DFT precoding FBMC system carrier synchronization method for air-ground communication

Abstract

The invention provides a DFT precoding FBMC system carrier synchronization method oriented to air-ground communication. The method comprises the following steps: acquiring FBMC symbol data to be transmitted; a special CAZAC sequence with the length of M / 4 is constructed, four synchronous sequence symbols are constructed, and corresponding sending signals of a time domain are output after the four synchronous sequence symbols and other data symbols are subjected to conjugate symmetry module, pre-FFT, IFFT subcarrier modulation and PPN multi-phase filtering; the method comprises the following steps: constructing a local sequence at a receiver end, respectively taking two continuous parts with the length of M / 4 from a received special time domain sequence signal, and respectively correlating the two continuous parts with corresponding parts of the local sequence to obtain a correlation result; and conjugating the correlation result of the first part of sequence, multiplying the conjugate with the correlation result of the second part of sequence to obtain a frequency offset measurement function, obtaining decimal frequency offset estimation according to the frequency offset measurement function, and carrying out carrier frequency offset compensation by using a frequency offset estimation value. According to the method, a synchronous training sequence design method is provided, and carrier frequency offset is estimated and corrected by using time domain signal construction characteristics of a training sequence.

Description

Technical Field

[0001] The invention relates to the technical field of orthogonal frequency division multiplexing, and in particular to a carrier synchronization method of a DFT precoding FBMC system for air-to-ground communication. Background Art

[0002] In recent years, multi-carrier transmission technologies such as Orthogonal Frequency Division Multiplexing (OFDM) have been widely used due to their advantages such as high spectrum efficiency, low transceiver complexity, simple equalization and easy combination with multi-antenna technology. With the heterogeneity and asynchronous nature of future communication networks, OFDM has some disadvantages such as high out-of-band (OOB) radiation and sensitivity to synchronization errors, which have gradually attracted people's attention. To meet these challenges, filter bank multi-carrier with offset quadrature amplitude modulation (FBMC-OQAM) is considered to be a very promising waveform technology due to its advantages such as extremely low OOB radiation, and has great advantages in asynchronous communication networks. For example, in massive IoT networks, due to the large number of terminals, synchronization between devices requires a lot of signaling resources. The FBMC system can use good protection subcarriers to greatly reduce interference between asynchronous users, thereby saving a lot of signaling resources and power consumption of terminal devices.

[0003] As a multi-carrier transmission technology, FBMC also faces a serious peak-to-average power ratio (PAPR) problem. FBMC-OQAM symbols are composed of multiple independent equal-bandwidth subcarrier modulated signals. When the signal phases on the subcarriers are the same, they can generate a larger peak power when added together, which will result in a larger ratio of the signal's peak power to the average power, which is referred to as the peak-to-average power ratio. In order to avoid the influence of power amplifier nonlinearity, the amplifier circuit needs to increase the power back-off value, which will seriously increase the power consumption of the terminal equipment and reduce the battery life cycle. This has a particularly serious impact on nonlinear satellite communication systems and low-cost Internet of Things communications. In order to reduce the PAPR of the FBMC system, the most effective way to reduce the signal peak-to-average power ratio is to perform discrete Fourier transform (DFT) precoding. Therefore, a FBMC-OQAM system solution based on data mapping and DFT precoding has been proposed, which can effectively reduce the peak-to-average power ratio, making it more suitable for transmission links such as satellites and Internet of Things terminals.

[0004] With the demand for the integration of space and earth, the application of multi-carrier modulation technologies such as FBMC to satellite communication systems will greatly promote this process. At the same time, due to the growing demand for broadband transmission and the increasing congestion of working frequency bands such as L and S, the working frequency bands of wireless communications, especially broadband satellite communications, may gradually shift to high-frequency bands such as Ka. The increase in frequency bands makes it easier for communication systems to have large carrier frequency offsets CFO. In addition, low-orbit satellite communications also have high Doppler frequency shifts due to their rapid motion characteristics. Therefore, it will become very important to improve the estimation accuracy of CFO, that is, to improve the performance of the synchronization system.

[0005] Currently, in OFDM systems, the synchronization training sequence is generally selected as the Constant Amplitude Zero Auto Correlation (CAZAC) sequence. However, in FBMC systems, due to the tailing phenomenon caused by the filter of FBMC symbols, multiple symbols before and after are superimposed, and the construction of the synchronization sequence becomes complicated. Therefore, how to design efficient synchronization sequences and estimation algorithms such as carrier frequency offset has always been an important issue. Summary of the invention

[0006] To solve the above problems, the present invention provides a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication. The method proposes a training sequence design method for synchronization applied to a data mapping DFT precoding FBMC-OQAM system, and utilizes the frequency domain characteristics of the training sequence to estimate and correct the carrier frequency offset. At the same time, the synchronization sequence has a lower peak-to-average ratio, which is conducive to low-power transmission.

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

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

[0009]

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

[0011] The four consecutively placed synchronization sequence symbols together with other data symbols are subjected to the conjugate symmetric module of the data mapping DFT precoding FBMC-OQAM system, pre-FFT, IFFT subcarrier modulation, and PPN multi-phase filtering to obtain the corresponding transmission signal in the time domain. The synchronization sequence symbols will construct a segment in the time domain in the form of [Z, (-j)·Z * ], where Z* is a specific conjugate symmetric sequence of Z;

[0012] At the receiver, a local sequence Z' = [Z, (-j)·Z * ], take two continuous parts of length M / 4 for the received special time domain sequence signal, perform correlation operation on the two parts of the sequence except the first value with the corresponding parts of the local sequence to obtain correlation results; take the conjugate of the correlation result of the first part of the sequence and multiply it with the correlation result of the second part of the sequence to obtain the frequency deviation measurement function, obtain the fractional frequency deviation estimation according to the frequency deviation measurement function, use the frequency deviation estimation value to perform carrier frequency deviation compensation to complete the fractional carrier frequency synchronization.

[0013] Preferably, the weight 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 include the following steps:

[0017] The A, B, C, and D sequences are combined with the Z sequence in a specific order to form a data sequence of four FBMC transmission symbols as the data in the nth to n+3th 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, the four sequences are subjected to specific conjugate symmetric mapping, and the mapped data is expressed as: 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 rule, in the sequences [Z, Z*, B, B*] and [Z, Z*, D, D*], g0 is z0, and g1 is the real part of the first sequence value of the B sequence or the D sequence; in the sequences [A, A*, Z, Z*] and [C, C*, Z, Z*], g0 is the conjugate of the first sequence value of the A sequence or the C sequence, and g1 is the imaginary part of z0;

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

[0022]

[0023] Among them, a0~a M / 4-1 is the data in the A sequence. The CC* sequence structure is the same as AA*, and c2 is the real part of z0.

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

[0025]

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

[0027] Preferably, the signal transmission process from the data mapping DFT precoding FBMC-OQAM system transmitting end to the receiving end comprises the following steps:

[0028] The original signal to be transmitted is grouped, converted into serial and parallel signals, a special training sequence is inserted, conjugate symmetric mapping is performed, and DFT precoding is performed to obtain a data sequence in which real and virtual signals are alternately arranged;

[0029] Perform IFFT conversion and multi-phase filtering on the data sequence and output the baseband transmission signal, which is then sent to the channel after carrier frequency modulation;

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

[0031] After adding phase factors, IFFT conversion, demapping and serial-to-parallel conversion operations to the real data, the data is output.

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

[0033] For any nth set of original data, use vector x n Indicates that the original data x n The nth group of data sequences mapped is represented by vector b n express;

[0034] The nth group of original data x nThe following formula is used to map the nth group of data sequence b that satisfies the specific conjugate symmetry property. n :

[0035]

[0036] Among them, the value range of n is a non-negative integer; x n (k) represents the nth group of original data x n The kth data in b n (k) is represented by the nth group of data sequence b n The kth data in ; those marked with superscript * represent conjugate values; is the real part operation; It is the operation of taking the imaginary part.

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

[0038]

[0039] Wherein, r(l) is the sampling 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 time, the estimated value of the frequency deviation v is By obtaining the phase of the frequency deviation metric function J(v), we can obtain:

[0041]

[0042] Among them, angle() is a phase operation.

[0043] Beneficial effects of the present invention:

[0044] The present invention proposes a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication. The method constructs a conjugate symmetric special training sequence, inserts the special training sequence into the signal conjugate symmetric mapping of the transmitting end of the FBMC-OQAM system or before the signal conjugate symmetric mapping, and forms a synchronization sequence with four conjugate symmetric transmission sequences for signal synchronous transmission, and uses the frequency domain characteristics of the special training sequence to estimate and correct the carrier frequency offset. The method provides a carrier frequency offset estimation synchronization training sequence design method with low system overhead for the data mapping DFT precoding system. At the same time, compared with the synchronization sequence design of the traditional FBMC-OQAM, the calculation process is simplified. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0046] Figure 2 This is the data mapping DFT precoding FBMC-OQAM system architecture diagram;

[0047] Figure 3 is an example of a mapping with signal conjugate symmetry;

[0048] Figure 4 It is a schematic diagram for generating single FBMC symbol data;

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

[0050] Figure 6 Schematic diagram of the prototype filter sequence block (overlap factor K = 4);

[0051] Figure 7 It is a graphical analysis diagram of the corresponding transmitted signal in the time domain to construct a complete ZZ* sequence;

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

[0053] Fig. 9 This is the performance simulation result of the carrier estimation using the training sequence of the present invention. DETAILED DESCRIPTION

[0054] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0055] Example 1

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

[0057] The transmitting end of the FBMC-OQAM system includes a preprocessing module and a transmitting module. The preprocessing module is used to preprocess the original data to be transmitted to obtain real and virtual alternating pretransmission data that meets the real orthogonality requirements of the FBMC-OQAM system. The transmitting module is used to send the preprocessed pretransmission data from the transmitting end. The present invention only describes the processing of the baseband system, and omits the intermediate frequency and radio frequency processing units such as analog-to-digital / digital-to-analog conversion, amplifier, mixer, radio frequency filter, antenna, etc. Among them, the preprocessing module includes a front serial-to-parallel conversion module 21, a conjugate symmetry module 22, a front FFT module 23, a front IFFT module 24 and a front multi-phase filtering module 25. The front serial-to-parallel conversion module 21 is used to group the original data to be transmitted and perform serial-to-parallel conversion on each group of original data. In one embodiment, the length of each group of original data is set to M / 2, where M is the number of subcarriers of the FBMC-OQAM system, and M=2 L , the value range of L is a natural number greater than 2.

[0058] The conjugate symmetry 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. n Indicates that the original data x n The nth group of data sequences mapped is represented by vector b n express;

[0059] The nth group of original data x n The following formula is used to map the nth group of data sequence b that satisfies the specific conjugate symmetry property. n :

[0060]

[0061] Among them, the value range of n is a non-negative integer; x n (k) represents the nth group of original data x n The kth data in b n (k) is represented by the nth group of data sequence b n The kth data in ; those marked with superscript * represent conjugate values; is the real part operation; To take the imaginary part operation, for example Figure 3 shown.

[0062] The pre-FFT module 23 is used to perform DFT precoding on each group of mapped data sequences to obtain real and imaginary alternating pre-transmission data that meets the real orthogonal requirements of the FBMC-OQAM system. The pre-IFFT module 24 is used to perform IFFT conversion on the data sequence after DFT precoding, that is, to perform subcarrier modulation of the FBMC-OQAM system. The pre-polyphase filter module 25 is used to perform polyphase filtering on the data sequence signal after subcarrier modulation and output the baseband transmission signal, which is sent to the channel 26 after carrier frequency modulation by the intermediate frequency and radio frequency processing unit.

[0063] The receiving end of the FBMC-OQAM system includes a receiving module and a post-processing module. The receiving module is used to obtain real data by subjecting the received baseband signal to multi-phase filtering, FFT subcarrier demodulation, equalization and OQAM demodulation operations. The post-processing module is used to add a phase factor, IFFT conversion, demapping and serial-to-parallel conversion operations to the real data obtained by the receiving module 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, which are respectively used to perform polyphase filtering, FFT subcarrier demodulation, equalization and OQAM demodulation operations on the signal received from the channel 26. In one embodiment, the demodulation output module 33 adopts the OQAM post-processing module of the traditional FBMC-OQAM system, such as Figure 2 As shown, the received real data is downsampled (decimated) by 2 and then delayed by one data point, the received imaginary data is downsampled (decimated) by 2 and then multiplied by j, and finally the real and imaginary parts are summed to restore the pre-transmitted complex data.

[0065] The invention discloses a carrier synchronization method for a DFT precoding FBMC system for air-to-ground communication, and proposes a special training sequence design method. After or before signal conjugate symmetric mapping, a synchronization sequence is formed, and the synchronization sequence is used to estimate and correct the carrier frequency offset.

[0066] The specific flow chart is as follows Figure 1 As shown, the following steps are included:

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

[0068] In the mapped DFT precoding FBMC system, the transmission signal is composed of the result after the previous IFFT module, which is copied to the same length as the prototype filter and then multiplied point-to-point with the prototype filter to generate an FBMC symbol data. Then, the different symbols before and after are offset and superimposed to obtain the final transmission signal. Figure 4 and Figure 5As shown, Figure 4 Generate schematics for single FBMC symbol data, Figure 5 This is a schematic diagram of the superposition of multiple signals. It can be seen that a filter is used for each subcarrier, and there is a tailing phenomenon caused by the filter, which causes the FBMC symbols to overlap in the time domain.

[0069] In this embodiment, the training sequence includes some user data, which is composed of four data sequences with a length of M / 4, respectively represented 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] Wherein, 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, that is, the inverse value of the superposition of the corresponding filter signal value, and its value for k=1, 2, ..., M / 4-1 is:

[0073]

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

[0075] S3: Combine A, B, C, and D with Z sequences in a specific order to form four FBMC transmission symbol data sequences as the data in the nth to n+3th 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, the four sequences are subjected to specific conjugate symmetric mapping, and the mapped data is expressed as: 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 rule of the data mapping DFT precoding FBMC system, in the [Z, Z*, B, B*] and [Z, Z*, D, D*] sequences, g0 is z0, and g1 is the real part of the first sequence value of the B sequence or the D sequence; and in the [A, A*, Z, Z*] and [C, C*, Z, Z*] sequences, g0 is the conjugate of the first sequence value of the A sequence or the C sequence, and g1 is the imaginary part of z0; although the values ​​of [Z, Z*] of different symbols are slightly different, in the present invention, for the sake of simplicity, they are uniformly represented by [Z, Z*];

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

[0080]

[0081] Among them, a0~a M / 4-1 is the data in the A sequence. The CC* sequence structure is the same as AA*, and c2 is the real part of z0.

[0082]

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

[0084] [A,A*,Z,Z*], [Z,Z*,B,B*], [C,C*,Z,Z*] and [Z,Z*,D,D*] are subjected to the pre-FFT, IFFT subcarrier modulation and PPN multi-phase filtering of the data mapping DFT precoding FBMC-OQAM system to obtain the corresponding transmission signals in the time domain.

[0085] In particular, due to the inverse transformation effect of the previous FFT and IFFT subcarrier modulation, and the superposition of the previous and next 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 that the sequence numbers of the four synchronous training FBMC symbols are n, n+1, n+2, and n+3, corresponding to the part related to the first half of the Z sequence (for simplicity of representation, assume that the sequence number in this part of the sequence in the received signal is k=1, 2, ..., M / 4-1):

[0087]

[0088] Among them, the coefficient

[0089] Corresponding to the part related to the second half of the Z* sequence:

[0090]

[0091] Among them, the last step in the above formula is derived due to the symmetry characteristics of the filter signal.

[0092] Graphical explanation process Figure 7 shown.

[0093] S4: At the receiver, construct a local sequence Z' = [Z, (-1j)·Z * ], take two continuous parts of the received signal with a length of M / 4 respectively, correlate the two parts of the sequence with the local sequence respectively (except the first value) to obtain the correlation results; take the conjugate of the correlation result of the first sequence and multiply it with the correlation result of the second sequence to obtain the frequency deviation measurement function.

[0094] S5: Obtain a fractional frequency offset estimate according to the frequency offset metric function.

[0095] In this embodiment, in S5, for the synchronization training sequence proposed in the present invention, the following method is used to estimate the carrier frequency offset:

[0096] The received sequence is divided into two parts of length M / 4, and correlated with the local sequence copy respectively. Then the correlation result of the first sequence segment is conjugated and multiplied with the correlation result of the second sequence segment to obtain a frequency deviation metric function:

[0097]

[0098] Wherein, r(l) is the sampling 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 time, the estimated value of the frequency deviation v is By obtaining the phase of the frequency deviation metric function J(v), we can obtain:

[0100]

[0101] Among them, angle() is the operation of obtaining the phase.

[0102] The Matlab computer simulation implementation effect of this embodiment is as follows Figure 8 , Fig. 9 shown. Figure 8 The variant sequence of the time domain [Z, Z*] constructed according to the method of the present application is shown, which has a constant amplitude. Fig. 9The root mean square error (RMSE) performance of the carrier frequency offset estimation completed according to the method of the present 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 channel and Rayleigh flat channel.

[0103] The method proposed in this embodiment constructs a special synchronization sequence for signal synchronization transmission, and uses the special training sequence to superimpose into a special conjugate symmetric ZC sequence in the time domain through multi-phase filtering and other effects, and uses the characteristics of the sequence to estimate the carrier frequency offset. This method provides a carrier frequency offset estimation synchronization training sequence design method with low system overhead for the data mapping DFT precoding system. At the same time, compared with the synchronization sequence design of the traditional FBMC-OQAM, the calculation process is simplified.

[0104] The above are only 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 in 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: The following steps are involved: Obtain FBMC symbol data to be transmitted, including four blocks of data sequences A, B, C and D to be transmitted with a length of M / 4, and construct four consecutive synchronization sequence symbols, wherein the data sequence structures of the four synchronization sequence symbols are [A, Z], [Z, B], [C, Z] and [Z, D], respectively, where Z is a special CAZAC sequence of length M / 4: Wherein, z(k) is the first M / 4-1 sequence of the CAZAC sequence of length M / 2-1, z0 is an arbitrary complex value; w(k) is a weight coefficient sequence, and when k=1, 2, ..., M / 4-1, the value is the reciprocal value of the weighted superposition of the corresponding filter signal value; The four consecutively placed synchronization sequence symbols together with other data symbols are subjected to the conjugate symmetric module of the data mapping DFT precoding FBMC-OQAM system, pre-FFT, IFFT subcarrier modulation, and PPN multi-phase filtering to obtain the corresponding transmission signal in the time domain. The synchronization sequence symbols will construct a segment in the time domain in the form of [Z, (-j)·Z * ], where Z* is a specific conjugate symmetric sequence of Z; At the receiver, a local sequence Z' = [Z, (-j)·Z * ], take two continuous parts of length M / 4 for the received special time domain sequence signal, perform correlation operation on the two parts of the sequence except the first value with the corresponding parts of the local sequence to obtain correlation results; take the conjugate of the correlation result of the first part of the sequence and multiply it with the correlation result of the second part of the sequence to obtain the frequency deviation measurement function, obtain the fractional frequency deviation estimation according to the frequency deviation measurement function, use the frequency deviation estimation value to perform carrier frequency deviation compensation to complete the fractional carrier frequency synchronization.

2. The carrier synchronization method of 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 of 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 A, B, C, and D sequences are combined with the Z sequence in a specific order to form a data sequence of four FBMC transmission symbols as the data in the nth to n+3th 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, the four sequences are subjected to specific conjugate symmetric mapping, and the mapped data is expressed as: 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 rule, in the sequences [Z, Z*, B, B*] and [Z, Z*, D, D*], g0 is z0, and g1 is the real part of the first sequence value of the B sequence or the D sequence; in the sequences [A, A*, Z, Z*] and [C, C*, Z, Z*], g0 is the conjugate of the first sequence value of the A sequence or the C sequence, and g1 is the imaginary part of z0; The sequence [A,A*] is: Among them, a0~a M / 4-1 is the data in the A sequence. The CC* sequence structure is the same as AA*, and c2 is the real part of z0. The [B,B*] sequence is: The DD* sequence structure is the same as BB*, and the value of g2 is the imaginary part of the first sequence value of the B sequence or the D sequence.

4. The carrier synchronization method of DFT precoding FBMC system for air-to-ground communication according to claim 1, characterized in that: The signal transmission process from the data mapping DFT precoding FBMC-OQAM system transmitting end to the receiving end includes the following steps: The original signal to be transmitted is grouped, converted into serial and parallel signals, a special training sequence is inserted, conjugate symmetric mapping is performed, and DFT precoding is performed to obtain a data sequence in which real and virtual signals are alternately arranged; Perform IFFT conversion and multi-phase filtering on the data sequence and output the baseband transmission signal, which is then sent to the channel after carrier frequency modulation; The signal received from the channel is subjected to polyphase filtering, FFT subcarrier demodulation, equalization and OQAM demodulation to obtain real data; After adding phase factors, IFFT conversion, demapping and serial-to-parallel conversion operations to the real data, the data is output.

5. The carrier synchronization method of DFT precoding FBMC system for air-to-ground communication according to claim 4, characterized in that: The conjugate symmetric mapping comprises the following steps: For any nth set of original data, use vector x n Indicates that the original data x n The nth group of data sequences mapped is represented by vector b n express; The nth group of original data x n The following formula is used to map the nth group of data sequence b that satisfies the specific conjugate symmetry property. n : Among them, the value range of n is a non-negative integer; x n (k) represents the nth group of original data x n The kth data in b n (k) is represented by the nth group of data sequence b n The kth data in ; those marked with superscript * represent conjugate values; is the real part operation; It is the operation of taking the imaginary part.

6. The carrier synchronization method of DFT precoding FBMC system for air-to-ground communication according to claim 5, characterized in that: The frequency deviation metric function is shown in the following formula: Wherein, r(l) is the sampling 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 time, the estimated value of the frequency deviation v is By obtaining the phase of the frequency deviation metric function J(v), we can obtain: Among them, angle() is a phase operation.

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