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Optical OFDM Design – Direct and Coherent Detection
As the demand for high transmission data rates rise, there becomes an increasing research interest in orthogonal frequency division multiplexing (OFDM) modulation formats. OFDM is a multi-carrier transmission technique which divides the available spectrum into several carriers, each one modulated by a lower data rate stream. Compared to conventional serial modulation schemes, optical OFDM [1] has the following advantages:
It reduces the symbol rate which makes optical OFDM robust against chromatic dispersion (CD) and polarization mode dispersion (PMD).
It transfers the complexity of transmitters and receivers from analog to digital domain. The implementations of inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) in the transceiver make electronic dispersion compensation (EDC) possible.
The subcarrier frequencies are selected so that signals are mathematically orthogonal over one OFDM symbol period. The spectra of individual subcarriers are partially overlapped, resulting in high optical spectral efficiency.
This Application Note demonstrates the simulation of direct & coherent-detection optical OFDM (DD-OOFDM/CO-OOFDM) systems using OptiSystem.
Fig. 1: 10 Gbps direct-detection 4-QAM OFDM system configuration (click to enlarge)
The schematic of the 10 Gbps direct-detection 512-subcarrier 4-QAM OFDM system is shown in Fig. 1. The input data for the OFDM modulator can have different modulation formats: BPSK, QPSK, QAM, etc. In this case 4-QAM is used.
After the OFDM modulator and quadrature modulator (where the RF signal is upconverted to the 7.5 GHz carrier frequency), the generated RF OFDM spectrum is shown in Fig. 2. The RF OFDM signal is then used to drive a Mach-Zehnder Modulator.
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Fig. 2: RF OFDM spectrum after OFDM & quadrature modulation
In Fig. 3, the double and single side band optical signals are presented (after filtering). For simplicity, a back-to-back case is simulated and the detected RF OFDM spectrum is shown in Fig. 4(a). The recovered signal constellation diagram (after OFDM & quadrature demodulation) is shown in Fig. 4(b).
Fig. 3: Optical OFDM spectrum (a) double side band; (b) single side band
Fig. 4: At the receiver: (a) RF OFDM spectrum; (b) signal constellation diagram
The schematic for a 10 Gbps coherent 512-subcarrier 4-QAM OFDM system is shown in Fig. 5. As proposed in [2], a generic CO-OOFDM system includes five basic functional blocks: RF OFDM transmitter, RF to optical (RTO) upconverter, optical link, optical to RF (OTR) donwnconverter, and RF OFDM receiver.
Fig. 5: 10 Gbps coherent 4-QAM OFDM system configuration (click to enlarge)
The RF spectrum for the in-phase component is shown in Fig 6.(a), and the generated optical OFDM spectrum after I/Q modulation is presented in Fig 6.(b). After propagating through 60 km SMF 28 fiber, the recovered signal constellation diagram at the RF OFDM receiver is given in Fig. 7.
Fig. 6: (a) RF spectrum for the in-phase component; (b) Optical OFDM spectrum
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Fig. 7: Signal constellation diagram at the RF OFDM receiver
For transmission of optical OFDM signal over a long length of fiber, electronic dispersion compensation is necessary. In one example of a 10 Gbps coherent 4-QAM OFDM and with 3000km SMF 28 fiber link, the received signal constellation diagram with/without electronic dispersion compensation is presented in Fig. 8.
Fig. 8: 10 Gbps coherent 4-QAM OFDM simulation with 3000km SMF 28 fiber link (a) without electronic dispersion compensation; (b) with electronic dispersion compensation.
Reference:
[1] J. Armstrong, "OFDM for Optical Communications", J. Lightwave Technology, vol. 27, pp. 189-204, Feb 2009.
[2] W. Shieh, X. Yi, Y. Ma, and Q. Yang, "Coherent optical OFDM: has its time come?", J. Optical Networking, vol. 7, pp. 234-255, Mar 2008.
