The rail industry requirement and desire to achieve an on-board ‘connected’ environment, for both customer and operational purposes, has resulted in the need for the installation of a wide variety of transmitting and receiving antennas on the rooftop of trains. The physical size constraints of the rooftop mean that the antennas often need to be installed in close proximity to each other, which has the potential to give rise to technical conflicts, particularly interaction and interference issues between the different radio communications systems.
These conflicts have the potential to limit the performance of the radio systems – of particular concern with safety and performance-related communications. Many problems with rail radio systems around the world have been identified to poor train antenna design and deployment.
With the introduction of internet connectivity for passenger Wi-Fi and operational purposes, together with wireless connections for train signalling and satellite positioning, the train antenna is becoming increasingly important and train to ground connectivity will be at the heart of digital rail. 5G radio will probably use both lower and higher frequencies than traditional radio systems. Methods of delivering higher data rates for customer internet use will also be required. All this makes efficient roof-top antennas a very important train body requirement, one that is important for the rail industry to get right.
What is an antenna?
Electromagnetic (or radio) waves are waves of energy that travel through the air at the speed of light. A radio wave can be visualised as a sine wave and the distance it travels to complete one cycle is known as the wavelength of the signal. A 2.4 GHz signal will travel 12.5cm every cycle.
Basically, the magnetic field that the transmitting antenna radiates produces an electric current on any metal surface that it strikes. In a receiving antenna, the applied electromagnetic field is distributed throughout the entire length of the antenna to receive the signal. If the metal that the wave strikes has a certain length relation to the wavelength, the induced current is much stronger. Hence antennas can be ‘tuned’ for the frequency they are required to transmit or receive.
Normally, a radio works on multiple frequencies. For example, the 2.4 GHz band, used by Wi-Fi and Bluetooth devices, has a range of 2,400-2,483MHz. In this band, many channels are used with a frequency-hopping technique, with typically 1MHz between each channel. This means that the antenna has to perform well over a range of frequencies.
Any train rooftop obstruction will ‘block’ the propagation of the signal transmitted or received by the antenna. This results in the signal being reduced and also increases the variability of the signal. Given that a trackside base station may be several kilometres from the train, the signal arrives at the train at a low angle of elevation above the horizontal. Therefore, any obstruction on top of the train impacts on the level of signal. In this situation, the signal relies on diffraction for it to be communicated.
Train antennas need to be certified in accordance with EN50155. This is an international standard covering electronic equipment used on rolling stock for railway applications. The standard includes temperature, humidity, shock, vibration, and other parameters. A train antenna will be typically 40mm to 80mm in height and allow the mutual use of different communication systems, which for example can include; 2m-Band, 70cm-Band/trunked radio/TETRA/UIC, GSM-R, GSM 1800, UMTS, LTE, 2×2 LTE MIMO, 4×4 LTE MIMO, Wi-Fi 2.4, Wi-Fi 5.8 and a Global Navigation Satellite System receiver, all via a single antenna!
Antennas on trains
The ideal position for an external antenna on a train is for it to be as high as possible, although this is likely to be constrained by gauge limitations. Some train classes have almost no space between the envelope of the train and the maximum permitted gauge, although antennas have been specially designed for this situation.
Mounting antennas underneath non-metallic train roofs can be used to resolve the gauge limitation issue. This was incorporated into the antenna systems design for the Class 390, where some antennas are visible on the roof and others are hidden under a non-metallic fairing. The obstructions on the train roofs provide a wide range of challenges to the siting of train antennas; ranging from simple to complex.
A gently curving, uncluttered shape will permit good omni-directional coverage from an antenna placed almost anywhere on or near the centre-line of the roof. Such a simple roof type allows good coverage at low angles in the direction of the front and back of the train for railway-specific communication systems where the base stations are likely to be positioned alongside the track.
The fitting of air-conditioning units can result in significant obstructions being present on the rooftop. Mounting an antenna near to these obstructions should be avoided, as considerable degradation of performance can be expected, particularly if the height of the units is significantly greater than the height of the antennas.
It is not always practical to elevate the antenna such that it is higher than other rooftop items, however the further an obstruction is from the antenna, the higher the obstruction can be before there is a noticeable impact on the signal level.
The tops of air-conditioning units on the Class 390 are about 300mm above roof-level and are typical of train rooftop obstructions in that they block the view from the antenna along the track, both forward and backwards. Any signal that is received via an antenna in the ‘well’ between the air-conditioning units comes predominantly from a signal that is diffracted over the edge of the air-conditioning unit or reflected from objects in the vicinity of the train.
On the Siemens Desiro Class 444, for example, the GSM shark’s fin antenna has been positioned away from the air- conditioning units taking advantage of the large roof area available.
The ground plane is a conductive area on which the antenna is mounted to maintain the correct functioning of most types of antenna. The ‘ideal’ ground plane would be a large, flat, horizontal sheet of metal located above the height of any other object on top of the train, with the antenna mounted centrally on it. In practise, the metal train body is generally used as the ground plane, although some train bodies fall short of this requirement. For example, train roofs are generally curved and, at the antenna mounting point, the roof is often sloped. Fibreglass and composite materials used on some trains do not help the provision of suitable ground planes.
There have reportedly been instances where space has been left on train roof for antennas in the future, but with air conditioning units positioned directly underneath, preventing antenna and cable feeder installation. This is a
good example of the many competing requirements for space on trains, and the need to thoroughly and robustly consider antenna requirements early in the design of a train.
For Wi-Fi inside train carriages, low profile antennas are available which can be located completely out of sight. Wireless Access Points (WAP) with integrated ‘Smart’ antennas can be deployed. These are more efficient than traditional antennas with higher gain and better interference mitigation which provides better coverage/ capacity. They require fewer and less- expensive cables (data cable vs radio frequency cable), and may be cascaded with a single connection to a switch.
Multiple Input and Multiple Output
A train’s radio channel will be affected by random fading (variation of the attenuation of a signal dependent on factors such as geography) and this will impact the signal to noise ratio, and in turn the data error rate.
In very simple terms the principle of Multiple Input and Multiple Output antennas (MIMO) is to provide the receiver with multiple versions of the same signal. The probability that they will all be affected at the same time is considerably reduced and this helps to stabilise a link and improves performance, reducing error rate. MIMO antenna technology effectively uses multiple antennas at the transmitter and receiver to enable a variety of signal paths to carry the data.
MIMO antenna systems are already widely deployed – an example is Ferrovie del Gargano, one of the largest commuter rail systems in Southern Italy. Its passenger Wi-Fi service uses a hybrid approach using a combination of a Fluidmesh-provided train-to-trackside Wi-Fi system and public LTE. It supports data, VoIP, as well as live video streaming to passengers.
The Ferrovie del Gargano project demonstrates how dedicated trackside wireless networks can be an effective solution to provide on-board connectivity in areas with limited or no mobile radio LTE coverage. The on-board system is provided by two Fluidmesh FM4200 MOBI radios per train connected with high-gain 2×2 MIMO roof antennas. The 5GHz Wi-Fi base stations are deployed along the track with a trackside spacing up to 5km (3.5 miles) delivering 100 per cent coverage. It is claimed that the system provides a Gigabit of usable bandwidth to a train traveling up to 250mph. So, the technology exists, the issue (as often) is how will the infrastructure provide and connect the base stations be funded?
Wireless communications in tunnels has for many years been provided using leaky feeder radiating cables or, for relatively short tunnels and UHF bands, free space antennas. There is now a growing interest on implementing MIMO systems in tunnels with the aim of delivering higher data rates and/or a decrease of the error rate as in free space.
Research by Lille University on propagation mode in tunnels as a means to deliver MIMO has concluded that the challenges can be overcome. In the UK a trial of MIMO over the radiating cable in the Waterloo & City Line is underway to evaluate communication links to customers.
Beamforming is a signal processing technique used in directional signal transmission or reception. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends of a link to improve performance.
Pivitol Commware is one company which is developing a metamaterials beamforming product for what is referred to as Access-in-MotionTM and Holographic Beam FormingTM (HBF). This is aimed at boosting the agility, range, capacity, and spectral efficiency of the communication links for transport sectors (rail, air and sea), while also reducing equipment cost, size, weight and power. HBF objective is to provide high-capacity, long range and interference- avoiding data links, combined with electronic speed beam switching.
Technologies such as massive MIMO, super-dense meshed cells and macro-assisted small cells are being discussed as possible 5G radio access technologies. These may use very-high-frequency bands in the ‘millimetre’ range of the radio spectrum, such as 15GHz through to 70GHz. This spectrum can better support the use of multiple, miniaturised antennas, and more bandwidth is available in these bands than in the bands below 1GHz.
However, millimetre-wave bands do not lend themselves to providing wide area coverage. Therefore, further spectrum below 1GHz is expected to be needed in order to improve mobile broadband coverage. 5G may therefore encompass a range of existing and new bands, which will potentially span a wide section of radio spectrum.
60GHz Wi-Fi for rail?
Traditional Wi-Fi uses either the 2.4 or 5GHz radio spectrum band, but 802.11ad is a relatively new Wi-Fi standard that can deliver speeds of up to 7Gbps using the unlicensed 60GHz radio spectrum band. Known as WiGig, 60GHz is five times faster than current.
Wi-Fi systems with speeds of up to 70Gbps and, by using such high frequencies, the antennas will be a few mm in size. It sounds great, but at these extremely high frequencies the range will only be a few metres and the signal will be absorbed by walls or other physical obstructions. In summary, it’s a very short-range, high capacity, line of sight same-room solution.
However, suppliers are developing micro beam-forming control technology. This rapidly modifies the communication module (in less than 1/3,000 second) should the signal be interfered with in any way, hence getting around the obstruction problem. It could therefore be ideal for use within a train, or from the ground to the train link, although a considerable number of Wi-Fi points would be required.
Fee space optical and terahertz communications to trains
With increasing demand for broadband communications, the radio frequency wireless spectrum is a finite resource that is fast running out. Free space optical (FSO) communications offers another possibility of Gbps data rates for mobile applications without using any licenced spectrum. FSO transmission can be limited by attenuation due to cloud and fog, plus small variations in the refractive index of the atmosphere, however it may be ideal for short- range systems.
The advent of high-power light emitting diodes and highly sensitive photo diodes and simultaneous use as a source of lighting and data communication, has helped the development of FSO as an attractive as well as energy-efficient technique for high-speed data communications.
Channel coding methods are being developed to address the fading issues and provide reliable and high-speed communication channels. Adaptive transmission methods have been used very successfully in fading RF channels for many years, and investigations indicate that significant performance gains may be possible. It is relatively early days for FSO communication, with many challenges to be overcome, but it is another technology for train-to-trackside communications which the rail industry may wish to adopt.
Train radio antenna design is important, but it can involve many compromises and is an area which will become more complex as requirements and technology evolves. It will be essential for both digital rail and customer requirements, and train designers and builders need to involve radio designers at the early stage of a train design. However, with a train body having a typical life of 30 to 40 years and radio frequency technology changing every few years, it is a very challenging subject for all the engineers involved.
Antennas or Antennae?
The word ‘antenna’ is Latin, and means the yard of a sail – the spar that stiffens the sail on a (usually) square-rigged sailing ship.
The Greek word for the same thing was ‘keraia’, but it had another meaning as well – horn. So it came to be applied to the feelers of insects, perhaps because they looked like small horns.
When a Greek text, supposedly by the philosopher Aristotle, was being translated from Greek into Latin, the plural ‘keraiai’ (feelers) was mistakenly translated as ‘antennae’, and the word stuck. So an insect’s feeler became an antenna – plural antennae.
When early radio aerials began to look like long metal feelers, the name ‘antenna’ was adopted, but the engineers involved either didn’t know the Latin source, or ignored it, so the plural became ‘antennas’.
FSO: Free-space optical communication – an optical communication technology.
GSM-R: Global System for Mobile Communications-Railway or GSM-Railway.
LAN: Local Area Network.
LTE: Long-Term Evolution telecommunications – similar to GSM 4G.
MIMO: Multiple-input and multiple-output.
TETRA: Terrestrial Trunked Radio (formerly Trans-European Trunked Radio).
UHF: Ultra High Frequency.
UIC: International Union of Railways.
UMTS: Universal Mobile Telecommunications System – a 3rd generation mobile cellular system.
VoIP: Voice over Internet Protocol – also Internet telephony.
WAP: Wireless Access Point.
WiGig: Wireless Gigabit Alliance. Subsumed by the Wi-Fi Alliance in March 2013.
This article was written by Paul Darlington.