Forward Error Correction (FEC): A Primer on the Essential Element for Optical Transmission Interoperability
Forward error correction (FEC) has been a powerful tool in the cable industry for many years. In fact, perhaps the single biggest performance improvement in the DOCSIS 3.1 specifications was achieved by changing the FEC being used in previous versions – Reed-Soloman (RS) – to a new coding scheme with improved performance: low-density parity check (LDPC). Similarly, FEC has also become an indispensable element for high-speed optical transmission systems, especially in current coherent optical transmission age.
FEC is an effective digital signal processing method that improves the bit error rate of communication links by adding redundant information (parity bits) to the data at the transmitter side so that the receiver side then uses the redundant information to detect and correct errors that may have been introduced in the transmission link. As the following figure shows, the signal encoding that takes place at the transmitter has to be properly decoded by the receiver in order to extract the original signal information. Precise definition and implementation of the encoding rules are required to avoid misinterpretation of the information by the receiver decoding the signal. Successful interoperability will only take place when both the transmitter and receiver follow and implement the same encoding and decoding rules.
As you can see, FEC is the essential element that needs to be defined to enable the development of interoperable transceivers using optical technology over point-to-point links. The industry trends are currently moving toward removing proprietary aspects and becoming interoperable when the operators advocate more open and disaggregated transport in high-volume short-reach applications.
When considering which FEC to choose for a new specification, you need to consider some key metrics, including the following:
- Coding overhead rate— The ratio of the number of redundant bits to information bits
- Net coding gain (NCG)— The improvement of received optical sensitivity with and without using FEC associated with increasing bit rate
- Pre-FEC BER threshold— A predefined threshold for error-free post-FEC transmission determined by NCG
Other considerations include hardware complexity, latency, and power consumption.
One major decision point for FEC coding and decoding is between Hard-Decision FEC (HD-FEC) and Soft-Decision FEC (SD-FEC). HD-FEC performs decisions whether 1s or 0s have occurred based on exact thresholds, whereas SD-FEC makes decisions based on probabilities that a 1 or 0 has occurred. SD-FEC can provide higher NCG to get closer to the ideal Shannon limit with the sacrifice of higher complexity and more power consumption.
The first-generation FEC code, standardized for optical communication, is RS code. RS is used for long-haul optical transmission as defined by ITU-T G.709 and G.975 recommendations. In this RS implementation, each codeword contains 255 code word bytes, of which 239 bytes are data and 16 bytes are parity, usually expressed as RS (255,239) with the name of Generic FEC (GFEC). Several FEC coding schemes were recommended in ITU-T G. 975.1 for high bit-rate dense wavelength division multiplexing (DWDM) submarine systems in the second-generation of FEC codes. The common mechanism for increased NCG was the use of concatenated coding schemes with iterative hard-decision decoding. The most commonly-implemented example is the Enhanced FEC (EFEC) from G.975.1 Clause I.4 for 10G and 40G optical interfaces.
At the 100 Gbps data rate, CableLabs has adopted Hard-Decision (HD) Staircase FEC, defined in ITU-T G.709.2 and included in the CableLabs P2P Coherent Optics Physical Layer v1.0 (PHYv1.0) Specification. This Staircase FEC, also known as high-gain FEC (HG-FEC), is the first coherent FEC that provides an NCG of 9.38 dB with the pre-FEC BER of 4.5E-3. The 100G line-side interoperability has been verified in the very first CableLabs’ Point-to-Point (P2P) Coherent Optics Interoperability Event.
At the 200 Gbps data rate, openFEC (oFEC) was selected in CableLabs most-recent release of P2P Coherent Optics PHYv2.0 Specification. The oFEC provides an NCG of 11.1 dB for Quadrature Phase-Shift Keying (QPSK) with pre-FEC BER of 2E-2 and 11.6 dB for 16QAM format after 3 soft-decision iterations to cover multiple use cases. This oFEC was also standardized by Open ROADM targeting metro applications.
Although CableLabs has not specified 400G coherent optical transport, the Optical Interworking Forum (OIF) has adopted a 400G concatenated FEC (cFEC) with soft-decision inner Hamming code and hard-decision outer Staircase code in its 400G ZR standard; this same FEC has been selected as a baseline proposal in the IEEE 802.3ct Task Force. This 400G implementation agreement (IA) provides an NCG of 10.8 dB and pre-FEC BER of 1.22E-2 for coherent dual-polarized 16QAM modulation format specially for the Data Center Interconnection (DCI).
The following table summarizes performance metrics for standardized FEC in optical fiber transmission systems.
CableLabs is the first specification organization to demonstrate 100G coherent optics interoperability with a significant level of participants. Please register for our next coherent optics interoperability testing.
What is Full Duplex Coherent Optics?
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.
Doubling up on Fiber Capacity: A Winning Strategy for Full Duplex Coherent Optics
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.
There are two fundamental topologies to achieve bidirectional P2P coherent transport:
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.
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.
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.
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.
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.
It’s Only Wireless for THIS long
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.