Adaptive constellations and decision regions for an optical transport system

An apparatus comprising an optical receiver configured to decode an optical input signal using a constellation, wherein:

• the constellation comprises at least a first constellation point and a second constellation point;
• the optical receiver is further configured to:
• decode a symbol received via the optical input signal using the first constellation point if the symbol falls within a first decision region;
• decode a symbol received via the optical input signal using the second constellation point if the symbol falls within a second decision region;
• measure a probability density function corresponding to the constellation based on a sequence of symbols received via the optical input signal;
• and
• define a shape of at least one of the first and second decision regions based on a thalweg of the probability density function;
• and
• the first decision region includes an area that is closer in distance to the second constellation point than to the first constellation point.