CN113114377B - QPSK signal frequency offset estimation method for spatial coherent laser communication - Google Patents

QPSK signal frequency offset estimation method for spatial coherent laser communication Download PDF

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CN113114377B
CN113114377B CN202110245514.3A CN202110245514A CN113114377B CN 113114377 B CN113114377 B CN 113114377B CN 202110245514 A CN202110245514 A CN 202110245514A CN 113114377 B CN113114377 B CN 113114377B
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frequency offset
frequency
phase
offset estimation
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CN113114377A (en
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杨成武
刘向南
赵�卓
林一
李晓亮
谌明
于勇
李凉海
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Beijing Research Institute of Telemetry
Aerospace Long March Launch Vehicle Technology Co Ltd
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Aerospace Long March Launch Vehicle Technology Co Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/548—Phase or frequency modulation
    • H04B10/556—Digital modulation, e.g. differential phase shift keying [DPSK] or frequency shift keying [FSK]
    • H04B10/5561—Digital phase modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/613—Coherent receivers including phase diversity, e.g., having in-phase and quadrature branches, as in QPSK coherent receivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H04B10/6165—Estimation of the phase of the received optical signal, phase error estimation or phase error correction
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • 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

A QPSK signal frequency offset estimation method for spatial coherent laser communication is characterized in that modulation information of an ADC sampling signal after balanced detection is eliminated in a quadratical mode; performing a small number of point FFT (fast Fourier transform) operations on the data with the modulation information eliminated to obtain a signal frequency spectrum; dividing the frequency value corresponding to the peak value of the signal frequency spectrum by 4 to obtain a preliminary frequency offset estimation value delta f1(ii) a Using preliminary frequency offset estimate Δ f1Performing preliminary frequency offset compensation on the sampling signal; obtaining signal phase information after preliminary frequency offset compensation through a lookup table; carrying out differential operation on the phase information to obtain a phase difference of adjacent symbols; processing the phase difference to obtain a phase; estimating the residual frequency offset value delta f by the obtained phase2(ii) a Finally, the frequency deviation estimated value delta f is used2And performing secondary frequency compensation on the signal subjected to the primary frequency offset compensation to obtain a final frequency offset compensated signal.

Description

QPSK signal frequency offset estimation method for spatial coherent laser communication
Technical Field
The invention relates to a QPSK signal frequency offset estimation method for spatial coherent laser communication, and belongs to the technical field of laser communication.
Background
Compared with microwave communication, space laser communication has the characteristics of high transmission rate, strong anti-interference capability, small terminal volume, low power consumption, light weight and the like, so that the space laser communication becomes one of the current high-speed inter-satellite and inter-satellite-ground communication technology development directions with the most potential. The laser communication technology system is divided into two major types, namely a coherent communication system and an incoherent communication system. At present, the main system of space laser communication mainly comprises two incoherent detection systems of OOK modulation/direct detection, PPM modulation/direct detection, and PSK modulation/coherent reception detection. The OOK modulation/direct detection system receiving and demodulating technology is simple, but the detection sensitivity is low; the PPM modulation/direct detection system has low bandwidth utilization rate and is not beneficial to high-speed and high-sensitivity communication among satellites; the PSK modulation/coherent detection system has higher receiving sensitivity and background interference resistance, and is the first choice of a high-orbit inter-satellite long-distance high-code-rate laser communication link. On the other hand, according to internationally recognized evaluation criteria, the performance of a laser link can be characterized by the communication rate multiplied by the square of the link distance:
Q＝Rb·L2
q is link performance and has a unit of Gbit/s.106km2，RbThe unit is communication speed, Gbit/s, L is link distance, and the unit is 103km2. When Q is more than 1000 Gbit/s.106km2When the method is used, a coherent laser communication system is more suitable to select; q < 1000 Gbit/s.106km2In this case, either a coherent communication scheme or a non-coherent communication scheme may be selected. But for high-orbit space link laser communication, Q is more than 100000Gbit/s 106km2Is inevitable.
Therefore, based on the future development requirement of inter-satellite links, the advantages of the coherent communication system in terms of code rate expansion, detection sensitivity, background light interference resistance and the like are synthesized compared with the direct detection, and the coherent system is an important scheme for building a future laser communication network.
With the successful on-track experiments of the high-speed high-order coherent laser communication terminal, the laser communication of the QPSK coherent system is verified, and the spatial coherent laser signal modulation system is changed from BPSK to QPSK.
As shown in fig. 1, in a spatial coherent laser communication system, at a transmitting end, laser generated by a narrow linewidth signal laser is used as a carrier, modulated into a QPSK signal by an electro-optical modulator, and the modulated optical signal is amplified by an erbium-doped fiber amplifier (EDFA) and transmitted to a free space channel by a transmitting antenna. After the spatial link transmission, the optical signal received by the receiving antenna at the receiving end is filtered by a narrow-band optical filter to remove part of background light noise, and then 4 paths of optical signals output after the signal light and the local oscillator light are mixed in a 2 × 490-degree optical mixer are converted into 2 paths of electric signals (in-phase I path and orthogonal Q path electric signals) after balanced detection. And finally, the ADC performs analog-to-digital conversion on the two paths of electric signals and sends the electric signals to the DSP data processing unit, and the laser communication signals are processed in an electric domain. Because the frequencies of the received light and the local oscillator are not consistent, the frequency mixing cannot be carried out perfectly, so that frequency offset exists, and the frequency offset estimation and compensation are required to be carried out on signals in a subsequent DSP data processing unit.
In the prior art (see Selmi M, Jaou n Y, circuit P. accurate digital frequency shift estimate for coherent Polmux QAM transmission systems [ C ].200935th European Conference on Optical Communication (ECOC 2009), IEEE,2009,1-2.), frequency offset estimation can be realized based on FFT, but the estimation accuracy is related to the number of FFT calculation points, and the number of FFT points must be increased to improve the estimation accuracy, which greatly increases the calculation complexity. The frequency offset Estimation method based on post-Phase-difference modulo described in the prior art (see Hoffmann S, Bhandare S, Pfau T, et al. frequency and Phase Estimation For Coherent QPSK Transmission With Unlocked DFB Lasers [ J ]. IEEE Photonics technologies Letters,2008,20(18):1569 and 1571.) has poor performance when the frequency offset Estimation value is large. The accuracy of the M-power based frequency offset Estimation method described in the prior art (see Leven A, Kaneda N, Koc U V, et al. frequency Estimation in Intradyne Reception. IEEE Photonics Technology Letters,2007,19(6): 366:. 368.) is affected by the sum average length, which increases the computational complexity significantly. Therefore, how to provide a frequency offset estimation method with high estimation accuracy and moderate computation complexity for QPSK signals suitable for spatial coherent laser communication is a problem to be solved urgently by those skilled in the art.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the method overcomes the defects of the prior art, provides a QPSK signal frequency offset estimation method for spatial coherent laser communication, and eliminates modulation information of an ADC sampling signal after balanced detection in a quadratical mode; performing a small number of point FFT (fast Fourier transform) operations on the data with the modulation information eliminated to obtain a signal frequency spectrum; dividing the frequency value corresponding to the peak value of the signal frequency spectrum by 4 to obtain a preliminary frequency offset estimation value delta f1(ii) a Using preliminary frequency offset estimate Δ f1Performing preliminary frequency offset compensation on the sampling signal; obtaining signal phase information after preliminary frequency offset compensation through a lookup table; carrying out differential operation on the phase information to obtain a phase difference of adjacent symbols; processing the phase difference to obtain a phase; estimating the residual frequency offset value delta f by the obtained phase2(ii) a Finally, the frequency deviation estimated value delta f is used2And performing secondary frequency compensation on the signal subjected to the primary frequency offset compensation to obtain a final frequency offset compensated signal.
The purpose of the invention is realized by the following technical scheme:
a QPSK signal frequency offset estimation method for spatial coherent laser communication comprises the following steps:
s1, eliminating modulation information of the ADC sampling signal after balance detection to obtain a sampling signal A;
s2, FFT operation is carried out on the sampling signal A to obtain the frequency corresponding to the peak value in the signal frequency spectrum, and then the preliminary frequency offset estimation value delta f is obtained1；
S3, utilizing Delta f1Performing frequency compensation on the ADC sampling signal after the balance detection to obtain a sampling signal B after the preliminary frequency offset compensation, wherein the phase data of the sampling signal B after the preliminary frequency offset compensation is C; obtaining a signal residual phase through phase searching;
s4, carrying out differential operation on the signal residual phase to obtain the phase difference of adjacent symbols; eliminating modulation information and noise of the phase data C by using the phase difference of adjacent symbols, and performing summation average operation on the phase data C after the modulation information and the noise are eliminated to obtain phase data D;
s5, carrying out phase frequency conversion on the phase data D to obtain a secondary frequency offset estimation value delta f2；
S6, according to Δ f1And Δ f2And obtaining a final frequency offset estimation value delta f.
The above-mentioned frequency offset estimation method for QPSK signal preferably uses Δ f2And carrying out frequency compensation on the sampling signal B to obtain a final frequency-compensated signal.
In the above QPSK signal frequency offset estimation method, preferably, in S1, the method for eliminating the modulation information of the ADC sampling signal after the balance detection includes: and performing fourth power on the ADC sampling signal after the balance detection.
In the above QPSK signal frequency offset estimation method, preferably, in S2, FFT operation is performed on partial data of the sampled signal a to obtain a frequency corresponding to a peak in a signal spectrum.
In the above QPSK signal frequency offset estimation method, preferably, in S2, the frequency corresponding to the peak in the signal spectrum is divided by 4 to obtain a preliminary frequency offset estimation value Δ f1。
In the above QPSK signal frequency offset estimation method, preferably, in S4, the method for removing the modulation information and noise of the phase data C by using the phase difference between adjacent symbols includes:
and adding pi/4 to the phase difference of adjacent symbols, then performing modulo operation on the pi/2, and finally subtracting the pi/4.
In the above QPSK signal frequency offset estimation method, preferably, in S6, Δ f is calculated1And Δ f2And adding to obtain a final frequency offset estimation value delta f.
A computer-readable storage medium, on which a computer program is stored, which, when executed by a processor, implements the above-described QPSK signal frequency offset estimation method.
A laser communication frequency offset estimation device comprises:
a processor; and
a memory for storing computer program instructions;
wherein when the computer program instructions are loaded and executed by the processor, the processor performs the above-described QPSK signal frequency offset estimation method.
Compared with the prior art, the invention has the following beneficial effects:
(1) the invention greatly improves the frequency offset estimation precision in the spatial coherent laser communication system and reduces the system calculation complexity.
(2) The invention improves the signal recovery effect and the error code performance of the system.
(3) The invention performs N-point (N is 256, 512 and 1024) FFT (less points) operation, thereby reducing the calculation complexity.
(4) The invention adopts the Homman algorithm to carry out secondary frequency offset estimation on the sampling signal B, thereby improving the frequency offset estimation precision.
(5) The invention uses a small number of points to carry out FFT operation during primary frequency offset estimation, thereby greatly reducing the calculation complexity, and then carries out secondary frequency offset estimation with lower complexity, thereby greatly improving the frequency offset estimation precision and being suitable for the field of spatial coherent laser communication.
Drawings
FIG. 1 is a diagram of a spatially coherent laser communication system of the present invention.
Fig. 2 is a flowchart of a frequency offset estimation method and a compensation apparatus according to the present invention.
FIG. 3 is a graph comparing the loss curves of Optical signal to noise ratio (OSNR) at different levels of error rate for the same error rate using the method proposed by the present invention and the method described in the prior art (see Selmi M, Jaou N Y, C. accurate digital frequency offset estimator for coherent Polmux QAM transmission systems [ C ].200935th European Conference on Optical Communication (ECOC 2009), IEEE,2009,1-2.) and the prior art (see Leven A, Kaneda N, Koc U V, et al. frequency Estimation in Intraradial reception. IEEE Photonics technologies Letters,2007,19(6): 366-.
FIG. 4 is a graph comparing error rate curves at different Optical signal to noise ratio (OSNR) values for the method proposed by the present invention and the method described in the prior art (see Selmi M, Jaou n Y, Ciblat P. accurate digital frequency offset estimator for coherent policy Mux QAM transmission systems [ C ].200935th European Conference on Optical Communication (ECOC 2009), IEEE,2009, 1-2.).
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
A QPSK signal frequency offset estimation method suitable for spatial coherent laser communication comprises the following steps:
s1: performing the fourth power on the ADC sampling signal after the balance detection, eliminating modulation information, and executing S2;
s2: performing N-point (N is 256, 512, 1024) FFT operation on the data from which the modulation information is removed to obtain a frequency spectrum with frequency offset data, and executing S3;
s3: performing peak value search on the frequency spectrum obtained in the step S2, finding a corresponding frequency value at the peak value, and executing a step S4;
s4: dividing the frequency value searched by S3 by 4 to obtain a preliminary frequency offset estimation value delta f1Executing S5;
s5: obtaining the preliminary frequency offset estimation value delta f obtained in S41Performing frequency compensation on the ADC sampling signal after the balance detection to obtain a signal after the preliminary frequency offset compensation, and executing S6;
s6: performing phase search on the preliminary frequency offset compensation signal through a lookup table to obtain a signal residual phase, and executing S7;
s7: carrying out differential operation on the signal residual phase to obtain a phase difference of adjacent symbols, and executing S8.1;
s8.1: adding pi/4 to the phase difference, then performing modulo operation on pi/2, and finally subtracting pi/4 to obtain phase data for eliminating modulation information and noise, and executing S8.2;
s8.2: performing a sum-average operation on the phase data from which the modulation information and the noise are removed, and performing S9;
s9: performing phase frequency conversion on the phase data after the summation average operation to obtain a secondary frequency offset estimation value delta f2Executing S10 and S11;
s10: obtaining the estimated value of the second frequency offset delta f obtained in S92After compensating for preliminary frequency offsetPerforming frequency compensation on the signal to obtain a final frequency-compensated signal;
s11: the preliminary frequency deviation estimated value delta f1And a secondary frequency offset estimate Δ f2And adding to obtain a final frequency deviation estimated value delta f.
In order to illustrate the technical solution of the present invention, the following description will be made by way of specific implementation.
As shown in fig. 1, which is a diagram of an exemplary spatial coherent laser communication system according to the present invention, at a transmitting end, laser generated by a narrow linewidth signal laser is used as a carrier, modulated into a QPSK signal by an electro-optical modulator, and the modulated optical signal is amplified by an erbium-doped fiber amplifier (EDFA) and transmitted to a free space channel by a transmitting antenna. After transmission through an inter-satellite link, optical signals received by a receiving antenna at a receiving end are filtered by a narrow-band optical filter to remove part of background light noise, and then 4 paths of optical signals output after frequency mixing of signal light and local oscillator light in a 2 x 490-degree optical mixer are converted into 2 paths of electric signals (in-phase I path and quadrature Q path electric signals) after balanced detection. And finally, the ADC performs analog-to-digital conversion on the two paths of electric signals and sends the electric signals to the DSP data processing unit, and the laser communication signals are processed in an electric domain. In the DSP data processing unit, firstly, the frequency offset estimation and compensation of the data are carried out.
In the above process, the signal light E incident to the coherent laser receiverSThe following were used:
Figure BDA0002963951210000061
Asto receive the complex amplitude of the signal light, f1To receive the optical signal carrier frequency, thetas(t) is phase information of the modulated QPSK signal, θs(t)∈{±π/4,±3π/4}，θn(t) is laser phase noise, subject to the wiener process;
Figure BDA0002963951210000071
for the original phase of the signal, n1(t) is white Gaussian noise.
Produced by local oscillator laserOptical signal ELOAnd signal light ESCarry out mixing, in which ELOThe following were used:
Figure BDA0002963951210000072
ALOis the complex amplitude of the local oscillator light, f2Is the local oscillator optical carrier frequency, θn1(t) local oscillator laser phase noise obeying the wiener process;
Figure BDA0002963951210000073
is the original phase of the local oscillator light, n2(t) is white Gaussian noise. After 90-degree frequency mixing and balanced detection, I, Q two paths of data I (t) are obtained, Q (t) is as follows:
Figure BDA0002963951210000074
Figure BDA0002963951210000075
a is the complex amplitude of the mixing signal, Δ f is the residual carrier frequency of the mixing signal, θn2(t) is the difference between the phase noise of the two lasers, and the two lasers also obey the wiener process;
Figure BDA0002963951210000076
to mix the original phases of the signal, n (t) is white gaussian noise. The nth complex signal symbol Sn, which arrives at fig. 2 via AD sampling, is as follows:
Figure BDA0002963951210000077
as shown in the process S1, the process proceeds to the fourth power of Sn to remove the modulation signal information, then N-point (N-256, 512, 1024) FFT operations are performed on the data after the modulation information is removed according to the process S2, then peak search is performed on the frequency spectrum according to the process S3 to find the corresponding frequency value at the peak, and finally, according to the process S3, the frequency value is foundProcedure S4 for obtaining preliminary frequency offset estimation value Δ f1S1-S4 can be described as the following mathematical process:
Figure BDA0002963951210000078
as shown in the process S5, the preliminary frequency offset estimation value Δ f is used1Carrying out frequency compensation on Sn to obtain a signal S 'subjected to preliminary frequency offset compensation'nIt can be expressed as the following mathematical process:
S'n＝Sn×exp(-j2πΔf1t)
as shown in the process S6, the signal S 'after the preliminary frequency offset compensation is performed'nPhase is searched to obtain phase psinIt can be expressed as the following mathematical process:
ψn＝arg{S'n}
then, according to the process S7, performing a difference operation on the phase information to obtain a phase difference between adjacent symbols, then, as described in the process S8.1, adding pi/4 to the phase difference, then performing a modulo operation on pi/2, and finally subtracting pi/4 to obtain phase data for eliminating modulation information and noise, then, according to the step of the process S8.2, performing a sum-average operation on the phase data for eliminating modulation information and noise, and finally, according to the process S9, performing phase-frequency conversion on the phase data after the sum-average operation to obtain a secondary frequency offset estimation value Δ f2S7-S9 can be described as the following mathematical process:
Figure BDA0002963951210000081
as shown in procedure S10, a quadratic frequency offset estimation value Δ f is used2To S'nPerforming frequency compensation to obtain a final frequency offset compensated signal S'n' can be expressed as the following mathematical process:
S″n＝S'n×exp(-j2πΔf2t)
as shown in the process S11, the preliminary frequency offset estimation value Δ f is obtained1And a secondary frequency offset estimation value delta f2Carrying out summation operation to obtain the final frequency deviation estimated value delta f ═ delta f1+Δf2。
FIG. 3 shows that the error rate is 10 by the method of combining 256-point FFT and Hoffman, the Leven estimation method and the Hoffman estimation method provided by the invention-4And comparing the optical signal to noise ratio (OSNR) loss curves at different frequency deviation degrees. It can be seen that, when the frequency offset is large (greater than 500MHz), the OSNR value loss by the method of combining 256-point FFT with Hoffman is much smaller than that by the Hoffman estimation method, and compared with the Leven estimation method, the performance is improved by about 0.2 dB.
Fig. 4 is a comparison graph of the error rate curves of the method of the present invention and the method using only N-point FFT at different optical signal-to-noise ratio (OSNR) values. It can be seen that the 256-point FFT combined with Hoffman method is superior to the 1024-point FFT only method. And the more the FFT method points, the better the error code performance.
Those skilled in the art will appreciate that those matters not described in detail in the present specification are well known in the art.
Although the present invention has been described with reference to the preferred embodiments, it is not intended to limit the present invention, and those skilled in the art can make variations and modifications of the present invention without departing from the spirit and scope of the present invention by using the methods and technical contents disclosed above.

Claims (8)

1. A QPSK signal frequency offset estimation method for spatial coherent laser communication is characterized by comprising the following steps:
s1, eliminating modulation information of the ADC sampling signal after balance detection to obtain a sampling signal A;
s2, FFT operation is carried out on the sampling signal A to obtain the frequency corresponding to the peak value in the signal frequency spectrum, and then the preliminary frequency offset estimation value delta f is obtained1；
S3, utilizing Delta f1After detecting the balancePerforming frequency compensation on the ADC sampling signal to obtain a sampling signal B after preliminary frequency offset compensation, wherein the phase data of the sampling signal B after the preliminary frequency offset compensation is C; obtaining a signal residual phase through phase searching;
s4, carrying out differential operation on the signal residual phase to obtain the phase difference of adjacent symbols; eliminating modulation information and noise of the phase data C by using the phase difference of adjacent symbols, and performing summation average operation on the phase data C after the modulation information and the noise are eliminated to obtain phase data D;
s5, carrying out phase frequency conversion on the phase data D to obtain a secondary frequency offset estimation value delta f2；
S6, converting delta f1And Δ f2And adding to obtain a final frequency offset estimation value delta f.
2. The method of estimating frequency offset of QPSK signal according to claim 1, wherein Δ f is used2And carrying out frequency compensation on the sampling signal B to obtain a final frequency-compensated signal.
3. The QPSK signal frequency offset estimation method according to claim 1, wherein in S1, the method for removing the modulation information of the ADC sampled signal after the balanced detection comprises: and performing fourth power on the ADC sampling signal after the balance detection.
4. The method for estimating frequency offset of QPSK signal according to claim 1, wherein in S2, FFT operation is performed on the partial data of the sampled signal a to obtain the frequency corresponding to the peak in the signal spectrum.
5. The method for frequency offset estimation of QPSK signal according to claim 1, wherein in S2, the frequency corresponding to the peak in the signal spectrum is divided by 4 to obtain a preliminary frequency offset estimation value Δ f1。
6. The QPSK signal frequency offset estimation method of claim 1, wherein in S4, the method for removing the modulation information and noise of the phase data C by using the adjacent symbol phase difference comprises:
and adding pi/4 to the phase difference of adjacent symbols, then performing modulo operation on the pi/2, and finally subtracting the pi/4.
7. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out the steps of the method according to any one of claims 1 to 6.
8. A laser communication frequency offset estimation device comprises:
a processor; and
a memory for storing computer program instructions;
wherein when the computer program instructions are loaded and executed by the processor, the processor performs the method of any of claims 1 to 6.
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