Impairment compensation techniques for high performance coherent optical transceivers
A method and structure for compensation techniques in coherent optical receivers. The present invention provides a coherent optical receiver with an improved 8×8 adaptive MIMO (Multiple Input, Multiple Output) equalizer configured within a digital signal processor (DSP) to compensate the effects of transmitter I/Q skew in subcarrier multiplexing (SCM) schemes. The 8×8 MIMO equalizer can be configured such that each of the 8 outputs is electrically coupled to 3 of 8 inputs, wherein each of the input-output couplings is configured as a filter. The method includes compensating for impairments to the digital conversion of an optical input signal via the 8×8 MIMO equalizer following other signal processing steps, such as chromatic dispersion (CD)/polarization-mode dispersion (PMD) compensation, carrier recovery, timing synchronization, and cycle slip correction.
1. A digital signal processor (DSP) device configured within a coherent optical receiver, the device comprising:
a first subcarrier path configured to receive an optical input signal and to output 4 component baseband signals;
a second subcarrier path configured to receive the optical input signal input signal and to output 4 component baseband signals; and
an 8×8 MIMO (Multiple Input, Multiple Output) equalizer electrically coupled to the first subcarrier path and the second carrier path, the 8×8 MIMO equalizer having 8 inputs and 8 outputs;
wherein the 8×8 MIMO equalizer is characterized by the following operation:
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where ⊗ denotes sum of convolution, f ij is an impulse response of the equalizer corresponding to an i-th output due to a j-th input, where δ denotes a delayed impulse function, s x and s y are the component baseband signals for a first subcarrier and a second subcarrier, and r x and r y are component baud-rate samples for the first subcarrier and the second subcarrier.
2. The device of claim 1 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by a plurality of digital filters between the 8 inputs and the 8 outputs of the 8×8 MIMO equalizer.
3. The device of claim 1 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by using 16 digital filters between the inputs and outputs of the 8×8 MIMO equalizer and a tap configured to each of the 8 outputs.
4. The device of claim 1 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by using 24 digital filters between the inputs and outputs of the 8×8 MIMO equalizer such that each of the 8 outputs is coupled to 3 of the inputs according to the operation.
5. The device of claim 1 wherein the 8×8 MIMO equalizer is configured to operate at symbol rate.
6. The device of claim 1 further comprising
a chromatic dispersion (CD)/polarization-mode dispersion (PMD) equalizer module;
a carrier recovery module;
a timing synchronization module; and
a cycle slip correction module;
wherein the 8×8 MIMO equalizer is configured following the CD/PMD equalizer module, the carrier recovery module, the timing synchronization module, and the cycle slip correction module.
7. A multi-subcarrier coherent optical receiver device having at least a first subcarrier and a second subcarrier, the device comprising:
a front-end module configured to receive an input signal;
an analog-to-digital converter (ADC) electrically coupled to the front-end module; and
a digital signal processor (DSP) electrically coupled to the ADC, wherein the DSP includes
a chromatic dispersion (CD)/polarization-mode dispersion (PMD) equalizer module;
a carrier recovery module;
a timing synchronization module;
a cycle slip correction module; and
an 8×8 MIMO (Multiple-Input, Multiple-Output) equalizer;
wherein the 8×8 MIMO equalizer is characterized by the following operation:
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⊗
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where ⊗ denotes sum of convolution, f ij is an impulse response of the equalizer corresponding to an i-th output due to a j-th input, where δ denotes a delayed impulse function, s x and s y are component baseband signals for the first subcarrier and the second subcarrier, and r x and r y are component baud-rate samples for the first subcarrier and the second subcarrier; and
wherein the 8×8 MIMO equalizer is configured following the CD/PMD equalizer module, the carrier recovery module, the timing synchronization module, and the cycle slip correction module.
8. The device of claim 7 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by a plurality of digital filters between the 8 inputs and the 8 outputs of the 8×8 MIMO equalizer.
9. The device of claim 7 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by using 16 digital filters between the inputs and outputs of the 8×8 MIMO equalizer and a tap configured to each of the 8 outputs.
10. The device of claim 7 wherein the operation characterizing the 8×8 MIMO equalizer is implemented by using 24 digital filters between the inputs and outputs of the 8×8 MIMO equalizer such that each of the 8 outputs is coupled to 3 of the inputs according to the operation.
11. The device of claim 7 wherein the 8×8 MIMO equalizer is configured to operate at symbol rate.
12. A method of impairment compensation in a multi-subcarrier coherent optical receiver device having at least a first subcarrier and a second subcarrier, the method comprising:
receiving, by a front-end module, an optical input signal;
converting, by an analog-to-digital converter (ADC) electrically coupled to the front-end module, the optical input signal to a digital input signal;
processing, by a digital signal processor (DSP) electrically coupled to the ADC, the digital input signal; and
compensating, by an 8×8 MIMO equalizer within the DSP, the digital input signal;
wherein the 8×8 MIMO equalizer is characterized by the following operation:
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⊗
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,
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]
where ⊗ denotes sum of convolution, f ij is an impulse response of the equalizer corresponding to an i-th output due to a j-th input, where δ denotes a delayed impulse function, s x and s y are component baseband signals for the first subcarrier and the second subcarrier, and r x and r y are component baud-rate samples for the first subcarrier and the second subcarrier.
13. The method of claim 12 wherein processing, by the DSP, the digital input signal comprises compensating, by a chromatic dispersion (CD)/polarization-mode dispersion (PMD) equalizer, the digital input signal.
14. The method of claim 12 wherein processing, by the DSP, the digital input signal comprises compensating, by a carrier recovery module, the digital input signal.
15. The method of claim 12 wherein processing, by the DSP, the digital input signal comprises correcting, by a timing synchronization module, the digital input signal.
16. The method of claim 12 wherein processing, by the DSP, the digital input signal comprises synchronizing, by a cycle slip correction module, the digital input signal.
17. The method of claim 12 wherein the 8×8 MIMO equalizer is configured to operate at symbol rate.