# Smart Bandwidth Assignation in an Underlay Cellular Network for Internet of Vehicles
## 2 Literature  
D2D communications have been proposed to allow nearby users to communicate with each other without passing through the infrastructure. It has enabled the devices to transmit at higher data rates, with lower power consumptions, lower latencies and improved spectral efficiency. Based on these advantages, the applicability of D2D techniques for V2V communications has been analyzed in different works, such as [7, 8]. [PERSON] et al. [8] evaluated the suitability of existing D2D techniques for V2V communications and concluded that, in order to meet the stringent QoS of ITS applications, a careful adaption of the legacy D2D concepts is necessary to exploit them in the road safety domain. A similar understanding prevails among the 3 GPP community. As a consequence, in December 2015 a WI (Work Item) was approved [9] whose primarily goal was to standardize V2V system operating on the basis of LTE sidelink. D2D, specified in 3 GPP Release 12 and 13, has been chosen to serve as a core technology but numerous areas for enhancement have been identified with the aim to fulfill V2V requirements. The most straightforward aspects that distinguish V2V from the pure D2D are related to the V-UEs' velocity, the density of V-UEs and to the latency constraints. For example, one of the objectives of the V2V WI was to develop a solution capable of supporting relative velocities of up to 500 kph what inevitably implied changes to the physical layer structure. To cater for such rigorous requirements, reference signals' density in time has been doubled in comparison to the legacy D2D. Other issues addressed in the course of this WI include, e.g., partitioning the relevant area into the zones with associated resource pools, targeted at minimizing the interference level and mitigating the \"near-far\" effect, or standardizing SPS (Semi-Persistent Scheduling) for sidelink transmissions in order to effectively serve a periodical part of V2V traffic. According to the official time-line, the WI is on the verge of completion (i.e., approval at September's 3 GPP RAN Plenary meeting), at least with respect to the feature subset concerning UE autonomous resource selection (so-called \"Mode 2\"). Inside the extensive research carried out in the context of traditional D2D systems, e.g., [10, 11, 12] to refer to a few relevant papers, the work presented in [13] offers an excellent introduction to the underlay RRM problem. In this paper, authors propose an RRM solution which performs admission control and power allocation for each admissible D2D pair and its potential C-UE partners. Once they get all the admissible pairs and their corresponding power levels, they apply a maximum weight bipartite matching to define the final pairs. Focusing our attention on V2X communications, [PERSON] et al. [7] study the coexistence of V2I and V2V communications in a D2D underlay mode and conclude that D2D in ITS exhibit transmission rate advantage with respect to the traditional V2V-only mode, the V2I-only mode, or the V2V overlay mode. Nevertheless, only a few studies have applied this underlay mode possibility to vehicular environments [7, 8, 14, 15, 16, 17]. [PERSON] et al. [14] solve the interference problem originating from the reuse gain by dividing the cell coverage area into zones and assigning a pool of resources to each zone. With such approach, they guarantee a maximum acceptable interference level caused by C-UE in the V2V underlay. However, it suffers from a lack of scalability, it does not consider QoS requirements and its performance depends entirely on the zone definition and associated pool assignment. [PERSON] et al. [15], instead of dividing the coverage area into zones and assigning pools, group together the road vehicles into multiple clusters. In their approach, each uplink resource can be allocated to only one cluster, that is, the number of resources that are reused by vehicles is in fact equal to the amount of clusters.