A polarization recovery apparatus, a method thereof and an optical receiver. The method includes: performing adaptive equalization processing and polarization recovery on a received signal, wherein a polarization state of the received signal, after the adaptive equalization processing and polarization recovery being performed, is aligned with a principal axis of polarization of an optical receiver.

A polarization change tracking apparatus, a processing apparatus for a received signal and methods thereof. The polarization change tracking method includes: estimating a polarization change speed in a link according to a received signal; setting a response coefficient of polarization tracking according to a relationship between an estimated polarization change speed and a predetermined value, to make a response of polarization tracking and a response of adaptive equalization be consecutive; and performing compensation for polarization change on the received signal according to a set response coefficient of polarization tracking.

A digital receiver is configured to process a polarization multiplexed carrier from a communication network. The polarization multiplexed carrier includes a first polarization and a second polarization. The receiver includes a first lane for transporting a first input signal of the first polarization, a second lane for transporting a second input signal of the second polarization, a dynamic phase noise estimation unit disposed within the first lane and configured to determine a phase noise estimate of the first input signal, a first carrier phase recovery portion configured to remove carrier phase noise from the first polarization based on a combination of the first input signal and a function of the determined phase noise estimate, and a second carrier phase recovery portion configured to remove carrier phase noise from the second polarization based on a combination of the second input signal and the function of the determined phase noise estimate.

An adaptive equalizer (70) according to this invention includes an adaptive equalization filter (71) configured to adaptively compensate for a waveform distortion caused by a polarization fluctuation of a received signal (61) by updating a tap coefficient, a first tap coefficient updater (72) configured to calculate the tap coefficient according to the polarization fluctuation of the received signal (61) using a variable step size and update the tap coefficient of the adaptive equalization filter (71), a second tap coefficient updater (73) configured to calculate the tap coefficient according to the polarization fluctuation of the received signal (61) using a fixed step size μ.sub.0, a polarization state estimator (74) configured to estimate a polarization state of the received signal (61) using the tap coefficient calculated by the second tap coefficient updater (73), and a step size updater (75) configured to obtain the step size corresponding to the polarization state estimated by the polarization state estimator (74) and update the variable step size. According to this invention, it is possible to provide an adaptive equalizer that always implements stable followability to various SOP fluctuations.

Disclosed is a receiver for receiving an optical signal comprising a plurality of carriers within a predetermined frequency band. The receiver comprises means for sampling and converting each of the carriers into a set of corresponding digital signals, and a digital processing unit for processing said digital signals of said set of digital signals such as to mitigate transmission impairments of the corresponding optical carriers based on corresponding processing parameters. The digital processing unit is configured for determining such processing parameters by carrying out a corresponding parameter derivation procedure based on one of the digital signals of said set of digital signals. The processing unit is configured for sharing thus determined processing parameters for processing of other digital signals among said set of digital signals based on said shared determined processing parameters, or processing parameters derived from said shared determined processing parameters.

The present disclosure provides example method for sending a signal in optical communication, an example optical transmitter, and an example optical communication system. One example method for sending a signal in optical communication includes generating, by an optical transmitter, multiple first subcarriers based on a bit stream. State of polarization (SOP) rotation is performed by the optical transmitter on at least one subcarrier in the multiple first subcarriers to generate multiple second subcarriers, wherein the multiple second subcarriers comprise multiple subcarriers each with a relative SOP rotation angle, and the relative SOP rotation angle is not zero and is not an integer multiple of 90 degrees. The multiple second subcarriers are modulated to an optical signal. The modulated optical signal is sent by the optical transmitter.

A digital coherent receiver includes: an adaptive equalizer configured to execute, using a first tap coefficient, adaptive equalization processing on a digital signal that corresponds to a signal; a first coefficient updating unit configured to update the first tap coefficient based on the digital signal on which the adaptive equalization processing has not been executed, the digital signal on which the adaptive equalization processing has been executed, and a first step size; a second coefficient updating unit configured to update a second tap coefficient based on the digital signal on which the adaptive equalization processing has not been executed, the digital signal on which the adaptive equalization processing has been executed, and a second step size; and a control unit configured to detect a fluctuation speed of a state of polarization of the digital signal based on the second tap coefficient, and change the first tap coefficient to the updated second tap coefficient if it is determined that the fluctuation speed is higher than or equal to a speed threshold.

A method and apparatus for monitoring the change of state of polarization and a receiver. The method for monitoring the change of state of polarization includes: extracting a matrix of equalization filter tap coefficients of a receiver; performing an FFT operation of N taps on each path of equalization filter tap coefficients of the matrix to obtain a converted matrix, a sum of each path of equalization filter tap coefficients of the converted matrix being a value of non-zero frequency; and estimating an amount of change of state of polarization by using at least one path of equalization filter tap coefficients of the converted matrix. Thus, as the sum of the tap coefficients is shifted in the frequency domain from the zero frequency to a higher frequency (non-zero frequency), an effect of imperfection of the transmitter on a monitoring result of the change of state of polarization may be avoided.

A transmitter maps an N-bit sequence to a point selected from a four-dimensional (4D) constellation consisting of 2.sup.N points which form a subset of a Cartesian product of first and second two-dimensional (2D) constellations, the first constellation consisting of M1 points divided into first, second, and third points, and the second constellation consisting of M2 points divided into fourth, fifth, and sixth points, wherein M1, M2≥5, and wherein log.sub.2(M1×M2)>N. The subset includes any 4D point that is generated by combining any one of the M1 points and any one of the fourth points; includes any 4D point that is generated by combining any one of the first points and any one of M2 points; and excludes any 4D point that is generated by combining any third point and any sixth point. An optical signal representing the selected point is transmitted to a receiver.

A transmitter maps an N-bit sequence to a point selected from a four-dimensional (4D) constellation consisting of 2.sup.N points which form a subset of a Cartesian product of first and second two-dimensional (2D) constellations, the first constellation consisting of M1 points divided into first, second, and third points, and the second constellation consisting of M2 points divided into fourth, fifth, and sixth points, wherein M1, M2≥5, and wherein log.sub.2(M1×M2)>N. The subset includes any 4D point that is generated by combining any one of the M1 points and any one of the fourth points; includes any 4D point that is generated by combining any one of the first points and any one of M2 points; and excludes any 4D point that is generated by combining any third point and any sixth point. An optical signal representing the selected point is transmitted to a receiver.