Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : Medium

Frequency division multiplexing

• Frequency division multiplexing is a technique of separating the signals in frequency domain. In other words, many narrow bandwidth channels are combined and transmitted over a single wide bandwidth transmission system without interfering with each other. Thus FDM takes up a given bandwidth and subdivide it into narrower segments with each segment carrying different information.

• FDM is an analog multiplexing scheme where the information entering the FDM system must be analog and it remains analog throughout the transmission.

• If the original source information is digital then it must first be converted into equivalent analog signal and then multiplexed in frequency domain.

• A common example of FDM is the commercial AM broadcast band. 535 kHz to 1605 kHz is the frequency spectrum occupied by the AM band.

• Information signal at each broadcast station occupies a bandwidth between 0 Hz and 5 Hz. If we transmit information from each station with the original spectrum, then it would be difficult to differentiate one station’s transmissions from another. Thus to avoid this situation, each station amplitude modulates a different carrier frequency to produce a 10 kHz signal.

• Since the carrier frequencies of adjacent stations are separated by 10 kHz signal, the total commercial AM broadcast band is divided into 107 slots with 10 kHz frequency, which are arranged next to each other in the frequency domain.

• The receiver tunes in to a particular frequency band associated with the station’s transmission in order to receive that particular station.

• The Fig2 shows FDM technique applied to the commercial AM broadcast station for transmission on a common medium.

• FDM technique finds application in commercial FM and television broadcasting, telephone and communication systems etc.

  

  Fig.1 Frequency Division Multiplexing