# Survey and Perspectives of Vehicular Wi-Fi versus Sidelink Cellular-V2X in the 5G Era
## 2 Overview of the Technologies and Comparative Discussion
### Related Work  
Since the first experiments with IEEE 802.11p, studies to compare its performance to that of cellular communications have been carried out. Long-range cellular connection is considered in some early works, both using unicast or multicast transmissions [24; 25; 26; 27]. Although this solution appears possible, it is clear that it suffers from the following two issues: (1) it is not possible to guarantee a full coverage of the territory; and, (2) with large cells, it has limited scalability and, with small cells, the complexity increases due to frequent handovers. The mentioned problems motivated 3 GPP to add first D2D capabilities in Release 12 and specific short-range V2X in Release 14. In Release 12, the target was mainly on public safety, but its applicability to vehicular scenarios was already considered by a few works. For example, in [28], a non-standard frame structure is proposed for this aim. Since Release 14 was defined, several studies have been carried out to compare IEEE 802.11p and sidelink LTE-V2X, which can be separated into two major groups: (1) white papers published by those companies that have a direct interest in either technology, such as [3; 4; 5; 6]; and (2) research papers that have gone through a peer-review process and have in most cases no direct commercial interest (Table 4) [7; 8; 9; 10; 11; 12; 13; 14; 15]. Within white papers, some push C-V2X as a more efficient technology, supported by a stronger ecosystem and a clearer road-map for future evolutions. In [5], comparative experiments with real devices are shown, demonstrating 1.7x-3.4x range improvement of C-V2X with respect to IEEE 802.11p and verifying that the latency in C-V2X under congested conditions does not exceed 100 ms. Other white papers are instead in favour of IEEE 802.11p and against C-V2X, remarking that only the Wi-Fi technology is truly tested and reliable, and C-V2X products are not expected to be ready for the market within a few years. In [6], the mentioned results demonstrating the superiority of C-V2X are contested as biased by methodological weaknesses and issues with the PHY layer design are highlighted. Peer-review papers mainly use simulations with moving vehicles and compare the two technologies in terms of reliability [7; 8; 9; 11; 12; 13; 15]. Except for [15], addressing a rural area, all of them consider a highway scenario. Some also include an urban Manhattan case [8; 9]. Platooning as a specific application is addressed in [12]. In addition, analytical evaluations are provided in [10] and a focus on the link quality is posed in [14]. Focusing on Mode 4 of LTE-V2X, the standard protocol is always shown to guarantee the same performance at larger distance when the basic 6 Mb/s MCS is adopted by IEEE 802.11p, especially under low channel load conditions [7; 8; 9]. Adopting a different MCS in IEEE 802.11p, however, it is shown in [7] that the outcome might be different, especially when the generated data traffic increases. Results using Mode 3 are given in [8] using a not specified algorithm, in [10] under ideal conditions, in [11] adopting an algorithm that blocks some calls in order to guarantee a configurable reliability at a given distance, and in [15] with a simple round-robin based scheduling. Even if the mentioned algorithms all appear as simplified solutions, yet the more complete knowledge of the network status is shown to allow outperforming IEEE 802.11p in all cases. In summary, all of the mentioned works basically agree that C-V2X can provide a longer range than IEEE 802.11p. However, most of them adopt settings that might not be optimal for IEEE 802.11p. In addition, results are not able to distinguish the contribution from the PHY and MAC layers and it is difficult to infer what happens if the PHY layer is re-designed in IEEE 802.11p.