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Keely Portway looks at how probabilistic constellation shaping (PCS), could address the limitations associated with conventional coherent modulation, and why it is fast becoming an industry disruptor

It’s reasonable to say that the biggest disruptive influence this year has been the global pandemic that still currently has a grip on our lives. Cast your minds back to the start of the year, however, and the predictions – pre-Covid – were all pointing to high-capacity optics as a technology to watch.
 
In the run-up to this year’s OFC, organisers predicted that one of the major focus areas for attending-investors would be the race to 800G, with eyeballs particularly turned toward Ciena and Infinera’s developments in this area. This is in keeping with some of the key network trends for the sector, including 5G wireless, fibre-deep and high capacity long-haul and regional metro deployments.
 
The long game
The challenge with high-capacity optics has been their recognised limitation when it comes to longer-haul applications because they have relied upon higher order modulation, leaving the 200-400G wavelengths as the reserve of long-haul networks. The OFC organisers noted that ‘the ability to leverage 800G for longer distances could spark telecom customer interest for metro and long-haul applications.’
 
Despite some of the expected parties being unable to put in an appearance in San Diego this year, their technological developments in this area have continued at pace. Recently U.S. operator, Verizon completed a successful test in its live fibre network to move 800Gb/s of data on a single wavelength across longer distances.
 
Amongst the trial’s achievements, a 800Gb/s single-wavelength transmission was accomplished over 667km between Nashville and Atlanta; and a 600Gb/s single-wavelength transmission over 2,283km from Atlanta to Memphis with a loop back in Memphis. Using a combination of industry standard fibres, Verizon leveraged Infinera’s fifth-generation coherent ICE6 800G-generation optical engine, equipped in a GX Series platform.
 
ICE6 was selected thanks to a number of features that are useful for maximising spectral efficiency over longer distances. These include Nyquist subcarriers, a shared wavelocker, dynamic bandwidth allocation, SD-FEC gain sharing and long-codeword probabilistic constellation shaping (PCS).
 
What is PCS?
According to Paul Momtahan, director of solutions marketing at Infinera: ‘PCS, in a nutshell, is a modulation technique which enables you to very precisely control the number of bits-per-symbol by adjusting the probability of each constellation point.
 
Normally the constellation points all have the same probability, and what we’re doing is increasing the probability of the layer power in a constellation point and decreasing the probability of the outer constellation points that have a much higher power.’
 
This means, said Momtahan, that for the same amount of power and the same amount of information, there is a bigger gap between the constellation points, so they’re further apart. ‘That makes them easier to distinguish,’ he said, ‘so we can now go further with more capacity.’
 
Rob Shore, senior vice president, marketing at Infinera, added: ‘One thing to recognise is that when you look at constellation points on the map, the ones closest to the centre are the lowest power, and the lowest power ones are easier to transmit, they have less noise, less air, and so on. The more often you can transmit the inner constellations, the better the signal will perform.’
 
Under control
What makes PCS particularly interesting, explained Shore, is that you can’t usually control which constellations you transmit because the constellation point that is sent is very specifically related to the bit pattern that comes in. ‘What PCS does,’ he said, ‘is some weird translations of bit patterns so you can map bit patterns into the constellations you want. So you can then force the inner
constellations to be transmitted more frequently and it’s a really crazy maths formula that translated a bit pattern into another constellation than it otherwise would have been.’
 
Amongst the benefits offered by PCS, fine granularity is an option, which enables a smoother curve than traditional modulation. The enhanced granularity can significantly increase capacity and spectral efficiency if the reach requirement is beyond the capabilities of the higher-order modulation. ‘We can go in at almost Mb/s or Gb/s,’ explained Momtahan. ‘In reality, you don’t want a wavelength in individual gigabits so we do 25Gb/s increments but it gives a lot more granularity than you would get in conventional modulation.’
 
In addition, PCS offers greater tolerance to noise and/or fewer non-linearities, which are functions of the average power of the wavelength and its spectrum. This brings us closer to the Shannon limit. There is also the option to choose the optimal baud rate and then use PCS to get the desired wavelength speed, for greater baud rate flexibility.
 
It is also possible to enhance PCS using a long codeword, a higher quality transceiver, or adopting per-subcarrier PCS with dynamic bandwidth allocation, and the option of a super-Gaussian distribution for applications with high-power and non-linearities.
 

Find out more about the benefits of PCS, how it works, and the enhancements that can maximize its value.