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Reconfigurable Intelligent Surface-Enabled Channel Signature Modulation

M. A. Teeti

TL;DR

This work tackles spectral and energy efficiency in RIS-enabled wireless networks by removing the need for RIS-side beamforming or RIS CSI and introducing RIS-CSM. It partitions the RIS into $N_Q$ groups and assigns one of $K$ Hadamard-based patterns per group to generate $K^{N_Q}$ distinct channel signatures, embedding information in signature indices and detecting via ML at a SIMO receiver; channel estimation focuses on the $N_Q K$ effective channels. The analysis yields a closed-form upper bound on error probability showing diversity order $n_R$ and a coding gain that scales with $N$, along with an information-theoretic capacity bound and numerical validation; spatial correlation presents a trade-off between SNR gains and channel diversity. Simulations show RIS-CSM outperforming RIS-MIMO, RIS-GSM, and RIS-CIM at the same spectral efficiency, with practical ML complexity; correlations can boost performance at low SE but may hurt high-SE performance, providing valuable guidance for RIS-based IM design in real deployments.

Abstract

This work proposes RIS-enabled channel signature modulation (RIS-CSM), a lightweight index modulation scheme for reconfigurable intelligent surfaces (RIS). An N-element RIS is partitioned into disjoint groups, each employing predetermined binary reflection patterns to generate distinct channel signatures at an $n_R$-antenna receiver, without RIS-side beamforming. Information is embedded in the indices of these signatures, enabling simple channel estimation and scalable spectral efficiency. A closed-form upper bound on error probability and capacity analysis are derived, revealing diversity order $n_R$ and coding gain proportional to N. Simulation results under Rayleigh fading validate the theoretical analysis. Moreover, simulations indicate that spatial correlation among RIS elements can improve system performance at low spectral efficiency.

Reconfigurable Intelligent Surface-Enabled Channel Signature Modulation

TL;DR

This work tackles spectral and energy efficiency in RIS-enabled wireless networks by removing the need for RIS-side beamforming or RIS CSI and introducing RIS-CSM. It partitions the RIS into groups and assigns one of Hadamard-based patterns per group to generate distinct channel signatures, embedding information in signature indices and detecting via ML at a SIMO receiver; channel estimation focuses on the effective channels. The analysis yields a closed-form upper bound on error probability showing diversity order and a coding gain that scales with , along with an information-theoretic capacity bound and numerical validation; spatial correlation presents a trade-off between SNR gains and channel diversity. Simulations show RIS-CSM outperforming RIS-MIMO, RIS-GSM, and RIS-CIM at the same spectral efficiency, with practical ML complexity; correlations can boost performance at low SE but may hurt high-SE performance, providing valuable guidance for RIS-based IM design in real deployments.

Abstract

This work proposes RIS-enabled channel signature modulation (RIS-CSM), a lightweight index modulation scheme for reconfigurable intelligent surfaces (RIS). An N-element RIS is partitioned into disjoint groups, each employing predetermined binary reflection patterns to generate distinct channel signatures at an -antenna receiver, without RIS-side beamforming. Information is embedded in the indices of these signatures, enabling simple channel estimation and scalable spectral efficiency. A closed-form upper bound on error probability and capacity analysis are derived, revealing diversity order and coding gain proportional to N. Simulation results under Rayleigh fading validate the theoretical analysis. Moreover, simulations indicate that spatial correlation among RIS elements can improve system performance at low spectral efficiency.
Paper Structure (21 sections, 1 theorem, 60 equations, 12 figures, 4 tables)

This paper contains 21 sections, 1 theorem, 60 equations, 12 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Motivation and contribution
  3. Key features of the proposed scheme
  4. System model
  5. Proposed RIS-CSM system
  6. Properties and generation of phase-shift patterns
  7. Channel estimation
  8. Performance analysis
  9. Asymptotic analysis and capacity evaluation
  10. Asymptotic error performance and diversity order analysis
  11. Impact of RIS partitioning on error performance
  12. Capacity analysis
  13. Simulation Results
  14. SER performance with varying RIS size
  15. BER Improvement through receiver diversity
  16. ...and 6 more sections

Key Result

Proposition 1

The per-group average SER in the RIS-CSM system is approximately upper bounded as follows: where $F(\zeta)$ is defined by with $\mu(\zeta) = \sqrt{\frac{N E_s \zeta /\sigma^2}{2 N_Q + N E_s \zeta/\sigma^2}}.$

Figures (12)

  • Figure 1: Conceptual schematic of the RIS-CSM system in a SIMO RIS-assisted setup with a square $8 \times 8$ RIS divided into $N_Q=4$ groups, each containing $n=16$ elements. The RIS operates either as a backscatterer or under transmitter control in Rayleigh fading. For each group, $\log_2(K)$ index bits are mapped to one of $K=4$ phase-shift patterns derived from a $16 \times 16$ Hadamard matrix. Each reflection pattern produces a distinct effective channel signature at the receiver, illustrated with different colors. In total, $4\log_2(4)=8$ bits are embedded across the four group signatures. The receiver recovers the transmitted bits by identifying the tuple of four channel signatures.
  • Figure 2: MSE of the estimated effective channel for a system with $N = 128$ and $N_Q = 1$.
  • Figure 3: SER performance of RIS-CSM at spectral efficiency of 3 bpcu with $n_R=1$, $N_Q=1$, and $K=8$. Analytical results are based on the upper bound in \ref{['eq:thmbound']}.
  • Figure 4: BER performance of RIS-CSM at spectral efficiency of 4 bpcu with $N=32, 64$ and $n_R=1, 2$. Analytical results are derived from \ref{['eq:BER_bound']}.
  • Figure 5: Impact of partitioning on the error probability performance of RIS-CSM with $N=128$ and spectral efficiency of 4 bpcu. Analytical results are based on the upper bound in \ref{['eq:thmbound']}.
  • ...and 7 more figures

Theorems & Definitions (3)

  • Proposition 1
  • proof
  • Remark 1