In this paper, we propose a transmission mechanism for fluid antennas (FAs) enabled multiple-input multiple-output (MIMO) communication systems based on index modulation (IM), named FA-IM, which incorporates the principle of IM into FAs-assisted MIMO system to improve the spectral efficiency (SE) without increasing the hardware complexity. In FA-IM, the information bits are mapped not only to the modulation symbols, but also the index of FA position patterns. Additionally, the FA position pattern codebook is carefully designed to further enhance the system performance by maximizing the effective channel gains. Then, a low-complexity detector, referred to efficient sparse Bayesian detector, is proposed by exploiting the inherent sparsity of the transmitted FA-IM signal vectors. Finally, a closed-form expression for the upper bound on the average bit error probability (ABEP) is derived under the finite-path and infinite-path channel condition. Simulation results show that the proposed scheme is capable of improving the SE performance compared to the existing FAs-assisted MIMO and the fixed position antennas (FPAs)-assisted MIMO systems while obviating any additional hardware costs. It has also been shown that the proposed scheme outperforms the conventional FA-assisted MIMO scheme in terms of error performance under the same transmission rate.

翻译：本文针对流体天线辅助的多输入多输出通信系统，提出了一种基于索引调制的传输机制——FA-IM。该方案将索引调制原理融入FA辅助MIMO系统，在不增加硬件复杂度的前提下提升频谱效率。在FA-IM中，信息比特不仅映射到调制符号，还映射到FA位置模式索引。此外，通过最大化有效信道增益精心设计了FA位置模式码本，进一步增强了系统性能。随后，利用传输FA-IM信号矢量的固有稀疏性，提出了一种低复杂度检测器——高效稀疏贝叶斯检测器。最后，在有限路径与无限路径信道条件下，推导了平均误比特率上界的闭式表达式。仿真结果表明，与现有FA辅助MIMO及固定位置天线辅助MIMO系统相比，所提方案能够在避免额外硬件成本的同时提升频谱效率性能。同时，在相同传输速率下，所提方案的误码性能优于传统FA辅助MIMO方案。