# Survey and Perspectives of Vehicular Wi-Fi versus Sidelink Cellular-V2X in the 5G Era
## 1 Introduction  
Wireless communications and the wide distribution of the Internet have completely changed society over the last twenty years. The rapid evolution of communication technology has made it possible to implement the paradigm of _everyone communicating everywhere and any time_. The next step, which has already begun but is still in its infancy, is the interconnection and interoperability of robots, which will have a drastic impact in many fields, including industry, health, and mobility. With an unpredictable period of transition, someday vehicles will be all autonomous and connected, with the promise of no more deaths on the road, of more efficiency of traffic flows, and of more comfort for all passengers. One of the main first steps along this path is the transformation of cars and trucks from isolated road-users, adhering common rules, to communicating, coordinated, and cooperative parts of an ecosystem. Such transformation passes through the capability of vehicles to connect to each other and to the rest of the world, and a key role is played by wireless short-range technologies. In fact, short-range is required to allow scalability of the service as well as to avoid risks of areas where the connectivity is not available. From an industrial point of view, this represents an enormous opportunity, given that more than 90 million of vehicles are produced every year [1] and that the global connected car market is projected to grow tremendously in the next years (estimated in [2] to exceed $225,000.0 million by 2025). In this scenario, two technologies have been designed to be used on board of the vehicles for short-range connectivity and a tough debate is ongoing to demonstrate that either can be preferred to the other. The point is that vehiclesneed to use the same technology to be able to communicate, thus if vehicles from different manufacturers are equipped or use only one of them without agreements, it would have detrimental effects. The two technologies, largely discussed in the following, are the Wi-Fi solution, often referred to as IEEE 802.11p from the name of the first standard designed to this scope, and the cellular solution, named long-term-evolution-vehicle-to-anything (LTE-V2X) as part of the fourth generation of Third Generation Partnership Project (3 GPP) standards and under the broader umbrella of the so-called cellular-V2X (C-V2X). Whereas IEEE 802.11p has a relatively long history, with very large field trials performed worldwide, LTE-V2X is very recent. This gives to the latter solution a potential advantage in the improved physical (PHY) layer design, but makes its effective reliability and scalability still to be completely verified. Several works have recently discussed the two technologies from various perspectives and a number of performance comparisons have been provided both in white [3; 4; 5; 6] and peer-reviewed papers [7; 8; 9; 10; 11; 12; 13; 14; 15]. A common conclusion is normally that, given a target performance, the more advanced PHY layer of LTE-V2X allows for reaching a longer distance. However, as deeply discussed in the following, there are specific scenarios and settings where IEEE 802.11p provides similar or even better results. It is also worth noting that most papers showing a comparison assume given modulation and coding schemes (MCSs), which are not optimized and often penalize IEEE 802.11p. In addition, no specific discussion has been done about the role played by the medium access control (MAC) protocol. In this work, we aim at revising the performance comparison between IEEE 802.11p and sidelink LTE-V2X, first summarizing the related work and then providing original results in various realistic scenarios and optimizing the adopted MCSs.