# A Hybrid Route Selection Scheme for 5G Network Scenarios: An Experimental Approach
### Why Is a Hybrid Route Selection Scheme Crucial to 5G Networks?  
While separate centralized mechanisms [6; 7; 17] and distributed mechanisms [18; 19; 20] for route selection are shown to improve network performance, the benefits of combining both mechanisms are yet to be discovered in the context of 5G, and so a hybrid route selection scheme is the focus of this paper. Intuitively, a hybrid route selection scheme can address the intrinsic limitations of each mechanism. In the centralized mechanism, the computational complexity and routing overhead increase with the number of nodes in the network (or node density) as each source node in the _data plane_ must receive information from the CC and then discover and maintain routes (e.g., update the routing table) [9; 21]. In the distributed mechanism, the routing overhead increases as each node in the _data plane_ must report the information availability (i.e., the available channels and the bottleneck link rate) with neighboring source nodes [12; 22; 23]. The issues observed in both centralized and distributed mechanisms intensify with the increased dynamicity of channel availability, network heterogeneity, and ultra-densification. Our proposed hybrid mechanism uses both centralized and distributed mechanisms. The distributed mechanism performs the traditional centralized tasks, particularly those using highly dynamic information in a distributed manner to minimize high computational complexities at the CC. Naturally, the routing overhead reduces because BSs and nodes do not send dynamic information (e.g., the traffic level) to the CC. The centralized mechanism performs the traditional distributed tasks, particularly those requiring frequent information exchange among BSs and nodes in a centralized manner to minimize high routing overheads at BSs and nodes. Naturally, the routing overhead reduces because BSs and nodes do not exchange dynamic information among themselves.  
CenTri consists of _centralized_ and _distributed_ mechanisms for route selection. The centralized mechanism is embedded in a CC in the control plane, gathers and maintains network-wide and less dynamic information (i.e., available routes), and selects routes across different layers (i.e., the macrocell and small-cell layers). Subsequently, the source node makes routing decisions based on the list of routes provided by the CC. The routing decisions prioritize routes with fewer PUs in the data plane and D2D communication. The purpose is to minimize SU interference to PUs at the global level. In addition, being more stable, the routes can be established in a proactive manner to serve as backbone routes for different source-destination pairs, contributing to a reduced routing overhead required in the route discovery and maintenance. The distributed mechanism, which is embedded in each BS and node in the data plane, gathers and maintains neighborhood (or local) and more dynamic information (i.e., available routes and traffic levels of nodes in range), and selects intra- or inter-cell routes in the small-cell layer. The purpose is to maximize traffic offload from BSs, particularly the MC BSs, at the local level. Being more dynamic, the routes can be established via D2D among BSs and nodes in the small-cell layer in a reactive (or on-demand) manner to offload traffic from the macrocell layer.