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

## TL;DR

This paper investigates whether spatial path index modulation (SPIM) can theoretically improve spectral efficiency in millimeter-wave hybrid beamforming systems, extending existing models and providing fundamental performance conditions.

## Contribution

It introduces a generalized SPIM-mmWave system that includes existing models and derives conditions under which SPIM guarantees better spectral efficiency than conventional systems.

## Key findings

- SPIM-mmWave outperforms conventional mmWave-MIMO under certain channel conditions.
- A theoretical framework is established for performance comparison.
- Explicit channel and noise conditions for superiority are provided.

## Abstract

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.