The Concept of FDM

Frequency Division Multiplexing (FDM) is one of the classic techniques for sharing a single communication medium among multiple independent signals. The core idea is to divide the total available bandwidth of a channel into a series of smaller, non-overlapping frequency sub-channels. Each of these sub-channels is then assigned to a separate user or data stream.

The best analogy for FDM is broadcast radio. Many different radio stations can transmit simultaneously in the same area because each one is assigned a unique frequency on the FM or AM dial. When you tune your radio to "99.5 FM," you are selecting one specific sub-channel out of the entire available radio spectrum, ignoring all the others.

The FDM Process: From Baseband to Broadband

The process of combining multiple signals using FDM involves a series of steps at the transmitter (multiplexing) and receiver (demultiplexing).

Transmitter Side (Multiplexing)

1. Input Signals: We start with multiple independent input signals. Typically, these are baseband signals, like voice channels occupying 0-4 kHz.
2. Modulation: Each baseband signal is used to modulate a unique, high-frequency carrier wave. For example, signal 1 modulates a carrier at frequency $f_{c1}$, signal 2 modulates a carrier at $f_{c2}$, and so on. Amplitude modulation (AM), particularly Single-Sideband (SSB), is often used to conserve bandwidth.
3. Filtering: After modulation, band-pass filters are used to select the desired portion of the modulated signal (e.g., just the upper sideband in SSB) and to prevent it from spilling into adjacent channels.
4. Combining: Finally, all the modulated and filtered signals are summed together electrically to create a single, wideband composite FDM signal, which is then sent into the transmission medium.

Receiver Side (Demultiplexing)

1. Filtering: The incoming composite FDM signal is fed into a bank of band-pass filters. Each filter is precisely tuned to one of the carrier frequencies ($f_{c1}, f_{c2}, ...$) and only allows its designated channel to pass through, effectively separating the signals.
2. Demodulation: Each separated signal is then individually demodulated. This process strips away the high-frequency carrier, shifting the signal's spectrum back down to its original baseband range (e.g., 0-4 kHz).
3. Final Filtering: Low-pass filters are used to clean up the demodulated signals, removing any unwanted high-frequency artifacts left over from the demodulation process.
4. Output Signals: The original baseband signals are now recovered and sent to their respective destinations.

A Practical Example: Analog Telephony Hierarchy

FDM was the workhorse of long-distance analog telephone systems for decades. These systems were built on a standardized hierarchy to combine thousands of voice calls onto a single coaxial cable.

• Base Channel: A single voice channel was allocated a nominal bandwidth of 4 kHz (which included the ~3.1 kHz for speech plus guard bands).
• Channel Group (or Primary Group): The first level of multiplexing combined 12 voice channels using FDM to occupy the frequency range of 60-108 kHz.
• Supergroup: The next level combined five Channel Groups (5 x 12 = 60 voice channels).
• Mastergroup: A higher level combined ten Supergroups (10 x 60 = 600 voice channels).

This hierarchical stacking allowed carriers to efficiently scale their network capacity by combining groups into ever-larger composite signals for long-haul transmission.

Key Concepts and Trade-offs

• Guard Bands:To prevent signals from adjacent sub-channels from interfering with each other, small, unused frequency gaps known as guard bands are left between them. While necessary, they represent a source of bandwidth inefficiency.
• Analog vs. Digital: FDM is fundamentally an analog technique. While it can carry digitally modulated signals, the multiplexing process itself operates on continuous analog waveforms.
• FDM vs. WDM: Wavelength Division Multiplexing (WDM), used in fiber optics, is the direct optical equivalent of FDM. Instead of separating electrical signals by frequency (in MHz or GHz), WDM separates optical signals by wavelength/color (in nanometers). The underlying principle is identical.
• Rigid Allocation: A primary drawback of classic FDM is its rigid resource allocation. Each user is assigned a fixed bandwidth slice, which remains reserved for them even if they are not actively transmitting data. This can be inefficient compared to the dynamic allocation used in packet-switched networks.

Interactive FDM Demonstration