  To efficiently utilize the wireless spectrum and save hardware costs, the
fifth generation and beyond (B5G) wireless networks envisage integrated sensing
and communications (ISAC) paradigms to jointly access the spectrum. In B5G
systems, the expensive hardware is usually avoided by employing hybrid
beamformers that employ fewer radio-frequency chains but at the cost of the
multiplexing gain. Recently, it has been proposed to overcome this shortcoming
of millimeter wave (mmWave) hybrid beamformers through spatial path index
modulation (SPIM), which modulates the spatial paths between the base station
and users and improves spectral efficiency. In this paper, we propose an
SPIM-ISAC approach for hybrid beamforming to simultaneously generate beams
toward both radar targets and communications users. We introduce a low
complexity approach for the design of hybrid beamformers, which include
radar-only and communications-only beamformers. Numerical experiments
demonstrate that our SPIM-ISAC approach exhibits a significant performance
improvement over the conventional mmWave-ISAC design in terms of spectral
efficiency and the generated beampattern.
