• Over the past decade, the scaling of optical transport networks has been driven by successive generations of coherent optics technology. Much of this scaling has been predicated on leveraging the gains of Moore’s Law for silicon semiconductors – faster gate speeds and low power – into the electronic digital signal processors (DSPs) that power coherent optics.

The fifth and latest generation of coherent optics relies on 7 nm silicon CMOS node technology, which can enable scaling of coherent optics that are optimized for both metro-regional, and long-haul and subsea WDM network applications.

As they evaluate fifth-generation coherent optics, network operators will take a pragmatic approach that focuses on optimization for their specific network applications, performance on their actual fiber plant and ease of operationalization.

Operators that are undertaking greenfield deployments over new cables or fibers may benefit from greater latitude in optimizing their fiber links to eke out the best possible performance from these technologies. But many operators will be undertaking brownfield upgrades, and will need to insert fifth-generation optics into their existing networks with minimal operational disruption.

The optimization of coherent transport implementations across the breadth of network operators’ applications therefore needs to deliver more than “hero” performance in ideal conditions. It should focus on three key solution areas – application optimization, capacity–reach performance and minimally disruptive operationalization – all of which consider the real-world conditions in which they will operate.

Photonic Service Engines

It is into this context that Nokia introduced its fifth-generation PSE-V family of coherent optics solutions. The Photonic Service Engine (PSE) closely integrates the latest generation of DSPs and high-speed electro-optics into tightly optimized digital coherent optics (DCOs), as shown in Figure 1.

Figure 1: Nokia PSE – Coherent optical engines to power network scaling

Optimizing coherent solutions for the application

As a family of solutions, PSE-V coherent optics focus on the first aspect of coherent optimization: operators’ specific network applications. The PSE-V family includes Compact (PSE-Vc) and Super Coherent (PSE-Vs) options, which together optimize performance and operational benefits for a wide range of applications, from access and metro links all the way to the most challenging long-haul and subsea applications.

The PSE-Vc solutions are implemented as DCOs in a pluggable CFP2 form factor that operates from 33 to 67 Gbaud. They are optimized to provide low power, modularity and pluggability in a small form factor. Nevertheless, they provide high spectral efficiency by supporting 400 Gb/s transmission in 75 GHz channel spacings over hundreds of kilometers. Known as 400G Multihaul, this approach enables cost-effective network capacity upgrades for metro and regional network applications.

The PSE-Vs is designed to complement the PSE-Vc for challenging applications over very long spans. It operates at 90 Gbaud, utilizes the industry’s only-second generation probabilistic constellation shaping (PCS) algorithm and supports low-overhead, high-gain forward error correction (FEC). The PSE-Vs can be utilized as either an embedded or pluggable DCO to enable the highest capacities at the longest distances. These and other features leverage Nokia’s accumulated history of coherent innovation, with a pragmatic approach to solution optimization and the need to provide meaningful improvements in real-world long-haul, ultra-long-haul and subsea applications.

Narrowing the gap to the Shannon Limit

High-performance super coherent optics rely on three key technologies to maximize capacity–reach. The first is FEC, which utilizes powerful parity check algorithms to correct bit transmission errors and help maximize performance. The second is coherent modulation, which today supports high-order modulations such as 16QAM and 64QAM to transfer multiple data bits for each coherent symbol transmitted. Finally, PCS algorithms shape the coherent constellation to transmit symbols across the most efficient allocation of constellation points. All of these techniques work together in a complex interplay to maximize performance, and never has this more been the case than in the PSE-Vs.

Standard high-gain FEC techniques typically add 25 percent of overhead bits to a coherent signal. This means that what may nominally be an 800 Gb/s solution actually consists of 800 Gb/s of payload and 200 Gb/s of FEC overhead. The PSE-Vs uses fundamental Bell Labs research to implement FEC algoritms that have only 15 percent overhead while providing the same coding gain as standard 25 percent FEC. This enables the PSE-Vs to operate at 90 Gbaud signal rates without sacrificing performance. The results are lower power consumption and reduced noise and implementation penalties compared to coherent solutions that need to operate at a higher baud rate to achieve the same FEC gain.  

The lower FEC overhead, in turn, reduces the total number of bits that need to be coded by the PCS algorithm for a given payload size, which increases either capacity or reach (see Figure 2). If a specific payload size needs to be transmitted, for example, 400 Gigabit Ethernet (400GE), a lower FEC overhead means fewer total bits per symbol need to be transmitted. This enables stronger shaping by the PCS, which lowers the effective modulation order and provides better noise tolerance and performance. Put more simply, a given payload can be transmitted farther. Conversely, for a given fixed span length, the smaller FEC overhead means that a greater payload size, for example, 400GE + 100GE, can be transmitted over a given distance.

Figure 2:  Benefits of co-optimizing second-generation PCS with low-overhead FEC

This capacity–reach optimization is further enhanced by the unique ability of the second-generation PCS algorithm to finely optimize the baud rate of the coherent signal. By adjusting the coherent carrier to maximize spectral efficiency for every wavelength on a given link, operators can maximize total capacity on a fiber. Finally, the PSE-Vs implements a shaped-PCS 16QAM modulation to concentrate the coherent constellation points. This increases discrimination and signal-to-noise ratio compared to shaped 64-QAM modulations, which spread the coherent signal information much more broadly.

Together, these advances allow network operators to use PSE-Vs solutions to maximize capacity–reach performance upgrades for their real-world long-haul and subsea networks. For these real-world implementations, they must consider attributes such as propagation across very long links that have uneven or long spacings between optical line amplifiers, photonic pass-through across many intermediate reconfigurable optical add-drop multiplexers (ROADMs), and operation over a wide range of fiber types and losses. They must also accommodate system aging and repair margins.

In recent tests, PSE-Vs coherent linecards implemented in the Nokia 1830 PSI-M disaggregated compact modular system operated at 600 Gb/s per carrier over 1,800 km and across five ROADMs and 18 Raman-amplified fiber spans while multiplexed into 100 GHz WDM channels. The implementation delivered a spectral efficiency of 6.0 bits/sec/Hz over that distance.

Facilitating network upgrades

In many cases, network operators will seek to upgrade existing networks or spans to the latest generation of super coherent optics. This means adding new wavelengths onto existing line systems and ROADMs with existing WDM channels deployed using prior generations of coherent optics, and over wavelength channel plans optimized around 50 GHz or 100 GHz grids.

In this context, inserting new high-baud-rate super coherent optics that mandate a shift to new WDM channel plans, such as 112.5GHz or 125GHz, can require upgrades of existing ROADMs  necessitate complex re-tuning of existing coherent channels, and be operationally disruptive.

Designed with real-world applications in mind, the 90 Gbaud operation of the PSE-Vs delivers the full benefits of capacity–reach performance described above into a 100 GHz channel spacing that is fully compatible with existing ROADMs and n x 50 GHz channel plans.

Figure 3:  PSE-V super coherent maximizes capacity-reach for challenging long-haul and subsea applications

Taking light to the limit

The Nokia PSE-V family of fifth-generation coherent optics specifically aims to provide network operators with choice, performance and simplicity. With options optimized for both metro-regional and long-haul and subsea applications, it enables operators to choose the most efficient approach for scaling each part of their networks. For the most challenging applications, the PSE-V Super Coherent optics balance the need to maximize fiber capacity in real-world conditions over the longest fiber links, with capabilities specifically designed to enable seamless upgrades of existing networks with minimal operational disruption.

Learn more

Technology: PSE Super Coherent Technology
eBook: Beyond the limit: Coherent solutions for the next decade
Blog:  Nokia Innovation in single-carrier and multi-carrier coherent optics
White paper: The 400GE inflection point