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

Patent metadata

Field	Value
Publication number	US-11277207-B1
Application number	US-202017008402-A
Country	US
Kind code	B1
Filing date	Aug 31, 2020
Priority date	Aug 31, 2020
Publication date	Mar 15, 2022
Grant date	Mar 15, 2022

Abstract

Methods and apparatuses for IQ time skew calibration in a coherent transceiver are described. A four-channel signal is received. A set of inputs is constructed for a 4×8 MIMO equalizer by converting the four-channel signal into four complex inputs that each have a phase shift corresponding to an estimated carrier frequency offset. The set of inputs further includes conjugate replicas of the four complex inputs. Using output from the 4×8 MIMO equalizer, equalizer coefficients are calculated by minimizing error between the MIMO output and a reference signal. Receiver and transmitter IQ skew are estimated using the equalizer coefficients, by converting the equalizer coefficients form the time domain to the frequency domain to determine receiver and transmitter IQ differential phase responses, which are indicative of respective receiver and transmitter IQ skew in the time domain. Skew compensation is then performed.

First claim

The invention claimed is:

1. An apparatus comprising a receiver having a processor configured 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.

2. The apparatus of claim 1 , wherein the set of equalizer coefficients is calculated using a least squares algorithm to minimize the error between the MIMO output and the reference signal.

3. The apparatus of claim 1 , wherein the set of equalizer coefficients is calculated using an adaptive least mean square algorithm to minimize the error between the MIMO output and the reference signal.

4. The apparatus of claim 1 , wherein the set of inputs is constructed by:
multiplying the Q-channel signals of the four-channel signal by imaginary number j, to obtain imaginary-valued Q-channel signals;
pointwise multiplying each of real-valued I-channel signals and the imaginary-valued Q-channel signals with a carrier frequency offset phasor generated using the estimated carrier frequency offset, to obtain the four complex inputs; and
applying a conjugate operation to each of the four complex inputs to obtain the respective conjugate replicas.

5. The apparatus of claim 1 , wherein the receiver IQ skew and the transmitter IQ skew are estimated by:
converting the equalizer coefficients from the time domain to the frequency domain;
determining the receiver IQ differential phase response and the transmitter IQ differential phase response for each polarization; and
using a slope of the respective differential phase response as the estimated receiver IQ skew and the estimated transmitter IQ skew for each polarization.

6. The apparatus of claim 1 , wherein the set of equalizer coefficients is further used to estimate receiver XY skew and transmitter XY skew, and wherein the estimated receiver XY skew and the estimated transmitter XY skew are further used to perform skew compensation.

7. The apparatus of claim 6 , wherein the receiver XY skew and the transmitter XY skew are estimated by:
converting the equalizer coefficients from the time domain to the frequency domain;
determining the receiver XY phase difference and the transmitter XY phase difference; and
using a slope of the respective phase difference as the estimated receiver XY skew and the estimated transmitter XY skew.

8. The apparatus of claim 1 , wherein performing skew compensation comprises:
transmitting the estimated transmitter IQ skew to another apparatus.

9. The apparatus of claim 1 , wherein performing skew compensation comprises real-time equalization using the 4×8 MIMO equalizer.

10. 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.

11. The method of claim 10 , wherein the set of equalizer coefficients is calculated using a least squares algorithm to minimize the error between the MIMO output and the reference signal.

12. The method of claim 10 , wherein the set of equalizer coefficients is calculated using an adaptive least mean square algorithm to minimize the error between the MIMO output and the reference signal.

13. The method of claim 10 , wherein the set of inputs is constructed by:
multiplying the Q-channel signals of the four-channel signal by imaginary number j, to obtain imaginary-valued Q-channel signals;
pointwise multiplying each of real-valued I-channel signals and the imaginary-valued Q-channel signals with a carrier frequency offset phasor generated using the estimated carrier frequency offset, to obtain the four complex inputs; and
applying a conjugate operation to each of the four complex inputs to obtain the respective conjugate replicas.

14. The method of claim 10 , wherein the receiver IQ skew and the transmitter IQ skew are estimated by:
converting the equalizer coefficients from the time domain to the frequency domain;
determining the receiver IQ differential phase response and the transmitter IQ differential phase response for each polarization; and
using a slope of the respective differential phase response as the estimated receiver IQ skew and the estimated transmitter IQ skew for each polarization.

15. The method of claim 10 , wherein the set of equalizer coefficients is further used to estimate receiver XY skew and transmitter XY skew, and wherein the estimated receiver XY skew and the estimated transmitter XY skew are further used to perform skew compensation.

16. The method of claim 15 , wherein the receiver XY skew and the transmitter XY skew are estimated by:
converting the equalizer coefficients from the time domain to the frequency domain;
determining the receiver XY phase difference and the transmitter XY phase difference; and
using a slope of the respective phase difference as the estimated receiver XY skew and the estimated tra

Assignees

• Ng Wing Chau
• Tang Xuefeng
• Zhang Zhuhong
• Huawei Tech Co Ltd

Inventors

• Ng Wing Chau
• Tang Xuefeng
• Zhang Zhuhong

Classifications

• H04L25/03968 Primary

  mean-square error [MSE] · CPC title

• H04B7/0413

  MIMO systems · CPC title

• H04B10/616

  Details of the electronic signal processing in coherent optical receivers · CPC title

• H04L2025/03426

  transmission using multiple-input and multiple-output channels · CPC title

• H04B10/613

  including phase diversity, e.g., having in-phase and quadrature branches, as in QPSK coherent receivers · CPC title