## 4 Wireless Enabling Technologies
### Cellular-V2V (C-V2V)  
Despite much effort in the last 15 years and several projects with hundreds to thousands of vehicles, no connected vehicles based on IEEE 802.11p are on the field. In the meantime, the world of mobile communications has sprinted forward and C-V2X is now referred to as one future reference standard for connected vehicles. Inside the general world of C-V2X, short-range LTE-V2X represented the first step in this direction. Standardized in June 2017, LTE-V2X exploits the existing infrastructure/system and sketches out the road to 5G vehicular networks. The first step in this direction was made by LTE-Direct.  
\"You can think of LTE-Direct as a sixth sense that is always aware of the environment around you\", said few years ago [PERSON], technical marketing director at Qualcomm. LTE-Direct represented the first modification to the most wide diffuse communication technology, the cellular one. The massive growth in the number of connected devices and in traffic volume and the increasingly wide range of applications with varying requirements and characteristics pushed long term evolution (LTE) to face new challenges targeting device to device (D2D) communication. Hence, for the first time in history, a cellular system allows direct communication between peers. LTE-Direct is supported by the 3 GPP LTE Advanced Release 12 and provides D2D communications over cellular networks. It exploits the LTE infrastructure for resource allocation, timing, and authentication; then, it allows LTE-Direct based devices to communicate directly with one another when in proximity [24].  
However, it is only with 3 GPP Release 14, frozen in June 2017, that the support of short-range V2X communication was added to LTE [25; 26]. The new specifications define modifications to D2D: direct V2V communication adopts the uplink setup of LTE at the physical (PHY) and MAC layers, thus single carrier-frequency division multiple access (SC-FDMA) is applied. In the frequency domain, the channel bandwidth is subdivided into groups of orthogonal subcarriers equally spaced by 15 kHz while, in the time domain, the signal is split into frames of 10 ms, which are in turn made of 10 subframes of 1 ms. The subframe is additionally discretized in two time slots of 0.5 ms. Considering the time-frequency matrix of resources, one time slot and 12 contiguous subcarriers constitute a resource block (RB). The Sidelink interface has been tuned to counteract the severe Doppler experienced at high speed: out of the total 14 OFDM symbols carried by a subframe, the last is left unused to allow timing adjustment and Tx/Rx switch and 4 are dedicated to channel estimation, thus leaving only nine data symbols. Advanced coding techniques and an almost continuous variation of 21 MCS combinations are adopted which contribute, together with the possible use of portions of the bandwidth, to a higher reliability and range with respect to IEEE 802.11p.  
Release 14 also introduces two new functioning modes (Mode 3 and Mode 4) to solve the issue of resource allocation for V2V communications [27; 28]: in Mode 3, two vehicles directly communicate, but the reservation and scheduling of the resources is performed by the cellular infrastructure; in Mode 4, vehicles autonomously manage also the resources adopting a fully distributed procedure. Hence, in sidelink Mode 4, there is no control of the evolved NodeB (eNB) on direct links, as V2V public safety applications should also work in the absence of network coverage. The resource selection procedure is performed autonomously by vehicles that typically have to sense the medium before transmitting with semi-persistent scheduling (SPS); this means that the algorithm applies real-time measurements of the received power, with the aim of keeping a tolerable level of interference, while increasing the reuse of resources.