Carrier synchronization method for DFT precoding FBMC system for air-to-ground communication

Abstract

This invention provides a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication, comprising: acquiring FBMC symbol data to be transmitted; constructing a special CAZAC sequence of length M / 4, constructing four synchronization sequence symbols, and outputting the corresponding time-domain transmission signal after passing them together with other data symbols through a conjugate symmetric module, pre-FFT, IFFT subcarrier modulation, and PPN multiphase filtering; at the receiver end, constructing a local sequence, taking two consecutive parts of length M / 4 from the received special time-domain sequence signal, and correlating them with the corresponding parts of the local sequence to obtain correlation results; multiplying the conjugate of the correlation result of the first part of the sequence with the correlation result of the second part of the sequence to obtain a frequency offset metric function, obtaining a fractional frequency offset estimate based on the frequency offset metric function, and using the frequency offset estimate to compensate for the carrier frequency offset. This method proposes a synchronization training sequence design method, utilizing the time-domain signal construction characteristics of the training sequence to estimate and correct the carrier frequency offset.

Description

Technical Field

[0001] This invention relates to the field of orthogonal frequency division multiplexing (OFDM) technology, specifically to a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication. Background Technology

[0002] In recent years, multi-carrier transmission technologies such as Orthogonal Frequency Division Multiplexing (OFDM) have been widely used due to their advantages, including high spectral efficiency, low transceiver complexity, simple equalization, and ease of integration with multi-antenna technologies. However, with the heterogeneity and asynchronous nature of future communication networks, some drawbacks of OFDM, such as high out-of-band (OOB) radiation and sensitivity to synchronization errors, have gradually attracted attention. To address these challenges, Filter Bank Multi-Carrier with Offset Quadrature Amplitude Modulation (FBMC-OQAM) is considered a promising waveform technology due to its extremely low OOB radiation and other advantages, offering significant advantages in asynchronous communication networks. For example, in massive IoT networks, the large number of terminals and the resulting synchronization between devices consume substantial signaling resources. FBMC systems can effectively protect subcarriers, greatly reducing interference between asynchronous users, thereby significantly saving signaling resources and reducing the power consumption of terminal devices.

[0003] As a multi-carrier transmission technology, FBMC also faces a significant peak-to-average power ratio (PAPR) problem. FBMC-OQAM symbols are formed by adding signals modulated by multiple independent subcarriers of equal bandwidth. When the signals on the subcarriers are in phase, their addition can produce a large peak power, resulting in a high ratio of peak power to average power. To avoid the influence of power amplifier nonlinearity, the amplifier circuit needs a larger power back-off value, which significantly increases the power consumption of terminal devices and reduces battery life. This is particularly problematic for nonlinear satellite communication systems and low-cost IoT communications. Currently, the most effective way to reduce PAPR in FBMC systems is through Discrete Fourier Transform (DFT) precoding. Therefore, an FBMC-OQAM system scheme based on data mapping and DFT precoding has been proposed, which can effectively reduce PAPR and is more suitable for transmission links in satellites and IoT terminals.

[0004] With the growing demand for integrated space-ground communication, applying multi-carrier modulation technologies such as FBMC to satellite communication systems will greatly accelerate this process. Simultaneously, due to the increasing demand for broadband transmission and the growing congestion of L and S bands, wireless communication, especially broadband satellite communication, may gradually shift to higher frequency bands such as Ka. This shift in frequency bands makes communication systems more prone to significant carrier frequency offset (CFO). Furthermore, low-Earth orbit (LEO) satellite communication, due to its rapid motion, also exhibits high Doppler shift. Therefore, improving the accuracy of CFO estimation, and thus enhancing the performance of the geostationary system, will become crucial.

[0005] Currently, in OFDM systems, constant amplitude zero autocorrelation (CAZAC) sequences are generally selected for synchronization training. However, in FBMC systems, the trailing phenomenon caused by filters in FBMC symbols leads to the superposition of multiple preceding and following symbols, making the construction of synchronization sequences complex. Therefore, how to design efficient synchronization sequences and estimate carrier frequency offset has always been an important issue. Summary of the Invention

[0006] To address the aforementioned issues, this invention provides a carrier synchronization method for DFT precoding FBMC systems for air-to-ground communication. This method proposes a training sequence design approach for synchronization in data mapping DFT precoding FBMC-OQAM systems. It utilizes the frequency domain characteristics of the training sequence to estimate and correct carrier frequency offset. Furthermore, this synchronization sequence has a low peak-to-average power ratio, which is beneficial for low-power transmission.

[0007] To achieve the above objectives, this invention provides a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication, comprising the following steps:

[0008] Obtain the FBMC symbol data to be transmitted, including four data sequences A, B, C, and D of length M / 4. Construct four consecutive synchronization sequence symbols, with data sequence structures of [A,Z], [Z,B], [C,Z], and [Z,D], respectively, where Z is a special CAZAC sequence of length M / 4.

[0009]

[0010] Where z(k) is the first M / 4-1 sequences of a CAZAC sequence of length M / 2-1, and z0 is any complex value; w(k) is a weighted coefficient sequence, and when k = 1, 2, ..., M / 4-1, the value is the reciprocal of the weighted superposition of the corresponding filter signal values;

[0011] Four consecutively placed synchronization sequence symbols, along with other data symbols, are processed through the conjugate symmetric module of the FBMC-OQAM system using data mapping DFT precoding, pre-FFT, IFFT subcarrier modulation, and PPN multiphase filtering to obtain the corresponding transmitted signal in the time domain. The synchronization sequence symbols will construct a segment of the form [Z, (-j)·Z in the time domain. * A special time-domain sequence of Z, where Z* is a specific conjugate symmetric sequence of Z;

[0012] At the receiver, a local sequence Z' = [Z, (-j)·Z is constructed. * The received special time-domain sequence signal is divided into two consecutive parts of length M / 4. The two parts of the sequence, except for the first value, are correlated with the corresponding parts of the local sequence to obtain the correlation results. The correlation results of the first part of the sequence are conjugate and multiplied with the correlation results of the second part of the sequence to obtain the frequency offset metric function. The fractional frequency offset estimate is obtained based on the frequency offset metric function. The carrier frequency offset is compensated using the frequency offset estimate to complete the fractional carrier frequency synchronization.

[0013] Preferably, the weighted coefficient sequence w(k) is:

[0014]

[0015] Where k = 1, 2, ..., M / 4-1, p1, p2, ... p 2K The prototype filter signal is divided into 2K sequential blocks of length M / 2; K is the overlap factor.

[0016] Preferably, the construction and conjugate symmetric mapping of the four synchronization sequence symbols includes the following steps:

[0017] The sequences A, B, C, and D are combined with Z in a specific order to form the data sequences of four FBMC transmitted symbols, which are used as the data in the nth to (n+3rd)th FBMC symbols: [A,Z], [Z,B], [C,Z], and [Z,D], where Z is a special ZC sequence of length M / 4. According to the OQAM modulation phase requirements, these four sequences are subjected to a specific conjugate symmetric mapping, and the mapped data is represented as: j n [A,A*,Z,Z*]、j n+1 [Z,Z*,B,B*]、j n+2 [C,C*,Z,Z*] and j n+3 [Z,Z*,D,D*];

[0018] in:

[0019]

[0020] According to the data mapping rules, in the sequences [Z,Z*,B,B*] and [Z,Z*,D,D*], g0 takes the value of z0, and g1 takes the real part of the first sequence value of sequence B or sequence D; in the sequences [A,A*,Z,Z*] and [C,C*,Z,Z*], g0 takes the conjugate of the first sequence value of sequence A or sequence C, and g1 takes the imaginary part of z0.

[0021] The [A,A*] sequence is:

[0022]

[0023] Where a0~a M / 4-1 For the data in sequence A, the CC* sequence has the same structure as AA*, and c2 takes the real part of z0;

[0024] The [B,B*] sequence is:

[0025]

[0026] Among them, the DD* sequence structure is the same as BB*, and g2 takes the imaginary part of the first sequence value of the B or D sequence.

[0027] Preferably, the signal transmission process from the transmitter to the receiver in the data mapping DFT precoding FBMC-OQAM system includes the following steps:

[0028] The original signal to be transmitted is grouped, converted from serial to parallel, a special training sequence is inserted, a conjugate symmetric mapping is performed, and DFT precoding is applied to obtain a data sequence with alternating real and imaginary elements.

[0029] The data sequence is subjected to IFFT transformation and polyphase filtering, and the baseband transmission signal is output. After carrier frequency modulation, it is transmitted to the channel.

[0030] The signal received from the channel is subjected to polyphase filtering, FFT subcarrier demodulation, equalization, and OQAM demodulation to obtain real data.

[0031] The real data is processed by adding phase factor, IFFT transformation, demapping and serial-to-parallel conversion operations, and then used as the output data.

[0032] Preferably, the conjugate symmetric mapping includes the following steps:

[0033] For any nth set of original data, use vector x n This indicates that the original data x in this set... n The nth data sequence of the mapping is represented by vector b. n express;

[0034] The nth set of original data x nThe following formula maps the nth data sequence b to a set of data that satisfies a specific conjugate symmetry property. n :

[0035]

[0036] Where n takes the value of a non-negative integer; x n (k) represents the nth set of original data x. n The k-th data in; b n (k) represents the nth data sequence b n The k-th data in the array; the superscript * indicates the conjugate value; To perform the real part operation; This is an operation to extract the imaginary part.

[0037] Preferably, the frequency offset metric function is as follows:

[0038]

[0039] Where r(l) is the sampled value of the received signal, z'(l) is the sequence value of the local sequence Z', v is the frequency offset, Δl is the starting position of the sampling sequence number of the special time domain sequence, and F is the subcarrier spacing;

[0040] At this point, the estimated value of frequency offset v It is obtained by calculating the phase of the frequency offset metric function J(v):

[0041]

[0042] Here, angle() is the phase take operation.

[0043] The beneficial effects of this invention are:

[0044] This invention proposes a carrier synchronization method for DFT precoding FBMC systems in air-to-ground communication. This method constructs a special training sequence with conjugate symmetry. This special training sequence is inserted after or before the signal conjugate symmetry mapping at the transmitter of the FBMC-OQAM system, forming a synchronization sequence with four conjugate symmetric transmission sequences for signal synchronization transmission. The frequency domain characteristics of the special training sequence are utilized to estimate and correct the carrier frequency offset. This method provides a carrier frequency offset estimation synchronization training sequence design method with low system overhead for data mapping DFT precoding systems. Furthermore, compared to the traditional synchronization sequence design of FBMC-OQAM, the calculation process is simplified. Attached Figure Description

[0045] Figure 1 This is a flowchart of a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to an embodiment of the present invention;

[0046] Figure 2 This is a system architecture diagram of Data Mapping DFT Precoding FBMC-OQAM;

[0047] Figure 3 This is an example of a signal conjugate symmetric mapping;

[0048] Figure 4 This is a schematic diagram of the generation principle of a single FBMC symbol data;

[0049] Figure 5 This is a schematic diagram of multiple signals superimposed on each other;

[0050] Figure 6 This is a schematic diagram of the prototype filter sequence segmentation (overlap factor K = 4);

[0051] Figure 7 This is a graphical analytical diagram illustrating the construction of a complete ZZ* sequence from the corresponding transmitted signals in the time domain.

[0052] Figure 8 It is the effect of the transmitted signal generated after the training sequence is modulated;

[0053] Figure 9 This is the performance simulation result of carrier estimation using the training sequence of this invention. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0055] Example 1

[0056] This invention applies to a carrier synchronization method for a data mapping DFT precoding FBMC-OQAM system. The architecture of the FBMC-OQAM system is as follows: Figure 2 As shown.

[0057] The FBMC-OQAM system transmitter includes a preprocessing module and a transmission module. The preprocessing module preprocesses the raw data to obtain alternating real and virtual carriers that meet the real-orthogonality requirements of the FBMC-OQAM system. The transmission module transmits the preprocessed data from the transmitter. This invention only describes the baseband system processing, omitting intermediate frequency and radio frequency processing units such as analog-to-digital / digital-to-analog converters, amplifiers, mixers, RF filters, and antennas. The preprocessing module includes a pre-serial-to-parallel conversion module 21, a conjugate symmetry module 22, a pre-FFT module 23, a pre-IFFT module 24, and a pre-polyphase filtering module 25. The pre-serial-to-parallel conversion module 21 groups the raw data and performs serial-to-parallel conversion on each group. In one embodiment, the length of each group of raw data is set to M / 2, where M is the number of subcarriers in the FBMC-OQAM system, M = 2. L The value of L is a natural number greater than 2.

[0058] The conjugate symmetric module 22 is used to map each set of original data after serial-to-parallel conversion into a data sequence that satisfies a specific conjugate symmetry property. For any nth set of original data, the vector x... n This indicates that the original data x in this set... n The nth data sequence of the mapping is represented by vector b. n express;

[0059] The nth set of original data x n The following formula maps the nth data sequence b to a set of data that satisfies a specific conjugate symmetry property. n :

[0060]

[0061] Where n takes the value of a non-negative integer; x n (k) represents the nth set of original data x. n The k-th data in; b n (k) represents the nth data sequence b n The k-th data in the array; the superscript * indicates the conjugate value; To perform the real part operation; For the operation of retrieving the imaginary part, an example is shown below. Figure 3 As shown.

[0062] The pre-FFT module 23 performs DFT precoding on each mapped data sequence to obtain pre-transmission data with alternating real and virtual arrays that meet the real orthogonality requirements of the FBMC-OQAM system. The pre-IFFT module 24 performs IFFT conversion on the DFT-precoded data sequence, i.e., performs subcarrier modulation for the FBMC-OQAM system. The pre-polyphase filtering module 25 performs polyphase filtering on the subcarrier-modulated data sequence signal and outputs a baseband transmit signal, which is then transmitted to channel 26 after carrier frequency modulation by the intermediate frequency and radio frequency processing units.

[0063] The FBMC-OQAM system receiver includes a receiving module and a post-processing module. The receiving module is used to perform polyphase filtering, FFT subcarrier demodulation, equalization, and OQAM demodulation operations on the received baseband signal to obtain real data. The post-processing module is used to add phase factors, IFFT transformation, demapping, and serial-to-parallel conversion operations to the real data obtained by the receiving module, and then use it as the output data of the FBMC-OQAM system.

[0064] The receiving module includes a post-polyphase filtering module 31, a post-FFT module 32, and a demodulation output module 33. The post-polyphase filtering module 31, the post-FFT module 32, and the demodulation output module 33 are respectively used to perform polyphase filtering, FFT subcarrier demodulation, equalization, and OQAM demodulation operations on the signal received from channel 26. In one embodiment, the demodulation output module 33 adopts the OQAM post-processing module of a conventional FBMC-OQAM system, such as... Figure 2 As shown, the received real part data is downsampled by a factor of 2 (decimated) and then delayed by one data point. The received imaginary part data is downsampled by a factor of 2 (decimated) and then multiplied by j. Finally, the real and imaginary parts are summed to recover the pre-transmitted complex data.

[0065] This invention provides a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication. It proposes a special training sequence design method, which constructs a synchronization sequence after or before the signal conjugate symmetric mapping, and uses the synchronization sequence to estimate and correct the carrier frequency offset.

[0066] The detailed flowchart is as follows: Figure 1 As shown, it includes the following steps:

[0067] S1: Obtain the FBMC signal to be transmitted.

[0068] In a mapped DFT precoding FBMC system, the transmitted signal is constructed by copying the result from the pre-IFFT module. This copy is then multiplied point-to-point with the prototype filter to generate an FBMC symbol. Finally, the different symbols are offset and superimposed to obtain the final transmitted signal. For example... Figure 4 and Figure 5As shown, Figure 4 Generate a schematic diagram for a single FBMC symbol data. Figure 5 The diagram illustrates the superposition of multiple signals. It can be seen that each subcarrier uses a filter, resulting in a trailing phenomenon caused by the filter, which leads to the overlap of FBMC symbols in the time domain.

[0069] In this embodiment, the training sequence contains a portion of user data, which consists of four data sequences of length M / 4, denoted as A, B, C, and D, and placed in four FBMC symbols, where M is the number of working subcarriers.

[0070] S2: Construct a special ZC sequence Z of length M / 4:

[0071]

[0072] Where z0 is a complex value, z(k) is the first M / 4-1 sequence values ​​of the ZC sequence of length M / 2-1; w(k) is the weighting coefficient sequence, i.e., the reciprocal of the superposition of the corresponding filter signal values, and its values ​​for k = 1, 2, ..., M / 4-1 are:

[0073]

[0074] The prototype filter signal is divided into 2K (K is the overlap factor) sequence blocks of length M / 2, namely p1, p2, ... p... 2K .

[0075] S3: Combine sequences A, B, C, and D with Z in a specific order to form four FBMC transmitted symbols as the data sequences in the nth to (n+3rd)th FBMC symbols: [A,Z], [Z,B], [C,Z], and [Z,D], where Z is a special ZC sequence of length M / 4. According to the OQAM modulation phase requirements, perform a specific conjugate symmetric mapping on these four sequences, and represent the mapped data as: j n [A,A*,Z,Z*]、j n+1 [Z,Z*,B,B*]、j n+2 [C,C*,Z,Z*] and j n+3 [Z,Z*,D,D*];

[0076] in:

[0077]

[0078] It should be noted that, according to the data mapping rules of the DFT precoding FBMC system, in the sequences [Z,Z*,B,B*] and [Z,Z*,D,D*], g0 takes the value of z0, and g1 takes the real part of the first sequence value of the B or D sequence; while in the sequences [A,A*,Z,Z*] and [C,C*,Z,Z*], g0 takes the conjugate of the first sequence value of the A or C sequence, and g1 takes the imaginary part of z0; although the values ​​of [Z,Z*] differ slightly for different symbols, in this invention, for the sake of simplicity, [Z,Z*] is used uniformly.

[0079] The sequences [A,A*], [B,B*], [C,C*], and [D,D*] are composed as follows:

[0080]

[0081] Where a0~a M / 4-1 For the data in sequence A, the CC* sequence has the same structure as AA*, and c2 takes the real part of z0;

[0082]

[0083] Among them, the DD* sequence structure is the same as BB*, and g2 takes the imaginary part of the first sequence value of the B or D sequence.

[0084] The corresponding time-domain transmitted signals of [A,A*,Z,Z*], [Z,Z*,B,B*], [C,C*,Z,Z*], and [Z,Z*,D,D*] are obtained by performing pre-FFT, IFFT subcarrier modulation, and PPN polyphase filtering on the data mapping DFT precoding FBMC-OQAM system.

[0085] Specifically, due to the inverse transform effect of the pre-FFT and IFFT subcarrier modulation, and the superposition effect of the preceding and following symbols under the influence of the filter, a variant sequence of [Z,Z*] will be constructed in the time domain. The mathematical analysis process is as follows:

[0086] Assume the four synchronous training FBMC symbols are numbered n, n+1, n+2, and n+3, corresponding to the part related to the first half of the Z sequence (for simplicity, assume the sequence number k = 1, 2, ..., M / 4-1 in the received signal):

[0087]

[0088] Among them, coefficient

[0089] The part related to the latter half of the Z* sequence:

[0090]

[0091] The last step in the above formula is derived from the symmetry characteristics of the filter signal.

[0092] The process of visual explanation is as follows Figure 7 As shown.

[0093] S4: At the receiver, construct a local sequence Z' = [Z, (-1j)·Z * The received signal is divided into two consecutive parts of length M / 4. The two parts are correlated with the local sequence (except for the first value) to obtain the correlation results. The correlation result of the first part is conjugate and multiplied with the correlation result of the second part to obtain the frequency offset metric function.

[0094] S5: Obtain the fractional frequency offset estimate based on the frequency offset metric function.

[0095] In this embodiment, in step S5, the carrier frequency offset is estimated using the following method for the synchronous training sequence proposed in this invention:

[0096] The received sequence is divided into two parts of length M / 4, and each part is correlated with a local sequence copy. The correlation result of the first part is then multiplied by the conjugate of the correlation result of the second part to obtain a frequency offset metric function.

[0097]

[0098] Where r(l) is the sampled value of the received signal, z'(l) is the sequence value of the local sequence Z', v is the frequency offset, Δl is the starting position of the sampling sequence number of the special time domain sequence, and F is the subcarrier spacing;

[0099] At this point, the estimated value of frequency offset v It is obtained by calculating the phase of the frequency offset metric function J(v):

[0100]

[0101] Here, angle() is the phase-taking operation.

[0102] The Matlab computer simulation implementation effect of this embodiment is as follows: Figure 8 , Figure 9 As shown. Figure 8 A variant sequence of time domain [Z,Z*] constructed according to the method of this application is shown, which has a constant amplitude. Figure 9The performance of the root mean square error (RMSE) of carrier frequency offset estimation based on the method of this application is demonstrated. The simulated frequency offset is set to 0.9 times the subcarrier spacing, and good estimation accuracy is achieved in both AWGN and Rayleigh channels.

[0103] The method proposed in this embodiment constructs a special synchronization sequence for signal synchronization transmission. It utilizes this special training sequence, after undergoing polyphase filtering and other processes in the time domain, to superimpose a special conjugate symmetric ZC sequence. The characteristics of this sequence are then used to estimate the carrier frequency offset. This method provides a carrier frequency offset estimation synchronization training sequence design approach with lower system overhead for data mapping DFT precoding systems. Furthermore, compared to the traditional FBMC-OQAM synchronization sequence design, it simplifies the computation process.

[0104] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication, characterized in that, Includes the following steps: Obtain the FBMC symbol data to be transmitted, including four data sequences A, B, C, and D of length M / 4. Construct four consecutive synchronization sequence symbols, with data sequence structures of [A,Z], [Z,B], [C,Z], and [Z,D], respectively, where Z is a special CAZAC sequence of length M / 4. Where z(k) is the first M / 4-1 sequences of a CAZAC sequence of length M / 2-1, and z0 is any complex value; w(k) is a weighted coefficient sequence, and when k = 1, 2, ..., M / 4-1, the value is the reciprocal of the weighted superposition of the corresponding filter signal values; Four consecutively placed synchronization sequence symbols, along with other data symbols, are processed through the conjugate symmetric module of the FBMC-OQAM system using data mapping DFT precoding, pre-FFT, IFFT subcarrier modulation, and PPN multiphase filtering to obtain the corresponding transmitted signal in the time domain. The synchronization sequence symbols will construct a segment of the form [Z, (-j)·Z in the time domain. * A special time-domain sequence of Z, where Z* is a specific conjugate symmetric sequence of Z; At the receiver, a local sequence Z' = [Z, (-j)·Z is constructed. * The received special time-domain sequence signal is divided into two consecutive parts of length M / 4. The two parts of the sequence, except for the first value, are correlated with the corresponding parts of the local sequence to obtain the correlation results. The correlation results of the first part of the sequence are conjugate and multiplied with the correlation results of the second part of the sequence to obtain the frequency offset metric function. The fractional frequency offset estimate is obtained based on the frequency offset metric function. The carrier frequency offset is compensated using the frequency offset estimate to complete the fractional carrier frequency synchronization.

2. The carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to claim 1, characterized in that, The weighted coefficient sequence w(k) is: Where k = 1, 2, ..., M / 4-1, p1, p2, ... p 2K The prototype filter signal is divided into 2K sequential blocks of length M / 2; K is the overlap factor.

3. The carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to claim 1, characterized in that, The construction and conjugate symmetric mapping of the four synchronization sequence symbols include the following steps: The sequences A, B, C, and D are combined with Z in a specific order to form the data sequences of four FBMC transmitted symbols, which are used as the data in the nth to (n+3rd)th FBMC symbols: [A,Z], [Z,B], [C,Z], and [Z,D], where Z is a special ZC sequence of length M / 4. According to the OQAM modulation phase requirements, these four sequences are subjected to a specific conjugate symmetric mapping, and the mapped data is represented as: j n [A,A*,Z,Z*]、j n+1 [Z,Z*,B,B*]、j n+2 [C,C*,Z,Z*] and j n+3 [Z,Z*,D,D*]; in: According to the data mapping rules, in the sequences [Z,Z*,B,B*] and [Z,Z*,D,D*], g0 takes the value of z0, and g1 takes the real part of the first sequence value of sequence B or sequence D; in the sequences [A,A*,Z,Z*] and [C,C*,Z,Z*], g0 takes the conjugate of the first sequence value of sequence A or sequence C, and g1 takes the imaginary part of z0. The [A,A*] sequence is: Where, a0~a M / 4-1 For the data in sequence A, the CC* sequence has the same structure as AA*, and c2 takes the real part of z0; The [B,B*] sequence is: Among them, the DD* sequence structure is the same as BB*, and g2 takes the imaginary part of the first sequence value of the B or D sequence.

4. The carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to claim 1, characterized in that, The signal transmission process from the transmitter to the receiver in the data mapping DFT precoding FBMC-OQAM system includes the following steps: The original signal to be transmitted is grouped, converted from serial to parallel, a special training sequence is inserted, a conjugate symmetric mapping is performed, and DFT precoding is applied to obtain a data sequence with alternating real and imaginary elements. The data sequence is subjected to IFFT transformation and polyphase filtering, and the baseband transmission signal is output. After carrier frequency modulation, it is transmitted to the channel. The signal received from the channel is subjected to polyphase filtering, FFT subcarrier demodulation, equalization, and OQAM demodulation to obtain real data. The real data is processed by adding phase factor, IFFT transformation, demapping and serial-to-parallel conversion operations, and then used as the output data.

5. The carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to claim 4, characterized in that, The conjugate symmetric mapping includes the following steps: For any nth set of original data, use vector x n This indicates that the original data x in this set... n The nth data sequence of the mapping is represented by vector b. n express; The nth set of original data x n The following formula maps the nth data sequence b to a set of data that satisfies a specific conjugate symmetry property. n : Where n takes the value of a non-negative integer; x n (k) represents the nth set of original data x. n The k-th data in; b n (k) represents the nth data sequence b n The k-th data in the array; the superscript * indicates the conjugate value; To perform the real part operation; This is an operation to extract the imaginary part.

6. The carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication according to claim 5, characterized in that, The frequency offset metric function is shown in the following formula: Where r(l) is the sampled value of the received signal, z'(l) is the sequence value of the local sequence Z', v is the frequency offset, Δl is the starting position of the sampling sequence number of the special time domain sequence, and F is the subcarrier spacing; At this point, the estimated value of frequency offset v It is obtained by calculating the phase of the frequency offset metric function J(v): Here, angle() is the phase take operation.

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