New Electronics - May 2010

There are several reasons why engineers are working harder when designing high capacity optical systems – higher data rates mean fewer photons per optical pulse and there is greater signal dispersion. Adding to the challenge, service providers want the reach of 40 and 100Gbit/s systems to match that of existing networks designed for 10Gbit/s lightpaths.

"The optical signal to noise ratio goes down 10dB as line rates go from 10Gbit/s to 100Gbit/s," said Michael Scholten, strategy and technology manager at Vitesse Semiconductor.

Advanced signal processing must be used to compensate for the distortion, while system performance must be enhanced to claw back the lost dBs. "Something needs to change to maintain the same [optical] reach," said David Yeh, AppliedMicro's senior product marketing manager.

This is where forward error correction (fec) comes in and it explains the growing interest in an approach known as soft decision fec (sd-fec).
SD-FEC, which has already been used for networking interfaces such as 10GBaseT, and for radio and satellite communications, differs from traditional fec in that it uses analogue signals to show the likelihood of data being a 1 or a 0, rather than rigid binary values. "You get information from the channel as to a 1 or a 0 and how reliable that piece of info is," said Sameep Dave, principal engineer at ViaSat.

With sd-fec, extra information is fed to the decoder, which equates to an improved electrical coding gain. But this means greater design complexity, a lot more processing horsepower and, in the case of 100Gbit/s optical transmission, a/d converters operating at rates approaching 64Gsample/s.

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