University of Baghdad College of Engineering Ele. & Comm. Dept.

4

th

year

optical Orthogonal frequency division multiplexing (OOFDM)

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 single carrier 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 sub carriers. This waveform is modulated onto an RF-carrier, fRF, using an I-Q modulator, producing a real-valued waveform comprising a band of sub carriers 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 odd frequencies 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.