RIS-Parametrized Rich-Scattering Environments: Physics-Compliant Models, Channel Estimation, and Optimization

The tunability of radio environments with reconfigurable intelligent surfaces (RISs) enables the paradigm of smart radio environments in which wireless system engineers are no longer limited to only controlling the radiated signals but can in addition also optimize the wireless channels. Many practical radio environments include complex scattering objects, especially indoor and factory settings. Multipath propagation therein creates seemingly intractable coupling effects between RIS elements, leading to the following questions: How can a RIS-parametrized rich-scattering environment be modelled in a physics-compliant manner? Can the parameters of such a model be estimated for a specific but unknown experimental environment? And how can the RIS configuration be optimized given a calibrated physics-compliant model? This chapter summarizes the current state of the art in this field, highlighting the recently unlocked potential of frugal physical-model-based open-loop control of RIS-parametrized rich-scattering radio environments.Comment: 29 pages, 3 figures, author's version of chapter from forthcoming book "Reconfigurable Metasurfaces for Wireless Communications: Architectures, Modeling, and Optimization

Electromagnetic inverse scattering problems

Ph.DDOCTOR OF PHILOSOPH

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Iterative synthetic aperture radar imaging algorithms

Synthetic aperture radar is an important tool in a wide range of civilian and military imaging applications. This is primarily due to its ability to image in all weather conditions, during both the day and the night, unlike optical imaging systems. A synthetic aperture radar system contains a step which is not present in an optical imaging system, this is image formation. This is required because the acquired data from the radar sensor does not directly correspond to the image. Instead, to form an image, the system must solve an inverse problem. In conventional scenarios, this inverse problem is relatively straight forward and a matched lter based algorithm produces an image of suitable image quality. However, there are a number of interesting scenarios where this is not the case. Scenarios where standard image formation algorithms are unsuitable include systems with data undersampling, errors in the system observation model and data that is corrupted by radio frequency interference. Image formation in these scenarios will form the topics of this thesis and a number of iterative algorithms are proposed to achieve image formation. The motivation for these proposed algorithms is primarily from the eld of compressed sensing, which considers the recovery of signals with a low-dimensional structure. The rst contribution of this thesis is the development of fast algorithms for the system observation model and its adjoint. These algorithms are required by large-scale gradient based iterative algorithms for image formation. The proposed algorithms are based on existing fast back-projection algorithms, however, a new decimation strategy is proposed which is more suitable for some applications. The second contribution is the development of a framework for iterative near- eld image formation, which uses the proposed fast algorithms. It is shown that the framework can be used, in some scenarios, to improve the visual quality of images formed from fully sampled data and undersampled data, when compared to images formed using matched lter based algorithms. The third contribution concerns errors in the system observation model. Algorithms that correct these errors are commonly referred to as autofocus algorithms. It is shown that conventional autofocus algorithms, which work as a post-processor on the formed image, are unsuitable for undersampled data. Instead an autofocus algorithm is proposed which corrects errors within the iterative image formation procedure. The proposed algorithm is provably stable and convergent with a faster convergence rate than previous approaches. The nal contribution is an algorithm for ultra-wideband synthetic aperture radar image formation. Due to the large spectrum over which the ultra-wideband signal is transmitted, there is likely to be many other users operating within the same spectrum. These users can produce signi cant radio frequency interference which will corrupt the received data. The proposed algorithm uses knowledge of the RFI spectrum to minimise the e ect of the RFI on the formed image

A new method for holographic measurement of microwave antenna radiation patterns

The rate at which new communications technologies are being developed has been immense and will continue to intensify for the foreseeable future. This evolution is fuelled by the desire to meet the wants and needs of the global community, by developing devices able to offer ever-increasing functionality, with greater complexity. To achieve this designers are forced to move to higher and higher frequencies. The antenna, as one of the fundamental building blocks of any radiated wave system, and as such, must develop along with the evolution of the communication system be it for, mobile, satellite or point to point systems. Antenna designers need to be able to test antenna, to ensure they exhibit the characteristics to which they were designed. Antenna test becomes progressively more difficult and costly as both, the operating frequency and the size of the antenna increase, especially for the measurement of the antenna Far-Field radiation pattern. Either the distance over which the measurement must be made becomes unfeasibly large or expensive measurement equipment is required to attain the phase component of the antenna field, where traditional methods for measuring close to the antenna are applied. Techniques have been developed to eliminate the need for the expensive phase measurement at reduced distances. Specifically of interest in this thesis, is the optical process of Off-Axis holography. The process allows phase information to be retained in a scalar measurement by use of a phase coherent known reference source. The reference desired reference source is a plane wave, which although possible at optical frequencies with the use of lasers is problematic at microwave frequencies. To date the plane wave characteristic required has been approximated using conventional radiating elements, which degrades the quality of the recorded holographic image. This thesis proposes a novel implementation of the Off-Axis hologram system, for application in the microwave frequency region. The novel system developed here addresses the problem of generating the desired plane wave characteristic. The conventional radiating element used to provide the near plane wave reference is replaced by a synthetic equivalent, which allows the magnitude and phase of the reference beam to be directly controlled at every measurement location required. Practical verification of the novel system proposed has been performed, with comparisons made between the results obtained from the novel technique and standard techniques used in industry. The comparisons show that the novel implementation is valid and able to provide good repeatable results