Fast estimation of chromatic dispersion

A receiver apparatus comprising:

• a communication interface configured to receive a communications signal conveying symbols at a symbol frequency fS;
• and
• circuitry configured to
• calculate a representation of the communications signal, wherein the representation comprises a first frequency band and a second frequency band which are disjoint and have non-zero correlation;
• calculate a first term comprising a function of a phase difference between the representation at a first pair of frequencies separated by a gap Δ and comprised within the first frequency band;
• calculate a second term comprising a function of a phase difference between the representation at a second pair of frequencies separated by the gap Δ and comprised within the second frequency band, wherein the higher frequency of the first pair and the higher frequency of the second pair are separated by a gap G;
• and
• calculate an estimate of chromatic dispersion (CD) in the communications signal based on the first term and the second term.