Boosting Spectral Efficiency via Spatial Path Index Modulation in RIS-Aided mMIMO

TL;DR

This paper introduces spatial path index modulation (SPIM) in RIS-aided mMIMO systems to enhance spectral efficiency, coverage, and network performance by exploiting spatial diversity and designing low-complexity hybrid beamformers.

Contribution

It proposes a novel SPIM framework for RIS-aided mMIMO, including a low-complexity hybrid beamformer design and theoretical SE analysis, outperforming traditional fully digital beamforming.

Findings

SPIM improves spectral efficiency compared to conventional methods.

The proposed hybrid beamformer achieves near-FD performance with fewer RF chains.

Numerical results validate the theoretical SE gains of SPIM in RIS-aided mMIMO systems.

Abstract

Next generation wireless networks focus on improving spectral efficiency (SE) while reducing power consumption and hardware cost. Reconfigurable intelligent surfaces (RISs) offer a viable solution to meet these requirements. In order to enhance the SE, index modulation (IM) has been regarded as one of the enabling technologies via the transmission of additional information bits over the transmission media such as subcarriers, antennas and spatial paths. In this work, we explore the usage of spatial paths and introduce spatial path IM (SPIM) for RIS-aided massive multiple-input multiple-output (mMIMO) systems. Thus, the proposed framework improves the network efficiency and the coverage with the use of RIS while SPIM provides SE improvement. In order to perform SPIM, we exploit the spatial diversity of the millimeter wave channel and assign the index bits to the spatial patterns of the channel between the base station and the users through RIS. We introduce a low complexity approach for the design of hybrid beamformers, which are constructed by the steering vectors corresponding to the selected spatial path indices for SPIM-mMIMO. Furthermore, we conduct a theoretical analysis on the SE of the proposed SPIM approach, and derive the SE relationship between the SPIM-based hybrid beamforming and fully digital (FD) beamforming. Via numerical simulations, we validate our theoretical results and show that the proposed SPIM approach presents an improved SE performance, even higher than that of the use of FD beamformers while using a few RF chains.