Anton Tishchenko

Reconfigurable intelligent surfaces (RIS) are positioned as one of the key enabling technologies for 6G networks as they can provide ubiquitous coverage for areas with blocked line-of-sight (LOS) links. However, to be successfully integrated into functional networks such structures will require the addition of sensors and other radio network elements, thereby resulting in a multi-functional RIS (MF-RIS). These structures are expected to be deployed for integrated sensing and communications (ISAC) and radar and communication coexistence (RCC) in 6G, which will enhance the performance of radio communication and enable a smart wireless environment (SWE) that is programmable and self-reconfigurable. This survey provides an up-to-date summary of the state of the art. It considers applications for MF-RISs and the challenges associated with their deployment.

—This paper introduces the latest designed electromagnetic metasurfaces at the Institute for Communication Systems (ICS) for 5G-and-beyond networks. Various technologies and metasurfaces at different frequency ranges were developed to solve the drawbacks related to metasurfaces such as the limited bandwidth and Non-line-of-sight (NLoS) coverage issues. I. REFLECTIVE METASURFACES FOR 5G AND 6G COMMUNICATIONS One core objective of applying reflective metasurfaces in future communication systems is to provide electromagnetic (EM) coverage in the network's blind spots [1]. This happens by regulating the aperture response when it is illuminated by EM source(s), to purposefully reflect the incoming waves to the direction of interest. Controlling the aperture response can be done by the generalized Snell's law of reflection and holographic technique. In this section, we introduce two reflective metasurfaces based on these two techniques. A. Reconfigurable Intelligent Surface based on Generalized Snell's Law of Reflection A reconfigurable intelligent surface (RIS) is presented in [2] where the generalized Snell's law of reflection is applied to regulate the phase profile on the surface. This method requires knowledge about the location of the EM source and the direction of reflection, as well as the spacing between the unit cells on the surface. In a designed structure, the unit cell spacing (periodicity) is in general constant, but the location of the EM source and the direction of reflection can vary case by case. Hence, it is required to add a controllable component (varactor diode in [2]) to the physics of the unit cell to correspondingly customize the response of the surface and to make a reconfigurable structure. B. Reflective Metasurface based on Holography Technique A holographic-based reflective metasurface is presented in [3]. In the holography technique, the direction of the incoming waves must be known, and then, based on the direction of reflection, an interferogram will be obtained which is the so-called EM hologram. With this technique, it is possible to define more than one reflected beam, resulting in multi-spot coverage provisioning. Under this circumstance, the su-perposition of the desired reflecting beams will contribute to calculating the EM hologram. A dual-beam reflector is designed in [3] correspondingly. II. REFLECTIVE METASURFACE FOR OAM BEAMS GENERATION Orbital angular momentum (OAM) beams have been suggested as a strong solution to increase the channel capacity of a communication system by utilizing many orthogonal independent channels without using extra frequency resources [4]. Therefore, they can be used to solve the limited bandwidth drawback of metasurfaces. A. Reflective Metasurface with Steered OAM Beams Three environment-friendly reflective metasurfaces with single and dual-directed OAM beams to tackle the poor network coverage of THz waves in the absence of LoS communications are introduced in [5]. The integration between the OAM and THz RMTS technologies can improve spectral efficiency through a low-cost and low-profile solution. The presented metasurfaces of 90 × 90 mm were simulated, fabricated, and tested to verify the capability to control and steer the wavefront of the EM waves in the frequency range 90-110 GHz. B. THz reflectarray antenna with OAM multiplexing and beam-steering capabilities The unexplored potentials of reflectarray antennas to manipulate OAM beams are examined at 330 GHz in [6]. It investigated the maximum achievable angles by a planar meta-surface per single feed for a single OAM beam. That motivated the proposed work to investigate the possibility of generating multiple off-centered OAM beams of different modes with the maximal achievable angles for OAM multiplexing and beam-scanning applications through passive structures. The designed RAs can be envisaged for THz indoor communications. III. REPROGRAMMABLE GRAPHENE-BASED DIGITAL METASURFACE The metasurfaces using phase-only or amplitude-only engineering have limited the full functionality of the devices. In [7], a digital graphene-based metasurface simultaneously manipulating both amplitude and phase has been proposed to address this challenge in the terahertz (THz) band. As Fig. 1(c) presents conceptually, leveraging a 2/2-bit digital unit cell with independent control of 2-bit states of amplitude and phase, an efficient multi-focal meta-lens has been demonstrated. Moreover , the proposed metasurface has been applied to develop a

Sixth-generation cellular networks (6G) are expected to involve not only data communications but also sensing capabilities, enabling a wide range of applications. This paper proposes a novel Dual Functional Shared Aperture reconfigurable intelligent surface (RIS) design that enables the coexistence between radar sensing and millimeter wave (mm-wave) communication. Some RIS meta-elements integrate radio-frequency (RF)-feeds to support the transmission/reception of multiple-input multiple-output (MIMO) radar signals, permitting holographic beam focusing/forming in near/far fields without phase shifters. The designed Dual Functional RIS provides a synergy between communication and sensing modalities, particularly in scenarios where both far-field and near-field interactions play critical roles. Primarily, we emphasize far-field conditions and antenna-related aspects which serve as a foundational framework for future work considering that both communication and radar detection take place in the near field. We show adding an RF-feed to the meta-element incurs additional amplitude loss in the spectrum of interest. At the same time, increasing the number of radar antennas (i.e., elements with RF-feed) improves the radar angular accuracy.

The power consumption of reconfigurable intelligent surfaces (RIS) has not been addressed enough in the state-of-theart. This paper proposes a paradigm for converting RISs into ecofriendly structures that generate their needed power by the highefficiency multiple junctions (MJ) solar cells. This work considers the impact of adding solar cells to the RIS and investigates the amount of generated power. Moreover, the inclination angle of the solar irradiance is considered as well as the needed batteries and converters. Since the introduced RIS can be a potential candidate for the 5G-and-beyond-communications, two ranges of frequencies will be discussed in this work: sub-6GHz and 5G Millimeter wave (mmWave).

The power consumption of reconfigurable intelligent surfaces (RIS) has not been addressed enough in the state-of-the-art. This paper proposes a paradigm for converting RISs into eco-friendly structures that generate their needed power by the high-efficiency multiple junctions (MJ) solar cells. This work considers the impact of adding solar cells to the RIS and investigates the amount of generated power. Moreover, the inclination angle of the solar irradiance is considered as well as the needed batteries and DC-DC converters. Since the introduced RIS can be a potential candidate for the 5G-and-beyond-communications, two ranges of frequencies will be discussed in this work: sub-6GHz and 5G Millimeter wave (mmWave).

—The proposed intelligent reflective surface (IRS) is presented to compensate for the path loss and enhance the coverage of 5G networks at mm-wave band. A (π) shaped element with variable-sized dipoles, distributed in a certain way to maintain a phase length curve over 340 • in the range of 23-27 GHz, is addressed in this work. The proposed structure can be an ideal candidate for 5G mm-wave band n258.

Holographic Beamforming is a promising concept to reduce the power consumption of Multiple Input Multiple Output (MIMO) antenna arrays. In a holographic approach, the impedance of antenna patches is varied through the inclusion of tuning elements, such as varactor diodes, which allow electronic control of the phase and amplitude of each antenna. In this work, we provide the electromagnetic framework for the design of a Holographic MIMO Surface (HMIMOS). We analyze its performance and compare its power consumption to passive Reconfigurable Intelligent Surfaces (RIS) and MIMO Active Phased Arrays (APA) at 5G Frequency Range (FR) 2. The results show that the power consumption of HMIMOS is lower than of MIMO APAs, but significantly higher than of RISs. However, a combination of active and passive elements on a RIS can offer many benefits in terms of environmental awareness and intelligence for Integrated Sensing and Communication (ISAC) in Beyond 5G (B5G) networks.