OTFS system high-precision frequency synchronization method

Abstract

The invention belongs to the technical field of wireless communication, discloses a high-precision frequency synchronization method for an OTFS system, and provides a coarse and fine carrier frequency offset hierarchical estimation scheme for solving frequency offset by mainly using a differential phase between adjacent symbols and solving the problem of poor frequency offset estimation precision in the existing frequency synchronization algorithm. Coarse frequency offset estimation compensates carrier frequency offset to a working range of fine frequency offset estimation while ensuring a frequency offset estimation range, then the fine frequency offset estimation is utilized to further improve estimation precision, and the frequency offset estimation method provided by the invention achieves estimation precision close to a theoretical limit with low implementation complexity, and has a wide application prospect. Compared with a traditional single-stage frequency offset estimation method, the method has obvious advantages.

Description

Technical Field

[0001] This invention belongs to the field of wireless communication technology and is applicable to Orthogonal Time Frequency Space (OTFS) communication systems, specifically referring to a high-precision frequency synchronization method. Background Technology

[0002] Orthogonal Time-Frequency Synchronization (OTFS) technology has demonstrated excellent anti-multipath and anti-Doppler capabilities in 6G high-speed mobile communication scenarios with integrated air-space-ground systems, becoming a current research hotspot. Addressing the timing and frequency synchronization issues faced by OTFS systems, synchronization is fundamental for signal detection and channel estimation; without high-performance synchronization algorithms, the accuracy of subsequent channel estimation and signal detection cannot be guaranteed. Currently, domestic and international scholars have conducted considerable research on OTFS signal detection and channel estimation, but research on frequency synchronization remains relatively limited. Summary of the Invention

[0003] To address the issue that existing frequency synchronization algorithms primarily rely on differential phase between adjacent symbols to calculate frequency offset, resulting in poor frequency offset estimation accuracy, this invention proposes a coarse and fine carrier frequency offset hierarchical estimation scheme. The coarse frequency offset estimation, while maintaining the estimated range, compensates for the carrier frequency offset within the working range of the fine frequency offset estimation. Then, the fine frequency offset estimation is used to further improve the estimation accuracy. The frequency offset estimation method proposed in this invention achieves an estimation accuracy close to the theoretical limit with relatively low implementation complexity, demonstrating significant advantages over traditional single-stage frequency offset estimation methods. To achieve the above objectives, this invention employs the following technical solution:

[0004] A high-precision frequency synchronization method for an OTFS system, comprising the following specific steps:

[0005] (1) Data of the two-dimensional matrix to be modulated Perform IDFT processing to obtain The two-dimensional matrix data to be modulated includes a pilot sequence, a guard sequence, and a data sequence, with the pilot sequence placed in a specific row of the two-dimensional matrix data. The position, the value of the protection sequence is zero, and it is set on both the upper and lower sides of the pilot sequence. OK, Greater than the channel's maximum delay; , , M and N are the number of rows and columns of the two-dimensional matrix data to be modulated, respectively;

[0006] (2) Perform parallel-to-serial conversion on the data processed by IDFT to obtain... The signal is transmitted via a time-frequency dual-dispersion channel to obtain a time-delay-time domain received signal. ;

[0007]

[0008] In the formula, , , and These are the gain tap, delay index, and carrier frequency offset index of the time-frequency bidispersive channel impulse response, respectively, where p is the maximum number of multipaths.

[0009] (3) Receive signals according to the time domain Obtain the two-dimensional array of delay-Doppler domain Calculate the cyclic correlation function Then construct the timing measurement function. Complete the timing synchronization to obtain the timing offset estimate, and obtain the time delay of all paths;

[0010] (4) Construct the first Time index of the path's timed synchronization location Corresponding Doppler dimension cyclic correlation function ;

[0011] (5) Offset the carrier frequency index of the i-th multipath. Decompose integer multiple frequency offset index Coarse-small frequency offset index and small offset index The relationship between them is: ;

[0012] (6) When the cyclic shift amount and When they are closest in numerical value, The cyclic shift value reaches its maximum amplitude peak in the Doppler dimension. Recorded as Its amplitude is denoted as At this point, the integer frequency offset index estimate is obtained. The amplitudes of the spectral lines to the left and right of the peak are denoted as... and Calculate the parabolic interpolation offset. The estimated value of the coarse decimal frequency offset index is... ;

[0013] (7) Based on the timing synchronization position, the time delay-time domain pilot sequence signal is extracted to obtain:

[0014]

[0015] in, ,use right Perform coarse carrier frequency offset compensation to obtain ;

[0016] (8) Pilot sequence signal and local pilot sequence Divide the sequence into two equal subsequences;

[0017] (9) Perform point-by-point complex conjugate multiplication on the corresponding preceding and following subsequences of the received pilot sequence signal and the local pilot sequence respectively to obtain the product sequence. Integrate the product sequence to obtain the matched filter output. and ;

[0018] (10) Perform conjugate multiplication on the results of the matching filtering of the preceding and following subsequences to extract the phase difference. ;

[0019] (11) Based on phase difference By scaling the mapping, the estimated value of the minute frequency offset index can be obtained. ;

[0020] Step (3) specifically includes the following steps:

[0021] (301) Receive the signal in the time domain Perform serial-to-parallel conversion, and use Starting from a point, extract one symbol every M symbols in the time domain, for a total of N symbols. As a column vector, with The N symbols corresponding to each time point are used as row vectors to form a two-dimensional array of the received signal in the time domain. Then, after DFT transformation to the delay-Doppler domain, it becomes... ;

[0022] (302) Receive the signal in the time-delay-Doppler domain The row vectors and the local time-delay-Doppler domain pilot sequences are used to construct a correlation function through cyclic correlation operations. ;

[0023]

[0024] In the formula, ;

[0025] (303) From a two-dimensional array each line Find the maximum value among the elements and form a new one-dimensional array. ;

[0026] (304) For the time-delay-Doppler domain received signal Energy normalization is obtained :

[0027] ;

[0028] (305) Based on the new one-dimensional array Normalized time-delay-Doppler domain received signal Constructing a timing offset metric :

[0029] ;

[0030] (306) Constructing a timing measurement The dynamic decision threshold T is used to determine the search interval when the timing offset metric exceeds the threshold T. Then, within the search interval, all peak positions that meet the conditions are identified. The time index of the peak value is denoted as . The peak determination condition is that the amplitude measured at the current timing offset is greater than the amplitudes of the h adjacent points before and after it, where h is a set value.

[0031] (307) Based on the time index of the peak Timed offset and multipath delay taps The relationship between the time offset estimates is used to derive the timing offset values. Get the delay of all paths ;

[0032] Among them, the peak time index Timed offset and multipath delay The relationship between them is:

[0033] .

[0034] In step (1):

[0035]

[0036] The pilot sequence is:

[0037]

[0038] In the formula, Where N is the root index and N is the length of the ZC sequence. Coprime to N;

[0039] The pilot sequence is expressed in the time-delay domain as follows:

[0040] .

[0041] In step (4) The mathematical form is:

[0042] .

[0043] Among them, the parabolic interpolation offset in step (6) The calculation formula is:

[0044] .

[0045] In step (7) , This is a pilot sequence signal with only a residual fractional frequency offset.

[0046] The subsequence expression formed in step (8) is as follows:

[0047]

[0048]

[0049]

[0050] .

[0051] Among them, the estimated value of the fine number frequency offset index in step (11) for:

[0052] .

[0053] Among them, the estimated value of carrier frequency offset index in step (12) Represented as:

[0054] .

[0055] Compared with the prior art, the present invention has the following advantages:

[0056] The frequency offset estimation method proposed in this invention achieves an estimation accuracy close to the theoretical limit with very low computational complexity. Attached Figure Description

[0057] Figure 1 This is the construction of the OTFS pilot sequence for the present invention. Detailed Implementation

[0058] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0059] A high-precision frequency synchronization method for an OTFS system, comprising the following specific steps:

[0060] (1) Data of the two-dimensional matrix to be modulated Perform IDFT processing to obtain ;

[0061]

[0062] In the formula, , , M and N are the number of rows and columns of the two-dimensional matrix data to be modulated, respectively;

[0063] The two-dimensional matrix data to be modulated includes a pilot sequence, a guard sequence, and a data sequence, with the pilot sequence placed in a row of the two-dimensional matrix data to be modulated. The position, the value of the protection sequence is zero, and it is set on both the upper and lower sides of the pilot sequence. OK, Greater than the channel's maximum delay; such as Figure 1 As shown;

[0064] The pilot sequence is:

[0065]

[0066] in, Where N is the root index and N is the length of the ZC sequence. Coprime to N, the latter half of this sequence is the conjugate transform of the standard ZC sequence. The time-delay domain expression of the ZC pilot sequence is:

[0067] .

[0068] (2) Perform parallel-to-serial conversion on the data processed by IDFT to obtain The signal is transmitted via a time-frequency dual-dispersion channel to obtain a time-delay-time domain received signal. ;

[0069]

[0070] in, , , and These are the gain tap, delay index, and carrier frequency offset index of the time-frequency bidispersive channel impulse response, respectively, where p is the maximum number of multipaths.

[0071] (3) Receive the signal in the time domain Perform serial-to-parallel conversion, and use Starting from a point, extract one symbol every M symbols in the time domain, for a total of N symbols. As a column vector, with The N symbols corresponding to each time point are used as row vectors to form a two-dimensional array of the received signal in the time domain. Then, after DFT transformation to the delay-Doppler domain, it becomes... ;

[0072] (4) Receive the signal in the time-delay-Doppler domain The row vectors and the local time-delay-Doppler domain pilot sequence are used to construct a correlation function through cyclic correlation operations:

[0073]

[0074] in, ;

[0075] (5) From a two-dimensional array each line Find the maximum value among the elements and form a new one-dimensional array. ;

[0076] (6) For the received signal in the time-delay-Doppler domain Energy normalization is performed to obtain ;

[0077] ;

[0078] (7) Based on the new one-dimensional array Normalized time-delay-Doppler domain received signal Constructing a timing offset metric The formula is as follows:

[0079] ;

[0080] (8) Constructing a timed measurement The dynamic decision threshold T is used to determine the search interval when the timing offset metric exceeds the threshold T. Then, within the search interval, all peak positions that meet the conditions are identified. The time index of the peak value is denoted as . The peak value determination condition is that the amplitude measured at the current timing offset is greater than the amplitudes of the h adjacent points, where h is a set value. The mathematical expression for this determination principle is as follows:

[0081]

[0082] Among them, multipath sequence number , For the maximum number of multipaths, ;

[0083] (9) Based on the time index of the peak Timed offset and multipath delay taps The relationship between the time offset estimates is used to derive the timing offset values. Get the delay of all paths ;

[0084] Among them, the peak time index Timed offset and multipath delay The relationship between them is:

[0085] .

[0086] (10) Construct the first Time index of the path's timed synchronization location Corresponding Doppler dimension cyclic correlation function Its mathematical form can be expressed as:

[0087] ;

[0088] (11) Offset the carrier frequency index of the i-th multipath. Decompose integer multiple frequency offset index Coarse-small frequency offset index and small offset index The relationship between them is:

[0089] .

[0090] (12) When the cyclic shift amount and When they are closest in numerical value, The cyclic shift value reaches its maximum amplitude peak in the Doppler dimension. Recorded as Its amplitude is denoted as At this point, the integer frequency offset index estimate is obtained. The amplitudes of the spectral lines to the left and right of the peak are denoted as... , Parabolic interpolation offset The calculation formula is:

[0091]

[0092] Therefore, the estimated value of the coarse decimal frequency offset index is... ;

[0093] (13) Based on the timing synchronization position, extract the time delay-time domain pilot sequence signal to obtain...

[0094]

[0095] in, ,use right After performing coarse carrier frequency offset compensation, we obtain:

[0096] ,

[0097] The pilot sequence signal is received with only a residual fractional frequency offset;

[0098] (14) will Pilot sequence signal and local pilot sequence Divide the sequence into two equal subsequences, as shown in the following expression:

[0099]

[0100]

[0101]

[0102] .

[0103] (15) Perform point-by-point complex conjugate multiplication on the corresponding preceding and following subsequences of the received pilot sequence signal and the local pilot sequence respectively to obtain the product sequence. Integrate the product sequence to obtain the matched filter output, the mathematical expression of which is as follows:

[0104] ;

[0105] (16) Perform conjugate multiplication on the results of the matching filtering of the preceding and following subsequences to extract the phase. The mathematical representation is as follows:

[0106] ;

[0107] (17) Based on the above phase difference, the estimated value of the small number frequency offset index is obtained by scaling transformation mapping. :

[0108] .

[0109] (18) Carrier frequency offset index estimate (i.e., normalized Doppler frequency offset, with the normalization unit being Doppler resolution) ), represented as:

[0110] .

Claims

1. A high-precision frequency synchronization method for an OTFS system, characterized in that, The specific implementation steps include: (1) Data of the two-dimensional matrix to be modulated Perform IDFT processing to obtain The two-dimensional matrix data to be modulated includes a pilot sequence, a guard sequence, and a data sequence, with the pilot sequence placed in a specific row of the two-dimensional matrix data. The position, the value of the protection sequence is zero, and it is set on both the upper and lower sides of the pilot sequence. OK, Greater than the channel's maximum delay; , , M and N are the number of rows and columns of the two-dimensional matrix data to be modulated, respectively; (2) Perform parallel-to-serial conversion on the data processed by IDFT to obtain... The signal is transmitted via a time-frequency dual-dispersion channel to obtain a time-delay-time domain received signal. ; ； In the formula, , , and These are the gain tap, delay index, and carrier frequency offset index of the time-frequency bidispersive channel impulse response, respectively, where p is the maximum number of multipaths. (3) Receive signals according to the time domain Obtain the two-dimensional array of delay-Doppler domain Calculate the cyclic correlation function Then construct the timing measurement function. Complete the timing synchronization to obtain the timing offset estimate, and obtain the time delay of all paths; (4) Construct the first Time index of the path's time synchronization location Corresponding Doppler dimension cyclic correlation function ; (5) Offset the carrier frequency index of the i-th multipath. Decompose integer multiple frequency offset index Coarse-small frequency offset index and small offset index The relationship between them is: ； (6) When the cyclic shift amount and When they are closest in numerical value, The cyclic shift value reaches its maximum amplitude peak in the Doppler dimension. Recorded as Its amplitude is denoted as At this point, the integer frequency offset index estimate is obtained. The amplitudes of the spectral lines to the left and right of the peak are denoted as... and Calculate the parabolic interpolation offset. The estimated value of the coarse decimal frequency offset index is... ; (7) Based on the timing synchronization position, the time delay-time domain pilot sequence signal is extracted to obtain: ； in, ,use right Perform coarse carrier frequency offset compensation to obtain ; (8) Pilot sequence signal and local pilot sequence Divide the sequence into two equal subsequences; (9) Perform point-by-point complex conjugate multiplication on the corresponding preceding and following subsequences of the received pilot sequence signal and the local pilot sequence respectively to obtain the product sequence. Integrate the product sequence to obtain the matched filter output. and ; (10) Perform conjugate multiplication on the results of the matching filtering of the preceding and following subsequences to extract the phase difference. ; (11) Based on phase difference By scaling the mapping, the estimated value of the minute frequency offset index can be obtained. ; (12) Calculate the estimated value of carrier frequency offset index. That is, normalized Doppler frequency offset.

2. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, Step (3) specifically includes the following steps: (301) Receive the signal in the time domain Perform serial-to-parallel conversion, and use Starting from a point, extract one symbol every M symbols in the time domain, for a total of N symbols. As a column vector, with The N symbols corresponding to each time point are used as row vectors to form a two-dimensional array of the received signal in the time domain. Then, after DFT transformation to the delay-Doppler domain, it becomes... ; (302) Receive the signal in the time-delay-Doppler domain The row vectors and the local time-delay-Doppler domain pilot sequences are used to construct a correlation function through cyclic correlation operations. ; ； In the formula, ; (303) From a two-dimensional array each line Find the maximum value among the elements and form a new one-dimensional array. ; (304) For the time-delay-Doppler domain received signal Energy normalization is obtained : ； (305) Based on the new one-dimensional array Normalized delay-Doppler domain received signal Constructing a timing offset metric : ； (306) Constructing a timing measurement The dynamic decision threshold T is used to determine the search interval when the timing offset metric exceeds the threshold T. Then, within the search interval, all peak positions that meet the conditions are identified. The time index of the peak value is denoted as . The peak determination condition is that the amplitude measured at the current timing offset is greater than the amplitudes of the h adjacent points before and after it, where h is a set value. (307) Based on the time index of the peak Timed offset and multipath delay taps The relationship between the time offset estimates is used to derive the timing offset values. Get the delay of all paths ; Among them, the peak time index Timed offset and multipath delay The relationship between them is: 。 3. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, In step (1): ； The pilot sequence is: ； In the formula, Where N is the root index and N is the length of the ZC sequence. Coprime to N; The pilot sequence is expressed in the time-delay domain as follows: 。 4. The high-precision frequency synchronization method for an OTFS system according to claim 2, characterized in that, In step (4) The mathematical form is: 。 5. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, Parabolic interpolation offset in step (6) The calculation formula is: 。 6. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, In step (7) , This is a pilot sequence signal with only a residual fractional frequency offset.

7. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, The subsequence expression formed in step (8) is as follows: ； ； ； 。 8. The high-precision frequency synchronization method for an OTFS system according to claim 1, characterized in that, The estimated value of the decimal frequency offset index in step (11) for: 。 9. A high-precision frequency synchronization method for an OTFS system according to claim 10, characterized in that, Carrier frequency offset index estimate in step (12) Represented as: 。

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