ABSTRACT
SalimSidatMBA,DBA,BSc (Hons), CEng,MIEE Customer SystemsManager mmO2, Ltd
In an ideal situation, all potential viewers would be provided with television programmes by means of a central high power transmitter. This would be modulated with audio and video signals from the studio, connected by means of a landline. Such an arrangement would represent the most economical means of distribution in terms of cost per viewer. Unfortunately, such a simple situation is possible only at a
limited number of locations. Intervening hills, buildings and even trees attenuate direct reception of a high power transmitter, particularly at UHF, causing a shadow in its coverage. Substantial numbers of people can be affected. Rather than provide landlines and expensive modulators at
every transmitter site, a more satisfactory solution is to provide lower power relay transmitters to fill in the shadows. The principle is shown in Figure 6.3.1. The relay is sited at a carefully chosen location which can both see the target area to be served and also receive good signals from the main transmitter. Despite the shadow, in most cases there will actually be some residual signal in the target area from the principal station, so if the relay station were to transmit on the same channel, there would be unacceptable co-channel interference (cci) to viewers of the relay and of the main station. The equipment at the relay site therefore changes the frequency of (or transposes) the incoming signal from the parent transmitter on channel X and re-transmits on a clear channel (channel Y). In some areas it may be necessary to
build a chain of relay stations, each serving its own target area and providing a signal feed onwards to the next. In that case several different channels will be used by the various stations. Although this technique is simple enough in concept, in
countries like the UK (which has around 900 relay sites as well as 50 high power main sites) very complex planning is necessary. This is because each site transmits four different programmes (and possibly five), so groups of channels have to be transposed. As the network expands it becomes increasingly difficult to find four clear channels (even more so now with channels being used for Digital Terrestrial Television) for each site to use. In many cases no completely clear transmit channel can be found, and the service area is limited by co-channel interference from a remote station.
