27.09.07
KeTech offers ultimate safety package for one person operation
Following an extensive period of research and development, KeTech Systems has released the next generation of track-to-train transmission based on sophisticated leaky feeder technology, promising a revolution in passenger safety for metros and railways. Here the transport communications and information specialist presents a resume of its recent research aimed at highlighting the safest possible solution for track-to-train communication.
Safe train departure in the LUL environment has traditionally been ensured initially by guards, then later using mirror arrangements. The modern system continues to fulfil this fundamental requirement of safe departure, providing drivers with high quality real time images up to when the train has departed from the platform.
KeTech’s engineers have been heavily involved with track-to-train transmission applications for over 15 years, including research undertaken on behalf of London Underground. As a result, KeTech has gained a particularly thorough understanding of the often highly challenging issues involved. Research undertaken by the company has involved detailed assessment and trial of the various technologies available for track-to-train transmission, allowing an objective selection to be made of the most appropriate technology for the purpose.
As well as providing a safe method of platform departure during one person operation (OPO), track-to-train video transmission can deliver benefits such as reduced dwell times and improved efficiency. To enable drivers to operate trains without the need for a guard or external monitors/mirrors, CCTV cameras are located on the platform to provide total coverage of the platform/train interface. The output of these cameras is combined into one or two transmission channels depending on the number of cameras required. Pictures are passed to the train via the transmission system, decoded via the onboard receiver and displayed on LCD monitors in the driver’s console.
Key performance factors
Typical track-to-train video applications require pictures to be available to the driver as the train approaches the platform stopping point, remaining on until the entire train leaves the platform. To enable this facility it is essential that the system can deliver excellent dynamic performance, this being one of the key challenges in the provision of this type of system.
In order for a track-to-train system to be suitable for safe train departure applications, a number of key factors need to be considered:
• extremely high integrity is required - this is a safety-related system
• the system must provide reliable surface and sub-surface operation in all track configurations
• a high quality ‘transparent’ transmission link is essential
• a full bandwidth link with virtually zero latency is required
• continuous pictures are required over the entire length of the coverage area
• ease of installation and ease of maintenance must minimise disruption to service
• performance must be repeatable across the metro/rail network
• integration with train-borne systems is essential to provide highest integrity
• license free operation is preferable to minimise ongoing costs
• non-public transmission bands are preferable for secure communication
On an initial appraisal, it is tempting to think that wi-fi or similar ‘off-the-shelf’ technologies may be suitable for transmission. However, if these considerations are coupled with the necessity to comply with stringent railway EMC, environmental specifications and licensing issues, the overall requirements become very challenging indeed.
Track-to-train systems for OPO are regarded as safety related. The paramount issue is one of system integrity and the need to ensure that, under all circumstances, pictures from the correct platform are displayed to the driver and that crosstalk, reflections and picture freeze are never displayed. In an environment where platforms can appear almost identical, it would be disastrous to receive pictures from an incorrect platform. Special consideration must be given to this where highly complex track layouts exist, together with a multitude of station designs as is the case on many metro systems such as London Underground.
On initial consideration, there are various technologies that would provide a viable transmission solution:
• optical transmission - normally utilising infra-red LED or laser
• wi-fi - widely available off-the-shelf equipment
• microwave transmission - specialised equipment, adapted from point-to-point links
• leaky feeder cable transmission - KeTech’s chosen technology
After in-depth assessment of the available technology types, it became apparent that each of them exhibit characteristics which in some way make them challenging to integrate into a railway environment. The key issues are:
Optical systems
A number of optical systems have been used for this type of application. Generally, however, these have been utilised for minimal or static coverage applications. Difficulties with line of site, handover between transmitters and dust build are key limiting factors when using optical systems for this application. Two generic technologies exist:
Laser transmission
Laser transmitters have the advantage of multi-channel capability, however these generally exhibit very narrow beam patterns. This leads to difficulties when trying to accommodate coverage over curved track areas, which require multiple transmitters. Key points when considering the use of laser technology are:
• can provide multiple video channels
• suffers with critical alignment – installation and maintenance difficulties
• susceptible to dust build up and adverse weather conditions – reduction in system performance
• range issues and track curvature - requires the use of multiple transmitters giving rise to interference problems at the point where transmitters overlap
LED transmission
LED transmission is generally limited to a single video channel although this may be increased by the implementation of video multiplexing (reduces frame rate). For track-to-train applications, similar issues exist to those of laser systems. Some LED systems have employed tubular lenses to help improve acceptance angles.
• generally single channel devices transmitting baseband video signals.
• susceptible to dust build up and adverse weather conditions – reduction in system performance
• range issues and track curvature - requires the use of multiple transmitters giving rise to interference problems at the point where transmitters overlap
Wi-fi
Wi-fi has a number of prohibiting issues for this application, chiefly that the incoming camera signals need to be digitised in order to allow transmission. This introduces latency and the potential for ‘frozen’ pictures on the train operator’s display. Other significant issues are:
• utilises a public access band – open to interference from domestic Wi-Fi networks
• subject to reflections and multi-path effects
• inefficient use of transmission bandwidth
Microwave
Microwave transmission is capable of providing high quality video performance and multiple channels. There are, however, several key issues giving rise to implementation difficulties in a railway environment:
• requires active transmission equipment to be installed trackside leading to access, clearance and maintenance issues
• requires complex frequency management to prevent adjacent transmitters interfering with each other
• unproven for complex scenarios and therefore carries a significant implementation risk
• susceptible to reflections and multi-path effects (can be reduced by the use of COFDM)
• generally relies on received radio signals from beacons to provide channel switching which gives rise to the potential for crosstalk
• licensing regulations are unclear depending on frequency band utilised
• transmitter positioning is site-specific. Track clearance issues and local features may prevent ideal locations being achieved leading to coverage compromise
Leaky feeder
Leaky feeder video transmission has been in use for around 15 years on several London Underground lines and has continued to provide robust and reliable service. Utilising analogue modulation techniques, leaky feeder can provide high quality, multi-channel transmission with zero latency pictures.
The transmission link operates by feeding radio signals (containing camera pictures) into a specially designed slotted cable called leaky feeder. This cable is installed for the length of the platform, underneath the nosing. Train-borne antennas adjacent to the cable receive the signal which is then processed and displayed to the driver via LCD monitors in the driver’s console. Channel diversity is employed along with train-borne system intelligence to ensure that correct platform pictures are always displayed.
Key benefits of leaky feeder technology
• simple installation – only cabling is required trackside as transmitters are located in equipment rooms
• full bandwidth, non-compressed video transmission
• in-built channel switching intelligence - interfaces to train-borne signalling to provide highest integrity and eliminate crosstalk issues
• common solution for all sites - channel switching intelligence is train-borne
• works consistently at both surface and sub-surface sites
• very low power operation - no licence required
KeTech’s transmission system directly interfaces to train-borne management and signalling systems to facilitate channel switching, providing a ‘closed’ system not reliant on received signals for channel switching and ensuring the highest level of integrity. This entirely eliminates the possibility of incorrect platform pictures. In addition to providing systems for ‘live’ railways, KeTech has installed its track-to-train System on a local private railway which is invaluable for providing a realistic demonstration facility.
Following its extensive review of currently available technologies, KeTech is convinced that leaky feeder is the most reliable, flexible and proven technology to deliver track-to-train video communications for safe train departure, whilst the fundamental reliability of the technology ensures that whole life costs are kept to an absolute minimum.
Additionally, trackside installation requires nothing more than leaky feeder and co-axial cables to be installed under the platform nosing. Other technologies generally require active track-side equipment to achieve full coverage, leaving electronic equipment exposed to the elements and vulnerable to damage during permanent way works.
KeTech is currently implementing a leaky feeder system as part of the Victoria Line upgrade project, working with Bombardier to integrate receiving equipment onto new trains.
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