SEPTEMBER 22, 2010 -- Fujitsu Ltd., Fujitsu Laboratories Ltd., and Fujitsu Research and Development Center Co., Ltd. of China say that they have developed a digital signal processing (DSP) algorithm that compensates for waveform distortion in signals sent over hundreds of kilometers of fiber-optic lines and that they have also reduced the size of the required circuitry by approximately 70%.
The Fujitsu group asserts their work will enable long-haul fiber-optic transmission systems transmitting at more than 100 Gbps per wavelength at a lower cost than conventional 10-Gbps systems. Fujitsu's further asserts that the new technology will make transmission at rates greater than 100G possible within five years.
Some of this research was conducted as part of the "Universal Link Project R&D" sponsored by the National Institute of Information and Communications Technology (NICT) of Japan.
The problems nonlinear optical effects cause at very high speeds has prompted research into nonlinear compensation technology, which can restore the signal received with distortion to a clean waveform. Using conventional methods, however, the implementation of nonlinear compensation technology would require massive circuits with more than 100 million logic gates; chips that include such compensation circuits are only expected to become feasible around 2020, according to Fujitsu. Reducing the scale required of such circuits, therefore, has been a pressing issue.
Fujitsu's team developed a new signal-processing algorithm and also was able to dramatically reduce the circuit stages, implementing the algorithm to approximately one-quarter of what previously had been required. This was enabled by advances in two areas, Fujitsu says:
- Addition of weighted-average processing: In previous technologies, the amount of compensation used to compensate for the distortion in a signal over a given interval was computed based on the signal in only that interval. Fujitsu's new approach is to take into account adjacent intervals. Fujitsu says this approach enables fine-grained and accurate compensation using fewer circuit stages. This has been implemented using weighted-average processing.
- Decision algorithm for weighted-average weighting: To compensate the signal accurately and with good granularity using fewer circuit stages, the weight to be given to adjacent intervals can only be decided based on a number of factors, such as transmission distance. The Fujitsu team's development included the ability to automatically determine the ideal weighting at the receiver based on the characteristics of the incoming wave.
This technology was employed in an experiment to transmit a 112-Gbps signal over 1,200 km. The signal quality using the new five-stage circuit was found to be comparable to that resulting from a conventional 20-stage circuit, Fujitsu asserts. By reducing the circuit size, Fujitsu believes it will be possible to quickly commercialize the technology using 28-nm CMOS process technology, which is expected to be widely used by around 2013. By contrast, using existing technologies, it is anticipated that it would have been challenging to commercialize in less than 10 years, Fujitsu says.
Fujitsu adds it is working to develop next-generation long-distance optical communications systems that operate at speeds of over 100 Gbps and are scheduled for commercialization by 2015. The company is also considering expanding the technology's applications to include high-volume, short-distance transmissions within datacenters and access networks.