Understanding Q-Factor in Optical Communications
What is Q-Factor?
Q is the quality of a communication signal and is related to BER (Bit Error Rate). A lower BER gives a higher Q, and thus a higher Q indicates better performance. Q is primarily used for translating relatively large BER differences into manageable values.
Key Relationship: The Q-Factor is a metric to identify the attenuation in the receiving signal and determine a potential loss of signal (LOS). It provides an estimate of the Optical Signal-to-Noise Ratio (OSNR) at the optical receiver.
As attenuation in the receiving signal increases, the dBQ value drops and vice-versa. Hence, a drop in the dBQ value can mean that there is an increase in the Pre-FEC BER, and a possible LOS could occur if the problem is not corrected in time.
dBQ and FEC Integration
Pre-FEC signal fail and Pre-FEC signal degrade thresholds are provisionable in units of dBQ so that the user does not need to worry about FEC scheme when determining what value to set the thresholds to. The software will automatically convert the dBQ values to FEC corrections per time interval based on FEC scheme and data rate.
Forward Error Correction (FEC) Process
The quality of an optical Rx signal can be measured by determining the number of "bad" bits in a block of received data. The bad bits in each block of received data are removed and replaced with "good" zeros or ones such that the network path data can still be properly switched and passed on to its destination.
This strategy is referred to as Forward Error Correction (FEC) and prevents a complete loss of traffic due to small unimportant data-loss that can be resent again later. The process by which the "bad" bits are replaced with the "good" bits in an Rx data block is known as Mapping.
Pre-FEC vs Post-FEC
Pre-FEC Counts: The FEC counts of "bad" bits before the Mapper
Post-FEC Counts: The FEC counts after the Mapper has corrected the errors
Monitoring Network Health
The number of Pre-FEC counts for a given period of time can represent the status of the optical Rx network signal. An increase in the Pre-FEC count means that there is an increase in the number of "bad" bits that need to be replaced by the Mapper. Hence, a change in rate of the Pre-FEC count (Bit Error Rate - BER) can identify a potential problem upstream in the network.
Critical Threshold: At some point, the Pre-FEC count will be too high as there will be too many "bad" bits in the incoming data block for the Mapper to replace—this will then mean a Loss of Signal (LOS).
Why Use dBQ Values?
As the normal number of Pre-FEC counts are high (i.e., 1.35E-3 to 6.11E-16) and constantly fluctuate, it can be difficult for a network operator to determine whether there is a potential problem in the network. Hence, a dBQ value, known as the Q-Factor, is used as a measure of the quality of the receiving optical signal. It should be consistent with the Pre-FEC count Bit Error Rate (BER).
Q-Factor Definitions
The standards define the Q-Factor as:
where Xj and Nj are the mean and standard deviation of the received mark-bit (j=1) and space-bit (j=0).
Example: The linear Q range 3 to 8 covers the BER range of 1.35E-3 to 6.11E-16.
dBQ Calculation Standards
Nortel defines dBQ as:
where Qref is the pre-FEC raw optical Q, which gives a BER of 1E-15 post-FEC assuming a particular error distribution.
Important Note: Some organizations define dBQ as 20·log₁₀(Q/Qref), so care must be taken when comparing dBQ values from different sources.
The dBQ figure represents the dBQ of margin from the pre-FEC BERs (which are equivalent to a post-FEC BER of 1E-15). The equivalent linear Q value for these BERs are Qref in the above formula.
Practical Application: Signal Degrade Warning
Pre-FEC Signal Degrade: This can be used the same way a car has an "oil light" in that it states that there is still margin left but you are closer to the fail point than expected, so action should be taken.
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