# Three-Dimensional Resource Allocation in D2D-Based V2V Communication
## 1 Introduction  
Vehicular communication has received a great deal of attention from the research community in the recent past. The Dedicated Short-Range Communication (DSRC)-based Vehicular Adhoc+82 Networks (VANETs) have been thoroughly researched, and with the advent of Long-Term Evolution (LTE) Release 12 and onward, the research community's focus has been shifted to LTE-based vehicular communication, or Long-Term Evolution Vehicle to Everything (LTE V2X), specifically. LTE V2X, which is based on Device-to-Device (D2D) proximity-based services (Prose), is said to be a major cornerstone for the establishment of an Intelligent Transport System (ITS) and will facilitate autonomous driving in the future. D2D communication, which is the enabler of V2X, has a large number of applications in terms of increasing spectral efficiency, cellular offloading, emergency communication, and IoT enhancement, to name a few. It allows devices in the proximity of each other to communicate directly with each other, with or without the support of the infrastructure. The operating modes in D2D communication are divided into inband and outband. Inband allows the devices to communicate in the licensed spectrum in which the cellular users are the primary users, whereas D2D users become the secondary users. Inband D2D communication has mostly been used in recent research due to its better control over the licensed spectrum. In contrast to the inband communication, outband communication allows the devices to communicate in an unlicensed spectrum. Furthermore, both inband and outband are divided into underlay and overlay modes. With underlay communication, both D2D and cellular usersshare the whole spectrum, whereas for overlay communication, D2D users use a fraction of channels, and another fraction is used by cellular users. The underlay mode offers better spectrum utilization compared to the overlay mode. In terms of D2D-based V2X, the applications can vary under different communication scenarios. Vehicle-to-Vehicle (V2V) communication is mostly used for safety purposes. Vehicle to Infrastructure (V2I) and Vehicle to Network (V2N) are used for non-safety infotainment services and cloud-based storage services, respectively. Vehicle to Pedestrian (V2P) is again used for the safety of pedestrians and cyclists in the vicinity of the vehicles. Most of the research related to D2D is based on (but not limited to) resource allocation, power control, interference management, security, and peer discovery [1]. Although there have been several recent papers on the D2D resource allocation problems, there are still limited papers on resource allocation in D2D-based vehicular networks. Since the requirements of vehicular user equipment are completely different from those for cellular users, several restrictions are imposed on finding an optimum solution to the resource allocation problem. One such restriction is the mobility of vehicles, which makes it hard to track the channel state information (CSI) by the base station (BS). The major contributions of this paper are as follows:  
* Power allocation for the three types of users--cellular user equipment (CUE), V2V, V2I--keeping in mind the constraints and capacity calculation using 3-dimensional matching in graphs. * A hypergraph coloring-based RB allocation using an adjacency matrix. Recently, several articles have been published for resource management in D2D. Several factors have been put forward for the design of an efficient resource allocation algorithm for D2D communication. The authors in [1] have given an in-depth review of the resource management techniques in D2D communication from an optimization perspective and presented a methodical way of first identifying the problem and constraints and then applying the relevant optimization technique. Also, in [1], the three major techniques used for resource allocation were determined: game theory, graph theory, and heuristic approaches. One aspect of resource management is the selection of the mode between D2D and cellular users.