R&D Report 1993-12 : Predicted coverage of a COFDM Single Frequency Network for UHF digital terrestrial TV broadcasting M.B.R. Lee The Coded Orthogonal Frequency Division Multiplex (COFDM) transmission system provides a very high degree of tolerance towards multipath interference. Three factors contribute towards this tolerance; the inclusion of a guard interval, coding to provide error correction and the division of the digital data amongst a very large number of low bit rate carriers. It follows from this that a national network of co-channel transmitters can be developed such that not only do neighbouring transmitters not cause mutual interference but can combine in a constructive manner. This is the principle of the Single Frequency Network, or SFN. Experience with Digital Audio Broadcasting (DAB) suggests that an SFN could be utilised for Digital Video Broadcasting (DVB). Such a system would make very efficient use of the available UHF channels. The report presents the results of coverage predictions performed for a network of 128 transmitters sited at the locations of existing high and medium power UK UHF broadcast transmitters. Predictions were carried out for a range of values of guard interval, and for both directional and omnidirectional receiving antennas. Use of COFDM and the latest data compression techniques allows a High Definition Television (HDTV) programme to be transmitted in a standard UHF channel. However it will require a higher-order modulation scheme than is used for DAB. This will reduce the tolerance towards noise and interference. The guard interval may therefore have to be lengthened so that signals from more distant transmitters within the network arrive within the guard interval. The results of the predictions show that the ad hoc transmitter network examined will serve 90% or more of the UK population, using the same guard interval as has been chosen for Mode 1 of the Eureka DAB system, provided that a directional antenna is used. The directional antenna provides most of the extra protection required against interfering signals. The same coverage can be obtained using an omnidirectional antenna if the guard interval is doubled. Alternatively the data could be coded in such a way that a standard definition picture is produced if the level of interference is too high for HDTV.
