FDM versus TDM-Difference between FDM and TDM

This page on FDM versus TDM describes difference between FDM and TDM multiplexing very useful for telecom engineers. TDM (Time Division Multiplexing) and FDM (Frequency Division Multiplexing) are the two techniques of multiplexing. The common difference between TDM and FDM is that TDM share the timescale for the different signals; Whereas FDM shares the frequency scale for the different signals.FDM is the short form of frequency division multiplexing and TDM is the short form of time division multiplexing.

#FDM
In FDM each signal is modulated onto different unique RF carrier frequency and all carrier frequencies are separated significantly so that bandwidth of the signals do not overlap in frequency domain.

Fig.1 FDM

As shown in the fig.1 four signal sources are fed into a multiplexer and that each modulates onto different RF carriers i.e. f1,f2,f3 and f4. To prevent interference between adjacent channels i.e. adjacent channel interference, each RF carriers are separated by guard bands. To understand more on difference between FDM and TDM, also refer FDMA vs TDMA vs CDMA.

See Also: >>What is the Difference between AM and FM?  

#TDM
In TDM, more than one digital signals can be carried on single medium for transmission by interleaving each signals in time.






As shown in figure 2, four channels each having about 2.4 kbps data rate can accommodate/multiplexed onto a single line having capacity to carry 9.6kbps using TDM. TDM is not only used for digital signals but also used for analog signals also.

Combination of TDM and FDM scheme is also possible where in first frequency is divided into number of channels as required by system and later each channel is further divided into time intervals via TDM.

See Also: >>AM receiver vs FM receiver | Difference between AM receiver and FM receiver 

#Applications
1) TDM is employed in PCM transmission to obtain T1 at the rate of 1.544Mbps.
2) FDM is employed in satellite,Radio,HF and other wireless technologies.
3) Both TDM and FDM is employed in GSM cellular technology, Refer GSM Frame Structure for more information. GSM frame structure uses FTDMA (combination of FDMA and TDMA).

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