# Opportunistic Multi-Technology Cooperative Scheme and UAV Relaying for Network Disaster Recovery
## 3 Multi-Technology Cooperative Communication and Drone Data Relaying  
### Multi-Technology Network Architecture  
This work proposes COPE, a cooperative communication scheme that leverages mobile devices (here, each device (or node) is supposed to be owned by a survivor, therefore, the two terms are used interchangeably in the rest of the article) involving multiple network technologies and characterized by various energy levels. The considered network environment consists of mobile devices equipped with multiple network technologies, such as those available in off-the-shelf smartphones (e.g., Bluetooth, WiFi, and cellular). These technologies are characterized by different transmission ranges and energy consumption characteristics [17]. Accordingly, a multiple-tier architecture is spontaneously created by opportunistically grouping devices capable of reaching each other directly (i.e., in a single hop) into clusters, as illustrated in Figure 1. The devices in each tier all use the same communication technology, and tiers are layered depending on their features. In particular, the lowest tier is the one with the most energy-efficient communication technology, but also with the shortest range. The highest tier is the one with the most energy-hungry communication technology, which also corresponds to the highest range. Intermediate tiers are made by increasing levels of energy efficiency and decreasing transmission ranges. The proposed network structure is flexible enough to include a varying number of tiers. However, the figure shows a network composed of three tiers, corresponding for instance to communication technologies existing in nowaday mobile devices such as smartphones; i.e. Bluetooth (\(n_{1}\)), WiFi (\(n_{2}\)) and cellular (\(n_{3}\)) communication technologies. This is also the most realistic option in practice, given currently available smartphones.  
One node in each cluster is designated as cluster-head (CH). The CH is the node that is able to act as a bridge between different tiers: it collects data from one tier and relays them to the upper tier. The CHs in the highest tier communicate directly with a drone that hovers over them. For example, node \(s_{4}\) in Figure 1 is a CH for the cluster that includes nodes \(s_{5}\) and \(s_{6}\) in the \(n_{1}\) tier. Instead, node \(s_{2}\) is a CH in three clusters: the one that includes nodes \(s_{1}\) and \(s_{3}\) in the \(n_{1}\) tier, and the one that contains node \(n_{4}\) in the \(n_{2}\) tier.  
In addition to the mobile devices, the network also includes drones that are sent on-demand to the area of the disaster. In particular, drones are equipped with the same technology that constitutesthe highest tiers. For instance, they can be equipped with on-board femto-cells, and provide ad-hoc cellular communication to the nodes in the highest tier [18; 19]. In particular, a drone makes a tour of the network by reaching certain designated locations, where it collects data from one or more nodes, depending on the specific path planning algorithm employed (refer to Section 5 for more details). Going back to the previous example, node \(s_{2}\) is the only one able to communicate with the drone in the \(n_{3}\) tier among all nodes in the clusters it belongs to.