Thus far, we have only considered bandpass data modulation schemes
using binary signalling, and with only one property (amplitude,
frequency or
phase) of the carrier wave being altered. We have also seen that for binary ASK, FSK
or PSK, the minimum bandwidth required for transmission is at least twice the minimum
bandwidth required to pass the baseband binary data stream.
In order to improve on the bandwidth efficiency of bandpass data transmission, we can of
course increase the number of symbol states used (except for the case of FSK, where
increasing the number of frequencies can increase the occupied bandwidth). As a general
rule, however, we know that as the number of symbol states is increased, the tolerance to
noise is reduced. We shall see that there are two exceptions to this rule, QPSK and
orthogonal M-ary FSK.
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