## 1 Introduction
#### Objectives  
The goal of this work is to motivate the use of multi-UAV systems by a well-informed study of the constraints related to their implementation. We offer useful suggestions on how to choose the type of UAV, the communication architecture, as well as the type of protocol. These suggestions are based on a multi-criteria survey that considers the complexity of the mission, the environment, and the capabilities of the UAVs used. This work can serve as a background for all types of projects involving the use of multi-UAV systems. In this paper, we formalize the problem of collaborative target detection in large-scale environments. We conduct a comparative analysis of existing communication architectures and routing protocols for the efficient deployment of a swarm, especially for complex missions. The following are the key contributions.  
1. Types of UAVs: We review existing drones. The UAVs are classified into four categories according to their shape and size.
2. Problem formulation: We start by identifying the key features that allow us to recognize a multi-UAV ecosystem. We emphasize the complexities and major challenges associated with the collaborative target detection problem. Then, we propose a mathematical formulation of the problems of k-partition of the environment and path planning to reach the targets. The proposed models are based on multi-objective functions that satisfy the constraints of the workload balancing of the swarm and the optimization of the paths.
3. Architectures and protocols: We give cutting-edge knowledge on multi-UAV communication architectures as well as the existing routing protocols. Then, we evaluate the performance and draw a comparison for each communication architecture and routing protocol listed in this paper.
4. Suggestions: Based on the comparisons of the existing architectures and routing protocols, we give some suggestions for choosing the right architectures and routing protocols that are tailored to a well-defined mission.
5. Open challenges: We discuss the ongoing issues and potential future directions, such as collaborative decision making, communication infrastructure, spatial organization, and regulation for multi-UAV deployment.  
**Organization** The rest of this article is structured as follows. Section 2 provides useful information on multi-UAV ecosystems. Subsequently, in Section 3, we perform a comparative study of the types of autonomous uncrewed aerial vehicles. Based on this study, we highlight the advantages and drawbacks of different types of UAVs. In Section 4, we present formal definitions and mathematical explanations about the problem of collaborative target identification. Furthermore, in Section 5, we provide the state of the art of some applications of multi-UAV systems in the civilian domain. Meanwhile, in Sections 6 and 7, we discuss the existing communication architectures and provide a roadmap that helps to choose the most adapted. Similarly, Sections 8 and 9 provide a comparison of routing protocols, followed by a roadmap of how to choose the right protocols. Section 10 discusses the open difficulties and potential directions. Finally, in Section 11, we conclude this work.