Abstract-Unmanned Aerial Vehicles (UAVs) have enormous potential in the public and civil domains. These are particularly useful in applications where human lives would otherwise be endangered. Multi-UAV systems can collaboratively complete missions more efficiently and economically as compared to single UAV systems. However, there are many issues to be resolved before effective use of UAVs can be made to provide stable and reliable context-specific networks. Much of the work carried out in the areas of Mobile Ad Hoc Networks (MANETs), and Vehicular Ad Hoc Networks (VANETs) does not address the unique characteristics of the UAV networks. UAV networks may vary from slow dynamic to dynamic; have intermittent links and fluid topology. While it is believed that ad hoc mesh network would be most suitable for UAV networks yet the architecture of multi-UAV networks has been an understudied area. Software Defined Networking (SDN) could facilitate flexible deployment and management of new services and help reduce cost, increase security and availability in networks. Routing demands of UAV networks go beyond the needs of MANETS and VANETS. Protocols are required that would adapt to high mobility, dynamic topology, intermittent links, power constraints and changing link quality. UAVs may fail and the network may get partitioned making delay and disruption tolerance an important design consideration. Limited life of the node and dynamicity of the network leads to the requirement of seamless handovers where researchers are looking at the work done in the areas of MANETs and VANETs, but the jury is still out. As energy supply on UAVs is limited, protocols in various layers should contribute towards greening of the network. This article surveys the work done towards all of these outstanding issues, relating to this new class of networks, so as to spur further research in these areas.Index Terms-Unmanned Aerial Vehicle, UAV, Multi-UAV Networks, ad hoc networks, communication networks, wireless mesh networks, software defined network, routing, seamless handover, energy efficiency
I. INTRODUCTION
A. The growing importance of UAV networksUnmanned Aerial Vehicles (UAVs) are an emerging The latter ones are proving to be quite useful in civilian applications. As described by Daniel and Wietfeld in [1] they are likely to become invaluable inclusions in the operations of police departments, fire brigades and other homeland security organizations in the near future. Besides, advances in electronics and sensor technology have widened the scope of UAV network applications [2] to include applications as diverse as traffic monitoring, wind estimation and remote sensing [3].In this context it would be relevant to mention that the current FAA guidelines allow a government public safety agency to operate unmanned aircraft weighing 4.4 pounds or less, within the line of sight of the operator; less than 400 feet above the ground; during daylight conditions; within Class G airspace; and outside of 5 statute miles from any airport, heliport, seapl...
