CN116938324A - Satellite signal time-frequency double-synchronization method and device based on fractional Fourier transform - Google Patents
Satellite signal time-frequency double-synchronization method and device based on fractional Fourier transform Download PDFInfo
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Abstract
The application provides a satellite signal time-frequency double synchronization method and device based on fractional Fourier transform, wherein the method comprises the following steps: establishing a mapping relation between a satellite channel response frequency characteristic of a low-orbit satellite channel and each processing domain, determining an optimal processing domain of the low-orbit satellite channel, setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals; and (3) carrying out joint processing on each processing domain based on the optimal order of the fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal to jointly estimate the time delay and the Doppler frequency offset, and carrying out double enhancement processing on time synchronization and frequency synchronization of the linear frequency modulation signal. The application can select the fractional domain capable of effectively resisting noise and time delay influence aiming at the satellite channel with space-time-frequency non-stationary characteristic to process signals, optimize signal resource allocation under a 5GNR system and fully mine the performance mutual enhancement characteristic between time-frequency synchronization.
Description
Technical Field
The application relates to the technical field of satellite communication, in particular to a satellite signal time-frequency double-synchronization method and device based on fractional Fourier transform.
Background
Firstly, the traditional time-frequency synchronization technology mainly relies on the correlation of auxiliary signals to complete the design of an algorithm, the auxiliary signal sequences have good cross-correlation properties or good autocorrelation properties, a more commonly used PSS sequence for downlink initial synchronization carries ID important information in a physical cell group, the ID important information is placed in an SSB structure in the signal, and the sequence is generated by m sequences and has good autocorrelation properties and cross-correlation properties; the DMRS sequence for downlink tracking synchronization is responsible for important tasks such as channel estimation, signal demodulation and the like, is placed in the PBCH in the SSB, is generated by a Gold sequence, and has good auto-correlation and cross-correlation properties; the CP for time-frequency synchronization has the important function of eliminating the interference among carriers and ensuring the orthogonality of sub-carriers, and the sequence is copied from the tail part of an OFDM signal and is placed at the head part of the whole OFDM signal, so that the CP has good autocorrelation characteristic; the training sequence for time-frequency synchronization is obtained by modulating PN sequence and through special structural design, the training sequence after special design has a certain rule in structure, and certain arithmetic relation exists between the front part and the rear part, so that the training sequence has good autocorrelation characteristic. However, the conventional receiving end perceives the time-frequency state of the channel by adopting sequences such as PSS, CP, DMRS, which is mainly characterized by good correlation of the single domain based on the time domain or the frequency domain, but it is difficult to achieve the optimal correlation and performance compromise of the single domain in the space-time-frequency non-stationary channel.
And the performance of the traditional time-frequency synchronization technology is highly dependent on the design of the synchronization sequence, when the correlation capability of the synchronization sequence cannot adapt to the scene of large Doppler frequency offset change rate, the structure design and the sequence processing mode of the synchronization sequence are further optimized on the premise of not greatly changing the original synchronization sequence as much as possible, and the time-frequency synchronization algorithm is optimized. However, the time-frequency synchronization algorithm mainly uses sequence correlation as a reference, and controls the related precision, range and complexity through related sliding intervals, iterative sliding, sliding ranges and the like, so that in a space-time non-stationary channel, the performance of the algorithm is greatly influenced, and the accurate mapping and equivalent processing are carried out on the related processing of the non-independent time-frequency division domain.
In addition, the prior art of linear frequency modulation signals and fractional Fourier transform is widely studied and used in the radar monitoring field, the linear frequency modulation signals can play a larger role under the condition of occupying fewer resources by virtue of the uniqueness of the linear frequency modulation signals in the fractional domain, and the matching of the linear frequency modulation signals and the fractional Fourier transform can play a great role in time-frequency resource optimization and synchronization performance improvement. However, the current chirp signal is used as an independent sequence in a frame structure, occupies independent time-frequency resources, has limited application range, is tightly combined with the radar monitoring field at present, and is still further required to be promoted in combination with the communication field. In a 5G-fused low-orbit satellite scene, the acquisition and tracking optimization under different signal waveforms and uplink and downlink systems is required to be realized on the premise of using a 5G NR frame structure as much as possible.
Disclosure of Invention
In view of the foregoing, embodiments of the present application provide a method and apparatus for time-frequency double synchronization of satellite signals based on fractional fourier transform, so as to obviate or improve one or more disadvantages in the prior art.
One aspect of the present application provides a satellite signal time-frequency double synchronization method based on fractional fourier transform, comprising:
establishing a mapping relation between a satellite channel response frequency characteristic of a low-orbit satellite channel and each processing domain, and determining an optimal processing domain of the low-orbit satellite channel, wherein each processing domain comprises: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain;
setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals;
acquiring the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset;
and performing double enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
In some embodiments of the present application, the establishing a mapping relationship between the satellite channel response frequency characteristic of the low-orbit satellite channel and each processing domain includes:
Constructing a satellite channel response expression model based on fading influence parameters, frequency influence parameters, phase influence parameters and time delay influence parameters of a low-orbit satellite channel, wherein the satellite channel response expression model comprises a time domain response expression and a frequency domain response expression;
constructing a corresponding received signal time domain expression according to the time domain response expression, and constructing a corresponding received signal frequency domain expression according to the frequency domain response expression;
respectively constructing an initial time domain signal cross-correlation calculation expression, an initial frequency domain signal cross-correlation calculation expression and an initial fractional domain signal cross-correlation calculation expression of two signals which do not pass through a low orbit satellite channel;
and constructing a time domain signal cross-correlation calculation expression of two signals after passing through a low-orbit satellite channel based on the time domain expression of the received signals, constructing a frequency domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the frequency domain expression of the received signals, and constructing a fractional domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the time domain signal cross-correlation calculation expression so as to construct and obtain the mapping relation between the satellite channel response frequency characteristics of the low-orbit satellite channel and the time domain, the frequency domain and the fractional domain respectively.
In some embodiments of the application, the determining the optimal processing domain of the low-orbit satellite channel comprises:
setting a channel initial state of a low-orbit satellite channel, a transformation order of fractional Fourier transformation, a rotation angle of a fractional domain plane relative to a time-frequency plane under the transformation order and a corresponding relation between the rotation angle and the transformation order;
performing optimal order searching operation on a preset linear frequency modulation signal of a transmitting end and a receiving signal after passing through a low-orbit satellite channel, obtaining respective initial conversion optimal orders of the receiving end and the transmitting end corresponding to a fraction domain through initial configuration, obtaining an initial time-frequency plane rotation angle of the receiving end based on the initial conversion optimal order configuration of the receiving end, and obtaining the initial time-frequency plane rotation angle of the transmitting end based on the initial conversion optimal order configuration of the transmitting end;
executing a preset optimal processing domain determining step under the current channel state of the low-orbit satellite channel;
wherein the optimal processing domain determining step includes: performing cross-correlation calculation on the transmitted signal and the received signal in the time domain according to the initial time domain signal cross-correlation calculation expression to obtain a corresponding time domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in the frequency domain according to the frequency domain signal cross-correlation calculation expression to obtain a corresponding frequency domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in a fractional domain according to the initial fractional domain signal cross-correlation calculation expression to obtain a corresponding fractional domain signal cross-correlation calculation result; selecting a maximum value from the time domain signal cross-correlation calculation result, the frequency domain signal cross-correlation calculation result and the fractional domain signal cross-correlation calculation result, and taking a processing domain corresponding to the maximum value as an optimal processing domain in the current channel state;
And if the channel state of the low-orbit satellite channel is changed, executing the optimal processing domain determining step again under the changed new channel state so as to complete the iteration of the optimal processing domain.
In some embodiments of the present application, the setting of the chirp signal as a low-orbit satellite signal under the 5G NR system includes:
under a 5G NR system, setting key parameters of chirp signals of different structures under different links to establish a characteristic expression of the chirp signals, wherein the key parameters comprise: amplitude, modulation frequency, sampling point, sampling interval, initial frequency of the linear frequency modulation signal and rectangular function for defining time domain range of the linear frequency modulation signal;
performing sequence segmentation on each linear frequency modulation signal by taking a sampling point as a unit, and setting the insertion interval of the linear frequency modulation signal in a time-frequency resource grid;
and adjusting key parameters of each chirp signal.
In some embodiments of the present application, the combining and fusing heterogeneous chirp signals includes:
setting the dimension of a resource grid data matrix of each heterogeneous linear frequency modulation signal under different low-orbit satellite channels and different links to the same order to obtain a time-frequency resource grid matrix of each linear frequency modulation signal, wherein each data item in the time-frequency resource grid matrix comprises a data signal transmitted by the corresponding channel link and a physical signal except the data signal;
Completing the insertion work of each linear frequency modulation signal in the data signal based on the time-frequency resource grid matrix so as to realize the distribution of each linear frequency modulation signal in a resource grid;
and performing waveform shaping and sequence multiplexing processing on each linear frequency modulation signal by adopting a DFT-s-OFDM modulation mode so as to obtain a DFT-s-OFDM signal corresponding to the fused linear frequency modulation signal.
In some embodiments of the present application, the obtaining the optimal order of the fractional fourier transform corresponding to the optimal processing domain of the chirp signal includes:
setting a search interval for searching the optimal order of fractional Fourier transform and a search step length in the search interval;
in the search interval, respectively executing a preset optimal order search step on each linear frequency modulation signal based on the search step length so as to determine the optimal order of fractional Fourier transform corresponding to the optimal processing domain for processing each linear frequency modulation signal by the receiving end;
performing fractional Fourier transform on each linear frequency modulation signal by using the optimal order of the fractional Fourier transform to obtain the maximum value in fractional Fourier transform results corresponding to each linear frequency modulation signal respectively;
Wherein the best order searching step comprises: based on the current fractional Fourier transform of the current linear frequency modulation signal from the starting position of the search interval, recording the maximum value in the current corresponding fractional Fourier transform result and the currently used transform order; step-by-step increase of a preset step length is carried out on a currently used fractional Fourier transform for each step length, the maximum value in the fractional Fourier transform result corresponding to the current time is recorded, the maximum value in the fractional Fourier transform result is compared with the maximum value in the fractional Fourier transform result before the maximum value, if the maximum value in the fractional Fourier transform result before the maximum value is larger than the maximum value in the fractional Fourier transform result before the maximum value, the maximum value in the fractional Fourier transform result before the maximum value is replaced by the maximum value in the fractional Fourier transform result before the maximum value, the transformation order used when the maximum value in the fractional Fourier transform result before the maximum value is obtained is replaced by the transformation order used when the maximum value in the fractional Fourier transform result before the maximum value is obtained is adopted, and the optimal order of the fractional Fourier transform for processing the current linear FM signal by a receiving end is determined after the search interval is traversed.
In some embodiments of the present application, the performing joint processing of each of the processing domains based on the optimal order to perform delay and doppler frequency offset joint estimation includes:
setting a linear frequency modulation signal of a transmitting end and a linear frequency modulation signal extracted by a receiving end;
searching the optimal orders of the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end to obtain the optimal orders of the linear frequency modulation signals of the transmitting end and the optimal orders of the linear frequency modulation signals extracted by the receiving end respectively;
fractional Fourier transform corresponding to the optimal order is carried out on the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end respectively, and the position of the maximum value in the transformation results corresponding to the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end is recorded;
and determining the position difference between the maximum values in the conversion results respectively corresponding to the receiving end and the transmitting end according to the positions of the maximum values in the conversion results respectively corresponding to the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end, and obtaining a relational expression between the position difference and Doppler frequency offset and time delay.
In some embodiments of the present application, the performing dual enhancement processing on time synchronization and frequency synchronization of the chirp signal includes:
setting a timing synchronization result, a timing synchronization result error, a frequency synchronization result and a frequency synchronization result error, and setting an optimized timing synchronization result, an optimized timing synchronization result error, an optimized frequency synchronization result and an optimized frequency synchronization result error;
under the same channel state of a low orbit satellite channel, establishing a relational expression between the timing synchronization result and the timing synchronization result error and a relational expression between the frequency synchronization result and the frequency synchronization result error in a function fitting mode;
and determining a time delay estimation result change in the timing synchronization performance enhancement section according to the optimized timing synchronization result and the timing synchronization result, determining a time delay estimation result error change in the timing synchronization performance enhancement section according to the optimized timing synchronization result error and the timing synchronization result error, determining a frequency offset estimation result change in the frequency synchronization performance enhancement section according to the optimized frequency synchronization result and the frequency synchronization result, and determining a frequency offset estimation result error change in the frequency synchronization performance enhancement section according to the optimized frequency synchronization result error and the frequency synchronization result error, so as to establish a mapping function for representing a performance enhancement mapping relationship between time synchronization and frequency synchronization of the low-orbit satellite signal;
Setting an initial timing synchronization result, a timing synchronization result after performance enhancement, a starting point of an initial extracted linear frequency modulation signal and OFDM signal data, and generating a first original linear frequency modulation signal expression corresponding to the initial extracted linear frequency modulation signal, wherein the linear frequency modulation signal expression consists of data of a position corresponding to the OFDM signal;
determining a first new linear frequency modulation signal expression corresponding to the linear frequency modulation signal which is changed after the timing synchronization performance is enhanced;
setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the first original chirp signal expression and the first new chirp signal expression to obtain a first reliability factor expression;
performing frequency synchronization operation by adopting a first reliability factor expression, and setting the position offset, the time-frequency plane rotation angle and the actual frequency offset value of the front and rear spectrum peaks passing through the low-orbit satellite channel, so that the error between the calculated new frequency offset estimation result and the new frequency synchronization result is calculated;
comparing the new frequency synchronization result error with the frequency synchronization result error before the timing synchronization performance is enhanced to verify the enhancement of the frequency synchronization performance;
Setting an initial frequency synchronization result, a frequency synchronization result after performance enhancement, a linear frequency modulation signal in an OFDM receiving signal and a linear frequency modulation signal after Doppler frequency offset compensation, and determining a second original linear frequency modulation signal expression between the initial extracted linear frequency modulation signal and the receiving signal;
determining a second new linear frequency modulation signal expression corresponding to the linear frequency modulation signal subjected to Doppler frequency offset compensation after the frequency synchronization performance is enhanced;
setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the second original chirp signal expression and the second new chirp signal expression to obtain a second reliability factor expression;
performing timing synchronization operation by adopting a second reliability factor expression, and setting position offset, time-frequency plane rotation angle and actual time delay value of front and rear spectrum peaks passing through a low-orbit satellite channel, so as to calculate new time delay estimation result and new timing synchronization result error;
and comparing the new timing synchronization result error with the timing synchronization result error before the frequency synchronization performance is enhanced to verify the enhancement of the timing synchronization performance.
Another aspect of the present application provides a satellite signal time-frequency double synchronization device based on fractional fourier transform, comprising:
the system comprises an optimal processing domain determining module, a processing domain determining module and a processing domain determining module, wherein the optimal processing domain determining module is used for establishing a mapping relation between satellite channel response frequency characteristics of a low-orbit satellite channel and each processing domain, and determining the optimal processing domain of the low-orbit satellite channel, and each processing domain comprises: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain;
the signal fusion module is used for setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals;
the processing domain joint module is used for acquiring the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset;
and the synchronous double-enhancement module is used for carrying out double-enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
In a third aspect, the present application provides an electronic device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, where the processor implements the fractional fourier transform-based satellite signal time-frequency double synchronization method when executing the computer program.
A fourth aspect of the present application provides a computer readable storage medium having stored thereon a computer program which when executed by a processor implements the fractional fourier transform based satellite signal time-frequency double synchronization method.
The application provides a satellite signal time-frequency double synchronization method based on fractional Fourier transform, which establishes a mapping relation between satellite channel response frequency characteristics of a low-orbit satellite channel and each processing domain, and determines an optimal processing domain of the low-orbit satellite channel, wherein each processing domain comprises: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain; setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals; acquiring the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset; compared with the traditional correlation-based time-frequency synchronization technology, the time synchronization and frequency synchronization of the linear frequency modulation signals are subjected to severe influence of noise and frequency offset, and the matching of the fractional Fourier transform and the linear frequency modulation signals can achieve good resistance to severe noise and Doppler frequency offset interference in satellite channels, so that the receiving quality of the signals at a receiving end can be improved, and further the performance of a time-frequency synchronization module is improved. Meanwhile, the mutual enhancement relation of the performances of the time-frequency synchronization is noted, the mutual gains of the frequency synchronization and the timing synchronization are fully mined, and the performances of the frequency synchronization and the timing synchronization are feedback-improved on the basis of time delay/frequency offset estimation optimization.
Additional advantages, objects, and features of the application will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the application. The objectives and other advantages of the application may be realized and attained by the structure particularly pointed out in the written description and drawings.
It will be appreciated by those skilled in the art that the objects and advantages that can be achieved with the present application are not limited to the above-described specific ones, and that the above and other objects that can be achieved with the present application will be more clearly understood from the following detailed description.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate and together with the description serve to explain the application. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the application. Corresponding parts in the drawings may be exaggerated, i.e. made larger relative to other parts in an exemplary device actually manufactured according to the present application, for convenience in showing and describing some parts of the present application. In the drawings:
fig. 1 is a general flow chart of a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 2 is a logic flow diagram of a method for performing time-frequency double synchronization of satellite signals based on fractional fourier transform according to an embodiment of the application.
Fig. 3 is a flowchart illustrating a step 100 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 4 is a schematic diagram of step 100 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 5 is a flowchart illustrating a step 200 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 6 is a schematic diagram of step 200 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 7 is a flowchart illustrating a step 300 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 8 is a schematic diagram of step 300 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 9 is a flowchart illustrating a step 400 in a satellite signal time-frequency double synchronization method based on fractional fourier transform according to an embodiment of the application.
Fig. 10 is a schematic structural diagram of a satellite signal time-frequency double synchronization device based on fractional fourier transform according to an embodiment of the application.
Detailed Description
The present application will be described in further detail with reference to the following embodiments and the accompanying drawings, in order to make the objects, technical solutions and advantages of the present application more apparent. The exemplary embodiments of the present application and the descriptions thereof are used herein to explain the present application, but are not intended to limit the application.
It should be noted here that, in order to avoid obscuring the present application due to unnecessary details, only structures and/or processing steps closely related to the solution according to the present application are shown in the drawings, while other details not greatly related to the present application are omitted.
It should be emphasized that the term "comprises/comprising" when used herein is taken to specify the presence of stated features, elements, steps or components, but does not preclude the presence or addition of one or more other features, elements, steps or components.
It is also noted herein that the term "coupled" may refer to not only a direct connection, but also an indirect connection in which an intermediate is present, unless otherwise specified.
Hereinafter, embodiments of the present application will be described with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar components, or the same or similar steps.
In one or more embodiments of the application, satellite signal time-frequency double synchronization refers in particular to time-frequency double synchronization for low-orbit satellite signals.
Prior art related to the application-one: the method has the advantages of selecting and designing channel time-frequency state sensing capability auxiliary signaling.
In 2018, a learner proposes a design and application of a channel estimation sequence of a millimeter wave MIMO communication system, and on the basis of analyzing the design requirement of a frequency domain channel estimation sequence with low peak-to-average power ratio under the frequency spectrum constraint condition of direct current zero sub-carrier, high frequency zero sub-carrier and the like, two frequency domain channel estimation training sequence designs based on Zadoff-Chu sequences and Golay pair/mate sequences are provided, and simulation results show that the frequency domain channel estimation training sequences of the two designs have lower PAPR and can flexibly adapt to the channel estimation requirements under the scenes of large time delay, multiple antennas, channel bonding and the like. In the same year, students carry out 5G new air interface system parameter design and research, on the basis that the subcarrier is the second power of the original interval, the related simulation of the subcarrier interval and the CP length of different substrates is carried out, the throughput curve is analyzed, and the conclusion of how the CP length should be selected under different modulation modes, subcarriers and time delays is obtained; a method is presented of how to choose a parametric approach to achieve optimal performance in future 5G critical deployment scenarios. The final conclusion has guiding significance on the self-adaptive configuration of the system parameters in the design of the new air interface of 5G. In 2021, a learner proposes a 5G base station energy saving scheme research based on an SSB configuration policy, compares an SSB overhead ratio of NR with an overhead ratio of PSS, SSS, PBCH, CRS of LTE, and gives an SSB configuration policy, and suggests measures such as increasing SSB period, reducing SSB number, and intelligently turning off a slot and a symbol to reduce power consumption of the 5G base station. In 2021, a learner proposed a DMRS sequence generation method for computer sequence search, and the method first searches according to the self attribute of the sequence, that is, the autocorrelation value and the PAPR of the sequence, considering the self attribute of the sequence during the sequence search. Then, cross correlation of the sequence and other sequences is used as a search condition, so that the anti-interference capability of the DMRS signal is enhanced. And (3) the last search is carried out through demodulation performance in the communication system, and sequences with relatively good demodulation performance are screened out. Compared with the existing base sequence, the search base sequence has obvious advantages in PAPR value and autocorrelation, and also has better performance in cross correlation and demodulation performance than the existing sequence.
However, the conventional receiving end perceives the time-frequency state of the channel by adopting sequences such as PSS, CP, DMRS, which is mainly characterized by good correlation of the single domain based on the time domain or the frequency domain, but it is difficult to achieve the optimal correlation and performance compromise of the single domain in the space-time-frequency non-stationary channel.
The second prior art related to the present application: time-frequency synchronization algorithm based on auxiliary signaling related operation.
In 2014, when a learner uses the sequence to perform cross-correlation, the integral multiple frequency offset can be directly obtained according to the shift information of the timing measurement peak value relative to the ideal position. Therefore, CAZAC sequence cross-correlation class synchronization algorithms have been extensively studied. In 2019, a learner proposed a time synchronization algorithm based on an improved CAZAC sequence, in the past several timing synchronization algorithms using OFDM signal correlation showed worse performance due to the platform effect existing and the side lobe effect existing in the correlation peak, a new OFDM signal was designed based on the CAZAC sequence, and a new timing synchronization timing metric was proposed, and through a simulation experiment, it was confirmed that the method has better performance compared with the traditional algorithm.
The frequency synchronization of the cyclic prefix is utilized, the algorithm is simple, the calculated amount is small, the additional overhead is not needed to be added, the frequency band utilization rate is not reduced, but only the decimal frequency offset can be estimated, and the estimation accuracy is limited by the length of the CP and the condition of a channel. In 2016, a correlation algorithm using clustering has been proposed by a learner, the number of complex multiplications is reduced to 8, and part of the calculation complexity is reduced. In 2016, the scholars have increased the autocorrelation properties of the cyclic prefix by multiplying the PN sequence of the cyclic prefix.
The pilot frequency assisted estimation algorithm is utilized, so that the calculation accuracy is high and the compensation performance is good. In 2018, a learner proposes to complete the compensation of the doppler shift by using a symmetrical frame structure sequence aiming at the problem of the influence of the doppler shift on the frame synchronization, thereby reducing the influence of the integer multiple of the doppler shift on the synchronization process. In 2020, a learner aims at the problems of high dynamic and low signal-to-noise ratio of a satellite communication terminal in a high dynamic environment, considers the insertion of scattered pilots for tracking carriers in a signal design, and provides a frequency change rate estimation method based on fractional Fourier transform.
For a frequency offset correction algorithm of a non-signaling estimation class, in 2015, a learner proposes a channel response and carrier frequency offset joint estimation scheme based on a desired maximization algorithm, but the algorithm complexity is very high, and the method does not have engineering application prospect.
However, the conventional time-frequency synchronization algorithm mainly uses sequence correlation as a reference, and controls the precision, range and complexity of correlation through correlation sliding intervals, iterative sliding, sliding ranges and the like, so that in a space-time non-stationary channel, the performance of the algorithm is greatly affected, and accurate mapping and equivalent processing are carried out on correlation processing of a non-independent time-frequency division domain.
Three prior art related to the present application: and optimizing time-frequency resources and synchronizing performance of the multi-scene signals based on a 5G NR system.
At present, the linear frequency modulation signal and the fractional Fourier transform technology are widely researched and used in the radar monitoring field, the linear frequency modulation signal can play a larger role under the condition of occupying fewer resources by the uniqueness of the linear frequency modulation signal in the fractional domain, and the matching of the linear frequency modulation signal and the fractional Fourier transform technology can play a great role in time-frequency resource optimization and synchronization performance improvement.
In 2018, a new waveform design technique for implementing a communication channel in a pulse radar was proposed by a learner. The proposed waveform consists of quasi-orthogonal chirped subcarriers generated by fractional fourier transform, the purpose of which is to preserve the radar performance of typical chirps when embedding data to be transmitted to the cooperating system. Waveform generation and demodulation are described, along with techniques aimed at optimizing design parameters and mitigating inter-carrier interference caused by quasi-orthogonality of the chirped subcarriers. The proposed fractional fourier transform based communication radar waveform design is compared with OFDM based communication radars in terms of radar and communication operation. Radar performance is assessed by examining the ambiguity function and assessing the performance of a standard square law detector. Communication performance is expressed in terms of bit error rate under different channel conditions. The results show that the proposed fractional fourier transform waveform exhibits near chirp performance in terms of detection probability and false alarm probability in exchange for slightly worse range and doppler resolution.
In 2020, a learner uses fractional Fourier transform to replace Fourier transform in the MIMO-OFDM system to improve the correlation coefficient, alleviate the influence of inter-carrier interference and improve the bit error rate. Based on the fast fading model and incomplete channel state information, the correlation coefficient, ICI average power and closed bit error rate expression of the system are deduced. Compared with the conventional system, the error rate of the proposed system is increased by 40%, and the inter-carrier interference cancellation is increased by 70%.
In 2022, a learner approximated the chirp signal using a partial sum of generalized fourier series of fractional fourier series, which can give the best approximation with only a small number of fractional fourier coefficients, and calculate the fractional fourier coefficients of the sampling points with fractional fourier transforms. In addition, the team finds that the fractional parameter has an optimal value, where a small fourier coefficient of zero degrees has the largest magnitude, resulting in a rapid decrease in the highly fractional fourier coefficient, and that the fractional fourier series approximation with the optimal fractional parameter provides the smallest mean square error over the fractional fourier parameter domain.
In 2015, a learner proposes a timing synchronization and frequency synchronization algorithm based on fractional Fourier transform for a MIMO-OFDM system, a transmitting end adopts a linear frequency modulation signal as a training sequence, a receiving end suppresses mutual interference among all transmitting antennas by fractional Fourier transform and utilizing the characteristic of energy aggregation of the linear frequency modulation signal in a specific order fractional domain, and then timing synchronization and frequency offset capturing are completed. Simulation proves that compared with other traditional methods, the method has obvious synchronization performance improvement, and can obtain higher timing synchronization and frequency offset capturing precision.
However, the current chirp signal is used as an independent sequence in a frame structure, occupies independent time-frequency resources, has limited application range, is tightly combined with the radar monitoring field at present, and is still further required to be promoted in combination with the communication field. In a 5G-fused low-orbit satellite scene, the acquisition and tracking optimization under different signal waveforms and uplink and downlink systems is required to be realized on the premise of using a 5G NR frame structure as much as possible.
Based on the problems that the performance of the traditional time-frequency synchronization technology is greatly influenced and the like in the situation of the space-time non-stationary characteristic of a channel in the low-orbit satellite communication environment and the situation, the time-frequency synchronization algorithm based on fractional Fourier transform and the signal system design scheme based on DFT-s-OFDM and linear frequency modulation signals are innovatively used, the time-frequency synchronization module in the system is effectively optimized in algorithm, the accuracy, the success rate and the reliability of time-frequency synchronization are greatly improved, and the adaptability of the time-frequency synchronization technology to the low-orbit satellite communication environment is further improved. The fractional Fourier transform technology is matched with the linear frequency modulation signal to be widely applied in the radar echo detection field, the main idea is to use the characteristic that the linear frequency modulation signal presents a sharp peak in the fractional domain to realize the purpose of echo detection, and through practical verification, the scheme obtains obvious performance improvement compared with the traditional scheme. Currently, fractional fourier transform techniques and chirps are increasingly being combined with the field of communications.
In the satellite communication scene, the design of a time-frequency synchronization scheme by using the combination of fractional Fourier transform and chirp signals is a very potential development direction, and the starting point is as follows: 1) The perceptibility of the communication signal to the environment of the receiving end is one of the promise in 6G communication, the environment of a near-earth channel of LEO satellite communication is complex, and the connection reliability of the satellite in the complex environment can be improved by fusing the sequence of the perceptibility of the states of the multi-domain signals. 2) The high dynamic low orbit satellite has strong space-time-frequency non-stationary characteristic, so that the synchronization is difficult to realize in a single time domain or a frequency domain when the received signal is correlated, and therefore fractional order synchronization is urgently needed to realize in a non-independent time-frequency domain. 3) The synchronization signaling overhead is particularly critical to the reliability of LEO satellite scene high-speed communication, especially when the signal-to-noise ratio is low, and the enhancement of the synchronization sequence performance is critical to simplify the synchronization sequence without reducing the connection reliability.
In consideration of the fact that the matching of the fractional Fourier transform and the linear frequency modulation signal has excellent signal detection capability in a severe scene, the design of a synchronization scheme in a satellite communication scene is carried out by matching the fractional Fourier transform with the linear frequency modulation signal. Therefore, the method and the device are based on the characteristic of space-time unsteadiness of the analysis satellite channel and the 5G signal system, adopt a time-frequency synchronization algorithm combining fractional Fourier transform and linear frequency modulation signals and a signal structure design based on the original 5G signal system, optimize resource allocation, improve time-frequency synchronization performance and realize mutual enhancement of the time-frequency synchronization performance. For this reason, the important concerns are:
1) And determining and designing a sequence of the optimal perception domain of the space-time-frequency nonstationary satellite channel. In satellite channels with space-time-frequency non-stationary characteristics, signal analysis in the time and frequency domains is more difficult. Therefore, it is desirable to find a signal processing domain that is more suitable for this scenario. Meanwhile, the design work of a new synchronous signal system is guaranteed to be completed on the premise of not changing the original 5G NR signal system.
2) Satellite uplink and downlink time synchronization and frequency synchronization under limited signaling resources are enhanced. The performance between timing synchronization and frequency synchronization is interrelated, and the enhancement of one side performance can improve the other side. Therefore, under the condition of limited signaling resources, the design of the new synchronization signal system occupies as little signaling resources as possible, and the efficiency mutual gain of timing synchronization and frequency synchronization is fully mined.
For this purpose, our idea is: the method comprises the following steps of firstly, respectively analyzing and modeling satellite channel time-frequency characteristics in time domain, frequency domain and fractional domain to provide a channel theoretical basis for designing an integral time-frequency synchronization scheme; then, aiming at signals with different channels and different structures, a design scheme of fusing the linear frequency modulation signals and the 5G signal system is used, so that resource allocation is fully optimized; and then designing a time-frequency synchronization algorithm based on fractional Fourier transform and the linear frequency modulation signal, improving the overall performance of time-frequency synchronization, and performing deep mining aiming at the performance mutual enhancement characteristic between time-frequency synchronization.
The following examples are provided to illustrate the application in more detail.
Based on this, the embodiment of the present application provides a fractional fourier transform low-orbit satellite signal time-frequency double synchronization method that can be implemented by a fractional fourier transform-based satellite signal time-frequency double synchronization device, referring to fig. 1, the fractional fourier transform-based satellite signal time-frequency double synchronization method specifically includes the following contents:
step 100: establishing a mapping relation between a satellite channel response frequency characteristic of a low-orbit satellite channel and each processing domain, and determining an optimal processing domain of the low-orbit satellite channel, wherein each processing domain comprises: the optimal processing domain comprises a time domain, a frequency domain and a fraction domain.
In step 100, firstly, a mapping relation between response frequency characteristics of a satellite channel and a related processing domain is established, and different expression forms of related operations in different domains are researched; then, according to the configuration of the channel state initial fraction domain, initializing each key parameter of the fraction domain; and finally, according to the domain of the optimal processing of the channel state iteration, selecting and switching the domain of the optimal processing of the signal under different channel states which change at the moment.
Step 200: and setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals.
In step 200, the used chirp signal sequence needs to be designed in a related manner, including key parameter setting, sequence division mode design, parameter optimization, and the like, and then fusion work between the chirp signal and heterogeneous OFDM signals, including resource grid matching, filtering forming, sequence multiplexing, and the like, is performed to generate a complete OFDM signal for use.
Step 300: and obtaining the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset.
In step 300, an optimal order search algorithm is designed for the correlation processing of the fractional domain, the fractional Fourier transform order of the optimal processing domain is found by search operation for the linear frequency modulation signals under different parameter designs, then the correlation joint processing under the time-frequency fractional domain is performed on the basis of the optimal order, and the joint estimation of the delay and the Doppler frequency offset is completed by utilizing the spectral peak translation characteristic of the linear frequency modulation signals under the fractional domain.
Step 400: and performing double enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
In step 400, a mapping function of the influence of the performance enhancement between time synchronization and frequency synchronization on the performance of the other party is first established, the synchronization result of time-frequency synchronization and the internal relation between synchronization errors are studied, then the enhancement of frequency synchronization performance is realized through the enhancement of timing synchronization performance, and in turn, the enhancement of frequency synchronization performance is realized through the enhancement of frequency synchronization performance.
That is, referring to fig. 2, the modeling work of the space-time non-stationary satellite channel multi-domain analysis model and the analysis of the link synchronization reliability are performed first, and the response model after the joint influence of the signal through the satellite channel large-scale fading, small-scale fading, doppler frequency offset fading and the like is studied; then, a signal time-frequency resource allocation design of merging the linear frequency modulation signals and the 5G NR signal system is carried out, and an insertion mode of different linear frequency modulation signals in an OFDM signal time-frequency resource grid is provided for heterogeneous signals of an uplink and a downlink; and then, carrying out time-frequency synchronization algorithm design based on fractional Fourier transform, optimally designing a timing synchronization and frequency synchronization mutual enhancement scheme, utilizing special properties of the linear frequency modulation signal between the linear frequency modulation signal and time delay and frequency offset in a fractional domain to finish estimation of the time delay and the frequency offset, and establishing a synchronization error feedback model.
As can be seen from the above steps 100 to 400, the technical problem to be solved in the present application is to combine fractional fourier transform technology with the chirp signal to realize the design of the signal based on the 5G NR system and the design of the complete time-frequency synchronization scheme in the satellite communication scenario. The existing time-frequency synchronization technology based on auxiliary signaling mostly needs to change the 5G NR system to a certain extent, so that resource waste is caused, and therefore, signal design and time-frequency resource optimization based on the 5G NR system are key preconditions for fusing the satellite and the ground communication system. Meanwhile, compared with a ground channel, the satellite channel has more complex environmental factor influence, and generates non-negligible influence on the analysis processing of signals in the time domain and the frequency domain, so that finding a signal analysis angle which avoids influence factors to the minimum degree is a key problem for effectively adapting to the space-time-frequency non-stationary characteristic of the satellite channel. In addition, the performance between the timing synchronization and the frequency synchronization is closely related, and the performance enhancement of one party can improve the performance of the other party, so the performance mutual enhancement between the timing synchronization and the frequency synchronization is mined and is a key link for improving the performance of the time-frequency synchronization. The main key points of the time-frequency synchronization technology under the design of the satellite communication scene are as follows:
(1) And determining and designing a sequence of the optimal perception domain of the space-time-frequency nonstationary satellite channel. For satellite channels with space-time-frequency non-stationary characteristics, environmental factors which exist in the time domain and the frequency domain and have great influence on signal transmission quality are all changed along with time and scene transformation, and the situation of the satellite channels cannot be completely summarized through a unified model, so that the analysis of the channels and signals from the time domain and the frequency domain is very difficult. At this time, if the analysis can be performed from the fractional domain between the time domain and the frequency domain, the above problems can be effectively solved, the interference in all aspects of the time domain and the frequency domain channels can be well resisted, and the high adaptability can be achieved between the linear frequency modulation signal and the satellite channels by matching with the linear frequency modulation signal which has the characteristics of resisting frequency offset and multipath effect. The method is characterized in that the sequence design is in accordance with the principle of keeping the original 5G signal system unchanged, a linear frequency modulation signal is fused with an OFDM signal in a mode of being inserted into a time-frequency resource grid, the design of an optimal insertion mode is carried out aiming at different structures of signals under different links of different channels, and finally the generation of the signals is carried out by matching with a DFT-s-OFDM modulation technology.
(2) Satellite uplink and downlink time synchronization and frequency synchronization under limited signaling resources are enhanced. The timing synchronization and the frequency synchronization are always in succession in the communication system, and a tight connection with improved performance is formed between the timing synchronization and the frequency synchronization, so that a more accurate processing starting point can be found for the signal by the timing synchronization with better performance, thereby finding an accurate auxiliary signal position and completing extraction for the next frequency synchronization operation, and further estimating a more accurate frequency offset value; the frequency offset value accurately estimated by the frequency synchronization can better compensate the Doppler frequency offset influence of the signal in the channel, so that the received signal is more reliably restored, and more accurate time delay estimation is realized. By utilizing the principle, an optimization scheme for improving the time-frequency synchronization performance is needed to be researched, a time-frequency synchronization realization scheme with better synchronization performance is provided, and under the condition of limited signaling resources, the situation that the linear frequency modulation signals occupy as few signaling resources as possible is achieved as much as possible, and the efficiency mutual gain of the timing synchronization and the frequency synchronization is fully excavated.
In the method for time-frequency double synchronization of satellite signals based on fractional fourier transform provided in the embodiment of the present application, referring to fig. 3 and 4, step 100 of the method for time-frequency double synchronization of satellite signals based on fractional fourier transform first establishes a mapping relationship between response frequency characteristics of a satellite channel and a relevant processing domain, so that step 100 specifically includes the following:
Step 101: and constructing a satellite channel response expression model based on the fading influence parameter, the frequency influence parameter, the phase influence parameter and the time delay influence parameter of the low orbit satellite channel, wherein the satellite channel response expression model comprises a time domain response expression and a frequency domain response expression.
In step 101, to clarify the influence of the response frequency characteristic of the satellite channel on the relevant processing domain, a satellite channel response expression model needs to be established, where L is a comprehensive influence parameter of each fading of the satellite channel, F is a frequency influence parameter of the satellite channel, θ is a phase influence parameter of the satellite channel, τ is a time delay influence parameter of the satellite channel, C (t) and C (ω) correspond to the time domain and the frequency domain respectively, and the overall response model of the channel may include:
time domain response expression: c (t) =h (L, F, θ, τ);
frequency domain response expression: c (ω) =h (L, F, θ, τ).
Step 102: and constructing a corresponding received signal time domain expression according to the time domain response expression, and constructing a corresponding received signal frequency domain expression according to the frequency domain response expression.
In step 102, assuming that the original transmission signal is s (t), then for signals passing through the satellite channel, the signal is received The time-frequency characteristics of (a) will change as follows:
for a time-domain received signal, the result is equal to the convolution of the time-domain transmitted signal with the channel time-domain response function, as shown in the received signal time-domain expression:
for a frequency domain received signal, the result is equal to the multiplication of the frequency domain version of the transmitted signal with the frequency domain version of the channel response function, as shown in the received signal frequency domain expression:
step 103: and respectively constructing an initial time domain signal cross-correlation calculation expression, an initial frequency domain signal cross-correlation calculation expression and an initial fractional domain signal cross-correlation calculation expression of the two signals which do not pass through the low orbit satellite channel.
In step 103, in performing correlation processing of signals, conventional correlation calculation is performed for ordinary signals x (t) and y (t) according to the following expression.
The calculation is performed in the time domain by using a mode of shift multiplication and then addition summation between two signals, and the calculation expression of the initial time domain signal cross correlation is as follows:
the frequency domain is calculated by conjugate multiplication of the frequency domain forms of the two signals and then the form of Fourier inverse transformation, and the following initial frequency domain signal cross-correlation calculation expression is shown:
the fractional domain correlation calculation is completed by using the time domain cross correlation function of the two functions and then performing fractional domain processing under the corresponding order, and the following initial fractional domain signal cross correlation calculation expression is shown:
Where α is the angle by which the fractional domain plane rotates relative to the time-frequency plane.
Step 104: and constructing a time domain signal cross-correlation calculation expression of two signals after passing through a low-orbit satellite channel based on the time domain expression of the received signals, constructing a frequency domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the frequency domain expression of the received signals, and constructing a fractional domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the time domain signal cross-correlation calculation expression so as to construct and obtain the mapping relation between the satellite channel response frequency characteristics of the low-orbit satellite channel and the time domain, the frequency domain and the fractional domain respectively.
In step 104, for signals that have passed through the satellite channelFor example, under the influence of the time-frequency characteristics of the satellite channel, the following changes will occur in the signal correlation process:
substituting the received signal time domain expression into the time domain signal cross correlation calculation expression after the satellite channel is obtained:
substituting the frequency domain expression of the received signal, and obtaining the frequency domain signal cross-correlation calculation expression after passing through the satellite channel by utilizing the conjugated property:
substituting the time domain signal cross-correlation calculation expression into the corresponding fractional domain processing in the situation to obtain the fractional domain signal cross-correlation calculation expression after satellite channel passing:
In the method for time-frequency double synchronization of satellite signals based on fractional fourier transform provided in the embodiment of the present application, referring to fig. 3 and 4, step 100 of the method for time-frequency double synchronization of satellite signals based on fractional fourier transform further needs to pre-select a fractional domain according to a channel state configuration and iterate an optimal processing domain to establish a mapping relationship between a satellite channel response frequency characteristic and a related processing domain, so step 100 further specifically includes the following contents:
step 105: setting a channel initial state of a low orbit satellite channel, a transformation order of fractional Fourier transformation, a rotation angle of a fractional domain plane relative to a time-frequency plane under the transformation order and a corresponding relation between the rotation angle and the transformation order.
In step 105, the channel initial state is set to c 0 ＝h(L 0 ,F 0 ,θ 0 ,τ 0 ) The relation between alpha and p is shown as follows, when the transformation order of the fractional Fourier transform is p:
step 106: and carrying out optimal order searching operation on the preset linear frequency modulation signal of the transmitting end and the receiving signal after passing through the low-orbit satellite channel, obtaining respective initial conversion optimal orders of the receiving end and the transmitting end corresponding to the fractional domain through initial configuration, obtaining an initial time-frequency plane rotation angle of the receiving end based on the initial conversion optimal order configuration of the receiving end, and obtaining the initial time-frequency plane rotation angle of the transmitting end based on the initial conversion optimal order configuration of the transmitting end.
In step 106, the originating chirp signal is set to s (t), and the received signal after passing through the channel is r (t), where r (t) is equal to the convolution of s (t) with the channel:
r(t)＝s(t)*c(t)
the initial configuration of the fractional domain in this state includes the initial transformation of the transmitting end by the optimal order p 0 The receiving end transforms the optimal orderInitial time-frequency plane rotation angle alpha with transmitting end 0 Time-frequency plane rotation angle of receiving end>The best order search operation is performed on the signals s (t) and r (t):
p 0 ＝h search (s(t))
wherein h is search Representing the best order search operation for a linear fm signal, is described in detail below. Obtaining parameter p by searching optimal order 0 、And then calculate the parameter alpha 0 、
Step 107: executing a preset optimal processing domain determining step under the current channel state of the low-orbit satellite channel; wherein the optimal processing domain determining step includes: performing cross-correlation calculation on the transmitted signal and the received signal in the time domain according to the initial time domain signal cross-correlation calculation expression to obtain a corresponding time domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in the frequency domain according to the frequency domain signal cross-correlation calculation expression to obtain a corresponding frequency domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in a fractional domain according to the initial fractional domain signal cross-correlation calculation expression to obtain a corresponding fractional domain signal cross-correlation calculation result; and selecting a maximum value from the time domain signal cross-correlation calculation result, the frequency domain signal cross-correlation calculation result and the fractional domain signal cross-correlation calculation result, and taking a processing domain corresponding to the maximum value as an optimal processing domain in the current channel state.
In step 107, the correlation R of the signals in the time domain, frequency domain, fractional domain, respectively, is used XY 、And->To determine the optimal processing domain gamma e { time domain, frequency domain, fractional domain }. Setting a transmission signal of a transmitting end as x (t), setting a received signal after a channel as y (t), and performing cross-correlation operation on the transmission signal x (t) and the received signal as y (t) in each processing domain.
The calculation is performed in the time domain by using a mode of shifting multiplication between two signals and then adding and summing, and the formula is as follows:
the frequency domain is calculated by conjugate multiplication of the frequency domain forms of the two signals and then the form of Fourier inverse transformation, and the calculation is shown in the following formula:
the fractional domain correlation calculation is completed by using the time domain cross correlation function of the two functions and then performing fractional domain processing under the corresponding order, as shown in the following formula:
comparing the cross-correlation results in the three domains, the larger the result proves that the better the signal transmission quality in the processing domain is, and the maximum value is takenCorresponding processing domain->I.e. the optimal processing domain gamma. />
Step 108: and if the channel state of the low-orbit satellite channel is changed, executing the optimal processing domain determining step again under the changed new channel state so as to complete the iteration of the optimal processing domain.
In step 108, when the channel state c=h (L, F, θ, τ) changes, the correlation results R in the three domains need to be fed back continuously under the new channel state c ' =h (L ', F ', θ ', τ ') XY ′(τ)、And->To complete the iteration of the new optimal processing domain gamma:
in the fractional fourier transform-based satellite signal time-frequency double synchronization method provided by the embodiment of the present application, referring to fig. 5 and 6, step 200 of the fractional fourier transform-based satellite signal time-frequency double synchronization method needs to perform a linear frequency modulation signal sequence design: the step 200 specifically includes the following steps:
step 201: under a 5G NR system, setting key parameters of chirp signals of different structures under different links to establish a characteristic expression of the chirp signals, wherein the key parameters comprise: the amplitude, the modulation frequency, the sampling point, the sampling interval, the starting frequency and a rectangular function for defining the time domain range in which the chirp signal is located.
In step 201, each parameter of the chirp signal is set according to the actual requirement and performance, and a characteristic expression of the chirp signal is established:
s(t)＝s 1 (t)+s 2 (t)
Wherein a is 0 Representing the amplitude of the chirp signal; k represents the modulation frequency of the linear frequency modulation signal, and the parameter represents the change rate of the signal frequency f; t= [ t ] 0 t 1 t 2 t 3 …]Sample points representing a chirp signal, in particular Δt=t between sample points n+1 -t n n=0, 1,2 … represents the sampling interval of the signal, and the smaller the sampling interval is, the more the number of chirp signal points used isf 0 The starting frequency of the linear frequency modulation signal is represented, T represents the period of the linear frequency modulation signal, the rect (T/T) function is a rectangular function, the time domain range of the linear frequency modulation signal is defined, and the larger T is, the larger occupied time domain range is.
Step 202: and performing sequence division on each linear frequency modulation signal by taking a sampling point as a unit, and setting the insertion interval of the linear frequency modulation signal in a time-frequency resource grid.
In step 202, in the subsequent step we need to perform a division operation on the chirp signal in units of sampling points N in order to perform an insertion operation of the chirp signal in the time-frequency resource grid. For the chirp signal s (t), the surrounding parameter t= [ t ] 0 t 1 t 2 t 3 …]Dividing s (t) by a value corresponding to each point in t, wherein the value is shown in the following formula:
at this time, the original chirp signals (t) is rewritten as In the form of individual sampling points for use in subsequent insertion operations.
Step 203: and adjusting key parameters of each chirp signal.
In step 203, the characteristic parameter configuration of the chirp signal affects the performance of the final time-frequency synchronization algorithm, and the time-frequency synchronization result can be referred toAdjusting characteristic parameters a of a linear frequency modulation signal 0 ，K，Δt，T：
a 0 ＝a 0 ′ K＝K′ Δt＝Δt′ T＝T′
Obtaining an adjusted chirp signal:
s′(t)＝s 1 ′(t)+s 2 ′(t)
and performing iterative adjustment on the characteristic parameters aiming at the time-frequency synchronization result until the time-frequency synchronization performance is optimal.
In the method for time-frequency double synchronization of satellite signals based on fractional fourier transform provided in the embodiment of the present application, referring to fig. 5 and 6, step 200 of the method for time-frequency double synchronization of satellite signals based on fractional fourier transform further needs heterogeneous signal fusion: the resource grid matching, filtering shaping, and sequence multiplexing, therefore, the step 200 further specifically includes the following:
step 204: setting the dimension of the resource grid data matrix of each heterogeneous linear frequency modulation signal under different low orbit satellite channels and different links to the same order to obtain the time-frequency resource grid matrix of each linear frequency modulation signal, wherein each data item in the time-frequency resource grid matrix comprises the data signal transmitted by the corresponding channel link and the physical signal except for the data signal.
In step 204, for heterogeneous signals under different links of different channels, the D dimension of the resource grid data matrix is set to m×n order, where the data is represented by D ij A representation is made where i=1, 2,3, …, m, j=1, 2,3, …, n. The time-frequency resource grid D of the signal is shown as the following matrix:
wherein data d ij Comprising the data signal d transmitted by itself under the channel link ds Physical signal d other than the channel itself transmission signal ps Wherein the physical signal d ps Mainly comprises two types of signals: reference signal d rs And a synchronization signal d ss Such signals are preset with reference to the system protocol for performing a specific function, and are linear frequency modulated signal s= [ s ] 0 s 1 … s n ]It should not be destroyed during insertion. Thus only in the data signal d ds The insertion of the chirp signal is completed.
Step 205: and completing the insertion work of each chirp signal in the data signal based on the time-frequency resource grid matrix so as to realize the distribution of each chirp signal in the resource grid.
In step 205, the first data is setPosition (i) 0 ,j 0 ) For inserting the first position ind of the chirp signal 0 Then:
ind 0 ＝(i 0 ,j 0 )i 0 ＝1,2,3,…,m,j 0 ＝1,2,3,…,n
this time orderSubstitution with divided chirp signal s= [ s ] 0 s 1 … s n ]First data s of (a) 0 The method comprises the following steps:
at this time, the insertion of one chirp signal data is completed, and all the chirp signal data are inserted in the above manner:
ind 1 ＝(i 1 ,j 1 ) ind 2 ＝(i 2 ,j 2 ) … ind n ＝(i n ,j n )
wherein ind n ＝(i n ,j n ) All are the positions of the data signals, ensure that the data are arranged in the sequence of the preceding and the following columns, and simultaneously pay attention to avoid the positions ind of other reference signals rs And synchronization signal position ind ss ：
ind＝[(i 0 ,j 0 ),(i 0 +1,j 0 ),…,(m,j 0 ),(0,j 0 +1),…,(m,n)]
i＝1,2,3,…,m j＝1,2,3,…,n(i,j)≠ind rs ,ind ss
Step 206: and performing waveform shaping and sequence multiplexing processing on each linear frequency modulation signal by adopting a DFT-s-OFDM modulation mode so as to obtain a DFT-s-OFDM signal corresponding to the fused linear frequency modulation signal.
DFT-s-OFDM refers to Spread orthogonal frequency division multiplexing (Discrete Fourier Transform-Spread-Orthogonal Frequency Division Multiplexing) based on a discrete fourier transform.
In step 206, after the allocation of the chirp signals in the resource grid is completed, the DFT-s-OFDM modulation scheme is selected to complete the waveform modulation, the total number of subcarriers is set to N, and P users are multiplexed with signals. Let the m modulation symbol information of the first user be x (m, l), i.e
x(m,l)＝[x(0,l),x(1,l),…,x(M-1,l)] T
First, M-point DFT operation is performed on M modulation symbol information x (M, l):
for symbol information y (k, l) of the first user after the DFT operation, the symbol information y (k, l) is expanded into information s (k, l) of N points by inserting 0 symbols, k epsilon (0, N-1), and then the symbol information y (k, l) is converted into time domain information z (N, l) by the IDFT operation:
Finally, the operation of adding the CP is carried out on the signal, L is set as the length of the CP, and the data with the length of L at the tail of Z (k, L) is copied to the header of Z (k, L), so that the following signals are formed:
[z(N-L+1,l),…,z(N-1,l),z(0,l),z(1,l),…,z(N-1,l)] T
thus, the complete DFT-s-OFDM signal fused with the linear frequency modulation signal is obtained.
In the method for time-frequency double synchronization of satellite signals based on fractional fourier transform provided in the embodiment of the present application, referring to fig. 7 and 8, step 300 of the method for time-frequency double synchronization of satellite signals based on fractional fourier transform needs to perform the search of the optimal order of correlation processing, so that step 300 specifically includes the following contents:
step 301: setting a search interval for searching the optimal order of fractional Fourier transform and a search step length in the search interval;
in step 301, a search interval [ a, b ] for searching the best order and a search step Δp within the interval are defined.
Step 302: in the search interval, respectively executing a preset optimal order search step on each linear frequency modulation signal based on the search step length so as to determine the optimal order of fractional Fourier transform corresponding to the optimal processing domain for processing each linear frequency modulation signal by the receiving end; wherein the best order searching step comprises: based on the current fractional Fourier transform of the current linear frequency modulation signal from the starting position of the search interval, recording the maximum value in the current corresponding fractional Fourier transform result and the currently used transform order; step-by-step increasing the step length of the preset size for the currently used conversion order, carrying out fractional Fourier transform on the current linear frequency modulation signal once every time the step length is increased, recording the maximum value in the fractional Fourier transform result corresponding to the current time, comparing the maximum value of the current time with the maximum value of the previous time, if the maximum value of the current time is larger than the maximum value of the previous time, replacing the maximum value of the previous time by the maximum value of the current time, replacing the conversion order used when the maximum value of the current time is obtained by the conversion order used when the maximum value of the current time is adopted, and determining the optimal order of the fractional Fourier transform of the current linear frequency modulation signal by the receiving end after traversing the search interval.
In step 302, a fractional Fourier transform F is performed on the chirp signal s (t) from the interval start position a p [s(t)]Wherein F p [·]The fractional Fourier transform operator is represented, the transformation result is S (u), the used transformation order is p, and the transformation process is shown as the following formula:
where α is the angle by which the fractional domain rotates relative to the time-frequency plane, and the transformation order p is:
recording maximum max in fractional fourier transform result S (u) 1 ＝max(S (u)) and the transformation order p used now 1 . Then, the used order is gradually increased by p=p+Δp with Δp as step length, and the linear frequency modulation signal s (t) is subjected to fractional Fourier transformation once for every step length, and the maximum value max in the transformation result is taken 2 =max (S (u)). Comparing the maximum value max of the conversion result 2 Maximum value max of the last conversion result 1 If max is the size of 2 >max 1 Max is used 2 Substitution max 1 Recording the conversion order used at this time to replace p 1 And proceeds downwardly; otherwise, the process is directly carried out downwards. Expressed by the following expression:
p 1 ＝p if max 2 >max 1
step 303: and carrying out fractional Fourier transform on each linear frequency modulation signal by using the optimal order of the fractional Fourier transform to obtain the maximum value in the fractional Fourier transform results corresponding to each linear frequency modulation signal.
In step 303, the search interval [ a, b ] is traversed]After completion, p 1 The value in (a) is the optimal fractional Fourier transform order for processing the chirp signal, where p is used 1 Fractional Fourier transform is carried out on the linear frequency modulation signal, and the maximum value max in the transformation result 1 Will be greater than the maximum of all order transform results. I.e.
p 1 ＝argmax p (F p (s(t)))
In the fractional fourier transform-based satellite signal time-frequency double synchronization method provided by the embodiment of the present application, referring to fig. 7 and 8, step 300 of the fractional fourier transform-based satellite signal time-frequency double synchronization method further needs to perform time-frequency fractional domain correlation joint processing, so that step 300 further specifically includes the following contents:
step 304: setting a linear frequency modulation signal of a transmitting end and a linear frequency modulation signal extracted by a receiving end;
in step 304, the chirp signal at the transmitting end is set to s (t), and the chirp signal extracted at the receiving end is set to r (t).
Step 305: searching the optimal orders of the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end to obtain the optimal orders of the linear frequency modulation signals of the transmitting end and the optimal orders of the linear frequency modulation signals extracted by the receiving end respectively;
In step 305, the best orders p of the sender signal s (t) and the receiver signal r (t) are obtained by searching the best orders 1 、p 2 And the optimal order p of the receiver signal 3 、p 4 Wherein p is 2 ＝-p 1 ，p 4 ＝-p 3 。
Step 306: fractional Fourier transform corresponding to the optimal order is carried out on the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end respectively, and the position of the maximum value in the transformation results corresponding to the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end is recorded;
in step 306, fractional Fourier transforms corresponding to the optimal order are performed on the chirped signals at both endsAnd->
Wherein S (u) and R (u) are S (t) and R (t) at the optimum order p, respectively 1 And p 3 The result of the down conversion, alpha 1 And alpha 3 S (t) and r (t) are respectively p 1 And p 3 The rotation angle of the fractional domain relative to the time-frequency plane under the order, p 2 And p is as follows 4 The same applies to the order.
Next, the positions ind1, ind2, ind3, ind4 where the maximum value is located are recorded in the conversion results S (u) and R (u)
ind1＝argmax(S 1 (u))ind2＝argmax(S 2 (u))
ind3＝argmax(R 3 (u))ind4＝argmax(R 4 (u))
Step 307: and determining the position difference between the maximum values in the conversion results respectively corresponding to the receiving end and the transmitting end according to the positions of the maximum values in the conversion results respectively corresponding to the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end, and obtaining a relational expression between the position difference and Doppler frequency offset and time delay.
In step 307, the signal s (t) is subjected to Doppler frequency offset f after passing through the channel d And the influence of the time delay tau to obtain a received signal R (t), wherein the fractional Fourier transform expression R (u) of R (t) and the fractional Fourier transform expression S (u) of S (t), doppler frequency offset f d The relation between the time delay tau is:
it can be derived that ind3=ind1+τcosα+f d sinα，ind4＝ind2+τcosα-f d sin alpha, setting the maximum value position difference in the fractional Fourier transform results S (u) and R (u) at the transmitting and receiving ends as Deltau 1 ＝ind3-ind1，Δu 2 =ind4-ind2. Then Deltau 1 ＝τcosα+f d sinα，Δu 2 ＝τcosα-f d sin alpha. Can be given Deltau 1 、Δu 2 With Doppler frequency offset f d The relation between the time delay tau is:
in the fractional fourier transform-based satellite signal time-frequency double synchronization method provided by the embodiment of the present application, referring to fig. 9 and 10, step 400 of the fractional fourier transform-based satellite signal time-frequency double synchronization method needs to establish a time-frequency synchronization performance enhancement mapping relationship, so that step 400 specifically includes the following contents:
step 401: setting a timing synchronization result, a timing synchronization result error, a frequency synchronization result and a frequency synchronization result error, and setting an optimized timing synchronization result, an optimized timing synchronization result error, an optimized frequency synchronization result and an optimized frequency synchronization result error;
In step 401, a timing synchronization result T, a frequency synchronization result F, and a timing synchronization result error E are set t Error of result of synchronization with frequency E f The method comprises the steps of carrying out a first treatment on the surface of the Setting the result of timing synchronization after optimization asThe error of the timing synchronization result is +.>The frequency synchronization result is->The error of the frequency synchronization result is +.>
Step 402: under the same channel state of a low orbit satellite channel, establishing a relational expression between the timing synchronization result and the timing synchronization result error and a relational expression between the frequency synchronization result and the frequency synchronization result error in a function fitting mode;
in step 402, in the same channel state C, the timing is established by means of a function fitSynchronization result T, synchronization error E t And frequency synchronization result F, synchronization error E f The relational expression between:
(F,E f )＝h t-syn (T,E t )
(T,E t )＝h f-syn (F,E f )
h t-syn an influence function of the timing synchronization result on the frequency synchronization result is expressed, h f-syn Then the function of the frequency synchronization result on the timing synchronization result is represented, both fitted from the time-frequency synchronization result data, and as much data as possible should be used to reduce the error.
Step 403: and determining time delay estimation result change in the timing synchronization performance enhancement part according to the optimized timing synchronization result and the timing synchronization result, determining time delay estimation result error change in the timing synchronization performance enhancement part according to the optimized timing synchronization result error and the timing synchronization result error, determining frequency offset estimation result change in the frequency synchronization performance enhancement part according to the optimized frequency synchronization result and the frequency synchronization result error, and determining frequency offset estimation result error change in the frequency synchronization performance enhancement part according to the optimized frequency synchronization result error and the frequency synchronization result error so as to establish a mapping function for representing a performance enhancement mapping relation between time synchronization and frequency synchronization of the low-orbit satellite signal.
In step 403, the time-frequency synchronization algorithm is further optimized, and the influence functions of the performance enhancing parts of the time-frequency synchronization algorithm on each other are established. The delay estimation result in the timing synchronization performance enhancement section variesError variation of time delay estimation result>Frequency synchronization performance enhancement section frequency offset estimation result variation +.>Error of frequency offset estimation resultVariation of difference->The mapping relation for establishing the enhancement of the time-frequency synchronization performance is as follows:
h t-syn an influence function of the enhanced timing synchronization performance on the frequency synchronization result is expressed, h f-syn Then the function of the impact on the timing synchronization result after the frequency synchronization performance is enhanced is represented.
In the fractional fourier transform-based satellite signal time-frequency double synchronization method provided by the embodiment of the present application, referring to fig. 9 and 10, step 400 of the fractional fourier transform-based satellite signal time-frequency double synchronization method further needs to perform time synchronization to enhance frequency synchronization, so that step 400 further specifically includes the following contents:
step 404: setting an initial timing synchronization result, a timing synchronization result after performance enhancement, a starting point of an initial extracted linear frequency modulation signal and OFDM signal data, and generating a first original linear frequency modulation signal expression corresponding to the initial extracted linear frequency modulation signal, wherein the linear frequency modulation signal expression consists of data of a position corresponding to the OFDM signal;
In step 404, the initial timing synchronization result is set to be T, and the result of the performance-enhanced synchronization is set to beInitial extraction of chirp signal starting point is ind 0 If the OFDM signal data is D, the first original linear frequency modulation signal expression which is initially extracted is:
wherein D is i And data representing the corresponding position of the OFDM signal, wherein n is the length of the linear frequency modulation signal.
Step 405: determining a first new linear frequency modulation signal expression corresponding to the linear frequency modulation signal which is changed after the timing synchronization performance is enhanced;
in step 405, the starting point of the extraction of the chirp signal after the timing synchronization performance has been enhanced will be changed to ind 1 The extracted chirp signal s' (t) will become the first new chirp signal expression:
step 406: setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the first original chirp signal expression and the first new chirp signal expression to obtain a first reliability factor expression;
in step 406, the true chirp signal is set toEstablishing an error assessment function between the linear frequency modulation signal and the extracted linear frequency modulation signals s (t) and s'(s) as h err To indicate the reliability of the extracted chirp signal, let the reliability factor be R, the following first reliability factor expression is derived:
step 407: performing frequency synchronization operation by adopting a first reliability factor expression, and setting the position offset, the time-frequency plane rotation angle and the actual frequency offset value of the front and rear spectrum peaks passing through the low-orbit satellite channel, so that the error between the calculated new frequency offset estimation result and the new frequency synchronization result is calculated;
in step 407, the frequency synchronization operation is performed using s '(t) having a higher reliability factor R', and the positional shift of the spectral peaks before and after the channel is set to Δu 1 And Deltau 2 The rotation angle of the time-frequency plane is alpha, and the true frequency offset value is F d The obtained frequency offset estimation result F' and the frequency synchronization result error E f ' calculated as follows:
E f ′＝F d -F′
step 408: and comparing the new frequency synchronization result error with the frequency synchronization result error before the timing synchronization performance is enhanced to verify the enhancement of the frequency synchronization performance.
In step 408, the resulting error E of frequency synchronization before enhancing the performance of timing synchronization f Comparing to obtain delta E f ＝E′ f -E f The enhancement of the frequency synchronization performance was verified.
In the fractional fourier transform-based satellite signal time-frequency double synchronization method provided by the embodiment of the present application, referring to fig. 9, step 400 of the fractional fourier transform-based satellite signal time-frequency double synchronization method further needs to perform frequency synchronization enhancing time synchronization, so that step 400 further specifically includes the following contents:
Step 409: setting an initial frequency synchronization result, a frequency synchronization result after performance enhancement, a linear frequency modulation signal in an OFDM receiving signal and a linear frequency modulation signal after Doppler frequency offset compensation, and determining a second original linear frequency modulation signal expression between the initial extracted linear frequency modulation signal and the receiving signal;
in step 409, the initial frequency synchronization result is set to be F, and the result of the performance-enhanced synchronization is set to beThe linear frequency modulation signal in the OFDM receiving signal is x (t), the linear frequency modulation signal after Doppler frequency offset compensation is s (t), and the initial extracted linear frequency modulation signal s (t) is the receiving signal x (t) multiplied by e j2πFt I.e. the second original chirp signal expression:
s(t)＝x(t)·e j2πFt
step 410: determining a second new linear frequency modulation signal expression corresponding to the linear frequency modulation signal subjected to Doppler frequency offset compensation after the frequency synchronization performance is enhanced;
in step 410, after the frequency synchronization performance is enhanced, the chirp signal after the doppler shift compensation will change, and s' (t) is set, and the extracted chirp signal will become x (t) multiplied byI.e. the second new chirp signal expression:
step 411: setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the second original chirp signal expression and the second new chirp signal expression to obtain a second reliability factor expression;
In step 411, the true chirp signal is set toEstablishing an error assessment function between the linear frequency modulation signal and the extracted linear frequency modulation signals s (t) and s' (t) as h err To indicate the reliability of the extracted chirp signal, let the reliability factor be R, to derive the following second reliability factor expression:
step 412: performing timing synchronization operation by adopting a second reliability factor expression, and setting position offset, time-frequency plane rotation angle and actual time delay value of front and rear spectrum peaks passing through a low-orbit satellite channel, so as to calculate new time delay estimation result and new timing synchronization result error;
in step 412, the timing synchronization operation is performed using s '(t) with higher reliability factor R', and the positional shift of the spectral peaks before and after the channel is set to Δu 1 And Deltau 2 The time-frequency plane rotation angle is alpha, the real time delay value is tau, and then the time delay estimation result T' and the timing synchronization result error E are obtained t ' calculated as follows:
E t ′＝τ-T′
step 413: and comparing the new timing synchronization result error with the timing synchronization result error before the frequency synchronization performance is enhanced to verify the enhancement of the timing synchronization performance.
In step 413, the resulting error E of timing synchronization with the frequency synchronization performance before enhancement t Comparing to obtain delta E t ＝E′ t -E t The enhancement of timing synchronization performance was verified.
In summary, according to the low-orbit satellite signal time-frequency double-synchronization method based on fractional Fourier transform provided by the embodiment of the application, signal processing and analysis are performed on a fractional domain which has space-time-frequency non-stationary characteristics and can effectively resist noise and time delay influence by satellite channel selection; under a 5G NR system, different linear frequency modulation signal insertion modes are designed aiming at signals with different structures under different links, and signal resource allocation is optimized; the time-frequency synchronization algorithm design based on fractional Fourier transform and linear frequency modulation signals fully digs the performance mutual enhancement characteristic between time-frequency synchronization.
Based on the above, the low-orbit satellite signal time-frequency double-synchronization method based on fractional Fourier transform provided by the embodiment of the application has the following beneficial effects:
(1) The linear frequency modulation signal and the OFDM signal can be organically fused on the basis of not changing the original 5G signal structure, and the signal resource allocation is optimized.
Because the linear frequency modulation signal has good frequency deviation resistance, multipath resistance and energy aggregation property presented in a fractional domain, a scheme combining fractional Fourier transform and the linear frequency modulation signal can achieve better synchronization performance of the linear frequency modulation signal sequence under the condition of occupying fewer signaling resources, and compared with a DMRS/PSS signal sequence used in a traditional 5G system, a large amount of signaling resources can be saved, and the design work of related technology in a satellite communication system by using the scheme can better meet the requirements of a mobile phone direct connection satellite scene on the signaling resources.
(2) The method can effectively solve the problem of difficult analysis and processing of the time-frequency domain signal caused by the fact that the satellite channel has the space-time-frequency non-stationary characteristic.
For satellite channel environments with space-time-frequency non-stationary characteristics, the environment adaptation capability and environment perception capability of the traditional signal sequence cannot meet the requirements of users on signal transmission quality. Compared with the traditional auxiliary sequence, the linear frequency modulation signal has better environment sensing capability due to the characteristics of Doppler frequency offset resistance and multipath resistance, and can further achieve the purpose of eliminating time-frequency domain interference by matching with a fractional Fourier transform technology for signal analysis processing in a fractional domain, thereby further improving the environment adaptation and sensing capability of the linear frequency modulation signal and having very high compatibility with satellite communication scenes.
(3) The performance of the timing synchronization technology can be further improved in a satellite communication scene, and the performance mutual gain between time and frequency synchronization is realized.
Compared with the traditional correlation-based time-frequency synchronization technology, the performance of the time-frequency synchronization module is seriously affected by noise and frequency offset, the matching of the fractional Fourier transform and the linear frequency modulation signal can achieve good resistance to serious noise and Doppler frequency offset interference in a satellite channel, the receiving quality of the signal at a receiving end is improved, and further the performance of the time-frequency synchronization module is improved. Meanwhile, the application notices that the performance between the time and frequency synchronization has a mutual enhancement relationship, fully excavates the efficiency mutual gain of the frequency synchronization and the timing synchronization, and feeds back and improves the performance of the frequency synchronization and the timing synchronization on the basis of the time delay/frequency offset estimation optimization.
In terms of software, the present application further provides a fractional fourier transform-based satellite signal time-frequency double synchronization device for executing all or part of the content in the fractional fourier transform-based satellite signal time-frequency double synchronization method, referring to fig. 10, the fractional fourier transform-based satellite signal time-frequency double synchronization device specifically includes the following contents:
an optimal processing domain determining module 10, configured to establish a mapping relationship between a satellite channel response frequency characteristic of a low-orbit satellite channel and each processing domain, and determine an optimal processing domain of the low-orbit satellite channel, where each processing domain includes: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain;
the signal fusion module 20 is used for setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals;
a processing domain joint module 30, configured to obtain an optimal order of fractional fourier transform corresponding to an optimal processing domain of the chirp signal, and perform joint processing of each processing domain based on the optimal order, so as to perform joint estimation of delay and doppler frequency offset;
And the synchronous double enhancement module 40 is used for carrying out double enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
The embodiment of the fractional fourier transform-based satellite signal time-frequency double synchronization device provided by the application can be particularly used for executing the processing flow of the embodiment of the fractional fourier transform-based satellite signal time-frequency double synchronization method in the embodiment, and the functions of the embodiment of the fractional fourier transform-based satellite signal time-frequency double synchronization method are not repeated herein, and can be referred to for the detailed description of the embodiment of the fractional fourier transform-based satellite signal time-frequency double synchronization method.
The fractional Fourier transform-based satellite signal time-frequency double synchronization device can be used for implementing the fractional Fourier transform-based satellite signal time-frequency double synchronization in a server or in a client device. Specifically, the selection may be made according to the processing capability of the client device, and restrictions of the use scenario of the user. The application is not limited in this regard. If all operations are performed in the client device, the client device may further include a processor for performing a specific process of time-frequency double synchronization of satellite signals based on fractional fourier transform.
The client device may have a communication module (i.e. a communication unit) and may be connected to a remote server in a communication manner, so as to implement data transmission with the server. The server may include a server on the side of the task scheduling center, and in other implementations may include a server of an intermediate platform, such as a server of a third party server platform having a communication link with the task scheduling center server. The server may include a single computer device, a server cluster formed by a plurality of servers, or a server structure of a distributed device.
Any suitable network protocol may be used between the server and the client device, including those not yet developed on the filing date of the present application. The network protocols may include, for example, TCP/IP protocol, UDP/IP protocol, HTTP protocol, HTTPS protocol, etc. Of course, the network protocol may also include, for example, RPC protocol (Remote Procedure Call Protocol ), REST protocol (Representational State Transfer, representational state transfer protocol), etc. used above the above-described protocol.
As can be seen from the above description, compared with the conventional time-frequency synchronization technology based on correlation, the time-frequency double synchronization device for satellite signals provided by the embodiment of the application has serious influence of noise and frequency offset, and the matching of the fractional fourier transform and the linear frequency modulation signal can achieve good resistance to serious noise and doppler frequency offset interference in a satellite channel, so that the receiving quality of signals at a receiving end can be improved, and further, the performance of a time-frequency synchronization module is improved. Meanwhile, the mutual enhancement relation of the performances of the time-frequency synchronization is noted, the mutual gains of the frequency synchronization and the timing synchronization are fully mined, and the performances of the frequency synchronization and the timing synchronization are feedback-improved on the basis of time delay/frequency offset estimation optimization.
The embodiment of the application also provides an electronic device, which may include a processor, a memory, a receiver and a transmitter, where the processor is configured to perform the satellite signal time-frequency double synchronization method based on fractional fourier transform mentioned in the foregoing embodiment, and the processor and the memory may be connected by a bus or other manners, for example, by a bus connection. The receiver may be connected to the processor, memory, by wire or wirelessly.
The processor may be a central processing unit (Central Processing Unit, CPU). The processor may also be any other general purpose processor, digital signal processor (Digital Signal Processor, DSP), application specific integrated circuit (Application Specific Integrated Circuit, ASIC), field programmable gate array (Field-Programmable Gate Array, FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof.
The memory is used as a non-transitory computer readable storage medium and can be used for storing non-transitory software programs, non-transitory computer executable programs and modules, such as program instructions/modules corresponding to the satellite signal time-frequency double synchronization method based on fractional Fourier transform in the embodiment of the application. The processor executes various functional applications and data processing of the processor by running non-transitory software programs, instructions and modules stored in the memory, i.e., implementing the fractional fourier transform-based satellite signal time-frequency double synchronization method in the above method embodiments.
The memory may include a memory program area and a memory data area, wherein the memory program area may store an operating system, at least one application program required for a function; the storage data area may store data created by the processor, etc. In addition, the memory may include high-speed random access memory, and may also include non-transitory memory, such as at least one magnetic disk storage device, flash memory device, or other non-transitory solid state storage device. In some embodiments, the memory may optionally include memory located remotely from the processor, the remote memory being connectable to the processor through a network. Examples of such networks include, but are not limited to, the internet, intranets, local area networks, mobile communication networks, and combinations thereof.
The one or more modules are stored in the memory that, when executed by the processor, perform the fractional fourier transform based satellite signal time-frequency double synchronization method of an embodiment.
In some embodiments of the present application, a user equipment may include a processor, a memory, and a transceiver unit, which may include a receiver and a transmitter, the processor, the memory, the receiver, and the transmitter may be connected by a bus system, the memory being configured to store computer instructions, the processor being configured to execute the computer instructions stored in the memory to control the transceiver unit to transmit and receive signals.
As an implementation manner, the functions of the receiver and the transmitter in the present application may be considered to be implemented by a transceiver circuit or a dedicated chip for transceiver, and the processor may be considered to be implemented by a dedicated processing chip, a processing circuit or a general-purpose chip.
As another implementation manner, a manner of using a general-purpose computer may be considered to implement the server provided by the embodiment of the present application. I.e. program code for implementing the functions of the processor, the receiver and the transmitter are stored in the memory, and the general purpose processor implements the functions of the processor, the receiver and the transmitter by executing the code in the memory.
The embodiment of the application also provides a computer readable storage medium, on which a computer program is stored, which when being executed by a processor, is used for realizing the steps of the satellite signal time-frequency double synchronization method based on fractional Fourier transform. The computer readable storage medium may be a tangible storage medium such as Random Access Memory (RAM), memory, read Only Memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, floppy disks, hard disk, a removable memory disk, a CD-ROM, or any other form of storage medium known in the art.
Those of ordinary skill in the art will appreciate that the various illustrative components, systems, and methods described in connection with the embodiments disclosed herein can be implemented as hardware, software, or a combination of both. The particular implementation is hardware or software dependent on the specific application of the solution and the design constraints. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present application. When implemented in hardware, it may be, for example, an electronic circuit, an Application Specific Integrated Circuit (ASIC), suitable firmware, a plug-in, a function card, or the like. When implemented in software, the elements of the application are the programs or code segments used to perform the required tasks. The program or code segments may be stored in a machine readable medium or transmitted over transmission media or communication links by a data signal carried in a carrier wave.
It should be understood that the application is not limited to the particular arrangements and instrumentality described above and shown in the drawings. For the sake of brevity, a detailed description of known methods is omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method processes of the present application are not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications and additions, or change the order between steps, after appreciating the spirit of the present application.
In the present application, features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments
The above description is only of the preferred embodiments of the present application and is not intended to limit the present application, and various modifications and variations can be made to the embodiments of the present application by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.
Claims (10)
1. The satellite signal time-frequency double synchronization method based on fractional Fourier transform is characterized by comprising the following steps:
establishing a mapping relation between a satellite channel response frequency characteristic of a low-orbit satellite channel and each processing domain, and determining an optimal processing domain of the low-orbit satellite channel, wherein each processing domain comprises: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain;
setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals;
Acquiring the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset;
and performing double enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
2. The method for time-frequency double synchronization of satellite signals based on fractional fourier transform according to claim 1, wherein the establishing a mapping relationship between satellite channel response frequency characteristics of a low-orbit satellite channel and each processing domain comprises:
constructing a satellite channel response expression model based on fading influence parameters, frequency influence parameters, phase influence parameters and time delay influence parameters of a low-orbit satellite channel, wherein the satellite channel response expression model comprises a time domain response expression and a frequency domain response expression;
constructing a corresponding received signal time domain expression according to the time domain response expression, and constructing a corresponding received signal frequency domain expression according to the frequency domain response expression;
respectively constructing an initial time domain signal cross-correlation calculation expression, an initial frequency domain signal cross-correlation calculation expression and an initial fractional domain signal cross-correlation calculation expression of two signals which do not pass through a low orbit satellite channel;
And constructing a time domain signal cross-correlation calculation expression of two signals after passing through a low-orbit satellite channel based on the time domain expression of the received signals, constructing a frequency domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the frequency domain expression of the received signals, and constructing a fractional domain signal cross-correlation calculation expression of two signals after passing through the low-orbit satellite channel based on the time domain signal cross-correlation calculation expression so as to construct and obtain the mapping relation between the satellite channel response frequency characteristics of the low-orbit satellite channel and the time domain, the frequency domain and the fractional domain respectively.
3. The fractional fourier transform-based satellite signal time-frequency double synchronization method of claim 2, wherein the determining the optimal processing domain of the low-orbit satellite channel comprises:
setting a channel initial state of a low-orbit satellite channel, a transformation order of fractional Fourier transformation, a rotation angle of a fractional domain plane relative to a time-frequency plane under the transformation order and a corresponding relation between the rotation angle and the transformation order;
performing optimal order searching operation on a preset linear frequency modulation signal of a transmitting end and a receiving signal after passing through a low-orbit satellite channel, obtaining respective initial conversion optimal orders of the receiving end and the transmitting end corresponding to a fraction domain through initial configuration, obtaining an initial time-frequency plane rotation angle of the receiving end based on the initial conversion optimal order configuration of the receiving end, and obtaining the initial time-frequency plane rotation angle of the transmitting end based on the initial conversion optimal order configuration of the transmitting end;
Executing a preset optimal processing domain determining step under the current channel state of the low-orbit satellite channel;
wherein the optimal processing domain determining step includes: performing cross-correlation calculation on the transmitted signal and the received signal in the time domain according to the initial time domain signal cross-correlation calculation expression to obtain a corresponding time domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in the frequency domain according to the frequency domain signal cross-correlation calculation expression to obtain a corresponding frequency domain signal cross-correlation calculation result; performing cross-correlation calculation on the transmitted signal and the received signal in a fractional domain according to the initial fractional domain signal cross-correlation calculation expression to obtain a corresponding fractional domain signal cross-correlation calculation result; selecting a maximum value from the time domain signal cross-correlation calculation result, the frequency domain signal cross-correlation calculation result and the fractional domain signal cross-correlation calculation result, and taking a processing domain corresponding to the maximum value as an optimal processing domain in the current channel state;
and if the channel state of the low-orbit satellite channel is changed, executing the optimal processing domain determining step again under the changed new channel state so as to complete the iteration of the optimal processing domain.
4. A satellite signal time-frequency double synchronization method based on fractional fourier transform according to claim 3, wherein the setting of the chirp signal as a low-orbit satellite signal under the 5G NR system comprises:
under a 5G NR system, setting key parameters of chirp signals of different structures under different links to establish a characteristic expression of the chirp signals, wherein the key parameters comprise: amplitude, modulation frequency, sampling point, sampling interval, initial frequency of the linear frequency modulation signal and rectangular function for defining time domain range of the linear frequency modulation signal;
performing sequence segmentation on each linear frequency modulation signal by taking a sampling point as a unit, and setting the insertion interval of the linear frequency modulation signal in a time-frequency resource grid;
and adjusting key parameters of each chirp signal.
5. The fractional fourier transform-based satellite signal time-frequency double synchronization method of claim 4, wherein the merging heterogeneous chirp signals comprises:
setting the dimension of a resource grid data matrix of each heterogeneous linear frequency modulation signal under different low-orbit satellite channels and different links to the same order to obtain a time-frequency resource grid matrix of each linear frequency modulation signal, wherein each data item in the time-frequency resource grid matrix comprises a data signal transmitted by the corresponding channel link and a physical signal except the data signal;
Completing the insertion work of each linear frequency modulation signal in the data signal based on the time-frequency resource grid matrix so as to realize the distribution of each linear frequency modulation signal in a resource grid;
and performing waveform shaping and sequence multiplexing processing on each linear frequency modulation signal by adopting a DFT-s-OFDM modulation mode so as to obtain a DFT-s-OFDM signal corresponding to the fused linear frequency modulation signal.
6. The method for time-frequency double synchronization of satellite signals based on fractional fourier transform according to claim 5, wherein the obtaining the optimal order of fractional fourier transform corresponding to the optimal processing domain of the chirp signal comprises:
setting a search interval for searching the optimal order of fractional Fourier transform and a search step length in the search interval;
in the search interval, respectively executing a preset optimal order search step on each linear frequency modulation signal based on the search step length so as to determine the optimal order of fractional Fourier transform corresponding to the optimal processing domain for processing each linear frequency modulation signal by the receiving end;
performing fractional Fourier transform on each linear frequency modulation signal by using the optimal order of the fractional Fourier transform to obtain the maximum value in fractional Fourier transform results corresponding to each linear frequency modulation signal respectively;
Wherein the best order searching step comprises: based on the current fractional Fourier transform of the current linear frequency modulation signal from the starting position of the search interval, recording the maximum value in the current corresponding fractional Fourier transform result and the currently used transform order; step-by-step increase of a preset step length is carried out on a currently used fractional Fourier transform for each step length, the maximum value in the fractional Fourier transform result corresponding to the current time is recorded, the maximum value in the fractional Fourier transform result is compared with the maximum value in the fractional Fourier transform result before the maximum value, if the maximum value in the fractional Fourier transform result before the maximum value is larger than the maximum value in the fractional Fourier transform result before the maximum value, the maximum value in the fractional Fourier transform result before the maximum value is replaced by the maximum value in the fractional Fourier transform result before the maximum value, the transformation order used when the maximum value in the fractional Fourier transform result before the maximum value is obtained is replaced by the transformation order used when the maximum value in the fractional Fourier transform result before the maximum value is obtained is adopted, and the optimal order of the fractional Fourier transform for processing the current linear FM signal by a receiving end is determined after the search interval is traversed.
7. The fractional fourier transform-based satellite signal time-frequency double synchronization method as recited in claim 6, wherein the joint processing of each of the processing domains based on the optimal order for joint delay and doppler frequency offset estimation comprises:
setting a linear frequency modulation signal of a transmitting end and a linear frequency modulation signal extracted by a receiving end;
searching the optimal orders of the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end to obtain the optimal orders of the linear frequency modulation signals of the transmitting end and the optimal orders of the linear frequency modulation signals extracted by the receiving end respectively;
fractional Fourier transform corresponding to the optimal order is carried out on the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end respectively, and the position of the maximum value in the transformation results corresponding to the linear frequency modulation signal of the transmitting end and the linear frequency modulation signal extracted by the receiving end is recorded;
and determining the position difference between the maximum values in the conversion results respectively corresponding to the receiving end and the transmitting end according to the positions of the maximum values in the conversion results respectively corresponding to the linear frequency modulation signals of the transmitting end and the linear frequency modulation signals extracted by the receiving end, and obtaining a relational expression between the position difference and Doppler frequency offset and time delay.
8. The fractional fourier transform-based satellite signal time-frequency double synchronization method as recited in claim 7, wherein the performing double enhancement processing on time synchronization and frequency synchronization of the chirp signal comprises:
setting a timing synchronization result, a timing synchronization result error, a frequency synchronization result and a frequency synchronization result error, and setting an optimized timing synchronization result, an optimized timing synchronization result error, an optimized frequency synchronization result and an optimized frequency synchronization result error;
under the same channel state of a low orbit satellite channel, establishing a relational expression between the timing synchronization result and the timing synchronization result error and a relational expression between the frequency synchronization result and the frequency synchronization result error in a function fitting mode;
and determining a time delay estimation result change in the timing synchronization performance enhancement section according to the optimized timing synchronization result and the timing synchronization result, determining a time delay estimation result error change in the timing synchronization performance enhancement section according to the optimized timing synchronization result error and the timing synchronization result error, determining a frequency offset estimation result change in the frequency synchronization performance enhancement section according to the optimized frequency synchronization result and the frequency synchronization result, and determining a frequency offset estimation result error change in the frequency synchronization performance enhancement section according to the optimized frequency synchronization result error and the frequency synchronization result error, so as to establish a mapping function for representing a performance enhancement mapping relationship between time synchronization and frequency synchronization of the low-orbit satellite signal;
Setting an initial timing synchronization result, a timing synchronization result after performance enhancement, a starting point of an initial extracted linear frequency modulation signal and OFDM signal data, and generating a first original linear frequency modulation signal expression corresponding to the initial extracted linear frequency modulation signal, wherein the linear frequency modulation signal expression consists of data of a position corresponding to the OFDM signal;
determining a first new linear frequency modulation signal expression corresponding to the linear frequency modulation signal which is changed after the timing synchronization performance is enhanced;
setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the first original chirp signal expression and the first new chirp signal expression to obtain a first reliability factor expression;
performing frequency synchronization operation by adopting a first reliability factor expression, and setting the position offset, the time-frequency plane rotation angle and the actual frequency offset value of the front and rear spectrum peaks passing through the low-orbit satellite channel, so that the error between the calculated new frequency offset estimation result and the new frequency synchronization result is calculated;
comparing the new frequency synchronization result error with the frequency synchronization result error before the timing synchronization performance is enhanced to verify the enhancement of the frequency synchronization performance;
Setting an initial frequency synchronization result, a frequency synchronization result after performance enhancement, a linear frequency modulation signal in an OFDM receiving signal and a linear frequency modulation signal after Doppler frequency offset compensation, and determining a second original linear frequency modulation signal expression between the initial extracted linear frequency modulation signal and the receiving signal;
determining a second new linear frequency modulation signal expression corresponding to the linear frequency modulation signal subjected to Doppler frequency offset compensation after the frequency synchronization performance is enhanced;
setting an actually used chirp signal, and establishing an error evaluation function between the actually used chirp signal and the second original chirp signal expression and the second new chirp signal expression to obtain a second reliability factor expression;
performing timing synchronization operation by adopting a second reliability factor expression, and setting position offset, time-frequency plane rotation angle and actual time delay value of front and rear spectrum peaks passing through a low-orbit satellite channel, so as to calculate new time delay estimation result and new timing synchronization result error;
and comparing the new timing synchronization result error with the timing synchronization result error before the frequency synchronization performance is enhanced to verify the enhancement of the timing synchronization performance.
9. A fractional fourier transform-based satellite signal time-frequency double synchronization device, comprising:
the system comprises an optimal processing domain determining module, a processing domain determining module and a processing domain determining module, wherein the optimal processing domain determining module is used for establishing a mapping relation between satellite channel response frequency characteristics of a low-orbit satellite channel and each processing domain, and determining the optimal processing domain of the low-orbit satellite channel, and each processing domain comprises: a time domain, a frequency domain, and a fraction domain, the optimal processing domain including the fraction domain;
the signal fusion module is used for setting a linear frequency modulation signal serving as a low-orbit satellite signal under a 5G NR system, and fusing heterogeneous linear frequency modulation signals;
the processing domain joint module is used for acquiring the optimal order of fractional Fourier transform corresponding to the optimal processing domain of the linear frequency modulation signal, and carrying out joint processing of each processing domain based on the optimal order so as to carry out joint estimation of time delay and Doppler frequency offset;
and the synchronous double-enhancement module is used for carrying out double-enhancement processing on the time synchronization and the frequency synchronization of the linear frequency modulation signals.
10. An electronic device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, wherein the processor implements the fractional fourier transform-based satellite signal time-frequency double synchronization method of any one of claims 1 to 8 when the computer program is executed by the processor.
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