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High notes

Arguably one of the biggest topics in optical communications in the past two years has been fifth-generation coherent optics, with a raft of trials and demonstrations, highlighting 800G wavelength achievements taking place. 

With greater requirements for bandwidth in general, thanks to video, cloud, data centre interconnect (DCI) and anticipation of full 5G implementation, not to mention the impact felt by the pandemic, network operators are looking to scale capacity cost-effectively while also minimising power consumption and footprint. This has led optical vendors to evolve their coherent optics technology with ever-higher baud rates. 

According to Paul Momtahan, director of solutions marketing at Infinera, higher baud rates are a good thing but, he asked, are they always great or is there also a use case for lower baud rates? Looking at the benefits, he said: ‘When you increase the baud rate for the same data rate you get this big increase in the reach. And you get a much better increase in the reach when you dial-up the baud rate versus dialling-up the modulation because, when you dial-up the modulation, firstly, you get a big reduction in the reach and then the other problem you have is, as you keep dialling it up you’re subject to diminishing returns.


The big advantage of baud rates, continued Momtahan, is you get a big increase in the data rate. ‘As a rule of thumb,’ he said, ‘if you double the baud rate, you double the data rate and you maybe take a 10 per cent hit in terms of the reach, so a little bit but not a big hit. Part of that is because, when you double the baud rate, you can double your sensitivity to noise nonlinearities that would harm your reach. But, you get most of that back because, when you increase the baud rate, you increase the amount of spectrum that wavelength ensues.’ 

This means, he continued, the power of that wavelength can be pumped up without getting lots of nonlinearities, because the power is spread out over more spectrum. ‘That gives you most of the reach back,’ he explains. ‘So, you take a very small hit in terms of the reach when you increase the baud rate compared to bigger hits when you increase the modulation. Again, you have diminishing returns, where going from 32 to 64 QAM only gives you an extra 20 per cent. But it halves the reach.’ 


But the downside for higher baud rates, explained Momtahan, is that it involves increasing the spectrum of the wavelengths in proportion to the baud rate. ‘Higher baud rates can’t help you with spectral efficiency,’ he said. ‘If anything, you get a slight dip in spectral efficiency, you’re getting a little bit of a reduced reach. High baud rates are more sensitive to nonlinearities at the receiving end.’ However, there is a solution to this in Nyquist subcarriers, which can help to increase the reach of higher-baud-rate wavelengths by reducing this sensitivity. 

Nyquist subcarriers can also help to reduce the blur or distortion caused by chromatic dispersion. ‘The Nyquist subcarrier essentially chops up the wavelength and makes it look like a bunch of smaller, lower baud rate subcarriers,’ Momtahan explained. ‘By doing this, the chromatic dispersion is proportional to the baud rate of that subcarrier. That’s one of the reasons we are Nyquist subcarriers in ICE6 and we’re starting to see those in high baud rate engines.’ 

Likewise, the use of probabilistic constellation shaping (PCS) can offer the option to ‘overclock’ the baud rate while dialing down the number of bits-per-symbol to maximise reach. ‘Previously, you had to match the modulation and the baud rate to get a specific data rate,’ explained Momtahan. ‘With PCS, you can pick your best baud rates, and then use PCS to get the exact number of bits-per-symbol to get the data rate, so it gives you that flexibility.’ 

In addition to this, Momtahan reasoned that there are some scenarios for which the highest baud rate is not necessarily the best option. ’You need to look at the amount of spectrum you have available, you need to look at the passband of the filters and the ROADM that you have. There are subsea scenarios where you want to get very long distances and you want to reduce the baud rate there. There are many examples where there’s a benefit from having a lower baud rate. And therefore, you also need tunability to address all the use cases.’ There is more detail on the use cases for lower baud rates available in Infinera’s latest white paper, The Ultimate Guide to Higher Baud Rates.

That said, Momtahan believes that baud rates will be key when it comes to increasing wavelength capacity reach. ‘Baud rates are probably the key trend moving forward,’ he said. ’But there are some disadvantages and there are also some tools like Nyquist subcarriers and probabilistic constellation shaping that can be used to really take advantage of the baud rates.'


Optical vendors are evolving coherent optics technology with ever-higher baud rates to help operators provide scalable, efficient networks. This white paper looks at the advantages, disadvantages and alternatives to higher bad rates in one handy ultimate guide.


Read the white paper


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