*2.3.5 Applications of FDM*

• FDM is used for FM and AM radio broadcasting.


### **2.4 Time division multiplexing**

*Multiplexing*

of the corresponding modulator.

*Block diagram of FDM receiver.*

*2.3.3 Advantage of FDM multiplexing*

• Demodulation of FDM is easy.

• Intermodulation distortion takes place.

• FDM suffers from the problem of cross talk.

• FDM is used for FM and AM radio broadcasting.

proper operation.

*2.3.4 Disadvantages of FDM*

*2.3.5 Applications of FDM*

signal.

**Figure 7.**

demodulator will be given to the band-pass filters which are well designed with the central frequencies of the carriers as used individually at the transmitting side. Thus the output of each BPF will be the same as that of the originally modulated output

Then we use the corresponding individual demodulators to recover the original

• A large number of signals (channels) can be transmitted simultaneously.

• Due to slow narrowband fading, only a single channel gets affected.

• The communication channel must have a very large bandwidth.

• A large number of modulators and filters are required.

• All the FDM channels get affected due to wideband fading.

• FDM does not need synchronization between its transmitter and receiver for

**14**

In time division multiplexing (TDM) [1–4], all signals operate with the same frequency at different times, i.e., it is a technique of transmitting several signals over a single communication channel by dividing the time frame into equal slots. Here the signal transmitted can occupy the total bandwidth of the channel, and each signal will be transmitted in its specified time period only. In TDM all signal operates at same frequency at different time slots.

**Figure 8** shows the schematic diagram of implementation of TDM system. From this it is clear that a circular ring has been split into eight equal segments and is completely separated from one another. It is also noted that there is a movable arm attached to the inner ring, and it slides over the eight segments over the ring. The eight segments are eight inputs, and the selector moves in clockwise direction from A to H; after completing one revolution, it starts again. The output is taken from the inner ring that contains the signal from only one slot at a time.

The same arrangement is also made at the receiving side. The two inner rings of the transmitting and receiving stations are rotated at the same speed, and they are synchronized. Thus the signal at segment A of the transmitter will reach segment A of the receiver in the period the arm is contacting the segment A. The same is in the case of other segments.

Time division multiplexing is used when data transmission rate of media is greater than the total transmission rate of the sources. Here each signal is allotted to a definite amount of time. These slots are too small so that the transmission appears to be parallel. In TDM all the signals to be transmitted are not transmitted simultaneously. Instead, they are transmitted one by one. When all the signals are transmitted once on the transmission channel, it is said to be one cycle of completion.

Synchronization between the multiplexer and demultiplexer is a major issue in TDM. If the multiplexer and the demultiplexer are not properly synchronized, a bit belonging to one channel may be received by another channel. Therefore, one or more synchronization bits are generally added to the beginning of each frame. These bits, called framing bits, allow the demultiplexer to synchronize with the incoming

**Figure 8.** *Schematic diagram of TDM operation.*

### *Multiplexing*

stream so that it can separate the time slots accurately. Normally, this synchronization information consists of 1 bit per frame, alternating between 0 and 1.

There are two types of TDM multiplexing, synchronous TDM and asynchronous or statistical TDM or intelligent TDM.
