# Survey and Perspectives of Vehicular Wi-Fi versus Sidelink Cellular-V2X in the 5G Era
## 2 Overview of the Technologies and Comparative Discussion
### Sidelink LTE-V2X  
Short-range LTE-V2X was defined in Release 14, based on the device-to-device (D2D) of Release 12. It is also called sidelink (opposed to downlink/uplink) and its communication interface is named PC5. At the PHY and MAC layers, sidelink LTE-V2X (hereafter simply LTE-V2X) relies on the same single carrier frequency division multiple access (SC-FDMA) used by LTE uplink: in the frequency domain, the subcarrier spacing is fixed to 15 kHz and subcarriers are used in groups of 12 (i.e., 180 kHz); in the time domain, 14 symbols form a subframe of 1 ms, also called transmission time interval (TTI), and include nine data symbols, four demodulation reference signal (DMRS) symbols, and one empty symbol for Tx-Rx switch and timing adjustment. LTE-V2X has a high number of MCSs, with 4-QAM and 16-QAM modulations and an almost continuous coding rate. As an example, the MCSs, with modulation and coding rate, are listed in Table 3 referring to packets of 300 bytes. The minimum resource for the allocation of messages in LTE-V2X is the subchannel in the frequency domain, corresponding to a multiple of the 12 subcarriers groups, and the TTI in the time domain. One packet normally occupies one or more subchannels in one TTI. In principle, resources are orthogonal and two transmitters either fully or negligibly interfere, except for in-band emission (IBE) due to the use of different frequencies in the same TTI. Regarding the upper layers, LTE-V2X is stated compatible with ETSI-Intelligent Transportation System (ITS), ISO, SAE and IEEE standards. Given the time-frequency structure with orthogonal resources, the resource allocation algorithm plays a crucial role to optimize the spatial reuse. Since LTE-V2X was mainly designed to enable the cooperative awareness service, which is the transmission of periodic messages by each vehicle to informabout its status and movements, the resource allocation is normally associated with a semi-persistent scheduling (SPS) mechanism. Two different approaches are defined by 3 GPP for the reservation of resources, namely Mode 3 and Mode 4, depending on the entity in charge of the allocation. In Mode 3, also known as scheduled or controlled, the resources to be used by each vehicle for short-range communications are defined by the network. This strategy, on the one hand, clearly requires that the vehicles are within the coverage of some eNodeB and that some information is exchanged between nodes and controllers to allow efficient transmissions. On the other hand, it can take advantage of large processing capabilities and wider view of the state of the network, thus is expected to enable improved performance. Mode 3 algorithms are not defined in the specifications and their implementation is left to the operators. Software-defined networking (SDN) is also expected to play a relevant role in this case [21]. In Mode 4, also known as autonomous or out-of-coverage, each node selects the resources to use based on a sensing procedure and an SPS mechanism. The algorithm is defined in details by 3 GPP (see also [7; 22]) and can be summarized as follows. At the MAC layer, a resource is randomly selected within the set received from the PHY and maintained (i.e., the same subchannels at periodic TTIs, with a period equal to the periodicity of the messages) for a period normally taken within 5 to 15 times the packet generation interval; after that time, the same resource is kept for another random interval with probability \(p_{\text{k}}\) (set by the operator within 0 and 0.8) and changed otherwise. At the PHY layer, the resources are monitored within a window of 1 s, comparing the received power measurements with a given threshold and reading sidelink control information (SCI) messages that advertise future reservations. Among the resources that are assumed not used in the next packet interval (or a portion of it), the 20% less interfered are selected and passed to the MAC. If less than 20% resources are estimated free, the mentioned threshold is increased by 3 dB and the process is repeated.