The future of rail depends on reliable critical mobile broadband connectivity that is powered by innovative 5G network solutions for faster, safer and greener travel. We look at how railway communications depends on great network design.
FRMCS and Gigabit in-train passenger communication
The rail transport sector is exploring digitalization to enhance customer experience, increase efficiency in its day-to-day operations, and maintain network security and data integrity.
From improved onboard monitoring systems to trains equipped with IoT connected sensors and more, the coming years will usher in a new era of highly connected trains and railways. For rail operators, this means greater management over the day-to-day services that passengers rely on to always reach their destinations on time.
Network-level infrastructural changes are required for this to happen. GSM-R, the legacy communications technology for railways, will be replaced by 5G. A new framework known as the Future Railway Mobile Communications System (FRMCS) is planned for deployment for about 200,000 kilometers of mainline railways. This 5G based framework will modernize train services and help keep railway operators on track for business success. 5G can also help deliver on the increasing requirements and expectations from passengers to be able to work online and have a flawless communication experience on trains.
Figure 1: Future Railway Mobile Communications.
Service level requirements
Stringent requirements for FRMCS
FRMCS is based on dedicated 5G bands for railways. However, the amount of spectrum dedicated to FRMCS is not sufficient for the railway’s needs. 5G though, also enables the CSP based Gigabit train connectivity for passengers. This will allow CSPs to deliver the needed stringent and heterogeneous SLAs through network slices and supplement the capacity required by railways for their applications. This will be a game changer when it comes to railways, as the same technology can serve both FRMCS applications as well as advanced consumer requirements, such as video streaming, video-calling, or gaming – all this while on a train moving at up to 500km/h.
Have you ever noticed that an ambulance sounds with a higher pitch as it moves towards you, but with a lower pitch as it moves away? This is called Doppler effect and it also affects radio waves, making communication with trains moving at high speed particularly complex.
Figure 2 summarizes the SLAs for consumer applications together with specific railway related SLAs to support various voice, video and data services applicable to rail communication scenarios:
- Voice communication for operational purposes impacting train safety.
- Critical video communication for observation purposes with indirect impact on train operation, e.g. passenger surveillance.
- Very critical video communication with direct impact on train safety-related critical train control and operation, for example used in driverless (e.g. Grade of Automation – GoA3/GoA4) operation for automated detection of objects (no human in the loop) or video-based remote control (human in the loop).
- Standard Data Communication: used for the exchange of train diagnostic information or communication relevant information.
- Critical Data Communication: for present rail traffic management systems.
- Very Critical Data Communication: for enhanced intelligent rail traffic management systems e.g. full automated train control systems (driverless – remote control); requires high reliable transmission and preservation of the response pattern.
- Messaging: for the reliable exchange of short information e.g. train departure procedure.
Figure 2: Sample performance requirements for railway scenarios, based on 3GPP TS 22.289. Consumer application requirements based on Ericsson Smartphone Lab studies. Size of the bubble illustrates the required reliability.
Network design is the process to obtain the best infrastructure layout for your network while fulfilling the service level requirements for all use cases. Reliable coverage and sufficient capacity are the foundation for high performance, high availability and future-proof mobile network covering railway tracks.
To achieve the stringent service level requirements for FRMCS and to guarantee high performance in-train passenger communication, meticulous network design is critical, and several aspects need to be considered:
- Various service level requirements: Rail signaling (high availability, low latency) and consumer services (high bandwidth).
- High-speed movement across a highly varied landscape: Trains can move, very fast, across very different environments through which the radio waves propagate differently (urban, bridges, tunnels…).
- Hundreds of users moving at once: The mobile network needs to be ready to handle hundreds of connected passengers moving in sync.
- Indoor-outdoor effects: While signaling related antennas are located outside the train, subscribers are inside. For both use case the service level requirements should be fulfilled considering different propagation paths and reflections.
- Interaction with other fast passing trains and subscribers.
What are the high-level steps in network design?
The first step in the network design process is the “dimensioning” phase, in which service level requirements, traffic models, radio technologies and spectrum, result in signal strength and signal to noise ratio targets. Already during this step an estimation of the required number of sites can be made and can indicate the scale and needed invest.
To make the planning for the deployment of a network, a modelling of radio environment characteristics along the railway track and inside the coaches is required. For that, a radio planning simulation tool is used that needs to be set up and appropriately calibrated.
Reliable railway planning using a simulation tool consists of three pillars: a propagation prediction model, accurate geo-data, and railway specific radio aspects.
Pillar 1: Propagation model calibration
The propagation model is the heart of a cell planning tool. It estimates how the waves will propagate in a certain environment, making it possible to come up with the optimal site locations to deliver on the required service level requirements. As radio waves propagate differently depending on their frequency and the landscape, propagation models should be tuned based on field measurements in typical railway environments like embankments, soundproof walls, bridges, road over rail bridges, as well as urban and rural areas, to ensure they represent a given reality in the most accurate way possible.
Figure 3: Sample data for propagation model tuning.
The propagation model tuning/calibration procedure follows a standard process where continuous wave measurements are used to adjust the coefficients of the selected propagation model. Typically, plenty of measurement samples are needed to calibrate the model and a subset of samples are used to verify the empirical propagation model.
The measurement collection can be time-consuming work and precise telecom engineering expertise is required to adjust the model.
Figure 4: Steps in model tuning: filtering (left) and statistics (right).
Pillar 2: Accurate geodata to model the environmental influence along the rail track.
In telecom, accurate 3D maps that are used in cell planning tools, as part of the network design process, are known as geodata. High quality geodata models building heights, building materials, terrain types (referred to as clutter) and even vegetation – basically anything across which the radio waves propagate differently.
Map errors, wrong clutter heights and wrong clutter types can jeopardize the process. When it comes to the network design of railways, it is important to procure extremely accurate geodata, with high resolution along the railway track itself.
Aerial photographs make it possible to automatically detect any radio technical obstacles and incorporate them into the geodata. In an automatic process, clutter types, clutter heights, 3D buildings and vegetation are generated. A manual control is necessary to make corrections and to ensure that the results are based on a realistic scenario.
Figure 5: 3D propagation prediction along railway track.
Pillar 3: Modelling Railway specific radio aspects.
For GSM-R and FRMCS the train can have a receiver antenna on top of the roof and therefore an outdoor planning threshold can be sufficient to guarantee a seamless layer for critical communication. Defensive, conservative planning is appropriate to ensure a high-quality network combined with reliable redundancy solutions.
Different approaches to provide in-wagon public customer broadband network exist. One approach is using receiver antennas on the roof and distribute the mobile signal with repeater antennas inside the train, illustrated in Figure 6. This solution is proven, but requires high maintenance and, due to frequency dependency, it is neither flexible nor future proof.
Figure 6: Traditional repeater solution.
In recent times, train companies have been testing and installing so called “RF windows”, these RF windows are optimally lasered for the commonly used frequencies, resulting in less attenuation than the regular train windows. In this case the installation of repeaters can be prevented.
To guarantee the service level requirements inside the train with RF windows, this solution needs to be properly modelled. For example, the angle of incidence between the antenna location and the train position must be respected. As illustrated in Figure 7, depending on the angle at which the signal impinges the windows of the train, a corresponding attenuation is to be considered in the planning tool.
Figure 7: RF window solution.
A solid network design is the foundation to deliver on stringent performance requirements associated with mission-critical railway communications and to deliver on consumer expectations, which remain unchanged regardless of being at home or sitting on a train moving at 500km/h.
Network design has the potential to identify the optimal site locations to deliver the target performance at the best TCO, but its complexity cannot be overlooked. While cell planning tools exist, operating them for the right outcome is not trivial and requires highly skilled experts connected to a global knowledge base to keep up to date with the latest industry developments and realize the potential of 5G-based FRMCS.
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