A Clustering Routing Algorithm Based on Improved Ant Colony Optimization for an Urban Rail Transit Ad Hoc Network  
[PERSON] School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, China; [EMAIL_ADDRESS] (Z.S. ); 22125031@bjtu. deployment and maintenance costs of trackside equipment and the core network. Based on this, decentralized networks have recently been researched. An ad hoc network is a typical decentralized network formed by wireless nodes that can communicate without pre-existing infrastructure. They have been widely used in the internet of things, internet of vehicles, and satellite internet. The train autonomous circumambulate system (TACS) is a development direction of the urban rail transit system, which simplifies the trackside equipment and adopts the intelligent onboard controller as the core, achieving the autonomous and safe operational control of trains [6]. Therefore, it is necessary to utilize the ad hoc network to optimize the vehicle-ground network. By arranging communication nodes reasonably, key areas such as turnouts and curves can be covered, and essential information can be exchanged between each node directly, which realizes the robust and dynamic networking of trains, trackside nodes, and station nodes. Compared with the traditional urban rail communication network, the ad hoc network has the following advantages:  
1. Decentralized network. Trains and trackside equipment can communicate directly under the ad hoc network without being forwarded by the central nodes;
2. Lower cost. Construction and maintenance costs of the base station and the core network can be reduced due to the simplification of the infrastructure, and the cost of deploying cables can also be reduced because of the wireless communication between each trackside node;
3. Lower time delay. Direct communication between trains and trackside equipment can reduce the time delay of forwarding;
4. Strong network robustness. Because of the cancellation of central nodes, damage to some nodes will not seriously affect the whole network's performance, leading to the network having strong survivability. Ad hoc networks can be divided into flat networks and hierarchical clustering networks [7]. Flat networks have a peer-to-peer structure, making it difficult to uniformly manage and maintain each node with the movement of nodes in an extensive network. Therefore, the flat topology results in the easy interruption of network links. In addition, each node under this topology maintains routes to all other nodes, resulting in high overheads and low network scalability. Under the hierarchical clustering structure, the network is separated into groups called clusters. Each cluster contains one cluster head node and several member nodes. This architecture has two levels: all cluster head nodes constitute the upper-layer network, and normal nodes constitute the lower-layer network. The upper-layer network is responsible for forwarding the data and managing the nodes of the lower-layer network. The clustering structure has the following advantages: (1) Only the cluster heads form the backbone network, rusting in simple topology; (2) Only part of the network is affected when nodes are changed, making the topology more stable; (3) Only the cluster heads need to maintain route information, which reduces the network overhead. Due to these advantages, researchers have proposed many clustering schemes. Low-energy adaptive clustering hierarchy (LEACH) is a critical and widely considered clustering protocol in ad hoc networks. LEACH chooses the cluster heads based on a random probability number and aims to reduce energy consumption in wireless sensor networks (WSNs). Because the wireless sensor node is battery-powered, an energy-efficient routing protocol should be considered in a WSN to extend network lifetime. Although LEACH has been proposed for a long time, it has also been researched widely in recent years and has good performance. CH-leach is proposed to optimize cluster head selection in ref.