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Overview

Optical OFDM to Combat Chromatic Dispersion in Long-Haul Systems

Description

Orthogonal Frequency Division Multiplexing (OFDM) is a widespread technology in broadband communication (wired and wireless) because of its ability to cope strong channel distortions (interference, frequency fading, multipath propagation). OFDM takes advantage of the Fast Fourier Transform (FFT) to achieve a high spectral efficiency and perform simple channel equalization.
For these reasons, optical OFDM has been identified as an attractive solution for optical long-haul transmission, as it offers a reduced signal bandwidth and enables simple digital equalization of chromatic dispersion.

Typical Results

The simulation setup is displayed in Figure 1. It is similar to the one reported in [1]: Single-side band optical OFDM is used to transmit a 10Gb/s signal over 1000 km of SSMF without the need for inline dispersion compensation. The OFDM signal consists of 64 sub-carriers, each being encoded with Quadrature Amplitude Modulation (QAM). Optical to electrical conversion is realized with a single photodiode. The detected signal is then passed to an electrical OFDM receiver that performs down-conversion, electrical filtering, analog to digital conversion (ADC) and demodulation (cyclic-prefix removal and inverse-FFT). The phase of each sub-carrier can be equalized separately to compensate for chromatic dispersion. The received constellation diagram with and without equalization are displayed in Figure 2 (ASE-noise from inline EDFAs is switched-off).
One drawback of this technique is its high OSNR requirement. Indeed, most of the power is located in the optical carrier (necessary for direct detection). This limits the achievable signal-to-noise ratio of the individual sub-carriers. The impact of optical noise on system performance is illustrated in Figure 3, where the spectrum and constellation diagram of the received signal are displayed for the case that ASE-noise from inline EDFAs is switched-on.

Keywords

Orthogonal Frequency Division Multiplexing (OFDM), Chromatic Dispersion, Digital Equalization