  The combination of millimeter wave (mmWave) multiple-input multiple-output
(MIMO) systems and index modulation (IM) technique has recently constituted a
novel form of mmWave hybrid beamforming. Lots of studies have been conducted to
show that such system has the potential to outperform conventional mmWave-MIMOs
with respect to spectral efficiency (SE). Most of the current works only
focused on designing hybrid beamforming structures to empirically achieve
higher SE performance. However, a fundamental question that whether IM
technique can truly improve the SE of current mmWave hybrid beamforming is
still left unanswered. Against such background, in this work we firstly extend
the IM-assisted mmWave system to a more generalized version, i.e. a spatial
path index modulation aided mmWave (SPIM-mmWave) system, which subsumes both
IM-assisted and conventional mmWave-MIMO systems as its special cases. Based on
the framework of SPIM-mmWave, a fundamental study is conducted towards a
theoretic condition under which SPIM-mmWave guarantees to outperform
conventional mmWave-MIMO schemes. It is demonstrated that, under a specific
channel condition and noise level (which will be explicitly given by our work),
SPIM-mmWave guarantees to outperform conventional mmWave-MIMO systems.
