PLT and broadcasting - can they co-exist?
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 trails in Drieff. 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 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-existance 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.
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