FDM vs TDM: Key Differences Explained

This article breaks down the differences between Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). These are two fundamental multiplexing techniques used in telecommunications.

What is FDM?

FDM stands for Frequency Division Multiplexing. In FDM, each signal is modulated onto a unique Radio Frequency (RF) carrier. These carrier frequencies are carefully chosen and spaced apart so that the bandwidths of the signals don’t overlap in the frequency domain. This prevents interference between the signals.


Fig.1 FDM

As illustrated in Figure 1, several signal sources are fed into a multiplexer. Each source modulates a different RF carrier (f1, f2, f3, and f4). Guard bands are added between the RF carriers to further minimize interference between adjacent channels, also known as Adjacent Channel Interference (ACI).

For a deeper dive into the comparisons, it’s useful to refer to the comparison of FDMA vs TDMA vs CDMA.

What is TDM?

TDM stands for Time Division Multiplexing. With TDM, multiple digital signals are transmitted over a single medium by interleaving each signal in time.


Fig.2 TDM

As shown in Figure 2, four channels, each with a data rate of approximately 2.4 kbps, can be multiplexed onto a single line with a capacity of 9.6 kbps using TDM.

TDM isn’t limited to just digital signals; it can also be used with analog signals. It’s also possible to combine TDM and FDM. In such a hybrid scheme, the frequency spectrum is first divided into channels using FDM, and then each channel is further subdivided into time slots using TDM.

Applications

• TDM: Commonly used in PCM (Pulse Code Modulation) transmission to achieve T1 at a rate of 1.544 Mbps.
• FDM: Widely used in satellite communications, radio broadcasting, HF (High Frequency) radio, and other wireless technologies.
• Both TDM and FDM: Employed in GSM (Global System for Mobile communications) cellular technology. Refer to GSM Frame Structure for more details. The GSM frame structure utilizes FTDMA, which is a combination of FDMA and TDMA.

This article offers a basic overview of the differences between TDM and FDM multiplexing techniques, valuable for telecom engineers.