# DRL-Based Load-Balancing Routing Scheme for 6G Space-Air-Ground Integrated Networks
## 2 Related Work
### LEO Satellite Routing  
The LEO satellite constellation can be divided into two categories, namely the inclined-orbit-based Walker Delta constellation and the polar-orbit-based Walker Star constellation. As shown in Figure 1, the Iridium constellation [9], a typical polar-orbit constellation, has been studied by many scholars as a low-orbit constellation because of its representative constellation configuration and easy mathematical model construction.  
The orbit and topology of the Iridium constellation are shown in Figure 2. The topology changes rapidly due to the mobility of the satellites and the changing connections. The satellites running in the reverse orbit cannot establish communication links. In addition, the communication links change when the satellites travel across the pole. Due to the  
Figure 1: Illustration of the Iridium constellation.  
challenges introduced by the dynamic topology, the routing algorithm for satellite networks has attracted a lot of research interest. This field of work mainly falls into the following two kinds, the centralized routing algorithm and the distributed routing algorithm.  
The distributed routing algorithm can adapt to the dynamic scenario of the satellite network. This is because the algorithm determines the next hop based on the status of the neighbor satellites, e.g., the remaining bandwidth and the queue utilization rate. Thus, when the status of the neighbor satellites changes, the algorithm can become aware of it rapidly and make a quick decision on the routing strategy according to the dynamic environment. By taking the design idea of the existing ground-distributed routing algorithm into account, the authors in [10] investigated the routing methods for the satellite network. The onboard buffer space was improved by fully considering the features of the LEO satellites. The data packets were classified, and the corresponding routing method was designed in [11].  
Different from the distributed routing algorithm, the centralized one needs to derive the global information of the satellite network [12]. The master control nodes first collect the global information and then perform the routing path computation. After obtaining the routing results, they transmit the whole routing strategy to the other nodes. The authors in [13] designed an improved Distributed Hierarchical Routing Protocol (DHRP) for the satellite network. This protocol set up master and candidate nodes and therefore showed superior routing performance compared with the traditional Discrete Relaxation Algorithm (DRA). The authors in [14] proposed a Hybrid Global-Local Load Balancing Routing (HGL) algorithm. However, it was ineffective when the large-scale traffic flows changed suddenly. The authors in [15] proposed a probability ISL Routing (PIR) algorithm in which the communication delay was leveraged to evaluate the path selection performance. This algorithm also considered the cost of the inter-satellite links.  
Although the aforementioned algorithms have made great progress in the adaptability to the dynamics of the LEO satellites, their failure to consider the satellite load remains a significant drawback.