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  What is Full Duplex Coherent Optics?

Steve Jia
Distinguished Technologist, Wired Technologies

May 24, 2018

A brand new innovation, Full Duplex Coherent Optics uses the same wavelength, in two different directions, over the same fiber at the same time. As a result, Full Duplex Coherent Optics technology supports over 200 times more capacity compared to non-coherent digital transmission over a single fiber. This makes Coherent Optics technology well suited for deployment in many more cable access network fibers. Watch our video to see how this technology will significantly increase the value of the currently deployed fiber infrastructure.

Click below to learn more about Full Duplex Coherent Optics. 

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Doubling up on Fiber Capacity: A Winning Strategy for Full Duplex Coherent Optics

Steve Jia
Distinguished Technologist, Wired Technologies

Mar 8, 2018

During our 2017 Winter Conference, CableLabs announced the launch of the point-to-point (P2P) Coherent Optics specification project, potentially multiplying the capacity of each existing cable access network fiber by over 100 times and possibly indefinitely deferring new fiber builds on existing routes. Now, a new CableLabs innovation, Full Duplex Coherent Optics:

  • Doubles the bi-directional capacity of each fiber
  • Multiplies the capacity of each existing access network fiber by over 200 times
  • Simultaneously makes Coherent Optics technology well suited for deployment in many more cable access network fibers

Why CableLabs Began the Coherent Optics Project

Most cable operators have a somewhat limited fiber count between the headend and the fiber node, so maximizing the capacity provided by this scarce resource has real economic advantages for cable operators.  Getting more capacity out of the existing fibers can eliminate the need to dig more trenches to lay more fiber. This allows operators to best leverage the existing fiber infrastructure to withstand the exponential growth in capacity and services for residential and business subscribers.

Transport Methodologies

There are two fundamental topologies to achieve bidirectional P2P coherent transport:

  • dual-fiber
  • single-fiber

According to a recent operators survey, 20 percent of existing cable access networks use a single-fiber topology. That means that downstream and upstream transmission to nodes takes place on a single strand of fiber. It is estimated that over the next 5 years, this number will grow to 60 percent. Therefore, bidirectional transmission over a single fiber is needed for coherent signals to support single-fiber topologies and to facilitate the redundancy of optical links.

Full Duplex Coherent Optics Transport

The Dual-Fiber Approach

Today, achieving bidirectional transmission in an optical domain with a single laser requires two fibers. This is the standard practice using today’s coherent optical technology. One laser in a transceiver performs two functions:

  • as the optical signal source in the transmitter
  • as the reference local oscillator signal in the receiver

Because of the use of the same wavelength from the same laser, a second fiber must be available for the other direction—one fiber for downstream and a second fiber for upstream.

Full Duplex Coherent Optics Dual Fiber Approach

The Single-Fiber Approach

The second typical approach is to use a single fiber but transmit at different frequencies or wavelengths, similar to the upstream and downstream spectrum split that we implement in our HFC networks. To accomplish this frequency/wavelength multiplexing approach, two lasers operating at different wavelengths are needed. Wavelength multiplexers and demultiplexers following a wavelength management and allocation strategy are needed to combine these different wavelengths over the same fiber. The second laser ends up costing a lot more than money—increasing power consumption, operational complexity, and transceiver footprint.

Full Duplex Coherent-Optics-Single-Fiber Approach

CableLabs’ Full Duplex Coherent Optics Approach

CableLabs proposes an alternative method to achieve full duplex coherent optics. We leverage two optical circulators on each end in a special configuration. The circulator is a low-cost, passive, but directional device—much like a traffic roundabout for cars, but this is an optical roundabout. Instead of using two fibers, a single fiber is connected for bidirectional transmission. Most importantly, instead of using two lasers, a single laser is employed for single-fiber coherent systems.

CableLabs Full Duplex Coherent-Optics Approach

How Does It Work in a Cable?

Many scenarios in cable focus on the access environment with limited transmission distances. Unlike backbone and metropolitan coherent optical networks, access networks don’t require multiple directional optical amplifiers in cascade. By definition, the introduction of directional components hampers bidirectional transmission.

When dealing with coherent signals, we have much higher Optical Signal to Noise Ratio (OSNR) sensitivity and higher tolerance to the impairments from the spontaneous Rayleigh backscattering than intensity-modulated systems. In addition, the threshold of the stimulated Brillouin scattering (SBS) nonlinear effect is much higher because of the nature of phase-modulated signals on the reduction of optical carrier power and the increase of effective linewidth.

With this new dimension of direction-division multiplexing (DDM) in the optical domain, any coherent wavelength can be used twice, once in each direction, thus doubling the whole fiber system capacity. This full duplex implementation is not bandwidth-limited. It works for 100G, 200G and future 400G. It is also not wavelength-selective. It works for short wavelengths and for long wavelengths, and it would cover not only the entire C-Band but, with different optical sources, the entire fiber spectrum. All these features have been experimentally verified in CableLabs’ Optical Center of Excellence (OCE) over distances of up to 100 kilometers.

Impacts/Benefits of Full Duplex Coherent Optics

Full duplex coherent optics will significantly increase the value of the currently-deployed fiber infrastructure. It has been implemented in an elegant way, without the requirement of redesigning new chips for digital signal processing. This scheme can be seamlessly incorporated into the ongoing CableLabs’ P2P Coherent Optics specification effort, which will be issued in mid-2018.

Dr. Alberto Campos, a CableLabs Fellow, also contributed to this article.


Interested in learning more about our point-to-point (P2P) Coherent Optics specification project? A follow-up video containing more information on the technology will be posted next week. Click below to join our working group.

Join our Working Group


It’s Only Wireless for THIS long

Steve Jia
Distinguished Technologist, Wired Technologies

Aug 17, 2016

Why the underlying fiber network is critical to mobile communications. -- The explosion in popularity of the mobile game Pokémon Go has triggered unprecedented attention on virtual reality (VR) and augmented reality (AR). Many believe that Pokémon Go is just the first step into the fully-immersive VR and AR applications, which, from a bandwidth demand perspective, are on the high end of the Internet of Things (IoT) connections. By the end of this decade, analysts predict that 50 billion IoT sensors will connect to mobile networks consuming 1000 times as much data as today’s mobile gadgets alone. Along with cloud, machine to machine, and new video streaming applications, the underlying network infrastructure that enables such constant high-quality connectivity is critical to ultimate user experiences. None of the existing radio access technologies will be able to individually provide the capabilities to effectively meet market demands. The next generation 5G mobile system is being designed specifically to support this vision of satisfying the increasing demand for higher data rates, lower network latencies, better energy efficiency, and reliable ubiquitous connectivity.


However, the success of 5G will not just be about new wireless technologies! The deployment of 5G technologies will be dependent on the ability of the wireline transport network connected to the radio access networks (RANs). This is because all the air bits will be transported from the wireline systems, most likely high-speed fiber optic networks. The network architecture and topology are evolving too (see Figure) and we expect to see a proliferation of small cells deeper in the network closer to the end-user. Small cells have a range of 10 to 200-meter cell radius within urban and in-building locations, to 1 or 2 km in rural areas. Centralized or Coordinated-RAN solutions, where the baseband units are placed together and share information at a centralized location, require the extremely high speed and low latency only available using fiber networks. The reality is… in mobile networks the bits are only air bits for a very short part of their life!

Historically, the transition to new mobile technologies has resulted in the need for a fourfold to fivefold increase in backhaul capacity. With the advancement from 3G to 4G, RANs reached a capacity of 1 Gb/s to 10 Gb/s per cell. If you consider the effective throughput for each user and the deployment of multi-antenna technologies, future 5G RANs will require ten times the backhaul capacity of today’s networks. If 5G network technology is deployed at scale, wireless networks will have to xhaul (backhaul, midhaul, and fronthaul) massive amounts of data between cell sites and core networks.

Compared to alternatives like mm-wave self-backhauling, deploying optical fiber provides a superior technical solution due to bandwidth scaling, low and deterministic latency and jitter, and high system reliability. Optical techniques can also provide the leverage to enhance inter-cell coordination, achieve wide network coherence, and also decrease timing jitter in high order vector modulation and simplify remote radio head architecture. In this sense, fiber is not only the transmission pipe, it can integrate with wireless systems for end-to-end seamless networking purpose to affect network control and power efficiency, minimize latency, provide network system protection and restoration, and decrease digital data processing overhead.

Fiber and optical transport technologies are expected to play more and more important roles in the RANs to meet the aggressive performance goals of 5G. CableLabs is heavily involved in both the wireless and wireline portions of RANs. On the wireless portion, CableLabs is contributing to the ongoing formation of 5G technologies and network architectures including multi-gigabit wireless transmission over millimeter-waves and dense mobile and fixed wireless access. On the wireline portion, CableLabs is exploring new fiber optic technologies that increase capacity and lower latency, while also leveraging the unique characteristics of Hybrid Fiber Coax networks. Leveraging our expanded efforts toward university research, we are exploring the melding of wireless and wireline through collaboration with the National Science Foundation’s Fiber-Wireless Integration and Networking (FiWIN) center led by Georgia Tech.