Apparatus and method for in-phase and quadrature skew calibration in a coherent transceiver

A method, comprising:

• receiving a measured signal, the measured signal being a four-channel signal corresponding to in-phase (I) and quadrature (Q) channels for each of two orthogonal polarizations;
• estimating a carrier frequency offset between the measured signal and a reference signal;
• constructing a set of inputs for a 4×8 multiple-input multiple-output (MIMO) equalizer implemented by the receiver, the set of inputs being constructed by converting the four-channel signal into four complex inputs, the Q-channel signals of the four-channel signal being converted to imaginary inputs and each of the four complex inputs having a phase shift corresponding to the estimated carrier frequency offset, the set of inputs including the four complex inputs and further including respective conjugate replicas of the four complex inputs;
• using MIMO output from the 4×8 MIMO equalizer, calculating a set of equalizer coefficients by minimizing error between the MIMO output and the reference signal;
• estimating receiver IQ skew and transmitter IQ skew using the equalizer coefficients, by converting the set of equalizer coefficients from a time domain to a frequency domain to determine receiver IQ differential phase response and transmitter IQ differential phase response in the frequency domain, the receiver and transmitter IQ differential phase response being indicative of respective receiver and transmitter IQ skew in the time domain;
• and
• performing skew compensation using the estimated receiver IQ skew and estimated transmitter IQ skew.

A non-transitory computer-readable medium having instructions thereon, wherein the instructions, when executed by a processor of a receiver, cause the receiver to:

• receive a measured signal, the measured signal being a four-channel signal corresponding to in-phase (I) and quadrature (Q) channels for each of two orthogonal polarizations;
• estimate a carrier frequency offset between the measured signal and a reference signal;
• construct a set of inputs for a 4×8 multiple-input multiple-output (MIMO) equalizer implemented by the receiver, the set of inputs being constructed by converting the four-channel signal into four complex inputs, the Q-channel signals of the four-channel signal being converted to imaginary inputs and each of the four complex inputs having a phase shift corresponding to the estimated carrier frequency offset, the set of inputs including the four complex inputs and further including respective conjugate replicas of the four complex inputs;
• using MIMO output from the 4×8 MIMO equalizer, calculate a set of equalizer coefficients by minimizing error between the MIMO output and the reference signal;
• estimate receiver IQ skew and transmitter IQ skew using the equalizer coefficients, by converting the set of equalizer coefficients from a time domain to a frequency domain to determine receiver IQ differential phase response and transmitter IQ differential phase response in the frequency domain, the receiver and transmitter IQ differential phase response being indicative of respective receiver and transmitter IQ skew in the time domain;
• and
• perform skew compensation using the estimated receiver IQ skew and estimated transmitter IQ skew.