**2.3 Frequency division multiplexing**

FDM [1–4] is based on sharing of the available bandwidth of a communication channel among the signals to be transmitted. It is an analog multiplexing technique

**13**

**Figure 6.**

*Block diagram of FDM transmitter.*

*Multiplexing*

*DOI: http://dx.doi.org/10.5772/intechopen.85866*

*2.3.1 FDM transmitter block diagram*

*2.3.2 FDM receiver*

that uses a single transmission medium which is divided into several frequency channels. Here the total bandwidth of the channel must be higher than the sum of the individual bandwidth. If the channels are closer to each other, then cross talk may occur; thus, it is necessary to implement channel synchronization. For that purpose some bandwidth is allocated as guard band; these are unused channels

For frequency division multiplexing if the input signal is digital, it must be

In FDM signals are generated by sending devices and there are multiple input lines. From the block diagram (**Figure 6**), channel 1 to channel n are taken as the input channels. These signals reach at the input of the corresponding modulator where it receives another signal from a crystal oscillator known as carrier signal, which is a high-frequency high-amplitude signal. The carrier signal is modulated with the input signal. Different modulators use different carrier signals for modulation. It should be noted that the frequency band of one modulator will not make any

Each of the modulator produces the corresponding modulated signal at their output. All the output of the modulators will be given to an adder or mixer circuit; from there it is given to another modulator for further shift of total bandwidth. Finally, this higher-frequency signal will be transmitted over the channel.

The following block diagram (**Figure 7**) shows the concept of demodulation of FDM signal at the receiving side. The antenna receives the multiplexed modulated signal from the transmitter. This signal will be weak at the receiver. Therefore it is necessary to amplify the signal. This is done at the initial stage of the receiver. The amplified signal is then forwarded to the demodulator. The output of the

placed between the successive transmission channels to avoid cross talk.

converted to analog before giving it as the input to the modulator.

interference to the frequency band of other modulators.

### *Multiplexing DOI: http://dx.doi.org/10.5772/intechopen.85866*

*Multiplexing*

**Figure 5.**

*Bidirectional WDM.*

Wavelength division multiplexing is divided into two types, unidirectional WDM and **bi-directional WDM**. In unidirectional WDM, the data is sent only from one side and received on the other side. Multiplexing of the wavelength occurs on the sender side, and demultiplexing of the wavelengths takes place on the receiver side. In bi-directional WDM the data can be sent from both sides which means that both

sides can do multiplexing and demultiplexing as shown in **Figure 5**.

• Provide high security and faster access to new channel.

• Optical components are more reliable and provide higher bandwidth.

• Scalability is a concern as optical line termination (OLT); optical line termination has to have transmitter array with one transmitter for each optical network unit (ONU). Adding a new ONU could be a problem unless transmitters were provisioned in advance. Each ONU must have a wavelength-specific laser.

• Inefficiency in BW utilization, difficulty in wavelength tuning, and difficulty

FDM [1–4] is based on sharing of the available bandwidth of a communication channel among the signals to be transmitted. It is an analog multiplexing technique

• The cost of the system increases with addition of optical components.

• In WDM full-duplex transmission is possible.

• Low cost and easy system expansion.

• Simultaneous transmission of various signals.

*2.2.1 Advantages of WDM*

• It is easier to reconfigure.

*2.2.2 Disadvantages of WDM*

in cascaded topology.

**2.3 Frequency division multiplexing**

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that uses a single transmission medium which is divided into several frequency channels. Here the total bandwidth of the channel must be higher than the sum of the individual bandwidth. If the channels are closer to each other, then cross talk may occur; thus, it is necessary to implement channel synchronization. For that purpose some bandwidth is allocated as guard band; these are unused channels placed between the successive transmission channels to avoid cross talk.

For frequency division multiplexing if the input signal is digital, it must be converted to analog before giving it as the input to the modulator.
