Apparatus and methods for timing tone based transmitter skew alignment in an optical communication system
Apparatus and method for transmitter alignment in an optical communication system are provided. In certain configurations, a method of correcting for transmitter skew is provided. The method includes generating an optical signal using a transmitter based on an in-phase (I) component and a quadrature-phase (Q) component of a transmit signal, the optical signal having a baud rate that is based on a timing tone. The method further includes receiving the optical signal as an input to a receiver, and generating a signal vector representing the optical signal using the receiver. The signal vector includes an I component and a Q component. The method further includes calculating a power of the timing tone based on processing the signal vector using a tone power calculator of the receiver, and correcting for a skew of the transmitter based on the calculated power.
1. A transmitter error compensation device for a digital signal processing circuit in a receiver for an optical communications system, the device comprising:
a MIMO equalizer configured to receive a digital signal vector from an analog front-end and to generate a compensated digital vector, wherein the digital signal vector comprises a digital representation of an in-phase (I) component and a quadrature-phase (Q) component of an optical signal received from a transmitter;
wherein the MIMO equalizer is configured to compensate for a modulating biasing error of the transmitter;
a first SISO equalizer configured to generate an I component of the compensated digital vector;
a second SISO equalizer configured to generate a Q component of the compensated digital vector;
wherein the first and second SISO equalizers are configured to compensate for a transmit skew error of the transmitter;
an I signal path including the MIMO equalizer and the first SISO equalizer; and
a Q signal path including the MIMO equalizer and the second SISO equalizer.
2. The device of claim 1 further comprising a slicer configured to generate an output digital signal vector based on slicing the compensated digital signal vector.
3. The device of claim 2 further comprising a first digital adaption engine configured to control a plurality of coefficients of the first and second SISO equalizers based on a first error signal comprising a difference between the compensated digital signal vector and the output digital signal vector.
4. The device of claim 3 further comprising an error back propagation system configured to generate a second error signal based on the first error signal and the plurality of coefficients of the first and second equalizers.
5. The device of claim 4 further comprising a second digital adaption engine configured to control a plurality of coefficients of the MIMO equalizer based on the second error signal.
6. The device of claim 1 configured to adapt a plurality of coefficients of the first and second SISO equalizers based on a least mean squares (LMS) stochastic gradient algorithm.
7. The device of claim 1 wherein the digital signal vector is derived from an optical signal comprising a multi-level quadrature modulation (QAM) signal, a discrete multi-toned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM) signal, or a phase-shift keying (PSK) signal.
8. The device of claim 1 wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.
9. A method of operating an transmitter error compensation device for a digital signal processing circuit in a receiver for an optical communications systems, the method comprising:
receiving, from an analog front-end, a digital signal vector, wherein the digital signal vector comprises a digital representation of an in-phase (I) component and a quadrature-phase (Q) component of an optical signal received from a transmitter;
generating, by a MIMO equalizer, a compensated digital vector by compensating for a modulating biasing error of the transmitter;
generating, by a first SISO equalizer, an I component of the compensated digital vector, wherein the first SISO equalizer is configured in an I signal path with the MIMO equalizer;
generating, by a second SISO equalizer, a Q component of the compensated digital vector, wherein the second SISO equalizer is configured in a Q signal path with the MIMO equalizer; and
compensating, by the first and second SISO equalizers, for a transmit skew error of the transmitter.
10. The method of claim 9 further comprising generating, by a slicer, an output digital signal vector based on slicing the compensated digital signal vector.
11. The method of claim 10 further comprising controlling, by a first digital adaptation engine, a plurality of coefficients of the first and second SISO equalizers based on a first error signal comprising a difference between the compensated digital signal vector and the output digital signal vector.
12. The method of claim 11 further comprising generating, by an error back propagation system, a second error signal based on the first error signal and the plurality of coefficients of the first and second equalizers.
13. The method of claim 12 further comprising controlling, by a second digital adaptation engine, a plurality of coefficients of the MIMO equalizer based on the second error signal.
14. The method of claim 9 further comprising adapting a plurality of coefficients of the first and second SISO equalizers based on a least means square (LMS) stochastic gradient algorithm.
15. The method of claim 9 wherein the digital signal vector is derived from an optical signal comprising a multi-level quadrature modulation (QAM) signal, a discrete multi-toned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM) signal, or a phase-shift keying (PSK) signal.
16. The method of claim 9 wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.
17. A receiver for an optical communications system, the device comprising:
an analog front-end configured to convert an analog signal vector representing an optical signal into a digital signal vector, the optical signal having a baud rate that is based on a timing tone, wherein the digital signal vector comprises a digital representation of an in-phase (I) component and a quadrature-phase (Q) component of an optical signal received from a transmitter;
a digital signal processing circuit configured to process the digital signal vector to recover data from symbols of the optical signal, wherein the symbols change at the baud rate;
wherein the digital signal processing circuit further comprises a transmitter error compensation system configured to compensate the digital signal vector for at least one of a transmit skew error or a modulator biasing error of the transmitter;
wherein the transmitter error compensation system comprises:
a MIMO equalizer configured to receive the digital signal vector from the analog front-end and to generate a compensated digital vector;
wherein the MIMO equalizer is configured to compensate for a modulating biasing error of the transmitter;
a first SISO equalizer configured to generate an I component of the compensated digital vector;
a second SISO equalizer configured to generate a Q component of the compensated digital vector;
wherein the first and second SISO equalizers are configured to compensate for a transmit skew error of the transmitter;
an I signal path including the MIMO equalizer and the first SISO equalizer; and
a Q signal path including the MIMO equalizer and the second SISO equalizer.
18. The device of claim 17 further comprising a slicer configured to generate an output digital signal vector based on slicing the compensated digital signal vector.
19. The device of claim 18 further comprising a first digital adaption engine configured to control a plurality of coefficients of the first and second SISO equalizers based on a first error signal comprising a difference between the compensated digital signal vector and the output digital signal vector.
20. The device of claim 19 further comprising an error back propagation system configured to generate a second error signal based on the first error signal and the plurality of coefficients of the first and second equalizers.
21. The device of claim 20 further comprising a second digital adaption engine configured to control a plurality of coefficients of the MIMO equalizer based on the second error signal.
22. The device of claim 17 configured to adapt a plurality of coefficients of the first and second SISO equalizers based on a least mean squares (LMS) stochastic gradient algorithm.
23. The device of claim 17 wherein the digital signal vector is derived from an optical signal comprising a multi-level quadrature modulation (QAM) signal, a discrete multi-toned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM) signal, or a phase-shift keying (PSK) signal.
24. The device of claim 17 wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.