A carrier synchronization method based on QAM signal

Abstract

The present application proposes a carrier synchronization method based on a QAM signal, which is mainly realized by an equalization algorithm and a phase-locked loop decision corresponding to a constellation diagram. The process includes preprocessing to extract a carrier recovery signal, and then completing a rough adjustment of the carrier frequency offset through a constellation equalization algorithm, and then using a phase-locked loop decision to realize frequency phase error estimation, adjusting the carrier error through a numerically controlled oscillator, and correcting the error through feedback from a loop filter to achieve a carrier recovery effect. The constellation equalization decision algorithm of the present invention mainly completes the equalization decision of the constellation points of the carrier recovery signal and the demodulated signal to extract the error weight vector, and completes the adjustment of the carrier frequency offset. The present invention has a low degree of complexity and a simple implementation method, and does not limit whether the constellation diagram of the MQAM signal is a cross or a square, and can achieve carrier frequency and phase correction.

Description

Technical Field

[0001] The invention belongs to the technical field of carrier recovery, and in particular relates to a carrier synchronization method based on a QAM signal. Background Art

[0002] In digital communication systems, carrier synchronization is a very important link. Carrier synchronization is also called carrier recovery, which is to generate a local oscillation with the same frequency and phase as the received signal in the receiving device and supply it to the demodulator for coherent demodulation.

[0003] Multiple Quadrature Amplitude Modulation (MQAM) is a modulation and demodulation technology with high spectrum utilization. During the transmission process, the signal is affected by factors such as Doppler frequency shift, and the received signal has frequency error and phase deviation. In order to ensure the accuracy of the demodulation result, carrier recovery processing must be performed to meet the bit error rate requirements.

[0004] The existing modulation and demodulation technology makes phase correction according to the constellation diagram of the MQAM signal, which is divided into cross and square types. It has low adaptability and high complexity. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a carrier synchronization method based on QAM signals. The technical problem to be solved by the present invention is achieved by the following technical solutions:

[0006] The present invention provides a QAM signal carrier synchronization method, which is applied to a carrier system. The QAM signal carrier synchronization method includes:

[0007] S100, receiving any QAM received signal r at time t within a period QAM (t), and the QAM receiving signal r QAM (t) preprocessing to obtain the carrier signal r(t) to be recovered;

[0008] S200, multiplying the carrier signal to be recovered r(t) by the carrier frequency signal u(t) at time t to obtain a carrier recovery signal S(t) at time t;

[0009] S300, using an equalization algorithm to perform coarse deflection correction on the carrier recovery signal S(t) at the time t, to obtain a deflection-corrected symbol V(t) at the time t;

[0010] S400, performing polarity determination on the constellation point Tr / Tq of the corrected symbol V(t) at the time t, and obtaining a phase error FreqEr(t) between the constellation point Tr / Tq at the time t and an ideal constellation point;

[0011] S500, extracting a low-frequency component θ(t) from the phase error FreqEr(t), and using the low-frequency component θ(t) to adjust the carrier frequency signal u(t) at the time t to obtain an adjusted carrier frequency signal;

[0012] S600, using the adjusted carrier signal as the carrier frequency signal at time t+1, and repeating S200 to S500.

[0013] Beneficial effects:

[0014] The present application proposes a carrier synchronization method based on a QAM signal, which is mainly realized by an equalization algorithm and a phase-locked loop decision corresponding to a constellation diagram. The process includes preprocessing to extract a carrier recovery signal, and then completing a rough adjustment of the carrier frequency offset by a constellation equalization algorithm, and then using a phase-locked loop decision to realize frequency phase error estimation, adjusting the carrier error by a numerically controlled oscillator (NCO), and performing error correction by feedback from a loop filter to achieve a carrier recovery effect. The constellation equalization decision algorithm of the present invention mainly completes the equalization decision of the constellation points of the carrier recovery signal and the demodulated signal to extract the error weight vector, and completes the adjustment of the carrier frequency offset. The present invention has a low degree of complexity, a simple implementation method, and does not limit whether the constellation diagram of the MQAM signal is a cross or a square, and can achieve correction of the carrier frequency and phase.

[0015] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of a carrier system provided by the present invention;

[0017] Figure 2 It is a decision schematic diagram of the phase-locked loop provided by the present invention;

[0018] Figure 3 It is a flow chart of a QAM signal carrier synchronization method provided by the present invention;

[0019] Figure 4 It is a schematic diagram of a QAM receiver provided by the present invention;

[0020] Figure 5 is a schematic diagram of the equalization decision algorithm provided by the present invention;

[0021] Figure 6 It is a 16QAM equalization decision grid diagram provided by the present invention;

[0022] Figure 7 It is a decision grid diagram of phase error provided by the present invention;

[0023] Figure 8It is a carrier deviation convergence diagram provided by the present invention;

[0024] Fig. 9 It is a system test diagram provided by the present invention;

[0025] Fig.10 This is the simulation test constellation diagram provided by the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0027] Before introducing the present invention, a brief introduction to the technical concept of the present invention is first given.

[0028] The present invention proposes a new digital carrier frequency estimation method based on the basic characteristics of MQAM signals by using symbol data information. The method adopts a closed-loop method, based on the basic structure of a phase-locked loop, and an equalization algorithm adjusts its convergence characteristics, guides the symbol information of the received data to make a judgment, and judges it to the constellation point closest to the information characteristics to complete carrier phase tracking. The method has low complexity and simple implementation, and does not limit whether the constellation diagram of the MQAM signal is a cross or a square, and can achieve carrier frequency and phase correction. The effectiveness of the algorithm has been verified by simulation in actual system applications.

[0029] The specific details of the scheme of the present invention are introduced below.

[0030] The present invention provides a QAM signal carrier synchronization method, which is applied to a carrier system. Figure 1 The carrier system includes a mixer, a loop filter, a phase detector, a decision device and an NCO. Figure 2 The phase-locked loop is a phase feedback control system commonly used in digital demodulation. The local carrier is adjusted through the feedback structure of the phase deviation in the loop, so that the output carrier is infinitely close to the actual modulated carrier; the phase-locked loop decision algorithm flow chart in this application is as follows Figure 2 As shown, the design adopts a closed-loop structure and realizes accurate carrier recovery by continuously adjusting the feedback system parameters.

[0031] Combination Figures 1 to 3 The QAM signal carrier synchronization method of the present invention comprises:

[0032] S100, receiving any QAM received signal r at time t within a period QAM (t), and the QAM receiving signal r QAM (t) preprocessing to obtain the carrier signal r(t) to be recovered;

[0033] Fully Digital QAM Receiver Reference Figure 4The preprocessing of this step includes down-conversion, bandpass filtering and other processing; down-conversion converts the signal to baseband signal, and the signal contains a certain amount of frequency deviation, which is caused by clock deviation, Doppler frequency shift and other factors; bandpass filtering is to filter out other signals and retain the signal for demodulation as much as possible. The reception of this step meets any QAM received signal r at time t within the period QAM (t), expressed as

[0034]

[0035] Where, X n , Y n is the mutually independent data carrying information in the QAM constellation diagram, and its value set is {X n +jY n}, and Xn, ω c is the carrier frequency; g(t) is the impulse response of the raised cosine shaping filter; T s is the symbol period, t is a moment in the symbol period;

[0036] The QAM received signal r QAM (t) is down-converted and band-pass filtered to obtain the carrier signal r(t) to be recovered at time t.

[0037] S200, multiplying the carrier signal to be recovered r(t) by the carrier frequency signal u(t) at time t to obtain a carrier recovery signal S(t) at time t; the carrier recovery signal S(t) is expressed as:

[0038] S(t)=r(t)*u(t);

[0039] u(t)=cos(θt)+sin(θt);

[0040] Where θ is the phase error.

[0041] S300, using an equalization algorithm to perform coarse deflection correction on the carrier recovery signal S(t) at the time t, to obtain a deflection-corrected symbol V(t) at the time t;

[0042] Step S300 of the present invention includes:

[0043] S310, using an equalization algorithm, mapping the carrier recovery signal S(t) at the time t onto a constellation diagram to obtain a real symbol point Z(t);

[0044] refer to Figure 5 , S310 of the present invention includes:

[0045] S320, using the error weighted vector W(t-1) at time t-1 and the corrected symbol V(t-1) to perform phase compensation on the symbol point Z(t) to obtain the corrected symbol V(t) at time t, which is expressed as:

[0046] V(t)=V(t-1)+W(t-1)*Z(t).

[0047] In this step, the carrier recovery signal S(t) is correlated with the symbol point to obtain Z(t), and Z(t) is weighted correlated with the error weight vector W(t-1). By iterative processing of fixed point equalization, the signal constellation point Tr / Tq and the corrected symbol V(t) are output.

[0048] Among them, Tr / Tq=V(t)*Aeqstep, Aeqstep is the equalization normalization value, Tr / Tq represents Tr and Tq, Tr represents the constellation point data of the Ith path in the symbol V(t) after correction, Tq represents the constellation point data of the Qth path in the symbol V(t) after correction, and the Ith path and the Qth path are parallel.

[0049] As a specific embodiment of the present invention, S300 includes:

[0050] S311, according to the constellation diagrams of QAM signals of different modulation orders and the positions of the ideal symbol points on the constellation diagrams, the plane coordinate system is divided into a plurality of grid units, represented as 0, ±2, ±4…±2 n ;

[0051] S312, for any grid unit, determine whether the real symbol point mapped to the grid unit by the carrier recovery signal S(t) at time t exceeds the boundary of the grid unit. If so, the real symbol point does not belong to the grid unit. If not, the real symbol point belongs to the grid unit, and obtain the real symbol point Z(t) mapped to the grid unit by the carrier recovery signal S(t) at the current time t.

[0052] As a specific embodiment of the present invention, the calculation process of the error weighted vector W(t-1) at time t-1 in S320 includes:

[0053] a. Map the carrier recovery signal S(t-1) at time t-1 onto the constellation diagram to obtain the real symbol point Z(t-1);

[0054] b, calculate the deviation Err(t-1) between the real symbol point Z(t-1) and the ideal symbol point at time t-1;

[0055] refer to Figure 6According to the QAM signals with different modulation orders, the plane coordinate system is divided into decision grids, i.e., 0, ±2, ±4, etc., according to the positions of the ideal constellation points; it is determined whether the weighted symbol data V(t-1) is greater than or less than the decision grid threshold, and Figure 6 If it is less than the threshold, it converges to the two ideal constellation points on the left; if it is greater than the threshold, it converges to the two constellation points on the right. The positive and negative areas in the figure, that is, the positive deviation or negative deviation Err(t-1), can be obtained.

[0056] c. Using the error weighted vector W(t-2), the deviation Err(t-1) and the true symbol point Z(t-1) at time t-2, calculate the error weighted vector W(t-1) at time t-1, expressed as:

[0057] W(t)=W(t-1)-step*Err(t-1)*Z(t);

[0058] Among them, step is the balancing step size of the balancing algorithm.

[0059] Taking 16QAM as an example, the four constellation points in the first quadrant are as follows: Figure 6 As shown, the carrier recovery signal is mapped to obtain the symbol point Z(t-1), and the deviation from the ideal constellation point is obtained after judgment is made according to the divided grid diagram, the error weighted vector is calculated, and the signal is phase compensated to obtain the correction effect.

[0060] As a specific embodiment of the present invention, a includes:

[0061] a1, according to the constellation diagram of QAM signals with different modulation orders and the position of the ideal symbol point on the constellation diagram, the plane coordinate system is divided into multiple grid units, represented as 0, ±2, ±4…±2 n ;

[0062] a2. For any grid unit, determine whether the real symbol point mapped to the grid unit by the carrier recovery signal S(t-1) at time t-1 exceeds the boundary of the grid unit. If so, the real symbol point does not belong to the grid unit. If not, the real symbol point belongs to the grid unit. The real symbol point Z(t-1) mapped to the grid unit by the carrier recovery signal S(t-1) at time t-1 is obtained.

[0063] S400, performing polarity determination on the constellation point Tr / Tq of the corrected symbol V(t) at the time t, and obtaining a phase error FreqEr(t) between the constellation point Tr / Tq at the time t and an ideal constellation point;

[0064] refer to Figure 7 According to the principle of the all-digital QAM receiver, the ideal QAM constellation point occupies The coordinate system can be divided into Figure 7 In the grid diagram shown, the black points are ideal constellation points.

[0065] Carrier error extraction includes two parts: the decision device and the phase detector; the decision device makes symbol decisions based on the output constellation point Tr / Tq, that is, according to Figure 7 The grid diagram is used to determine the error between the red data points and the black ideal constellation points, and the positions of the positive and negative areas marked in the diagram are obtained. The decision formula is expressed as:

[0066]

[0067] Among them, Sgn represents the sign;

[0068] The phase detector calculates the relevant data of the constellation point and obtains the phase error FreqEr(t) expressed as:

[0069] FreqEr(t)=sgn(Tr)Tq-Trsgn(Tq) ；

[0070] S500, extracting a low-frequency component θ(t) from the phase error FreqEr(t), and using the low-frequency component θ(t) to adjust the carrier frequency signal u(t) at the time t to obtain an adjusted carrier frequency signal;

[0071] In this step, the loop filter filters out the high-frequency component of the phase error in the phase detector output signal, retains the low-frequency component, and converts the phase error into a corresponding phase adjustment parameter.

[0072] S600, using the adjusted carrier signal as the carrier frequency signal at time t+1, and repeating S200 to S500.

[0073] In this step, the output phase of the digital controlled oscillator is adjusted multiple times by changing the phase parameter to eliminate the carrier deviation. The carrier deviation convergence diagram finally obtained by the present invention is as follows: Figure 8 shown.

[0074] The effect of the present invention is explained below through simulation experiments.

[0075] In actual system equipment applications, such as Fig. 9 As shown; the signal source outputs the modulated signal and sends it through the transmitting antenna, the device antenna receives the signal, and after the ADC analog-to-digital conversion, the FPGA chip receives the digital modulated signal. The new joint carrier recovery algorithm designed by the present invention is run to test 16QAM, 32QAM, 64QAM, and 128QAM. The test results are shown in Fig.10 As shown. Fig.10It can be seen that the carrier recovery algorithm of the present invention achieves a better convergence effect for different QAM signal carriers, the algorithm is not affected by the signal type, the implementation complexity is low, and the convergence effect is good.

[0076] Although the present application is described herein in conjunction with various embodiments, in the process of implementing the claimed application, those skilled in the art may understand and implement other variations of the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude a plurality of components or steps.

[0077] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A QAM signal carrier synchronization method, characterized in that: Applied to a carrier system, the QAM signal-based carrier synchronization method includes: S100, receiving any QAM received signal r at time t within a period QAM (t), and the QAM receiving signal r QAM (t) preprocessing to obtain the carrier signal r(t) to be recovered; S200, multiplying the carrier signal to be recovered r(t) by the carrier frequency signal u(t) at time t to obtain a carrier recovery signal S(t) at time t; S300, using an equalization algorithm to perform coarse deflection correction on the carrier recovery signal S(t) at the time t, to obtain a deflection-corrected symbol V(t) at the time t; S400, performing polarity determination on the constellation point Tr / Tq of the corrected symbol V(t) at the time t, and obtaining a phase error FreqEr(t) between the constellation point Tr / Tq at the time t and an ideal constellation point; S500, extracting a low-frequency component θ(t) from the phase error FreqEr(t), and using the low-frequency component θ(t) to adjust the carrier frequency signal u(t) at the time t to obtain an adjusted carrier frequency signal; S600, using the adjusted carrier signal as the carrier signal at time t+1, and repeating S200 to S500; S300 includes: S310, using an equalization algorithm, mapping the carrier recovery signal S(t) at the time t onto a constellation diagram to obtain a real symbol point Z(t); S320, using the error weighted vector W(t-1) at time t-1 and the corrected symbol V(t-1) to perform phase compensation on the symbol point Z(t) to obtain the corrected symbol V(t) at time t, which is expressed as: V(t)=V(t-1)+W(t-1)*Z(t); The carrier recovery signal S(t) is subjected to symbol point correlation processing to obtain Z(t), and a weighted correlation operation is performed on Z(t) and the error weight vector W(t-1). Through iterative processing of fixed point equalization, the signal constellation point Tr / Tq and the corrected symbol V(t) are output; Among them, Tr / Tq=V(t)*Aeqstep, Aeqstep is the equalization normalization value, Tr / Tq represents Tr and Tq, Tr represents the constellation point data of the Ith path in the symbol V(t) after correction, Tq represents the constellation point data of the Qth path in the symbol V(t) after correction, and the Ith path and the Qth path are parallel.

2. The QAM signal carrier synchronization method according to claim 1, characterized in that: S100 includes: S110, receiving a QAM received signal r at time t within any matching period QAM (t), expressed as: Where, X n , Y n is the mutually independent data carrying information in the QAM constellation diagram, and its value set is {X n +jY n },and M represents the modulation order; ω c is the carrier frequency; g(t) is the impulse response of the raised cosine shaping filter; T s is the symbol period, t is a moment in the symbol period; S120, receiving the QAM signal r QAM (t) is down-converted and band-pass filtered to obtain the carrier signal r(t) to be recovered at time t.

3. The QAM signal carrier synchronization method according to claim 1, characterized in that: The carrier recovery signal S(t) in S200 is expressed as: S(t)=r(t)*u(t); u(t)=cos(θt)+sin(θt); Where θ is the phase error.

4. The QAM signal carrier synchronization method according to claim 1, characterized in that: S310 includes: S311, dividing the plane coordinate system into a plurality of grid units according to constellation diagrams of QAM signals of different modulation orders and positions of ideal symbol points on the constellation diagrams; S312, for any grid unit, determine whether the real symbol point mapped to the grid unit by the carrier recovery signal S(t) at time t exceeds the boundary of the grid unit. If so, the real symbol point does not belong to the grid unit. If not, the real symbol point belongs to the grid unit, and obtain the real symbol point Z(t) mapped to the grid unit by the carrier recovery signal S(t) at the current time t.

5. The QAM signal carrier synchronization method according to claim 1, characterized in that: The calculation process of the error weighted vector W(t-1) at time t-1 in S320 includes: a. Map the carrier recovery signal S(t-1) at time t-1 onto the constellation diagram to obtain the real symbol point Z(t-1); b, calculate the deviation Err(t-1) between the real symbol point Z(t-1) and the ideal symbol point at time t-1; c. Using the error weighted vector W(t-2), the deviation Err(t-1) and the true symbol point Z(t-1) at time t-2, calculate the error weighted vector W(t-1) at time t-1, expressed as: W(t)=W(t-1)-step*Err(t-1)*Z(t); Among them, step is the balancing step size of the balancing algorithm.

6. The QAM signal carrier synchronization method according to claim 5, characterized in that: a include: a1, according to the constellation diagram of QAM signals with different modulation orders and the position of the ideal symbol point on the constellation diagram, the plane coordinate system is divided into multiple grid units, represented as 0, ±2, ±4…±2 n ; a2. For any grid unit, determine whether the real symbol point mapped to the grid unit by the carrier recovery signal S(t-1) at time t-1 exceeds the boundary of the grid unit. If so, the real symbol point does not belong to the grid unit. If not, the real symbol point belongs to the grid unit. The real symbol point Z(t-1) mapped to the grid unit by the carrier recovery signal S(t-1) at time t-1 is obtained.

7. The QAM signal carrier synchronization method according to claim 1, characterized in that: S400 includes: Performing polarity judgment on the constellation point Tr / Tq of the corrected symbol V(t) at the time t to obtain a phase error between the constellation point Tr / Tq at the time t and an ideal QAM constellation point; Among them, the ideal QAM constellation point occupies Position; the phase error FreqEr(t) is expressed as: FreqEr(t)=sgn(Tr)Tq-Trsgn(Tq); Among them, sgn represents the symbol, M represents the modulation order, and Tr and Tq both represent constellation points.

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