# Underlay Loosely Coupled Model for Public Safety Networks Based on Device-to-Device Communication
## 2 Related Works  
To minimize the impact of possible cellular network overload, or coverage limitation, the research community and the industry have recently considered the deployment of Unmanned Aerial Vehicles (UAVs) during a rescue mission. UAVs can play a fundamental role in supporting new value-added services and applications. Among these, we can mention remote sensing in surveillance operations [4], collaborative search and rescue [5; 6], building aerial sensor networks that aid disaster management [7], and supporting backbone communications to mobile ground stations [1].  
Instead of using UAVs, the author in [8] proposes the deployment of a vehicle-mounted mobile base station to provide services like file distribution among group communications over heterogeneous architecture with the aid of a group Application Server and Proximity Service links, as illustrated in Figure 1. In this solution, a vehicle-mounted mobile base station is placed close to the disaster area and works in a cell-breathing manner to expand or shrink the cell coverage periodically to support group communications. Even though the proposed solutions can provide effective support to extend/minimize the overload of the cellular network, the deployment of land and air vehicles is limited to accessible disaster areas, and their commissioning may take additional delay not usually suited for emergency situations.  
In [9], the author uses the multi-hop function, where the device can play the role of the relay to connect out-of-range devices using a D2D connection. This device-based solution can extend networks using multi-level relay functionality. However, this does not resolve the overload problem since the deployed relays will only connect the out-of-coverage device to the access point.  
In [10], the author creates a virtual mesh networking using multi-hop D2D, where routing and packet forwarding are split into two separate mechanisms in the control plane and the user plane, respectively. In this solution, base stations collect links and topology information from terminals and then form a virtual mesh network among terminals accordingly. On the other hand, the terminal should establish and maintain a direct connection with its neighbors. This can prove very costly in terms of device autonomy. Moreover, the terminal does not participate in the decision to choose its neighbors, which complicates the task for the central server.  
Figure 1: Overview of some solutions for extending cellular coverage.  
It is possible to build an optimized underlay communication model that allows each participant to maintain a minimal number of links with its neighbors and to assist the central server during link management. ProSe technology allows UEs in proximity to discover themselves and even have direct communication as soon as telecom operators enable the public safety mode. Direct links are then created between UEs rather than having their radio data (user plane) travel through the base station (BS) or over the core network. D2D communications can effectively improve overall throughput and minimize packet latency/ jitter as well as optimize radio spectrum utilization and even energy consumption. The next section explores how to use D2D and ProSe technology to build underlay networks.