Frequency Division Multiplexing : Block Diagram, Working & Its Applications

The multiplexing technique was developed in 1870, however in the late 20th century; it became much more applicable for digital telecommunications.  In telecommunications, the Multiplexing technique is used to combine and send multiple data streams over a single medium. So, the hardware which is used for multiplexing is known as a multiplexer or MUX that merges n input lines to produce a single o/p line. The multiplexing method is widely used in telecommunications where numerous telephone calls are carried throughout a single wire. Multiplexing is classified into three types such as; frequency division, wavelength division (WDM), and time division. At present, these three multiplexing techniques have become a very significant asset in telecommunication processes & they have very much improved the way that we send & receive independent signals over telephone lines, AM & FM radio, and also optical fibers. This article discusses one of the types of multiplexing known as FDM or frequency division multiplexing – working & its applications.

What is Frequency Division Multiplexing?

Frequency division multiplexing definition is: a multiplexing technique that is used to combine more than one signal over a shared medium. In this type of multiplexing, signals with different frequencies are merged for concurrent transmission. In FDM, multiple signals are merged for transmission over a channel or single communications line where every signal is allocated to a different frequency in the main channel.

Frequency Division Multiplexing Block Diagram

The frequency division block diagram is shown below which includes a transmitter and a receiver. In FDM, the different message signals like m1(t), m2(t) & m3(t) are modulated at the different carrier frequencies like fc1, fc2 & fc3. In this manner, the different modulated signals are separated from each other within the frequency domain. These modulated signals are merged together to shape the composite signal which is transmitted over the channel/transmission medium.

To avoid interference between the two message signals, a guard band is also kept in between these two signals. A guard band is used to separate two wide ranges of frequencies. This ensures that communication channels that are used simultaneously do not experience interference which would affect in reduced quality of transmissions.

As shown in the above figure, there are three different message signals are modulated at various frequencies. After that, they are merged into a single composite signal. Each signal’s carrier frequencies must be chosen so that there is no overlapping of modulated signals. Like this, each modulated signal within the multiplexed signal is simply separated from each other within the domain of frequency.

At the receiver end, bandpass filters are used to separate each modulated signal from the composite signal & demultiplexed. By transmitting the demultiplexed signal through the LPF, it is achievable to recover every message signal. This is how a typical FDM (Frequency Division Multiplexing) method is.

How Does Frequency Division Multiplexing Work?

In the FDM system, the transmitter end has several transmitters & the receiver end has several receivers. In between the transmitter & receiver, the communication channel is there. In FDM, at the transmitter end, every transmitter transmits a signal with a different frequency. For instance, the first transmitter transmits a signal with 30 kHz frequency, the second transmitter transmits a signal with 40 kHz frequency & third transmitter transmits a signal with 50 kHz frequency.

After that, these signals with different frequencies are combined with a device known as a multiplexer which transmits the multiplexed signals through a communication channel. FDM is an analog method which is a very popular multiplexing method. At the receiver end de-multiplexer is used to separate the multiplexed signals then it transmits these separated signals to the particular receivers.

A typical FDM has a total of n channels, where n is an integer greater than 1. Each channel carries one bit of information and has its own carrier frequency. The output of each channel is sent at a different frequency from all other channels. The input to each channel is delayed by an amount dt, which may be measured in units of time or cycles per second.

The delay through each channel can be calculated as follows:

dI(t) = I(t) + I(t-dt)/2 − I(t-dt)/2, where I(t) = 1/T + C1 * 

I(t) = 1/T + C2 * 

I(t) = 1/T + C3 * 

where T = period of signal in units of time (in our case this is nanoseconds). C1, C2 and C3 are constants that depend on the type of signal being transmitted and its modulation scheme.

Each channel consists of an array of photonic crystals that act as filters for light waves passing through them. Each crystal can pass only certain wavelengths of light; others are blocked out entirely by their structure or by reflection from an adjacent crystal.

FDM requires the use of an additional receiver for each user, which can be expensive and difficult to install in mobile devices. This problem has been solved by using frequency modulation techniques such as orthogonal frequency division multiplexing (OFDM). OFDM transmission reduces the required number of receivers by assigning different subcarriers to different users on a single carrier frequency.

It requires additional receivers because the base station and each mobile unit have to be synchronized over time. In this multiplexing data cannot be sent in burst mode so the data is sent continuously, so that the receiver must wait until the next packet is received before it can start receiving the next one. It requires special receivers to be able to receive packets at different rates from different base stations, otherwise they would not be able to decode them correctly.

The number of transmitters and receivers involved in FDM systems is called the “transmitter-receiver pair” or TRP for short. The number of TRPs that must be available can be calculated by using the following formula:

NumberOfTRPs = (# Transmitters) (# Receive Points) (# Antennas)

For example if  we have three transmitters and four receive points (RPs), we will have nine TRPs because there are three transmitters and four RPs. To keep things simple, let’s assume that each RP has an RP antenna and each TRP has two RP antennas; this means we will need nine more TRPS:

This multiplexing can be either point to point or point to multi point. In the point-to-point mode, each user has its own dedicated channel with its own transmitter, receiver and antenna. In this case, there could be more than one transmitter per user and all users would use different channels. In the point-to-multipoint mode, all users share the same channel, but each user’s transmitter and receiver are connected to those of other users on the same channel.

Frequency Division Multiplexing Vs Time Division Multiplexing

The difference between frequency division multiplexing and time division multiplexing is discussed below.

Advantages and Disadvantages

The advantages of frequency division multiplexing include the following.

• The transmitter & receiver of FDM does not need any synchronization.
• It is simpler & its demodulation is easy.
• Only one channel will get effect because of the slow narrow band.
• FDM is applicable for analog signals.
• A large number of channels can be simultaneously transmitted.
• It is not expensive.
• This multiplexing has high reliability.
• Using this multiplexing, it is possible to transmit multimedia data with low noise & distortion and also with high efficiency.

The disadvantages of frequency division multiplexing include the following.

• FDM has a cross-talk problem.
• FDM is applicable only when a few less-speed channels are preferred
• Intermediation distortion occurs.
• FDM circuitry is complex.
• It needs more bandwidth.
• It gives fewer throughputs.
• As compared to TDM, the latency provided by FDM is more.
• This multiplexing doesn’t have dynamic coordination.
• FDM needs a large number of filters & modulators.
• The channel of this multiplexing can get affected by wideband fading
• The channel’s complete bandwidth cannot be utilized on the FDM.
• The system of FDM requires a carrier signal.

Applications

The applications of frequency division multiplexing include the following.

• Earlier, FDM is used in the cellular telephone system and harmonic telegraphy communication system.
• Frequency division multiplexing is mainly used in radio broadcasting.
• FDM is also used in TV broadcasting.
• This type of multiplexing is applicable in the telephone system to help in transmitting several phone calls over a single link or single transmission line.
• FDM is used in a satellite communication system for transmitting various data channels.
• It is used in FM transmission systems or stereo frequency modulation.
• It is used in AM radio transmission systems/Amplitude Modulation.
• It is used for public telephones and cable TV systems.
• It is used in broadcasting.
• It is used in AM and FM broadcasting.
• It is used in wireless networks, cellular networks, etc.
• FDM is used in broadband connection systems and also in DSL (Digital Subscriber Line) modems.
• FDM system is mainly used for multimedia data like audio, video & image transmission.

Thus this is an overview of Frequency division multiplexing or FDM. This is a multiplexing technique that separates the existing bandwidth into several sub-bands where each can carry a signal. So, this multiplexing allows simultaneous transmissions above a shared communication medium. This multiplexing allows the system to transmit a huge amount of data throughout a number of segments transmitted above independent frequency sub-bands. Here is a question for you, what is time division multiplexing?