Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems

HUANG Fuchun; ZHU Han; TANG Xiaoqing; YANG Fan; HUANG Jie

doi:10.11999/JEIT251289

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HUANG Fuchun, ZHU Han, TANG Xiaoqing, YANG Fan, HUANG Jie. Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT251289

Citation:

HUANG Fuchun, ZHU Han, TANG Xiaoqing, YANG Fan, HUANG Jie. Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT251289

Citation:

HUANG Fuchun, ZHU Han, TANG Xiaoqing, YANG Fan, HUANG Jie. Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems[J]. Journal of Electronics & Information Technology. doi: 10.11999/JEIT251289

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Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems

doi: 10.11999/JEIT251289 cstr: 32379.14.JEIT251289

1.
Guangzhou College of Commerce, Guangzhou 511363, China
2.
University of Coimbra, Portugal 3030290
3.
Hubei University, Wuhan 430062, China
4.
Chongqing University of Technology, Chongqing 400054, China

Funds: National Natural Science Foundation of China (62301094), Hubei Provincial Key Research and Development Program Project (2023BAB082), Chongqing Special Key Project of Technological Innovation and Application Development (CSTB2024TIAD-STX0034), Guangdong Provincial University Characteristic and Innovation Project (2025KTSCX202)

Accepted Date: 2026-04-17
Rev Recd Date: 2026-04-17

Available Online: 2026-05-04

Abstract

Abstract

Objective This paper aims to address two main challenges in RIS-assisted MIMO index modulation (IM) systems: (1) the practical deployment difficulty of using a single large-scale RIS panel, and (2) the high complexity of designing efficient transmit spatial signal vectors. To overcome these issues, this paper proposes a joint design of sparse spatial constellation and dynamic multi-RIS block selection to enhance spectral efficiency, bit error rate (BER) performance, and deployment flexibility. Methods Inspired by the extended space index modulation (ESIM) paradigm, a new design of sparse spatial constellation with two active antennas (SCTA) is proposed, which leads to the SCTA-RIS-SM system. The idea is to mix primary and secondary PAM constellations to form a spatial constellation vector[x₁,x₂]^T and modulated onto two active antennas. Thus, it not only maximizes the minimum Euclidean distance between transmit vectors but also significantly enhances the anti-interference capability. To get around the deployment difficulties of a single large RIS panel, an enhanced scheme of SCTA-MBRIS-SM is further proposed. This system employs a distributed array of multiple small RIS blocks and dynamically selects a subset of blocks for cooperative reflection, treating different “RIS block selection combinations” as a new index modulation dimension. Finally, theoretical analysis of spectral efficiency and average bit error rate is carried out, and Monte Carlo simulations are conducted to compare the proposed systems with several existing schemes. Results and Discussions Simulation results demonstrate that the proposed SCTA-RIS-SM system achieves notable signal-to-noise ratio (SNR) gains over RIS-SIM, RIS-SM, and DH RIS-SM systems under the same spectral efficiency (e.g., 10–12 bits/s/Hz) in near-field wideband scenarios. For instance, at BER = 10⁻³, SCTA-RIS-SM outperforms RIS-SIM by about 1.5–2.5 dB and DH RIS-SM by more than 6 dB. Furthermore, the SCTA-MBRIS-SM system, by exploiting additional index modulation from RIS block selection, further improves the BER performance and spectral efficiency compared to SCTA-RIS-SM without increasing the number of radio frequency chains. With total numbers of reflecting elements kept identical, the proposed multi-block scheme achieves up to 5 dB gain over RIS-SIM at BER = 10⁻³. Theoretical BER curves match well with simulation results in the high SNR region, validating the analytical derivations. The results also show that the performance advantage is maintained as the number of transmit antennas increases, and the system exhibits good compatibility with channel coding. Conclusions This paper addresses the challenges of large-scale RIS deployment and high-complexity spatial signal design in RIS-assisted MIMO systems. The proposed sparse spatial constellation with two active antennas optimizes the Euclidean distance distribution in the signal space, effectively improving system reliability. The introduction of dynamic multi-RIS block selection transforms hardware deployment constraints into a new dimension for spectral efficiency enhancement, offering a feasible path for practical large-scale RIS applications. Simulation results confirm that jointly optimizing the transmit spatial vector and the degrees of freedom of RIS reflections is an effective strategy for performance improvement. Future work will focus on robustness under imperfect channel state information, construction of higher-dimensional sparse constellations, extension to extremely large-scale MIMO scenarios, and multi-user communications.
- Index Modulation,
- Reconfigurable Intelligent Surface (RIS),
- Sparse Spatial Constellation,
- Minimum Euclidean Distance,
- Multi-RIS Block Selection

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References(20)

References

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