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I'm comparing frequency spectrum of FSK, ASK, QPSK, and MSK. And I'm asked which has the least and greatest spectral coverage. Could someone enlighten me on how to get it?

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  • $\begingroup$ Could you define spectral coverage? $\endgroup$ – Naresh Mar 14 '13 at 16:10
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Spectral coverage, I assume, means the Bandwidth required to transmit the incoming signal. A simple formula(approximate) for the Spectral Efficiency of Memoryless Modulation Schemes(MSK is not one) which gives the ratio between Data Rate and Bandwidth required is:

R/W = (2*logM)/N

where R is the incoming Data Rate and W is the required Bandwidth; log is in Base 2, M is the number of points on the Signal Constellation and N is the number of dimensions required to use the particular signalling.

  1. FSK requires as many as Dimensions as the number of Points you want to transmit.
  2. ASK is one-dimensional.
  3. QPSK is two-dimensional.
  4. MSK is more spectrally efficient than the other 3 and I think the formula for it's Power Spectral Density and its 3dB value can be found in all Digital Communication Texts.
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