COFDM
Of the three media for DTV signals considered here, terrestrial broadcasting in the UHF band presents the most difficult passage from transmitter to receiver. This kind of transmission is vulnerable to multipath reception, fading, interference and co-channel effects, which vary between channels, even closely spaced ones. The receiving aerial arrangements are the most unpredictable, and more at the mercy of the viewer’s whim, situation and budget than for other delivery systems. ‘Ruggedness’ is the main requirement here, then, and COFDM (Coded Orthogonal Frequency Division Multiplexing) is used in conjunction with the 64-QAM system described above for cable DTV distribution.
Fig. 12.14 illustrates the spectrum of a 2K COFDM transmission as used for UK DTV transmissions. The channel slot is tightly and efficiently packed with 1705 separate carriers, each of which is 64-QAM modulated at a relatively low symbol rate. This latter – in conjunction with an inter-symbol guard period – gives increased protection against signal echoes due to multipath reception. The car- rier cycles are in quadrature, so that sampling any one at peak catches the others passing through zero. An elaborate data-protection system (convolutional coding) is used for transmission; it cross-references data codes so that those lost by fading or corruption can be recovered. The result is a very ‘benign’ signal, amenable to reception on inefficient aerial systems, even set-top types in many cases.
The low carrier-to-noise (C/N) ratio required for successful DTV reception, plus the efficient and even use of the spectrum slot (Fig. 12.14) permits the use of much lower transmitter powers for digital services, and for a similar coverage area the E.R.P. (Effective Radiated Power) of a terrestrial signal can typically be reduced by 20 dB for DTV; this represents only 1% of that for an analogue signal.