Proposal regarding Power Line Telecommunications (PLT or PLC)

By R C Marshall, Richard Marshall Ltd

 

Introduction

This proposal has been stimulated by attendance at  “EMC 2003” - the 15th International Zurich Symposium February 18-20, 2003, and especially by the papers presented there.

 

Thierry Brefort of DG Enterprise of the European Commission said in paper 2W2 that PLT was interesting because an EU strategic goal was to become the most dynamic economy in the world,  and communication was vital to that.  PLT offered competition in the “last mile”, but it raised difficult regulatory questions that should be solved within the framework set  by Standards and the EMC Directive.

 

Many speakers made the point that the range of emission limit figures that are being advocated is so wide (60 to 70 dB - up to three thousand to one!) that a compromise is not sensible because it would suit no-one.  They all sought a more radical approach and a more serious technical engagement between the advocates of cable communications and the radio communications community.  In these circumstances prior proposals may be too simplistic to provide a practical solution. For example, the range from 150KHz to 30MHz encompasses seven and a half octaves of frequency, over which the radiation properties of antennas and the propagation characteristics of the earth and the ionosphere vary greatly.  Statements applied to more than one octave are almost certainly open to challenge.

 

This document is intended to encourage wider debate and a more radical approach.  It may be freely copied and used for this purpose. 

 

Comments and additions will be welcomed by the author richard.marshall@iee.org

 

Problem Analysis and Overview

The EC goal of competition in the local loop by means of PLT (It is not the only competition:  Radio and CATV cabling are valid alternatives) can only be met without upset to radio communications if the two can be separated.  The following forms of separation occur to the writer.  Can you think of more?

 

1.            Separation in Space

 

1.1          Installation of PLT might be avoided near sensitive receivers.  This appears impossible within the home (near field effects), within the city (far field effects), or within the world (skywave effects).

 

1.2          PLT signals could be restricted to underground or metal-sheathed cables, and absorbers applied wherever conductors leave this environment.

 

1.3          Separation of in-house and access wiring is a form of spatial separation.

 

1.4          There are various types of Network that should be considered.  This is inherent in 1.1 and 1.2 but is important in its own right.

 

These last three are considered further in the detailed commentaries below.

 

2.            Separation in Time

 

2.1          Minimising PLT emission when no useful data is being transmitted would be one such measure.  Appropriate PLT system design could be encouraged by including a long-term average emission limit in the Standard.

 

3.            Separation in Frequency

 

3.1          There is an ISM band at 13.41 to 13.57MHz.  By analogy with the 2.4GHz ISM band this could be used for PLT at relatively high power without causing difficulty to other spectrum users.  Whilst it is very narrow because of the value of this part of the spectrum it might be used to provide a high-integrity synchronising (clock) signal to allow PLT system design to cope at other frequencies with lower signal/noise ratios, and hence lower powers.

 

4.            Separation by Polarisation

 

4.1          Power line distribution is primarily by horizontal cables:  Radio communication below 30MHz largely (but not entirely) uses vertical antennas and so vertical polarisation.  There is therefore a real possibility of some limited separation provided that the emission of PLT signals from vertical cables is minimal.

 

Detailed Commentaries on the above Analysis

1.2          Distribution of PLT signals on a network restricted to underground or metal-sheathed supply cables is unlikely to cause serious interference;  It is customer cabling such as that in domestic premises and street-lighting that is likely to radiate.

    

If absorbers were provided at every transition point from supply to customer then a class of network would be created that could meet the objective of an alternative local loop with little impact upon radio communications.  It would also display much superior PLT transmission characteristics, with fewer and more stable reflection and attenuation effects,  and should be able to offer a higher data rate.  At least one PLT contender already uses a modem at the customer’s meter point as a translator between in-home and access signalling.

    

A class of PLT modem could therefore be standardised at a relatively high power level provided that it was only attached to a network that met specified emission requirements.  It may be expected that such a network would be wholly underground with absorbers at every customer metering point.

 

Low cost absorbers are evidently required in considerable quantities.  It may be envisaged that easily-installed devices developed from today’s clip-on ferrite sleeves would be suitable; there would be strong financial incentives to develop these.  (The ferrite materials available in this construction today saturate too easily for use on single power conductors as would be required to attenuate differential-mode signals).  Installation when reading the billing meter would be the aim.

 

Responsibility for establishing and maintaining network classification must be the responsibility of the network owner, and a network standard will be required.  Technically this is a task closely akin to the work currently in hand within CISPR/I to specify the Longuitudinal Conversion Loss (LCL) for incorporation into the CISPR22 “T” Network for use with multipurpose ports.

 

1.3          The provision of absorbers at each transition point from supply to customer should directly remove the near-field interference problem within the homes of customers and non-customers alike. 

 

1.4          It is entirely possible that other classes of network with adequate electromagnetic compatiblity at lower data rates might be identified that is still commercially attractive (a network including wide-spaced overhead conductors, for example, or two-wire networks with multiple grounds.)

 

4.1          It would be sensible to base PLT emission requirements upon the emission level of vertically polarised interference.

 

Since vertical cable runs are largely confined to customer premises much of the advantage of separation by polarisation should be achieved without further effort if 1.2 and 1.3 are adopted.

 

5/03/2003                                                                  

R.C Marshall, MA, CEng, FIEE,  EMC Consultant

30 Ox Lane, Harpenden,  Herts. AL5 4HE,  UK