Nokia targets 400G era with PSE-V coherent DSP launch

Nokia has unveiled its latest coherent digital signal processor (DSP) family, its fifth-generation Photonic Service Engine dubbed the PSE-V. 

Kyle HollaschTwo devices make up the family: the high-end super coherent PSE-Vs and the compact PSE-Vc for use in pluggable modules.

The PSE-Vc chip is already sampling, the PSE-Vs will sample later this year. 

The PSE-Vs, operating at a 90 gigabaud (GBd) symbol rate, supports transmission distances from local data centres to ultra-long-haul and sub-sea networks while the 64GBd PSE-Vc implements the OIF’s 400ZR standard, ZR+ and beyond.

Nokia has also expanded its coherent optics strategy having completed the acquisition of Elenion Technologies. It is now vertically integrated and is offering coherent optics and its DSPs to partners that include module makers and contract manufacturers.

The 400G era 

The last decade has been primarily based on 100 gigabit, with 100-gigabit client signals appearing around 2010 and coherent optics starting at 100 gigabit. “We like to forget about 40 gigabit,” quips Kyle Hollasch, director of optical networking product marketing at Nokia. 

This decade marks the start of the 400-gigabit era. “Just about this year and, in earnest, next year, we will see 400 gigabit adopted as the dominant data centre client-rate, timed along with 400-gigabit coherent,” says Hollasch.        

During the last decade, traffic has continued to grow exponentially while cost-per-bit has declined, first at 30 per cent per annum and more recently at 15 per cent. Overall, the optical market spend has been largely flat over the last decade (see diagram).  

 Source: Nokia

Meanwhile, the technologies underpinning coherent and optical transport are becoming more challenging.

Optical performance using coherent has improved significantly with advances in baud rate, modulation schemes and forward-error correction (FEC). So much so that optical performance is approaching the theoretical Shannon limit. “We don’t know where it is but we know we are close,” says Hollasch. 

Moore’s law is also slowing down. The gains in processing performance and power reduction are harder to achieve with each advance in CMOS process geometry. The latest PSE-V devices are implemented using 7nm CMOS. 

“The question we are trying to answer is that, with this new era and certain technology trends slowing down, how are we going to keep scale and cost-containment going,” says Hollasch.  

Coherent strategy

Nokia highlights three elements to address scale and reduce cost.

In addition to its existing coherent DSP expertise, Nokia has added an optical engine design capability, and integration and packaging know-how.

The CSTAR optical engine, gained with the Elenion acquisition, includes silicon-photonics-based optics and transmitter and receiver ICs that are parcelled in a BGA chip package   

“The existing CSTAR portfolio is already shipping to several customers and there is a lot of development and opportunities going forward,” says Hollasch. 

The PSE-Vc and the optical engine are being integrated into various form factors to deliver different price points and optical performances. “And the market is rapidly shifting towards pluggable [coherent] transceivers,” says Hollasch. 

Being vertically integrated brings cost-advantages, says Hollasch, as well as an ability to innovate faster by ‘owning’ the entire development and manufacturing process. 

The approach has been adopted by other optical transport companies. Infinera has always developed its DSPs and indium-phosphide optics; several years ago Ciena brought indium-phosphide and silicon photonics expertise in-house to complement its DSPs, Huawei also has both DSP and optics expertise, while Cisco Systems gains both with its Acacia acquisition. 

Nokia’s partners will be its customers, using its components and taking the resulting products to market. In turn, Nokia will also be a customer of modules and subsystems.

“Elenion was a corporate acquisition,” says Hollasch. “It lives under the optical unit but it was acquired for the benefit of the entire corporation.” After the optical unit, Nokia’s IP division will be the next largest consumer of transceivers, particularly at 400 gigabit, while mobile and access “are at the cusp of coherent adoption”, he says.   

Hollasch does not rule out its modules being sold via partners and resellers: “It is still early in the process but it is absolutely part of the roadmap.”    

Source: Gazettabyte, Nokia 

PSE-Vs

Upping the symbol rate of the PSE-Vs to 90GBd delivers a significant performance benefit.

In the current-generation PSE-3, 400-gigabit transmission at 64GBd requires 16-ary quadrature amplitude modulation (16-QAM) whereas operating at 90GBd, quadrature phase-shift keying (QPSK) can be used.

“Instead of a regional distance of 1,000km [for 400-gigabit 16-QAM], you can send QPSK almost anywhere on the planet,” he says (see table).

The PSE-Vs is Nokia’s second-generation DSP  to use probabilistic constellation shaping (PCS). The technique allow the transmission data rate to be defined with fine granularity, tailored for a specific link. Nokia was the first vendor to implement PCS with its PSE-3.

The 90GBd baud rate is used for all the data rates, from 200-600 gigabits-per-second (Gbps) using PCS. A reach of 1,000km is possible at 600Gbps whereas 200Gbps is used to address the longest sub-sea spans. 

Hollasch highlights what he believes is a key benefit of Nokia’s technology: the joint optimisation of PCS and its proprietary FEC scheme. 

Hollasch says its FEC reduces the extra bits that are appended to the data - the overhead payload - from 25 per cent to 16 per cent: “We believe that gives a performance enhancement not available to the rest of the market.”

Nokia says silicon photonics and indium phosphide will be used for PSE-Vs-based coherent designs. 

“We believe that we can achieve similar performance with both technologies,” says Hollasch. “The reason for using both is for diversification of supply; the COVID-19 event has shown that we need diversification in all areas of our supply chain.”

PSE-Vc 

The PSE-Vc will be used to implement 400ZR and ZR+ in a QSFP-DD pluggable module.

Nokia refers to the two extremes of coherent, as exemplified by the ‘super coherent’ and ‘compact’ names for its PSE-V chips, as high-end performance and profile, by which it means form factor. 

Whereas performance is all about maximum reach and reducing cost/bit/km, profile is concerned with cost, interoperability and being pluggable.

But Hollasch stresses there is an important middle region where the goal of the interfaces - partly the ZR+ and the rest the CFP2-DCO - is to balance these two extremes. 

“The middle does not get the attention; all the attention goes to the high-end,” says Hollasch. “But the vast majority of optical networking is happening in the middle.”

Nokia is implementing 400ZR and ZR+ using the QSFP-DD form factor. ZR+ extends the performance of 400ZR by supporting traffic protocols other than Ethernet as well as data rates of 400 gigabits and below. Such rates - 100 to 300 gigabits - extend reach beyond 400ZR’s 120km.  

Nokia is not adopting the OSFP form factor for coherent. “We don’t see a huge demand for the OSFP,” says Hollasch.  

The CFP2-DCO using the PSE-Vc is what Hollasch refers to as the ‘ZR++’. The greater volume and power envelope of the CFP2 module means its optical performance can exceed greatly that of 400ZR and ZR+. “The CFP2 will be a large part of our optical platforms.” 

The CFP2 and the QSFP-DD modules will be available towards year-end.       

No 800 gigabit    

Unlike its rivals Ciena, Huawei and Infinera, Nokia has chosen not to support 800-gigabit wavelengths with its latest high-end DSP. 

The company argues that at data rates greater than 600 gigabit, the reach is limited. “It is not that 800 gigabit isn't important, it is just that it lives in a world of many options,” says Hollasch. 

The 800-gigabit rate is not economical to pursue with this generation, he says: “For data centre interconnect, it is much more economical to pursue shorter distances with 400ZR and ZR+.”  

He argues that 400-gigabit coherent pluggables will be hard to beat in terms of power consumption and cost, given the demand from the webscale players and the many companies making them. Equally, since 400ZR is a standard, modules are interoperable.         

Nokia also highlights the engineering issues involved in implementing rates above 600 gigabit. 

At 600 gigabit, the PSE-Vc uses 16-QAM whereas 800 gigabit requires 64-QAM. Processing four times as many constellation points loads the DSP for what is a limited use case. “It puts lots of strain on the DSP; we know, we did 64-QAM in both the PSE-2 and PSE-3,” he says.

Confining an 800-gigabit wavelength to a 100GHz channel limits its reach to 100-200km, says Hollasch. Extended the channel to 125GHz, the reach of an 800-gigabit wavelength is extended. But two 400-gigabit pluggables, each using a 75GHz channel, can also be used to transmit the same capacity over long reaches. 

In other words, a 125GHz channel is needed ‘to get anywhere’ while saving very little in terms of spectral efficiency, concludes Hollasch.