# Topology Duration Optimization for UAV Swarm Network under the System Performance Constraint
## 1 Introduction  
The high-speed development of distributed collaborative techniques represented by multi-node joint detection, high dynamic networking, distributed cooperative control and intelligent decision making, has promoted the application of unmanned aerial vehicle (UAV) evolved from early single or small-scale to large-scale swarm, namely, UAV swarm system. A UAV swarm is the combination of not only the multiple UAV platforms, but also the discrete functionality of distributed notes. Multiple discrete UAVs are organized and coordinated in a distributed and collaborative operation with the goal of accomplishing the same task [1]. With outstanding advantages of distributed multifunctional fusion, rapid dynamic reconfiguration, strong survivability and expansion, and high task effectiveness, UAV swarm systems has been extensively applied in various field [2; 3; 4; 5; 6; 7; 8; 9; 10]. In the UAV swarm system, a large number of UAVs are interconnected to each other by establishing UAV-to-UAV wireless link, which integretally forms a UAV swarm network. In fact, the swarm network is the foundation of collaboration between UAV nodes, and its performance directly affects the effectiveness of information interaction, joint computation, cooperative control and other aspects of the system [11; 12; 13]. One of the most important designs for swarm network is the construction of network topology, which can be described as the connection state between UAV nodes constructed based on wireless links. Many researches on the construction methods of swarm network topology have been proposed. Generally, according to different construction criteria, we can classify the construction methods of network topology of swarm system into three categories: (i) under the criterion of topology construction speed [14; 15; 16]: the first category is the methods aiming at topology construction speed, which mainly focuses on the low complexity and fast convergence of algorithms, but considers less about the system performance after construction; (ii) under the criterion of energy consumption [17; 18; 19; 20; 21]: based on the consideration that UAV swarm is sensitive to energy consumption, the second category takes node energy consumption as the optimization target in the step of topology construction; (iii) under the criterion of network performance [22; 23; 24]: the network topology is essentially constructed to serve the network interaction of UAV nodes, so the third category of methods perform the topology construction in terms of the network performance (network throughput, delay, etc.) under the constructed topology. However, another important criterion, i.e., the stability of swarm network topology, has not been fully considered. Due to the variability of task objectives and its own dynamic attributes of a swarm system [25], the relative position or motion relationships of UAV nodes change frequently, which results in dynamically changed swarm network topology [24]. This will lead to high frequency maintenance or reconfiguration of network topology. While frequent maintenance or reconfiguration will increase the network overheads and energy consumption of swarm system, the interruption of network topology will cause a sharp decrease in the efficiency of collaborative tasks. Besides, an unstable network topology will generate isolated offline nodes, which will bring extra difficulty to the flight control of UAV swarm. Therefore, a reliable swarm network topology has a vital influence on the performance of the swarm network and even the operation of whole swarm system. It is worth mentioning that the network topology stability is considered in reference [17; 18; 19; 20; 21]. To be more precise, most of researches considered enhancing topology stability from the view of reducing the energy consumption of nodes (improving the node survival time).