenging due to the dependence on spatial angle. We propose a series of
baseband waveform processing methods which address the challenge specifically
for the legitimate user (Bob) and meanwhile scrambles constellations almost
omnidirectionally. We discover a deterministic sign rule from the signals
processed by the proposed methods. Based on the rule, we develop accurate
algorithms for information decoding at Bob. Confirmed by simulation, our design
achieves substantially high physical layer security for FH-MIMO DFRC, improves
decoding performance compared with existing designs and reduces mutual
interference among radar targets.
