Reducing variance in reach of WDM channels in an optical network

1. A method for reducing variance in reach of wavelength division multiplexed (WDM) channels in optical transport networks, comprising:

selecting, for each of a plurality of WDM channels in an optical network, each assigned to a respective wavelength within a range of wavelengths, a respective initial modulation format with an initial distribution of constellation points in a complex plane;

determining a target reach for the plurality of WDM channels that is achievable by one or more WDM channels assigned to wavelengths in an upper portion of the range using their respective initial modulation formats but is not achievable by one or more WDM channels assigned to wavelengths in a lower portion of the range using their respective initial modulation formats, the wavelengths in the lower portion of the range being shorter than the wavelengths in the upper portion of the range;

applying one or more reach extension techniques to a given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range to extend its reach to at least the target reach;

refraining from applying reach extension techniques to the one or more WDM channels assigned to wavelengths in the upper portion of the range;

transmitting or receiving traffic over the given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range using its initial modulation format and using the one or more reach extension techniques applied to the given channel; and

transmitting or receiving traffic over the one or more WDM channels assigned to wavelengths in the upper portion of the range using their respective initial modulation formats and without using reach extension techniques.

2. The method of claim 1 , wherein determining the target reach comprises calculating an expected reach for a channel assigned at a wavelength within a center portion of the range between the upper portion of the range and the lower portion of the range.

3. The method of claim 1 , wherein applying the one or more reach extension techniques to the given channel comprises activating symbol rate optimized subcarrier multiplexing for the given channel.

4. The method of claim 3 , wherein applying the one or more reach extension techniques to the given channel further comprises:

selecting, dependent on characteristics of transmission media for the given channel and the target reach, a symbol rate for the given channel; and

determining, dependent on the selected symbol rate, a number of subcarriers for the given channel.

5. The method of claim 1 , further comprising applying one or more reach extension techniques to another one of the one or more WDM channels assigned to wavelengths in the lower portion of the range to extend its reach to at least the target reach.

6. The method of claim 5 , wherein at least one of the reach extension techniques applied to the other channel is different than the one or more reach extension techniques applied to the given channel.

7. The method of claim 1 , wherein applying the one or more reach extension techniques to the given channel comprises applying probabilistic constellation shaping to the given channel and activating symbol rate optimized subcarrier multiplexing for the given channel.

8. The method of claim 1 , further comprising configuring one or more transponders to:

transmit or receive traffic over the given channel using its initial modulation format and using the one or more reach extension techniques applied to the given channel; and

transmit or receive traffic over the one or more channels assigned to wavelengths in the upper portion of the range using their respective initial modulation formats and without using reach extension techniques.

9. The method of claim 1 , wherein applying the one or more reach extension techniques to the given channel comprises applying geometric shaping to the given channel.

10. An optical transport network for constellation shaping of modulation formats, the optical transport network comprising:

a plurality of wavelength division multiplexed (WDM) channels, each assigned to a respective wavelength within a range of wavelengths;

a network management system configured to:

select, for each of the plurality of WDM channels, a respective initial modulation format with an initial distribution of constellation points in a complex plane;

determine a target reach for the plurality of WDM channels that is achievable by one or more WDM channels assigned to wavelengths in an upper portion of the range using their respective initial modulation formats but is not achievable by one or more WDM channels assigned to wavelengths in a lower portion of the range using their respective initial modulation formats, the wavelengths in the lower portion of the range being shorter than the wavelengths in the upper portion of the range;

apply one or more reach extension techniques to a given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range to extend its reach to at least the target reach; and

refrain from applying reach extension techniques to the one or more WDM channels assigned to wavelengths in the upper portion of the range; and

one or more transponders configured to:

transmit or receive traffic over the given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range using its initial modulation format and using the one or more reach extension techniques applied to the given channel; and

transmit or receive traffic over the one or more WDM channels assigned to wavelengths in the upper portion of the range using their respective initial modulation formats and without using reach extension techniques.

11. The optical transport network of claim 10 , wherein to determine the target reach, the network management system is configured to calculate an expected reach for a channel assigned at a wavelength within a center portion of the range between the upper portion of the range and the lower portion of the range.

12. The optical transport network of claim 10 , wherein to apply the one or more reach extension techniques to the given channel, the network management system is configured to activate symbol rate optimized subcarrier multiplexing for the given channel.

13. The optical transport network of claim 12 , wherein to apply the one or more reach extension techniques to the given channel, the network management system is further configured to:

select, dependent on characteristics of transmission media for the given channel and the target reach, a symbol rate for the given channel; and

determine, dependent on the selected symbol rate, a number of subcarriers for the given channel.

14. The optical transport network of claim 10 , wherein the network management system is further configured to apply one or more reach extension techniques to another one of the one or more WDM channels assigned to wavelengths in the lower portion of the range to extend its reach to at least the target reach.

15. The optical transport network of claim 14 , wherein at least one of the reach extension techniques applied to the other channel is different than the one or more reach extension techniques applied to the given channel.

16. The optical transport network of claim 10 , wherein to apply the one or more reach extension techniques to the given channel, the network management system is configured to apply probabilistic constellation shaping to the given channel and to activate symbol rate optimized subcarrier multiplexing for the given channel.

17. A method for reducing variance in reach of wavelength division multiplexed (WDM) channels in optical transport networks, comprising:

selecting, for each of a plurality of WDM channels in an optical network, each assigned to a respective wavelength within a range of wavelengths, a respective initial modulation format with an initial distribution of constellation points in a complex plane;

determining a target reach for the plurality of WDM channels that is achievable by one or more WDM channels assigned to wavelengths in an upper portion of the range using their respective initial modulation formats but is not achievable by one or more WDM channels assigned to wavelengths in a lower portion of the range using their respective initial modulation formats, the wavelengths in the lower portion of the range being shorter than the wavelengths in the upper portion of the range;

applying a reach extension technique by applying probabilistic constellation shaping to a given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range to extend its reach to at least the target reach, the probabilistic shaping includes

selecting a higher order modulation format for the given channel than its initial modulation format, and

selecting, dependent on a target spectral efficiency for the given channel, a weak probabilistic shaping technique or a strong probabilistic shaping technique;

refraining from applying the reach extension technique to the one or more WDM channels assigned to wavelengths in the upper portion of the range;

transmitting or receiving traffic over the given one of the one or more WDM channels assigned to wavelengths in the lower portion of the range using the reach extension technique applied to the given channel; and

transmitting or receiving traffic over the one or more WDM channels assigned to wavelengths in the upper portion of the range using their respective initial modulation formats and without using the reach extension technique.