Application Of 5G in Urban Rail Transit

I. 5G Overview

( I ) 5G key technology First, large-scale antenna array technology. By arranging a large number of antennas, large-scale antenna arrays can realize high-density spatial multiplexing and improve the spectral efficiency and capacity of the system. This enables 5G to provide high-speed, stable connections for a large number of users and meet their growing communication needs. Second, millimeter wave technology. Millimeter wave refers to electromagnetic waves with a frequency of 30-300 GHz, which has a higher transmission speed and a shorter transmission distance. 5G application of millimeter wave technology can achieve higher transmission speeds and lower delays, providing users with faster data transmission speeds and better service experience. Third, ultra-dense network. While traditional mobile communication networks mainly use macro base station coverage, 5G uses more micro base stations and small base stations to form an ultra-dense network. This kind of network can realize higher spectrum reuse and better coverage performance, providing better service quality and user experience. Fourth, network slicing technology. Network slicing refers to the division of network resources into multiple independent virtual networks, each of which can be optimally configured according to different application requirements. This technology can provide personalized services and customized network experience for different application scenarios according to the specific needs of the application scenarios. ( ii ) 5G Network Architecture Compared with 4G, 5G has several significant differences. First, 5G adopts an innovative distributed core network architecture, which sinks the core network functions close to the end devices. This change helps reduce communication latency and provides better support for real-time applications. Second, 5G introduces mobile edge computing (MEC), which sinks application server functions into the central host room. This design enables real-time processing of massive amounts of data, which improves data processing efficiency and reduces network load. 5G also redefines the functions of the building baseband unit (BBU) and the remote radio unit (RRU). Separating the physical layer chip from the BBU and sinking it into the RRU reduces the pre-transmission capacity and lowers the cost, making the network layout more flexible and efficient. 5G also applies network functions virtualization (NFV) technology to transfer the hardware and software functions of dedicated telecom equipment to a virtual machine, which further enhances the flexibility and scalability of the network. This further improves the flexibility and scalability of the network. Finally, 5G establishes a connection between the core cloud and the edge cloud, mapping virtual machines in the edge and core clouds through a software defined network (SDN) controller. This design enables centralized control of network slicing, further improving the intelligence of the network. Based on NFV/SDN technology, 5G divides the physical network into multiple virtual networks (network slices), each of which is oriented to different application scenarios. This design realizes logical independence, mutual non-interference, and centralized management by SDN controllers, making the network more efficient.

II. 5G Application Paths in Urban Rail Transportation

( a ) Train control and dispatching system In urban rail transit, train control and dispatching system is the core part, which directly affects the safety and efficiency of train operation. Train control and dispatching system mainly relies on traditional communication equipment such as rail circuit, leaky cable and wireless spread spectrum, which has the shortcomings of low transmission speed, high delay, etc., and it is difficult to meet the needs of modern urban rail transportation, and the application of 5G can bring revolutionary changes to the train control and dispatching system. With 5G, trains can receive and send control signals in real time, realizing precise control. Using 5G's low-latency characteristics, trains can provide timely feedback on their operating status and receive dispatching commands, enabling the dispatching center to grasp real-time information such as train position and speed, and thus carry out dispatching more accurately. At the same time, 5G also supports high-definition video transmission, providing more reliable data support for train monitoring and safety warning. ( II ) Passenger information service system Passenger information service system is an important service system directly facing passengers in urban rail transit. Passenger information service system mainly relies on traditional media such as subway TV, radio, etc., which has strong unidirectional information transmission and limited information content, while the application of 5G can provide passengers with richer, real-time information services. First, the application of 5G, the subway train can receive and send high-definition video, audio and other multimedia information in real time, providing passengers with richer information services. Passengers can know the arrival time of the train, station information, weather forecast and other information in real time through the display screen in the subway train, cell phones and other terminal devices. Second, using the characteristics of wide coverage of 5G, multiple information collection points can be set up along the subway, real-time collection and transmission of information on the surrounding environment, commercial advertisements, etc., to provide passengers with more personalized information services. For example, when the train is close to the commercial area, commercial advertisements and preferential information can be sent to passengers to facilitate independent consumption. ( III ) Safety Monitoring and Emergency Response System Safety monitoring and emergency response system is an indispensable part of urban rail transit. The safety monitoring and emergency response system mainly relies on traditional means such as video monitoring and system alarms, which have shortcomings such as low transmission speed and slow response speed, while the application of 5G can improve the data transmission and processing capability of the safety monitoring and emergency response system. By applying 5G, the monitoring system can transmit high-definition video, audio and other data to the monitoring center in real time, providing a more reliable video source for security monitoring. The monitoring center can carry out safety warning and abnormality detection based on the real-time monitoring screen, so that safety problems can be discovered and dealt with in a timely manner. In the event of an emergency, the metro train, using the low latency characteristics of 5G, can quickly activate the emergency response system, send emergency signals to the dispatch center, requesting support and guidance. The dispatching center can make a judgment and carry out dispatching quickly according to the real-time situation, start the relevant emergency plan to protect the lives of passengers. ( four ) car-ground communication system car-ground communication system is the key part of the subway train and ground equipment, facilities for data interaction. Vehicle-ground communication system mainly relies on leaky cable, Wi-Fi and other traditional communication equipment, there are signal penetration difficulties, low transmission speed and other shortcomings, and the application of 5G can provide a more reliable and efficient data transmission channel for the vehicle-ground communication system. By applying 5G, subway trains can interact with ground equipment and facilities in real time, realizing vehicle-ground integrated information transmission and processing. For example, the subway train can send request signals to the intelligent traffic signals along the line, requesting to adjust the signal length to meet the needs of train operation. Ground equipment can also send control instructions and status information to the train, realizing vehicle-ground cooperative operation. Using the low-power characteristics of 5G, power supply equipment with longer continuous power supply time can be designed or the power supply method of the equipment can be optimized to extend the operation time of the equipment and improve the stability of vehicle-ground communication.

III. Key Issues and Solutions for 5G Application in Urban Rail Transportation

(I) Signal coverage problem Due to the complex environment of urban rail transit, special environments such as tunnels and underground stations have put forward higher requirements for signal coverage. 5G signals may experience severe signal attenuation in these special environments, resulting in the inability to continuously cover the signal, which in turn affects the stability of communication. In order to solve the signal coverage problem, 5G signals should be enhanced, and the signal coverage range should be expanded by increasing the number of base stations and optimizing antenna layout. In special environments, signal enhancement technology is used, such as leaky cables, leaky cables combined with macro base stations, etc., to compensate and enhance the signal. Optimize for different scenarios and comprehensively consider factors such as frequency band, antenna type, and transmission power to achieve the best signal coverage effect. Antennas can be used for 5G signal coverage in urban rail transit vehicle depots, stations, parking lots, and indoor and outdoor scenarios. The free space loss is: L=32.4+20lg(F)+20lg(D) Where: L is free space loss, dB; F is frequency, MHz; D is distance, km. When designing a wireless signal coverage solution for urban rail tunnel sections, multiple factors need to be considered comprehensively. First, in order to ensure the quality and stability of signal transmission, the leakage cable transmission loss needs to be considered. In the design of the section coverage solution, coaxial cables with lower leakage cable transmission loss should be selected to reduce signal attenuation. Second, coupling loss needs to be considered. Coupling loss refers to the signal attenuation caused by mutual interference between coaxial cables. In the design solution, coaxial cables with lower coupling loss should be selected to reduce signal interference. Third, vehicle shielding and other additional coupling losses need to be considered. Vehicle shielding refers to the shielding effect of ground vehicles in the tunnel on wireless signals, which may cause signal weakening. In the design solution, appropriate signal enhancement equipment should be selected to compensate for the loss caused by vehicle shielding. At the same time, other possible additional coupling losses should also be considered, such as interference of tunnel structure and materials on signals. (II) Section signal switching problem In the urban rail transit system, trains will pass through different sections, and the signal coverage and interference conditions of each section are also different. Therefore, when a train enters from one section to another, signal switching is required. In the urban rail transit system using 5G, the following two key issues need to be paid attention to in section signal switching. First, seamless signal switching. During the train's travel, the signal conditions in each section are different, so the train needs to switch signals. This requires that the signal switching process should be as seamless as possible without affecting the communication services on the train. Seamless switching requires solving the delay problem in the switching process to ensure accurate data transmission during the switching process. Second, optimization of section switching. In urban rail transit systems, vehicles usually pass through multiple sections, and the signal conditions and network quality of each section may be different. Therefore, it is necessary to select the best section for switching through intelligent algorithms or optimization algorithms to improve communication quality and efficiency. This requires that during the switching process, the system can monitor and evaluate the signal conditions and quality of each section in real time, and make switching decisions based on this information. (III) Band resource issues In urban rail transit, the band resources of the communication system are limited. Compared with 4G, 5G has higher data transmission speed, lower latency, and more connections, so it requires wider band resources. However, the existing band resources are limited and it is difficult to meet the large-scale application of 5G in urban rail transit. In urban rail transit, different services have different requirements for band resources. How to reasonably allocate and manage frequency band resources, avoid conflicts and interference, and ensure the normal operation of various services are the main problems faced by the application of 5G in urban rail transit. In order to solve the frequency band resource problem, the corresponding frequency band resources should be reasonably allocated according to the needs and characteristics of different services. For example, for train control and safety services, the stability of their frequency band resources should be guaranteed first. In addition, the optimization and management of frequency band resources should be strengthened, and intelligent scheduling and management systems should be applied to dynamically allocate and manage frequency band resources. At the same time, some new frequency band resource technologies, such as high frequency bands and millimeter waves, can be applied to improve the utilization efficiency of frequency band resources. (IV) Equipment compatibility issues Compared with 4G, 5G has great differences in spectrum, modulation mode, network architecture, etc., making it difficult for many existing 4G devices to be directly upgraded to 5G. Therefore, a large number of 5G-specific equipment needs to be purchased, which increases the cost of transformation. There are many types of communication equipment in urban rail transit, such as train control systems, signal systems, safety monitoring systems, etc., and each system has its own communication protocols and standards. When introducing 5G, how to ensure the compatibility of these systems with 5G equipment is an issue worthy of attention. If there are compatibility issues with the equipment, it may lead to blocked signal transmission, data exchange errors, etc., affecting the punctual operation of trains. There are the following measures to solve the problem of equipment compatibility. First, the compatibility testing and verification of 5G equipment with existing systems should be strengthened, which includes compatibility evaluation of equipment and systems from different manufacturers, and the formulation of unified communication protocols and standards to ensure stable communication between the two and achieve interoperability. Secondly, the application of 5G in urban rail transit should be actively promoted, and equipment manufacturers and scientific research institutions should be encouraged to cooperate to jointly develop equipment and systems with higher compatibility. Finally, in the process of equipment procurement and selection, the compatibility of equipment with 5G should be fully considered to avoid various problems caused by later transformation and replacement.

In-depth research on the application of 5G in urban rail transit shows that 5G has brought unprecedented opportunities to urban rail transit, making the communication system more efficient, safe and reliable. However, there are still some problems in the application of 5G in urban rail transit, such as signal coverage, interval signal switching, frequency band resources and equipment compatibility. In practical applications, it is necessary to fully consider these factors, strengthen technology research and development and innovation, and formulate reasonable solutions.