Clock recovery method and clock recovery arrangement for coherent polarization multiplex receivers
Component signal values are derived from component signals and fed to at least one fixed equalizer which generates equalizer output signals. The signals are fed to phase error detectors generating phase error signals. The phase error signals are combined with further phase error signals derived by further error detectors receiving signal values from further equalizers and/or the component signal values directly from sample units.
1. Clock recovery method for coherent polarization multiplex receivers, comprising the steps of
coherent demodulating a received polarization multiplex signal and deriving orthogonal signal components,
sampling and converting the orthogonal signal components into digital component values,
feeding said digital component values to a same equalizer, said equalizer being configured to effectively reverse the effect of polarization mixing of the received orthogonal signals for one specific polarization mixing angle,
deriving a phase error value from output values of said equalizer and another phase error value from the digital component values,
calculating resulting phase error values from at least two of the derived phase error values,
deriving an oscillator control signal from said resulting phase error values, and
controlling at least one controllable oscillator to generate a sample signal for sampling said orthogonal signal components or re-sample the digital component values.
2. Clock recovery method according to claim 1 ,
wherein said resulting phase error values are calculated by combining phase error values directly derived from the digital component values with phase error values derived from the equalizer output values.
3. Clock recovery method according to claim 1 , further comprising:
generating different resulting phase error values for two separate clock recovery loops, and
generating two sample signals, each for sampling the orthogonal signal components or re-sampling the digital component values.
4. Clock recovery method according to claim 1 , comprising the step of
generating phase error values according to
X PE1 [n]=XI Ei [n− ½]( XI Ei [n]−XI Ei [n− 1])+ XQ Ei [n− ½]( XQ Ei [n]−XQ Ei [n− 1]), Y PE1 [n]=YI Ei [n− ½]( YI Ei [n]−YI Ei [n− 1])+ YQ Ei [n− ½]( YQ Ei [n]−YQ Ei [n− 1])
wherein XI Ei , XQ Ei ; YI Ei , YQ Ei resemble equalizer output values or component signal values; n is an integer resembling a sample instant; and i=1, 2, . . . , N.
5. Clock recovery arrangement for a coherent polarization multiplex receiver with at least one phase locked loop deriving a sample signal, including a phase error detector unit receiving sampled component values, a loop filter, and a controllable oscillator, comprising
a fixed equalizer receiving sampled component values and outputting equalizer output values, said equalizer being configured to effectively reverse the effect of polarization mixing of the received orthogonal signals for one specific polarization mixing angle,
at least one phase error detector receiving equalizer output values and generating at least one phase error value,
at least one phase error detector receiving sampled component values and generating at least one phase error value, and
means for combining said phase error values to derive resultant phase error values for controlling at least one controllable oscillator of the at least one phase locked loop.
6. The clock recovery arrangement according to claim 5 , comprising
a first phase error detector unit generating a first control signal,
a first phase locked loop receiving said first control signal and generating a first sample signal sampling x signal components, and
a second phase error detector unit generating a second control signal, and
a second phase locked loop receiving said second control signal and generating a second sample signal sampling y signal components.