Frequency Division Multiplexing (FDM) is a technique the carrier bandwidth is divided into sub-channels of different frequency widths, each carrying a signal at the same time in parallel.Each channel is 30 kHz. All the signals may be amplified, conducted, translated in frequency and routed toward a destination as a single signal, resulting in economies which are the motivation for multiplexing. Receivers at the receiving end separates the multiplexed signals by means of frequency passing or rejecting filters, and demodulates the results individually, each in the manner appropriate for the modulation scheme used for that band or group.
Wavelength Division Multiplex (WDM) and Frequency Division multiplex (FDM) are both based on the same principles but WDM applies to digitized wavelengths of light in optical fiber while FDM is used in analog transmission such as twisted pair telephone line, cable access, cellular, radio and TV communications. TDMA and CDMA are always used in combination with FDMA, i.e., a given frequency channel may be used for either TDMA or CDMA independently of signals on other frequency channels.
Where frequency division multiplexing is used as to allow multiple users to share a physical communications channel, it is called frequency division multiple access (FDMA). FDMA analog transmissions are the least efficient networks since each analog channel can only be used one user at a time. Analog channels don't take full advantage of band-width. Not only are these FDMA channels larger than necessary given modern digital compression, but they are also wasted whenever there is silence during communication. Analog signals are especially susceptible to noise and the extra noise cannot get filtered out.
FDM: Frequency Division Multiplex and FDMA