Nortel unveils another 40G modulation format

By Stephen Hardy


Nortel has unveiled dual-polarization binary phase-shift keying, which the company asserts is more resilient to chromatic and polarization-mode dispersion than other formats.

The alphabet soup of 40G modulation formats has been thickened. Nortel reveals that the record transmission distance touted in the August 3 announcement of its high-speed terrestrial networking trial with Telstra, as well as the 8,000-km undersea trial with Southern Cross Cables described in a subsequent announcement, derived from a prototype application of dual-polarization binary phase-shift keying (DP-BPSK) with coherent detection. The company plans to make the technology available commercially in the first quarter of 2010 for applications requiring reaches on the order of 4,000 km or greater.

Like the DP quadrature phase-shift keying (QPSK) with coherent detection that Nortel already has on the market for 40 Gbps and plans to have commercially available for 100 Gbps by the end of this year, the new modulation format transmits signals in phase rather than amplitude. Combined with dual polarization, standard DP-BPSK would result in a transmission rate of 23 Gbaud, according to John Sitch, Nortel senior advisor, optical systems, and Helen Xenos, optical solutions marketing manager. However, as is the case with Nortel’s current 100G approach, the prototype line cards combine the modulation format with optical frequency-division multiplexing, splitting the 40-Gbps transmission into two subcarriers that fit within a 50-GHz window. This results in a transmission rate of 11.5 Gbaud.

The appeal of DP-BPSK is that it is significantly more robust to impairments than DP-QPSK—or, as Sitch put it, “It will talk through dirt.” The use of binary rather than quadrature encoding provides 3 dB more noise margin and 6 dB greater margin for phase distortion, he says. Nortel will continue to pair its proprietary eDCO electronic dispersion compensation technology with the new format, however.

While acknowledging that DP-BPSK line cards will be more expensive to produce than Nortel’s current 40-Gbps offerings, Sitch and Xenos declined to speculate about what, if any, cost premium the commercial offering might carry. Sitch pointed out that the ability to skip regeneration sites might soften any potential sticker shock.

Both the Telstra and Southern Cross Cable trials demonstrated the use of DP-BPSK with coherent detection using Nortel’s Optical Multiservice Edge (OME) 6500 metro platform equipped with the company’s 40G Adaptive Optical Engine. In the case of the Telstra trial, both 40- and 100-Gbps signals were transmitted using the same platform.

The 100-Gbps stream did not use the new format. As to whether Nortel will leverage the DP-BPSK for 100-Gbps applications, Sitch would only allow that the company is interested in supporting ultralong-haul deployments of 100G.

Links to more information

LIGHTWAVE ONLINE: Telstra Touts 100G Trials with Nortel
LIGHTWAVE ONLINE: Nortel and Southern Cross Achieve 40G at 8,000 km on Submarine Cable
LIGHTWAVE ONLINE:OIF Issues 100G Framework Document

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