Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

A support vector machine (SVM)-based classification nonlinear equalizer (NLE) is demonstrated, for the first time, in coherent optical orthogonal frequency-division multiplexing (CO-OFDM). For a 40-Gb/s 16 quadrature amplitude modulated (16QAM) CO-OFDM system at 400 km of transmission, SVM-NLE reduces the fiber-induced nonlinearity penalty by about 1 dB in comparison to the benchmark artificial-neural-network (ANN)-based and inverse-Volterra-series-transfer-function-based NLEs.

Fiber Nonlinearity Equalizer Based on Support Vector Classification for Coherent Optical OFDM

The nonlinear effects of Mach-Zehnder modulator (MZM) and self phase modulation (SPM), in optical orthogonal frequency division multiplexing (OFDM) under the presence of high peak power is studied. A full coherent optical communication system is presented and analyzed. Standard method to reduce the peak to average power ratio (PAPR) values combined with improved technique to mitigate the nonlinear effect, by means of optimized digital pre-distortion, is analyzed and a full performance analysis is presented.

Nonlinear Effects Mitigation in Coherent Optical OFDM System in Presence of High Peak Power

In coherent optical OFDM systems, the large peak-to-average power ratio (PAPR) gives rise to signal impairments through the nonlinearity of modulator and fiber. We review the most prominent PAPR reduction techniques that have been proposed for mitigating the impairments with regard to their reduction capability, computational complexity and redundancy. Simulation results are presented for Clipping, Selected Mapping, Active Constellation Extension and Trellis Shaping.

PAPR reduction techniques for coherent optical OFDM transmission

Coherent optical orthogonal frequency-division multiplexed (OFDM) systems with uniform chromatic dispersion can efficaciously combat both fiber dispersion by utilizing the properties of the cyclic prefix and four-wave mixing (FWM) among the individual subcarriers via the phased-array effect in dispersive fiber links. Such excellent performance, however, often implies appreciable sacrifices in data rate, since a long cyclic prefix is required to compensate for the dispersion accumulated at the receiver. The spectral efficiency of such OFDM systems may be substantially improved by dispersion management. Dispersion-compensating fibers placed periodically along the transmission line can significantly shorten the channel memory thereby allowing a reduction in the cyclic prefix overhead. However, the FWM tolerance of such dispersion-managed (DM) links may suffer considerably. In this work, the application of optical phase conjugation (OPC) to DM OFDM systems is investigated. Several systems set-ups are considered, and the degrees of inter-subcarrier FWM suppression are estimated analytically for arbitrary dispersion map parameters. A comparison is also made against links with uniform chromatic dispersion. Despite their inherently inferior FWM tolerance properties, DM OFDM systems can be made quite competitive with the application of OPC.

On Four-Wave Mixing Suppression in Dispersion-Managed Fiber-Optic OFDM Systems With an Optical Phase Conjugation Module

We propose a frequency-domain method with good structural properties for reduction of the PAPR in coherent optical OFDM systems. Numerical results indicate that the nonlinear threshold can be increased considerably at modest redundancy.

Trellis shaping for reduction of the peak-to-average power ratio in coherent optical OFDM systems

We show that signal shaping schemes for the reduction of nonlinearities in optical OFDM systems need to take into account the signal evolution along the dispersive link. Rather than minimizing the transmit PAPR, a good precoding scheme should minimize the cumulative power variance.

Link-aware precoding for nonlinear optical OFDM transmission

We describe how simulations based on the Split-Step Fourier Method can be significantly accelerated by using commodity graphics hardware. Results for a benchmark scenario are provided to highlight performance and accuracy.

Stephan Hellerbrand

Papers

PAPR reduction techniques for coherent optical OFDM transmission

Trellis shaping for reduction of the peak-to-average power ratio in coherent optical OFDM systems

Link-aware precoding for nonlinear optical OFDM transmission

Fast implementation of the Split-Step Fourier Method using a graphics processing unit

Advanced Modulation Formats for Electronic Predistortion of Intrachannel Nonlinearities at 40 Gbit/s