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

Ahmet M. Elbir, Abdulkadir Celik, Asmaa Abdallah, Ahmed M. Eltawil

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.

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

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.
Paper Structure (23 sections, 3 theorems, 72 equations, 11 figures, 2 algorithms)

This paper contains 23 sections, 3 theorems, 72 equations, 11 figures, 2 algorithms.

Table of Contents

  1. Introduction
  2. Prior Work
  3. Our Contributions
  4. Notation
  5. System Model
  6. SE Analysis & Problem Formulation
  7. SE for SPIM mMIMO
  8. SE for mMIMO
  9. Problem Formulation
  10. SPIM for RIS-mMIMO
  11. Channel Acquisition and Beamformer Design
  12. Channel Acquisition
  13. Hybrid Beamformer Design for SPIM
  14. RIS Design
  15. Receiver Design
  16. ...and 8 more sections

Key Result

Lemma 1

The SE of the SPIM-aided system is equivalent to the SE of conventional mMIMO system if $S = 1$.

Figures (11)

  • Figure 1: The illustration of the IM schemes over various transmission entities. In addition to the transmission of conventional APM bits, the index bits are assigned to the indices of a) subcarriers, b) antennas, c) spatial paths and d) RIS configurations.
  • Figure 2: SPIM for RIS-aided mMIMO system, wherein the BS processes the incoming data-streams and employs spatial path index information $s_0$ in a switching network, which connects $N_\mathrm{RF}$ RF chains to $L$ taps on the analog beamformer to exploit $L_\mathrm{S}$ out of $L$ paths for communication.In UL, the estimated spatial path directions at the user are fed-back to the BS which performs SPIM beamformer design. In DL, the user measures the path strengths to detect the spatial patterns.
  • Figure 3: SE versus $\mathrm{SNR}$ when $L=8$ for $L_\mathrm{S} = \{1,2\}$.
  • Figure 4: SE versus $\mathrm{SNR}_\mathbf{H}$ when $L=8$ for $L_\mathrm{S} = 1$.
  • Figure 5: SE versus number of paths $L$ for $\mathrm{SNR}=\{-10,0\}$ dB and $L_\mathrm{S}= N_\mathrm{S} = 1$.
  • ...and 6 more figures

Theorems & Definitions (6)

  • Lemma 1
  • proof
  • Theorem 2
  • proof
  • Lemma 3
  • proof