China Academy of Information and Communications Technology (CAICT), Huawei Technologies Co., Ltd.
For more than a decade, metro trains have used Wi-Fi-based technology for communication, and to guarantee critical services. Normally, two Wi-Fi networks are set up to carry critical services and non-critical services respectively. In order to simplify communication network deployment, the metro industry needs to look for a next generation solution for the wireless communication in metro systems. This testbed seeks to address the concerns of the metro industry when looking for a next generation wireless communication technology, including:
- What functions should be supported?
- What are the requirements (performance, reliability, security, etc.)?
- How can these requirements be fulfilled (by what architecture, features, configuration, or new innovations)?
The testbed will establish and validate a profile for the use of LTE (Long-Term Evolution) in mission critical metro environments, so as to support the multiple types of services (critical and non-critical) in a metro system, replacing the existing non-standard multiple types of Wi-Fi networks.
Use LTE to achieve the following objectives:
- One technology to support all of the services, rather than multiple technologies
- One single network to support all of the services, rather than multiple networks of one technology
- Mainstream standard based technology, rather than technology with many vendor-specific extensions
The business objective is to simplify overall system architecture, reduce maintenance expense, and improve operation efficiency and reliability.
How It Works:
The testbed will conduct tests under various test conditions and configurations, to work out a system that can support the services for metro control and operation with proper performance and availability.
The tests are implemented in both a lab environment and on a real segment of pilot metro railways, covering the key requirements and specifications of critical services like CBTC (communications-based train control), PIS, CCTV (closed circuit television), and trunking.
The most important outcome of the testbed is to establish and validate a profile for use of LTE in metro systems. This will specify system architecture, functional requirements, performance requirements (delay, jitter, packet loss rate, etc.).
The LTE profile validated through this testbed brings the following benefits for metro operators:
- Provide a benchmark solution for new metro design and technology selection
- Enable the use of 3GPP standard based technology for metro industry
- Mature ecosystem, more suppliers, easy to get good components during the construction and find backup components over the long term, especially when there are multiple metro lines in a city.
- Good interoperability and mobility guaranteed by 3GPP protocols, easy to achieve shared-line operation; metro lines in a city can be organized and operated as a “network”, improve operation efficiency through optimized scheduling within or across metro lines.
- Simplified system architecture and less maintenance work, reduce OPEX (operating expenses).
- Enable more automation, even FAO (fully-automated operation), and improve operation efficiency.
- To have secure and reliable metro service, reduce unexpected train stops.
- Improve passenger experience, more indication, news broadcasting, etc.
The testbed will evaluate the feasibility of using LTE to support train-track communication in metros, carrying multiple types of services over a single network, including critical service and non-critical service.
For example, in the lab environment, it uses simulators to test the QoS (quality of service) including CBTC, voice and video trunking, train status information, emergency text, and CCTV monitoring under various conditions.
In addition, filed tests are performed on two stations and one metro section in the eastern Chinese city of Ningbo. This section is 3.8km long, about 1km underground and about 2km elevated. It can cover both ground and underground scenes.
Two sets of LTE equipment are used to construct A and B networks in the test section of LTE vehicle-ground integrated communication system.
Each network includes core network (EPC), baseband processing unit (BBU), radio frequency remote unit (RRU), vehicle-borne wireless terminal (TAU). BBU directly accesses two sets of LTE core network equipment through Ethernet switch.
The underground section is mainly covered by RRU and a leaky coaxial cable, while the ground is covered by an elevated RRU and antenna structure.
To verify that the LTE system meets the functional requirements of signal systems, LTE transmission performance under BBU handover is tested. In order to increase the number of handovers, RRU is cross-connected to the corresponding BBU in A and B networks, which makes the handover between BBU's occur once every RRU passes through the vehicle wireless terminal and increases the number of test samples.
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