Ofdm
Ofdm
Ofdm
Division Multiplexing
Orthogonal frequency division (ADSL) as well as wireless systems
multiplexing (OFDM) is a multi-carrier such as IEEE 802.11a/g (Wi-Fi*) and
transmission technique that has been IEEE 802.16 (WiMAX*). It is also used
recently recognized as an excellent for wireless digital audio and video
method for high speed bi-directional broadcasting.
wireless data communication. Its
history dates back to the 1960s, but it It is based on frequency division
has recently become popular because multiplexing (FDM), which is a
economical integrated circuits that can technology that uses multiple
perform the high speed digital frequencies to simultaneously transmit
operations necessary have become multiple signals in parallel. Each signal
available. OFDM effectively squeezes has its own frequency range (sub-
multiple modulated carriers tightly carrier) which is then modulated by
together, reducing the required data. Each sub-carrier is separated by
bandwidth but keeping the modulated a guard band to ensure that they do
signals orthogonal so they do not not overlap. These sub-carriers are
interfere with each other. Today, the then demodulated at the receiver by
technology is used in such systems as using filters to separate the bands.
asymmetric digital subscriber line
9 Subcarriers
Intel in
Communications
Frequency
Figure 1: FDM with Nine Sub-carriers Using Filters
OFDM is similar to FDM but much with it. In Figure 2, the effect of this is
more spectrally efficient by spacing seen as the required bandwidth is
the sub-channels much closer greatly reduced by removing guard
together (until they are actually bands and allowing signals to overlap.
overlapping). This is done by finding In order to demodulate the signal, a
frequencies that are orthogonal, which discrete Fourier transform (DFT) is
means that they are perpendicular in a needed. Fast Fourier transform (FFT)
mathematical sense, allowing the chips are commercially available,
spectrum of each sub-channel to making this a relatively easy
overlap another without interfering operation.
9 Subcarriers
Frequency
individual data sub-carrier and modulated using during the transmission while null carriers allow for
either PSK or QAM. Pilot sub-carriers provide a guard bands and the DC carrier (center
reference to minimize frequency and phase shifts frequency).
Nulls
… … Nulls
Frequency Channel
Bandwidth
Orthogonal frequency division multiple access Another advantage of OFDM is its resilience to
(OFDMA) allows some sub-carriers to be assigned multipath, which is the effect of multiple reflected
to different users. For example, sub-carriers 1, 3 signals hitting the receiver. This results in
and 7 can be assigned to user 1 and sub-carriers interference and frequency-selective fading which
2, 5 and 9 to user 2. These groups of sub-carriers OFDM is able to overcome by utilizing its parallel,
are known as sub-channels. Scalable OFDMA slower bandwidth nature. This makes OFDM ideal
allows smaller FFT sizes to improve performance to handle the harsh conditions of the mobile
(efficiency) for lower bandwidth channels. This wireless environment.
applies to IEEE 802.16-2004 which can now
reduce the FFT size from 2048 to 128 to handle OFDM’s high spectral efficiency and resistance to
channel bandwidths ranging from 1.25–20 MHz. multipath make it an extremely suitable technology
This allows sub-carrier spacing to remain constant to meet the demands of wireless data traffic. This
independent of bandwidth which reduces has made it not only ideal for such new
complexity while also allowing larger FFT for technologies like WiMAX* and Wi-Fi* but also
increased performance with wide channels. currently one of the prime technologies being
considered for use in future fourth generation (4G)
networks.
Intel Access
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