## Disdvantage of FDMA

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. |

Frequency stability and the need for guard-bands has traditionally been a bigger problem for FDMA use, requiring very costly and high stability oscillators in the modems if the guard-bands are to be kept to a minimum. In recent years, the use of a broadcast off-air reference has been exploited to allow designers to dispense with these costly oscillator components and achieve much greater stability than hither too possible. (Today, it is possible to buy watches that take their timing reference 'off-air' for millisecond precision accuracy.) The major disadvantage of FDMA in a wireless environment is the susceptibility of any individual narrow frequency slot to frequency selective fading which can cause loss of signal for that user – usually on a temporary basis.

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