RISs, in a Nutshell…

In this context, RIS has emerged as a new enabling candidate future wireless technology for shaping and controlling the electromagnetic waves between transmitters and receivers, in a dynamic and goal-oriented way, thus turning the wireless environment into a service [11]. As illustrated in Figure 1, an RIS is a planar surface that consists of an array of passive scattering elements, each of which can independently impose the required phase shift, and possibly an amplitude gain, on the incident electromagnetic waves. By carefully adjusting the phase shifts (and the amplitudes) of all the scattering elements, the reradiated electromagnetic waves can be shaped to propagate towards specified directions. Broadly speaking, an RIS is a planar (or, more in general, conformal) structure that is engineered to have properties that enable one to dynamically control the electromagnetic waves, via, e.g., signal reflections, refractions, focusing, collimation, modulation, and their combinations.

As shown in Figure 1, a typical RIS mainly consists of a planar surface and a controller. The planar surface is usually made of a single or, in general, a multiple layer structure. Often utilized three-layer structures are constituted by an outer layer with a large number of scattering elements printed on a dielectric substrate, which directly act on the incident electromagnetic waves; a middle layer that can be a copper panel to avoid signal/energy leakage; and a third layer that includes the necessary circuitry for controlling the scattering properties of the RIS elements. Figure 1 shows an example of the conceptual structure of each scattering element, in which a positive-intrinsic negative (PIN) diode enables the reconfigurability of the surface. By controlling the voltage through the biasing line, the PIN diode can switch between “on” and “off” modes, as shown in the equivalent circuit of the figure, which can realize a phase shift difference of 180 degrees. To increase the number of phase shift levels, several PIN diodes need to be integrated on the surface for each RIS element. Figure 2 reports an example of manufactured RIS that enables a continuous control of the phase of the reradiated electromagnetic waves by using four varactors for each available RIS element.

RISs, Pros, & Cons…

An RIS can be implemented using mostly passive elements, without requiring high-cost active components, such as power amplifiers, thus possibly resulting in a low implementation cost and energy consumption. An RIS is not expected to possess sophisticated signal processing capabilities, but only the necessary low-power electronic circuits for enabling it reconfigurability and hence controlling the propagation environments. This may in turn result in an easier and more flexible deployment, with the possibility of RIS-equipped nodes to take any shapes and to be even integrated onto objects (e.g., walls, buildings, lamp posts, and indoor and outdoor advertising street furniture). Thanks to these features, an RIS is often referred to as a “nearly passive” device and hence it is unlikely to increase the exposure to electromagnetic fields. These associated characteristics suggest that an RIS may be considered as a sustainable and environmentally friendly technology solution. The absence of power amplifiers and digital signal processing capabilities naturally pose some design challenges, such as the impossibility of on-board channel estimation, signal regeneration, and amplification, which are currently being tackled by wireless researchers and engineers [12], [13]. In general, these design features of an RIS result in an inherent trade-off between the coverage range of the surface, its size, and the number of RIS elements that need to be deployed on it: The larger the desired coverage range is, the larger the size of the RIS or the larger the number of RIS elements on the same surface area need to be. Surfaces with densely deployed RIS elements whose sizes and inter-distances are smaller than half of the wavelength pose implementation challenges as well, e.g., due to non-local mutual coupling effects among the RIS elements and the need of designing sub-wavelength scattering particles and tuning circuits. Table 1 offers a non-exhaustive and qualitative comparison between an RIS and other technologies that are widely employed in wireless communications and networks.

From an implementation perspective, an RIS does not usually need active RF chains and only needs passive scattering elements and low-power electronics, which may potentially result in orders-of-magnitude lower hardware cost and energy consumption compared to 5G technologies based on active antenna arrays. The passive reradiation of an RIS mimics a full-duplex transmission mode that can be free of antenna noise amplification and self-interference. This may make an RIS more attractive than traditional active half-duplex relays that suffer from low spectral efficiency or full-duplex relays that require sophisticated techniques to cancel the self-interference out. Since an RIS is nearly-passive, it may be fabricated with light weight, low profile, and can be even made conformal to various objects. As a result, an RIS may be deployed in a wide range of scenarios, such as walls, ceilings, billboards, lampposts, and even on the surface of vehicles to support several applications for smart factories, stadiums, shopping centers, airports. Last but not the least, an RIS is often viewed as an energy-efficient auxiliary device that is transparent to the wireless users since it does not require modifications in the hardware configuration of the end-user devices. This offers high flexibility and compatibility with legacy wireless systems. As of today, however, the actual performance gains of RISs with respect to other widely deployed technologies, and the most promising use cases and applications for RISs are still under discussion. This is indeed one of the main objectives of currently ongoing collaborative research projects on this technology and of the recently established ETSI ISG on RISs. We will get back to this shortly…

RIS-empowered SREs

In wireless communication networks, RISs are intended to realize so-called programmable and reconfigurable wireless propagation environments, i.e., wireless environments that are not viewed and treated as random uncontrollable entities but become part of the network design parameters that are subject to optimization for supporting diverse performance metrics and quality of service needs, so as to fulfil the requirements of future 6G networks [8], [9], [12]. Recent applications of RISs in wireless communications include their use as nearly passive relay-type surfaces [14], signal-RF multi-stream multi-antenna transmitters [15], and reconfigurable ambient backscatters [16].

As illustrated in Figure 3, capillary deployed RISs may turn a wireless environment into a service by controlling the electromagnetic waves between specified transmitters and receivers, in a dynamic and goal-oriented way. This has motivated wireless researchers to evaluate new potential use cases targeting at (i) enhancements of various system key performance indicators, such as capacity, coverage, positioning, sustainability, and security and (ii) enabling new wireless technology applications, such as sensing, localization, wireless power transfer, and ambient backscattering. Broadly speaking, an RIS is viewed as a new system node that is dynamically configured by a system controller, turning a wireless environment from a passive to an intelligent actor such that the channel becomes programmable: An SRE. This trend is currently challenging basic wireless system design paradigms and is creating innovation opportunities which may progressively impact the evolution of wireless system architectures, access technologies, and networking protocols.

It is well understood that the RIS technology is not the panacea of future telecommunication standards. Several challenges need to be tackled for the possible deployment and integration of this technology in future wireless networks. At best, RISs can enhance current wireless networks that are designed based on active infrastructure elements and may turn future wireless systems into a hybrid network that comprises active and passive network elements and components, which are co-designed and co-work in an intelligent way for achieving a sustainable capacity growth at a low and affordable cost, implementation complexity, power consumption, in a sustainable environmentally friendly manner.

Collaborative Research Efforts and IEEE COMSOC Initiatives

RISs have attracted a lot of attention in the research community globally, including, noticeably, Europe, USA, and China. There exist several collaborative research and innovation projects on RISs and SREs. A non-exhaustive list of projects funded by national and supranational agencies is given in Table 2. By considering only the projects reported in the table, we can see that more than 20 million euro have been allocated by funding agencies to this technology, without considering industry-funded projects.

In the summer of 2020, the IEEE Communications Society launched several seed initiatives for promoting and supporting research activities in the emerging area of RIS-empowered SREs. This includes a Special Interest Group (SIG) within the Wireless Communications Technical Committee [18], an SIG within the Signal Processing and Computing for Communications Technical Committee [19], an Emerging Technology Initiative (ETI) on RIS [20], which was established by the IEEE Communications Society Emerging Technologies Committee (ETC), and a Best Readings in RISs [21]. These initiatives have fueled several opportunities for nurturing the RIS technology, which encompass the first special issue published in the IEEE Journal on Selected Areas in Communications in November 2020 [22], the latest forthcoming multidisciplinary special issue in the Proceedings of the IEEE [23], the first thematic workshop on RISs at the 2020 IEEE Global Communications Conference [24], a special workshop on RISs at the 2022 IEEE International Conference on Communications [25], and the first dedicated Selected Areas in Communications symposium at the 2022 IEEE Global Communications Conference [26].

Furthermore, the RIS technology is featured as a key technology trend in the International Telecommunication Union Radiocommunication Sector (ITU-R) - IMT-2030 Future Technology Trends report, and early attempts to bring it for exploratory studies in Standards Development Organizations including the 3rd Generation Partnership Project have recently been made.

The Road to Standardization: ETSI SIG on RISs

While all these extensive research efforts on RISs are ongoing, the global standardization of this technology is still at its very early stages [17]. There are many technical challenges that need to be adequately addressed before the RIS technology may be adopted and integrated into future telecommunication standards towards its eventual commercialization.

On June 30^th, 2021, an initiative to constitute a new ETSI ISG on RISs was approved by the ETSI Director-General. This initiative, jointly led by InterDigital Europe (UK) and CNRS-UPSaclay (France) along with other founding members (British Telecommunications, CNIT, IMEC, NPL, NEC Europe, UK DCMS, University of Oulu, University of Surrey, and ZTE Corporation), was formed to coordinate and streamline pre-standardization research efforts on this emerging technology [4]. The mission statement of the ETSI ISG on RISs reads as

“To provide an opportunity for ETSI members to coordinate their pre-standards research efforts on RIS technology across various EU/UK collaborative projects, extended with relevant global initiatives, towards paving the way for future standardization of the technology”

The ETSI ISG on RISs, which will kick start on September 30^th, 2021 and will run for a duration of two years, will focus on pre-standardization work, including the definition of use cases, application scenarios, and relevant system requirements. The activities of the ISG on RISs will be complementary to the existing work of ETSI. Relationships with other ETSI bodies and industrial organizations will be established to avoid duplications, to maximize synergies, and to ensure the broad industrial adoption of the RIS technology. The outcomes of the ETSI ISG on RISs will be made available through publicly accessible deliverables for wide dissemination and feedback [4].

RIS, Quo Vadis?

It is difficult to predict. Many research and deployment challenges need to be addressed, including the realization of sub-wavelength digitally controllable metamaterials operating at 6G frequencies, the design of efficient channel estimation algorithms, the definition of efficient control channels, and the conception of interference management schemes and communication protocols, just to mention a few. The ETSI SIG on RISs will kickstart on September 30^th, 2021 and will run for two years. At the same time, the major European-funded projects on RISs will end by 2023 and 2024. Hopefully, we will be able to answer this question in the coming couple of years…

References

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