An important point to note with modulation schemes is that although the carrier signal is periodic, the resultant modulated signal is generally not periodic. (It would be periodic if the modulating signal were periodic, for example if it consisted of the repeating series 1, 0, 1, 0, etc.) Therefore, in frequency terms the modulated carrier wave occupies not just one frequency but a range of frequencies. The signal is said to extend over a certain bandwidth, measured in Hz. (Note that the word ‘bandwidth’ is also commonly used to mean the data rate of a digital signal, but here I am talking about analogue bandwidth.)
The bandwidth of an ASK signal is approximately BASK = 2B, where B is the bandwidth of the modulating signal. This is also approximately true for PSK; BPSK = 2B. The bandwidth of FSK depends how far apart the two frequencies used are. It is approximately:
where 2Δf is the frequency separation of the highest- and lowest-frequency symbols.
It follows that any channel conveying useful information has to use a section of the available frequency spectrum, not just one point on it. For a shared medium such as radio, this means there are limits to the number of channels that can be used at the same time and in the same place. This is a fundamental limitation in practical communications. Spectrum use is thus a major resource allocation problem.