Frequency Division Multiplexing(FDM)

Frequency Division Multiplexing (FDM) is a technique used in communication systems to transmit multiple signals simultaneously over a single communication channel. FDM is based on the principle of dividing the available bandwidth of a communication channel into multiple frequency bands, each of which can carry a separate signal.

Frequency Division Multiplexing(FDM)

In FDM, the signals to be transmitted are modulated onto separate carrier frequencies, each of which is separated by a fixed frequency interval known as the guard band. The modulated signals are then combined and transmitted over the communication channel. At the receiver end, the received signal is demodulated to recover the individual signals.

The basic block diagram of an FDM system includes a modulator, a channel, and a demodulator. The modulator modulates the individual signals onto separate carrier frequencies, and the channel transmits the modulated signals over the communication channel. The demodulator separates the individual signals from the received signal.

FDM is commonly used in analog communication systems such as radio and television broadcasting, where multiple channels of information need to be transmitted simultaneously over a limited bandwidth. However, FDM has some limitations, including the need for precise frequency control to avoid interference between the different frequency bands and the requirement for a large guard band to prevent overlapping of the frequency bands.

To address these limitations, other multiplexing techniques such as Time Division Multiplexing (TDM) and Code Division Multiplexing (CDM) have been developed. TDM divides the available time slots of a communication channel between multiple signals, while CDM assigns a unique code to each signal to allow multiple signals to share the same frequency band.

Types of Multiplexing

There are several types of multiplexing techniques used in communication systems, including:

1. Frequency Division Multiplexing (FDM): As explained earlier, FDM divides the available bandwidth of a communication channel into multiple frequency bands, each of which can carry a separate signal.

2. Time Division Multiplexing (TDM): In TDM, the available time slots of a communication channel are divided between multiple signals. Each signal is given a dedicated time slot, and the signals are transmitted sequentially.

3. Statistical Time Division Multiplexing (STDM): STDM is a variation of TDM in which the time slots are dynamically allocated to the signals based on their data rate and traffic requirements.

 4. Code Division Multiplexing (CDM): CDM assigns a unique code to each signal to allow multiple signals to share the same frequency band. The receiver uses the code to extract the desired signal from the received signal.

 5. Wavelength Division Multiplexing (WDM): WDM is used in optical fiber communication systems to transmit multiple signals simultaneously over different wavelengths of light.

6. Space Division Multiplexing (SDM): SDM is used in multiple-input, multiple-output (MIMO) wireless communication systems to transmit multiple signals simultaneously over different antennas.

Each type of multiplexing has its advantages and limitations, and the choice of a particular multiplexing technique depends on the specific requirements of the communication system. 

Transmission bandwidth

Transmission bandwidth refers to the range of frequencies required to transmit a signal over a communication channel. The bandwidth of a signal is defined as the difference between the highest and lowest frequencies present in the signal.

In analog communication systems, the bandwidth of a signal depends on its modulation type. For example, in Amplitude Modulation (AM), the bandwidth of the modulated signal is twice the maximum frequency present in the message signal. In Frequency Modulation (FM), the bandwidth of the modulated signal is directly proportional to the maximum frequency deviation of the carrier frequency.

In digital communication systems, the bandwidth of a signal depends on its data rate and the modulation technique used. For example, in Pulse Amplitude Modulation (PAM), the bandwidth of the modulated signal is directly proportional to the data rate and the number of amplitude levels used.

The available transmission bandwidth of a communication channel is limited, and different communication channels have different bandwidths depending on their physical characteristics and regulatory restrictions. For example, radio frequency bands allocated for broadcasting have larger bandwidths compared to those allocated for mobile communication. Optical fiber communication systems have much larger bandwidths than traditional copper cable systems, which is one reason why fiber optics is often used for high-speed data transmission.

The choice of modulation and multiplexing techniques in a communication system is often influenced by the available transmission bandwidth and the required data rate. Efficient use of the available bandwidth is critical to maximizing the capacity and performance of a communication system.

B = 24*4 = 96 kHz

Using FDM for telephone lines:

FDM can be used for transmitting multiple telephone signals over a single communication channel. In the context of telephone lines, FDM is typically used in a technique called Frequency Division Multiplexing Access (FDMA).

In FDMA, the available bandwidth of a telephone line is divided into multiple frequency bands, each of which can carry a separate telephone conversation. The bandwidth of each frequency band is typically around 4 kHz, which is the bandwidth required for transmitting a single voice signal.

To implement FDMA, the telephone signals are first modulated onto separate carrier frequencies, each of which corresponds to a separate frequency band. The modulated signals are then combined and transmitted over the telephone line. At the receiver end, the received signal is demodulated to recover the individual telephone signals.

FDMA is an efficient way of utilizing the available bandwidth of a telephone line to transmit multiple telephone conversations simultaneously. However, it has some limitations, including the need for precise frequency control to avoid interference between the different frequency bands and the requirement for a large guard band to prevent overlapping of the frequency bands.

To address these limitations, other multiplexing techniques such as Time Division Multiplexing (TDM) and Code Division Multiplexing (CDM) have been developed. TDM divides the available time slots of a communication channel between multiple signals, while CDM assigns a unique code to each signal to allow multiple signals to share the same frequency band.

Block Diagram of FDM System

The block diagram of a Frequency Division Multiplexing (FDM) system typically includes the following components:

Block Diagram of FDM System

1. Multiple input signals: These are the signals that need to be transmitted over the communication channel. In the case of telephone lines, these would be the individual voice signals.

2. Modulators: Each input signal is modulated onto a separate carrier frequency. The modulators typically use analog circuits such as mixers to combine the input signal with a carrier frequency.

3. Bandpass filters: Each modulated signal is passed through a bandpass filter to select the corresponding frequency band for transmission. The bandpass filters remove any unwanted frequencies from the signal.

4. Adder: The bandpass filtered signals are combined using an adder to form a single composite signal for transmission.

5. Amplifier: The composite signal is then amplified to increase its power level for transmission over the communication channel.

 6. Transmission channel: This is the communication channel over which the composite signal is transmitted, such as a telephone line or a radio frequency band.

 7. Receiver: At the receiving end, the received signal is first passed through a bandpass filter to select the frequency band of interest. The filtered signal is then demodulated using a demodulator to recover the original input signals.

 8. Demodulators: Each demodulator extracts the individual input signal from the modulated carrier signal. The demodulators typically use analog circuits such as mixers to mix the received signal with a local oscillator frequency to recover the input signal.