I am not sure about channel spacing. You may refer to few threads here that mention channel spacing be it equal channel spacing or unequal channel spacing.
Here are few links. You may refer to them
http://link.springer.com/chapter/10.1007%2F978-1-4020-8741-7_1#page-1
https://www.researchgate.net/publication/261320396_Duobinary_modulation_format_and_unequal_channel_spacing_integration_to_suppress_four-wave_mixing_crosstalk_in_WDM_systems
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5876926&abstractAccess=no&userType=inst
Also go through these threads https://optiwave.com/forums/topic/bande-width/
Hope this will be useful.
Regards
Burhan