# Survey and Perspectives of Vehicular Wi-Fi versus Sidelink Cellular-V2X in the 5G Era
## 2 Overview of the Technologies and Comparative Discussion
### IEEE 802.11p  
The first Wi-Fi based standard specifically designed for vehicular communications was approved under the name IEEE 802.11p in 2010. IEEE 802.11p was later included in the IEEE 802.11-2012, now superseded by the IEEE 802.11-2016. The peculiarities of the vehicular based technology with respect to the other associated standards are mainly a smaller bandwidth of 10 MHz (less noise and longer cyclic prefix) and the possibility to work without the need to join a basic service set (BSS), which is called outside of the context of a BSS (OCB) mode.  
IEEE 802.11p defines the PHY and MAC layer protocols. Above IEEE 802.11p, a number of other standards have been defined, creating two different pillars in the US and in Europe [17]. In the US, IEEE 1609 standards focus on the protocols below the application layer, which is instead defined by the Society of Automotive Engineers (SAE). In Europe, IEEE 802.11p was adopted by ETSI under the name ITS-G5, together with a large number of other documents dealing with all layers above it.  
At the PHY layer, IEEE 802.11p adopts orthogonal frequency division multiplexing (OFDM) with 48 useful and four pilot subcarriers. The OFDM symbol lasts 8 \(\upmu\)s and the subcarrier spacing is 156.25 kHz, bringing to a raw bandwidth of 10 MHz. The packet is preceded by a preamble for synchronization and training. Eight MCSs are possible, with modulations going from binary phase shift keying (BPSK) to 16-quadrature amplitude modulation (QAM) and (possibly punctured) convolutional coding. The eight combinations are reported in Table 2.  
At the MAC layer, carrier sensing multiple access with collision avoidance (CSMA/CA) is adopted. A node needing to transmit senses the medium to check if it is idle, and a mechanism based on random backoff is performed to reduce the probability of collisions. In the vehicular scenario, the acknowledgement is not foreseen and thus the exponential backoff introduced in other versions for retransmissions does not apply.  
The CSMA/CA mechanism introduces significant overhead, especially with short packets and high level modulations. In addition, it is known that communications are severely affected by collisions when the channel occupation raises 50-60% [18], thus mechanisms to avoid overloads are necessary. This indeed brought both SAE through SAE J2945/1 and ETSI through decentralized congestion control (DCC) to add algorithms specifically addressing congestion control.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
1999 & Frequencies are allocated in the US & General \\
2004 & IEEE 802.11p Task Group is formed & IEEE 802.11p \\
2008 & Frequencies are allocated in Europe & General \\
2010 & IEEE 802.11p is approved & IEEE 802.11p \\
2012 & The Ann Arbor experiment starts with thousands of devices & IEEE 802.11p \\
2016, October & The first part of 3 GPP Release 14 is published & C-V2X \\
2016, November & The Livrono Plug test: ITS-G5 is declared ready & IEEE 802.11p \\
2016, November & The European Commission publishes the “Strategy” & General \\
2017, January & The proposed rule is issued in the US to mandate short-range & General \\
2017, March & 3 GPP Release 14 is frozen & C-V2X \\
2018, May & IEEE 802.11 Next Generation V2X is announced (IEEE 802.11 bd) & IEEE 802.11p \\
2019, March & 3 GPP Release 15 is frozen & C-V2X \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chronology of the main facts in the US and Europe.