Index Modulation Design with Sparse Spatial Constellation and Dynamic Multi-RIS Block Selection for RIS-MIMO Systems
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摘要: 针对可重构智能表面(RIS)辅助多输入多输出(MIMO)索引调制系统中单块大规模RIS部署困难和发射端空间信号设计复杂度高的挑战，本文研究一种联合稀疏空间星座-双激活天线(SCTA)与多RIS块(MBRIS)选择的索引调制设计。本研究首先提出了一种基于稀疏空间星座-双激活天线的RIS空间调制(SCTA-RIS-SM)系统，其核心是在发射端构造一种基于双激活天线的稀疏空间矢量，通过混合主、次级脉冲幅度调制(PAM)与次级PAM(SPAM)星座设计，优化发射矢量之间的最小欧氏距离，从而显著提升了系统的抗干扰能力。为克服单块RIS的部署瓶颈，本研究进一步提出一种增强型方案：基于稀疏空间星座-双激活天线的多RIS块空间调制(SCTA-MBRIS-SM)系统。该系统采用分布式多RIS块阵列替代传统单块面板，通过动态选择激活一组特定的RIS块进行协同反射，将不同的“RIS块选择组合”状态作为一个新的索引调制维度。此增强型方案在不增加射频链路的条件下，额外提升了频谱效率，同时增强了部署的灵活性。理论分析与蒙特卡洛仿真结果表明，所提的两种系统在误比特率性能与频谱效率方面均优于现有典型方案，为未来高能效、高灵活性的RIS-MIMO通信系统提供了有效的解决方案。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. -
表 1 归一化发射矢量之间平方MED比较
Nt=4 Nt=8 矢量 R1 R2 R3 R4 R5 R6 XQSM 0.20 0.10 0.05 0.20 0.10 0.05 XESIM 0.33 0.20 0.10 0.33 0.20 0.10 XSCTA 0.38 0.24 0.19 0.44 0.31 0.19 
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表 2 IBI比特与4块RIS激活状态的映射关系
输入比特LBI 开关索引矢量$ {\mathbf{k}}_{g} $ B块反射面板编号 0 0 [1,1,1,1,0]T 1,2,3,4 0 1 [1,1,1,0,1] T 1,2,3,5 1 0 [1,1,0,1,1] T 1,2,4,5 1 1 [1,0,1,1,1] T 1,3,4,5
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