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LTE TDD: A Better Choice for Metro Wireless

| By Chen Yu, Communications Engineer with Xiamen Rail Transit Group

As the ideal technology for metro rail communication, Long-Term Evolution Time-Division Duplexing (LTE TDD) assures reliable voice and data services for trains running on overhead rails, on the ground, or even in underground tunnels. This 4G wireless technology supports basic telephony, dedicated trunking dispatch, broadband data services, and other services required by metro rail operations.

LTE TDD thus merges the capabilities of conventional voice-dispatch systems with those of broadband data systems such as Terrestrial Trunked Radio (TETRA) and Wi-Fi into a single wireless system. With this versatile set of capabilities in one system, LTE TDD offers improvements over legacy communications systems while minimizing Capital Expenditure (CAPEX) and Operating Expense (OPEX).

Public transportation systems such as rail transit need highly reliable communications systems to ensure safe and efficient operation as well as provide a comfortable experience for passengers. In emergencies, a reliable communications platform is essential to assuring quick response times and precise instructions for dispatch, coordination, and command. LTE TDD can meet these requirements with an ideal set of technical advantages.

LTE TDD Advantages

● Stability and Security

LTE TDD uses an IP-based flat architecture that requires only base stations and a core network. Designed to support future upgrades, this simple network topology reduces transmission latency on the radio protocol user and control planes to less than 5 ms and 100 ms, respectively.

LTE TDD protocols are divided into an Access Stratum (AS) and Non-Access Stratum (NAS). LTE TDD provides AS security protection over air-interfaces and NAS security protection between terminals and the core network. The AS/NAS architecture has been designed to minimize security risks on LTE and System Architecture Evolution (SAE) networks.

● Licensed Spectrum and Advanced Technology Prevent Interference

LTE TDD operates in licensed frequency bands (400 MHz, 1.4/1.8/2.3 GHz, etc.), whose purpose includes interference prevention from consumer devices and other mainstream communication platforms.

In addition to the advantages of using licensed frequencies, LTE TDD includes interference prevention technologies such as Inter-Cell Interference Coordination (ICIC), NULL forming, Super Cell, and Interference Rejection Combining (IRC) that, in turn, optimize cell capacity and enhance overall system performance.

● High-Speed Transmission

LTE TDD supports a wide range of bandwidths (1.4, 3.0, 5.0, 10, 15, and 20 MHz) as well as carrier aggregation to combine multiple carriers and provide greater bandwidth. Based on these conditions, LTE TDD achieves 100 Mbit/s downlink and 50 Mbit/s uplink data transfer rates. When the system evolves to LTE-Advanced, it will be able to support 1,000 Mbit/s downlink and a 500 Mbit/s uplink rates.

An LTE TDD trunking system can support as many as 100 supplementary services, including group calling, private calling, late entry, dynamic group number assignment, floor queuing, floor preemption, and service priority. In addition, implementations can include a dedicated dispatch terminal and management console for enabling the full range of wireless mobile services.

● Long-Distance Coverage

LTE TDD covers tunnels using leaky cables (cables designed to radiate, in contrast to coax), and signals radiate perpendicularly to the direction in which the trains are moving. This design ensures that signals arriving at train-mounted stations are stable.

LTE TDD devices operate at high power on a relatively low frequency band and cover a range as wide as two kilometers on both sides of the railway. The system is built to dynamically adjust power based on radiated coverage.

● High Mobility

Speed dependent, LTE TDD networks offer the highest performance services on trains traveling up to 120 km/h, and through dynamic frequency band assignment, is able to provide basic wireless services and mobility at speeds from 120 km/h to 350 km/h, and up to 500 km/h.

● Maturity and Defined Evolution

Standards organizations, operators, system and device manufacturers, terminal chip manufacturers, and system test companies are collaborating throughout the LTE TDD supply chain in an effort to promote advances in the technology. As an international 4G-wireless communications standard, LTE TDD has a clearly-defined software roadmap that provides for the smooth evolution to the upcoming LTE-Advanced. LTE-Advanced, a developing 100 MHz multi-carrier protocol, is expected to address heightened requirements and significantly reduce investments in technology updates.

Application of LTE TDD to Rail Transit

A rail transit communication and transportation system consists of train dispatch, public security, emergency call, rail-yard facility management, vehicle management, and device maintenance subsystems. Train-to-ground communication requires fast, accurate, and reliable transmission of operating and management data, as well as the ability to support the transmission of voice and image data.

● Train-to-Ground Broadband

The Passenger Information System (PIS) needs to upload and download video surveillance and multimedia service data in real-time. LTE TDD addresses this requirement with wireless channel capacity as high as 50 Mbit/s.

In high-speed rail deployments, LTE TDD wireless access devices need to be installed in the front and rear compartments of the train in order to provide the throughput for live surveillance and display devices in interaction with the control center.

LTE TDD technology has been applied to many major projects in China, including the PIS on Zhengzhou Metro Line 1 and the Shuo Huang Railway (SHR). In the SHR project, LTE TDD technology is used for synchronous control of freight trains and transmission of multi-locomotive security service, video surveillance, and non-security service data.

● Trunking Dispatch

In support of operational command, wireless trunking dispatch is a core service provided by the wireless communications system for urban railways. In addition to dedicated trunking dispatch services, the LTE TDD platform provides for general-purpose broadband data and basic telephony services.

•The dispatch control center is connected to the LTE TDD broadband system through an IP network that delivers commands, and receives status and sensor data related to train operation, security, safety, maintenance, and rail-yard management.

•Automatic Train Supervision (ATS) services monitor movement and pinpoint on-track positions. The ATS is built to enable the development of custom applications to suit specific operator requirements.

Multi-service integration into a single LTE TDD solution dramatically reduces the CAPEX and OPEX for every metro rail company who chooses to migrate their system a unified trunking dispatch platform.

Summary

With high bandwidth, high data rates, and excellent mobility, 4G LTE TDD technology provides low-latency command and control plus real-time voice and data transmission services for high-speed metro trains. These capabilities support operational and safety functions as well as delivering an enhanced travel experience for passengers.