• The 130 gigabaud (GBd) PSE-6s coherent modem is Nokia's first in-house design for high-end optical transport systems 
• The PSE-6s can send an 800 gigabit Ethernet (800GbE) payload over 2,000km and 1.2 terabits of data over 100km.
• Two PSE-6s DSPs can send three 800GbE signals over two 1.2-terabit wavelengths

Nokia has unveiled its latest coherent modem, the super coherent Photonic Service Engine 6s (PSE-6s) that will power its optical transport platforms in the coming years.

The PSE-6s comes three years after Nokia announced its current generation of coherent digital signal processors (DSPs): the PSE-Vs DSP for the long-haul and the compact PSE-Vc for the coherent pluggable market.

Nokia is only detailing the PSE-6s; its next-generation coherent modem for pluggables will be a future announcement.

Nokia will demonstrate the PSE-6s at the upcoming OFC show in March while field trials involving systems using the PSE-6s will start in the year's second half. 

Reducing cost per bit

In 2020, Nokia bought Elenion, a silicon photonics company specialising in coherent optics.

The PSE-6s is Nokia's first in-house coherent modem - the coherent DSP and associated optics - targeting the most demanding optical transport applications.

Nokia points out that coherent systems started approaching the Shannon limit two generations ago.

In the past, operators could reduce the cost of optical transport by sending more data down a fibre; upgrading the optical signal from 100 to 200 to 400 gigabit required only a 50GHz channel.

"You were getting more fibre capacity with each generation," says Serge Melle, director of product marketing, optical networks at Nokia. And this helped the continual reduction of the cost-per-bit metric.

But with more advanced DSPs, implemented using 16nm, 7nm, and now 5nm CMOS, going to a higher symbol rate and hence data rate requires more spectrum, says Melle.

Increasing the symbol rate is still beneficial. It allows more data to be sent using the same modulation scheme or transmitting the same data payload over longer distances.

"So one of the things we are looking to do with the PSE-6s is how do we still enable a lower total cost of ownership even though you don't get more capacity per wavelength or fibre," says Melle.

Nokia's various generations of PSE designs. The PSE-6s is a Class 3 coherent modem while at ECOC 2022, Nokia Bell Labs and partners showed a lab experiment sending 1.84Tbps at 185GBd and 800Gbps at 260GBd (Class 4).

Symbol rate classes

Coherent optics from the leading vendors use a symbol rate of 90-107 gigabaud (GBd), while Cisco-owned Acacia's latest 1.2-terabit coherent modem in a CIM-8 module operates at 140GBd.

Acacia uses a classification system based on symbol rate. First-generation coherent systems operating at 30-34GBd are deemed Class 1. Class 2 doubles the baud rates to 60-68GBd, the symbol rate window used for 400ZR coherent optics, for hyperscalers to connect equipment across their data centres up to 120km apart.

The DSPs from the leading optical transport systems vendors operating at 90-107GBd are an intermediate step between Class 2 and Class 3 using Acacia's classification. In contrast, Acacia has jumped directly from Class 2 to Class 3 with its 140GBd CIM-8 coherent modem.

Competitors view Acacia's classification scheme as a marketing exercise and counter that their 90-107GBd optical transport systems benefited customers for over two years.

Nokia's 90GBd PSE-Vs can send 400 gigabits using quadrature phase-shift keying (QPSK) over 3,000km. This contrasts with its earlier 67GBd PSE-3s that sends 400GbE up to 1,000km using 16-QAM.

However, with the PSE-Vs, Nokia, unlike its optical transport competitors, Infinera, Ciena and Huawei, decided not to support 800-gigabit wavelengths.

Nokia argued that 7nm CMOS, 90-100GBd coherent optics tops out at 600 gigabit when used for distances of several hundred kilometers, while metro-regional distances are more economically served using 400-gigabit pluggable optics such as the CFP2 implementing 400ZR+.

With the 130Gbd PSE-6s, Nokia has a Class 3 coherent modem with the PSE-6s capable of sending 800 gigabits more than 2,000km.

The PSE-6s also doubles the maximum data rate of the PSE-Vs to 1.2 terabits per wavelength. However, at 1.2 terabits, the reach is 100-plus km, valuable for very high capacity metro transport and data centre interconnect.

Scale, reach and power consumption per bit

Nokia highlights the PSE-6s' main three performance metric improvements.

First, the coherent modem delivers scaling: two coherent optical engines fit on a line card to deliver 2.4 terabits to transport emerging high-speed services such as 800GbE.

The two PSE-6s are linked using a dedicated interface to share the client-side signals (see diagram).

Source: Nokia

"We are not the only ones introducing a 5nm solution, but I think we are the only ones that allow two DSPs to work together," says Melle.

Without the interface, a single 800GbE and up to four 100GbE clients or a 400GbE client can be sent over each DSP's 1.2-terabit wavelength. Adding the interface, an operator can send three uniform 800GbE clients, with the interface splitting the third 800GbE client between the two DSPs.

"In a single line card, you can stripe the three 800-gigabit services rather than have to deploy three separate line cards in the network," says Melle.

Nokia is not detailing the interface used to link the DSPs but said that the interface is used for data only and not to share signal processing resources between the ASICs.

"There is an extra amount of circuitry to share the client bandwidth across the two DSPs, but it is not high power consuming, and most transponders have some circuitry between the clients and the DSP," says Melle. "So the incremental 'power tax' is marginal; it doesn't add any significant power overhead."

The resulting 2.4-terabit transmission is sent as two 1.2-terabit wavelengths, each occupying a 150GHz-wide channel. Existing systems that operate at 90-107GBd typically use a 112.5GHz channel for an 800-gigabit transmission, so the PSE-6s delivers a fibre capacity benefit.

The two wavelengths can be bonded, as in a two-channel 'super-channel', or sent to separate locations.

The second improvement is optical performance. For example, an 800-gigabit payload can travel over 2,000km. Nokia claims this is 3x the reach of existing commercial optical transport systems.

The improved transmission performance is achieved using a combination of the 130GBd baud rate, probabilistic constellation shaping (PCS), and improved forward error correction (FEC). Melle says the contributions to the improvement are 90 per cent baud rate and 10 per cent due to coherent modem algorithm tweaks.

"Baud rate is king; that is what really drives this improved performance," says Melle.

Source: Nokia

The third benefit is reduced power consumption at the device and system (networking) levels.

Using a 5nm finFET CMOS process to make the PSE-6s DSP ASIC and developing denser line cards (two modems per card) means systems will consume 60 per cent less power than Nokia's existing coherent technology.

According to Nokia, the PSE-6s optical engine consumes 40 per cent fewer Watts per bit compared to the PSE-Vs.

Nokia 1830 transport systems

The PSE-6s line cards fit into Nokia's existing range of 1830 transport platforms.

Source: Nokia