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Multiplexing combines multiple information streams down a single cable, fibre or radio link.
Frequency Division Multiplexing modulates many frequencies sent by one transmitter.

Broadband F.D.M.

• Many channels (such as phone conversations) can be sent down a single cable or across a radio link.
• Each channel is transmitted on its own carrier frequency but all the frequencies share the same cable.
• The receiver filters out each frequency and decodes each signal recreating the original channels.
• The receiver normally uses digital filtering. This avoids the need for expensive analogue filters.
• DSP - Digital Signal Processing chips perform the digital filtering.
• Fibre-optic systems can use different colours (frequencies of light). This is another example of frequency division multiplexing.
• OFDM is a widely used technique where multiple carriers on different frequencies each carry digital data at a relatively low bit rate. The combined data rate is large because so many carriers are used. This is used for ...
  • Mobile Phones
  • ADSL Broadband. (Asymmetric Digital Subscriber Line)
  • DAB Digital Radio. (Digital Audio Broadcasting)
  • Digital TV
  • Satellite Digital TV
  • DRM MF and HF radio broadcasting. (Digital Radio Modiale)

Multi-Carrier FDM

Each carrier is modulated with digital data. Using many carriers with error correction techniques improves the reliability of the communication link. If a few of the carriers get damaged, the link still works.

ADSL Broadband Internet

Asymmetric Digital Subscriber Line (ADSL) uses a simple form of Frequency Division Multiplexing. Voice / phone call data arrives at frequencies between 300 Hz and 4000 Hz. Computer data arrives at a much higher frequency. Line splitters (filters) are used to separate these two signals. The low frequencies go to phone handsets. The high frequencies go to the ADSL router or modem. These are nice examples of low and high pass filters.

ADSL Spectrum

This diagram shows the audio, the upstream and the downstream spectra. All three look as if they have the same bandwidth but notice the logarithmic graph scale. In fact the downstream spectrum is about ten times the upstream bandwidth. Both are many times the audio bandwidth.

Here is a zoomed in view showing many overlapping channels each occupying about 4.3 kHz. This is called Orthogonal Frequency Division Multiplexing (OFDM) and Digital Signal Processing (DSP) is able to extract the data from the overlapping spectra.

More Details

The ADSL computer data is sent using many carriers. Each carrier is modulated with digital data. The higher frequency carriers suffer more from attenuation (signal loss). If the distance to the exchange is too great, the highest frequencies become unusable. This is why connection speeds get worse as the distance to the exchange increases. The ADSL router automatically negotiates with the exchange to decide on the highest usable frequency.

ADSL and most other digital signals transmitted using Frequency Division Multiplexing are decoded using a mathematical technique called "Fast Fourier Transforms (FFT)" using DSP (Digital Signal Processing) chips.

The signal is sampled at regular TIME intervals at more than twice the highest FREQUENCY. This is called the TIME domain. Then the FFT analysis transforms the data into the FREQUENCY domain. This allows the many carriers to be separated out and the data they are carrying can be recovered.

Here is a cartoon which completely fails to explain the technique. Thank you xkcd.

[Elizabeth Basha] wrote this paper on the use of Field Programmable Gate Arrays for Fast Fourier Transforms.