Metasurface-based Fluid Antennas: from Electromagnetics to Communications Model

Fluid antenna systems (FASs) have become a popular topic in the wireless community as an effective yet simple means of exploiting spatial diversity. Due to the limitations of physically moving radiating elements, electronically reconfigurable antennas are emerging as practical implementations of FASs, since changing the radiation pattern is functionally equivalent to physically moving the device. However, electronically reconfigurable antennas pose a challenge in terms of analytical modeling, often requiring full-wave simulations or measurements for their characterization; this severely limits the extraction of theoretical insights useful for system design. Motivated by these difficulties and the growing interest in FASs, we propose in this paper a complete analytical model for metasurface-based embodiments of FASs. Specifically, we advocate for the implementation of the FAS concept through dynamic metasurface antennas (DMAs), hitherto proposed as array replacements in multiple-input multiple-output (MIMO) systems. We leverage circuit theory to rewrite the conventional signal model of FASs in terms of admittance matrices accounting for the electromagnetic effects inherent to metasurfaces. The model is validated with full-wave simulations, showing good agreement. We further illustrate how to apply the model for standard performance analysis, and provide closed-form expressions for key metrics, including the resulting signal covariance matrix. Results confirm that practical DMA-based FASs can achieve similar performance to that of idealized implementations of position-flexible antennas.