Background
In recent years, high dynamics are ubiquitous in satellite communications, aerospace measurement and control, and navigation systems. In a satellite communication system, with rapid development of satellite communication technology, mobile communication between constellations and between the earth and between space vehicles (communication satellites, navigation satellites, deep space probes, etc.) is receiving more and more attention; because of the high-speed motion of the satellite constellation relative to the ground and the high-speed motion between the constellations, the receiving and transmitting sides are under the high-dynamic communication condition, and the satellite constellation shows serious time selective fading in the time domain, and shows that the signals have large Doppler frequency offset and high-order Doppler frequency offset change rate in the frequency domain.
In the satellite navigation field, the application of low-medium dynamic receivers in the civil field is quite wide, such as vehicle and ship navigation; however, the research on the high dynamic receiver in China is less, and the high dynamic navigation system plays a role in improving the military capability; therefore, research into signal receiving technology in a low signal-to-noise ratio, high dynamic environment has been increasingly urgent.
In a high dynamic application environment, high-speed relative motion between two communication parties often introduces larger Doppler frequency offset and high-order change rate thereof on carrier frequency of signals received by a receiver; in order to adapt to the high dynamic environment, the loop bandwidth of the carrier tracking loop of the medium-low dynamic receiver must be widened to capture and track the doppler frequency offset and the change rate of the received signal, however, the increase of the loop bandwidth also tends to reduce the carrier tracking accuracy, and especially when the tracking loop is in a low signal-to-noise ratio communication environment, the introduction of noise can even cause the carrier tracking loop to lose lock.
Carrier synchronization in a high dynamic environment mainly has two technical difficulties; the absolute frequency offset is larger, and in a high dynamic environment, the communication carrier often has extremely high movement speed, so that the communication carrier has extremely large Doppler frequency offset at a communication receiving end, and serious challenges are brought to the design of a receiver. The occurrence of large frequency offset forces the receiver to have to relax the front-end bandwidth so that the useful signal can completely enter the post-stage signal processing module; however, a large amount of out-of-band noise is inevitably introduced while the bandwidth is amplified, so that the input signal-to-noise ratio of the receiver is obviously reduced, and great challenges are brought to the subsequent synchronization and demodulation modules; meanwhile, in the capturing module, a larger frequency offset range also means a larger frequency search interval, and the too large search interval may cause practical engineering problems such as shortage of hardware resources of a receiver.
Another technical difficulty in a high dynamic environment is the large rate of frequency change; in the actual space, the relative movement speed of the communication carrier often changes drastically, so that the corresponding Doppler frequency offset also generates drastic fluctuation, namely, a larger frequency change rate exists; the higher rate of frequency change has a stronger impact on the communication system than the frequency offset; the method not only needs the receiver synchronization module to give a relatively accurate estimated value during capturing, but also requires the carrier tracking system to have extremely high dynamic tracking capability, and can give an accurate estimated value in real time under the condition of rapid change of carrier frequency and phase, thereby bringing serious tests to the capturing and tracking modules in carrier synchronization.
Disclosure of Invention
The invention aims to provide a satellite signal carrier synchronization method and a system, which can realize accurate capturing and tracking of a satellite signal carrier and complete second synchronization of the carrier signal under the environment of high dynamic and large frequency offset.
The invention is realized by the following technical scheme:
in a first aspect, a satellite signal carrier synchronization method is provided, including the steps of:
performing mixing filtering on the received carrier signal to obtain a sampling signal;
Calculating an in-phase component and a quadrature component of the sampling signal based on the sampling signal;
capturing carrier frequency offset in the sampling signal based on the in-phase component and the quadrature component and compensating to realize carrier first synchronization;
The carrier signal after the first synchronization is subjected to frequency mixing filtering by utilizing a pre-designed double-layer phase-locked loop, the carrier frequency and phase information of the carrier signal after the frequency mixing filtering are extracted for the first time, and the carrier frequency offset and the phase deviation are compensated for the first time through the carrier frequency and the phase information, so that the carrier signal after the first compensation is obtained;
the carrier signal after the first compensation is subjected to frequency mixing filtering by utilizing a pre-designed double-layer phase-locked loop, carrier frequency and phase information of the carrier signal after the first compensation and frequency mixing filtering are extracted for the second time, and carrier frequency deviation and phase deviation are compensated for the second time through the carrier frequency and the phase information, so that the carrier signal after the second compensation is obtained;
And according to the carrier signals after the first compensation and the second compensation, completing the phase tracking of the carrier signals and realizing the second synchronization of the carrier.
With reference to the first aspect, further, the in-phase component and the quadrature component of the sampled signal are calculated by the following method:
Where I (k) is an in-phase component, Q (k) is a quadrature component, r (k) is a sampled signal, nI (k) is an in-phase component of noise, nQ (k) is a quadrature component of noise,Is the phase of the local oscillator after mixing with the received carrier signal, deltaf (k) is the frequency estimate residual, tk-1 is the start time,Is the initial phase of the received carrier signal.
With reference to the first aspect, further, the carrier frequency offset in the sampled signal is captured by the following method:
where f (k) is the carrier frequency offset, I (k) is the in-phase component, Q (k) is the quadrature component,Is the phase of the local oscillator after mixing with the received carrier signal, deltaf (k) is the frequency estimate residual, tk-1 is the start time,Is the initial phase of the received carrier signal,Is the phase difference between the phase of the carrier signal after mixing and the initial phase;
the carrier frequency offset in the sampling signal is compensated by the DDS.
With reference to the first aspect, further, the double-layer phase-locked loop performs two-time mixing filtering on the carrier signal through the down-converter and the matched filter.
With reference to the first aspect, further, the carrier frequency and phase information of the filtered carrier signal are extracted for the first time and extracted for the second time by the following method:
the known code sequence is used for carrying out inverse modulation to obtain carrier signal prior information, and the carrier frequency and phase information of the carrier signal are obtained by carrying out integral accumulation and zero clearing on the prior information.
With reference to the first aspect, further, the first compensation and the second compensation for the carrier frequency offset and the phase offset are implemented by the following methods:
And the carrier frequency and phase information of the carrier signal are utilized to realize the first compensation and the second compensation of the carrier frequency difference and the phase deviation through DDS and filtering.
In a second aspect, the invention also provides a satellite signal carrier synchronization system, which comprises a down converter, a matched filter, a frequency locking loop and a double-layer phase locking loop;
down converter and matched filter: the device is used for carrying out mixing filtering on the received carrier signals to obtain sampling signals;
Frequency locking ring: the carrier frequency offset is used for acquiring an in-phase component and a quadrature component of the carrier frequency offset based on the sampling signal and the in-phase component and the quadrature component of the carrier frequency offset based on the sampling signal and compensating the carrier frequency offset, so that the carrier first synchronization is realized;
Double-layer phase-locked loop: the carrier frequency offset compensation method comprises the steps of carrying out frequency mixing filtering on carrier signals after primary synchronization, extracting carrier frequency and phase information of the carrier signals after the frequency mixing filtering for the first time, and carrying out primary compensation on carrier frequency offset and phase deviation through the carrier frequency and the phase information to obtain carrier signals after primary compensation;
Mixing and filtering the carrier signal after the first compensation, extracting carrier frequency and phase information of the carrier signal after the first compensation and mixing and filtering for the second time, and carrying out the second compensation on carrier frequency offset and phase deviation through the carrier frequency and phase information to obtain the carrier signal after the second compensation;
And according to the carrier signals after the first compensation and the second compensation, completing the phase tracking of the carrier signals and realizing the second synchronization of the carrier.
Compared with the prior art, the invention has the following beneficial effects:
According to the satellite signal carrier synchronization method and system provided by the invention, the received carrier signal is subjected to mixing filtering to obtain the sampling signal, and the carrier frequency offset in the sampling signal is captured and compensated by the pre-designed frequency locking ring, so that the first carrier synchronization is realized; the carrier frequency and the phase information of the carrier signal are firstly extracted through the pre-designed double-layer phase-locked loop, the carrier frequency deviation and the phase deviation of the carrier signal are firstly compensated according to the carrier frequency deviation and the phase deviation, the carrier signal after the first compensation is obtained, the carrier signal is filtered again, the carrier frequency and the phase information of the carrier signal are accurate, the carrier frequency deviation and the phase deviation are secondly compensated according to the carrier frequency deviation and the phase deviation, the carrier signal after the second compensation is obtained, the phase tracking of the carrier signal is completed, and the carrier secondary synchronization is realized;
The invention provides a satellite signal carrier synchronization method and a system, which provides a carrier synchronization algorithm combining a frequency-locking loop and a double-layer phase-locking loop, and utilizes an improved frequency-locking loop to capture larger carrier frequency deviation and compensate to realize carrier first synchronization.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for more clearly illustrating the technical aspects of the present invention, and are not intended to limit the scope of the present invention.
Example 1
With reference to fig. 1, fig. 2, and fig. 3, the method for synchronizing satellite signal carriers provided by the invention includes the following steps:
Mixing filtering is carried out on the received carrier signals to obtain sampling signals:
The invention provides a satellite signal carrier synchronization method, which adopts a down converter and a matched filter to carry out mixing filtering on a received carrier signal; after the carrier signal passes through the down converter, the mixed signal is lower than the original signal, the carrier frequency in the carrier signal is reduced, the carrier signal has the maximum signal-to-noise ratio after passing through the matched filter, the noise in the carrier signal is reduced, and the sampling signal is obtained.
The in-phase component and the quadrature component of the sampling signal are calculated based on the sampling signal:
According to the satellite signal carrier synchronization method provided by the invention, after the sampling signal is obtained, the sampling signal is calculated to obtain the in-phase component and the quadrature component of the sampling signal.
The in-phase component and the quadrature component of the sampled signal are calculated by:
Where I (k) is an in-phase component, Q (k) is a quadrature component, r (k) is a sampled signal, nI (k) is an in-phase component of noise, nQ (k) is a quadrature component of noise,Is the phase of the local oscillator after mixing with the received carrier signal, deltaf (k) is the frequency estimate residual, tk-1 is the start time,Is the initial phase of the received carrier signal.
Capturing carrier frequency offset in the sampling signal based on the in-phase component and the quadrature component and compensating, so as to realize carrier first synchronization:
According to the satellite signal carrier synchronization method provided by the invention, after the in-phase component and the quadrature component of the sampling signal are obtained through calculation, the carrier frequency offset in the sampling signal is captured through the frequency discriminator based on the in-phase component and the quadrature component of the sampling signal.
The carrier frequency offset is captured by the following method:
where f (k) is the carrier frequency offset, I (k) is the in-phase component, Q (k) is the quadrature component,Is the phase of the local oscillator after mixing with the received carrier signal, deltaf (k) is the frequency estimate residual, tk-1 is the start time,Is the initial phase of the received carrier signal,Is the phase difference between the phase of the carrier signal after mixing and the initial phase; and each symbol in the formula is irrelevant to the change of the data symbol and only changes along with the frequency offset, x (k) in fig. 2 is a denominator in the formula, and y (k) is a numerator in the formula.
After the carrier frequency offset in the sampling signal is calculated, the carrier frequency offset in the sampling signal is compensated through the DDS; the invention provides a satellite signal carrier synchronization method, which utilizes an improved frequency locking ring to capture larger carrier frequency offset and compensates to realize carrier first synchronization.
The carrier signal after the first synchronization is filtered by utilizing a pre-designed double-layer phase-locked loop, the carrier frequency and phase information of the carrier signal after the filtering are extracted for the first time, and the carrier frequency offset and the phase deviation are compensated for the first time through the carrier frequency and the phase information, so that the carrier signal after the first compensation is obtained:
The invention provides a satellite signal carrier synchronization method, which adopts a structure of a double-layer phase-locked loop, wherein the phase-locked loop 1 (PLL 1) is used for phase-locking a carrier signal so as to acquire frequency information of the carrier signal, and the carrier signal is reversely rotated by utilizing the frequency information acquired by the PLL1, so that the newly acquired carrier signal has only a small frequency difference, and the first compensation of the carrier signal is realized.
The specific process flow of the PLL1 for the first compensation of the carrier signal is as follows:
carrying out mixing filtering on the carrier signal, reducing noise in the carrier signal, and obtaining the carrier signal with the maximum signal-to-noise ratio;
in PLL1, a priori information of the carrier signal is obtained by inverse modulation with a known code sequence (UW);
Integrating, accumulating and clearing prior information of the carrier signal through a low-pass filter to acquire carrier frequency and phase information of the carrier signal;
And the carrier frequency and phase information acquired in the PLL1 is utilized to realize the first compensation of carrier frequency offset and phase deviation of the carrier signal through the DDS and the loop filter.
The carrier signal after the first compensation is subjected to frequency mixing filtering by utilizing a pre-designed double-layer phase-locked loop, carrier frequency and phase information of the carrier signal after the first compensation and frequency mixing filtering are extracted for the second time, and carrier frequency deviation and phase deviation are compensated for the second time through the carrier frequency and the phase information, so that the carrier signal after the second compensation is obtained:
The invention provides a satellite signal carrier synchronization method, which adopts a double-layer phase-locked loop structure, and after the carrier signal is compensated for the first time by a phase-locked loop (PLL) 1, the carrier signal is subjected to low-delay filtering by a phase-locked loop (PLL) 2, and the carrier signal is compensated for the second time until a loop is locked.
The second compensation specific processing flow of the PLL2 to the carrier signal is as follows:
Carrying out mixing filtering on the carrier signal after the first compensation, reducing noise in the carrier signal, and obtaining the carrier signal with the maximum signal-to-noise ratio;
in PLL2, the prior information of the first compensated and mixed filtered carrier signal is obtained by inverse modulation with a known code sequence (UW);
Integrating, accumulating and clearing prior information of the carrier signal through a low-pass filter to acquire carrier frequency and phase information of the carrier signal;
and the carrier frequency and phase information acquired in the PLL2 is utilized to realize the second compensation of carrier frequency offset and phase deviation of the carrier signal through the DDS and a loop filter.
According to the carrier signals after the first compensation and the second compensation, the phase tracking of the carrier signals is completed, and the second synchronization of the carrier is realized:
After the carrier signal is compensated for the first time by the PLL1 and the carrier signal is compensated for the second time by the PLL2, the phase tracking of the carrier signal is completed according to the carrier signal after the first compensation and the second compensation, and the carrier secondary synchronization is realized.
The invention provides a satellite signal carrier synchronization method, which provides a carrier synchronization algorithm combining a frequency-locking loop and a double-layer phase-locking loop, and utilizes an improved frequency-locking loop to capture larger carrier frequency offset and compensate to realize carrier first synchronization, the double-layer phase-locked loop is utilized to track the phase, the carrier tracking capture range is increased under the environment of high dynamic and large frequency offset, the convergence speed of the tracking loop is improved, the carrier accurate capture and tracking of satellite signals are realized, and the carrier secondary synchronization is rapidly realized.
Example two
In order to verify the feasibility of the carrier synchronization method designed in the invention, the feasibility of a time service signal link is analyzed on the premise of not affecting the communication performance of the carrier synchronization method, and the satellite signal carrier synchronization method provided by the invention is explained by taking the communication reception of a high-speed aircraft in a certain satellite communication system as an example with reference to fig. 4 and 5.
The Doppler frequency fd (t) of the received carrier signal can be expressed as:
In the above equation, the first term fd(t0 on the right of the equation) represents the constant doppler frequency of the received carrier signal, which corresponds to the relative radial movement velocity between the satellite and the receiver, and the coefficients f'd(t0) and f "d(t0) on the remaining two terms on the right of the equation represent the first and second order rates of change of the doppler frequency of the received carrier signal, respectively, which correspond to the acceleration and jerk of the relative radial movement between the satellite and the receiver.
Assuming a Doppler frequency offset of 4.4kHz (GPS Doppler frequency offset of about 929 Hz), the Doppler frequency change rate is 40Hz/s.
Firstly, setting a frequency locking loop parameter and an initial frequency difference: fd(t0)＝4.4KHz,f'd(t0)＝40Hz/s,f"d(t0) =0, eb/n0= -3dB.
An open loop test of the Frequency Locked Loop (FLL) is performed, and the lock detection indication signal can be detected after 50000 measurement samples (50000/16=3125 symbols=0.35 ms). As shown in fig. 4.
The design and verification of a phase-locked loop (PLL) is performed, in which a second-order loop is to be used for a loop filter, and the transfer function F(s) of the loop filter can be expressed as:
For convenience of representation, letThe parameter values of the three branches of the loop filter can be obtained as follows:
In the above formula, b is a real value coefficient of a real pole, and b is 1; k is the gain of the PLL, k=1; xi is a damping coefficient, and 0.707 is taken; omegan is the undamped oscillation frequency,BL is the loop bandwidth.
According to the design parameters, simulation experiments are performed to verify that fig. 5 shows the loop tracking performance (with larger phase deviation) in the capturing state.
Example III
The invention provides a satellite signal carrier synchronization system, which adopts the satellite signal carrier synchronization method in the embodiment one.
The invention provides a satellite signal carrier synchronization system, which comprises a down converter and a matched filter, wherein the down converter is used for carrying out mixing filtering on a received carrier signal to obtain a sampling signal; after the carrier signal passes through the down converter, the mixed signal is lower than the original signal, the carrier frequency in the carrier signal is reduced, the carrier signal has the maximum signal-to-noise ratio after passing through the matched filter, the noise in the carrier signal is reduced, and the sampling signal is obtained.
The invention provides a satellite signal carrier synchronization system, which also comprises a frequency locking ring, wherein the frequency locking ring is used for calculating an in-phase component and a quadrature component of the satellite signal based on a sampling signal, capturing carrier frequency offset in the sampling signal based on the in-phase component and the quadrature component and compensating the carrier frequency offset to realize first carrier synchronization; for first synchronization of the mixed filtered sampled signal, the first synchronization being achieved by capturing a larger carrier frequency offset and compensating for the larger carrier frequency offset using an improved frequency-locked loop.
The invention provides a satellite signal carrier synchronization system, which also comprises a double-layer phase-locked loop, wherein the double-layer phase-locked loop is used for filtering a carrier signal after the first synchronization, extracting carrier frequency and phase information of the carrier signal after the filtering for the first time, and carrying out the first compensation on carrier frequency deviation and phase deviation through the carrier frequency and phase information to obtain the carrier signal after the first compensation; and carrying out frequency mixing filtering on the carrier signal after the first compensation, extracting carrier frequency and phase information of the carrier signal after the first compensation and the frequency mixing filtering for the second time, and carrying out the second compensation on carrier frequency offset and phase deviation through the carrier frequency and the phase information to obtain the carrier signal after the second compensation.
The double-layer phase-locked loop is used for carrying out secondary synchronization on the sampling signals after primary synchronization, the double-layer phase-locked loop comprises a phase-locked loop 1 (PLL 1) and a phase-locked loop 2 (PLL 2), the phase-locked loop 1 (PLL 1) is used for extracting carrier frequency and phase information of carrier signals for the first time, carrying out primary compensation on carrier frequency offset and phase deviation, the phase-locked loop 2 (PLL 2) is used for extracting carrier frequency and phase information of the carrier signals for the second time, carrying out secondary compensation on carrier frequency offset and phase deviation, carrying out phase tracking by utilizing the double-layer phase-locked loop, increasing the carrier tracking capture range in the environment of high dynamic and large frequency offset, improving the convergence speed of a tracking loop, realizing carrier accurate capture and tracking on satellite signals, and rapidly realizing carrier secondary synchronization.
The foregoing is merely a preferred embodiment of the present invention, and it should be noted that modifications and variations could be made by those skilled in the art without departing from the technical principles of the present invention, and such modifications and variations should also be regarded as being within the scope of the invention.