Carson's rule does not apply well when the modulating signal contains discontinuities, such as a square wave.
(Geographically close FM broadcast transmitters are almost always assigned nominal center frequencies at least 400 kHz apart).
Carson's bandwidth rule is often applied to transmitters, antennas, optical sources, receivers, photodetectors, and other communications system components.
Any frequency modulated signal will have an infinite number of sidebands and hence an infinite bandwidth but, in practice, all significant sideband energy (98% or more) is concentrated within the bandwidth defined by Carson's rule.
less than the carrier inside, therefore Carson's Rule is of little use in spectrum planning.