Design of Reconfigurable Intelligent Surfaces by Using S-Parameter Multiport Network Theory -- Optimization and Full-Wave Validation

Multiport network theory has been proved to be a suitable abstraction model for analyzing and optimizing reconfigurable intelligent surfaces (RISs), especially for studying the impact of the electromagnetic mutual coupling among radiating elements that are spaced less than half of the wavelength. Both representations in terms of $Z$-parameter (impedance) and $S$-parameter (scattering) matrices are widely utilized. In this paper, we embrace multiport network theory for analyzing and optimizing the reradiation properties of RIS-aided channels, and provide four new contributions. (i) First, we offer a thorough comparison between the $Z$-parameter and $S$-parameter representations. This comparison allows us to unveil that the typical scattering models utilized for RIS-aided channels ignore the structural scattering from the RIS, which results in an unwanted specular reflection. (ii) Then, we develop an iterative algorithm for optimizing, in the presence of electromagnetic mutual coupling, the tunable loads of the RIS based on the $S$-parameters representation. We prove that small perturbations of the step size of the algorithm result in larger variations of the $S$-parameter matrix compared with the $Z$-parameter matrix, resulting in a faster convergence rate. (iii) Subsequently, we generalize the proposed algorithm to suppress the specular reflection due to the structural scattering, while maximizing the received power towards the direction of interest, and analyze the effectiveness and tradeoffs of the proposed approach. (iv) Finally, we validate the theoretical findings and algorithms with numerical simulations and a commercial full-wave electromagnetic simulator based on the method of moments.Comment: Submitted for journal publicatio

Analysis and Optimization of Reconfigurable Intelligent Surfaces Based on SS-Parameters Multiport Network Theory

In this paper, we consider a reconfigurable intelligent surface (RIS) and model it by using multiport network theory. We first compare the representation of RIS by using $Z$-parameters and $S$-parameters, by proving their equivalence and discussing their distinct features. Then, we develop an algorithm for optimizing the RIS configuration in the presence of electromagnetic mutual coupling. We show that the proposed algorithm based on optimizing the $S$-parameters results in better performance than existing algorithms based on optimizing the $Z$-parameters. This is attributed to the fact that small perturbations of the step size of the proposed algorithm result in larger variations of the $S$-parameters, hence increasing the convergence speed of the algorithm.Comment: Submitted to a conference (invited

Flexible Unidirectional PTD-Symmetric Waveguide

The transmission and reflection characteristics of a bent square transverse electromagnetic waveguide constituted by two opposite perfect electric conductor and perfect magnetic conductor walls are analyzed. This waveguide exhibits a parity, timereversal, duality (PTD) symmetry with respect to both the diagonal axes. It is found that this property is maintained even when the structure is bent in a plane orthogonal to one of the two PTD symmetry axes (PTDbend). As a consequence, the transverse electromagnetic mode propagation is protected against backscattering by this class of discontinuities. The preservation of the PTD symmetry in presence of a geometric flexibility of the bend is also analyzed, thus introducing a new class of bendable waveguides that are largely immune to backscattering by bend discontinuities

On the safety design of radar based railway level crossing surveillance systems

Recent accidents experienced at railway level crossings are pushing researchers to design surveillance systems able to grant safety of passengers and structural integrity of trains at level crossings. The challenge is represented by granting at the same time an appropriate reliability, availability and maintainability degree despite the high safety requirements imposed by the application. The approach proposed in this paper takes into consideration the most common suggested standards used in designing this kind of systems and introduces new general concepts which demystify the use of such standards in actual applications. This paper illustrates the roadmap to be followed in general when designing level crossing monitoring systems, to minimize the risk due to object misdetection occurring on barrier closure when exploiting radar technology

An incremental theory of diffraction for objects with local cylindrical shape

In this paper, a quite general systematic procedure is presented for defining incremental field contributions, that may provide effective tools for describing a wide class of scattering and diffraction phenomena at any aspect, within a unitary, self-consistent framework. This is based on a generalization of the localization process for cylindrical canonical problems with elementary source illumination and arbitrary observation aspects. In particular, it is shown that the spectral integral formulation of the exact solution may also be represented as a spatial integral convolution along the axis of the cylinder. Its integrand is then directly used to define the relevant incremental field contribution. This procedure, that will be referred to as a ITD (Incremental Theory of Diffraction) Fourier transform convolution localization process, is explicitly applied to both wedge and circular cylinder canonical configurations, to define incremental diffraction and scattering contributions, respectively. These formulations are asymptotically approximated to find closed form high-frequency expressions for the incremental field contributions. This generalization of the ITD localization process may provide a quite general, systematic procedure to find incremental field contributions that explicitly satisfy reciprocity and naturally lead to the UTD ray field representation, when it is applicable