# Underlay Loosely Coupled Model for Public Safety Networks Based on Device-to-Device Communication
###### Abstract  
In several emergency situations, during natural or human-caused disasters, frontline responders need to be able to communicate and collaborate to properly carry out relief missions. Some countries build their national Public Safety Mobile Broadband based on cellular LTE technology to provide fast, safe, and secure emergency services. However, in several emergency situations, cellular antennas can be overloaded or partially damaged in a manner that affects group communication services. In the last few years, direct device-to-device (D2D) communications have been proposed by the 3 GPP as an underlay of long-term evolution (LTE) networks based on proximity, reuse, and hop gains. This paper focuses on a loosely coupled model based on direct D2D communication in a public safety context. Many scenarios related to user membership and network management are detailed. Both the \"less cost\" and \"optimized tree\" approaches are proposed and implemented, and their performance is evaluated in terms of the network update number and the resulting average Channel Quality Indicator (CQI). Other optimization approaches, with different CQI thresholds and optimization interval parameters, are simulated to compare their performance with the \"optimized tree\" approach. By conducting simulations that combine a CQI threshold = 1 and optimization interval = 2 s, it becomes possible to keep an average CQI level close to the \"optimized tree\" approach, while the costs related to network updates significantly decrease by almost 35%. Other simulations are also carried out to measure the bandwidth required by the control messages between the server and active users. It was found that both inbound and outbound traffic on the server side can be well supported with LTE and 5G networks. public safety network; D2D; proximity service; underlay networks; loosely coupled model; network simulation +
Footnote †: journal:  
0000-0001-6697-22000/00000-0000-0000-0000-00000-0000-00000-00000-00000-0000-00000-  
for example. Therefore, attention is paid to the use of cellular long-term evolution (LTE) technology and beyond as access network support for public safety. Such a solution is known as Public Safety Mobile Broadband (PSMB), like FirstNet in the United States, the Emergency Service Network (ESN) in the United Kingdom, or SafeNet in South Korea [2]. The operating principle of PSMB is to allocate a wide and low-frequency band (in the 700 MHz range) that will be reserved exclusively for emergency situations. However, in many disaster scenarios, the proposed frequency band can be saturated, or the affected area might not be covered by 4G/5G networks. Moreover, deploying such solutions is restricted to only registered professional first responders, and there is no means for victims or disaster witnesses to take part or to collaborate by identifying survivors and their location, for example. With the aim to provide extended LTE coverage to a large geographic area, some PSMB solutions, like FirstNet, deploy aerostats from 300 m height that can cover roughly 360 km\({}^{2}\). However, the slowness of aerostats' movement speed (10 km/h) to enter the disaster area, combined with their sensitivity to wind, means their deployment is not usually obvious. Some complementary PSMB mobile-vehicle-based solutions might also be deployed directly in the disaster area to create/extend coverage or even to increase existing network capacity using drones, light trucks, or minivans that can carry equipment based on both LTE and Satellite. However, most emergency situations require urgent action for rapid communication establishment between responders. The proposed solutions are not usually easy to deploy since heavy logistics tasks are required and a considerable availability of qualified human resources is necessary to drive or command land/air vehicles.