GSM-R: An Innovative Railway Wireless Communications System

Promising Prospects for GSM-R

During the 1990s, cable and analog communications systems were widely used in European railways. There were 35 different platforms; in Germany alone, eight analog wireless systems existed. However, railway communications were plagued with problems such as limited functionality, scattered systems, serious interference, poor confidentiality, and high maintenance costs. As high-speed rail developed in line with the global economy, analog systems were unable to meet the requirements of train dispatch and control.

Since 1997, the International Union of Railways (UIC) has been constructing GSM-R trial networks in France, Germany, and Italy to solve the problems of analog communications systems. GSM-R is a secure, efficient digital radio system with robust anti-interference, error control, and easy to use encryption features. It provides a variety of data communication services that support rail dispatch and control, as well as the development of information empowered railways. GSM-R improves railway use and dispatch efficiency. By 2009, more than 30 countries had deployed it in their railway infrastructure.

Statistics show that in 2006, railways stretched 1,370,782km across the globe. But by January 2009, only 72,293km was being operated under GSM-R—a mere 6% of total global railway. The UIC predicts that in the future, GSM-R will be used in 149,210km (nearly 60%) of  Europe’s total 221,025km of railway.

In recent years, environmental issues have put immense pressure on air and road transportation. High-speed electric rail does not use oil and satisfies the need for low-carbon economic development. In the coming low-carbon era, governments are turning to high-speed, efficient, energy-saving railway systems.

In 2009, China put 2,319km of high-speed rail into operation. The 1,068km Wuhan-Guangzhou high-speed line is the longest of its kind in the world, and has been operated at 350km/h. The US government is also planning to spend eight billion US dollars to construct 17,000 miles of high-speed rail throughout the country. Britain intends to spend 34 billion pounds on a 2,400km Scotland-London high-speed rail to be completed by 2030. France expects its high-speed rail will extend 2,500 miles by 2020. The Union of European Railway Industries (UNIFE) predicts total investment in railway-related devices will reach 122 billion Euros by 2010. UNIFE expects global investment in high-speed rail will increase to 150 billion Euros by 2016, propelled by infrastructure projects and environmental concerns. Opportunities arising from the development of GSM-R―the core of railway communications systems―are great.

ZTE’s Innovative GSM-R Solution

Railway mobile communications systems operate along rail lines and in train stations. Complex terrain, poor radio environment, and fast train speed are some of the problems associated with railway mobile communication. Transmission continuity and reliability are of key importance because dispatch and control require extremely high immediacy and reliability. Drawing on its experience in R&D, ZTE has developed an innovative GSM-R solution for railway network operators.

High reliability

GSM-R is responsible for key commands in rail dispatch and control, and a fault in the GSM-R system could lead to commuter chaos or a serious accident. Therefore, extremely stringent reliability requirements are imposed on every GSM-R network.

 With an understanding of railway telecommunications requirements, ZTE has developed a highly reliable GSM-R solution that includes:

■   Strict quality control flow to ensure hardware is stable and highly reliable. Premium quality electronic components are selected,  screened, and tested throughout the manufacturing process;

■    Comprehensive software management and fault tolerance technologies to guarantee high software reliability;

■    Adoption of redundant backup (including 1+1 backup, N+1 backup, resource pooling, and load sharing) inside the equipment to ensure high reliability;

■    Redundant connections, ring networking, and transmission bypass in the network design to enhance system reliability;

■    MSC/SGSN POOL solution to handle service interruption caused by MSC/SGSN or MSC/BSC single point failure, and to improve CN reliability;

■    GSM-R dual-layer coverage (dual-site co-address redundancy coverage and deep interweaving redundancy coverage) to provide BTS redundancy and to prevent single point fault in the wireless network.

Weak field coverage

Railways often pass through weak field areas such as mountains, bridges, and tunnels, and this is one of the most difficult aspects of GSM-R system planning. Traditional weak field coverage solutions employ base stations and repeaters combined with leaky cables or directional antennas. But ZTE also provides an innovative distributed base station (RRU) solution.

Compared with conventional integrated base stations, ZTE’s distributed BBU and RRU modules are small, lightweight, and convenient to transport and install. Thus, they are especially suitable for coverage in tunnels, on bridges, and in mountainous regions along the line. Using multi-RRU co-cell sharing technology, ZTE’s unique distributed base station solution tackles the problem of coverage in weak field areas and has the following advantages:

■    The RRU module controls uplink noise, avoiding degradation of receiver sensitivity as the cascaded number increases;

■    The RRU module supports ring networking, ensuring reliable transmission;

■    RRUs and BBUs support unified remote management and provide performance statistics, making equipment monitoring and management easier;

■    Each RRU has automatic time delay correction, so they are not restricted by repeaters that need to be installed within a 15us time delay.

■    The RRU module has higher power output, thereby reducing the number of RF units needed in tunnels and saving costs. 

High-speed mobility

Especially with trains running at 350 to 500Km/h, issues arise in GSM-R wireless network design and construction. The most critical are:

■    Severe Doppler shift on uplink and downlink signals is caused by high speed. This leads to fast signal fading and degradation of terminals’ receiving and sending capabilities. It also causes frequent call dropout, poor voice quality, poor call success rate, high bit error, and throughput reduction;

■    High speed shortens the time a terminal remains within a cell, and this increases instances of cell reselection and handover. If the overlapping area between cells is irrational or relatively small, the inter-cell handover success rate is reduced, and the call drop rate increased. 

With these challenges in mind, ZTE has developed a complete solution that ensures quality communication in 500Km/h trains. 

■    A unique frequency offset correction algorithm counteracts signal fading caused by the Doppler shift, and compensates for receiving performance degradation;

■    Multi-RRU co-cell sharing technology enables multiple RRUs to be configured into one logical cell and to transmit the same signals. This extends coverage of the logical cell and reduces handover attempts by terminals on a high-speed train;

■    An optimized handover algorithm reduces the number of handover decisions, shortens handover  time, and enhances handover success rate;

■    A speed- and direction-based handover algorithm reduces the possibility of ping-pong handover, further shortens handover time, and improves handover success rate.

As governments sharpen their focus on energy saving, opportunities presented by GSM-R development in a new era of railway construction are great. Leading the world in wireless telecom solutions, ZTE is ready to construct railway systems around the world with its innovative GSM-R solution.