# Device-To-Device Communication in 5G Environment: Issues, Solutions, and Challenges
## 1 Introduction  
New technologies fundamentally change the way people exchange information with each other, especially in wireless communication and mobile computing. Despite this, cellular mobile environment is still infrastructure-dependent. The mobile users' connection is restricted depending on the base station (BS) coverage, and does not permit direct communication among mobile devices [1]. Even though the source and the destination are in close proximity to each other, the routing traffic is routed through the core network. Due to this inability, the possibility of exchanging data among mobile users is limited, especially considering the transformation process of personal computing from desktop computers to laptops and finally to mobile devices. Due to the trend of shifting toward mobile devices, the traffic for mobile data is expected to increase to 30.6 exabytes per month by 2020, estimated an eight-time increment over 2015 [2]. New communication technologies must be capable of exchanging data on-demand over proper network connections, and must be able to scale the network capacity. Device-to-device (D2D) communications are considered to be a promising technique that allow mobile devices to communicate with one another directly without going through access points or BSs [1]. There are several studies that have been conducted to analyze the use of D2D in cellular networks. For example, the authors in [3] proposed a machine-learning-based code dissemination (MLCD) scheme to reduce the costs of ground control stations (GCSs) arising from the need to organize vehicles in large quantities as code disseminators. The scheme works by selecting the higher coverage ratio and reliability of vehiclesas code disseminators. [PERSON] et al., in [PERSON] _et al._ (2018), reduced the service response delay and redundancy of data by proposing a service orchestration and data aggregation framework (SODA) that is able to provide services in organizing the data and combining data packets. However, a conventional cellular system requires all that communication to go through the BSs and does not allow devices to communicate directly with one another ([PERSON] and [PERSON], 2018). D2D aims to use the communicating devices' physical range to increase the signal for mobile devices in a sparse environment. To complement each other, D2D communication should work together with cellular networking services. The critical factor that must be considered when designing D2D is resource-sharing, in terms of energy and spectrum, between D2D and cellular communication ([PERSON] and [PERSON], 2018). For example, the authors in ([PERSON] and [PERSON], 2018) use a security disjoint routing-based verified message (SDRVM) scheme to set up two separate connected dominating sets (CDSs), which are v-messages and data CDSs. The sender nodes send data packets and the v-message to destination nodes at the same time using data CDS and v-message CDS. Only when the sensor nodes' energy is high enough to improve the successful arrival ratio of data will the number of data packets that are retransmitted be increased. One of the benefits derived from D2D is content privacy and strong anonymity. These are provided because the central storage is not responsible for storing the shared information. D2D communications also have the potential to improve energy efficiency, throughput, fairness, and delay ([PERSON] and [PERSON], 2018). Furthermore, by enhancing the system throughput and spectrum reusage due to the D2D traffic direct routing, the performance of D2D improves significantly ([PERSON] and [PERSON], 2018). D2D offloads the cellular traffic by switching the path from infrastructures to direct transmissions. These attributes produce low transmission delay, energy savings, and a high data rate ([PERSON] and [PERSON], 2018).