Network Diagramming Channel Feature

Lightwaveonline Explains Ways to Address Issues with DP-QPSK Channels on Metro/Regional Networks

By Rajani Baburajan
TMCnet Contributor

The deployment of coherent technology in metropolitan and regional networks will address the capacity demands facing carriers, according to an article from Lightwaveonline.com.  

Metro/regional mesh networks feature a large number of relatively short (10-80 km) spans, interconnected by ROADM (News - Alert) switches, with optical amplifiers and dispersion compensation incorporated to give essentially zero loss and zero dispersion between nodes.

While 40G coherent DP-QPSK signals offer several advantages in metro/regional applications, there’s one hurdle that must be overcome when deploying this technology on legacy networks. Cross-phase modulation (XPM) can impair DP-QPSK signals when combined with 10G on/off-keyed (OOK) channels on fully dispersion compensated WDM links.

This issue can be addressed through possible mitigation techniques, including skipping channels to reduce XPM or assigning 10G OOK signals and 40G DP-QPSK channels to different bands, separated by guardbands.

However, such methods waste valuable spectrum and reduce flexibility of network deployment, according to Lightwaveonline.com.

Another method to deal with these impairments is the forward error correction (FEC) codes at 40 Gbps. Originally developed for submarine channels, FEC can drive very high pre-FEC error rates essentially to zero. The high overhead of these codes is not problematic with DP-QPSK because of its relatively narrow spectrum.

According to Lightwaveonline.com, the high overhead can actually help alleviate XPM as it increases the baud rate difference between the 10G and 40G channels. These codes are commercially available today in Optical Transport Network (OTN) network diagramming/mapper/framer chips that are already deployed in many existing telecom systems.

A recent experiment was performed to quantify the advantage of combining DP-QPSK with high-gain FEC. The result shows the optical signal-to-noise ratio (OSNR) required to drive the post-FEC bit error rate (BER) essentially to 0 for different FEC coding schemes of various overheads, for 50-GHz and 100-GHz channel spacing, respectively.

In the 50-GHz channel spacing results, the launch power of the wavelengths increased, the OSNR required to close the link increased due to XPM. With 7 percent FEC, DQPSK is less sensitive to this effect than DP-QPSK is. But the higher-gain FECs improved the performance of DP-QPSK, reducing the penalty significantly.

The crossover launch power at 13 percent overhead is approximately -1.5 dBm (News - Alert). At 20 percent and 25 percent overhead, the crossover launch power is in the -0.8 dBm to -0.6 dBm range.

In the 100-GHz channel spacing, the 7 percent coded DQPSK again outperformed the 7 percent coded DP-QPSK at all launch powers tested due to XPM-induced phase noise. But with the 13 percent code, the DP-QPSK showed good tolerance to the XPM aggressors and outperformed the 7 percent DQPSK channel up to a launch power of +0.5 dBm.

Further, using the 20 percent and 25 percent overhead codes, the DP-QPSK XPM tolerance is excellent, outperforming the 7 percent coded DQPSK channel at all launch powers tested, including launch powers of +1 dBm.

The study concludes that use of high-gain FEC solves the XPM sensitivity of 40G coherent DP-QPSK and enables combination with 10G OOK links on legacy metro and regional networks. While it has been argued that coherent 40G requires use of more expensive BPSK, FEC can mitigate the effects of XPM and enable use of cost-effective and bandwidth-efficient DP-QPSK. 

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