I/Q imbalance correction for an optical-transport system

Patent metadata

Field	Value
Publication number	US-9847841-B1
Application number	US-201615379154-A
Country	US
Kind code	B1
Filing date	Dec 14, 2016
Priority date	Dec 14, 2016
Publication date	Dec 19, 2017
Grant date	Dec 19, 2017

Abstract

A coherent optical receiver that is capable of obtaining separate estimates of the I/Q phase imbalances caused by the front-end circuits of the receiver and transmitter. In an example embodiment, the receiver's I/Q imbalance is estimated using equalization coefficients of a first digital equalizer located upstream from the carrier-recovery module in the train of digital-signal processing implemented at the receiver, whereas the transmitter's I/Q imbalance is estimated using equalization coefficients of a second digital equalizer located downstream from the carrier-recovery module. The receiver DSP can then use the first estimate to carry out signal processing that reduces adverse effects of the receiver's I/Q imbalance on data recovery at the receiver. The receiver can also provide the estimate of the transmitter's I/Q phase imbalance to the transmitter, which can then perform digital signal pre-distortion directed at compensating that I/Q imbalance at the transmitter.

First claim

What is claimed is:

1. An apparatus comprising:
a front-end circuit configured to mix an optical input signal and an optical local-oscillator signal to generate a first electrical digital signal and a second electrical digital signal, the first electrical digital signal being a digital measure of a first I component of the optical input signal, and the second electrical digital signal being a digital measure of a first Q component of the optical input signal; and
a digital signal processor configured to:
process the first and second electrical digital signals to recover data encoded onto the first I and Q components of the optical input signal;
generate an estimate of a first I/Q phase imbalance using one or more digital signals generated therein, the first I/Q phase imbalance being an I/Q imbalance caused by the front-end circuit; and
generate an estimate of a second I/Q phase imbalance using one or more digital signals generated therein, the second I/Q phase imbalance being an I/Q imbalance caused by an optical transmitter configured to apply the optical input signal to the front-end circuit.

2. The apparatus of claim 1 , wherein the digital signal processor is further configured to generate the estimate of the first I/Q phase imbalance in a frequency-dependent manner within a bandwidth of the optical input signal.

3. The apparatus of claim 2 , wherein the digital signal processor is further configured to generate the estimate of the second I/Q phase imbalance in a frequency-dependent manner within the bandwidth of the optical input signal.

4. The apparatus of claim 2 , wherein the digital signal processor is further configured to generate an estimate of a group delay corresponding to the first and second electrical digital signals in the front-end circuit.

5. The apparatus of claim 1 , wherein the digital signal processor is further configured to generate the estimate of the second I/Q phase imbalance in a frequency-dependent manner within a bandwidth of the optical input signal.

6. The apparatus of claim 5 , wherein the digital signal processor is further configured to generate an estimate of a group delay corresponding to the first I and Q components of the optical input signal at the optical transmitter.

7. The apparatus of claim 1 , wherein the digital signal processor is further configured to generate an estimate of a phase difference between the first and second electrical digital signals at a carrier frequency of the optical input signal.

8. The apparatus of claim 1 , wherein the digital signal processor is further configured to generate an estimate of a phase difference between the first I and Q components of the optical input signal at a carrier frequency of the optical input signal.

9. The apparatus of claim 1 ,
wherein the digital signal processor comprises a carrier-recovery module configured to compensate a non-zero frequency difference between a carrier frequency of the optical local-oscillator signal and a carrier frequency of the optical input signal; and
wherein the digital signal processor is further configured to:
generate the estimate of the first I/Q phase imbalance using one or more digital signals generated therein before the non-zero frequency difference is compensated by the carrier-recovery module; and
generate the estimate of the second I/Q phase imbalance using one or more digital signals generated therein after the non-zero frequency difference is compensated by the carrier-recovery module.

10. The apparatus of claim 9 , wherein:
the front-end circuit is further configured to mix the optical input signal and the optical local-oscillator signal to generate a third electrical digital signal and a fourth electrical digital signal, the third electrical digital signal being a digital measure of a second I component of the optical input signal, and the fourth electrical digital signal being a digital measure of a second Q component of the optical input signal;
the digital signal processor is further configured to process the third and fourth electrical digital signals to recover data encoded onto the second I and Q components of the optical input signal;
the first I and Q components of the optical input signal have a first polarization of light; and
the second I and Q components of the optical input signal having a second polarization of light that is different from the first polarization of light.

11. The apparatus of claim 10 , wherein the digital signal processor is further configured to:
generate an estimate of a third I/Q phase imbalance using one or more digital signals generated therein before the non-zero frequency difference is compensated by the carrier-recovery module, the third I/Q phase imbalance being another I/Q imbalance caused by the front-end circuit; and
generate an estimate of a fourth I/Q phase imbalance using one or more digital signals generated therein after the non-zero frequency difference is compensated by the carrier-recovery module, the fourth I/Q phase imbalance being another I/Q imbalance caused by the optical transmitter.

12. The apparatus of claim 11 ,
wherein the digital signal processor is further configured to generate the estimate of the third I/Q phase imbalance in a frequency-dependent manner within the bandwidth of the optical input signal; and
wherein the digital signal processor is further configured to generate the estimate of the fourth I/Q phase imbalance in a frequency-dependent manner within the bandwidth of the optical input signal.

13. The apparatus of claim 1 , wherein the digital signal processor comprises an I/Q-imbalance correction circuit configured to reduce an adverse effect of the first I/Q phase imbalance on data recovery.

14. The apparatus of claim 1 , wherein the digital signal processor is connectable to a control link and is configured to send out the estimate of the second I/Q phase imbalance by way of the control link.

15. The apparatus of claim 14 , further comprising the optical transmitter;
wherein the optical transmitter is connectable to the control link; and
wherein the optical transmitter comprises a digital pre-distortion circuit configured to reduce an adverse effect of the second I/Q phase imbalance on the first I and Q components of the optical input signal using the estimate of the second I/Q phase imbalance received from the digital signal processor by way of the control link.

16. The apparatus of claim 1 , wherein the digital signal processor comprises:
a carrier-recovery module configured to compensate a non-zero frequency difference between a carrier frequency of the optical local-oscillator signal and a carrier frequency of the optical input signal;
a first equalizer circuit configured to generate a first set of equalized digital signals corresponding to the first and second electrical digital signals and apply the first set of equalized digital signals to the carrier-recovery module; and
a first I/Q-imbalance estimation circuit operatively connected to the first equalizer circuit and configured to generate the estimate of the first I/Q phase imbalance using a first set of equalization coefficients, the first set of equalization coefficients being used by the first equalizer circuit to generate the first set of equalized digital signals.

17. The apparatus of claim 16 , wherein the first equalizer circuit is configured to determine the first set of equalization coefficients using one or both of a CMA algorithm and an MMA algorithm.

18. The apparatus of claim 16 , wherein the digital signal processor further comprises:
a second

Assignees

• Alcatel Lucent Usa Inc

Inventors

• Kaneda Noriaki

Classifications

• H04B10/65

  Intradyne, i.e. coherent receivers with a free running local oscillator having a frequency close but not phase-locked to the carrier signal · CPC title

• H04B10/616

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

• H04B10/5561

  Digital phase modulation · CPC title

• H04B10/548 Primary

  Phase or frequency modulation · CPC title

• H04B10/40

  Transceivers · CPC title