# A Reliable Low-Latency Multipath Routing Algorithm for Urban Rail Transit Ad Hoc Networks
## 1 Introduction  
Urban rail transit is steadily increasing its position in the field of public transportation because of its large capacity, fast speed, and high safety. With the deep integration of new information technologies and urban rail transit, such as Big Data, Satellite Communication, and Block Chain, the demand for urban rail transit communication is growing rapidly, and achieving a more intelligent, safe, efficient, and economical urban rail transit system is the future development direction [1]. In this paper, the main research is how to improve the network communication performance of urban rail transit vehicle-ground communication systems with multiple services. The traditional urban rail transit generally used communication-based train control (CBTC), which has the advantages of small departure intervals and high operating efficiency, but with the increasing demand for operation efficiency, the existing vehicle-ground communication system is unable to meet the communication requirements in the future [2]. The network architecture design of vehicle-ground communication systems is an important research object for the next-generation urban rail transit. At present, the urban rail vehicle-ground communication network adopts WLAN or LTE-M technology to carry CBTC, PIS, CCTV, and other services [3]. In this wireless communication network, there are a large number of network facilities, such as the need to set up multiple pieces of trackside equipment and base stations at the trackside; the base stations are connected to the station's network switches, and the terminals need to establish connections to the core networkthrough the base stations to complete the communication with the station. Therefore, this network architecture is complex to build and costly to operate and maintain, and when the central node is disturbed or damaged, the vehicle-ground communication will be broken, which will cause problems such as emergency braking, delayed running, and the stopping of trains. For this reason, it is difficult to meet the demand for low-latency and high-reliability vehicle-ground communication in the next-generation urban rail transit system. There are many ways to improve the performance of vehicle-ground communication. Reference [4] balances energy usage and passenger appearance time by considering the joint optimization of passenger and energy allocation, thereby improving the performance of train-ground communication. Reference [5] focuses on train scheduling and proposes three optimization models for train scheduling to improve the efficiency of train-ground operations. This article focuses on the impact of wireless communication on vehicle-ground communication. There has been research on distributed communication network technology for urban rail transportation. Based on the design idea of the \"Starlink Project\" [6], the researchers propose a communication scheme for applying ad hoc network technology to an urban rail transit communication network, which is called \"Rail Starlink\", which removes the ground facilities, upgrades the trackside equipment to communication nodes, and enables direct communication among the nodes through the reasonable placement of communication nodes. Finally, the urban rail transit ad hoc networks composed of mobile nodes, trackside nodes, and sink nodes are realized to carry out urban rail transit communication services. Compared with the traditional urban rail wireless communication network, the urban rail transit ad hoc network has the following characteristics:  
1. Decentralized network. Add a routing function in the trackside equipment and use a Device-to-Device (D2D) interface without intermediate node control forwarding to realize the direct communication between the vehicle-ground and trackside equipment 2. Lower cost. It does not need to rely on ground facilities, reducing the construction and maintenance costs of base stations and core networks, and vehicle and trackside equipment use wireless communication, with no need to deploy and maintain cables. 3.