  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.
