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BBC R&D White Paper WHP099

PLT and broadcasting - can they co-exist?
J.H. Stott

Keywords
PLT, PLC, BPL, EMC, radio broadcasting

Abstract
The use of Power-Line Transmission (PLT) to convey data signals along mains wiring, for internet access or home networking, is briefly described.

The paper then considers the threat that emissions from PLT system pose to reception of radio broadcasting, with a description of the use made by broadcasting of spectrum below 30 MHz, the way in which reception is in principle protected from interference by international agreement, and considers whether existing regulatory proposals actually deliver this protection.

Examples are then presented of the deleterious impact of several access-PLT systems on reception of HF radio broadcasting, as noted in brief field trials in Crieff. Separate experiments on a commercially-available home networking system show that the notches it implements with the intention of protecting the frequency bands allocated to radio amateurs perform as specified, but also show that it too interferes with in-home reception of radio broadcasting – and that in certain circumstances it can function as a wireless network!

A proposal is made for using notches in the spectrum used by PLT systems in order to facilitate co-existence with reception of broadcasting. It is explained that this would require near-instantaneous flexibility in the placing of notches, as broadcasting makes flexible, constantly-changing use of spectrum in the HF band. This would make human control of the placing of notches impracticable; furthermore, it would in any case be undesirable since it would raise questions of censorship.

A possible way forward is proposed: the PLT equipment should determine which parts of the spectrum are in use, and avoid them. An experiment demonstrates the principle, showing that measurements of the RF signal present on the mains can indicate the use of the spectrum by broadcasting.

It is concluded that regulation of emissions by a simple limit alone cannot achieve co-existence of broadcasting and PLT, but an approach in which PLT equipment operates as proposed – avoiding spectrum in use – could maximise the capacity of PLT systems while simultaneously protecting reception of radio broadcasting.

This document was originally presented to the EMC UK 2004 Conference in Newbury on 12 & 13 October 2004. It contains the paper, as printed in the Conference Proceedings, with its page numbering, together with the presentation aids: slides, some audio samples and a brief video.
The paper is also reprinted in the EMC & Compliance Journal, issue 55, November 2004, pp16 to 26.

Recordings downloads
The author's presentation included some audio samples and a brief video:

Slide 7
Audio recordings of interference caused to short-wave broadcast reception by the Main.Net PLT system, as recorded in a PLT subscriber's house.

(a) with PLT modem quiescent (wav format 979 Kb)

(b) with PLT modem busy (wav format 836 kb)

Slides 13 and 14
Audio recordings demonstrating the benefits of digital broadcasting using the DRM system.

(a) reception of AM (analogue in Erlangen, Germany of a broadcast from Orford Ness (wav format 3.4 Mb)

(b) reception of DRM (digital), over the same path on the same frequency, at nearly the same time (wav format 3.4 Mb)

Slide 24
Video demonstrating use of a HomePlug PLT network, its impact on radio reception, and that the network can also function as a Wireless LAN. (Real Media format 19 Mb)

(Note that this file has been reduced in size for your convenience and is therefore of inferior picture quality to that used for the presentation itself).