CN110971302A - A device and method for estimating fiber dispersion by low-speed delay sampling - Google Patents
A device and method for estimating fiber dispersion by low-speed delay sampling Download PDFInfo
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Abstract
本发明公开了一种低速采样与延时采样结合估计光纤色散的装置与方法，该方法包括：步骤1：建立残余色散‑代价函数值查找表：用低速延时相干探测单元对光信号进行低速探测，经过等效采样单元获得等效过采样信号，再经色散补偿单元和计算单元，调整搜索判断单元与调整单元至建表状态，建立残余色散‑代价函数值查找表；步骤2：以步骤1中所获参数设置搜索判断单元并调整其与调整单元至估测状态；步骤3：用步骤2中装置对光信号进行低速探测，经过色散补偿迭代和查表得到色散估计值。本发明公开的方法及装置能实现对低速率采样的信号进行色散估计，降低了监测成本、运算复杂度和前期准备的时间成本。
The invention discloses a device and method for estimating optical fiber dispersion by combining low-speed sampling and time-delay sampling. The method includes: step 1: establishing a residual dispersion-cost function value look-up table: using a low-speed delay coherent detection unit to perform low-speed detection on an optical signal Detecting, obtaining an equivalent oversampling signal through the equivalent sampling unit, and then adjusting the search judgment unit and the adjustment unit to the table-building state through the dispersion compensation unit and the calculation unit, and establishes the residual dispersion-cost function value look-up table; Step 2: step The parameters obtained in 1 set the search judgment unit and adjust it and the adjustment unit to the estimation state; Step 3: Use the device in step 2 to detect the optical signal at low speed, and obtain the dispersion estimation value through dispersion compensation iteration and table look-up. The method and device disclosed in the present invention can realize dispersion estimation for signals sampled at a low rate, and reduce monitoring cost, computational complexity and time cost of preliminary preparation.
Description
Technical Field
The invention relates to the technical field of communication, in particular to a device and a method for estimating optical fiber dispersion by low-speed delay sampling.
Background
With the increasing demand for data services, the capacity of optical transmission networks is increasing, the transmission distance is also increasing, and the optical signals are affected by Chromatic Dispersion (CD) and nonlinearity in the transmission process of the optical fiber. In order to better manage and monitor the Optical network and ensure the reliability and stability of the system, Optical Performance Monitoring (OPM) gets more and more attention, and among numerous Monitoring indexes, chromatic dispersion is an important parameter for measuring the quality of an Optical fiber link, and has an important meaning for estimating and measuring the system.
The estimation and compensation of the dispersion are the premise for processing the nonlinear effect, and the precision of the dispersion compensation directly influences the processing process of the subsequent signals. The digital signal processing technology can be used for effectively compensating the dispersion, and accurate and quick dispersion estimation can provide a basis for subsequent dispersion compensation, thereby being greatly helpful for improving the transmission performance of the system.
In the prior art, the delay sampling technology is considered to be an effective, reliable and low-cost dispersion estimation technology. The signal waveform is sampled in the form of sampling pairs (Xi, Yi), sampling points Xi and Yi are delayed by tau, asynchronous delay sampling is achieved, distortion of the signal can be represented by drawing a scatter diagram (delay sampling diagram) of the sampling points obtained by two ADC taps, and the function of the sample is similar to that of an eye diagram. Since the desired signal is sampled in a Digital Signal Processing (DSP) based receiver, delay tap sampling can be easily implemented and is naturally suitable for dispersion estimation. However, as the transmission rate is continuously increased, if the conventional delay tap sampling technique is adopted, since it requires 2 times of symbol rate sampling, there will be a relatively high requirement for the sampling rate of the dispersion estimation device, and the increase of the sampling rate means the increase of the cost.
In recent years, researchers have attracted attention on how to reduce the cost of dispersion monitoring equipment, and the application of low-speed sampling technology in dispersion monitoring equipment is the most direct way to reduce the cost. The low-speed sampling technique is a technique with a sampling rate lower than the nyquist sampling rate, and at a low sampling rate, a periodic training sequence can be used, which is equivalent to an oversampled signal. The equivalent sampling is implemented under the low-speed sampling, and the cost of the dispersion monitoring equipment can be greatly reduced on the premise of ensuring that the performance is not obviously degraded by combining the existing dispersion estimation means.
The current methods for performing dispersion estimation under low-speed sampling technology mainly include two methods: a is for sampling under the low speed and carrying on the equivalent sampling after the fractional Fourier transform (FrFT) blind dispersion estimation method, utilize training sequence, sample at low speed, the signal got is transformed into the fractional domain, the energy concentration can be represented by the concentrated function of energy of different transformation orders, search its extreme value in order to obtain the optimum order, utilize the optimum order in the fractional domain to estimate the cumulative dispersion value finally, its disadvantage is that it is high to calculate the complexity when transforming to the search optimum order of the fractional domain; the other method relies on a neural network, a large amount of data (training set) in the system is utilized to train the neural network to extract data characteristics, finally, a test set is input into a trained neural network model, and weighted average is carried out on the output of a neural network unit and reference dispersion, so that a dispersion estimation value is obtained.
Disclosure of Invention
The invention provides a device and a method for estimating optical fiber dispersion by low-speed delay sampling, aiming at overcoming the defects of higher operation complexity of a fractional Fourier transform (FrFT) -based blind dispersion estimation method under the existing low-speed sampling and high time cost and poor mobility of a dispersion estimation method combining a neural network under the low-speed sampling, and greatly reducing the time cost before a dispersion estimation device is put into use and the operation complexity of the dispersion estimation device.
In a first aspect, an apparatus for estimating fiber dispersion by low-speed delay sampling is provided, the apparatus comprising:
the low-speed delay coherent detection unit is used for carrying out delay sampling on the signal at a set low sampling rate to obtain a delay under-sampled signal;
the equivalent sampling unit is connected with the low-speed coherent detection unit, and performs down-sampling and splicing on a frame where a training sequence is located by using the periodicity of the training sequence in a signaling to equivalently obtain an over-sampling signal;
the dispersion compensation unit is connected with the equivalent sampling unit so as to perform Overlapping Frequency Domain Equalization (OFDE) on the equivalent sampling signal by the step length fed back by the adjustment unit; wherein, the sampling rate required by the overlapped frequency domain equalization is the oversampling rate of the equivalent oversampling signal;
the calculation unit is connected with the dispersion compensation unit and calculates a function value related to dispersion for the signal processed by the dispersion compensation unit by using a designed cost function;
the searching and judging unit is connected with the calculating unit, the estimation state is that the cost function value output by the calculating unit is searched in a built dispersion-cost function value lookup table, so that whether the table lookup is finished to estimate the dispersion and judge whether the dispersion compensation step length needs to be adjusted or not is judged, if the cost function value is higher than a small step length adjustment threshold, the dispersion compensation step length is still judged to be an initial large step length, if the cost function value is smaller than a set small step length adjustment threshold, the dispersion compensation step length needs to be adjusted to the small step length, and if the cost function value falls into a table lookup interval, the residual dispersion value obtained by the table lookup is sent to the adjusting unit and is not searched any more; the table building state is that no processing is carried out on the signal;
and the adjusting unit is connected with the searching and judging unit and the dispersion compensation unit and adjusts the step length of dispersion compensation or jumps out of the dispersion estimation process according to the judging result transmitted by the searching and judging unit.
Wherein, the low-speed time delay coherent detection unit comprises:
the system comprises a local oscillator laser, two polarization beam splitters, eight photoelectric detectors, four time delays and eight low-speed analog-to-digital converters; wherein, the time delay generated by the time delay device is a low-speed sampling period; the two low-speed analog-to-digital converters respectively output two paths of low-speed sampling signals in an X polarization state I, Q, the two delayers are connected with the two low-speed analog-to-digital converters and respectively output two paths of low-speed delay sampling signals in an X polarization state I, Q, the two low-speed analog-to-digital converters respectively output two paths of low-speed sampling signals in a Y polarization state I, Q, and the two delayers are connected with the two low-speed analog-to-digital converters and respectively output two paths of low-speed delay sampling signals in a Y polarization state I, Q;
wherein, the meterThe calculation unit is specifically used for calculating the distance between the delayed sampling power pair and the diagonal line on the delayed sampling graph according to the two polarization states and a formula
Determining a function value related to the dispersion to obtain a dispersion-cost function value lookup table; wherein the delayed sampling power is relative to the distance from the diagonal line on the delayed sampling diagram
In a second aspect, a method for estimating fiber dispersion for low-speed delay sampling is provided, the method comprising:
step 1: the method comprises the steps that a low-speed delay coherent detection unit is used for carrying out low-speed delay detection on optical signals transmitted in an optical fiber link at an estimated position, the signals are subjected to down-sampling and splicing through an equivalent sampling unit and are equivalent to oversampled signals, a dispersion compensation unit is used for carrying out dispersion compensation on the signals subjected to equivalent sampling in a set small step length, a calculation unit calculates a function value related to dispersion on the signals processed by the dispersion compensation unit, and a search judgment unit and an adjustment unit are set to be in a table building state;
the table building states of the searching and judging unit and the adjusting unit are as follows: the searching and judging unit does not process the signal, the adjusting unit jumps out of the compensation dispersion estimation flow when the compensated dispersion reaches at least 2 times of the compensated dispersion corresponding to the lowest value of the cost function, and a residual dispersion-cost function value lookup table is established.
Wherein, the step 1 specifically comprises the following substeps:
step 1.1: and performing low-speed time delay detection on the optical signals of the same modulation format after being transmitted by the optical fiber links under different conditions by using a low-speed time delay coherent detection unit to obtain I, Q two-path discrete digital sequences in two polarization states under different transmission conditions.
The symbol period of the optical signal transmission training sequence in the same modulation format is T;
wherein, the different transmission conditions refer to the transmission span number of the optical digital signals;
step 1.2: and (3) carrying out equivalent sampling on the signal obtained in the step (1.1) by using an equivalent sampling unit to obtain an equivalent sampling signal.
Wherein, the calculation formula of the down sampling interval in the equivalent sampling is characterized by (1):
where M denotes the down-sampling interval, T denotes the period of the training sequence, TsIndicating the low-speed sampling period, and M is the result of rounding up after calculation.
Wherein, the equivalent sampling rate calculation formula in the equivalent sampling is characterized by (2):
wherein f iseqvRepresenting the equivalent sampling rate, M the down-sampling interval, T the period of the training sequence, TsIndicating a low-speed sampling period.
Step 1.3: and performing dispersion compensation on the signal subjected to the equivalent sampling by using a dispersion compensation unit in a set small step length.
The dispersion compensation method is an Overlapped Frequency Domain Equalization (OFDE), the number of overlapped points of the frequency domain is the number of corresponding points of the OFDE under the highest optical fiber span section in different transmission conditions of the system, the number of FFT points is 2 times of the number of overlapped points of the frequency domain, and the sampling rate is set to be the equivalent sampling rate calculated in the step (2);
step 1.4: the calculation unit calculates a function value related to dispersion for the signal processed by the dispersion compensation unit, and establishes a corresponding relation between accumulated dispersion and the function value.
Wherein the dispersion-related cost function is characterized by (3):
wherein std represents a standard deviation, dx(y)Expressed as x-polarization state or y-polarization stateDelayed sample power versus distance from diagonal, d, on the delayed sample plotx(y)Is characterized by (4):
wherein, the power of the equivalent post-sampling point forms a delay sampling power pair according to every two continuous groups, and is respectively Px(y)[2n]And Px(y)[2n-1]The delay time of the delayed sample is 1 period of the equivalent sample symbol.
Step 1.5: the adjusting unit is set to jump out of the compensation dispersion estimation process when the compensated dispersion reaches at least 2 times of the compensated dispersion corresponding to the lowest value of the cost function, average different cost function values of the same residual dispersion under different transmission conditions, select a proper size step length adjusting threshold avoiding a limiting point in a range with positive residual dispersion, select a range with higher coincidence degree of dispersion-cost function value curves under different transmission conditions as a table building range in a range with negative residual dispersion, and build a residual dispersion-cost function value lookup table suitable for different transmission conditions.
Wherein the residual dispersion is the difference between the known dispersion and the accumulated compensation dispersion.
Wherein the limit point is the lowest point of the cost function value before the residual dispersion and the cost function value are in obvious linear relation.
Step 2: and (2) setting a search judgment unit according to the size step length adjustment threshold and the table lookup table interval in the step (1), putting the table lookup in the step (1) into the search judgment unit, and adjusting the search judgment unit and the adjustment unit to an estimation state.
Wherein, the estimation states of the search judging unit and the adjusting unit are as follows: the searching and judging unit searches the cost function value output by the calculating unit in the established dispersion-cost function value lookup table so as to judge whether the table is looked up to complete dispersion estimation and judge whether the dispersion compensation step length needs to be adjusted or not, if the cost function value is higher than a small step length adjustment threshold, the dispersion compensation step length is still the initial large step length, if the cost function value is smaller than the set small step length adjustment threshold, the dispersion compensation step length needs to be adjusted to the small step length, if the cost function value falls into the table look-up interval, the residual dispersion value obtained by table look-up is sent to the adjusting unit and is not searched again, the adjusting unit adjusts the dispersion compensation step length or jumps out of the dispersion estimation flow according to the judgment result transmitted by the searching and judging unit, if the judgment result transmitted by the searching and judging unit is the large step length, the dispersion compensation step length is adjusted to the large step, if the judgment result transmitted by the searching and judging unit is a small step length, the step length of dispersion compensation is adjusted to be the small step length, if the judgment result transmitted by the searching and judging unit is the corresponding dispersion after table look-up, the step length of dispersion compensation is adjusted to be the table look-up value, the judgment result transmitted by the searching and judging unit at the next time is ignored, and an estimation process is skipped;
and step 3: and (3) placing the device in the step (2) into an estimation position to perform low-speed detection on the optical signal transmitted in the optical fiber link, so as to obtain a dispersion estimation value represented by a formula (5):
CDest＝KL×Slarge+KS×Ssmall+Slookup(5)
wherein, CDestFor estimated dispersion values, SlargeAnd SsmallRespectively representing a large step size and a small step size, SlookupRepresents the dispersion value obtained by looking up the table, and is a negative value. KLAnd KSRepresenting the number of large and small step iterations, respectively.
So far, from step 1 to step 3, a method for estimating fiber dispersion by low-speed delay sampling is completed.
Advantageous effects
Compared with the existing dispersion estimation system and method of low-speed sampling, the device and method for estimating the optical fiber dispersion by low-speed delay sampling have the following beneficial effects:
1. the existing blind dispersion estimation method based on fractional Fourier transform (FrFT) under low-rate sampling needs to perform fractional Fourier transform and search for an optimal order at dispersion estimation equipment, has high operation complexity, and can greatly reduce the operation complexity of dispersion estimation under low-rate sampling;
2. the existing dispersion estimation method based on the neural network under low-rate sampling needs a large number of training processes before deployment, the time cost is high, and the requirement on the performance of a trained computer is high.
Drawings
Fig. 1 is a schematic diagram of an apparatus and a method for estimating optical fiber dispersion by low-speed delay sampling according to the present invention and an equivalent sampling under low-speed sampling in embodiment 1;
FIG. 2 is a delay sampling diagram after equivalent sampling in the apparatus and method for estimating fiber dispersion by low-speed delay sampling and embodiment 1 of the present invention;
FIG. 3 is a schematic diagram of an apparatus and method for estimating fiber dispersion by low-speed delay sampling according to the present invention and the apparatus in embodiment 1;
FIG. 4 is a flowchart of an iterative dispersion estimation search process in the embodiment 1 of the apparatus and method for estimating optical fiber dispersion by low-speed delay sampling according to the present invention;
FIG. 5 is a diagram of a simulation system in the embodiment 2 of the present invention and an apparatus and method for estimating fiber dispersion by low-speed delay sampling;
fig. 6 is a graph of cost function value-residual dispersion curve under 5-20 span transmission conditions in a low-speed delay sampling and estimation device and method for optical fiber dispersion and embodiment 2 of the present invention;
FIG. 7 is a diagram showing the result of the chromatic dispersion estimation of an optical signal independent and unrelated to the optical signal used for the table establishment in the apparatus and method for estimating the chromatic dispersion of an optical fiber by low-speed delay sampling according to the present invention and embodiment 2;
FIG. 8 is a diagram showing the result of estimating the chromatic dispersion of the periodic training sequence signal in the PDM-QPSK signal transmitted through the optical fiber link under different ASE and nonlinear noise according to the apparatus and method for estimating the optical fiber chromatic dispersion by low-speed delay sampling of the present invention and embodiment 3;
fig. 8 is a graph showing (a) a result of performing dispersion estimation on a periodic training sequence signal in a PDM-QPSK signal by changing ASE noise, and (b) a result of performing dispersion estimation on a periodic training sequence signal in a PDM-QPSK signal by changing nonlinear noise;
fig. 9 is a diagram of a result of chromatic dispersion estimation of a periodic training sequence signal in a PDM-QPSK signal in an actual system in an embodiment 4 and an apparatus and a method for estimating optical fiber chromatic dispersion by low-speed delay sampling according to the present invention;
Detailed Description
Example 1
The embodiment 1 of the invention elaborates the principle, the device composition and the function of the method for estimating the optical fiber dispersion by low-speed delay sampling.
Fig. 1 shows the principle of equivalent sampling under low-rate sampling in this embodiment 1. Although the sampling rate of the receiver is much lower than the nyquist sampling, due to the periodicity of the training sequence, the reassembly of the sampled data is equivalent to oversampling, where T issDenotes the sampling period, T0Denotes the symbol period, T-64T0Representing the period of the training sequence, the obtained equivalent sampling period is MTs-T. FIG. 2 is a diagram of delayed sampling after equivalent sampling in this embodiment 1, where dxExpressed as the time-delayed sampled power in the x-polarization state versus the distance from the diagonal on the delayed sample plot. Fig. 3 is a schematic view of the apparatus in embodiment 1, wherein the apparatus 300 includes: the system comprises a low-speed delay coherent detection unit 301, an equivalent sampling unit 302, a dispersion compensation unit 303, a calculation unit 304, a search judgment unit 305 and an adjustment unit 306.
The low-speed delay coherent detection unit 301 is used for performing low-speed delay detection on an optical signal transmitted in an optical fiber link at an estimation position, the signal is subjected to down-sampling and splicing through the equivalent sampling unit 302 and is equivalent to an oversampled signal, the dispersion compensation unit 303 is used for performing dispersion compensation on the equivalently sampled signal with the step length adjusted by the adjustment unit, the calculation unit 304 calculates a function value related to dispersion on the signal processed by the dispersion compensation unit, and searching iteration is performed through the searching judgment unit 305 and the adjustment unit 306 until table lookup is performed to complete dispersion estimation. FIG. 4 is a flowchart illustrating an iterative search process for chromatic dispersion estimation in this embodiment 1, where a is a step length adjustment factor, SlargeAnd SsmallAre respectively provided withCorresponding to large step length and small step length, SlookupThe dispersion value obtained by looking up the table is negative. t is t1For small step size threshold, when the function value F is related to chromatic dispersiondBelow t1Then, change from large step length to small step length, t2And t3For looking up the table window, when FdEntry into this window will trigger a table lookup operation. Setting the iteration times of the large step length and the small step length as K respectivelylAnd KsThen, the estimated value CD of the dispersionestCan be calculated from the following formula:
CDest＝Kl×Slarge+Ks×Ssmall+Slookup
example 2
In order to make the purpose, technical scheme and advantages of the apparatus and method for estimating optical fiber dispersion by low-speed delay sampling more clear, the technical scheme in the embodiment of the present invention is clearly and completely described below with reference to the accompanying drawings in embodiment 2 of the present invention:
fig. 5 is a diagram of a simulation system of this embodiment 2, which is seen to include an optical transmitting end 501, an EDFA 502, M optical fiber spans (one span includes 100km of standard single-mode fiber and one EDFA)503, a Set OSNR module 504, and the present apparatus 505.
The light emitting end 501 is configured to emit a PDM-QPSK signal, where a symbol period of a training sequence is 64; the input interface of the optical amplifier 502 is connected with the output interface of the transmitting terminal 501, and is used for adjusting the transmitting power of the transmitting terminal; the M optical fiber spans 503 are connected to the optical amplifier 502, and are used for transmitting PDM-QPSK signals generated by the transmitting end and generating cumulative dispersion; the input of the Set OSNR module 504 is connected to the outputs of the M optical fiber spans 503, and is configured to Set an optical signal-to-noise ratio of an optical signal; the input of the present apparatus 505 is connected to the output of the Set OSNR module 504 for estimating the dispersion in the optical fiber communication link.
The embodiment of the invention builds the simulation system for estimating the optical fiber communication link dispersion by the low-speed delay sampling device and the method, and is used for acquiring the optical signals sampled at low speed under different transmission conditions. PDM-QPSK signalThe transmission symbol rate of the periodic training sequence signal is 20.14 GB; the symbol period of the transmitted training sequence is 64; the low-speed sampling rate of the device is 1.25 GSa/s; the transmitting power is set to 0 dBm; OSNR is set to 18 dB; the number of the cross sections is within the range of 1-5 and 10-20, and the intervals are 1 and 5 respectively (namely, the transmission distance is within the range of 100-500 km and 1000-2000 km, and the intervals are 100km and 1000km respectively). No Amplifier Spontaneous Emission (ASE) noise is added to each EDFA, and the OSNR is controlled by Set OSNR modules at the end of the fiber link. Using Standard Single Mode Fiber (SSMF) with 16e dispersion coefficient and nonlinear coefficient- 6s/m2And 1.31W-5km-5. Therefore, the reference dispersion values are 1600ps/nm to 8000ps/nm and 16000ps/nm to 32000ps/nm, and the intervals are 1600ps/nm and 8000ps/nm respectively.
In embodiment 2 of the present invention, the simulation experiment platform is vpitrismision maker 9.5;
this embodiment 2 specifically includes the following steps:
step 1: the light emitting end equipment outputs a PDM-QPSK optical signal, and the periodic training sequence symbol period of the PDM-QPSK optical signal is 64;
step 2: the modulated optical signals are transmitted in optical fiber links with different transmission conditions;
and step 3: establishing a residual dispersion-cost function value lookup table;
wherein, the step 3 is specifically the following substeps:
step 3.1: setting the searching and judging unit and the adjusting unit to be in a table building state, and carrying out low-speed detection on optical signals transmitted by optical fiber links under different conditions by using a low-speed time-delay coherent detection unit to obtain I, Q paths of discrete digital sequences in two polarization states under different transmission conditions;
step 3.2: performing equivalent sampling on the signal obtained in the step 3.1 by using an equivalent sampling unit to obtain an equivalent sampling signal;
step 3.3: performing dispersion compensation on the signal subjected to the equivalent sampling by using a dispersion compensation unit in a set small step (lower than 16 ps/nm);
the larger the dispersion is, the higher the needed FFT point number and the Overlap point number are, and the highest simulation midspan section is 20, so that the point number of frequency domain overlapping is uniformly set to be 512, and the FFT point number is 1024;
step 3.4: the calculation unit calculates a function value related to dispersion for the signal processed by the dispersion compensation unit, and establishes a corresponding relation between accumulated dispersion and the function value;
step 3.5: the adjusting unit is set to jump out of the compensation dispersion estimation process when the compensated dispersion reaches at least 2 times of the compensated dispersion corresponding to the lowest value of the cost function, different cost function values of the same residual dispersion under different transmission conditions are averaged, a proper size step length adjusting threshold is selected in a region where the residual dispersion is positive while avoiding a limiting point, a region with higher coincidence degree of dispersion-cost function value curves under different transmission conditions is selected as a table building region in a region where the residual dispersion is negative, and a residual dispersion-cost function value lookup table suitable for different transmission conditions is built;
fig. 6 is a graph of cost function value-residual dispersion curve under the transmission condition of 5-20 spans, which illustrates the limit point and the table building interval in step 3.5;
and 4, step 4: setting a search judgment unit according to the size step length adjustment threshold and the table lookup table interval in the step (3), putting the table lookup in the step (3) into the search judgment unit, adjusting the search judgment unit and the adjustment unit to an estimation state, and performing low-speed detection on the periodic training sequence optical signal which is independent and irrelevant to the periodic training sequence optical signal used for table establishment to obtain a dispersion estimation value;
fig. 7 is a diagram illustrating a result of chromatic dispersion estimation on periodic training sequence optical signals independent and unrelated to the periodic training sequence optical signals used for table creation in embodiment 2 of the present invention, where a horizontal axis represents a theoretical value of chromatic dispersion, a left vertical axis represents an estimated value of chromatic dispersion, and a right vertical axis represents an absolute error of estimated chromatic dispersion. Fig. 7 shows that the present invention has a good effect on the dispersion estimation of the periodic training sequence signal in the PDM-QPSK signal.
Example 3
This example 3 verifies that the present invention is robust to ASE and nonlinear noise when estimating dispersion in an optical fiber communication link. The specific implementation mode is as follows: the OSNR and the transmit Power (Launch Power) are varied, respectively, thereby varying the ASE and the nonlinear noise.
Fig. 8 is a diagram illustrating a result of performing dispersion estimation on a periodic training sequence signal in a PPDM-QPSK signal transmitted by an optical fiber link under different ASE and nonlinear noise. FIG. 8(a) is a graph showing the result of varying ASE noise in the dispersion estimation of the periodic training sequence signal in the PDM-QPSK signal; fig. 8(b) is a diagram showing the result of performing dispersion estimation on a periodic training sequence signal in a PDM-QPSK signal by changing nonlinear noise. In fig. 8(a), the abscissa is OSNR, the left vertical axis is absolute error of dispersion estimation, and the right vertical axis is relative error of dispersion estimation. The abscissa of fig. 8(b) is the transmit power, the left vertical axis is the absolute error of the dispersion estimate, and the right vertical axis is the relative error of the dispersion estimate. Figure 8 demonstrates that the present invention is robust to ASE and nonlinear noise when estimating dispersion in an optical fiber communications link.
Example 4
This example 4 demonstrates the performance of the present invention in estimating dispersion in an optical fiber communication link in a practical system. The specific implementation mode is as follows: building an actual system, wherein the signal transmission symbol rate of the PDM-QPSK periodic training sequence is 20.14 GB; the symbol period of the transmitted training sequence is 64; the sampling rate of a receiving end is 1.25 GSa/s; the transmitting power is set to 0 dBm; OSNR is set to 18 dB; the number of the cross sections is within the range of 1-5, and the interval is 1 (namely the transmission distance is 100-500 km, and the interval is 100 km). Using Standard Single Mode Fiber (SSMF) with 16e dispersion coefficient and nonlinear refractive index-6s/m2And 2.4e-20m2and/W. Therefore, the reference dispersion value is 1600ps/nm to 8000ps/nm, and the interval is 1600 ps/nm. The data is processed off-line to estimate the dispersion of the actual system signal using the lookup table constructed in example 2 and the method of the present invention.
Fig. 9 is a diagram of a result of estimating chromatic dispersion of an optical signal in an actual system in embodiment 4 of the present invention, in which a horizontal axis is a theoretical value of chromatic dispersion, a left vertical axis is an estimated value of chromatic dispersion, and a right vertical axis is an absolute error of estimated chromatic dispersion. Fig. 8 shows that, when the lookup table established in the simulation system is used, the method has a good effect on the dispersion estimation of the periodic training sequence signal in the PDM-QPSK signal in the actual system.
While the foregoing is directed to the preferred embodiment of the present invention, it is not intended that the invention be limited to the embodiment and the drawings disclosed herein. Equivalents and modifications may be made without departing from the spirit of the disclosure, which is to be considered as within the scope of the invention.
Claims (8)
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