What is Frequency Division Multiplexing (FDM)?

In the domain of telecommunications, Frequency Division Multiplexing (FDM) is a technique that divides the total accessible bandwidth into numerous distinct, non-overlapping frequency sub-bands. Each of these sub-bands caters to a specific communication channel, thereby facilitating simultaneous transmission of multiple signals over a solitary communication line or channel. An integral aspect of this technique is that each individual signal is apportioned a specific frequency sub-band, paving the way for exclusive transmission of its information.

Dissecting Frequency Division Multiplexing (FDM)

The conceptual groundwork for FDM was laid by George Owen Squier, an American inventor and Major General in the U.S. Signal Corps. In the 1910s, Squier developed a system for transmitting multiple telephone calls on the same line, a precursor to FDM called multiplexing. He received a patent for this invention in 1917, and it was later used as a foundational concept for FDM.

FDM emerged in response to a practical necessity: the efficient use of available communication mediums, particularly over long distances. Initially, telephone lines could only support one conversation at a time. With the increasing demand for telecommunication services, this became severely limiting. The industry needed a way to accommodate multiple conversations over a single line without cross-talk and interference.

How Frequency Division Multiplexing (FDM) Works

Frequency Division Multiplexing (FDM) functions through a series of well-ordered steps. These procedures range from initial signal preparation and frequency allocation, to final reception and signal processing. 

1. Signal Preparation: The process begins with the preparation of each data signal for transmission. If the data is digital, it often needs to be converted into an analog signal because FDM works in the frequency domain, which is a concept associated with analog signals. This conversion can be accomplished through modulation techniques like Frequency Shift Keying (FSK) or Phase Shift Keying (PSK). In FSK, the discrete digital data changes the frequency of the carrier wave, while in PSK, it changes its phase.
2. Frequency Allocation: After signal preparation, each signal is assigned a unique frequency band within the total frequency range of the communication channel. These frequency bands are carefully chosen to be non-overlapping, thus ensuring that signals do not interfere with each other during transmission.
3. Bandpass Filtering: Once frequency allocation is complete, each signal is then passed through a bandpass filter. A bandpass filter allows only a certain range of frequencies to pass through it, in this case, the frequency band assigned to the signal. This step ensures that the signal's frequency is confined to its assigned band.
4. Combination: Following bandpass filtering, all signals, now securely in their respective frequency bands, are combined into a single composite signal. This composite signal contains all the data to be transmitted and occupies the full frequency range of the channel.
5. Transmission: The composite signal, containing all individual data signals, is then transmitted over the communication channel. The channel could be a physical medium like a cable or a fiber-optic link, or it could be a wireless medium like air.
6. Reception: At the receiving end, the composite signal is picked up by a receiver, which is usually an antenna for wireless communication, or an appropriate interface for wired communication.
7. Demultiplexing: After reception, the composite signal is passed through a set of bandpass filters, each corresponding to the frequency band assigned to each original signal. This process, also known as demultiplexing, effectively separates the composite signal back into the individual data signals.
8. Signal Processing: Finally, each separated signal is demodulated, which is the reverse of the modulation process that occurred during signal preparation. This converts the analog signals back into their original form, usually digital data, ready to be used by the receiving system or user.

Frequency Division Multiplexing (FDM) Application

Frequency Division Multiplexing (FDM) is utilized in various sectors of the telecommunications industry due to its capacity to maximize the use of available bandwidth. Some specific FDM applications:

• Broadcast Radio and Television: One of the most prevalent applications of FDM is in broadcast radio and television. Different stations broadcast on different frequency bands, allowing you to tune into a specific frequency band to listen to a particular radio station or watch a specific TV channel.
• Telephony: In telephony, FDM is used to divide the available bandwidth into several channels, each capable of carrying a separate voice conversation. This allows multiple simultaneous phone calls over the same physical medium.
• Satellite Communication: FDM is widely used in satellite and space communications. Multiple signals from different sources can be multiplexed and transmitted over a single channel to the satellite, where they are then demultiplexed and routed to their respective destinations.
• Cable Modem Systems: FDM is also used in cable modem systems where multiple data signals from different users are transmitted over the same cable network.
• Cellular Networks: FDM plays a vital role in cellular networks such as 3G, 4G, and even the latest 5G networks. Different frequency bands are assigned to different users, allowing multiple users to make calls or transmit data simultaneously without interference.
• WiFi Networks: In WiFi networks, FDM helps multiple devices connect to the same router and use the internet simultaneously. Each device communicates with the router on a separate frequency, preventing interference between devices.
• DSL (Digital Subscriber Line) Services: FDM is used in DSL services to divide the available bandwidth of the physical line into separate channels for data, voice, and sometimes even video services. This allows users to talk on the phone, surf the web, and watch television simultaneously using the same physical line.