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The main design principles in computer architecture have recently shifted from a monolithic scaling-driven approach to the development of heterogeneous architectures that tightly co-integrate multiple specialized processor and memory chiplets. In such data-hungry multi-chip architectures, current Networks-in-Package (NiPs) may not be enough to cater to their heterogeneous and fast-changing communication demands. This position article makes the case for wireless in-package networking as the enabler of efficient and versatile wired-wireless interconnect fabrics for massive heterogeneous processors. To that end, the use of graphene-based antennas and transceivers with unique frequency-beam reconfigurability in the terahertz band is proposed. The feasibility of such a wireless vision and the main research challenges toward its realization are analyzed from the technological, communications, and computer architecture perspectives., This publication is part of the Spanish I+D+i project TRAINER-A (ref. PID2020-118011GB-C21), funded by MCIN/AEI/10.13039/501100011033. This work has been also supported by the European Commission under H2020 grants WiPLASH (GA 863337), 2D-EPL (GA 952792), and Graphene Flagship (GA 881603); the FLAGERA framework under grant TUGRACO (HA 3022/9-1, LE 2440/3-1), the European Research Council under grants WINC (GA 101042080), COMPUSAPIEN (GA 725657), and PROJESTOR (GA 682675), the German Ministry of Education and Research under grant GIMMIK (03XP0210) and the and the German Research Foundation under grant HIPEDI (WA 4139/1-1)., Peer Reviewed, Article signat per 21 autors/es: Sergi Abadal, Robert Guirado, Hamidreza Taghvaee, and Akshay Jain are with the Universitat Politècnica de Catalunya, Spain; Elana Pereira de Santana and Peter Haring Bolívar are with the University of Siegen, Germany; Mohamed Saeed, Renato Negra, Kun-Ta Wang, and Max C. Lemme are with RWTH Aachen University, Germany. Zhenxing Wang, Kun-Ta Wang, and Max C. Lemme are also with AMO GmbH, Germany; Joshua Klein, Marina Zapater, Alexandre Levisse, and David Atienza are with the Swiss Federal Institute of Technology, Switzerland. Marina Zapater is also with the University of Applied Sciences and Arts Western Switzerland; Davide Rossi and Francesco Conti are with the University of Bologna,Italy; Martino Dazzi, Geethan Karunaratne, Irem Boybat, and Abu Sebastian are with IBM Research Europe, Switzerland, Postprint (author's final draft)

Reconfigurable Intelligent Surfaces (RIS) are well established as a promising solution to the blockage problem in millimeter-wave (mm-wave) and terahertz (THz) communications, envisioned to serve demanding networking applications, such as 6G and vehicular. HyperSurfaces (HSF) is a revolutionary enabling technology for RIS, complementing Software Defined Metasurfaces (SDM) with an embedded network of controllers to enhance intelligence and autonomous operation in wireless networks. In this work, we consider feedback-based autonomous reconfiguration of the HSF controller states to establish a reliable communication channel between a transmitter and a receiver via programmable reflection on the HSF when Line-of-sight (LoS) between them is absent. The problem is to regulate the angle of reflection on the metasurface such that the power at the receiver is maximized. Extremum Seeking Control (ESC) is employed with the control signals generated mapped into appropriate metasurface coding signals which are communicated to the controllers via the embedded controller network (CN). This information dissemination process incurs delays which can compromise the stability of the feedback system and are thus accounted for in the performance evaluation. Extensive simulation results demonstrate the effectiveness of the proposed method to maximize the power at the receiver within a reasonable time even when the latter is mobile. The spatiotemporal nature of the traffic for different sampling periods is also characterized., This work was supported in part by the European Union through the Horizon 2020: Future Emerging Topics Call (FETOPEN), under Grant EU736876; in part by the Project VISORSURF (http://www.visorsurf.eu), Research and Innovation Programme, under Agreement 739578; and in part by the Government of the Republic of Cyprus through the Directorate General for European Programmes, Coordination, and Development., Peer Reviewed, Postprint (published version)

Reconfigurable Intelligent Surfaces (RIS) are well established as a promising solution to the blockage problem in millimeter-wave (mm-wave) and terahertz (THz) communications, envisioned to serve demanding networking applications, such as 6G and vehicular. HyperSurfaces (HSF) is a revolutionary enabling technology for RIS, complementing Software Defined Metasurfaces (SDM) with an embedded network of controllers to enhance intelligence and autonomous operation in wireless networks. In this work, we consider feedback-based autonomous reconfiguration of the HSF controller states to establish a reliable communication channel between a transmitter and a receiver via programmable reflection on the HSF when Line-of-sight (LoS) between them is absent. The problem is to regulate the angle of reflection on the metasurface such that the power at the receiver is maximized. Extremum Seeking Control (ESC) is employed with the control signals generated mapped into appropriate metasurface coding signals which are communicated to the controllers via the embedded controller network (CN). This information dissemination process incurs delays which can compromise the stability of the feedback system and are thus accounted for in the performance evaluation. Extensive simulation results demonstrate the effectiveness of the proposed method to maximize the power at the receiver within a reasonable time even when the latter is mobile. The spatiotemporal nature of the traffic for different sampling periods is also characterized. Author

The next generation of wireless communications within the framework of 6G will be operational at the low THz frequency band. Although THz systems will dramatically enhance several performance indicators such as the data rate, spectral efficiency, and latency, exploiting such technology is challenging. Electromagnetic waves confront severe propagation losses including atmospheric attenuation and diffraction. Thus, such communications are limited to line-of-sight scenarios. In 5G networks, Reconfigurable Intelligent Surfaces (RISs) are intro-duced to solve this issue by redirecting the incident wave toward the receiver and implement virtual-line-of-sight communications. In this paper, we aim to employ this paradigm for 6G networks and design a graphene-based RIS optimized to perform at multiple low atmospheric attenuation channels. We investigate the performance of this multi-wideband design through numerical and analytical analysis., Peer Reviewed, Postprint (author's final draft)

The next generation of wireless networks is expected to tap into the terahertz (THz) band (0.1–10 THz) to satisfy the extreme latency and bandwidth density requirements of future applications. However, the development of systems in this band is challenging as THz waves confront severe spreading and penetration losses, as well as molecular absorption, which leads to strong line-of-sight requirements through highly directive antennas. Recently, reconfigurable intelligent surfaces (RISs) have been proposed to address issues derived from non-line-of-sight (non-LoS) propagation, among other impairments, by redirecting the incident wave toward the receiver and implementing virtual-line-of-sight communications. However, the benefits provided by a RIS may be lost if the network operates at multiple bands. In this article, the suitability of the RIS paradigm in indoor THz scenarios for 6G is assessed grounded on the analysis of a tunable graphene-based RIS that can operate in multiple wideband transparency windows. A possible implementation of such a RIS is provided and numerically evaluated at 0.65/0.85/1.05 THz separately, demonstrating that beam steering and other relevant functionalities are realizable with excellent performance. Finally, the challenges associated with the design and fabrication of multiwideband graphene-based RISs are discussed, paving the way to the concurrent control of multiple THz bands in the context of 6G networks., Peer Reviewed, Postprint (author's final draft)

The reconfigurable intelligent surface is a promising technology for the manipulation and control of wireless electromagnetic signals. In particular, it has the potential to provide significant performance improvements for wireless networks. However, to do so, a proper reconfiguration of the reflection coefficients of unit cells is required, which often leads to complex and expensive devices. To amortize the cost, one may share the system resources among multiple transmitters and receivers. In this paper, we propose an efficient reconfiguration technique providing control over multiple beams independently. Compared to time-consuming optimization techniques, the proposed strategy utilizes an analytical method to configure the surface for multi-beam radiation. This method is easy to implement, effective and efficient since it only requires phase reconfiguration. We analyze the performance for indoor and outdoor scenarios, given the broadcast mode of operation. The aforesaid scenarios encompass some of the most challenging scenarios that wireless networks encounter. We show that our proposed technique provisions sufficient improvements in the observed channel capacity when the receivers are close to the surface in the indoor office environment scenario. Further, we report a considerable increase in the system throughput given the outdoor environment., The work of Gabriele Gradoni was supported by the Royal Society under Grant INF\R2\192066. This work was supported in part by the European Commission through the H2020 RISE-6G Project under Grant 101017011 and in part by the EPSRC under Grant EP/V048937/1., Peer Reviewed, Postprint (author's final draft)

Sixth Generation (6G) of wireless networks will be even more heterogeneous and dense as compared to Fifth Generation (5G) and other legacy networks. Thus, the 6G architecture will need to be adapted to serve the ever-evolving capacity and quality of service requirements. To satisfy these ever-increasing demands, multiple enablers such as visible light communication, light fidelity, Reconfigurable Intelligent Surfaces (RISs), THz communications, etc., have been proposed. Specifically, RISs, through their programmable characteristics, can perform the fine-grained manipulation of the radio signals being generated by the myriad transmitter devices/access points for their corresponding receivers. Such manipulations include absorption of certain components of the impinging radio signals, as well as fine-grained control of these signals in terms of direction, polarization, phase, and power in a frequency-selective manner. An RIS consists of a device that controls the behavior of the Electromagnetic (EM) waves, alongside circuits that provide the tuning mechanism and the intelligence to control it. This device that controls the EM wave behavior can be realized using Metasurfaces (MSs), which are electromagnetically thin-film and planar artificial structures that enable the control of EM fields in engineered and even atypical ways. Hence, the MS is a component of the RIS. On a more granular level, an MS is composed of subwavelength building blocks known as unit cells or meta-atoms. The design of these unit cells depends on the required EM functionality, reconfigurability, or accuracy. The promises of the RIS paradigm, therefore, come at the expense of a non-trivial complexity in the MS. On the one hand, the performance of an RIS depends on the size of the unit cells, the number of unit cell states, or the size of the whole MS. On the other hand, there are costs and energy overheads associated with the fabrication and operation of RISs that also scale with the aforementioned, La sisena generació (6G) de xarxes sense fils seran encara més denses i heterogènies que les xarxes de cinquena generació. Per tant, la arquitectura de les xarxes 6G necessitaran adaptar-se a necessitats de capacitat i qualitat de servei en constant canvi. Per servir aquestes necessitats creixents, s’han proposat diverses tecnologies com comunicacions a l’espectre visible, Light Fidelity (LiFi), superfícies reconfigurables intel·ligents (RIS, per les sigles en anglès), comunicacions a la banda dels THz, etc. En concret, les RIS han despertat interès degut a la seva capacitat de manipular de forma precisa i programàtica les senyals de ràdio generades per diversos transmissors o punts d’accés i dirigir-les a múltiples receptors. Les manipulacions esmentades inclouen l’absorció de certs components de les ones incidents, o el control de la seva direcció, polarització, fase, o amplitud de forma selectiva en freqüència. En aquest context, les metasuperfícies (MSs) són part essencial de les RIS com a estructures artificials i planars que implementen la funció electromagnètica u òptica que la RIS necessita, que pot ser fins i tot anti-natural i atípica. Així doncs, els RIS consisteixen de dispositius que controlen les ones electromagnètiques (que podrien ser MS) co-integrades amb dispositius que doten a la MS de reconfigurabilitat i intel·ligència. Les MS, perla seva banda, estan composades de elements menor de la longitud d’ona que es coneixen coma “unit cells” o “meta-atoms”. El problema radica en que la promesa de les RIS, però, comporta una complexitat a nivell de MS que no és trivial. Per una banda, el rendiment de la RIS depèn de la mida de les unit cells, el número d’estats que pugui prendre la unit cell, o la mida de la MS completa. D’altra banda, hi ha problemes de costos i consum energètic associat amb la fabricació i operació de les RIS, que també depenen dels factors abans esmentats. Aquesta tesi té com a objectiu adreçar aquests problemes proposant un mètode pe, Postprint (published version)

This paper presents the research directions pursued by the WiPLASH European project, pioneering on-chip wireless communications as a disruptive enabler towards next-generation computing systems for artificial intelligence (AI). We illustrate the holistic approach driving our research efforts, which encompass expertises and abstraction levels ranging from physical design of embedded graphene antennas to system-level evaluation of wirelessly-communicating heterogeneous systems., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 863337 (WiPLASH)., Peer Reviewed, Article signat per 22 autors/es: - Joshua Klein, Alexandre Levisse, Giovanni Ansaloni, David Atienza (EPFL, Lausanne, Switzerland) - Marina Zapater (HEIG-VD/HES-SO Yverdon-les-Bains, Switzerland / EPFL, Lausanne, Switzerland) - Martino Dazzi, Geethan Karunaratne, Irem Boybat, Abu Sebastian (IBM Research Europe, Zurich, Switzerland) - Davide Rossi, Francesco Conti (Università di Bologna, Bologna, Italy) - Elana Pereira de Santana, Peter Haring Bolívar (University of Siegen, Siegen, Germany) - Mohamed Saeed, Renato Negra (RWTH, Aachen, Germany) - Zhenxing Wang, Kun-Ta Wang (AMO GmbH, Aachen, Germany) - Max C. Lemme (RWTH, Aachen, Germany / AMO GmbH, Aachen, Germany) - Akshay Jain, Robert Guirado, Hamidreza Taghvaee, Sergi Abadal (Universitat Politècnica de Catalunya, Barcelona, Spain), Postprint (author's final draft)

Metasurface-based intelligent reflecting surfaces constitute a revolutionary technology which can serve the purpose of alleviating the blockage problem in mmwave communication systems. In this work, we consider the hypersurface paradigm complementing the software defined metasurface with an embedded controller network in order to facilitate the dissemination of reconfiguration directives to unit cell controllers. For the first time, we describe the methodology with which to characterize the workload within this embedded network in the case of the metasurface tracking multiple users and we use a vehicular communications setting to showcase the methodology. Beyond that, we demonstrate use cases of the workload analysis. We show how the workload characterization can guide the design of information dissemination schemes achieving significant reduction in the network traffic. Moreover, we show how the workload, as a measure of the consumed power, can be used in designing energy efficient communication protocols through a multi-objective optimization problem maximizing the achieved utilization while at the same time minimizing the workload incurred., Peer Reviewed, Postprint (author's final draft)

As the current standardization for the 5G networks nears completion, work towards understanding the potential technologies for the 6G wireless networks is already underway. One of these potential technologies for the 6G networks is reconfigurable intelligent surfaces. They offer unprecedented degrees of freedom towards engineering the wireless channel, i.e., the ability to modify the characteristics of the channel whenever and however required. Nevertheless, such properties demand that the response of the associated metasurface is well understood under all possible operational conditions. While an understanding of the radiation pattern characteristics can be obtained through either analytical models or full-wave simulations, they suffer from inaccuracy and extremely high computational complexity, respectively. Hence, in this paper, we propose a neural network-based approach that enables a fast and accurate characterization of the metasurface response. We analyze multiple scenarios and demonstrate the capabilities and utility of the proposed methodology. Concretely, we show that this method can learn and predict the parameters governing the reflected wave radiation pattern with an accuracy of a full-wave simulation (98.8–99.8%) and the time and computational complexity of an analytical model. The aforementioned result and methodology will be of specific importance for the design, fault tolerance, and maintenance of the thousands of reconfigurable intelligent surfaces that will be deployed in the 6G network environment., This research was funded by the European Commission grant number H2020-FETOPEN736876 (VISORSURF) and by ICREA under the ICREA Academia program., Peer Reviewed, Postprint (published version)

Metasurfaces, the ultrathin, 2D version of metamaterials, have recently attracted a surge of attention for their capability to manipulate electromagnetic waves. Recent advances in reconfigurable and programmable metasurfaces have greatly extended their scope and reach into practical applications. Such functional sheet materials can have enormous impact on imaging, communication, and sensing applications, serving as artificial skins that shape electromagnetic fields. Motivated by these opportunities, this progress report provides a review of the recent advances in tunable and reconfigurable metasurfaces, highlighting the current challenges and outlining directions for future research. To better trace the historical evolution of tunable metasurfaces, a classification into globally and locally tunable metasurfaces is first provided along with the different physical addressing mechanisms utilized. Subsequently, coding metasurfaces, a particular class of locally tunable metasurfaces in which each unit cell can acquire discrete response states, is surveyed, since it is naturally suited to programmatic control. Finally, a new research direction of software-defined metasurfaces is described, which attempts to push metasurfaces toward unprecedented levels of functionality by harnessing the opportunities offered by their software interface as well as their inter- and intranetwork connectivity and establish them in real-world applications., This work was supported by the European Union’s Horizon 2020 research and innovation programme-Future Emerging Topics (FETOPEN) under grant agreement No 736876 (VISORSURF). Financial support by the National Priorities Research Program grant No. NPRP9-383-1-083 from the Qatar National Research Fund is also acknowledged. O.T. acknowledges the financial support of the Stavros Niarchos Foundation within the framework of the project ARCHERS (“Advancing Young Researchers’ Human Capital in Cutting Edge Technologies in the Preservation of Cultural Heritage and the Tackling of Societal Challenges”)., Peer Reviewed, Postprint (published version)

Programmable metasurfaces have garnered significant attention as they confer unprecedented control over the electromagnetic (EM) response of any surface. Such feature has given rise to novel design paradigms such as Software-Defined Metamaterials (SDM) and Reconfigurable Intelligent Surfaces (RIS) with multiple groundbreaking applications. However, the development of programmable metasurfaces tailored to the particularities of a potentially large application pool becomes a daunting task because the design space becomes remarkably large. This paper aims to ease the design process by proposing a methodology that employs a semi-analytical formulation to model the response of a metasurface and, then, derives performance scaling trends as functions of a representative set of design and application-specific variables. Although the methodology is amenable to any EM functionality, this paper explores its use for the case of beam steering at 26 GHz for 5G applications. Conventional beam steering metrics are evaluated as functions of the unit cell size, number of unit cell states, and metasurface size for different incidence and reflection angles. It is shown that metasurfaces 5¿×5¿ or larger with unit cells of ¿/3 and four unit cell states ensure good performance overall. Further, it is demonstrated that performance degrades significantly for angles larger than ¿>60o and that, to combat this, extra effort is needed in the development of the unit cell. These performance trends, when combined with power and cost models, will pave the way to optimal metasurface dimensioning., This work was supported in part by the European Commission (VISORSURF) under Grant H2020-FETOPEN-736876, and in part by Catalan Institution for Research and Advanced Studies (ICREA) through the ICREA Academia Programme. Odysseas Tsilipakos acknowledges support by the Stavros Niarchos Foundation within the framework of the project Advancing Young Researchers Human Capital in Cutting Edge Technologies in the Preservation of Cultural Heritage and the Tackling of Societal Challenges (ARCHERS)., Peer Reviewed, Postprint (published version)

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Recent years have seen the emergence of programmable metasurfaces, where the user can modify the electromagnetic (EM) response of the device via software. Adding reconfigurability to the already powerful EM capabilities of metasurfaces opens the door to novel cyber-physical systems with exciting applications in domains such as holography, cloaking, or wireless communications. This paradigm shift, however, comes with a non-trivial increase of the complexity of the metasurfaces that will pose new reliability challenges stemming from the need to integrate tuning, control, and communication resources to implement the programmability. While metasurfaces will become prone to failures, little is known about their tolerance to errors. To bridge this gap, this paper examines the reliability problem in programmable metamaterials by proposing an error model and a general methodology for error analysis. To derive the error model, the causes and potential impact of faults are identified and discussed qualitatively. The methodology is presented and exemplified for beam steering, which constitutes a relevant case for programmable metasurfaces. Results show that performance degradation depends on the type of error and its spatial distribution and that, in beam steering, error rates over 20% can still be considered acceptable., This work has been supported by the European Commission under grant H2020-FETOPEN-736876 (VISORSURF) and by ICREA under the ICREA Academia programme. The person and base station icons in Figure 1 were created by Jens Tärningand Clea Doltz from the Noun Project., Peer Reviewed, Postprint (author's final draft)

Hypersurface, a software defined metasurface (SDM) paradigm constitutes a revolutionary technology aiming at offering programmatic control over all aspects of a propagating wave, altering its direction, power, polarization and phase. In the absence of line of sight (LOS) between a transmitter and a receiver, HSF can provide seamless connectivity via programmatic reflection. However, such an application requires fine tuning of the metasurface reconfiguration parameters, which may not be effective when done in an open loop fashion due to model uncertainties. In this work, we consider a feedback-based formulation of the problem, which involves maximization of the received power and propose the use of Extremum Seeking Control (ESC) for the controller implementation due to fact that it is model-free and adheres to the maximization problem. Extensive simulations indicate that the proposed scheme is successful in guiding the impinging wave to the receiver within reasonable time even in the presence of time varying delays incurred by message propagation. In addition, a discrete time implementation is considered, investigating its limitations as we increase the sampling time while also characterizing the traffic within the controller network., This work was funded by the European Union via the Horizon 2020: Future Emerging Topics call (FETOPEN), grant EU736876, project VISORSURF (http://www.visorsurf.eu)., Peer Reviewed, Postprint (author's final draft)