Channel Spacing and Necessary Bandwidth in FDM-FM Systems

Necessary bandwidth is defined by the International Telecommunications Union as: ". . . bandwidth sufficient to ensure the transmission of information at the rate and with the quality required for the system employed . . . Z'.1 Radio channel assignments are based on necessary bandwidth, i.e., a user is authorized a band at least equal to the necessary bandwidth. If he wants two adjacent channels, he gets another band equally wide and the two are usually not allowed to overlap. For many signals this is a straightforward and accurate way to assign channels. For instance an AIM broadcast signal is well contained within a specified frequency band. In FM systems however the case is not so clear. An FM signal theoretically has infinite bandwidth and bandlimiting will cause distortion. On the other hand putting two FM signals next to each other in the frequency band and not bandlimiting them will cause mutual interference. The present way of calculating the necessary bandwidth for FM signals is to use Carson's rule which states that Bn = 2(/, + fv) where 869