AUGUST 6, 2009 By Stephen Hardy -- The alphabet soup of 40G modulation formats just got thicker. Nortel has revealed that the record 40G transmission distance touted in the August 3 announcement of its high-speed networking trial with Telstra 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 next year 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 said. 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.
As to whether Nortel will leverage the new modulation format for 100-Gbps applications, Sitch would only allow that the company is interested in supporting ultra-long-haul deployments of 100G.