OFDM - Orthogonal Frequency Division Multiplex, the modulation
concept being used for many wireless and radio communications radio
applications from DAB, DVB, Wi-Fi and Mobile Video.
Orthogonal Frequency Division Multiplex or OFDM is a modulation
format that is finding increasing levels of use in today's radio
communications scene. OFDM has been adopted in the Wi-Fi arena where the
802.11a standard uses it to provide data rates up to 54 Mbps in the 5
GHz ISM (Industrial, Scientific and Medical) band. In addition to this
the recently ratified 802.11g standard has it in the 2.4 GHz ISM band.
In addition to this, it is being used for WiMAX and is also the format
of choice for the next generation cellular radio communications systems
including 3G LTE and UMB.
If this was not enough it is also being used for digital terrestrial
television transmissions as well as DAB digital radio. A new form of
broadcasting called Digital Radio Mondiale for the long medium and short
wave bands is being launched and this has also adopted COFDM. Then for
the future it is being proposed as the modulation technique for fourth
generation cell phone systems that are in their early stages of
development and OFDM is also being used for many of the proposed mobile
phone video systems.
OFDM, orthogonal frequency division multiplex is a rather different
format for modulation to that used for more traditional forms of
transmission. It utilises many carriers together to provide many
advantages over simpler modulation formats.
OFDM concept
An OFDM signal consists of a number of closely spaced modulated
carriers. When modulation of any form - voice, data, etc. is applied to a
carrier, then sidebands spread out either side. It is necessary for a
receiver to be able to receive the whole signal to be able to
successfully demodulate the data. As a result when signals are
transmitted close to one another they must be spaced so that the
receiver can separate them using a filter and there must be a guard band
between them. This is not the case with OFDM. Although the sidebands
from each carrier overlap, they can still be received without the
interference that might be expected because they are orthogonal to each
another. This is achieved by having the carrier spacing equal to the
reciprocal of the symbol period.
Traditional view of receiving signals carrying modulation
To see how OFDM works, it is necessary to look at the receiver. This
acts as a bank of demodulators, translating each carrier down to DC. The
resulting signal is integrated over the symbol period to regenerate the
data from that carrier. The same demodulator also demodulates the other
carriers. As the carrier spacing equal to the reciprocal of the symbol
period means that they will have a whole number of cycles in the symbol
period and their contribution will sum to zero - in other words there is
no interference contribution.
OFDM Spectrum
One requirement of the OFDM transmitting and receiving systems is
that they must be linear. Any non-linearity will cause interference
between the carriers as a result of inter-modulation distortion. This
will introduce unwanted signals that would cause interference and impair
the orthogonality of the transmission.
In terms of the equipment to be used the high peak to average ratio
of multi-carrier systems such as OFDM requires the RF final amplifier on
the output of the transmitter to be able to handle the peaks whilst the
average power is much lower and this leads to inefficiency. In some
systems the peaks are limited. Although this introduces distortion that
results in a higher level of data errors, the system can rely on the
error correction to remove them.
Data on OFDM
The data to be transmitted on an OFDM signal is spread across the
carriers of the signal, each carrier taking part of the payload. This
reduces the data rate taken by each carrier. The lower data rate has the
advantage that interference from reflections is much less critical.
This is achieved by adding a guard band time or guard interval into the
system. This ensures that the data is only sampled when the signal is
stable and no new delayed signals arrive that would alter the timing and
phase of the signal.
Guard Interval
The distribution of the data across a large number of carriers in the
OFDM signal has some further advantages. Nulls caused by multi-path
effects or interference on a given frequency only affect a small number
of the carriers, the remaining ones being received correctly. By using
error-coding techniques, which does mean adding further data to the
transmitted signal, it enables many or all of the corrupted data to be
reconstructed within the receiver. This can be done because the error
correction code is transmitted in a different part of the signal.
OFDM variants
There are several other variants of OFDM for which the initials are
seen in the technical literature. These follow the basic format for
OFDM, but have additional attributes or variations:
- COFDM: Coded Orthogonal frequency division multiplex. A form of OFDM where error correction coding is incorporated into the signal.
- Flash OFDM: This is a variant of OFDM that was
developed by Flarion and it is a fast hopped form of OFDM. It uses
multiple tones and fast hopping to spread signals over a given spectrum
band.
- OFDMA: Orthogonal frequency division multiple
access. A scheme used to provide a multiple access capability for
applications such as cellular telecommunications when using OFDM
technologies.
- VOFDM: Vector OFDM. This form of OFDM uses the
concept of MIMO technology. It is being developed by CISCO Systems. MIMO
stands for Multiple Input Multiple output and it uses multiple
antennas to transmit and receive the signals so that multi-path effects
can be utilised to enhance the signal reception and improve the
transmission speeds that can be supported.
- WOFDM: Wideband OFDM. The concept of this form of
OFDM is that it uses a degree of spacing between the channels that is
large enough that any frequency errors between transmitter and receiver
do not affect the performance. It is particularly applicable to Wi-Fi
systems.
Each of these forms of OFDM utilise the same basic concept of using
close spaced orthogonal carriers each carrying low data rate signals.
During the demodulation phase the data is then combined to provide the
complete signal.
OFDM Summary
OFDM and COFDM have gained a significant presence in the wireless
market place. The combination of high data capacity, high spectral
efficiency, and its resilience to interference as a result of multi-path
effects means that it is ideal for the high data applications that are
becoming a common factor in today's communications scene.
