Tuesday, 18 June 2013

Frequency Division Multiplexing (FDM)

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The bandwidth or range of a medium such as coaxial cable exceeds that of any one given signal like single telephone subscriber frequency. This fact is utilized for frequency division multiplexing. In FDM, the available bandwidth of a physical medium is split up (divided) into several smaller, disjoint logical bandwidths. Each component is used by bandwidths as a separate communication line (channel). Figure 1 illustrates the concept of FDM. Here, signals 1 to 5 on the left get multiplexed through a modulator. The combined frequency of the signal can move through single media. At the receiving end, this combined frequency signal is separated out and flows to the receiver as signals 1 to 5.

The best example of FDM is the way we receive various stations in a broadcast radio receiver. Each radio station is assigned a frequency range within a bandwidth of radio frequencies. Several radio stations may be transmitting electromagnetic signals simultaneously over the physical channel which is “ether” (air) in this case. A radio receiver’s antenna receives signals transmitted by all the stations. Finally, the tuning dial in the radio is used to isolate the specifies signal of the station tuned.

     Ø  In FDM, the signals to be transmitted must be analog signals. Thus , digital signals must be converted to analog form if they are to use FDM.
In Frequency Division Multiplexing (FDM), we share a transmission channel by dividing the bandwidth into several parallel paths, defined and separated by guard bands of different frequencies. All signals are carried simultaneously in the medium.

How FDM is done ?

The analog signal is impressed on another analog signal of different frequency a carrier, altering the carrier’s shape so that it bears the pattern of the message. The carrier frequency generally remains constant. Only its amplitude varies, at the rate corresponding to that of the message signal.

            Since each carrier has a different frequency, carriers can be stacked on top the other and sent together over a cable or microwave radio link capable of carrying a broad range of frequencies. The carriers are then separated at the other end. The greater the medium’s bandwidth, the more carries it can transmit, and more messages it can handle simultaneously.

            Figure 2 shows the process of frequency division multiplexing (FDM) of three voice channels. The telephone exchange takes each voice channel and modulates the signal to a higher frequency as shown.

            The FDM technique of multiplexing requires guard bands to keep signals from interfering with each other. If the signals are modulated to new frequency ranges and without sufficient separation, then the extraneous signals would create noise called cross talk. Thus, as seen as Figure 3, adequate guard band allows several telephone connections to take place through the same trunk circuits using coaxial cable or microwave communication links.   

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