A disadvantage that has often been levelled at FDM/FDMA is the
inflexibility to accommodate variable user data rates within a fixed bandwidth frequency
slot. This claim is nowadays unfounded for two reasons. Firstly, it is practical to vary
the data rate in a given frequency slot by increasing the number of symbol states used.
Secondly, it is possible to assign a user more than one frequency slot, or introduce the
concept of a variable bandwidth slot in order to vary the user data rate. Both of these
solutions rely heavily on the advent of powerful digital signal processing devices that
can implement adaptive rate multi-symbol modems (these are now commonplace in line modem
cards), and variable bandwidth matched channel filters again a simple function for
today's DSP devices. |
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