Viignesh has an MS in information technology.
Frequency Division Multiplexing
Frequency division multiplexing (FDM) is a networking method of sharing the total available bandwidth of any communication channel by dividing them into many non-overlapping bands of frequency. Let's consider an example.
FDM (frequency division multiplexing) can be compared to the operation of radio broadcasting where multiple frequencies are shared among the users and one can tune to any of the available frequency bands, to enjoy their favorite broadcast from that particular station. Frequency division multiplexing allows many users to share the communication channel.
Multiplexing & De-Multiplexing
In communication networks, multiplexing is a technique that refers to combining many signals (analog or digital) into one and sending them through a communication channel. The main objective of multiplexing is sharing of a scarce resource.
De-multiplexing, on the other hand, refers to the technique of separating these signals or extracting individual channels. Transmitters are devices that perform multiplexing at the sender's end and the receivers perform de-multiplexing at the receiving end.
Many signals get transmitted simultaneously and each signal occupies a different slot of frequency within the available bandwidth (common bandwidth).
Each signal that needs to be sent over a communication channel undergoes modulation with various carrier frequencies, as shown clearly in the diagram here.
There are different kinds of modulation such as amplitude modulation, pulse modulation, frequency modulation, etc. As the name suggests, the modulation done here is frequency modulation by the FDM transmitter. These modulated signals are then added up using a linear adder or a mixer, forming a composite signal which gets transmitted over a communication channel (single channel).
At the receiving end, the single composite signal is received by the FDM receiver. The receiver then passes the composite signal through various band pass filters. Each of these band pass filters has a frequency corresponding to the frequencies of one of the carrier waves. Each band pass filter will accept the signal whose frequency matches with the frequency of the carrier signal and rejects all other channels. The signals coming out of band pass filters pass through a demodulator. The demodulator does the work of separating the original signal from the carrier signal.
Frequency overlap is a serious issue when it comes to frequency division multiplexing and it must be completely avoided. Two frequency ranges can be separated by using some narrow frequency ranges called guard bands. The guard bands avoid signal interference and enhance the quality of communication.
Here's an example of frequency division multiplexing and de-multiplexing: Cable televisions use FDM technology. The FDM multiplexer combines multiple channels for maximizing usage of the available bandwidth. The FDM de-multiplexer, found either in a set-top box or a television, separates the transmitted signals and allows the viewers to watch their favorite channel.
Frequency vs Time
Frequency division multiplexing is preferred over time division multiplexing (TDM) for analog signal communication. Synchronization is needed in time division multiplexing but not in frequency division multiplexing. FDM requires a complex circuitry with multiple filters and demodulators. TDM, on the other hand, needs a simple circuitry.
The only advantage of time division multiplexing over frequency division multiplexing is that cross-talk is less in TDM than FDM. FDM has cross-talk issues due to BPF (band pass filter) imperfections. All the channels get affected in TDM by narrow-band fading whereas only the channel that has narrow-band fading is affected in FDM. While bandwidth fading occurs in the medium, all FDM channels are affected, whereas only a particular channel is affected in the case of TDM channels.
Let's summarize the lesson and what we learned. As we learned, frequency division multiplexing (FDM) is a networking method of sharing the total available bandwidth of any communication channel by dividing them into many non-overlapping bands of frequency. We looked at some of the differences between multiplexing, which is a technique that refers to combining many signals (analog or digital) into one and sending them through a communication channel, and de-multiplexing, which refers to the technique of separating these signals or extracting individual channels. We also looked at the use of guard bands, in which two frequency ranges can be separated by using some narrow frequency ranges. We also learned the advantages and disadvantages of FDM, along with its applications.
The advantages included the following:
- FDM doesn't require synchronization of transmitters and receivers for operating
- Allows propagation of multiple signals at once
- Only a particular channel is affected by fading of slow narrow bands
- Demodulation in frequency modulation is relatively easy
The disadvantages of frequency division multiplexing included the following:
- Wide-band fading affects all the channels of FDM
- Many filters and modulators are needed
- A higher bandwidth is needed by the communication channel
- Cross-talk is high in FDM
- There are chances for inter-modulation distortion
And, finally, its applications included the following:
- FDM is used in television networks
- It's used for AM and FM radio broadcasting
- It's also used in first generation telephones (cellular)
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