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 Reconfigurable Intelligent Surface (RIS)-assisted Multiple-Input Multiple-Output (MIMO) Index Modulation (IM) systems face two main challenges: the difficult deployment of a single large-scale RIS panel and the high design complexity of efficient transmit spatial signal vectors. To address these issues, a joint design that combines sparse spatial constellation and dynamic multi-RIS-block selection is proposed. The design improves spectral efficiency, Bit Error Rate (BER) performance, and deployment flexibility. Methods Inspired by the Extended Space Index Modulation (ESIM) paradigm, a sparse spatial constellation with two active antennas (SCTA) is proposed, forming the SCTA-RIS-SM system. In this design, Pulse Amplitude Modulation (PAM) and Secondary PAM (SPAM) constellations are combined to construct the spatial constellation vector [x₁,x₂]^T, which is modulated onto two active antennas. This design maximizes the Minimum Euclidean Distance (MED) between transmit vectors and improves the anti-interference capability of the system. To address the deployment difficulty of a single large-scale RIS panel, an enhanced SCTA-MBRIS-SM system is further proposed. The system uses a distributed array of small RIS blocks and dynamically selects a subset of blocks for cooperative reflection. Different RIS block selection combinations are used as a new IM dimension. Spectral efficiency and average BER are then analyzed theoretically. Monte Carlo simulations are conducted to compare the proposed systems with several existing schemes. Results and Discussions The simulation results show that the proposed SCTA-RIS-SM system achieves clear Signal-to-Noise Ratio (SNR) gains over RIS-SIM, RIS-SM, and DHRIS-SM systems at the same spectral efficiency, such as 10-12 bits/(s·Hz). For instance, when BER = 10^–3, SCTA-RIS-SM outperforms RIS-SIM by approximately 1.5-2.5 dB and DHRIS-SM by more than 6 dB. By using additional IM from RIS block selection, SCTA-MBRIS-SM further improves BER performance and spectral efficiency compared with SCTA-RIS-SM, without increasing the number of Radio Frequency (RF) chains. With the same total number of reflecting elements, the proposed multi-RIS-block scheme achieves an SNR gain of up to 5 dB over RIS-SIM when BER = 10⁻³. The theoretical BER curves agree well with the simulation results in the high-SNR region, confirming the validity of the analytical derivations. The results also indicate that the performance advantage is maintained as the number of transmit antennas increases. In addition, the proposed design is compatible 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 SCTA design improves system reliability by optimizing the Euclidean distance distribution in the signal space. Dynamic multi-RIS-block selection transforms hardware deployment constraints into a new dimension for improving spectral efficiency, providing a feasible path for practical large-scale RIS applications. Simulation results confirm that joint optimization of transmit spatial vectors and RIS reflection degrees of freedom is an effective strategy for improving system performance. Future work will focus on robust design under imperfect channel state information, construction of higher-dimensional sparse constellations, extension to extremely large-scale MIMO scenarios, and multi-user communications.
- Index Modulation(IM),
- Reconfigurable Intelligent Surface (RIS),
- Sparse spatial constellation,
- Minimum Euclidean Distance(MED),
- Multi-RIS block selection

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References(20)

References

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