Today, optical fiber telecommunication systems are based on high-order modulation formats, coherent detection schemes, digital signal processing and channel coding techniques to fulfill the ever increasing demands for growing capacity and transmission reach. While coherent detection in combination with digital signal processing enables the compensation of linear fiber effects like dispersion and polarization mode dispersion, signal distortions due to fiber nonlinearities are still a limiting factor. Optics-based methods such as midspan spectral inversion or phase-conjugated twin-wave transmission could not find their way into commercial systems. Digital signal processing methods like digital backward propagation are still too complex for real-time implementation, especially for multi-channel transmission in wavelength-division multiplexed systems.

In this article, Temprana et al. investigate a transmitter side digital backward propagation method that is combined with frequency-referenced carriers for the individual wavelength channels. Two major advantages follow from this concept. On the one hand the frequency-referenced carriers enable effective compensation of cross-phase modulation induced distortions, and on the other hand the signal processing is not influenced by noise generated by the inline amplifiers. As real-time implementation of the signal-processing unit is not possible, the authors use an off-line processing of the data before they modulate the predistorted 16-quadrature amplitude modulation signals onto the optical carriers. In a recirculating loop they propagate three channels up to 40 spans of 85 km before detection in a coherent receiver.
The system length where the forward error correction limit of 1.1x10^-3 bit error probability was reached could be shifted from 1530 km to 3016 km by applying the transmitter-side backward propagation instead of electronic dispersion compensation only.

This article discusses a novel concept of nonlinearity compensations. Even though the digital signal processing could not be demonstrated in a real-time setup and will need a strong reduction in numerical complexity, the authors made an important contribution to the field. It is obvious that a synergetic combination of digital signal processing techniques and sophisticated optical concepts give way for further progress in optical communication systems that is needed to fulfill the demand for ever increasing transport capacity and reach.

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