Orthogonal frequency division multiplexing

University of Baghdad
College of Engineering
Ele. & Comm. Dept.
4th year
optical Orthogonal frequency division multiplexing
By: Hassan abd-alhadi Ali
Introduction : Orthogonal frequency division multiplexing (OFDM) is a multicarrier
transmission technique which is based on frequency division multiplexing
(FDM) . In conventional FDM multiple-frequency signals are transmitted
simultaneously in parallel where the data contained in each signal is
modulated onto subcarriers and therefore the subcarrier multiplexed
signal typically contains a wide range of frequencies. Each subcarrier is
separated by a guard band to avoid signal overlapping. The subcarriers
are then demodulated at the receiver by using filters to separate the
frequency bands. By contrast OFDM employs several subcarrier
frequencies orthogonal to each other ( i.e. Perpendicular) and therefore
they do not overlap. Hence this technique can squeeze multiple modulated
carriers tightly together at a reduced bandwidth without the requirement
for guard bands while at the same time keeping the modulated signals
orthogonal so that they do not interfere with each other , as illustrated in
figure below.
In the upper spectral diagram 10 nonoverlapping subcarrier frequency
signals arranged in parallel depicting conventional FDM are shown, each
being separated by a finite guard band. OFDM is displayed in the bottom
spectral diagram where the peak of one signal coincides with the trough
of another signal. Each subcarrier ,however , must maintain the Nyquist
criterion separation with the minimum time period of T ( i.e. a frequency
spread of 1/T) for each subcarrier.
 OFDM uses the inverse fast Fourier transform ( IFFT ) for the purpose of
modulation and the fast Fourier transform ( FFT ) for demodulation.
Moreover , this is a consequence of the FFT operation by which
subcarriers are positioned perpendicularly and hence the reason why the
technique is referred to as orthogonal FDM . It may be observed that a
large bandwidth saving in comparison with conventional FDM is identified
in figure above resulting from the orthogonal placement of the
subcarriers.Since the orthogonal feature allow high spectral efficiency
near the Nyquist rate where efficient bandwith use can be obtained, OFDM
generally exhibits a nearly white frequency spectrum ( i.e. without
electromagnetic interference between the adjacent channels). OFDM, also
being tolerant to the signal dispersion , thus enables high-speed data
transmission across a dispersive channel and it has been widely used in
high-bit-rate cable and wireless communication systems.
Optical OFDM is currently a hot topic in the fiber-optic research
community and the number of optical OFDM research papers published in
international conferences and journals has grown exponentially over the
last couple of years . However, the question if this modulation format is
really a viable candidate for next generation fiber-optic transmission
systems is up till now only partially answered. There exist several
different definitions of OFDM in the fiber-optic community. In this paper,
we refer to OFDM as a digital multicarrier technique. As the subcarriers
are generated in the digital domain, these systems typically consist of
many subcarriers (typically more than 50) where channel estimation is
realized by periodically inserting training symbols We therefore exclude in
the paper coherent WDM systems typically have few subcarriers that are
generated in the optical domain. These systems typically do not use
training symbols, but rely on blind channel estimation instead. Such
systems have more in common with singlecarrier coherent systems and
its evaluation is out of scope of this manuscript. In this paper we assess
how the performance of OFDM scales with coherently detected singlecarrier QPSK, another promising modulation formats for long-haul
transmission. Even though many different OFDM systems have been
proposed, we restrict ourselves to coherent detected OFDM, as this
modulation format is most suited for long-haul transmission.
OFDM transmits a serial high-speed data channel by dividing it into blocks
of data then using Fourier transform techniques to encode the data on
separate subcarriers in the frequency domain. Our system using OFDM
over an optical channel is shown in Fig. 1. Each block of data is presented
as N parallel data paths to the OFDM transmitter. The N paths are
modulated onto N equally-spaced subcarriers using Quadrature-Amplitude
Modulation (QAM). This is similar to Hui’s subcarrier multiplexed system;
however, it overcomes the complexities and practicalities of multiple
microwave mixers by using an inverse-FFT (IFFT) to generate a dense
comb of OFDM sub-carrier frequencies: each QAM data channel is
presented to an input of the IFFT; the IFFT produces a complex-valued
time domain waveform containing a superposition of all of the subcarriers. This waveform is modulated onto an RF-carrier, fRF, using an I-Q
modulator, producing a real-valued waveform comprising a band of subcarriers displaced from DC Next, this band is modulated onto an optical
carrier using a linear optical modulator. In contrast to our earlier system,
the output of the optical modulator is filtered to remove all frequencies
other than the upper
 side-band (or lower sideband if preferred) and an attenuated
(suppressed) optical carrier.
After propagation through the fiber link, the photodiode produces an
electrical waveform. This is converted to I and Q components by mixing
with 0º and 90º phases of a local oscillator at fRF. The I and Q waveforms
are then converted to OFDM subcarriers using a FFT, which, if the
transmitter and receiver FFT windows are synchronized in time, acts as a
set of closelyspaced narrowband filters. The periodic boundary conditions
of the simulator enforce this synchronization. In a real system, a cyclic
prefix is added to each transmitted block after the IFFT, so that the
relative delays between the received OFDM-subcarriers (due to fiber
 dispersion) can be accommodated without destroying the orthogonality of
the OFDM subcarriers . For a 4000-km link of S-SMF at 1550 nm, the
relative delay over the OFDM band is 2560 ps, requiring prefixes that
extend the block by only a few percent. Once in the frequency-domain,
each channel is equalized to compensate for phase and amplitude
distortion due to the optical and electrical paths. This is easily achieved by
using a separate complex multiplication for each channel.
The multiplication coefficients can be determined by training the system
with a known data sequence or by introducing pilot channels to the OFDM
band to estimate the dependence of optical phase on frequency. After
 equalization, each QAM channel is demodulated to produce N parallel
data channels. These can be converted into a single data channel by
parallel to serial conversion.
Applying OFDM to Optical Signals:
Electrical OFDM signals are usually bipolar – containing both positive and
negative peaks. It is possible to modulate an RF OFDM signal onto an
optical carrier provided a sufficient bias is added to the electrical OFDM
signal to ensure that negative peaks become positive optical powers . This
technique imposes a receiver sensitivity penalty as it requires high mean
optical powers compared to the signal content. Typically the receiver
sensitivity (the required optical power for a given bit rate) will be
degraded by over 6 dB compared with non-return to zero
 modulation. We have previously demonstrated two solutions to this
problem, which are :
• Remove all excursions below the mean level of the electrical OFDM
signal when converting to an optical OFDM signal. This is called
‘Asymmetrically-Clipped Optical OFDM’. As expected from simple
Fourier theory, ACO-OFDM causes distortion of the OFDM signals, giving
rise to distortion products. However, we have shown that if only oddfrequencies are used, the distortion falls only even frequencies of the
OFDM sub-carrier grid so can be completely rejected at the receiver.
Alternatively, we can upconvert the OFDM spectrum (of bandwidth C) by
a frequency C, so that the lowest ‘clipping noise’ on the received
subcarriers, but the signal quality per unit optical power is improved :We
have shown analytically and by simulations that the receiver sensitivity to
be 1.8 dB better than NRZ.
• Optically modulate using a strong bias then suppress the optical carrier
using an optical filter. This also introduces clipping noise due to
intermixing of the subcarriers upon photodetection. The solution is again
to upconvert the OFDM band. A variation is to completely remove the
optical carrier at the transmitter and reintroduce it at the receiver, but this
requires a ‘coherent’ receiver design which should be insensitive to
sideband so that fiber dispersion does not polarization. In both systems it
is desirable to suppress one optical cause strong nulls in the baseband
spectrum after photodetection.

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