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Data communication systems in current use generally require substantially more bandwidth than the Nyquist minimum of one-half cycle per symbol. This comes about for two main reasons: first, the baseband signal spectrum has a gradual roll-off beyond the theoretical minimum;1 second, the modulation process needed to translate the baseband spectrum to the bandpass channel generates additional side frequencies which must be preserved to permit recovery of the signal. For example, a recently described vestigial-sideband system2 uses an extra 50 per cent of bandwidth for each of these two reasons. Consequently, such a system handles one symbol per cycle, and each symbol can convey as many levels as the signal-to-noise ratio permits — independent of all adjacent symbols. Thus, with n levels, each symbol yields log 2 n binary digits. |
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