capacity Archives - EFFECT Photonics

29 - Reducing the Cost per Bit with Coherent Technology

Insights

July 2, 2024

Reducing the Cost per Bit with Coherent Technology

The cost per bit is a metric directly impacting network operators’ economic viability and competitive…

The cost per bit is a metric directly impacting network operators’ economic viability and competitive positioning. It represents the expense of transmitting a single bit of information across a network, encompassing infrastructure, operations, and maintenance costs. Lower costs per bit enable providers to offer more data-intensive services at competitive prices, attracting more customers and increasing revenue. Additionally, optimizing this metric helps achieve higher efficiency and sustainability in network operations.

Different segments of a telecommunication network—core, metro, and access—prioritize the cost per bit differently due to their distinct roles and technical requirements. The core network, which connects major cities and data centers, handles high data volumes over long distances. Economies of scale play a significant role here, as reducing the cost per bit is crucial for maintaining profitability on the vast amount of data transmitted. This segment typically invests in high-capacity, longer-haul technologies that, while expensive, reduce the cost per bit through enhanced efficiency and higher data throughput.

Conversely, edge networks face different challenges and priorities. They must prioritize flexibility and adaptability to handle varying traffic loads efficiently. Reducing the cost per bit involves deploying technologies that can scale quickly and cost-effectively. The access segment of edge networks, which brings connectivity directly to end-users, focuses on maximizing coverage and reliability. Here, the cost per bit needs to be managed against the need for extensive physical infrastructure that reaches individual customers.

Coherent technology is often seen as more expensive than direct detection (IM-DD), but in this article, we will explore some ways in which the initial investment in coherent technology can help networks reduce their cost per bit.

Coherent Increases Transmission Reach

The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The fiber also adds other distortions due to nonlinear optical effects.

These effects get worse as the input power of the light signal increases, leading to a trade-off. You might want more power to transmit over longer distances, but the nonlinear distortions also become larger, which beats the point of using more power.

dispersion in optical fibers — Figure 1: Explaining dispersion compensation in optical fibers. A pulse of light will spread out and distort due to dispersion effects.

Coherent systems use sophisticated digital signal processing (DSP) technologies to automatically compensate for signal impairments, including chromatic and polarization mode dispersion. Coherent receivers are also highly sensitive, allowing them to detect signals over longer distances with higher fidelity than is possible with IM-DD systems. The dispersion compensation and increased sensitivity reduces the number of regenerative repeaters and other physical modules needed to boost the signal over longer distances.

Fewer repeaters mean lower energy consumption, reduced maintenance, and fewer operational disruptions, all contributing to a lower cost per bit. Additionally, the ability to transmit over extended distances without degradation in signal quality allows for more straightforward network architectures with longer point-to-point connections, simplifying the overall network design and further reducing costs

Coherent Increases Transmission Efficiency

Coherent systems use complex modulation formats that encode data on a light wave’s amplitude, phase, and polarization. By encoding multiple bits per symbol, these systems can transmit more data over a single wavelength than IM-DD, which primarily uses amplitude-only modulation, rather than just the intensity. This allows more bits to be transmitted per symbol, effectively increasing the data-carrying capacity of a single fiber.

Such efficiency can allow a single coherent channel to replace the work of several IM-DD channels. In our previous article, we provided an example of a single 100Gbps coherent channel replacing the link aggregation of four 10Gbps IM-DD channels found in some access and aggregation network architectures.

Figure 4x10G link aggregation — Figure 2: Comparison between 4x10G link aggregation and using a single coherent 100ZR link.

This substitution would replace eight SFP+ transceivers with just two coherent 100G transceivers, simplifying network configuration and operation. It more than doubles the capacity of 4x10Gbps link aggregation, allowing the network to handle more data traffic while reducing the required physical infrastructure, effectively reducing the cost per bit.

You can consult the recent Cignal AI report on 100ZR technologies to gain further insight into this link aggregation upgrade’s potential market and reach.

The Synergy with WDM Technology

Dense Wavelength Division Multiplexing (DWDM) is an optical technology that dramatically increases the amount of data transmitted over existing fiber networks. Data from various signals are separated, encoded on different wavelengths, and put together (multiplexed) in a single optical fiber.

The wavelengths are separated again and reconverted into the original digital signals at the receiving end. In other words, DWDM allows different data streams to be sent simultaneously over a single optical fiber without requiring the expensive installation of new fiber cables. In a way, it’s like adding more lanes to the information highway without building new roads!

Optical fiber with DWDM Channels — Figure 3: Example of a bidirectional DWDM system. At the central office, transceiver modules pick different wavelength channels for transmission, and these channels are combined into a single fiber through a device called a multiplexer. A demultiplexer splits the wavelength channels into separate fibers and the receiving modules on the remote end. Since this is a bidirectional system, transmission can go opposite too, from the remote end to the central office.

The tremendous expansion in data volume afforded with DWDM can be seen compared to other optical methods. A standard transceiver, often called a grey transceiver, is a single-channel device – each fiber has a single laser source. You can transmit 10 Gbps with grey optics. Coarse Wavelength Division Multiplexing (CWDM) has multiple channels, although far fewer than possible with DWDM. For example, with a 4-channel CWDM, you can transmit 40 Gbps. DWDM can accommodate up to 100 channels. You can transmit 1 Tbps or one trillion bps at that capacity – 100 times more data than grey optics and 25 times more than CWDM.

While the upgrade to DWDM requires some initial investment in new and more tunable transceivers, the use of this technology ultimately reduces the cost per bit transmitted to the network. Demand in access networks will continue to grow as we move toward IoT and 5G, and DWDM will be vital to scaling cost-effectively. Self-tuning modules have also helped further reduce the expenses associated with tunable transceivers.

Takeaways

Coherent systems minimize the need for frequent signal regeneration by compensating signal dispersion and enhancing signal reach. This simplifies network architecture and contributes to a lower cost per bit. Additionally, by employing complex modulation techniques, coherent technology maximizes the data capacity per wavelength, potentially replacing multiple IM-DD systems with a single coherent channel. This can further streamline network operations and reduce expenses.

The synergy of coherent technology with Dense Wavelength Division Multiplexing (DWDM) can multiply the data throughput of existing fiber infrastructures without requiring new infrastructure installations. Overall, while coherent technology involves a higher upfront investment compared to IM-DD systems, it can lower the cost per bit by enhancing the efficiency, reach, and capacity of data transmission.

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Tags: capacity, channels, Coherent technology, cost per bit, data, dispersion, DWDM, EFFECT Photonics, efficiency, fiber, IM-DD, infrastructure, modulation, multiplexing, network, operations, reach, signal, systems, technology, transmission

Insights

March 26, 2024

Transceiver Customization for Flexible Access Networks

Let’s be honest: not every optical network problem can be solved by scaling up capacity.…

Let’s be honest: not every optical network problem can be solved by scaling up capacity. It’s not always cost-effective, and it’s not always sustainable. Providers and operators who want to become market leaders must scale up while also learning to allocate their existing network resources most efficiently and dynamically. For example, they must monitor their network performance frequently, providing more energy and capacity to high-traffic links while reducing power and capacity in areas with little traffic. They must find hardware that fits the rest of their network devices to a tee instead of equipment that is over- or under-specified.

Operators usually change settings in the higher network layers to adjust their networks dynamically. For example, artificial intelligence in network layer management will become a major factor in reducing the energy consumption of telecom networks. However, as networks get increasingly complex, operators need more degrees of freedom and knobs to adjust. They must customize the network and physical layers to fit the network best.

Fortunately, the new generation of pluggable transceivers gives operators more customization options than ever to change physical layer settings and adapt to these changing and growing network requirements and use cases. In this article, we will provide examples from our own pluggable transceivers and NarroWave technology.

Customization for Remote Diagnostics and Management

NarroWave sets up a separate low-frequency communication channel between two modules. This channel allows the headend module to remotely modify certain aspects of the tail-end module, effectively enabling several remote management and diagnostics options.

For example, the operator can remotely measure metrics such as the transceiver temperature and power transmitted and received. These metrics can provide a quick and useful health check of the link. The headend module can also remotely read alarms for low/high values of these metrics.

Even after buying the module, our customers can customize several variables, such as the low/high transmitter power levels, NarroWave self-tuning variables, memory behavior when turning on/off, or also the temperature and power flags and alarms. Customers also have the freedom to rewrite administrative information such as the vendor name, organization unique identifier (OUI), serial and part numbers, and passwords to access certain memory registers. These features are useful for system integrators and OEM sellers.

Customization for Easier Installation

Some of our transceiver features are customizations that make installation tasks easier. For example, setting up a network sometimes requires fibers to be repatched, which creates a loss of signal (LOS). Our NarroWave procedures include a customizable Ignore LOS (in seconds) flag that holds up the self-tuning scan and allows operators to perform maintenance duties without causing error messages at the host equipment.

These remote diagnostics and management features can eliminate certain truck rolls and save more operational expenses. They are especially convenient when dealing with very remote and hard-to-reach sites (e.g., an underground installation) that require expensive truck rolls.

Customization for Energy Sustainability

To talk about the impact of transceiver customization on energy sustainability, we first must review the concept of performance margins. This number is a vital measure of received signal quality and determines how much room there is for the signal to degrade without impacting the error-free operation of the optical link.

In the past, network designers have played it safe, maintaining large margins to ensure a robust network operation in different conditions. However, these higher margins usually require higher transmitter power and power consumption. Network management software can develop tighter, more accurate optical link budgets in real time that require lower residual margins through the remote diagnostics provided by this new generation of pluggable transceivers. This could lower the required transceiver powers and save valuable energy.

Another related sustainability feature is deciding whether to operate on low- or high-power mode depending on the optical link budget and fiber length. For example, if the transceiver needs to operate at its maximum 10G speed, it will likely need a higher performance margin and output power. However, if the operator uses the transceiver for just a 1G link, the transceiver can operate with a smaller residual margin and use a lower power setting. The transceiver uses energy more efficiently and sustainably by adapting to these circumstances.

Takeaways

Thanks to the advances in photonics and electronic integration, the new generation of pluggable transceivers has packaged many knobs and variables that previously required additional, specialized hardware that increased network complexity and costs. These advances give designers and operators more degrees of freedom than ever.

There are customizations that enable simpler remote management and diagnostics or easier installations. There are also customizable power settings that help the transceiver operate more sustainably. These benefits make access networks simpler, more affordable, and more sustainable to build and operate.

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Tags: access networks, alarms, capacity, channel, cost-effective, customization options, customize, dynamically, EFFECT Photonics, efficiently, electronic integration, energy sustainability, fiber length, hardware, health check, high-traffic links, installation tasks, low-traffic areas, management, metrics, NarroWave technology, network devices, network management software, network resources, operational expenses, operators, optical network, performance margins, Photonics, Pluggable Transceivers, power, power consumption, providers, remote diagnostics, remote sites, Scaling up, Signal degradation, Sustainable

Coherent technology can give free-space optical systems greater reach and capacity, enabling these new applications in space and terrestrial links.

Insights

January 12, 2024

Coherent Free Space Optics for Ground and Space Applications

In a previous article, we described how free-space optics (FSO) could impact mobile fronthaul and…

In a previous article, we described how free-space optics (FSO) could impact mobile fronthaul and enterprise links. They can deliver a wireless access solution that can be deployed quickly, with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links.

*Figure 1: Free space optical system diagram*

However, there’s potential to do even more. There are network applications on the ground that require very high bandwidths in the range of 100 Gbps and space applications that need powerful transceivers to deliver messages across vast distances. Microwaves are struggling to deliver all the requirements for these use cases.

By merging the coherent technology in fiber optical communications with FSO systems, they can achieve greater reach and capacity than before, enabling these new applications in space and terrestrial links.

Reaching 100G on the Ground for Access Networks

Thanks to advances in adaptive optics, fast steering mirrors, and digital signal processors, FSO links can now handle Gbps-capacity links over several kilometers. For example, a collaboration between Dutch FSO startup Aircision and research organization TNO demonstrated in 2021 that their FSO systems could reliably transmit 10 Gbps over 2.5 km.

However, new communication technologies emerge daily, and our digital society keeps evolving and demanding more data. This need for progress has motivated more research and development into increasing the capacity of FSO links to 100 Gbps, providing a new short-reach solution for access networks.

One such initiative came from the collaboration of Norweigan optical network solutions provider Smartoptics, Swedish research institute RISE Acreo, and optical wireless link provider Norwegian Polewall. In a trial set-up at Acreo’s research facilities, Smartoptics’ 100G transponder was used with CFP transceivers to create a 100 Gbps DWDM signal transmitted through the air using Polewall’s optical wireless technology. Their system is estimated to reach 250 meters in the worst possible weather conditions.

Fredrik Larsson, the Optical Transmission Specialist at Smartoptics, explains the importance of this trial:

“Smartoptics is generally recognized as offering a very flexible platform for optical networking, with applications for all types of scenarios. 100Gbps connectivity through the air has not been demonstrated before this trial, at least not with commercially available products. We are proud to be part of that milestone together with Acreo and Polewall,”

Meanwhile, Aircision aims to develop a 100 Gbps coherent FSO system capable of transmitting up to 10km. To achieve this, they have partnered up with EFFECT Photonics, who will take charge of developing coherent modules that can go into Aircision’s future 100G system.

In many ways, the basic technologies to build these coherent FSO systems have been available for some time. However, they included high-power 100G lasers and transceivers originally intended for premium long-reach applications. The high price, footprint, and power consumption of these devices prevented the development of more affordable and lighter FSO systems for the larger access network market.

However, the advances in integration and miniaturization of coherent technology have opened up new possibilities for FSO links. For example, 100ZR transceiver standards enable a new generation of low-cost, low-power coherent pluggables that can be easily integrated into FSO systems. Meanwhile, companies like Aircision are working hard in using technologies such as adaptive optics and fast-steering mirrors to extend the reach of these 100G FSO systems into the kilometer range.

Coherent Optical Technology in Space

Currently, most space missions use radio frequency communications to send data to and from spacecraft. While radio waves have a proven track record of success in space missions, generating and collecting more mission data requires enhanced communications capabilities.

Coherent optical communications can increase link capacities to spacecraft and satellites by 10 to 100 times that of radio frequency systems. Additionally, optical transceivers can lower the size, weight, and power (SWAP) specifications of satellite communication systems. Less weight and size means a less expensive launch or perhaps room for more scientific instruments. Less power consumption means less drain on the spacecraft’s energy sources.

Space military satellite, a weapon in space shoots a laser against the background of the earth. Attack, technology, space war. mixed medium, copy space. image furnished by NASA — Figure 2: Optical data transmission from a space satellite to earth.

For example, the Laser Communications Relay Demonstration (LCRD) from NASA, launched in December 2021, aims to showcase the unique capabilities of optical communications. Future missions in space will send data to the LCRD, which then relays the data down to ground stations on Earth. The LCRD will forward this data at rates of 1.2 Gigabits per second over optical links, allowing more high-resolution experiment data to be transmitted back to Earth. LCRD is a technology demonstration expected to pave the way for more widespread use of optical communications in space.

Making Coherent Technology Live in Space

Integrated photonics can boost space communications by lowering the payload. Still, it must overcome the obstacles of a harsh space environment, with radiation hardness, an extreme operational temperature range, and vacuum conditions.

The James Webb Telescope. Space Observatory for the Study of the Universe and exploration of deep space. Elements of this image furnished by NASA — *Figure 3: The James Webb Telescope. Space Observatory for the Study of the Universe and exploration of deep space*

For example, the Laser Communications Relay Demonstration (LCRD) from NASA, launched in December 2021, aims to showcase the unique capabilities of optical communications. Future missions in space will send data to the LCRD, which then relays the data down to ground stations on Earth. The LCRD will forward this data at rates of 1.2 Gigabits per second over optical links, allowing more high-resolution experiment data to be transmitted back to Earth. LCRD is a technology demonstration expected to pave the way for more widespread use of optical communications in space.

*Mission Type*	*Conditions*
Pressurized Module	+18.3 ºC to 26.7 °C
Low-Earth Orbit (LEO)	-65 ºC to +125°C
Geosynchronous Equatorial Orbit (GEO)	-196 ºC to +128 °C
Trans-Atmospheric Vehicle	-200 ºC to +260 ºC
Lunar Surface	-171 ºC to +111 ºC
Martian Surface	-143 ºC to +27 ºC

Table 1. Temperature Extremes by Mission Type. Source: NASA

The values in Table 1 are unmanaged environmental temperatures and would decrease significantly for electronics and optics systems in a temperature-managed area, perhaps by as much as half.

A substantial body of knowledge exists to make integrated photonics compatible with space environments. After all, photonic integrated circuits (PICs) use similar materials to their electronic counterparts, which have already been space qualified in many implementations.

Much research has gone into overcoming the challenges of packaging PICs with electronics and optical fibers for these space environments, which must include hermetic seals and avoid epoxies. Commercial solutions, such as those offered by PHIX Photonics Assembly, Technobis IPS, and the PIXAPP Photonic Packaging Pilot Line, are now available.

Takeaways

Whenever you want to send data from point A to B, photonics is usually the most efficient way of doing it, be it over a fiber or free space.

This is why EFFECT Photonics sees future opportunities in the free-space optical (FSO) communications sectors. In mobile access networks or satellite link applications, FSO can provide solutions with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links.

These FSO systems can be further boosted by using coherent optical transmission similar to the one used in fiber optics. Offering these systems in a small package that can resist the required environmental conditions will significantly benefit the access network and space sectors.

Tags: 100G, access capacity, access network, capacity, certification, coherent, free space optics, FSO, GEO, ground, LEO, lunar, Photonics, reach, satellite, space, SWAP, temperature, Transceivers

Insights

November 20, 2023

Remote Provisioning and Management for Edge Networks

Smaller data centers near the end user can reduce latency, overcome inconsistencies in connectivity, and…

Smaller data centers near the end user can reduce latency, overcome inconsistencies in connectivity, and store and compute data closer to the end user. According to PricewaterhouseCoopers, these advantages will drive the worldwide market for edge data centers to more than triple from $4 billion in 2017 to $13.5 billion in 2024. With the increased use of edge computing, more high-speed transceivers are required to link edge data centers. According to Cignal AI, the number of 100G equivalent ports sold for edge applications will double between 2022 and 2025, as indicated in the graph below.

Figure 1: Forecast of 100G port equivalents shipped for edge applications. These shipments are overwhelmingly 400ZR standard technology. Source: Cignal AI Transport Applications Report Q42021.

The increase in edge infrastructure comes with many network provisioning and management challenges. While typical data centers were built in centralized and controlled environments, edge deployments will live in remote and uncontrolled environments because they need to be close to where the data is generated. For example, edge infrastructure could be a server plugged into the middle of a busy factory floor to collect data more quickly from their equipment sensors.

This increase in edge infrastructure will provide plenty of headaches to network operators who also need to scale up their networks to handle the increased bandwidths and numbers of connections. More truck rolls must be done to update more equipment, and this option won’t scale cost-effectively, which is why many companies simply prefer not to upgrade and modernize their infrastructure.

Towards Zero-Touch Provisioning

A zero-touch provisioning model would represent a major shift in an operator’s ability to upgrade their network equipment. The network administrator could automate the configuration and provisioning of each unit from their central office, ship the units to each remote site, and the personnel in that site (who don’t need any technical experience!) just need to power up the unit. After turning them on, they could be further provisioned, managed, and monitored by experts anywhere in the world.

The optical transceivers potentially connected to some of these edge nodes already have the tools to be part of such a zero-touch provisioning paradigm. Many transceivers have a plug-and-play operation that does not require an expert on the remote site. For example, the central office can already program and determine specific parameters of the optical link, such as temperature, wavelength drift, dispersion, or signal-to-noise ratio, or even what specific wavelength to use. The latter wavelength self-tuning application is shown in Figure 2.

Once plugged in, the transceiver will set the operational parameters as programmed and communicate with the central office for confirmation. These provisioning options make deployment much easier for network operators.

Figure 2: Short description and comparison of different approaches to wavelength tuning in optical transceivers, with the last one on the right being the self-tuning module.

Enabling Remote Diagnostics and Management

The same channel that establishes parameters remotely during the provisioning phase can also perform monitoring and diagnostics afterward. The headend module in the central office could remotely modify certain aspects of the tail-end module in the remote site, effectively enabling several remote management and diagnostics options. The figure below provides a visualization of such a scenario.

Figure 3: Visualization of remote diagnostics in an optical link. The headend module can check read tail-end module diagnostics such as wavelength channel number, temperature, transmitter, and receiver power.

The central office can remotely measure metrics such as the transceiver temperature and power transmitted and received. These metrics can provide a quick and useful health check of the link. The headend module can also remotely read alarms for low/high values of these metrics.

These remote diagnostics and management features can eliminate certain truck rolls and save more operational expenses. They are especially convenient when dealing with very remote and hard-to-reach sites (e.g., an antenna tower) that require expensive truck rolls.

Remote Diagnostics and Control for Energy Sustainability

To talk about the impact of remote control on energy sustainability, we first must review the concept of performance margins. This number is a vital measure of received signal quality. It determines how much room there is for the signal to degrade without impacting the error-free operation of the optical link.

In the past, network designers played it safe, maintaining large margins to ensure a robust network operation in different conditions. However, these higher margins usually require higher transmitter power and power consumption. Network management software can use the remote diagnostics provided by this new generation of transceivers to develop tighter, more accurate optical link budgets in real time that require lower residual margins. This could lower the required transceiver powers and save valuable energy.

Figure 4: Simplified example of signal degradation in a fiber link between two modules. Sources of loss include fiber connectors, splices, and intrinsic fiber attenuation. The performance margin is the difference between the expected Rx power and the receiver sensitivity. Higher-speed links (green) need a high Tx power and performance margin. Lower-speed links (red) can use a lower Tx power and performance margin.

Another related sustainability feature is deciding whether to operate on low- or high-power mode depending on the optical link budget and fiber length. For example, if the transceiver needs to operate at its maximum capacity, a programmable interface can be controlled remotely to set the amplifiers at their maximum power. However, if the operator uses the transceiver for just half of the maximum capacity, the transceiver can operate with a smaller residual margin and use lower power on the amplifier. The transceiver uses energy more efficiently and sustainably by adapting to these circumstances.

If the industry wants interoperability between different transceiver vendors, these kinds of power parameters for remote management and control should also be standardized.

Takeaways

As edge networks get bigger and more complex, network operators and designers need more knobs and degrees of freedom to optimize network architecture and performance and thus scale networks cost-effectively.

The new generation of transceivers has the tools for remote provisioning, management, and control, which gives optical networks more degrees of freedom for optimization and reduces the need for expensive truck rolls. These benefits make edge networks simpler, more affordable, and more sustainable to build and operate.

Tags: access network, capacity, cost, distributed access networks, DWDM, inventory stock, Loss of signal, maintenance, monitor, optical networks, plug-and-play, remote, remote control, scale, scaling, self-tuning, time

Insights

August 31, 2023

The Future of 5G Fronthaul

The 5G network revolution promises to fulfill the capacity needs that previous cellular generations could…

The 5G network revolution promises to fulfill the capacity needs that previous cellular generations could no longer provide to the ever-increasing customer demands. This network generation is expected to revolutionize the concept of telecommunication and bring the most anticipated services of larger bandwidth, higher speed, and reduced latency as part of the modern cellular network. The upgrade from 4G to 5G has shifted the radio access network (RAN) from a two-level structure with backhaul and fronthaul in 4G to a three-level structure with back-, mid-, and fronthaul:

Fronthaul is the segment between the active antenna unit (AAU) and the distributed unit (DU)
Midhaul is the segment from DU to the centralized unit (CU)
Backhaul is the segment from CU to the core network.

Figure-backhaul-fronthaul — Figure 2: Simplified diagram that shows and compares the building blocks of 4G (left) and 5G (right) access networks and the links between these blocks.

5G promises to interconnect exponentially more devices than before, with higher speed and latencies. As a result, 5G edge network bandwidth requirements can reach up to 100 times more than 4G. These requirements will put more pressure on the existing fiber infrastructure, and mobile network operators are expected to deliver their 5G promises with relatively little expansion in their fiber infrastructure.

The initial rollout of 5G has already happened in most developed countries, with operators switching from 1G SFP transceivers and grey transceivers to 10G SFP+ or wavelength division multiplexing (WDM). Mobile networks must move to the next phase of 5G deployments, which will exponentially increase the number of devices connected to the network. These more mature phases of 5G deployment will require operators to scale capacity cost-effectively. The move to 25G tunable optics in fronthaul networks will enable this expansion in capacity in an affordable way and help promote a long-awaited convergence between mobile and fixed access networks.

Enhancing Mobile Fronthaul Capacity

The move from 4G to 5G networks saw many operators upgrade their 10G grey transceivers to tunable 10G transceivers and 25G grey transceivers to make the most of their fiber infrastructure. However, as the 5G rollout moves into a more mature phase, the demands of fronthaul networks will often require even greater capacity from fiber infrastructure.

These future demands are a key reason why South Korea’s service providers decided to future-proof their fiber infrastructure and invested in 10G and 25G WDM technology since the early stages of their 5G rollout. Over time, providers in other countries will find themselves fiber-limited and turn towards tunable technologies. These trends are why LightCounting forecasts that the 25G DWDM market will provide the most significant revenue opportunity in the coming five years.

These 25G tunable transceivers support a typical reach of 10 km. It can reach up to 15 or even 20 km with extra amplification and compensation. Maximizing the capacity of fronthaul fiber will not only be beneficial for mobile network providers and telecom business developers, system architectures, equipment manufacturers, and product developers.

Graph DWDM Annual Sales — Figure 2: Fronthaul tunable optics transceiver revenues. Source: LightCouting Next-Gen Access Optics Report (2021)

DWDM Solutions For Fronthaul Aggregation

The transitions to 3G and 4G relied heavily on more efficient use of broader RF spectrum blocks. These transitions were as simple for many cell sites as changing the appropriate radio line card at a base station unit. The same cannot be said about the transition to 5G. Particularly this second phase of 5G deployment will require a more profound restructuring of mobile network architecture. These mature 5G networks will use higher frequency bands, which require the deployment of more cell sites and antennas to cover the same geographical areas as 4G. In contrast, existing antennas must upgrade to denser antenna arrays.

The characteristics of this second 5G deployment mean that operators must deploy larger-bandwidth channels and more total channels due to the additional base stations. DWDM is an excellent fit for interconnecting these new smaller cell sites since it allows operators to quickly increase their number of channels without having to lay out new and expensive fiber infrastructure. Thanks to the 25G capacity, these additional channels can be easily aggregated into the fronthaul transport network without being limited by fiber infrastructure.

Figure-5g-frount-haul-deep-fiber — Figure 3. Comparison between 4G legacy fiber deployments in access networks vs. future deep fiber deployments in 5G. The latter needs significantly more fiber capacity and infrastructure that can handle this aggregation, such as DWDM.

The Dream of Fixed-Mobile Access

Carriers that provide both fixed and mobile network services may often have to deal with situations in which their fixed and mobile access networks compete against each other. Since these networks often use different technology standards and even transmission media (e.g., legacy coaxial networks in fixed access), these carriers often have to build additional and arguably redundant optical infrastructure.

These carriers have long dreamed of merging their fixed and mobile network infrastructures under the same standards and transmission pipes. Such solutions reduce the need to build and manage redundant infrastructure. The expansion of fiber infrastructure and WDM technology might finally provide them with the opportunity to do so.

Figure-metro-core-network — Figure 4: Simplified example of some critical technologies and network architectures that could be merged in a converged fixed-mobile access network solution. These include radio access networks (RAN) with DWDM technology and Common Public Radio Interface (CPRI) protocols, as well as passive optical networks that combine time-division multiplexing (TDM) and WDM technology.

Passive optical networks (PON) have become a popular solution to implement fiber-to-the-home solutions. These networks’ bandwidth and latency requirements are leading to the standardization of 25G PONs and WDM-PON technology. Now that both mobile and fixed access are considering 25G WDM solutions, it might be a good time to revisit the subject of convergence, as together, this network will offer various quality services such as communication, entertainment, and data acquisition without any terminal, application, network, and location limitation.

Takeaways

25G tunable optics will become an industry standard for mobile fronthaul in the coming years. They allow operators to make the most of their existing fiber infrastructure, maximizing bandwidth and increasing the network’s ability to aggregate more signals from smaller and more numerous 5G base stations. For certain carriers, they could enable a future converged fixed-access network that simplifies the installation and management of their infrastructure. As shown by the example of South Korean network operators, it pays off to anticipate all these future demands and invest in a future-proof network that can scale up quickly.

Tags: 5G, access, aggregation, backhaul, capacity, DWDM, fronthaul, Integrated Photonics, LightCounting, metro, midhaul, mobile, mobile access, network, optical networking, optical technology, photonic integrated chip, photonic integration, PIC, PON, programmable photonic system-on-chip, solutions, technology

Insights

July 28, 2023

The Next Bright Lights of Eindhoven

Paris may be the more well-known City of Light, but we may argue that Eindhoven…

Insights

April 17, 2023

When will the Network Edge go Coherent?

Article first published 27 July 2022, updated 12 April 2023. Network carriers want to provide…

Article first published 27 July 2022, updated 12 April 2023.

Network carriers want to provide communications solutions in all areas: mobile access, cable networks, and fixed access to business customers. They want to provide this extra capacity and innovative and personalized connectivity and entertainment services to their customers.

Deploying only legacy direct detect technologies will not be enough to cover these growing bandwidth and service demands of mobile, cable, and business access networks with the required reach. In several cases, networks must deploy more 100G coherent dense wavelength division multiplexing (DWDM) technology to transmit more information over long distances. Several applications in the optical network edge could benefit from upgrading from 10G DWDM or 100G grey aggregation uplinks to 100G DWDM optics:

Mobile Mid-haul benefits from seamlessly upgrading existing uplinks from 10G to 100G DWDM.
Mobile Backhaul benefits from upgrading their links to 100G IPoDWDM.
Cable Access links could upgrade the uplinks of existing termination devices such as optical line terminals (OLTs) and Converged Cable Access Platforms (CCAPs) from 10G to 100G DWDM.
Business Services could scale enterprise bandwidth beyond single-channel 100G grey links.

However, network providers have often stuck to their 10G DWDM or 100G grey links because the existing 100G DWDM solutions could not check all the required boxes. “Scaled-down” coherent 400ZR solutions had the required reach and tunability but were too expensive and power-hungry for many access network applications. Besides, ports in small to medium IP routers used in most edge deployments often do not support the QSFP-DD form factor commonly used in 400ZR modules but the QSFP28 form factor.

Fortunately, the rise of 100ZR solutions in the QSFP28 form factor is changing the landscape for access networks. “The access market needs a simple, pluggable, low-cost upgrade to the 10G DWDM optics that it has been using for years. 100ZR is that upgrade,” said Scott Wilkinson, Lead Analyst for Optical Components at market research firm Cignal AI. “As access networks migrate from 1G solutions to 10G solutions, 100ZR will be a critical enabling technology.”

In this article, we will discuss how the recent advances in 100ZR solutions will enable the evolution of different segments of the network edge: mobile midhaul and backhaul, business services, and cable.

How Coherent 100G Can Move into Mobile X-haul

The upgrade from 4G to 5G has shifted the radio access network (RAN) from a two-level structure with backhaul and fronthaul in 4G to a three-level structure with back-, mid-, and fronthaul:

Fronthaul is the segment between the active antenna unit (AAU) and the distributed unit (DU)
Midhaul is the segment from DU to the centralized unit (CU)
Backhaul is the segment from CU to the core network.

Figure 1: Simplified diagram that shows and compares the building blocks of 4G (left) and 5G (right) access networks and the links between these blocks.

The initial rollout of 5G has already happened in most developed countries, with many operators upgrading their 1G SFPs transceiver to 10G SFP+ devices. Some of these 10G solutions had DWDM technology; many were single-channel grey transceivers. However, mobile networks must move to the next phase of 5G deployments, which requires installing and aggregating more and smaller base stations to exponentially increase the number of devices connected to the network.

These mature phases of 5G deployment will require operators to continue scaling fiber capacity cost-effectively with more widespread 10G DWDM SFP+ solutions and 25G SFP28 transceivers. These upgrades will put greater pressure on the aggregation segments of mobile backhaul and midhaul. These network segments commonly used link aggregation of multiple 10G DWDM links into a higher bandwidth group (such as 4x10G). However, this link aggregation requires splitting up larger traffic streams and can be complex to integrate across an access ring. A single 100G uplink would reduce the need for such link aggregation and simplify the network setup and operations. If you want to know more about the potential market and reach of this link aggregation upgrade, we recommend reading the recent Cignal AI report on 100ZR technologies.

Cable Migration to 10G PON Will Drive the Use of Coherent 100G Uplinks

According to Cignal AI’s 100ZR report, the biggest driver of 100ZR use will come from multiplexing fixed access network links upgrading from 1G to 10G. This trend will be reflected in cable networks’ long-awaited migration from Gigabit Passive Optical Networks (GPON) to 10G PON. This evolution is primarily guided by the new DOCSIS 4.0 standard, which promises 10Gbps download speeds for customers and will require several hardware upgrades in cable networks.

To multiplex these new larger 10Gbps customer links, cable providers and network operators need to upgrade their optical line terminals (OLTs) and Converged Cable Access Platforms (CCAPs) from 10G to 100G DWDM uplinks. Many of these new optical hubs will support up to 40 or 80 optical distribution networks (ODNs), too, so the previous approach of aggregating multiple 10G DWDM uplinks will not be enough to handle this increased capacity and higher number of channels.

Anticipating such needs, the non-profit R&D organization CableLabs has recently pushed to develop a 100G Coherent PON (C-PON) standard. Their proposal offers 100 Gbps per wavelength at a maximum reach of 80 km and up to a 1:512 split ratio. CableLabs anticipates C-PON and its 100G capabilities will play a significant role not just in cable optical network aggregation but in other use cases such as mobile x-haul, fiber-to-the-building (FTTB), long-reach rural scenarios, and distributed access networks.

Towards 100G Coherent and QSFP28 in Business Services

Almost every organization uses the cloud in some capacity, whether for development and test resources or software-as-a-service applications. While the cost and flexibility of the cloud are compelling, its use requires fast, high-bandwidth wide-area connectivity to make cloud-based applications work as they should.

Similarly to cable networks, these needs will require enterprises to upgrade their existing 1G Ethernet private lines to 10G Ethernet, which will also drive a greater need for 100G coherent uplinks. Cable providers and operators will also want to take advantage of their upgraded 10G PON networks and expand the reach and capacity of their business services.

The business and enterprise services sector was the earliest adopter of 100G coherent uplinks, deploying “scaled-down” 400ZR transceivers in the QSFP-DD form factor since they were the solution available at the time. However, these QSFP-DD slots also support QSFP28 form factors, so the rise of QSFP 100ZR solutions will provide these enterprise applications with a more attractive upgrade with lower cost and power consumption. These QSFP28 solutions had struggled to become more widespread before because they required the development of new, low-power digital signal processors (DSPs), but DSP developers and vendors are keenly jumping on board the 100ZR train and have announced their development projects: Acacia, Coherent/ADVA, Marvell/InnoLight, and Marvell/OE Solutions. This is also why EFFECT Photonics has announced its plans to co-develop a 100G DSP with Credo Semiconductor that best fits 100ZR solutions in the QSFP28 form factor.

Figure : Comparison between QSFP-DD and QSFP-28 form factors for 100G applications. — Figure 2 : Comparison between QSFP-DD and QSFP-28 form factors for 100G applications.

Takeaways

In the coming years, 100G coherent uplinks will become increasingly widespread in deployments and applications throughout the network edge. Some mobile access networks use cases must upgrade their existing 10G DWDM link aggregation into a single coherent 100G DWDM uplink. Meanwhile, cable networks and business services are upgrading their customer links from 1Gbps to 10Gbps, and this migration will be the major factor that will increase the demand for coherent 100G uplinks. For carriers who provide converged cable/mobile access, these upgrades to 100G uplinks will enable opportunities to overlay more business services and mobile traffic into their existing cable networks.

As the QSFP28 100ZR ecosystem expands, production will scale up, and these solutions will become more widespread and affordable, opening up even more use cases in access networks.

Tags: 5G, access, aggregation, backhaul, capacity, DWDM, fronthaul, Integrated Photonics, LightCounting, metro, midhaul, mobile, mobile access, network, optical networking, optical technology, photonic integrated chip, photonic integration, Photonics, PIC, PON, programmable photonic system-on-chip, solutions, technology

Technologies and trends and their impact

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April 14, 2023

Scaling Up Live Event Coverage with 5G Technology

Everyone who has attended a major venue or event, such as a football stadium or…

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November 22, 2022

Free Space Optics for Access Networks

Optical signals are moving deeper and deeper into access networks. Achieving the ambitious performance goals…

Optical signals are moving deeper and deeper into access networks. Achieving the ambitious performance goals of 5G architectures requires more optics than ever between small cell sites. As stated in a recent report by Deloitte, “extending fiber optics deeper into remote communities is a critical economic driver, promoting competition, increasing connectivity for the rural and underserved, and supporting densification for wireless.”

However, there are cases in which fiber is not cost-effective to deploy. For example, a network carrier might need to quickly increase their access network capacity for a big festival, and there is no point in deploying extra fiber. In many remote areas, the customer base is so small that the costly deployment of fiber will not produce a return on investment. These situations must be addressed with some kind of wireless access solution. Carriers have used fixed microwave links for the longest time to handle these situations.

However, radio microwave frequencies might not be enough as the world demands greater internet speeds and simply changing over to higher carrier frequencies will limit the reach of microwave links. On top of that, the radio spectrum is quite crowded, and a carrier might not have the available licensed spectrum to deploy this wireless link. Besides, microwave point-to-point links produce plenty of heat while struggling to deliver capacity beyond a few Gbps. This is where free-space optics (FSO) comes into play.

FSO is a relatively straightforward technology to explain. A high-power laser source converts data into laser pulses and sends them through a lens system and into the atmosphere. The laser travels to the other side of the link and goes through a receiver lens system and a high-sensitivity photodetector converts those laser pulses back into electronic data that can be processed. In other words, instead of using an optical fiber as a medium to transmit the laser pulses, FSO uses air as a medium. The laser typically operates at an infrared wavelength of 1550nm that is safer on the eye.

FSO has often been talked about as some futuristic technology to be used in space applications, but it can be used more than that, including ground-to-ground links in access networks.FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, there is no need to secure spectrum licenses.

Overcoming the challenges of alignment, and atmospheric turbulence

FSO struggled to break through into practical applications despite these benefits because of certain technical challenges. Communications infrastructure, therefore, focused on more stable transmission alternatives such as optical fiber and RF signals. However, research and innovation over the last few decades are removing these technical barriers. One obstacle to achieving longer distances with FSO had to do with the quality of the laser signal.

Over time, FSO developers have found a solution to this issue in adaptive optics systems. These systems compensate for distortions in the beam by using an active optical element—such as a deformable mirror or liquid crystal—that dynamically changes its structure depending on the shape of the laser beam. Dutch startup Aircision uses this kind of technology in its FSO systems to increase their tolerance to atmospheric disruptions.

Figure 2: How an adaptive optical system works.

Another drawback of FSO is aligning the transmitter and receiver units. Laser beams are extremely narrow, and if the beam doesn’t hit the receiver lens at just the right angle, the information may be lost. The system requires almost perfect alignment, and it must maintain this alignment even when there are small changes in the beam trajectory due to wind or atmospheric disturbances.

FSO systems can handle these alignment issues with fast steering mirror (FSM) technology. These mirrors are driven with electrical signals and are fast, compact, and accurate enough to compensate the disturbances in the beam trajectory. However, even if the system can maintain the beam trajectory and shape, atmospheric turbulence can still degrade the message and cause interference in the data. Fortunately, FSO developers also use sophisticated digital signal processing techniques (DSP) to compensate for these impairments.

These DSP techniques allow for reliable, high-capacity, quick deployments even through thick clouds and fog. FSO links can now handle Gbps capacity over several kilometers thanks to all these technological advances. For example, a collaboration between Aircision and TNO demonstrated in 2021 that their FSO systems could reliably transmit 10 Gbps over 2.5 km. Aircision’s Scientific Director John Reid explained, “it’s an important milestone to show we can outperform microwave E-band antennas and provide a realistic solution for the upcoming 5G system.”

An alternative for safe, private networks

An understated benefit of FSO is that, from a physics perspective, they are arguably the most secure form of wireless communication available today. Point-to-point microwave links transmit a far more directional beam than mobile antennas or WiFi systems, which reduces the potential for security breaches. However, even these narrower microwave beams are still spread out enough to cover a wide footprint vulnerable to eavesdropping and jamming.

At a 1km distance, the beam can spread out enough to cover roughly the length of a building, and at 5km, it could cover an entire city block. Furthermore, microwave systems have side- and back lobes radiating away from the intended direction of transmission that can be intercepted too. Finally, if an attacker is close enough to the source, even the reflected energy from buildings can be used to intercept the signal.

Figure 3: Geometry of an FSO system vs. a microwave link. The microwave link has a much wider spread than FSO, which makes eavesdropping and jamming easier.

Laser beams in FSO are so narrow and focused that they do not have to deal with these issues. At 1km, a typical laser beam only spreads out about 2 meters, and at 5km, only about 5 meters. There are no side and back lobes to worry about and no near-zone reflections. The beam is so narrow that intercepting the transmission becomes an enormous challenge. An intruder would have to get within inches of a terminal or the line of sight, making it easier to get discovered. To complicate things further, the intruder’s terminal would also need to be very well aligned to pick up enough of a signal.

Using Highly-Integrated Transceivers in Free Space Optics

Even though fiber optical communications drove the push for smaller and more efficient optical transceivers, this progress also has a beneficial impact on FSO. As we have explained in previous articles, optical transmission systems have been miniaturized from big, expensive line cards to small, affordable pluggables the size of a large USB stick. These compact transceivers with highly integrated optics and electronics have shorter interconnections, fewer losses, and more elements per chip area. These features all led to a reduced power consumption over the last decade. At EFFECT Photonics, we achieve even further efficiency gains by an optical System-On-Chip (SoC) that integrates all photonic functions on a single chip, including lasers and amplifiers.

Figure 4: Diagram of an FSO system connected to an EFFECT Photonics 10G SFP+ pluggable transceiver via optical fiber.

FSO systems can now take advantage of affordable, low-power transceivers to transmit and receive laser signals in the air. For example, a transceiver based on an optical SoC can output a higher power into the FSO system. By using this higher laser power, the FSO does not need to amplify the signal so much before transmitting it, improving its noise profile. Furthermore, this benefit happens with both direct detect and coherent transceivers. This is a key reason why Aircision has partnered up with EFFECT Photonics to create both direct detect and coherent free-space optical systems, since the startup ultimately aims to reach transmission speeds of 100 Gbps over the air.

Takeaways

FSO has moved from the domain of science fiction to a practical technology that now deserves a place in access networks. FSO can deliver a wireless access solution that can be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. Furthermore, since it does not use the RF spectrum, it is unnecessary to secure spectrum licenses. Affordable direct detect and coherent transceivers based on SoC can further improve the quality and affordability of FSO transmission.

Tags: access networks, adaptive optics, affordable, capacity, coherent, cost-effective, deployments, free space optics, integration, license, miniaturized, photonic integration, Photonics, pluggable, power consumption, private network links, quick deployments, radio spectrum, remote communities, security, SFP, signal processing, turbulence

Insights

November 4, 2022

What’s Inside a Coherent DSP?

Coherent transmission has become a fundamental component of optical networks to address situations where direct…

Coherent transmission has become a fundamental component of optical networks to address situations where direct detect technology cannot provide the required capacity and reach.

While direct detect transmission only uses the amplitude of the light signal, coherent optical transmission manipulates three different light properties: amplitude, phase, and polarization. These additional degrees of modulation allow for faster optical signals without compromising the transmission distance. Furthermore, coherent technology enables capacity upgrades without replacing the expensive physical fiber infrastructure on the ground.

The digital signal processor (DSP) is the electronic heart of coherent transmission systems. The fundamental function of the DSP is encoding the electronic digital data into the amplitude, phase, and polarization of the light signal and decoding said data when the signal is received. The DSP does much more than that, though: it compensates for impairments in the fiber, performs analog-to-digital conversions (and vice versa), corrects errors, encrypts data, and monitors performance. And recently, DSPs are taking on more advanced functions such as probabilistic constellation shaping or dynamic bandwidth allocation, which enable improved reach and performance.

Given its vital role in coherent optical transmission, we at EFFECT Photonics want to provide an explainer of what goes on inside the DSP chip of our optical transceivers.

There’s More to a DSP Than You Think…

Even though we colloquially call the chip a “DSP”, it is an electronic engine that performs much more than just signal processing. Some of the different functions of this electronic engine (diagram below) are:

*Figure 1: Layout of the electronic engine used in a coherent optical transceiver, including the sequence the signal follows when processed by the engine.*

Analog Processing: This engine segment focuses on converting signals between analog and digital formats. Digital data is composed of discrete values like 0s and 1s, but transmitting it through a coherent optical system requires converting it into an analog signal with continuous values. Meanwhile, a light signal received on the opposite end requires conversion from analog into digital format.
Digital Signal Processing: This is the actual digital processing. As explained previously, this block encodes the digital data into the different properties of a light signal. It also decodes this data when the light signal is received.
Forward Error Correction (FEC): FEC makes the coherent link much more tolerant to noise than a direct detect system and enables much longer reach and higher capacity. Thanks to FEC, coherent links can handle bit error rates that are literally a million times higher than a typical direct detect link. FEC algorithms allow the electronic engine to enhance the link performance without changing the hardware. This enhancement is analogous to imaging cameras: image processing algorithms allow the lenses inside your phone camera to produce a higher-quality image.

Figure 2: Simplified diagram of a network link with forward error correction (FEC). The FEC encoder adds a series of redundant bits (called overhead) to the transmitted data stream. The receiver can use this overhead to check for errors without asking the transmitter to resend the data.

Framer: While a typical electric signal sent through a network uses the Ethernet frame format, the optical signal uses the Optical Transport Network (OTN) format. The framer block performs this conversion. We should note that an increasingly popular solution in communication systems is to send Ethernet frames directly over the optical signal (a solution called optical Ethernet). However, many legacy optical communication systems still use the OTN format, so electronic engines should always have the option to convert between OTN and Ethernet frames.
Glue Logic: This block consists of the electronic circuitry needed to interface all the different blocks of the electronic engine. This includes the microprocessor that drives the electronic engine and the serializer-deserializer (SERDES) circuit. Since coherent systems only have four channels, the SERDES circuit converts parallel data streams into a single serial stream that can be transmitted over one of these channels. The opposite conversion (serial-to-parallel) occurs when the signal is received.

We must highlight that each of these blocks has its own specialized circuitry and algorithms, so each is a separate piece of intellectual property. Therefore, developing the entire electronic engine requires ownership or access to each of these intellectual properties.

So What’s Inside the Actual DSP Block?

Having clarified first all the different parts of a transceiver’s electronic engine, we can now talk more specifically about the actual DSP block that encodes/decodes the data and compensates for distortions and impairments in the optical fiber. We will describe some of the critical functions of the DSP in the order in which they happen during signal transmission. Receiving the signal would require these functions to occur in the opposite order, as shown in the diagram below.

*Figure 3. Layout of the digital signal processing blocks and the sequence followed by the signal when transmitted and received.*

Signal Mapping: This is where the encoding/decoding magic happens. The DSP maps the data signal into the different phases of the light signal—the in-phase components and the quadrature components—and the two different polarizations (x- and y- polarizations). When receiving the signal, the DSP will perform the inverse process, taking the information from the phase and polarization and mapping it into a stream of bits. The whole process of encoding and decoding data into different phases of light is known as quadrature modulation. Explaining quadrature modulation in detail goes beyond the scope of this article, so if you want to know more about it, please read the following article.
Pilot Signal Insertion: The pilot signal is transmitted over the communication systems to estimate the status of the transmission path. It makes it easier (and thus more energy-efficient) for the receiver end to decode data from the phase and polarization of the light signal.
Adaptive Equalization: This function happens when receiving the signal. The fiber channel adds several distortions to the light signal (more on that later) that change the signal’s frequency spectrum from what was initially intended. Just as with an audio equalizer, the purpose of this equalizer is to change specific frequencies of the signal to compensate for the distortions and bring the signal spectrum back to what was initially intended.

*Figure 4: How the equalizer in a communication systems work. The example in this figure is an equalizer for RF coaxial transmission (not optical), but the principle is the same. Source:* *Broadband Library*

Dispersion and Nonlinear Compensation: This function happens when receiving the signal. The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The fiber also adds other distortions due to nonlinear optical effects. These effects get worse as the input power of the light signal increases, leading to a trade-off. You might want more power to transmit over longer distances, but the nonlinear distortions also become larger, which beats the point of using more power. The DSP performs several operations on the light signal that try to offset these dispersion and nonlinear distortions.
Spectrum Shaping: Communication systems must be efficient in all senses, so they must transmit as much signal as possible within a limited number of frequencies. Spectrum shaping is a process that uses a digital filter to narrow down the signal to the smallest possible frequency bandwidth and achieve this efficiency.

When transmitting, the signal goes through the digital-to-analog conversion after this whole DSP sequence. When receiving the signal, it goes through the inverse analog-to-digital conversation and then through the DSP sequence.

Recent Advances and Challenges in DSPs

This is an oversimplification, but we can broadly classify the two critical areas of improvement for DSPs into two categories.

Transmission Reach and Efficiency

The entire field of communication technology can arguably be summarized with a single question: how can we transmit more information into a single frequency-limited signal over the longest possible distance?

DSP developers have many tools in their kit to answer this question. For example, they can transmit more data using more states in their quadrature-amplitude modulation process. The simplest kind of QAM (4-QAM) uses four different states (usually called constellation points), combining two different intensity levels and two different phases of light.

By using more intensity levels and phases, more bits can be transmitted in one go. State-of-the-art commercially available 400ZR transceivers typically use 16-QAM, with sixteen different constellation points that arise from combining four different intensity levels and four phases. However, this increased transmission capacity comes at a price: a signal with more modulation orders is more susceptible to noise and distortions. That’s why these transceivers can transmit 400Gbps over 100km but not over 1000km.

One of the most remarkable recent advances in DSPs to increase the reach of light signals is probabilistic constellation shaping (PCS). In the typical 16-QAM modulation used in coherent transceivers, each constellation point has the same probability of being used. This is inefficient since the outer constellation points that require more power have the same probability as the inner constellation points that require lower power.

*Figure 5: Comparison of conventional 16-QAM and a 16-QAM that uses probabilistic constellation shaping (PCS). Original diagram source:* *Infinera*.

PCS uses the low-power inner constellation points more frequently, and the outer constellation points less frequently, as shown in Figure 5. This feature provides many benefits, including improved tolerance to distortions and easier system optimization to specific bit transmission requirements. If you want to know more about it, please read the explainers here and here.

Energy Efficiency

Increases in transmission reach and efficiency must be balanced with power consumption and thermal management. Energy efficiency is the biggest obstacle in the roadmap to scale high-speed coherent transceivers into Terabit speeds.

Over the last two decades, power ratings for pluggable modules have increased as we moved from direct detection to more power-hungry coherent transmission: from 2W for SFP modules to 3.5 W for QSFP modules and now to 14W for QSSFP-DD and 21.1W for OSFP form factors. Rockley Photonics researchers estimate that a future electronic switch filled with 800G modules would draw around 1 kW of power just for the optical modules.

Figure 6: Normalized power breakdown of the internal blocks of a 400ZR pluggable module. Although this breakdown may vary a bit among different vendors, the authors find it to be generally valid for various DSP-based modules, both IM-DD and coherent. Source: *R. Nagarajan et al., IEEE JLT (2021)*

Around 50% of a coherent transceiver’s power consumption goes into the DSP chip. Scaling to higher bandwidths leads to even more losses and energy consumption from the DSP chip, and its radiofrequency (RF) interconnects with the optical engine. DSP chips must therefore be adaptable and smart, using the least amount of energy to encode/decode information. You can learn more about this subject in one of our previous articles. The interconnects with the optical engine are another area that can see further optimization, and we discuss these improvements in our article about optoelectronic co-design.

Takeaways

In summary, DSPs are the heart of coherent communication systems. They not only encode/decode data into the three properties of a light signal (amplitude, phase, polarization) but also handle error correction, analog-digital conversation, Ethernet framing, and compensation of dispersion and nonlinear distortion. And with every passing generation, they are assigned more advanced functions such as probabilistic constellation shaping.

There are still many challenges ahead to improve DSPs and make them transmit even more bits in more energy-efficient ways. Now that EFFECT Photonics has incorporated talent and intellectual property from Viasat’s Coherent DSP team, we hope to contribute to this ongoing research and development and make transceivers faster and more sustainable than ever.

Tags: 100G, 5G, 6G, access, access networks, aggregation, backhaul, capacity, coherent, DWDM, fronthaul, Integrated Photonics, live events, metro, midhaul, mobile, mobile access, mobile networks, network, optical networking, optical technology, photonic integrated chip, photonic integration, Photonics, PIC, PON, programmable photonic system-on-chip, solutions, technology, VR, WDM

Sub-Assemblies: pITLA

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capacity

Coherent Increases Transmission Reach

Coherent Increases Transmission Efficiency

The Synergy with WDM Technology

Takeaways

Customization for Remote Diagnostics and Management

Customization for Easier Installation

Customization for Energy Sustainability

Takeaways

Reaching 100G on the Ground for Access Networks

Coherent Optical Technology in Space

Making Coherent Technology Live in Space

Takeaways

Towards Zero-Touch Provisioning

Enabling Remote Diagnostics and Management

Remote Diagnostics and Control for Energy Sustainability

Takeaways

Enhancing Mobile Fronthaul Capacity

DWDM Solutions For Fronthaul Aggregation

The Dream of Fixed-Mobile Access

Takeaways

From Matches to the Light Bulb Factory

From ASML and Photonics Research to Philips

Bringing Photonics into the Real World from the Lab

How Coherent 100G Can Move into Mobile X-haul

Cable Migration to 10G PON Will Drive the Use of Coherent 100G Uplinks

Towards 100G Coherent and QSFP28 in Business Services

Takeaways

A Higher Bandwidth Experience

New Ways of Interaction

Strengthening the Transport Network

Takeaways

Overcoming the challenges of alignment, and atmospheric turbulence

An alternative for safe, private networks

Using Highly-Integrated Transceivers in Free Space Optics

Takeaways

There’s More to a DSP Than You Think…

So What’s Inside the Actual DSP Block?

Recent Advances and Challenges in DSPs

Transmission Reach and Efficiency

Energy Efficiency

Takeaways

Sub-Assemblies: pITLA

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