Understanding OFDM - Part 1

Understanding OFDM - Part 1

By CWNP On 09/23/2009 - 12 Comments

The 48-string Guitar

Like the sidewalk under your feet, always solid, never doubted, Orthogonal Frequency Division Multiplexing (OFDM) is the rock that supports current and near-term wireless technologies, including 802.11a, 802.11g, 802.11n, WiMAX, and LTE. As a wireless professional, you’ll be working with OFDM-based technologies for the foreseeable future. An understanding of OFDM will give you an edge in designing and maintaining the networks under your care.

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802.11a and 802.11g are the “tried and true” members of the 802.11 family of wireless local area network (WLAN) products. Both 802.11a and 802.11g (802.11a/g) make use of OFDM. The manner in which these two technologies exploit the features of OFDM is relatively uncomplicated, which makes them good choices with which to begin our exploration of OFDM. Once we have a good working understanding of OFDM and are able to visualize its operations it will be much easier to understand the changes that 802.11n encompasses.
802.11n is the most recent addition to the 802.11 family of WLAN technologies. In development since 2002, it has now officially been ratified by IEEE ballot and is published for use by the networking community. Because of its advanced features and higher communications rates, we can expect a gold rush of activity surrounding this new amendment. 802.11n has already proven that it can provide much higher data transfer rates than were possible using earlier forms of wireless technologies. The higher speeds of 802.11n are realized thanks to innovations such as Multiple Input Multiple Output (MIMO), wider channel bandwidth options (40 MHz), and improvements to existing mechanisms such as OFDM. One specific improvement to OFDM is the way in which 802.11n takes advantage of previously unused (by 802.11a/g) subcarriers. To appreciate these improvements, it’s necessary to explore what subcarriers are in the first place and how they were used previously, with 802.11a/g.

WiMAX and LTE are cutting-edge, broadband wireless technologies that hold great promise for delivering high-speed connectivity within wireless metropolitan area networks (WMANs). These are important “next gen” technologies and the way they use OFDM is a bit complex. For that reason we’ll postpone our examination of WiMAX’s and LTE’s use of OFDM until later in this commentary.

Before there was Orthogonal Frequency Division Multiplexing (OFDM), there was Frequency Division Multiplexing (FDM), a reliable but slower wireless signaling method. These two multiplexing techniques will be compared, against two common forms of rendition used in guitar performances, namely lead and rhythm guitar. The reason to do that is to give a recognizable analogy of something invisible and alien (OFDM) to something familiar and pleasant (music). The first comparison makes the assertion that FDM is comparable to lead guitar, while the second is of OFDM to the chords produced by a rhythm guitarist.
For the first example, we’ll compare the use of FDM, with a single-carrier, to the performance of a lead guitarist fingering the notes of a riff. Each string is sounded clearly and separately as the guitarist presses briefly behind the frets, while plucking the strings with a pick. This action continues in a swiftly moving serial progression to the end of the score. 
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That’s similar to the way in which FDM single-carrier* signals are transmitted. In FDM, an input data stream arrives at the transmitter radio chain to be modulated serially, onto a single-carrier channel by means of some form of phase modulation (BPSK, QPSK, 16-QAM, or 64-QAM). Each modulation event induces a clear and separate pulse, called a “modulation symbol”, which represents a select portion of the bits that make up the input data stream.
Just as the sound from the guitar travels through the air to reach a listener’s ears, FDM induced signals also propagate over the air interface where they likely encounter a receiving antenna. With music, the listener’s ears pass the guitar sound onto auditory nerves, which deliver the sounds to the brain, where the sounds may be interpreted pleasantly (or not). In wireless, the receiving antenna, directs the FDM signal to the receiver’s radio chain where processor chips decompose the modulation symbols back into the original information bits and send them to the higher network layers for interpretation.
In the second example, a rhythm guitarist presses down on several strings at once, while strumming the pick across all of the strings. This action produces a “chord” which is a composite sound made up of multiple individual notes played together. Each note still maintains its individuality, but what’s heard by the listener is a melodic, composite sound containing all of the individual notes superpositioned as one. 
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To play chords, the rhythm guitarist positions fingers on the strings in pre-defined patterns much like an OFDM radio prepares the data stream to be transmitted by performing a serial-to-parallel conversion. The guitarist strums across all of the guitar’s strings to sound the chord, while in OFDM, subcarriers (analogous to the guitar’s strings) are created through a mathematical function called Inverse Fast Fourier Transforms (IFFT). 802.11a/g allows 48 subcarriers to be individually phase modulated to represent bits from the input data stream. Once induced, the individual modulation symbols combine into a single transmission burst known as an “OFDM symbol”, which is comparable to the chord produced by a rhythm guitar. At the receiving side, the OFDM symbol is recovered through a Fast Fourier Transform (FFT) operation, the reverse of the IFFT operation, which splits the composite OFDM symbol back into its component, modulation symbols. Like the guitar chord, an OFDM symbol efficiently packs more information into each burst. The bits represented by the modulation symbols are queued and presented to a parallel-to-serial conversion step, resulting in a bit stream that can be copied to baseband and sent up the protocol stack.

You can think of the way that OFDM is used in 802.11a/g, as a rhythm guitar with 48 strings. That gives a clear, high-level, auditory and visual analogy of this use of OFDM. But to be more precise, there are really 64 subcarriers created by the 802.11a/g IFFT/FFT functions. It’s just that, only 48 of them are used to carry user data. In order to take this investigation deeper and to prepare for more complex versions of OFDM, it’s now necessary to leave our guitar analogy and talk about OFDM straight-up.

* for FDM multi-carrier, try picturing a jam session with several lead guitars 
Check out Understanding OFDM - Part 2

Summary of Part 1

  • OFDM is used in current and soon-to-be released wireless communications technologies.
  • FDM can be compared to the single notes played sequentially by a lead guitarist
  • OFDM can be compared to the multi-note chords produced by a rhythm guitarist
  • Like a chord produced by a guitar, an OFDM symbol contains multiple information components
  • 802.11a/g uses a form of OFDM which creates 64 subcarriers
  • Only 48 of these subcariers are used to represent bits from the input user data stream.

Rick Murphy's Homepage - http://www.wirelesstrainingsolutions.com

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