The concept of fluid antennas (FAs) has emerged as a promising solution to enhance the spectral efficiency of wireless networks, achieved by introducing additional degrees of freedom, including reconfigurability and flexibility. In this paper, we investigate the use of index-modulated (IM) transmissions within the framework of FA systems, where an FA position is activated during each transmission interval. This approach is motivated by the common characteristics exhibited by FAs and IM transmissions, which entails the use of a single radio-frequency chain. From this perspective, we derive a closed-form expression for the bit error rate of IM-FAs considering spatial correlation, and demonstrating superior performance compared to conventional IM systems. To enhance the performance of IM-FAs in correlated conditions, channel coding techniques are applied. We first analyze a set partition coding (SPC) scheme for IM-FAs to spatially separate the FA ports, and provide a tight performance bound over correlated channels. Furthermore, the spatial SPC scheme is extended to turbo-coded modulation where the performance is analyzed for low and high signal-to-noise ratios. Our results reveal that through the implementation of channel coding techniques designed for FAs and IM transmission, the performance of coded IM-FAs exhibits notable enhancements, particularly in high correlation scenarios.

翻译：液态天线（FA）的概念通过引入可重构性和灵活性等额外自由度，已成为提升无线网络频谱效率的一种有前景的解决方案。本文研究了在FA系统框架下采用索引调制（IM）传输的方案，其中每个传输间隔内激活一个FA位置。该方法的动机源于FA与IM传输的共同特性，即均使用单条射频链路。基于此视角，我们推导了考虑空间相关性的IM-FA系统误码率的闭式表达式，并证明了其性能优于传统IM系统。为提升相关条件下IM-FA系统的性能，本文应用了信道编码技术。首先，我们分析了一种针对IM-FA的集合分割编码（SPC）方案以在空间上分离FA端口，并给出了相关信道上的紧凑性能界。此外，我们将空间SPC方案扩展至涡轮编码调制，并分析了其在低信噪比与高信噪比下的性能。结果表明，通过采用针对FA和IM传输设计的信道编码技术，编码IM-FA系统的性能在强相关场景下表现出显著提升。