**Frequency Division Multiplexing (FDM)**
Frequency Division Multiplexing (FDM) is a technique that divides the total available bandwidth of a communication channel into multiple sub-channels, each carrying a separate signal. Each sub-channel operates at a different frequency, allowing multiple signals to be transmitted simultaneously without overlapping. To prevent interference between these signals, isolation bands—also known as guard bands—are placed between the sub-channels. This ensures that the signals remain distinct and do not interfere with one another.
One key characteristic of FDM is that all sub-channels operate in parallel, meaning that each signal can be transmitted independently without considering transmission delays. This makes FDM highly efficient and widely used in various applications. In addition to traditional FDM, a more advanced form called Orthogonal Frequency Division Multiplexing (OFDM) is also commonly used, especially in modern digital communication systems.
**Concept and Advantages**
Multiplexing is a method of combining multiple independent signals into a single channel for transmission. The main advantage of multiplexing is improved channel utilization, allowing more information to be sent over the same medium. There are two primary types of multiplexing: Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM).
FDM is used primarily in analog communication systems, where signals are separated by frequency. Each signal modulates a different carrier frequency, ensuring they do not overlap. TDM, on the other hand, separates signals in time, making it suitable for digital systems.
**System Block Diagram and Signal Structure**
A typical FDM system uses Single Sideband (SSB) modulation. At the transmitter, each message signal is first passed through a low-pass filter before being modulated onto a different carrier frequency. These carriers are carefully chosen so that their frequency ranges do not overlap, avoiding interference. The combined signal is then transmitted through the channel.
At the receiver, bandpass filters separate the individual frequency components, which are then demodulated to recover the original signals. Figure 1 shows a block diagram of an SSB FDM system, while Figure 2 illustrates the spectrum structure of the multiplexed signal. A guard band, denoted as "fe," is included between channels to prevent crosstalk.
The minimum total bandwidth required for n single-sideband signals is given by:
$$ B_{\text{total}} = \sum_{i=1}^{n} B_i $$
where $ B_i $ is the bandwidth of each individual signal.
In some cases, the multiplexed signal may be modulated again to better match the characteristics of the transmission medium.
**Application Example: FM Stereo Broadcasting**
An example of FDM in practice is FM stereo broadcasting. In this system, the audio signal is split into left and right channels, both operating within the 0–15 kHz range. The sum of the two signals, $ M_L(t) + M_R(t) $, is transmitted as a baseband signal, while the difference, $ M_L(t) - M_R(t) $, is modulated using Double-Sideband Suppressed Carrier (DSB-SC) AM at 38 kHz. A pilot tone at 19 kHz is added to help with coherent demodulation at the receiver.
Figure 3 shows the block diagram of an FM stereo transmitter and receiver. The receiver demodulates the signal, separates the sum and difference components, and reconstructs the original left and right audio signals. This allows for high-quality stereo sound transmission via FM radio.
SCSI Piercing Connector Section
Small computer system interface (SCSI) is an independent processor standard for system level interfaces between computers and intelligent devices (hard disks, floppy drives, optical drives, printers, scanners, etc.). SCSI is an intelligent universal interface standard.
SCSI Piercing Connector Section
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