Adaptive equalization method for joint compensation of carrier frequency offset and channel linear damage

Abstract

The invention discloses an adaptive equalization method for combined compensation of carrier frequency offset and channel linear damage. The adaptive equalization method specifically comprises the following steps: taking data subjected to matched filtering, clock synchronization, orthogonality and normalization as input signals; for an input signal, using the filter linear damage compensation weight vector and the frequency offset compensation weight coefficient to obtain output data of the filter; obtaining an expected value of filter output data, and calculating to obtain a theoretical error value of adaptive equalization; and obtaining a filter weight vector used for next adaptive equalization according to the theoretical error value. The adaptive equalization method for combined compensation of carrier frequency offset and channel linear impairment is suitable for a coherent optical transmission system, uses an adaptive equalizer to compensate signal frequency offset and channel linear impairment at the same time, does not need an additional frequency offset estimation algorithm, meets the advantages of multiple rates and multiple modulation formats, and is high in convergence speed and high in robustness. And the calculation complexity is low.

Description

Technical Field

[0001] The present invention relates to the field of optical communications, and in particular to an adaptive equalization method for joint compensation of carrier frequency offset and channel linear damage. Background Art

[0002] With the rapid development of emerging services such as high-definition video, the internet, and cloud computing, coherent optical communications has become a key technology for the next generation of high-speed optical communication backbone networks. Coherent optical communication systems utilize high-order modulation formats to improve spectral efficiency and communication capacity, and use digital signal processing techniques to compensate for signal impairments during transmission.

[0003] Since high-order modulation formats are more sensitive to frequency offset and phase noise, it is of great significance to study fast and low-complexity frequency offset estimation algorithms.

[0004] At the same time, the performance of high-baud-rate and high-order modulation format systems is easily affected by IQ amplitude imbalance, phase mismatch, IQ delay, and device bandwidth limitations within the coherent optical transceiver module. Therefore, how to compensate for various impairments through adaptive equalizers (AEQ) remains a research hotspot.

[0005] In addition, since data centers have strict requirements on the power consumption of optical modules, how to minimize the complexity of AEQ and frequency offset estimation algorithms and reduce the power consumption of optical modules has also become an urgent problem to be solved. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the technical problem to be solved by the present invention is that existing frequency offset estimation algorithms are computationally complex, have low estimation accuracy, are unsuitable for high-order modulation formats, and have poor compatibility. The present invention provides an adaptive equalization method for jointly compensating for carrier frequency offset and channel linear impairments, applicable to coherent optical transmission systems. This method uses an adaptive equalizer to simultaneously compensate for signal frequency offset and channel linear impairments, eliminating the need for additional frequency offset estimation algorithms. It meets the advantages of multiple rates and multiple modulation formats, offers fast convergence speed, and reduces computational complexity.

[0007] To achieve the above-mentioned objectives, the present invention provides an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment, which is applied to a coherent optical transmission system. The coherent optical transmission system includes an analog signal generation module, an electro-optical conversion module, a coherent receiver module and a digital signal processing module. The analog signal generation module is used to convert the original digital signal into an analog signal, and is connected to the electro-optical conversion module via a radio frequency line. The electro-optical conversion module is used to modulate the analog signal onto an optical carrier to obtain a modulated optical signal. The optical signal is connected to the coherent receiver module via an optical fiber. The coherent receiver module converts the modulated optical signal into a digital signal again, and then performs signal compensation on the digital signal through the digital signal processing module.

[0008] Furthermore, the coherent receiver module includes an optical coupling module, a photoelectric detection module, and an analog-to-digital conversion module. The optical coupling module is connected to the photoelectric detection module, and is used to receive the optical transmission signal and the local oscillator signal, and mix the optical transmission signal and the local oscillator signal; the photoelectric detection module is used to convert the mixed signal into an analog signal; the analog-to-digital conversion module is connected to the photoelectric detection module, and is used to sample the analog signal and convert the analog signal into a digital signal, and the digital signal is processed by the digital signal processing module.

[0009] Furthermore, the digital signal processing module includes a matched filtering module, a clock synchronization module, an orthogonal and normalization module, an adaptive equalization module and a phase recovery module, wherein the received signal is passed through the matched filtering module to obtain a baseband signal; the matched filtering module is connected to the clock synchronization module and the orthogonal and normalization module, and then the adaptive equalization module is followed to simultaneously compensate for linear damage such as signal frequency deviation damage, dispersion, and device bandwidth limitation; the phase recovery module performs carrier phase noise compensation on the equalized output signal.

[0010] Furthermore, a data-assisted adaptive equalization algorithm is used.

[0011] Further, the following steps are included:

[0012] The data after matched filtering, clock synchronization, orthogonalization and normalization is used as input signal;

[0013] For the input signal, the filter linear damage compensation weight vector and frequency offset compensation weight coefficient are used to obtain the filter output data;

[0014] Obtain the expected value of the filter output data and calculate the theoretical error value of the adaptive equalization;

[0015] According to the theoretical error value, the filter weight vector used for the next adaptive equalization is obtained.

[0016] Furthermore, the adaptive equalization algorithm includes the following steps:

[0017] The data after matched filtering, clock synchronization, orthogonalization and normalization is used as the input signal x(n), where n is a positive integer;

[0018] The equalizer weight coefficients are denoted as weight vector ω and weight coefficient ω f ;

[0019] Input x(n) into the adaptive equalizer and obtain the output data y(n) of the equalizer;

[0020]

[0021] Obtain the expected value of the n-channel output data of the equalizer and record it as d(n), and then calculate the theoretical error value e(n) of the adaptive equalization;

[0022] e(n)=d(n)-y(n)

[0023] Get the step size factor used for adaptive equalization and record it as μ1 and μ2; get the sampling interval of the system and record it as T s ;

[0024] According to the theoretical error value e(n), step factors μ1 and μ2, and the sampling interval T of the system s , the filter weight coefficient ω′ used for the next adaptive equalization is determined by the optimization method i and ω′ f ;

[0025] The filter weight coefficient ω′ i and ω′ f As the filter weight coefficient ω i and ω f , repeat the above steps until the adaptive equalization process is completed.

[0026] Furthermore, the LMS algorithm is used as an example to train the equalizer weight vector ω and weight coefficient ω f , the cost function of the algorithm is:

[0027] J(n)=E[|e(n)| 2 ]

[0028] E[·] represents the statistical expectation.

[0029] Furthermore, the equalizer weight vector ω and weight coefficient ω f The nth iteration vector is:

[0030]

[0031] ω f ←ω f +μ2·4πnT s ·Im{d(n)·y * (n)}

[0032] Among them, μ1 and μ2 are step size factors, and Im{·} represents the imaginary part operation.

[0033] Note that other optimization algorithms may also be used to update the tap coefficients. The present invention uses the LMS algorithm as an example to explain the present invention, but is not intended to limit the present invention.

[0034] Furthermore, the equalizer input data x(n) also contains linear impairments such as frequency offset impairment, dispersion, and device bandwidth limitation.

[0035] Technical Effects

[0036] The present invention provides an adaptive equalization method for jointly compensating for carrier frequency offset and channel linear impairments. The method is applicable to coherent optical transmission systems, uses an adaptive equalization algorithm, and is applicable to multiple rates and multiple modulation formats. It does not require an additional frequency offset estimation module, and only requires a single adaptive equalizer to simultaneously compensate for linear impairments such as signal frequency offset impairments, dispersion, and device bandwidth limitation, with low computational complexity. The method is applicable to multiple rates and multiple modulation formats, and has a fast convergence speed, capable of converging within 1000 training symbols.

[0037] The concept, specific structure and technical effects of the present invention will be further described below in conjunction with the accompanying drawings to fully understand the purpose, characteristics and effects of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of a coherent optical transmission system using an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to a preferred embodiment of the present invention;

[0039] Figure 2 It is a schematic diagram of a coherent receiver module and an optical transmission system of an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to a preferred embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of a digital signal processing module of an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment in a preferred embodiment of the present invention;

[0041] Figure 4 1 is a schematic diagram of an adaptive equalization method for jointly compensating for carrier frequency offset and channel linear impairment according to a preferred embodiment of the present invention;

[0042] Figure 5 This is a constellation diagram of input data of an adaptive equalizer of an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment in a preferred embodiment of the present invention;

[0043] Figure 6 This is a constellation diagram of output data of an adaptive equalizer of an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment in a preferred embodiment of the present invention;

[0044] Figure 7 It is a constellation diagram of output data of a phase recovery module of an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment in a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0045] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0046] In the following description, specific details, such as certain internal procedures and techniques, are provided for purposes of illustration and not limitation to facilitate a thorough understanding of the embodiments of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

[0047] The embodiment of the present invention provides an adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment, which is applicable to coherent optical transmission systems, such as Figure 1 As shown, the coherent optical transmission system includes an analog signal generation module, an electro-optical conversion module, a coherent receiver module and a digital signal processing module. The analog signal generation module is used to convert the original digital signal into an analog signal, and is connected to the electro-optical conversion module through a radio frequency line. The electro-optical conversion module is used to modulate the analog signal onto an optical carrier to obtain a modulated optical signal. The optical signal is connected to the coherent receiver module through an optical fiber. The coherent receiver module converts the modulated optical signal into a digital signal again, and then performs signal compensation on the digital signal through the digital signal processing module.

[0048] The coherent receiver module includes an optical coupling module, a photoelectric detection module, and an analog-to-digital conversion module. The optical coupling module is connected to the photoelectric detection module and is used to receive the optical transmission signal and the local oscillator signal and mix the optical transmission signal and the local oscillator signal; the photoelectric detection module is used to convert the mixed signal into an analog signal; the analog-to-digital conversion module is connected to the photoelectric detection module and is used to sample the analog signal and convert the analog signal into a digital signal, and the digital signal processing module is used to process the digital signal. Figure 2 As shown, the signal generated by the arbitrary waveform generator (AWG) can be a variety of high-order modulation formats, which are modulated onto an optical carrier through an IQ modulator. The signal light is transmitted in a single-mode optical fiber. The polarization state of the signal light is adjusted by manually adjusting the polarization controller so that it is consistent with the polarization state of the local oscillator light in the coherent receiver module. The coherent receiver module is used to receive the optical transmission signal and the local oscillator signal, and obtain a coupled signal based on the optical transmission signal and the local oscillator signal, and convert the coupled signal into a digital signal. The frequency value of the local oscillator signal differs from the frequency value of the optical transmission signal by a preset offset frequency value, and the preset offset frequency value is half the symbol rate value of the optical transmission signal.

[0049] like Figure 3As shown, the digital signal processing module includes a matched filter module, a clock synchronization module, an orthogonalization and normalization module, an adaptive equalization module, and a phase recovery module. The received signal passes through the matched filter module to obtain a baseband signal. Due to various factors such as the optical device manufacturing process, the frequency between the local oscillator (LO) and the transmitting laser (LD) cannot be completely consistent, which introduces cumulative phase noise into the signal. The matched filter module is connected to the clock synchronization module and the orthogonalization and normalization module, and then follows the adaptive equalization module to simultaneously compensate for linear impairments such as signal frequency offset, dispersion, and device bandwidth limitations. The phase recovery module compensates for carrier phase noise in the equalized output signal.

[0050] In the embodiment of the present invention, a data-assisted adaptive equalization algorithm is used.

[0051] An adaptive equalization method for jointly compensating for carrier frequency offset and channel linear impairments, provided by embodiments of the present invention, takes matched filtering, clock synchronization, orthogonalization, and normalization as input. It uses the filter's linear impairment compensation weight vector and frequency offset compensation weight coefficient to obtain the filter's output data. The expected value of the filter's output data is then calculated to determine the theoretical error value for adaptive equalization. Finally, the filter weight vector used for the next adaptive equalization operation is determined, completing one adaptive equalization cycle. This adaptive equalization algorithm is suitable for various rates and modulation formats, and achieves joint compensation for linear impairments such as frequency offset estimation and dispersion with low computational complexity.

[0052] An embodiment of the present invention provides an adaptive equalization method for jointly compensating for carrier frequency offset and channel linear impairments, including the following steps: The input data x(n) of the adaptive equalization algorithm is an n-channel data block consisting of a training sequence and a signal sequence. The training sequence serves as the expected value of the equalizer output data. The tap coefficients of the adaptive equalizer are adjusted using the training sequence, and the signal sequence is then equalized. x(n) is a signal that has undergone matched filtering, clock synchronization, orthogonalization, and normalization, and also contains frequency offset impairments, phase noise, dispersion, and other linear impairments. The output of the equalizer is:

[0053]

[0054] ω and ω f is the coefficient of the adaptive filter, where ω represents the compensation weighting coefficient for linear impairments such as dispersion and bandwidth limitation, and is initialized to [0,…,0,1+1i,0,…,0]; f Represents the compensation coefficient for frequency offset error, initialized to 0; T sis the sampling interval, which is equal to the sampling interval of the analog-to-digital conversion module of the coherent receiver module and must satisfy the Nyquist sampling theorem. N represents the order of the adaptive equalizer, which should be based on minimizing the error after equalization. The expected value of the n-channel output data of the equalizer is obtained, that is, the training sequence agreed upon by the transmitter and receiver of the coherent optical transmission system to assist in the tap update of the adaptive equalizer, and is recorded as d(n). The theoretical error value e(n) of the adaptive equalization is then calculated.

[0055] e(n)=d(n)-y(n)

[0056] Get the step size factor used for adaptive equalization and record it as μ1 and μ2; get the sampling interval of the system and record it as T s , the selection of sampling interval must satisfy the Nyquist sampling theorem.

[0057] In this embodiment, the LMS algorithm is used as an example to train the tap coefficients ω and ω f , the cost function of the algorithm is:

[0058] J(n)=E[|e(n)| 2 ]

[0059] E[·] represents the statistical expectation. The goal of the algorithm is to minimize the radial error using the stochastic gradient descent (SGD) method. The tap coefficient vector for the nth iteration is:

[0060]

[0061] ω f ←ω f +μ2·4πnT s ·Im{d(n)·y * (n)}

[0062] μ1 and μ2 are step size factors, and Im{·} represents the imaginary part operation. The equalizer outputs y(n) and e(n) typically converge within 1000 iterations.

[0063] Note that other optimization algorithms may also be used to update the tap coefficients. The present invention uses the LMS algorithm as an example to explain the present invention, but is not intended to limit the present invention.

[0064] According to the theoretical error value e(n), step factors μ1 and μ2, and the sampling interval T of the system s , the filter weight coefficient ω′ used for the next adaptive equalization is determined by the optimization method i and ω′ f ;

[0065] The filter weight coefficient ω′ i and ω′ f As the filter weight coefficient ω i and ω f , repeat the above steps until the adaptive equalization process is completed.

[0066] The above equalization structure can effectively eliminate signal linearity impairments such as inter-symbol crosstalk caused by chromatic dispersion and device bandwidth limitations, while compensating for phase imbalance caused by the laser's additional frequency drift.

[0067] The following uses a 20 GBaud 16QAM signal with a 21-order equalization filter coefficient as an example to illustrate the technical effects of the embodiments of the present invention.

[0068] like Figure 1 The coherent optical transmission system of this embodiment is shown, consisting of an analog signal generation module, an electro-optical conversion module, a single-mode optical fiber, a coherent receiver module, and a digital signal processing module. The analog signal generation module is used to generate an analog signal in a 16QAM modulation format and is connected to the electro-optical conversion module via an RF line. The electro-optical conversion module is used to modulate the analog signal onto an optical carrier to obtain a modulated optical signal. This optical signal is then connected to the coherent receiver module via a single-mode optical fiber. The coherent receiver module obtains the optical transmission signal and a local oscillator signal. The frequency of the local oscillator signal differs from that of the optical transmission signal by half the symbol rate. A coupled signal is generated based on the optical transmission signal and the local oscillator signal. The coupled signal is then converted into an analog signal. The analog signal is then sampled at a sampling frequency of 160 GS / s to obtain a digital signal. The digital signal is then compensated by the digital signal processing module.

[0069] like Figure 2 As shown, an arbitrary waveform generator (AWG) generates a 20GBaud 16QAM signal, which is modulated onto an optical carrier through an IQ modulator. The signal light is transmitted in a single-mode optical fiber. The polarization state of the signal light is adjusted by manually adjusting the polarization controller so that it is consistent with the polarization state of the local oscillator light in the coherent detection. The coherent detection module is used to receive the optical transmission signal and the local oscillator signal, and obtain a coupled signal based on the optical transmission signal and the local oscillator signal, and convert the coupled signal into a digital signal. The optical power of the local oscillator signal is 10dBm, and the optical power of the received optical transmission signal is -6dBm. The frequency value of the local oscillator signal differs from the frequency value of the optical transmission signal by a preset offset frequency value, and the preset offset frequency value is half of the symbol rate value of the optical transmission signal.

[0070] The digital signal is input into Figure 3In the digital signal processing flow shown in the figure, the received signal passes through the matched filter module to obtain the baseband signal. Due to the influence of various factors such as the optical device manufacturing process, it is difficult for the frequency between the local oscillator light source LO and the transmitting laser LD to maintain complete consistency, which introduces cumulative phase noise into the signal. The matched filter module is connected to the clock synchronization module and the orthogonal and normalization module, and then follows Figure 4 The adaptive equalization module shown also compensates for linear impairments such as signal frequency offset impairments, dispersion, and device bandwidth limitations.

[0071] like Figure 4 Figure 1 shows the adaptive equalization algorithm of an embodiment of the present invention. The equalizer input data x(n) is an n-channel data block. x(n) is a signal that has undergone matched filtering, clock synchronization, orthogonalization, and normalization. It also contains 100 MHz frequency offset impairments, phase noise, dispersion, and other linear impairments. Figure 5 The constellation diagram of the equalizer input data x(n) is shown. Because the equalizer input data x(n) also contains linear impairments such as frequency offset impairments, dispersion, and device bandwidth limitations, the constellation points rotate and diverge. Figure 6 The constellation diagram of the output data of the adaptive equalization algorithm is shown. Compared with the constellation diagram of the data x(n) before equalization, the output data of the adaptive equalization algorithm has significantly converged, and the 100MHz frequency offset is also compensated. No additional frequency offset estimation module is required. Only a single adaptive equalizer can simultaneously compensate for linear impairments such as signal frequency offset, dispersion, and device bandwidth limitations. The computational complexity is low, and convergence can be achieved within 1000 training symbols. However, the output signal of the adaptive equalizer still contains some phase noise, which is caused by the linewidth of the laser. The phase recovery module is required to compensate for the carrier phase noise.

[0072] Finally, the phase recovery module performs carrier phase noise compensation on the equalized output signal. Figure 7 The constellation diagram of the output signal of the phase recovery module is shown. The constellation points of the 16QAM signal are clearly visible, and the bit error rate after judgment is 0.

[0073] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. An adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment, characterized in that: Applied to coherent optical transmission systems, the coherent optical transmission system includes an analog signal generation module, an electro-optical conversion module, a coherent receiver module and a digital signal processing module. The analog signal generation module is used to convert the original digital signal into an analog signal and is connected to the electro-optical conversion module through a radio frequency line. The electro-optical conversion module is used to modulate the analog signal onto an optical carrier to obtain a modulated optical signal. The optical signal is connected to the coherent receiver module through an optical fiber. The coherent receiver module converts the modulated optical signal into a digital signal again, and then performs signal compensation on the digital signal through the digital signal processing module.

2. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 1, characterized in that: The coherent receiver module includes an optical coupling module, a photoelectric detection module, and an analog-to-digital conversion module. The optical coupling module is connected to the photoelectric detection module, used to receive the optical transmission signal and the local oscillator signal, and mix the optical transmission signal and the local oscillator signal; the photoelectric detection module is used to convert the mixed signal into an analog signal; the analog-to-digital conversion module is connected to the photoelectric detection module, used to sample the analog signal and convert the analog signal into a digital signal, and use the digital signal processing module to process the digital signal.

3. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 2, characterized in that: The digital signal processing module includes a matched filtering module, a clock synchronization module, an orthogonal and normalization module, an adaptive equalization module and a phase recovery module. The received signal is passed through the matched filtering module to obtain a baseband signal; the matched filtering module is connected to the clock synchronization module and the orthogonal and normalization module, and then the adaptive equalization module is followed to simultaneously compensate for linear damage such as signal frequency deviation damage, dispersion, and device bandwidth limitation; the phase recovery module compensates for carrier phase noise of the equalized output signal.

4. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 3, characterized in that: Use data-assisted adaptive equalization algorithm.

5. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 4, characterized in that: The following steps are involved: The data after matched filtering, clock synchronization, orthogonalization and normalization is used as input signal; For the input signal, using the filter linear damage compensation weight vector and the frequency offset compensation weight coefficient to obtain the output data of the filter; Obtain the expected value of the filter output data and calculate the theoretical error value of the adaptive equalization; According to the theoretical error value, the filter weight vector used for the next adaptive equalization is obtained.

6. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 5, characterized in that: The adaptive equalization algorithm comprises the following steps: The data after matched filtering, clock synchronization, orthogonalization and normalization is used as the input signal x(n), where n is a positive integer; The equalizer weight coefficients are denoted as weight vector ω and weight coefficient ω f ; Input x(n) into the adaptive equalizer and obtain the output data y(n) of the equalizer; Obtain the expected value of the n-channel output data of the equalizer and record it as d(n), and then calculate the theoretical error value e(n) of the adaptive equalization; e(n)=d(n)-y(n) Get the step size factor used for adaptive equalization and record it as μ1 and μ2; get the sampling interval of the system and record it as T s ; According to the theoretical error value e(n), step factors μ1 and μ2, and the sampling interval T of the system s , the filter weight coefficient ω′ used for the next adaptive equalization is determined by the optimization method i and ω′ f ; The filter weight coefficient ω′ i and ω′ f As the filter weight coefficient ω i and ω f , repeat the above steps until the adaptive equalization process is completed.

7. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 6, characterized in that: Use optimization method to train the equalizer weight vector ω and weight coefficient ω f .

8. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 7, characterized in that: Equalizer weight coefficient weight vector ω and weight coefficient ω f The nth iteration vector is: oh f ←oh f +μ2·4πnT s ·Im{d(n)·y * {noun} Among them, μ1 and μ2 are step factors, and Im{.} represents the imaginary part operation.

9. The adaptive equalization method for joint compensation of carrier frequency offset and channel linear impairment according to claim 8, characterized in that: The equalizer input data x(n) also contains linear impairments such as frequency offset impairment, dispersion, and device bandwidth limitation.

Images