(QAM) where both the amplitude and
phase of the carrier are changed to
transmit multiple bits per symbol.
A symbol is just one of several
amplitude-phase combinations that
represent multiple bits.
CDMA permits higher speeds by
spreading the data over a very wide
bandwidth. OFDM also uses a wide
bandwidth, but in a different way.
What OFDM does is generate many
closely spaced carriers side-by-side in
the spectrum. Then it divides up the
high speed data to be transmitted into
multiple, slower serial data streams
and uses those to modulate the
multiple carriers. The number of
carriers depends upon the application, but can be less than 100 on up to
Figure 1 shows what the OFDM
signal looks like in the frequency
domain. OFDM gets its name from
the fact that it looks like a frequency
division multiplexed (FDM) signal
that puts multiple channels on a
common bandwidth. A good
example is the multiple channels on a
common cable TV coax. A given
bandwidth is divided up into channels or subcarriers sometimes called
bins. Only a few subcarriers are
shown here, but each has a carrier at
its center with the surrounding
channel width providing some spectrum for the sidebands created by the
modulation. The carriers are frequency multiplexed into a wide channel,
thus the name. The type of modulation can vary, but it is usually some
form of phase shift keying like BPSK,
QPSK, and even QAM.
The big question is how do you
keep all the adjacent channels from
interfering with one another? In the
usual FDM application, the channels
are spaced apart from one another
with a so-called guard band between
them. In OFDM, the channels are
directly adjacent with no guard band.
■ FIGURE 2. Each sub-band
overlaps the adjacent bands.
The signals in each are
orthogonal such that the
sidebands are nulled out at the
adjacent carriers so they do not
interfere with one another,
making all signals recoverable
at the receiver.
That is where the term
orthogonal comes in.
Orthogonal usually refers to
two signals that are vectors
90 degrees out-of-phase. That
means they won’t bother one another and can be recovered individually
when combined. In OFDM, orthogonal means that the individual carriers
are spaced by a frequency that is the
reciprocal of the symbol duration.
What that means is that each carrier
frequency will have an integer
number of carrier cycles during one
bit period of the data. The result is a
spectrum that looks like that in Figure
2. Here, the spectrum response
creates a null or zero at each of the
adjacent carrier frequencies. With
this arrangement, the individual
modulated carriers can be recovered
and demodulated without interference from one another.
You are probably wondering just
how all those carriers can be generated, modulated, then multiplexed
on the output, then transmitted. At
the other end, how do you demulti-plex the carriers, demodulate each,
and then recombine the slower data
streams back into the fast data
stream you started with? Assuming
hundreds or thousands of subcarriers
and the needed modulators and
demodulators, the circuits would be
alarmingly large and complex.
Even with integrated circuits, it would
be a daunting task to make all that
In real OFDM, the whole process
is done with software on a digital
signal processor (DSP). A special
mathematical technique known as the
fast Fourier transform (FFT) is used to
convert a digitized OFDM signal back
into individual demodulated carriers.
An inverse FFT (IFFT) is used at the
transmitter to generate the OFDM
signal. Literally, the whole process is
done with mathematical algorithms
that run on a DSP. You simply
program it to give you the desired
number of subcarriers with the
Figure 3 shows the transmitter.
The fast serial data is divided into
hundreds or thousands of slower
serial data paths. For example, a 100
Mbps serial signal could be divided
into 1,000 channels each with a data
rate of 100 Mbps/1,000 = 100 kbps.
■ FIGURE 3. An OFDM transmitter. The
fast serial data is converted to slower
parallel data streams that are processed
by a DSP using the inverse FFT into
multiple modulated carriers. The DSP
outputs are translated into signals that
are upconverted to the desired output
frequency, then amplified and applied
to the antenna.
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