63 Offset QPSK method of generating an MSK waveform.
This method will produce an MSK signal S(t) at the output, with an apparent carrier frequency
of and deviations and . The impulse response of the conversion
filter, g(t), is given as
(2.40)
This filter is offset, in frequency, from the center of the BPSK signal generated at its input. This
method was introduced for a few reasons: first, for the sake of completeness to show the special properties
this modulation scheme has and, secondly, to introduce the relationship MSK has to the PSK family.
Another method used to generate MSK waveforms is treating this modulation scheme as if it were
a member of the PSK family, specifically OQPSK. We will present a method used to generate MSK
as treating it as OQPSK with sinusoidal pulse shaping [28]. This can be shown by the modulator block
diagram in Fig. 2.63.
In Fig. 2.64, we present the waveforms at various points in the modulator block diagram given above.
In comparing these baseband waveforms to those shown earlier, one can easily see that they are
different. Hence if the waveforms are detected using the same receiver, then the resulting bits would
be different as well. However, pre- and/or post-processing can be performed to extract the correct
information [29].
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