On the Diversity and Coded Modulation Design of Fluid Antenna Systems

TL;DR

This paper analyzes fluid antenna systems' outage probability, proposes a delay compensation scheme, and designs space-time coded modulations to maximize diversity and performance in reconfigurable communication networks.

Contribution

It introduces an analytical framework for fluid antennas' outage performance, proposes a linear prediction scheme to mitigate delays, and designs space-time codes exploiting FA reconfigurability.

Findings

Fluid antennas achieve maximum diversity in ideal conditions.

Delay effects prevent full diversity realization without compensation.

The proposed space-time coding scheme attains maximum diversity with low complexity.

Abstract

Reconfigurability is a desired characteristic of future communication networks. From a transceiver's standpoint, this can be materialized through the implementation of fluid antennas (FAs). An FA consists of a dielectric holder, in which a radiating liquid moves between pre-defined locations (called ports) that serve as the transceiver's antennas. Due to the nature of liquids, FAs can practically take any size and shape, making them both flexible and reconfigurable. In this paper, we deal with the outage probability of FAs under general fading channels, where a port is scheduled based on selection combining. An analytical framework is provided for the performance with and without estimation errors, as a result of post-scheduling delays. We show that although FAs achieve maximum diversity, this cannot be realized in the presence of delays. Hence, a linear prediction scheme is proposed that overcomes delays and restores the lost diversity by predicting the next scheduled port. Moreover, we design space-time coded modulations that exploit the FA's sequential operation with space-time rotations and code diversity. The derived expressions for the pairwise error probability and average word error rate give an accurate estimate of the performance. We illustrate that the proposed design attains maximum diversity, while keeping a low-complexity receiver, thereby confirming the feasibility of FAs.