13246_CAI_Feedback_Mar2018_COMPLETE_v1

received from it looks like noise, and is called co- channel interference. If too much of this signal is received, then it may not be possible to meet the 20dB requirement to successfully receive the wanted signal. In most cases, this means that a channel cannot be used again in an adjacent area; it can only be used again at some greater distance. This is illustrated in the hexagon diagram, which shows an idealised set of coverage areas, each with a transmitter at the centre. Choose any coloured hexagon (also numbered for clarity) and you can see that it is not re-used until about two hexagon diameters away. This is the basis of a multi-frequency network (MFN); several RF channels are needed to cover a region (i.e. an area larger than one transmitter can cover) with just one multiplex. In this configuration, to provide complete coverage of the whole region, with each area having just one multiplex, a total of seven channels are needed. Frequency planners would call this a re-use 7 network. In reality of course coverage areas are not hexagonal, and the effects of terrain, receiving aerial patterns, and many other factors come into play. Typical real DTT networks are more likely to be around re- use 5. Single Frequency Networks There is a particular case where the interference from an adjacent transmitter does not appear like noise. We already looked at how DVB-T was designed to ignore reflections that fall within the Guard Interval. What if the signal from the adjacent area’s transmitter could be made to look like a reflection? It can look like a reflection if the adjacent area’s transmitter:- n is carrying exactly the same content; and n is configured in exactly the same way as the intended transmitter for that area; and n radiates its signals timed so that when they arrive at the receiver, they are within the Guard Interval of the intended transmitter’s signals. If these three conditions are met, then a transmitter for an adjacent area can use the same channel as the intended transmitter, because the receiver sees its interference as being no different from a reflection. Clearly there’s a problem for much of the UK network: regional content. The conditions above require that all the transmitters in a SFN carry exactly the same content; any differences in content would act like harmful interference. Fortunately the content of the COM7 and COM8 multiplexes can be arranged to be the same over the entire country, so they can be set up as SFNs, using just two channels to cover much of the country. SFNs are said to be spectrally efficient, meaning that they can cover large regions with one RF channel (re-use 1). With pressure on broadcast spectrum from the 800 MHz and 700 MHz clearances, multiplexes are being packed more tightly into the remaining spectrum. Without SFNs, COM7 and COM8 would have been much more restricted in the coverage they could have achieved. A set of idealised coverage areas with re-use 7 39 CAI Feedback Spring 2018