Towards Higher Spectral Efficiency: Spatial Path Index Modulation Improves Millimeter-Wave Hybrid Beamforming

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. Numerous studies have been conducted to show that such a system has the potential to outperform conventional mmWave-MIMOs with respect to spectral efficiency (SE). Most of the current works focused only on designing hybrid beamforming structures to empirically achieve higher SE performance. However, a fundamental question whether IM technique can truly improve the SE of current mmWave hybrid beamforming is still left unanswered. Against such a background, in this paper, we first 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 study), the SPIM-mmWave guarantees to outperform conventional mmWave-MIMO systems.