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## Spectrum of M-ary PSK

#### 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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