Top Ten
FEC in Optical Transmission
By Andrew
Schmitt
Interest in forward error correction (FEC) for optical data transport applications has surged particularly in SONET/SDH transport using wavelength division multiplexing (WDM). FEC is widely deployed in undersea cable applications to solve the difficult engineering challenge of an ocean-spanning optical link. The aggressive cost and capacity targets of terrestrial systems are forcing engineers to look for new techniques like FEC to increase the distance and capacity of optical transmission
equipment.
1. Sources of optical bit errors.
Optical links are affected primarily by two phenomena: optical attenuation and dispersion. Optical attenuation is the loss of optical energy that the signal experiences as it travels through optical fiber. In effect, the light transmitted gets darker the further it’s transmitted. Techniques such as WDM divide the output power of amplifiers among multiple channels, attenuating them further. If a 16-channel WDM system is converted to a
32-channel system, each of the signals will be half as strong as before, or more accurately, attenuated by 3 dB. The various bit error rates (BERs) of a link are directly proportional to the optical power of the received signal. Therefore, more attenuation due to WDM (or a longer transmit distance) shows up as an increase in the BER.
2. Dispersion.
Dispersion affects the signal by causing the optical signal to stretch and become less sharp. An ideal laser emits light at a single precise
wavelength; what actually occurs is a band of wavelengths centered on a desired frequency. The speed of light through a substance is a function of the wavelength. Thus, each of these wavelengths propagates through the fiber at a slightly different speed. This results in a wider pulse at the receiver than was transmitted, making the signal harder to resolve because the received data eye pattern is less open.
3. ASE noise.
Amplified spontaneous emission (ASE) produces noisy 1s in a signal passed
through one or more optical amplifiers. These noise effects become more pronounced when a signal becomes more attenuated (as a logic 1 could mistakenly become a logic 0, if noise corrupts the signal). ASE noise is the dominant noise source in optical networks, though nonlinear noise effects are increasingly dominant as bit rates and WDM channel densities increase.
4. The effects of bit errors.
Communication systems must be engineered to deliver a guaranteed error rate, such as 10
-12
in SONET/ SDH systems. The amount of noise in an optical communication channel has a direct effect on the BER, and more noise equals more bit errors. The amount of optical attenuation, dispersion, and noise introduced into a system due to wavelength spacings or transmit distances must be limited to the point where a particular BER can be guaranteed. If a communication channel’s BER is too high, the channel must be taken out of service.
5. FEC.
FEC improves the BER by
detecting and correcting bit errors in a digital communication channel. Extra information is included along with the signal in order to provide redundancy for correcting up to a certain amount of bit errors. For example, if someone in a conversation at a noisy restaurant is only able to hear every 10th word from the person speaking to them, they will probably still understand what is being said because extra information received during communication (from body language, for example) provides contextual clues to
the words that are missing or garbled. The message is clear even if the communication channel is not. This same thing is accomplished in the digital domain by the use of a FEC encoder and decoder.
6. How does it work?
FEC improves the bit-error characteristics of a digital communication link. Bit errors (not present at the transmitter) incurred during transmission are corrected at the receiver. Parties on either end of the link never see the bit errors take place. The encoder takes the
transmitted data and divides it into digital sequences of equal lengths that are mathematically transformed to generate a new and longer bit sequence called a code word. Each encoded bit sequence has an error signature (syndrome) that the decoder can use to provide clues about the true nature of the sent message. The data rate coming out of the encoder is higher than the data rate going into the encoder because it adds extra FEC bits to the original signal. It’s important to remember that the information
rate remains the same. The extra bits are just there for the benefit of the receiver and error decoder.
7. How are errors corrected?
The decoder receives the code word, and then evaluates the syndrome to check that the transmitted code word matches the received code word. If there is no match, a bit error must have occurred during transmission. The decoder uses the information in the syndrome to reverse engineer (and correct) the original code word. The mathematics involved allow the
decoder to detect if a bit error was received, compute the number of errors, and determine their location. The strength of the FEC algorithm determines how many errors can be corrected. If the error rate is high enough, some errors will slip through. The decoder fixes the bit errors and allows the receiver of the signal to believe that the rate of errors taking place is lower than it actually is.
8. In-band vs. out-of-band FEC.
Out-of-band FEC increases the data rate by appending extra
codes along with the protocol. In-band FEC adds the codes, but they are embedded during idle periods of the transmitted protocol. Synchronous protocols (such as SONET) have a determinate idle period, which limits the strength of the FEC algorithm (the number of errors that can be corrected before the decoder begins to fail). In-band FEC protocols have the advantage of not changing the serial bit rate or protocol and can be made interoperable with non-FEC systems. Out-of-band FEC provides increased
error-correction capability and is protocol-independent. This makes it a good choice for solving the problems of emerging optical networks.
9. Optical channel overhead.
Protocol independence is highly desired by optical network architects. They want the ability to send any type of protocol through their equipment while providing the performance-monitoring functions people have come to expect from traditional networks like SONET/SDH. The problem is the difficulty in implementing network
management in a system that can theoretically transmit any type of protocol. A means for monitoring link integrity and intra-equipment messaging is required — an optical channel overhead. This data can be added in as an extra byte when a signal is being encoded and stripped off once a signal is decoded. This allows an end-to-end method for allowing network management traffic, while being completely protocol-independent.
10. The value of FEC.
FEC allows an optical link to withstand
more optical attenuation, dispersion, and noise while delivering a BER that is identical to a link without FEC. This means a signal can be transmitted further, the wavelength spacing in a WDM system can be tighter, or fewer optical amplifiers can be used without compromising the quality of the link. Improving the BER will increase the amount of bandwidth carried, or decrease the cost of delivering a fixed amount of bandwidth, dramatically improving the economics of optical communications.
Andrew Schmitt is a telecom applications engineer at Vitesse Semiconductor Corp. He received his BSEE from the University of California at Santa Barbara. He can be reached at
schmitt@vitesse.com
.
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