# An Experimental Study on D2D Route Selection Mechanism in 5G Scenarios
## 1 Introduction  
Fifth generation (5G) is a promising next-generation cellular mobile network that aims to enhance network performance, including higher quality of service (QoS) (e.g., ultra-low latency, and high throughput), higher reliability (e.g., better network connectivity with lower network congestion), and larger coverage, to cater for future network requirements, particularly handling a massive amount of data transmission and heterogeneous devices. 5G is equipped with new features, including device-to-device (D2D) communication and small cell (SC) e.g., femtocell (_fc_) deployment that help to increase network capacity to cope with ultra-dense networks that consist of a massive number of users and amount of data. Nevertheless, the new network requirements and features have brought some new issues,including higher co-channel interference among SCs [1]. Hence, 5G consists of a control plane, which is managed by a central controller (CC) in the core network that manages and optimizes network resources [2; 3]. For instance, the CC manages secondary users' (or SUs') (or unlicensed users') access to white spaces, which are underused licensed channels (or cognitive channels) owned by the primary users (or PUs) (or licensed users) [4]. Each node can access both cellular channels (in which the node is the PU) and cognitive channels (in which the node is the SU). D2D enables direct communication between two nodes without traversing the base station (BS), particularly the macrocell (MC) BS (or a central controller) that has the largest coverage in 5G networks [5] using D2D communication, a node can access both cellular and cognitive channels; as a result, it improves spectrum efficiency and QoS [6; 7]. Theoretical analysis has been conducted in [1; 4; 8; 9]. As an example, in [8], the handover process is investigated to determine the optimal number of SCs in the network. Data streams can be transmitted through either SCs or macrocells (MCs). Although a higher number of SCs in the network may seem to offload traffic from MCs and reduce the congestion level of MCs, it increases interference among SCs and the need for data transmission through MCs. As another example, in [4], a routing algorithm is proposed to select routes based on the CC's network-wide information, including intermediate nodes, the traffic of each link, and the number of hops of a route, to reduce interference among routes and achieve better load balancing. Meanwhile, simulation has been conducted in [3; 4; 9; 10; 11; 12]. In [3; 12], traffic offload from the CC and backbone routes [Backbone route is not defined yet prior to this] to SC nodes has shown to reduce access and end-to-end delays. In [9], a multihop D2D communication to deliver a high-quality video stream to a destination has shown to increase network capacity at the expense of a higher end-to-end delay. In [10], a virtual BS is proposed to determine the available channels, so that a source node can use this information to reduce the delay of a route. In [11], CC manages D2D communication among user devices in different channels, and it has shown to improve data transmission. Nevertheless, there is lack of experimental study using a testbed. In this paper, an experimental study is conducted to investigate route selection in the presence of PUs in the network layer of a 5G scenario. The testbed consists of universal software radio peripheral with GNU radio (USRP/GNU radio) [13] connected to Raspberry Pi3 B+ (RP3) [14], which serves as the decision-making engine at a source node. Equipped with RP3, our testbed is first of its kind to remove some significant assumptions used in previous testbeds, such as the use of a single processing unit (i.e., a personal computer, PC) via a switch, in route selection investigations. The CC receives PUs' activities information from source nodes and ranks available routes based on this information (or the number of white spaces) which is then informed to the source node.