This occurs because the higher frequency components become reduced in
amplitude due to more filtering performed on the information signal and secondly because memory
is inserted into the modulation symbol. The term memory used to describe the present phase change
is dependent not only on the present information bits but also on the past and filtered bits as well.
An alternative way to describe the same behavior is to consider the effects of the band limiting on
the signal constellation diagram (see Fig. 2.69). The constellation on the left shows the unfiltered
MSK phase states. When a Gaussian LPF is used, the phase transitions have been slowed down to
obtain a spectrally efficient transmit emission spectrum. This inserted memory causes ISI which we
have shown as large dark circles to emphasize a scatter of points around each state. As we decrease
the BTb product, the radius of the four large circles increases, resulting in BER performance degradation
since the minimum distance dmin decreases. The values of K have been investigated for various
values of BTb. (Consult [34] for further details.)
In Fig. 2.70, we plot the I-channel eye diagram of MSK, assuming a 16-time-oversampled signal.
This diagram clearly shows the absence of ISI.
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