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EP-4362356-B1 - OPTICAL TRANSMISSION SYSTEM, TRANSMISSION APPARATUS, RECEPTION APPARATUS, AND DEVICE CHARACTERISTIC ESTIMATION METHOD

EP4362356B1EP 4362356 B1EP4362356 B1EP 4362356B1EP-4362356-B1

Inventors

  • NAKAMURA MASANORI
  • SASAI TAKEO
  • YAMAZAKI ETSUSHI
  • KISAKA YOSHIAKI

Dates

Publication Date
20260506
Application Date
20210623

Claims (7)

  1. An optical transmission system comprising: a transmission apparatus; and a receiving apparatus, wherein: the transmission apparatus includes a pre-equalization unit (23) configured to pre-equalize transmission signals by using a compensation filter, a digital-to-analog conversion unit (24) configured to convert the transmission signals pre-equalized by the pre-equalization unit (23) from digital signals into analog signals, and an output unit configured to convert the transmission signals converted by the digital-to-analog conversion unit (24) from electrical signals into optical signals and output the converted optical signals; the receiving apparatus includes a receiving unit configured to receive the optical signals output by the output unit and convert the received optical signals into electrical signals, an analog-to-digital conversion unit (32) configured to convert the received optical signals converted by the receiving unit from analog signals into digital signals, and a demodulation unit (33) configured to demodulate the digital signals converted by the analog-to-digital conversion unit (32); and the optical transmission system further includes a scale estimation unit (34) configured to calculate a slope near an amplitude 0 in a correlation between an amplitude distribution of the transmission signals pre-equalized by the pre-equalization unit (23) and an amplitude distribution of the digital signals converted by the analog-to-digital conversion unit (32), a scale unit (35,61) configured to scale an amplitude of the digital signals converted by the analog-to-digital conversion unit (32) on the basis of the slope calculated by the scale estimation unit (34), a coefficient estimation unit (36) configured to calculate a filter coefficient of the compensation filter on the basis of the transmission signals pre-equalized by the pre-equalization unit (23) and the digital signals scaled by the scale unit (35,61), and an output amplitude level determination unit (26, 26a, 26b) configured to determine_an output level of the transmission signals from the digital-to-analog conversion unit (24) on the basis of a correlation between a distribution of amplitudes of the transmission signals input to the pre-equalization unit (23) and a distribution of amplitudes of the transmission signals pre-equalized by the pre-equalization unit (23).
  2. A transmission apparatus comprising: a pre-equalization unit (23) configured to pre-equalize_transmission signals by using a compensation filter; a digital-to-analog conversion unit (24) configured to convert the transmission signals pre-equalized by the pre-equalization unit (23) from digital signals into analog signals; an output unit configured to convert the transmission signals converted by the digital-to-analog conversion unit (24) from electrical signals into optical signals and output the converted optical signals; and an output amplitude level determination unit (26, 26a, 26b) configured to determine output levels of the transmission signals output from the digital-to-analog conversion unit (24) on the basis of a correlation between a distribution of amplitudes of the transmission signals input to the pre-equalization unit (23) and a distribution of amplitudes of the transmission signals pre-equalized by the pre-equalization unit (23).
  3. The transmission apparatus according to claim 2, wherein: a receiving apparatus that has received the optical signals output from the output unit is configured to convert the received optical signals into electrical signals of analog signals and scale the received optical signals on the basis of a slope near an amplitude 0 in a correlation between an amplitude distribution of the transmission signals pre-equalized by the pre-equalization unit (23) and an amplitude distribution of the received optical_signals; and the pre-equalization unit (23) is configured to set, to the compensation filter, a filter coefficient calculated on the basis of the transmission signals pre-equalized by the pre-equalization unit (23) and the received optical signals scaled.
  4. The transmission apparatus according to claim 2 or 3, further comprising a clipping unit (51) configured to clip the transmission signals pre-equalized by the pre-equalization unit (23) at the output levels determined by the output amplitude level determination unit (26, 26a, 26b).
  5. The transmission apparatus according to claim 4, further comprising a scale unit (35, 61) configured to scale the transmission signals to be input to the pre-equalization unit (23) on the basis of a ratio clipped by the clipping unit (51).
  6. A receiving apparatus comprising: a conversion unit configured to receive optical signals output from a transmission apparatus and convert the received optical signals into electrical signals; an analog-to-digital conversion unit (32) configured to convert the electrical signals from analog signals into digital signals; a demodulation unit (33) configured to demodulate the digital signals converted by the analog-to-digital conversion unit (32); a scale estimation unit (34) configured to calculate a slope near an amplitude 0 in a correlation between an amplitude distribution of transmission signals pre-equalized by a pre-equalization unit (23) of the transmission apparatus by using a compensation filter and an amplitude distribution of the digital signals converted by the analog-to-digital conversion unit (32); a scale unit (35, 61) configured to scale an amplitude of the digital signals converted by the analog-to-digital conversion unit (32) on the basis of the slope calculated by the scale estimation unit (34); and a coefficient estimation unit (36) configured to calculate a filter coefficient of the compensation filter on the basis of the transmission signals pre-equalized by the pre-equalization unit (23) and the digital signals scaled by the scale unit (35, 61).
  7. A method for device characteristic estimation performed by an optical transmission system including a transmission apparatus and a receiving apparatus for optical signals, the method for device characteristic estimation comprising: a pre-equalization step of the transmission apparatus performing pre-equalization on transmission signals by using a compensation filter; a digital-to-analog conversion step of the transmission apparatus converting the transmission signals pre-equalized from digital signals into analog signals; an output step of the transmission apparatus converting the transmission signals from electrical signals into optical signals and outputting the converted optical signals; a receiving step of the receiving apparatus receiving the converted optical signals output in the output step and converting the received optical signals into electrical signals; an analog-to-digital conversion step of the receiving apparatus converting the signals electrical signals converted in the receiving step from analog signals into digital signals; a demodulation step of the receiving apparatus demodulating the digital signals converted in the analog-to-digital conversion step; a scale estimation step of the optical transmission system calculating a slope near an amplitude 0 in a correlation between an amplitude distribution of the transmission signals pre-equalized in the pre-equalization step and an amplitude distribution of the digital signals converted in the analog-to-digital conversion step; a scale step of the optical transmission system scaling an amplitude of the digital signals converted in the analog-to-digital conversion step on the basis of the slope calculated in the scale estimation step; a coefficient estimation step of the optical transmission system calculating a filter coefficient of the compensation filter on the basis of the transmission signals pre-equalized in the pre-equalization step and the digital signals_scaled in the scale step; and an output amplitude level determination step of the optical transmission system determining output levels of the transmission signals subjected to the digital-to-analog conversion step on the basis of a correlation between a distribution of amplitudes of the transmission signals which have not been pre-equalized in the pre-equalization step and a distribution of amplitudes of the transmission signals which have been pre-equalized in the pre-equalization step.

Description

Technical Field The present invention relates to an optical transmission system, a transmission apparatus, a receiving apparatus and a method for characteristic estimation. Background Art In order to generate a high-quality optical signal in an optical transmitter, compensation for nonlinearity occurring in a device has attracted attention. For the nonlinearity compensation, there have been proposed a method using a Volterra filter (see, for example, Non Patent Literature 1) and a method based on a Wiener-Hammerstein model (see, for example, Non Patent Literature 2) . Citation List Patent Literature WO 2020/235627EP 2 634 934 Non Patent Literature Non Patent Literature 1: P. W. Berenguer, et al., "Nonlinear Digital Pre-distortion of Transmitter Components," Journal of Lightwave Technology, Vol. 34, No. 8, pp. 1739-1745, 2016.Non Patent Literature 2: T. Sasai, et al., "Wiener-Hammerstein model and its learning for nonlinear digital pre-distortion of optical transmitters," Optics Express, Vol. 28, No. 21, pp. 30952-30963, 2020. Summary of Invention Technical Problem In the above conventional methods, an optical receiver analyzes a reception waveform of an optical signal to calculate a compensation coefficient, thereby feeding back the compensation coefficient to the optical transmitter. A level diagram of input and output in a compensation filter is important for compensation for a nonlinear response. In the optical transmitter, an electrical signal output from a digital-to-analog converter (DAC) is amplified by a driver amplifier, is converted into an optical signal by an optical modulator, and is then output to an optical transmission line. The optical signal transmitted through the optical transmission line is further amplified by an optical amplifier and is then input to the optical receiver. The optical receiver converts the input optical signal into an electrical signal and obtains a reception waveform of the electrical signal by using an analog-to-digital converter (ADC). In an actual optical transmitter/receiver, attenuation caused by an electric wire or the like occurs. Therefore, it is difficult to accurately obtain a level diagram necessary for calculating the compensation coefficient only on the basis of the waveform output from the ADC of the optical receiver and design information. In view of the above circumstances, an object of the present invention is to provide an optical transmission system, a transmission apparatus, a receiving apparatus and a method for characteristic estimation, each of which is capable of accurately compensating an optical signal. Solution to Problem An aspect of the present invention is an optical transmission system including: a transmission apparatus; and a receiving apparatus, in which: the transmission apparatus includes a pre-equalization unit that performs pre-equalization on transmission signals by using a compensation filter, a digital-to-analog conversion unit that converts the transmission signals which have been pre-equalized from digital signals into analog signals, and an output unit that converts the transmission signals of the analog signals from electrical signals into optical signals and outputs the converted optical signals; the receiving apparatus includes a receiving unit that receives the optical signals which have been output from the output unit and converts the received optical signals into received signals of electrical signals, an analog-to-digital conversion unit that converts the received signals from the analog signals into the digital signals, and a demodulation unit that demodulates the received signals of the digital signals; and the optical transmission system further includes a scale estimation unit that calculates a slope near an amplitude 0 in a correlation between an amplitude distribution of the transmission signals which have been pre-equalized by the pre-equalization unit and an amplitude distribution of the received signals of the digital signals, a scale unit that scales an amplitude of the received signals of the digital signals on the basis of the slope which has been calculated by the scale estimation unit, a coefficient estimation unit that calculates a filter coefficient of the compensation filter on the basis of the transmission signals which have been pre-equalized by the pre-equalization unit and the received signals which have been scaled by the scale unit, and an output amplitude level determination unit that determines an output level of the transmission signals from the analog-to-digital conversion unit on the basis of a correlation between a distribution of amplitudes of the transmission signals which have been input to the pre-equalization unit and a distribution of amplitudes of the transmission signals which have been pre-equalized by the pre-equalization unit. An aspect of the present invention is a transmission apparatus including: a pre-equalization unit that performs pre-equalization on transmission signals