Frequency Division Multiplexing (FDM) Explained

What is Multiplexing?

Multiplexing is a technique that allows the simultaneous transmission of multiple signals through a single channel or link. In multiplexing, several signals are combined into a single composite signal and transmitted over a single channel or medium. Using multiplexing the channel bandwidth can be utilized more efficiently or more signals can be transmitted through a channel at the same time.

Types of Multiplexing :

Although there are several types of multiplexing techniques, but mainly there are three multiplexing techniques.

  1. Frequency Division Multiplexing (FDM)
  2. Time Division Multiplexing (TDM)
  3. Wavelength Division Multiplexing (WDM)

What is Frequency Division Multiplexing?

In Frequency Division Multiplexing, the different message signals are modulated at the different carrier frequencies. In this way, the modulated signals are separate from each other in the frequency domain. The modulated signals are combined together to form the composite signal and this signal is sent over the shared medium or channel. To avoid the interference between the two message signal, some guard band is also kept between the two message signals.

Fig. 1 shows the general block diagram of the Frequency Division Multiplexing scheme.

Fig.1 General Block diagram of Frequency Division Multiplexing

As shown in Fig.1, three different message signals are modulated at different carrier frequencies. And then they are combined into a single composite signal. The carrier frequencies of each signal should be selected such that there is no overlapping of two modulated signals. In this way, in the multiplexed signal, each modulated signal is separated from each others in the frequency domain. (as shown in Fig.2)

Fig.2 Frequency Division Multiplexing Technique

At the receiver, using the bandpass filter, each modulated signal is separated from the composite signal and demultiplexed. And by passing the demultiplexed signal through the low pass filter, it is possible to recover each message signal. So, this is the typical Frequency Division Multiplexing scheme.

Applications of Frequency Division Multiplexing Technique

  1. AM and FM radio Broadcast
  2. Cable TV
  3. Satellite Communication
  4. Telemetry
  5. Telephony Systems
  6. Early Generation
  7. Cellular Networks

Frequency Division Multiplexing Hierarchy in Analog Telephony System

Voice signals typically contains the information from 300 to 3500 Hz. And including some guard band, each voice signal is assigned a bandwidth of 4 kHz in the telephony system. Each voice channel is single sideband modulated and then multiplexed. In the first level of multiplexing, 12 voice signals are multiplexed. The multiplexed signal occupies the band from 60 kHz to 108 kHz or the total bandwidth of 48 kHz. The carrier frequency of each voice signal is 4 kHz apart from each other. And the multiplexed signal is called group.

In the next level, 5 such groups are multiplexed and form the supergroup. The supergroup has bandwidth of 240 kHz contains total 60 voice channels.

In the next level, 10 such super groups are multiplexed and forms the Master group. The master group contains total 600 voice channels or voice signals and occupies the total the bandwidth of 2.52 MHz.

Similarly, in the next level, 6 such master groups are combined to form a Jumbo group which contains total 3600 voice channels.

The multiplexing hierarchy in analog telephony system is shown in Fig. 3

Fig. 3 Multiplexing Hierarchy in Analog Telephony System

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