# Load-Balancing Routing for LEO Satellite Network with Distributed Hops-Based Back-Pressure Strategy
## 2 Related Works  
In the early stage of the development of the LSN, due to the low traffic load, researchers assumed that communication delay was mainly determined by propagation delay. Most of the routing strategies focused on minimizing end-to-end propagation delay. However, with the increase of network traffic load, queuing delay in satellites becomes an important part of communication delay. An effective routing policy must prevent traffic from passing through congested nodes. Load-balancing routing can be classified into global information-based and local information-based routing. The former mainly includes satellite networks link state routing (SLSR) [14], agent-based load-balancing routing (ALBR) [15], a load-balancing routing algorithm based on congestion prediction (LBRA-CP) [9], and state-aware and load-balanced routing (SALB) [8], etc. SLSR, ALBR, LBRA-CP, and SALB all belong to single-path traffic balancing schemes, which can only allocate traffic to other paths with a lower link cost when the traffic is heavy, but cannot fundamentally eliminate congestion. [PERSON] et al. [16] proposed network coding-based multi-path cooperative routing (NCMCR), which employs multi-path cooperative routing based on network coding so that different parts of data flow are delivered along multiple disjoint paths. Traffic on each path is allocated based on path queuing delays. [PERSON] et al. [17] proposed two forwarding strategies based on alternate link routing (ALR) to divert traffic from the shortest path to the alternate shortest path. Different from ALR, compact explicit multi-path routing (CEMR) [18] uses an orbital speaker mechanism to periodically collect and exchange the link state information of the whole network, and calculates \(k\) shortest paths for data transmission. Through the PathID coding scheme, the balance of traffic load in the satellite network is supported with a lower signal overhead. Although ALR and CEMR utilize multiple paths for traffic distribution, the traffic on each path is allocated randomly. The congestion on the diversion link may be exacerbated after traffic distribution. Since the state of other links is not fully considered, it is difficult to achieve the desired equalization effect. Different from the global strategy, local load-balancing makes routing decisions only based on local information, mainly including Priority-based Adaptive Routing (PAR) [19], Explicit Load-Balancing (ELB) [20], Traffic-Light-Based Intelligent Routing (TLR) [21], etc. Most of the above load-balancing algorithms carry out the transmission rate or link adjustment after the onset of congestion. To address this problem, researchers proposed load balanced routing based on congestion prediction [22], including Hybrid Global-Local load-balancing Routing (HGL) [23], etc. HGL decomposes satellite network traffic into a predictable large-scale baseline and a series of unpredictable small-scale random fluctuations. Aiming at the large-scale traffic baseline, HGL adopts a global balancing strategy to actively distribute traffic across the whole network. After that, in order to deal with the sudden congestion caused by small-scale random fluctuations, HGL uses a local balancing strategy to quickly adjust routes and alleviate congestion. Therefore, there are limitations in both global and local strategies. In this work, a low cost and scalable distributed routing scheme is designed to enhance the load-balancing ability of the LSN. [PERSON] et al. [24] proposed a low computation complexity routing algorithm (LCRA). LCRA selects the less congested next hop according to the congestion information of neighboring satellites, which improved delay and data delivery ratio. BP routing, which adopts the congestion gradient of packet queues between nodes to deal with dynamic routing traffic and stabilizes the random network in the process of routing and resource allocation, is widely used in multi-hop networks. [PERSON] et al.