Pervasive OFDM

Reading this article on Wireless USB, prompted me to think about just how pervasive the RF modulation known as OFDM is becoming.

I'm someone who pretty much has to know how something works, once it enters my consciousness. So, in mid-2000, while researching Australia's chosen form of digital TV, Digital Video Broadcasting - Terrestrial (DVB-T), I discovered a fascinating form of RF modulation, known as Orthogonal Frequency Division Multiplexing (OFDM).

The Technology

Traditional forms of modulation send one stream of information over one channel. OFDM sends many streams of information at once, over multiple, slower, channels (sometimes thousands of channels), simultaneously. By doing so, it increases the robustness of the individual channels to effects like multipath distortion (reflections from buildings, hills, trees, etc. that manifest as ghosting in analogue TV). By using appropriate levels of error correction, it can also be relatively immune to effects like selective channel-fading.

The real magic of OFDM comes from the clever way it can be modulated and demodulated by applying lots of digital signal processing. Instead of building thousands of individual modulators and demodulators – one for each channel – you can use the Fast Fourier Transform to transform from the frequency domain (the individual channels) to the time domain (the modulated RF). The opposite transformation can be used at the receiving end to recover the original channels and the original modulating information.

I can't possibly do justice to this clever technology, but the following papers are more enlightening:

The Applications

These applications of OFDM are either a part of my daily life, or will soon become so:

  • DVB-T – Terrestrial television broadcasting in Australia and Europe uses a variant called COFDM. Investigating digital TV was what got me interested in OFDM in the first place.

  • DSL – About a year ago, I realized that the modulation used to cram megabits per second into the unused capacity of the venerable telephone copper-pair, used OFDM.

  • 802.11g/a – I recently upgraded my wireless network to the faster 802.11g 54Mbps standard. Guess what; more ODFM.

  • Wireless USB – It seems the WUSB people are shooting for an initial bitrate of 480Mbps using OFDM. I imagine my near-term future will hold a number of WUSB personal area networks.

The History

I'd assumed that OFDM was relatively new, but looking at the history of OFDM, I discovered that OFDM is nearly as old as I am, having been described in a patent application made in 1966. It seems to have it's roots in military HF communications systems from the late 1950's.

It may not be as old as Spread Spectrum, but it's no spring chicken.