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.
What is OFDM? - The 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
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.
With Regards
Jalandhar
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