Apparatus and methods for digital signal constellation transformation
Apparatus and method for digital signal constellation transformation are provided herein. In certain configurations, an integrated circuit includes an analog front-end that converts an analog signal vector representing an optical signal into a digital signal vector, and a digital signal processing circuit that processes the digital signal vector to recover data from the optical signal. The digital signal processing circuit generates signal data representing a signal constellation of the digital signal vector. The digital signal processing circuit includes an adaptive gain equalizer that compensates the signal data for distortion of the signal constellation arising from biasing errors of optical modulators used to transmit the optical signal.
1. A method of operating an integrated circuit device, the method comprising:
receiving, by a digital signal processing circuit, a digital signal vector; and
generating, by the digital signal processing circuit, signal data representing a signal constellation of the digital signal vector; and
wherein generating the signal data comprises
generating, by an adaptive gain equalizer of the digital signal processing circuit, an estimate of an angular tilt of the signal constellation; and
generating, by the adaptive gain equalizer, transformed signal data by compensating the signal data for distortion of the signal constellation based on the estimate of the angular tilt.
2. The method of claim 1 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; and wherein the generating the estimate of the angular tilt includes generating, by the adaptive gain equalizer, the estimate of the angular tilt based on a sum of a plurality of cross-correlations of I and Q components of the signal constellation.
3. The method of claim 2 wherein the sum of the plurality of cross-correlations is computed over a moving window of data samples obtained from the digital signal vector.
4. The method of claim 1 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; and wherein the generating the signal data further comprises iteratively revising, by the adaptive gain equalizer, the estimate of the angular tilt until I and Q components of the transformed signal data are substantially uncorrelated.
5. The method of claim 1 wherein generating the signal data further comprises iteratively revising, by the adaptive gain equalizer, the estimate of the angular tilt based on a step gain, wherein the step gain changes based on a number of times the angular tilt has been estimated.
6. The method of claim 1 wherein generating the transformed signal data includes generating, by the adaptive gain equalizer, the transformed signal data based on a transformation matrix that includes the estimate of the angular tilt.
7. The method of claim 6 wherein the transformation matrix is about equal to
1
cos
2
θ
-
sin
2
θ
[
cos
θ
-
sin
θ
-
sin
θ
cos
θ
]
,
wherein θ is the estimate of the angular tilt.
8. The method of claim 1 wherein generating the transformed signal data comprises separately compensating, by the adaptive gain equalizer, for distortion of a first portion of the signal data associated with a horizontal polarization of an optical signal and for distortion of a second portion of the signal data associated with a vertical polarization of the optical signal.
9. The method of claim 1 further comprising slicing, by a decision slicer, the transformed signal data; and
processing, by the digital signal processing circuit, the signal data for at least one of feed forward equalization or carrier recovery prior to compensating the signal data for distortion of the signal constellation;
wherein the distortion of the signal constellation comprises at least one of squeezing, shifting, or tilting; and
wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.
10. The method of claim 1 wherein the digital signal vector comprises a digital representation of an optical signal; and wherein the optical signal comprises a multi-level quadrature amplitude modulation (QAM) signal, a discrete multitoned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM), or a phase-shift keying (PSK) signal.
11. A method of operating an integrated optical module device, the method comprising:
receiving, by an optical receiver, an optical signal from an optical cable;
generating, by the optical receiver, an analog signal vector representing the optical signal;
generating, by a transceiver, a digital signal vector from the analog signal vector; and
generating, by the transceiver, signal data representing a signal constellation of the digital signal vector;
wherein generating the signal data comprises
generating, by an adaptive gain equalizer of a digital signal processing circuit, an estimate of an angular tilt of the signal constellation; and
generating, by the adaptive gain equalizer, transformed signal data by compensating the signal data for distortion of the signal constellation based on the estimate of the angular tilt.
12. The method of claim 11 wherein the digital signal vector comprises a digital representation of an in-phase (I) component and a quadrature-phase (Q) component of the optical signal; and wherein the generating the estimate of the angular tilt includes generating, by the adaptive gain equalizer, the estimate of the angular tilt based on a sum of a plurality of cross-correlations of I and Q components of the signal constellation.
13. The method of claim 11 wherein the digital signal vector comprises a digital representation of an in-phase (I) component and a quadrature-phase (Q) component of the optical signal; and wherein the generating the signal data further comprises iteratively revising, by the adaptive gain equalizer, the estimate of the angular tilt until I and Q components of the transformed signal data are substantially uncorrelated.
14. The method of claim 11 wherein generating the transformed signal data comprises separately compensating, by the adaptive gain equalizer, for distortion of a first portion of the signal data associated with a horizontal polarization of an optical signal and for distortion of a second portion of the signal data associated with a vertical polarization of the optical signal.
15. The method of claim 11 wherein the optical signal comprises a multi-level quadrature amplitude modulation (QAM) signal, a discrete multitoned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM), or a phase-shift keying (PSK) signal.
16. The method of claim 11 wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.
17. A method of operating an optical communication system, the method comprising:
providing, by an optical transmitter, optical transmissions over an optical network;
receiving, by an optical receiver, an optical signal over the optical network;
generating, by the optical receiver, an analog signal vector representing the optical signal; and
converting, by an analog front-end of a transceiver, the analog signal vector into a digital signal vector;
generating, by a digital signal processing circuit of the transceiver, signal data representing a signal constellation of the digital signal vector;
wherein generating the signal data comprises
generating, by an adaptive gain equalizer of the digital signal processing circuit, an estimate of an angular tilt of the signal constellation; and
generating, by the adaptive gain equalizer, transformed signal data by compensating the signal data for distortion of the signal constellation based on the estimate of the angular tilt.
18. The method of claim 17 further comprising biasing, by an automatic bias controller of the optical transmitter, one or more modulators of the optical transmitter.
19. The method of claim 17 wherein the optical signal comprises a multi-level quadrature amplitude modulation (QAM) signal, a discrete multitoned (DMT) modulation signal, an orthogonal frequency division multiplexing (OFDM), or a phase-shift keying (PSK) signal.
20. The method of claim 17 wherein the digital signal vector comprises data representing a single polarization modulation or a dual-polarization modulation.