Shown here is a comparison of the bandwidth occupancy for unfiltered
QPSK and Minimum Shift Keying. Both schemes have a
constant envelope property and thus are tolerant to TX/RX gain distortion. While MSK has a
slightly wider main lobe than QPSK, the side-lobe power falls off much more rapidly than
that for QPSK, making this system more attractive when adjacent
channel interference is an important design consideration.
Of course, the QPSK signal can be filtered to reduce the side-lobe energy to any value
desired, but will inevitably introduce amplitude variations
in the filtered waveform, requiring much more linear processing in the communications
link. The data feeding the MSK modulator can also be filtered to reduce the side-lobe
energy a little further, an example being GMSK.
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