CN110943950B - FFT frequency offset estimation method based on amplitude amplification and phase rotation - Google Patents
FFT frequency offset estimation method based on amplitude amplification and phase rotation Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/003—Arrangements for allocating sub-channels of the transmission path
- H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
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- H—ELECTRICITY
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- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/0014—Carrier regulation
- H04L2027/0024—Carrier regulation at the receiver end
- H04L2027/0026—Correction of carrier offset
Abstract
The invention relates to an FFT frequency offset estimation method based on amplitude amplification and phase rotation, and belongs to the technical field of optical communication. The method comprises the following steps: obtaining an off-line signal to be detected; step two: carrying out amplitude mean value normalization processing; step three: performing fourth power processing on the normalized data; step four: judging whether the signal can be subjected to frequency offset estimation directly by using the evaluation factor gamma; step five: the value of the evaluation factor gamma is increased using amplitude amplification and phase rotation. Step six: and obtaining a frequency offset value by using FFT. According to the method, the peak amplitude after Fourier transform can be improved in an amplitude amplification and phase rotation mode, so that the peak value can be found more easily. The method has better robustness to spontaneous radiation noise and phase noise in the frequency offset measurement process; the evaluation factor gamma is provided to judge whether various modulation formats can be directly used for frequency offset estimation by FFT, and the gamma value can be improved by means of amplitude amplification and phase rotation to meet the conditions under the condition that the modulation formats cannot be directly used.
Description
Technical Field
The invention relates to an FFT frequency offset estimation method based on amplitude amplification and phase rotation, and belongs to the technical field of coherent optical communication and frequency offset estimation.
Technical Field
In the field of coherent optical communication, frequency offset estimation is one of the key technologies, and more frequency offset estimation schemes are proposed. The current relatively mature schemes include a time domain frequency offset estimation method based on a differential frequency offset and a frequency domain frequency offset estimation method based on an FFT. However, in the existing frequency offset estimation methods, the time domain differential frequency offset estimation method is suitable for frequency offset estimation of M-PSK signals. Although the scheme of partition rotation is applied to frequency offset estimation of high-order QAM signals, the scheme is less robust to spontaneous emission noise or phase noise.
The complexity of the frequency offset estimation method based on FFT is high, and in order to ensure real-time processing of data at the receiving end, the length of the data point used in the frequency offset estimation is usually 512. In this case, frequency offset estimation can be performed on rectangular distributed signals such as QPSK and 8/16/64 QAM. But for signals with non-rectangular distribution of constellation points, such as 32-QAM, 128QAM, etc., the FFT-based frequency offset estimation scheme cannot be directly applied.
Aiming at the problems of non-rectangular distribution signals, the invention aims to provide an FFT frequency offset estimation scheme based on amplitude amplification and phase rotation to realize frequency offset estimation of 32-QAM and other constellation point non-rectangular distribution signals. The scheme has better robustness to spontaneous radiation noise or phase noise, and provides an evaluation factor gamma to judge whether signals of various modulation formats can be subjected to frequency offset estimation by using FFT.
Disclosure of Invention
The invention aims to provide an FFT frequency offset estimation method based on amplitude amplification and phase rotation, aiming at solving the problem that the existing frequency offset estimation scheme can not realize frequency offset estimation of non-rectangular distributed signals such as 32-QAM.
The core idea of the invention is as follows: based on the influence of the modulation amplitude and the modulation phase of the signal on the peak amplitude of the signal after Fourier transform, the amplitude of the peak value of the signal is improved in an amplitude amplification and phase rotation mode, so that the robustness of the signal to spontaneous radiation noise and phase noise is improved. After the processed signal is subjected to Fourier transform, the peak value is easier to detect, so that a frequency offset value corresponding to the peak value position is obtained, and frequency offset estimation is realized.
The FFT frequency offset estimation method based on amplitude amplification and phase rotation comprises the following steps:
the method comprises the following steps: obtaining an offline signal to be detected, obtaining the signal to be detected after the signal to be detected is subjected to analog-to-digital conversion, dispersion compensation, clock recovery and polarization mode dispersion compensation at a coherent communication receiving end, and expressing the signal to be detected by a formula (1):
wherein S isnIs the signal to be measured, N is the data serial number, N represents the length of the data,
representing a modulated signal, AnModulation amplitude, anModulation phase, Δ f denotes frequency offset, TsRepresenting the symbol sampling interval, θnIs phase noise caused by laser linewidth;
step two: carrying out amplitude mean value normalization processing on the signal to be measured to obtain normalized data S'nExpressed by formula (2):
step three: performing quartic processing on the normalized data to obtain quartic processed data
Specifically, it is expressed by formula (3):
step four: judging whether the signal can be subjected to frequency offset estimation directly by using the evaluation factor gamma, namely calculating the evaluation factor gamma by a formula (4) after the quartic operation;
the evaluation factor gamma is used for judging whether the signal can be subjected to frequency offset estimation by directly utilizing FFT;
step five: when the gamma value is judged to be low and the FFT cannot be directly used for frequency offset estimation, the evaluation factor gamma needs to be improved, and the processed data is obtained, specifically:
firstly, placing the data processed by the fourth power obtained in the third step in a plane rectangular coordinate system, and finding two sides of a Y-axis of data point distribution; extracting the side with fewer distributed data points through an interval with a set amplitude value to obtain an extracted constellation point;
secondly, amplitude amplification is carried out on data with smaller amplitude in consideration of difference of the amplitudes of the extracted constellation points, so that the extracted constellation points comprise amplified data points and unamplified data points;
wherein the magnification is more than or equal to 2 times;
thirdly, performing phase rotation processing on the amplified data points and the data points which are not amplified to obtain processed data, and increasing the value of an evaluation factor gamma of the processed data;
step six: performing frequency offset measurement on the processed data and other unprocessed data with amplitude by using FFT, specifically represented by formula (5):
wherein, the data with unprocessed other amplitudes, namely the data obtained after the fourth power processing in the step three, is the data except the extracted constellation points; in the formula (5)
For normalized frequency, its range is [ -1/2Ts,-1/2Ts]The left side delta f of the formula (5) is a frequency offset estimation value;
thus, the FFT frequency offset estimation method based on amplitude amplification and phase rotation is completed.
Advantageous effects
Compared with the existing frequency offset estimation method, the FFT frequency offset estimation method based on amplitude amplification and phase rotation has the following beneficial effects:
1. compared with the existing frequency offset estimation method, the method provided by the invention has better robustness to spontaneous radiation noise and phase noise;
2. the peak amplitude after Fourier transform can be improved by means of amplitude amplification and phase rotation, so that the peak value can be found more easily;
3. the evaluation factor gamma provided by the invention can judge whether various modulation formats can directly utilize FFT to carry out frequency offset estimation, and when the FFT cannot be directly utilized to carry out frequency offset estimation, the gamma value can be improved in an amplitude amplification and phase rotation mode to meet the condition;
4. the invention can be combined with a modern coherent optical communication receiving end DSP, and carries out real-time processing on data at the DSP part of the receiving end, thereby having great application potential.
Drawings
FIG. 1 is a flowchart of an FFT frequency offset estimation method and an embodiment of the invention based on amplitude amplification and phase rotation;
FIG. 2 is a schematic diagram of a system in an embodiment of an FFT frequency offset estimation method based on amplitude amplification and phase rotation according to the present invention;
FIG. 3 is a constellation diagram of a 32-QAM signal and constellation points after signal squaring in an embodiment of a FFT frequency offset estimation method based on amplitude amplification and phase rotation according to the present invention;
fig. 4 shows the result of an embodiment of an FFT frequency offset estimation method based on amplitude amplification and phase rotation.
Detailed Description
The following describes an FFT frequency offset estimation method based on amplitude amplification and phase rotation in detail with reference to specific embodiments.
Example 1
This example describes a specific implementation of an FFT frequency offset estimation method based on amplitude amplification and phase rotation.
The system is shown in fig. 2, which produces a 32-QAM signal at 28Gbaud, with the laser linewidth set at 100 kHz. PRBS length of 230-1. The Set OSNR module is used for controlling the optical signal-to-noise ratio of the signal to be detected.
The steps for receiving data are as shown in figure 1, which specifically comprises:
a, at a coherent communication receiving end, when a signal to be detected is subjected to analog-to-digital conversion, obtaining offline data, and firstly performing dispersion compensation, clock recovery and polarization mode dispersion compensation;
b, carrying out amplitude mean value normalization processing on the signal to be detected to obtain constellation points shown in the attached figure 3 (a);
and C: performing a fourth power processing on the normalized data to obtain constellation points as shown in fig. 3 (b);
step D: the gamma value of the data after the fourth power operation is calculated, the gamma value at the moment is 14.50 percent, and the frequency offset estimation is difficult to be directly carried out;
step E: analyzing the constellation point distribution of fig. 3(b), it can be seen that the data of the C2 and C4 circles are located at the right side of the Y axis; c2 circle data with the amplitude range of 0.1-0.32 and C4 circle data with the amplitude range of 1.6-2.6 are selected by using the amplitude values; because the amplitude of the C2 circle data is lower, firstly amplifying the C2 circle data to 2.11, and rotating the amplified C2 circle data and C4 circle data by 180 degrees; the value of gamma at this time was calculated to be 70.29%;
step F: the processed data is substituted into formula (5) to be solved to obtain a frequency offset value, the frequency offset value obtained by calculation is used for compensating frequency offset and calculating the Bit Error Rate (BER) of the data, and the result is shown in figure 4. From the results, it can be found that, in the conventional FFT frequency offset estimation method, even when the length of the processed data reaches 1024, the error rate after compensating the frequency offset is all above the soft decision error correction coding (SD-FEC) threshold of 20%; after the data is processed by the method, even if the length of the processed data is 512, the error rate of the data with the OSNR of 22dB is below the SD-FEC threshold, which shows that the FFT frequency offset estimation method based on amplitude amplification and phase rotation can effectively realize the frequency offset estimation of the non-rectangular distribution signal.
While the above-described embodiments are preferred embodiments of the FFT frequency offset estimation method based on amplitude amplification and phase rotation, the present invention should not be limited to the disclosure of the embodiments and the accompanying drawings. The embodiment is only used for helping to understand the method of the invention and the core idea thereof; various changes or modifications may be made by those skilled in the art within the scope of the claims without departing from the spirit of the invention.
For those skilled in the art, the invention can be modified in the specific embodiments and applications according to the spirit of the present invention, and the content of the present description should not be construed as limiting the invention. 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 (3)
1. An FFT frequency offset estimation method based on amplitude amplification and phase rotation is characterized in that: the method comprises the following steps:
the method comprises the following steps: at a coherent communication receiving end, after the signal to be detected is subjected to analog-to-digital conversion, dispersion compensation, clock recovery and polarization mode dispersion compensation, obtaining the signal to be detected;
expressed by formula (1):
wherein S isnIs the signal to be measured, N is the data serial number, N represents the length of the data,
representing a modulated signal, AnModulation amplitude, anModulation phase, Δ f denotes frequency offset, TsRepresenting the symbol sampling interval, θnIs phase noise caused by laser linewidth;
step two: carrying out amplitude mean value normalization processing on the signal to be measured to obtain normalized data S'nExpressed by formula (2):
step three: performing quartic processing on the normalized data to obtain quartic processed data
Specifically, it is expressed by formula (3):
step four: judging whether the signal can be subjected to frequency offset estimation directly by using the evaluation factor gamma, namely calculating the evaluation factor gamma after the quartic operation;
wherein the evaluation factor γ is represented by formula (4):
the evaluation factor gamma is used for judging whether the signal can be subjected to frequency offset estimation by directly utilizing FFT;
step five: when the gamma value is judged to be low and the FFT cannot be directly used for frequency offset estimation, the evaluation factor gamma needs to be improved, and the processed data is obtained, specifically:
firstly, placing the data processed by the fourth power obtained in the third step in a plane rectangular coordinate system, and finding two sides of a Y-axis of data point distribution; extracting the side with fewer distributed data points through an interval with a set amplitude value to obtain an extracted constellation point;
secondly, amplitude amplification is carried out on data with small amplitude in consideration of difference of the amplitudes of the extracted constellation points, so that the extracted constellation points comprise amplified data points and unamplified data points;
thirdly, performing phase rotation processing on the amplified data points and the data points which are not amplified to obtain processed data, and increasing the value of an evaluation factor gamma of the processed data;
step six: performing frequency offset measurement on the processed data and other unprocessed data with amplitude by using FFT, specifically represented by formula (5):
wherein, the data with unprocessed other amplitudes, namely the data obtained in the third step after the fourth power processing, is the data except the extracted constellation points; in the formula (5)
To normalize frequency, Δ f to the left of equation (5)estNamely, the frequency offset estimation value is obtained;
thus, the FFT frequency offset estimation method based on amplitude amplification and phase rotation is completed.
2. The method of claim 1, wherein the FFT frequency offset estimation method based on amplitude amplification and phase rotation comprises: and the second medium amplitude magnification of the step five is more than or equal to 2 times.
3. The method of claim 1, wherein the FFT frequency offset estimation method based on amplitude amplification and phase rotation comprises: in the formula (5)
In the range of [ -1/2Ts,1/2Ts]。
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Application publication date: 20200331 Assignee: Hengshui Hanwang Electronic Technology Co.,Ltd. Assignor: BEIJING INSTITUTE OF TECHNOLOGY Contract record no.: X2022990000971 Denomination of invention: A FFT Frequency Offset Estimation Method Based on Amplitude Amplification and Phase Rotation Granted publication date: 20201106 License type: Exclusive License Record date: 20221201 |