  As the demand for wireless connectivity continues to soar, the fifth
generation and beyond wireless networks are exploring new ways to efficiently
utilize the wireless spectrum and reduce hardware costs. One such approach is
the integration of sensing and communications (ISAC) paradigms to jointly
access the spectrum. Recent ISAC studies have focused on upper millimeter-wave
and low terahertz bands to exploit ultrawide bandwidths. At these frequencies,
hybrid beamformers that employ fewer radio-frequency chains are employed to
offset expensive hardware but at the cost of lower multiplexing gains. Wideband
hybrid beamforming also suffers from the beam-split effect arising from the
subcarrier-independent (SI) analog beamformers. To overcome these limitations,
this paper introduces a spatial path index modulation (SPIM) ISAC architecture,
which transmits additional information bits via modulating the spatial paths
between the base station and communications users. We design the SPIM-ISAC
beamformers by first estimating both radar and communications parameters by
developing beam-split-aware algorithms. Then, we propose to employ a family of
hybrid beamforming techniques such as hybrid, SI, and subcarrier-dependent
analog-only, and beam-split-aware beamformers. Numerical experiments
demonstrate that the proposed SPIM-ISAC approach exhibits significantly
improved spectral efficiency performance in the presence of beam-split than
that of even fully digital non-SPIM beamformers.
