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Published on November 26, 2007

Author: lawson

Source: authorstream.com

Radio Technology & Compatibility Group Whyteleafe Surrey:  Radio Technology & Compatibility Group Whyteleafe Surrey Leakage Emissions from ADSL & Power Line Telecommunication Networks 14 November 2003 ADSL & PLT Telecommunication Networks:  ADSL & PLT Telecommunication Networks Modern ‘Local Loop’ broadband access systems Based on secondary use of existing cables ADSL utilises existing twisted pair telephone lines PLT utilises the low voltage electricity distribution network Both compete with broadband coaxial cable systems ADSL & PLT Leakage Issues:  ADSL & PLT Leakage Issues Telephone & Power cables were not designed for high frequency use Leakage emissions can occur on ADSL lines under certain fault conditions but will occur inherently on PLT networks. Leakage emissions may be at significant levels and thus have the potential to cause serious interference to local radio reception ADSL & PLT Interference – RA’s obligations :  ADSL & PLT Interference – RA’s obligations ITU Radio Regulations – Chapter IV - Interference 15.12 § 8 Administrations shall take all practicable and necessary steps to ensure that the operation of electrical apparatus or installations of any kind, including power and telecommunication distribution networks, but excluding equipment used for industrial, scientific and medical applications, does not cause harmful interference to a radio communication service and, in particular, to a radio navigation or any other safety service operating in accordance with the provisions of these Regulations. The ADSL Situation:  The ADSL Situation ADSL emissions & interference mechanism:  ADSL emissions & interference mechanism ADSL frequency spectrum encompasses the LW and MW broadcast bands Leakage emissions can occur due to imbalance in the unscreened twisted-pair telephone lines The resultant flow of common mode current generates an electromagnetic field around the telephone line Domestic AM broadcast receivers with integral ferrite rod antennas couple into the electromagnetic field ADSL interference is audible as white noise, a heterodyne whistle or a combination of both ADSL Spectrum:  ADSL Spectrum Actions by RA to limit ADSL interference:  Actions by RA to limit ADSL interference Accepted the positioning of a domestic portable radio between 1 & 3 metres from internal telephone wiring as the default operating condition requiring protection Consulted with Broadcasters and Telcos during the development of draft emission limits intended to protect LW and MW broadcast reception Issued the limits as a draft standard called MPT1570 & conducted a regulatory impact assessment Studied the feedback received & recommended a set of limits for ministerial approval Published MPT1570 containing emission limits from 9 kHz to 1.6 MHz The MPT 1570 Standard – Points to note:  The MPT 1570 Standard – Points to note Not a compliance standard Limits to be applied only for adjudication purposes in cases of reported interference A compromise between the need to offer some protection to radio users whilst allowing Telcos to roll out ADSL without fear of undue expenditure on interference limiting measures So what protect does the MPT1570 limit offer ? ADSL interference at the MPT1570 limit:  ADSL interference at the MPT1570 limit Best demonstrated subjectively with audio recordings These recordings were made at Whyteleafe using BBC broadcast stat0ions adjusted to a ‘edge of service area’ field strength of 66 dBuV/m in LW & 60 dBuV/m in MW The heterodyne case - unmodulated ADSL carriers The white noise case - modulated ADSL carriers Did RA get it right ? An ADSL Interference complaint:  An ADSL Interference complaint ADSL Interference resolution:  ADSL Interference resolution ADSL Interference – The Facts:  ADSL Interference – The Facts Approaching 1.5 million ADSL lines in the UK MPT1570 limit offers only minimal protection to LW & MW broadcast reception RA aware of only 1 substantiated interference complaint so far So how do we reconcile this situation ? ADSL Interference - Observations:  ADSL Interference - Observations The majority of fault-free ADSL lines have leakage radiation well below the MPT1570 limit Any faulty lines are usually repaired quickly & before any interference being experienced has the potential to become a ‘persistent interference’ complainant White noise & heterodyne whistles may not be recognised as external interference by most listeners Most radio listeners do not know how to complain or who to complain to Conclusions:  Conclusions As the ADSL roll-out gathers pace and radio users become aware of the technology further interference complaints are deemed inevitable The situation is likely to be exacerbated by the current use of self-install ADSL products having no filter between the incoming telephone line and customer premises wiring Interference resolution is likely to be more reliant on the cooperation and goodwill of Telcos than the effectiveness of the MPT1570 emission standard ADSL interference is not expected to become a serious issue for Ofcom The PLT Situation:  The PLT Situation Current status of PLT in the UK:  Current status of PLT in the UK A recent competitor in the broadband local loop Scottish & Southern Energy plc is currently the only PLT operator in the UK S&SE has commercial trials underway at Crieff Campbeltown and Stonehaven in Scotland and most recently in Winchester In comparison with other broadband systems the current number of PLT users is still very small RA have received no interference complaints to date The PLT Network:  The PLT Network Comprises an electricity sub-station and those PLT users served by it Typically up to 200 electricity users per sub-station with a potential of up to 20 PLT users Typical sub-station network is 200 to 300 metres long PLT launch power is similar to ADSL at -40 dBm/Hz but high attenuation means that repeaters may be required for maximum reach Each sub-station requires a dedicated high capacity link to service its PLT users PLT signals & spectrum:  PLT signals & spectrum PLT standards and equipment still evolving Several competing systems exist - built around: Gaussian minimum shift keying Direct sequence spread spectrum Orthogonal frequency division multiplex PLT spectrum lies in the 2 – 30 MHz range Band can be split between access & in-house Local Loop access from 2 to 10 MHz In-house distribution from 15 to 30 MHz PLT leakage emissions:  PLT leakage emissions Power cables are not designed for high frequency use They are unscreened and unbalanced to RF Leakage emissions are therefore inherent and unavoidable under normal PLT operating conditions Common mode PLT current generates an electromagnetic field around all power cables connected to a PLT enabled sub-station All properties fed by a PLT enabled sub-station will have high frequency electromagnetic fields radiated by their internal wiring whether they take the PLT service or not Action by RA to investigate PLT emissions:  Action by RA to investigate PLT emissions Field Strength measurement programme during the original Nor.Web PLT trials in Manchester - 1998/9 Field Strength measurement programme on an electricity network using a Gaussian White Noise ‘generic PLT’ signal - 2001 Field Strength measurement programme during the current Scottish & Southern Energy PLT trials in Crieff and Campbeltown - 2002/3 Measurement programme planned for the S&SE PLT trials at Winchester early in 2004 ‘PLT’ Emissions inside a user premises:  ‘PLT’ Emissions inside a user premises ‘PLT’ Emissions ‘next door’ to a ‘PLT’ user :  ‘PLT’ Emissions ‘next door’ to a ‘PLT’ user PLT interference to SW Broadcast reception:  PLT interference to SW Broadcast reception Relatively high emission levels are to be expected in the general vicinity of a PLT network SW broadcast reception in the home using domestic receivers having whip antennas will be particularly susceptible to PLT interference which may also be conducted into the receiver via the mains lead Typical near field strength regression levels of 1/r or 20 dB per decade indicate that external antennas situated within or at the boundaries of normal size domestic premises may also prove inadequate for the interference free reception of SW broadcast signals PLT interference to Amateur Radio reception:  PLT interference to Amateur Radio reception Potential to affect all 10 Amateur Radio allocations between 1.8 & 30 MHz Likely to be particularly problematic due to the low field strength levels involved Nature and extent of interference will depend on the PLT signalling system in use Amateur radio operator may or may not be able to avoid interference by changing frequency or band RA consultation on PLT and VDSL interference:  RA consultation on PLT and VDSL interference RA’s UK technical working group met during 2001/2 to study compatibility between Radio Services and VDSL & PLT operation in the 1.6 to 30 MHz range The UKTWG comprised broadcasters radio users and network operators A full and comprehensive report on the study appears on the RA website The report concludes that neither VDSL or PLT emissions can be reduced sufficiently to meet radio users near-field protection requirements PLT Interference – Points to Ponder:  PLT Interference – Points to Ponder RA measurements and UKTWG studies have shown that PLT deployment is likely to be incompatible with 1.6 to 30 MHz reception within the domestic environment due to substantial near-field emission levels Although Short Wave reception is generally considered a minority interest enjoyed by the technically aware this may change with the introduction of Digital Radio Mondiale broadcasts As PLT networks continue to roll out any interference complaints that do arise will be potentially irresolvable Ofcom will therefore need to consider their policy on this issue as a matter of urgency

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