5G Private Networks Enable Stable and Intelligent Urban Rail Transit

According to statistics from Ministry of Transport, by the end of 2024, a total of 54 cities in China had opened 325 urban rail transit lines, with a total operational mileage of 10,945.6 kilometers. Throughout 2024, the actual number of trains operated reached 40.85 million, with a total passenger volume of 32.24 billion. The rapid expansion of urban rail transit networks has significantly enhanced urban operational efficiency and facilitated residents' travel.

However, the costs of constructing and operating urban rail transit are significant. Data shows that the average operating cost per kilometer for rail transit enterprises nationwide stands at a staggering 11.26 million yuan, rising to over 15 million yuan per kilometer in major cities like Beijing, Shanghai, Guangzhou, and Shenzhen.

With adjustments in government investment policies for municipal infrastructure, the era of extensive rail transit expansion has ended, marking the beginning of a more diversified, intelligent, and sustainable future. The deployment of 5G private networks in urban rail transit systems not only enhances operational quality and efficiency but also aids enterprises in achieving dual goals: improving both economic and social benefits.

5G Private Networks Enable Simplified System Architecture

Wireless communication networks serve as a crucial foundation for ensuring the efficient and safe operation of rail transit systems. The implementation of wireless private networks in urban rail transit has progressed through several stages:

• In the early 1990s, analog voice wireless communication networks were introduced.
• In 2005, Nanjing Metro Line 1 pioneered the use of TETRA digital trunking communication.
• In 2008, Beijing Subway Line 2 upgraded its CBTC signaling system by utilizing WLAN.
• In 2016, Wuhan Metro Line 6 achieved a milestone by implementing LTE-M-based CBTC services for the first time.

However, these wireless communication networks lack compatibility and cannot substitute for one another, resulting in a scenario where multiple networks overlap and coexist, leading to issues such as data silos and inefficient resource utilization. The 5G private network, with its high speed, low latency, large capacity, and extensive coverage, can effectively address these pain points in urban rail transit by replacing multi-network coexistence with unified network integration. The evolution of wireless private networks in urban rail transit is shown in Fig. 1.

5G Private Networks Offer Significant Technical Advantages

5G offers a range of unique features that can better adapt to the diverse application needs of the rail transit industry.

• Flexible 5G NR air interface meets uplink capacity requirements: Unlike the LTE network, the NR wireless frame structure offers greater flexibility in configuring  uplink and downlink timeslots, allowing the number and length of uplink and downlink timeslots to be adjusted based on service needs. In addition, it supports asymmetric spectrum allocation to improve system adaptability and resource utilization. This flexibility enables NR to better adapt to the uplink-dominant data transmission model on the vehicle side in the rail transit industry.
• End-to-end (E2E) 5G network slicing for comprehensive service bearing: Network slicing creates multiple virtual E2E networks on shared physical infrastructure. Each slice realizes logical isolation in terms of equipment, access network, transmission network and core network, better supporting secure production, internal management, and external services in the rail transit industry. With 5G, several dozen subsystems achieve comprehensive service bearing and independent operation.
• 5G time-sensitive networking (TSN) supports deterministic communication and guarantees critical services: By integrating 5G networks with TSN technology, and leveraging clock synchronization, intelligent perception of service message types and characteristics, dual sending & selective receiving, intelligent scheduling, and precise gating, end-to-end service-level agreement (SLA) can be assured. Highly reliable deterministic network access with 10 ± 1 ms latency is provided, ensuring the safe and stable operation of train signaling systems such as CBTC/TACS.
• 5G LAN simplifies network connectivity: 5G LAN can replace commonly used IPSec and L2TP tunnel protocols in the industry, providing flexible communication capabilities such as terminal interconnection or terminal separation. Point-to-point and multi-point communication modes based on IP or Ethernet can be applied within the same LAN group. The traditional TCP/IP network service architecture can access the 5G network without modification, making networking simpler and connection more reliable.
• 5G MEC edge computing enables AI applications: The 5G network supports deploying computing power closer to the edge of the network, enabling a wide range of video AI application scenarios in urban rail transit, such as visual detection of key vehicle components in depots and video-based detection of catenary systems. Incorporating RFID technology, it facilitates efficient management through big data applications such as personnel positioning and passenger flow analysis.
• 5G multicast and broadcast services (MBS) improves the travel experience: 5G MBS broadcasts multimedia content to users, ensuring a smooth viewing experience. This feature can be integrated with existing PIS services, saving air-interface transmission bandwidth and simplifying multi-screen networking on the vehicle side. It also supports a free-to-air mode, enabling SIM card-free reception so that passengers can use their own mobile phones to access rich media content provided by the PIS system.
• 5G mission critical communications (MCx) broadband trunking breaks communication barriers: 5G MCx mobile broadband trunking services meet the trunking communication needs across voice, video, data and industry-specific services. Featuring high reliability, high security and easy deployment, they support integration and interworking with systems such as TETRA, B-TrunC, and PDT, breaking traditional communication barriers, and improving the efficiency of cross-line, cross-network operations, and cross-departmental collaboration.

5G Private Network Deployment Modes

Frequency serves as the foundation of 5G wireless networks. Taking into account current policies, regulations, and industry standards, urban rail transit systems across China have adopted flexible and diverse business models to actively deploy 5G private networks tailored to local conditions.

• Operator-led deployment and maintenance: By the end of 2024, the Ministry of Industry and Information Technology had cumulatively allocated a total of 1109 MHz of radio frequency bandwidth to four major telecom operators, of which 86.5% can be used for 5G. For RAMS services (such as signaling and trunking) in the rail transit industry, operators can offer construction and maintenance of standalone private 5G networks on behalf of enterprise clients. For non-RAMS services (including passenger information systems, video surveillance systems, and intelligent maintenance systems), operators can provide logical private networks based on 5G public network slicing, ensuring business needs are met while reducing operational costs.
• Industry-built 5G private networks: Currently, aside from enterprises like China Railway and COMAC that have obtained trial authorization for dedicated 5G frequencies, no other industries have yet been granted such authorization. However, under national radio frequency allocation regulations, industries can deploy standalone private 5G networks using 5G New Radio Unlicensed (5G NR-U) in the 5 GHz band or millimeter-wave frequencies. These medium- to high-frequency 5G private networks can replace existing Wi-Fi-based vehicle-to-ground networks, addressing issues related to Wi-Fi security and mobility, while meeting the industry's need for self-construction and self-maintenance. The unauthorized spectrum-based 4G/5G network has been deployed and implemented in the second phase of Chengdu Metro Line 19, while millimeter-wave frequencies have been deployed on Shanghai Metro Line 4 and on Seoul Metro lines overseas.

With the adoption of 5G private networks to replace existing wireless systems in the rail transit sector in over 10 cities in China, including Guangzhou, Shanghai, Nanjing, Wuhan, Tianjin, and Suzhou, there has been a gradual shift from carrying non-core operational services to exploring the feasibility of supporting core business systems, which will ultimately lead to a comprehensive 5G-based solution for all types of operations. 5G private network technology is poised to play a pivotal role in urban rail transit, driving it towards greater automation, intelligence, and sustainability.