GPS Signal Re-Radiating in Tunnel Networks - ViaLite

GPS Signal Re-Radiating in Tunnel NetworksCase Study 011This case study describes the application of RF over fibre technology to overcomeproblems of transporting GPS, Galileo and Glonass signals through tunnel networks.IntroductionThe use of GPS signals is now pervasive in everydaytechnology as they provide high accuracy location ortiming data. This enables devices that range fromtracking systems through to cellular phone networks tofunction effectively over wide geographic areas. Takingthe example of a cellular network, the base stationsrequire accurate timing information to ensure successfulhand over of connections between the cell sites.Depending on the topology employed this timing can beprovided by either E1 sync source from the backhaulnetwork to which the remote site is connected or from aGPS signal directly received at the cell site. With theemergence of IP based networks, time synchronisationvia GPS will become more prominent as no timereference is provided within the core network structure.Network SynchronisationWhere GPS signals are used in network synchronisation,significant issues may arise when the base station doesnot have direct line of sight to the GPS satellite. Thisissue is overcome by fitting a remote antenna that has lineof sight and can feed the base station with the GPSsignal. Often the positioning of such remote antennas foroptimum signal reception can be some distance from thebase station. GPS signals are very weak which meansthat the signal maybe too weak to use when carried overcopper cabling. This problem is solved by routing thesignal over fibre from the remote antenna to the basestation equipment.Secondly, Metro GPS offers up to three RF channels sothat fully redundant systems can be accommodated insituations where multiple antennas feed one equipmentlocation. Metro GPS provides a point to point signaltransfer system, but PPM have also provided othersystems that cope with point to multi point requirements.Variants of the Metro GPS system can be used to transmita variety of RF signals up to 4GHz in bandwidth.GPS in Tunnel Networks:Case Study: European Train Control System (ETCS)ETCS is the automatic train control system and will be aunifying standard for rail networks across Europe, Russiaand beyond. The system utilises the GSM-R or TETRAradio network to communicate train position information toa remote control centre. Train position is currentlyidentified by trackside sensors and radio beacons(Eurobalise) but in future GPS and Galileo positioning willbe used as a primary reference as shown in figure 1. Animportant element of this system will be to provide GPSsignals within tunnels along the route of the railway sothat the train position is always known.GPSFigure 1General scheme for ETCS using the GPS or GalileoSatellite constellation for positioningGLONASSGALILEOApplicationsThe requirement for GPS re-radiation within buildings andsubterranean locations such as mines and tunnelscontinues to attract much attention. In response to marketneeds for remote GPS antennas within high-rise officeblocks, PPM successfully launched Metro GPS, a turnkeyfibre optic remoting solution. Metro GPS overcomes twoproblems presented in high-rise building GPSinstallations. First, by making use of a preinstalled singlemode fibre networks that normally exist in high-risebuildings the GPS signals can be routed long distancesfrom roof top antennas to equipment located in thebasement.SatellitePositioning DataData onTrainPosition& SpeedGSM-R /TETRATracksideBaseTransceiverStationETCS RBC(RadioBlockCentre)ViaLite – The Experts in RF-over-Fibre Connectivity

This creates problems for the weak GPS signals due tothe distances involved. PPM has developed an optical reradiationsystem to overcome this problemFor the ETCS project PPM designed and manufacturedthe fibre optic head-end shown schematically in Figure 2.The electrical signal is converted into an optical signalwhich is coupled into a passive optical splitter thatproduces a customer defined number of optical feeds fordistribution to the nodes in the tunnel. In this particularcase both primary and secondary GPS (RF) feeds wereused for redundancy purposes to maintain full signalintegrity.Figure 2Schematic of Fibre Optic GPS Distribution Head-EndThis technique enables GPS extension into the 1150mtunnel without any appreciable degradation to re-radiatorperformance. Figure 4 shows a representation of thedeployed multiple fibre channel point-to-multipointdistribution used by the rail operator.Figure 4Deployment used by European rail operator, showing theGPS head-end outside the tunnel and receiving nodesspaced at 150 metres within the tunnel.CoaxialCableElectro-Optical150 metersPrimaryRXRXGPS AntennasOpticalSplitterRXSecondaryRXPrimary RF FeedFibre Optic TxModulePrimaryFibreChannelsSecondary RF FeedFibre Optic TxModuleSecondaryFibreChannelsPPM Fibre Optic Head EndOpticalSplitterEach fibre channel feeds GPS signals over single modefibre optic cable to a GPS receiving node (Rx) shownschematically in Figure 3. The receive nodes are locatedinside the tunnel and spaced every 150 metres apart. Ateach receiver node, the GPS signal is converted back intoan electrical signal before being fed to GPS antennae forre-radiation within the tunnelThe fibre optic system outlined in this case study providesa transport platform that enables extended reach intotunnels, and can typically be deployed in tunnels over30km in length. The point-to-multipoint configuration hasa high level of redundancy already designed into thetransmission fabric. The head-end hardware comprises astandard outdoor enclosure (IP65 rated & NEMA 13)populated with redundant converters and fibre opticsplitters and distribution. Each re-radiation stage ishoused in a standard enclosure (IP66 rated, NEMA 13rated) that feed re-radiating antennae. This designprovides a primary and secondary path for the GPSsignals which are both in operation at all times.Figure 3Schematic of Fibre Optic GPS Receiver Node (as used ina Point-to-Multipoint Install)PrimaryFibreChannelFibre Optic RxModuleGPS Re-RadiatorsConclusionsThis case study has described the principle of GPStransmission over fibre and some of the large number ofapplications that benefit from using this technology. Theexample of the ViaLite Metro GPS product is given for usein building distribution, and the case of the European railoperator that required GPS distribution through tunnelnetworks has been given in some detail.SecondaryFibreChannelFibre Optic RxModulePPM Fibre Optic Receiver NodeThe following links provide more general information onthis topic.http://en.wikipedia.org/wiki/European_Train_Control_Systemhttp://www.ertms.comhttp://www.uic.asso.fr/uic/spip.php?rubrique850Pulse Power & Measurement Ltd, 65 Shrivenham Hundred Business Park, Watchfield, Swindon, Wiltshire SN6 8TY, UKTel: +44 (0)1793 784389 Fax: +44 (0)1793 784391 Email: sales@ppm.co.uk Web: www.vialite.com