**1. Introduction**

Listening to Alexander Lautz from Deutsche Telekom, the device that, 10 years from now, we will look back at as the device of the 5G era, will be the car [1]. This is not just a catchphrase, but it is a clear index of the new perception of mobility as a service and not only as a transportation from an origin to a destination. Today, our on board smart navigator suggests the best route as the shortest one (in time or distance); tomorrow the mobility manager (on board or distributed in the cloud) will plan our commutes on the best connected roads or following the combination of routes and transportation that better fulfill the user's preference and needs. The concept of Internet of vehicles (IoV) has recently emerged just to indicate the need to go beyond the potentialities of vehicular ad-hoc networks (VANETs) and pass from the concept of smartphone to that of smart car, that is a moving connected object in a connected environment [2–5].

To obtain reliable service along the travel route, we need not only connected and automated car, but also a smart environment, able to opportunistically contribute to the signals propagation and to an ubiquitous and reliable coverage.

Different wireless access technologies are running to come on the market of connected vehicles for vehicle-to-everything (V2X) communications [6,7]: on the one hand, the old fashioned IEEE 802.11p (or its European version ITS-G5) is only waiting for a mandatory and clear business model to be installed on board of all new vehicles [8,9] and, on the other hand, the newer cellular-V2X (C-V2X) proposed by 3GPP Release 14 is rushing into things promising better coverage, higher throughput and lower latency with respect to its competitor [10,11]. Recent works studied the performance of both IEEE 802.11p and C-V2X in different realistic scenarios, demonstrating their potentialities in terms of connectivity, packet reception ratio, latency and coverage, but also highlighting some limits in obstructed scenarios or congested roads [12–21].

What could be enhanced, looking forward, is the environment: a smart environment could drastically enhance the performance of wireless access technologies and having, as a consequence, an impact on connected vehicles related applications. Motivated by this, researchers from both the academical world and the industrial one, are proposing new solutions to smarten up the cities, starting from the street, buildings and citizen themselves [1]. This includes sensors embedded in the roadway [22], wireless access technologies on traffic lights or lamps along the roads [23], vehicular social networks [24], vehicles' routing [25–27], managemen<sup>t</sup> of vehicular communication [28,29], cameras, smart wearable devices, etc. In this context, for example, the pilot project Austria's Autobahn uses Cisco's devices to connect tens thousands sensors with the objective to monitor traffic and road conditions. This represents an important example of how cities and public administrations move toward smart connected environments to improve safety, traffic efficiency, road capacity and infotainment [30,31]. The interest in smart environment is demonstrated also by the 6G Wireless Flagship Program (the world's first 6G research program), which indicates beside new wireless communication and computer science topics, also the importance of new electronics materials.

However, in spite of the huge effort in this direction, there will still scenarios in which the communication is obstructed by strong obstacles, preventing good links and allowing poor performance in terms of data rate, error rate, coverage and latency. In this context, reconfigurable meta-surfaces can play an active role, opportunistically redirecting the radio waves to improve connectivity and enabling the establishing of new and potentially stronger links [32]. Meta-surfaces are thin electromagnetic meta-material with typically sub-wavelength thickness and large in transverse size [33,34]. They are composed of sub-wavelength scattering particles that can revise the Snell's law redirecting the radio waves in the desired direction and can do this run time, changing the redirection of the waves time by time, according to the generalized Snell's laws, thus providing different values for the angles of incidence and reflection. Beyond meta-surface, what it is really challenging and stimulating, is the use of a *reconfigurable* meta-surface, where the scattering particles are not fixed, but can be moved and modified depending on the input they receive from the external world [35,36]. The idea of reconfiguring the wireless propagation environment has emerged only recently with focus especially on the indoor environments, where reconfigurable meta surfaces become connected to the rest of the scenario interacting with the connected objects and serving the user needs in unprecedented ways [37,38].

Several works deal with antenna design and performance optimization, such as [39], where a meta-superstrate for two vertically polarized MIMO antenna elements at the base-station is proposed to reduce the inter-element spacing. Preliminary evaluations related to the use of meta-surfaces in outdoor scenarios are related to the proposal of algorithms to minimize the total transmit power at the base station of a cellular system conditioned to the users quality of service (QoS) constraints [40] or to maximize either the energy or the spectral efficiency of a reconfigurable meta-surface multi-user MISO system [41]. Instead of using sensors embedded in the roadway and on traffic lights, reconfigurable meta-surfaces could be exploited to extend coverage in the highly dynamic vehicular environment, by coating the environment with intelligent meta materials.

Hence, the objective of this work can be summarized as follows:


This work is organized as follows: in Section 2, the main advantages and limits of the two main candidates radio access technologies for vehicular networks are highlighted; in Section 3, the concept of reconfigurable meta surface is introduced and the use of reconfigurable meta-surfaces in vehicular networks is proposed and discussed especially referring to two case studies, cooperative driving and pedestrian detection. A simplified model for the evaluation of the impact of meta-surfaces on the IEEE 802.11p performance in terms of collision probability is presented in Section 4 and is validated by simulations. Finally, in Section 5 our conclusions are drawn.

## **2. Technologies for Vehicular Networks**

Nowadays, two are the candidate enabling technologies for vehicular communications: IEEE 802.11p (or its European version ETSI ITS G5) and C-V2X.

IEEE 802.11p dates back in 2004 when the IEEE 802.11 working group started a discussion on how to modify and adapt Wi-Fi for dynamic environment, reducing the signaling and overhead of the nomadic version to support a completely different environments. It was then standardized in 2010 and tested in different field trials all around the world also with thousands of vehicles, demonstrating good performance in different use cases, thus representing a commercially available technology. On the other hand, C-V2X has been defined by 3GPP in 2016 within long term evolution (LTE) Release 14 and frozen in 2017 with the first plug test that took place in December 2019 in Malaga, Spain, demonstrating the 95% of success in terms of interoperability issues [42]. Hence, it can be observed that, while the IEEE 802.11p community spent several years from the first discussions to standardization and then on how and when set it up it on board, the cellular world sprinted forward and in a couple of years, test devices are ready for interoperability tests using a widespread technology, as it is demonstrated by the summary of timeline in Figure 1, with different colors showing the timeline of the different technologies. As it can be observed, the speed of development of C-V2X is much higher than Wi-Fi for mobility (IEEE 802.11p) and, despite being frozen in 2017, it will be ready for commercial installation on boards from 2020.

**Figure 1.** Summary of timeline of Wi-Fi for mobility and cellular vehicle-to-everything (C-V2X), showing the different sprint of the two standardization and experimentation processes.

## *2.1. IEEE 802.11p*

IEEE 802.11p defines the physical (PHY) and medium access control (MAC) layer protocols. At the PHY layer, IEEE 802.11p adopts orthogonal frequency division multiplexing (OFDM) with 52 subcarriers of which 48 used for data and 4 for pilots. The OFDM symbol lasts 8 μs and the

subcarrier spacing is 156.25 kHz, bringing to a raw bandwidth of 10 MHz. Eight modulation and coding schemes (MCSs) are possible, with modulations going from BPSK to 16-QAM with convolutional coding. At the MAC layer, carrier sensing multiple access with collision avoidance (CSMA/CA) is adopted, hence, when a node has to transmit a packet, it senses the medium; if the medium is idle the packet is transmitted after an Arbitration inter-frame spacing (AIFS) interval time (that takes into account potential delays in the propagation due to distant nodes), otherwise a mechanism based on random backoff is performed to reduce the probability of collisions by letting nodes to randomly start the next sensing phase for transmission. In addition, in the vehicular scenario, the acknowledgement and request to send/clear to send—RTS/CTS—mechanism are not foreseen to accelerate connection.

CSMA/CA has the advantage of being completely distributed and does not need any synchronization procedure, but, on the other hand, it suffers from collisions in dense vehicular environments [43], thus mechanisms to avoid overloads are necessary, such as decentralized congestion control (DCC) algorithm proposed by ETSI and SAE or new algorithm proposed in the literature [44], such as full duplex carrier sensing multiple access with collision detection (CSMA/CD) mechanism [45].
