I reviewed how coherent optics allowed 40 Gbps waves to be dropped into existing 10 Gbps DWDM systems without major modifications. That was good news for network operators who had a much more difficult upgrade path from 2.5 to 10 Gbps. There’s more good news: the optical magicians have pulled another rabbit out of their hat. The new generation of 100 Gbps transponders will also play with 10 and 40 Gbps waves in existing 50 Ghz DWDM windows. The bad news is that it looks like there are no more rabbits in the hat.
At 100 Gbps, the optical to electrical conversion is problematic, because processing a 100 Gbps native stream would require very specialized electronics today. One way to mitigate these problems is to divide the 100 Gbps stream using wavelength division multiplexing into 10 x 10 Gbps or 4 x 25 Gbps optical channels. These lower speed streams can be transmitted by separate lasers and processed using less specialized optoelectronics. This works well for short-range links where fiber capacity is relatively inexpensive. For longer reach where system capacity is valuable, and suitable lasers are expensive, a native 100 Gbps optical channel using a single laser is desirable.
But increasing modulation rate using the same method is not a viable option for upgrading existing networks, making more sophisticated modulation schemes necessary. Encoding two bits per symbol doubles the data rate without increasing the optical bandwidth, or sensitivity to dispersion. Encoding two of these signals, one in each polarization mode of the fiber, allows a further doubling of the data rate, still with the same bandwidth and dispersion tolerance. This scheme, known as dual-polarization quadrature phase-shift keying (DP-QPSK), is now the standard for commercial development of long-haul 100 Gbps on a single wavelength.
Encoding four bits per symbol interval not only enables transmission using a single channel, it also facilitates signal processing without expensive ultra high-speed electronics. The four bits can be processed as four parallel and uncorrelated 25 Gbps payloads on the line side, and then multiplexed into a single 100 Gbps serial handoff on the drop side.
Decoding a polarization multiplexed signal presents a problem, though. Ordinary single mode fiber does not maintain polarization state along its length. So, complex and expensive dynamic polarization controllers were needed in the past to align the receiver with the transmitted polarization state in the optical domain. A coherent detector moves the polarization state into the electrical domain, allowing it to be estimated by the DSP algorithm. The problem of receiving the two scrambled polarization modes is analogous to transmitting data in free space using two antennas and two receivers, known in wireless communications as multiple-input, multiple-output (MIMO). Algorithms developed for MIMO have been adapted to decode the scrambled polarization state in a coherent receiver, making polarization multiplexing feasible.
With these advancements, 100 Gbps DP-QPSK waves can be added to an existing DWDM system engineered for 10 Gbps. In fact, 100 Gbps transponders using all digital dispersion compensation could be used on links that would require dispersion compensation just to pass 10 Gbps. This can bring new life to older fiber routes that are capacity limited and not easily upgraded, or add value to old fiber obtained on long-term IRU.
Of course, there has to be a down side, and naturally it’s cost. Dual polarization adds optical elements and doubles the number of transmitter and receiver elements. The coherent detector doubles the number of receiver elements again. Each of the four receiver elements must employ high speed ADC and sophisticated real-time DSP. So the cost of 100 Gbps DP-QPSK transponders will probably not be too much less than ten times the cost of 10 Gbps, when they become available. Right now the standard is just a multi-source agreement to develop common components that each optical equipment vendor can use in their proprietary implementation. These components are just entering production now.
That does not mean that you can’t deploy 100 Gbps over a single DWDM wave now. Ciena has 100 Gbps line cards for the OME 6500 platform that have been deployed for more than a year. The former Nortel engineers who developed these had to use an additional trick to split the payload into 12.5 Gbps slices so readily available integrated circuit technology could be used to decode the data. In addition to splitting the signal in phase and polarization, they also split the optical carrier into two sub-carriers using frequency division multiplexing in the optical domain. Each sub-carrier carries half the data a la WDM, but the two carriers are only separated by 20 GHz so they can fit in a single 50 Ghz DWDM window.
Technology adapted from wireless to optical communication has allowed an order of magnitude growth in capacity of existing DWDM networks, without costly and disruptive upgrades to the installed plant. But this has taken us pretty close to the theoretical throughput limit under the Shannon–Hartley theorem given the typical parameters of existing large-scale networks. It is possible to get higher data rates with better OSNR, or with more bandwidth; but it’s doubtful that we will see a 400 Gbps transponder suitable for general deployment in existing 50 Ghz DWDM amplified networks originally engineered to carry only 10 Gbps.