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DSP rises to PON challenge

Posted: 11 Feb 2008 ?? ?Print Version ?Bookmark and Share

Keywords:PON? DSP? optical communications?

Next-generation optical communications, including Passive Optical Networks (PONs), will implement mature DSP techniques to provide cost-effective voice, data and video to the home via fiber, according communications experts.

"The speeds of DSP technologies have finally caught up with those of optical communications, where we can take advantage of them to provide the needed performance," said Naresh Shanbhag, research professor in the Coordinated Science Laboratory, University of Illinois at Urbana-Champaign. "Silicon technologies, with its fine-line CMOS, significantly extended the boundaries of digital signal processing versus analog, and what was impractical a few years ago is now aiming for production in optical systems."

Shanbhag moderated a panel at last week's International Solid State Circuits Conference that discussed the challenges of implementing PONs, which enable "first-mile customer access that is fast, flexible and cost-effective."

DSP advantages
"DSP-based CMOS solutions have emerged to address modal dispersion, offering advantages of predictability, scalability and SoC integration," said Norman Swenson, chief technology officer and co-founder of ClariPhy Communications, a fabless communications IC vendor.

The IEEE 10GBASE-LRM standard, recently adopted for 10GbE, requires electronic dispersion compensation (EDC) to combat modal dispersion on multimode fiber. 10G telecom links are increasingly relying on EDC to mitigate the effects of polarization mode dispersion and chromatic dispersion. The conventional method of implementing EDC is an analog tapped delay line or an analog implementation of a decision feedback equalizer. Swenson said that DSP's contribution to limiting EDC is now feasible. John Sitch, senior manager at Nortel, agreed: "Silicon technology has now progressed to the point where it's possible to perform comprehensive signal processing on signals at 40Gbit/s or even 100Gbit/s."

Sitch said that the possibility can be turned into reality because it's cost-effective to replace expensive optics with electronic signal processing. "Electronic technology leads to more operational flexibility, for it's easier to make the equipment self-adapting."

Andrew Singer, research professor at the Computer and Systems Research Laboratory, University of Illinois, Urbana, said that the design of EDC receivers has been enabled by the relentless progress in integration of complex signal processing architecture and circuitry at optical line rates: "The industry is looking forward to 40Gbit/s and 100Gbit/s systems, where it is expected that new photonic materials together with advanced modulation and coding techniques, signal processing and mixed-signal circuits will all be leveraged."

- Nicolas Mokhoff
EE Times

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