How do you calculate/compute/determine the spectral coverage of a frequency spectrum? 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?
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.


FSK requires as many as Dimensions as the number of Points you want to transmit.
ASK is one-dimensional.
QPSK is two-dimensional.
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.