What is Channel Margin?

What is Channel Margin? : The Channel Margin (CM) is the difference, expressed if terms of Q-factor (dB), between the current measured BER before FEC-correction and the minimum BER before FEC-correction needed to guarantee an error-free transmission after FEC-correction.

When a link is commissioned or upgraded, all channels must be set at a channel margin level larger than the “Channel Margin target”, which is the CM value sufficient to compensate for all possible time-varying transmission impairments (such as polarization-mode dispersion, terminal stability, polarization-dependent loss, polarization-dependent gain…). In this way, it is possible to guarantee that the channel BER before FEC-correction will never decrease below the minimum required value.

For example, Alcatel Enhanced-SFEC requires a BER before FEC-correction equal to 4x10-3 to guarantee error-free transmission after FEC-correction. This BER corresponds to a Q-factor of 8.5dB. It can be demonstrated that a CM target of 3.1 dB is sufficient to compensate for the PMD and all the other time-varying effects. In case of particular customer requirements, the CM target used for the commissioning can be further increased to 4 or 5 dB.

Optical Channels Margin

Optical channels experience different transmission parameters along the optical line, which induce differences in transmission performances. The main wavelength-dependent optical parameters include:

➤ Fiber chromatic dispersion

➤ Amplifier noise figure

➤ Amplifier gain

➤ Nonlinear effects (SPM, XPM, FWM, …)

➤ Filtering shape

An adjustment based on the “channel margin”, optimizing the received BER for all channels at the same time, intrinsically accounts for all these effects on each different wavelength, and finds the best compromise between linear impairments (such as OSNR, dispersion, filtering…), which can be partially or totally compensated for by increasing the optical channel power, and the nonlinear impairments, which, conversely, increase with the power itself.

An adjustment based on the “channel margin” can be performed manually or automatically (through the APA algorithm).

At the commissioning, the manual adjustment is performed in two steps: during the first step, the channel power is set to a value such that its OSNR is around 13dB, so that the FEC-correction starts to be effective with a minimum margin. Then, the power of all the different channels is adjusted to reach the target channel margin for all the channels.

Similarly, the APA algorithm works in two steps: during the first, called “channel transmission recovery“, the power of a newly loaded channel is increased from the minimum power value to the point where FEC-correction becomes effective with a minimum margin. In the meanwhile, all other channels are controlled to avoid any impact on the pre-existing traffic.  Then, during the “fine-grain automatic pre-emphasis adjustment“ step, the power of the different channels is adjusted to reach the target channel margin for all the channels.

Is system based on channel margin?

After the previous description, it is clear that a system not based on channel margin but on a simple channel power equalization can experience, because of all the wavelength-dependent parameters listed before, significant differences in performances among the channels, limiting its domain of application to metro and LH distances.

Also a system equalized in terms of OSNR, even if able to cope with the differences of amplifier gain and noise figure, cannot provide optimized performances, not being able to account for the nontrivial impact of wavelength-dependent impairments such as chromatic dispersion and nonlinear effects. Besides, an automatic algorithm based on OSNR require expensive integrated OSNR-monitoring devices, and suffer severe limitations in the application domain: for example, the spectral filtering produced by mux/demux functions prevents from a correct OSNR estimation in links with OADMs.

Differently from these methods, an adjustment based on the channel margin measurements (manual or automatic) is based on the optimization of the fundamental criterion for Quality-Of-Service evaluation, which is the signal BER. As a consequence:

➤ it allows guarantying the required performance margins with respect to all the possible transmission impairments at the same time (whereas any other algorithm would risk to cancel the margins versus nonlinear effects while trying to increase the ones versus optical noise and other linear effects, or vice-versa);

➤ it allows minimizing the amplifier output power for the given number of channels (thus extending amplifier lifetime beyond specifications), when an excess output power is not needed to guarantee the required performances;

➤ it allows optimizing the actual BER for all the channels simultaneously, bringing the best transmission performance out of a DWDM system.

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