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.