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.

翻译：下一代无线网络致力于提高频谱效率（SE），同时降低功耗和硬件成本。可重构智能表面（RIS）为满足这些要求提供了一种可行的解决方案。为了提升频谱效率，索引调制（IM）通过子载波、天线和空间路径等传输媒介传输额外信息比特，被视为一种使能技术。本文探索了空间路径的利用，并针对RIS辅助的大规模多输入多输出（mMIMO）系统引入了空间路径索引调制（SPIM）。因此，所提出的框架利用RIS提高了网络效率和覆盖范围，同时SPIM提供了频谱效率的改进。为实现SPIM，我们利用毫米波信道的空间分集，将索引比特分配给基站通过RIS与用户之间信道的空间模式。我们提出了一种低复杂度的混合波束成形设计方法，该波束成形器由对应于SPIM-mMIMO所选空间路径索引的导向矢量构成。此外，我们对所提出的SPIM方法的频谱效率进行了理论分析，并推导了基于SPIM的混合波束成形与全数字（FD）波束成形之间的频谱效率关系。通过数值仿真，我们验证了理论结果，并表明所提出的SPIM方法展现了改进的频谱效率性能，甚至在使用少量射频链时超过了FD波束成形器的性能。