OFDM synchronization method in high-speed environment

Abstract

The application discloses an OFDM synchronization method in high-speed environment. In the transmitting end, the OFDM parameters are determined according to the spectrum estimation range and the multipath time delay, the small m sequence is spread, the pilot sequence and two different pilot structures are designed, and the 8-phase quantization is carried out. In the receiving end, the timing synchronization and the spectrum rough estimation are carried out according to the pilot sequence, then the frequency offset fine estimation is carried out according to the two pilot structures, so that the total spectrum estimation result is obtained, and the frequency offset compensation is carried out. The application realizes the fast synchronization in high-speed environment, and greatly saves the resource consumption.

Description

Technical Field

[0001] This invention belongs to the field of communication signal processing technology, and specifically relates to an OFDM synchronization method in a high-speed environment. Background Technology

[0002] The main idea of ​​OFDM (Orthogonal Frequency Division Multiplexing) technology is to divide the channel into several orthogonal sub-channels in the frequency domain, and use subcarriers for modulation on each orthogonal sub-channel, with each subcarrier transmitted in parallel. Because narrowband transmission occurs on a single orthogonal sub-channel, and the carriers on each orthogonal sub-channel are mutually orthogonal, interference between signal waveforms can be greatly eliminated, and spectral efficiency is improved. OFDM technology is now widely used in mobile communications.

[0003] Synchronization is a crucial aspect of OFDM technology, directly impacting the overall performance of the communication system. It primarily includes timing synchronization and carrier synchronization. Currently used synchronization methods mainly fall into the following categories:

[0004] I. Synchronization Method Based on CP. Since CP and some data are the same, synchronization parameters can be obtained by calculating the correlation between the two. This method does not require additional framing overhead, but it is greatly affected by multipath propagation and has a small spectrum estimation range.

[0005] II. Pilot-based synchronization method. This method requires inserting pilot sequences into the frame structure for synchronization. It has low computational complexity and good synchronization performance, but the estimation range is small.

[0006] III. Synchronization Method Based on Training Sequence. This method can flexibly select training symbols according to system needs. Although there is a load problem caused by transmitting training symbols, it is not affected by multipath channels and has a large estimation range.

[0007] In high-speed environments, the Doppler frequency caused by rapid platform movement leads to rapid channel changes, resulting in symbol timing and carrier frequency deviations. This disrupts the orthogonality between subcarriers in the OFDM system, and system performance degrades due to inter-symbol interference (ISI) and inter-symbol interference (ICI). Existing OFDM synchronization methods have limited spectral estimation ranges and high computational costs. In high-speed environments, they are further affected by Doppler frequency offsets and multipath channels, leading to decreased synchronization accuracy and consequently impacting system performance. Summary of the Invention

[0008] The purpose of this invention is to provide an OFDM synchronization method in a high-speed environment. At the transmitting end, OFDM parameters are first determined based on the spectrum estimation range and multipath delay. After spreading the small m-sequence, a preamble sequence and two different pilot structures are designed, and 8-phase quantization is performed. At the receiving end, timing synchronization and coarse spectrum estimation are performed based on the preamble sequence. Then, fine frequency offset estimation is performed based on the two pilot structures to obtain the overall spectrum estimation result. Frequency offset compensation is then performed, achieving fast synchronization in a high-speed environment and greatly saving resource consumption.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] An OFDM synchronization method for high-speed environments includes the following steps at the transmitting end:

[0011] The subcarrier spacing is determined based on the frequency offset estimation range and multipath delay, and the OFDM symbol length and effective subcarrier number are calculated. The length of the ZC sequence is determined based on the OFDM symbol length and effective subcarrier number, and the ZC sequence is subjected to inverse fast Fourier transform (IFFT) to generate a time-domain spreading code. The generated spreading code is used to spread the small m-sequence, generate a preamble sequence, and perform 8-phase quantization.

[0012] The length of the ZC sequence and the length of the time-domain pilot symbol are determined based on the OFDM symbol length and the number of effective subcarriers. An IFFT transform is performed on the ZC sequence to obtain pilot symbols. Two cyclic prefixes of different lengths are added before the pilot symbols to generate the first and second pilots. The first and second pilots are then subjected to 8-phase quantization. Specifically, the cyclic prefix length of the first pilot is the same as that of the pilot symbol, while the cyclic prefix length of the second pilot is half that of the pilot symbol.

[0013] After scrambling, encoding, interleaving, modulation and OFDM modulation of the original transmitted data, data symbols are generated. A first pilot is inserted at the beginning of the data block, and a second pilot is inserted after every two data symbols. After framing, the data is transmitted.

[0014] The receiving end includes the following steps:

[0015] After receiving the transmitted signal, the received signal is quantized into 8 phases, and the signals in different angle ranges are mapped to 8 phases respectively. Then, the quantized received signal is correlated with the local ZC sequence to complete the timing synchronization.

[0016] After the timing is completed, a rough spectral estimate is performed using the preamble sequence;

[0017] The pilot symbols before and after the two data symbols are extracted and autocorrelation is calculated to perform fine frequency offset estimation. The coarse frequency offset estimation results are then used to correct the correlation results to obtain the total frequency offset.

[0018] Preferably, the method for correlating the quantized received signal with the local ZC sequence is to first perform a first correlation, that is, multiply the quantized received signal and the local ZC sequence by their conjugates and then sum them. The result of the first correlation is N peaks, where N is the length of the small m-sequence.

[0019] After dedifferentiating the first correlation result, i.e., multiplying by the conjugate of the length L of the spread code, and then performing a second correlation with the small m-sequence before local differential, i.e., taking N numbers at each interval L and multiplying them by the conjugate of the local small m-sequence and then adding them; the second correlation result will have a large peak value, which is used to set a threshold for capture, and then searching for the maximum value for timing.

[0020] Preferably, the coarse frequency estimation involves multiplying the N peaks from the first correlation result taken at the timing position with the locally differentiated small m-sequence, performing an M-point FFT on the result, taking the modulus to find the maximum value, and then calculating the frequency offset using the maximum value and its corresponding index to estimate the first-level frequency offset. .

[0021]

[0022] Where b is the position with the largest modulus of the FFT output sequence, Fsymb is the symbol rate, L is the spreading code length, and M is the number of FFT points.

[0023] Preferably, the spectrum estimation involves extracting the pilot symbols before and after the OFDM data symbol, performing correlation, and then using the first-level frequency offset. Correct the relevant results;

[0024]

[0025] Where LL is the pilot length, Fd is the data symbol rate, and D is the interval between the two pilot segments;

[0026] The angles of the corrected correlation results are calculated to obtain the second-level fine-esti angle (exact_esti_angle) and the second-level frequency offset (exact_offset_esti).

[0027]

[0028]

[0029] Adding the first-level frequency offset to the second-level frequency offset yields the final total frequency offset:

[0030]

[0031] Convert the calculated final frequency offset total_offset_esti into Then multiply it by the data after timing is complete to obtain the data after frequency offset correction:

[0032]

[0033] Where x represents the data sampling point.

[0034] The beneficial effects of this invention are as follows:

[0035] 1. At the transmitting end, the generation of the preamble sequence, pilot structure design and data block generation based on the ZC sequence can effectively reduce the influence of symbol timing deviation and carrier frequency deviation caused by Doppler frequency offset, so that the system has a certain anti-Doppler and anti-multipath capability.

[0036] 2. At the receiving end, autocorrelation-based timing synchronization and fast carrier synchronization can be used in a Doppler environment by combining the preamble sequence and two pilot structures at the transmitting end to perform timing synchronization using a quadratic correlation method, which improves the system performance in high-speed environments. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the leader sequence structure.

[0038] Figure 2 This is a schematic diagram of how the pilot structure is generated.

[0039] Figure 3 This is a schematic diagram of the data block structure. Detailed Implementation

[0040] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0041] This embodiment illustrates an OFDM synchronization method for high-speed environments, which includes a preamble sequence generation method, a pilot signal generation method, and a data block generation method at the transmitter.

[0042] Methods for generating leader sequences: such as Figure 1 and Figure 2 As shown, the subcarrier spacing is determined based on the frequency offset estimation range and multipath delay, and the OFDM symbol length and effective subcarrier number are calculated. Based on the OFDM symbol length and effective subcarrier number, a ZC sequence of appropriate length is selected, and the ZC sequence is subjected to inverse fast Fourier transform (IFFT) to generate a time-domain spreading code. The generated spreading code is used to spread the small m-sequence, generate a preamble sequence, and perform 8-phase quantization.

[0043] Pilot generation method: The length of the ZC sequence and the length of the time-domain pilot symbol are determined based on the OFDM symbol length and the number of effective subcarriers. An IFFT transform is performed on the ZC sequence to obtain pilot symbols. Two cyclic prefixes (CPs) of different lengths are added before the pilot symbols to generate the first and second pilots. The first and second pilots are then subjected to 8-phase quantization. Specifically, the CP length of the first pilot is the same as that of the pilot symbol, while the CP length of the second pilot is half that of the pilot symbol.

[0044] Data block generation method: The original transmitted data is scrambled, encoded, interleaved, modulated, and OFDM modulated to generate data symbols. A first pilot is inserted at the beginning of a data block, followed by a second pilot after every two data symbols. After framing, the data is transmitted.

[0045] At the receiving end, autocorrelation-based timing synchronization and fast carrier synchronization are achieved through the following steps:

[0046] Timing Synchronization: After receiving the transmitted signal, the receiver performs 8-phase quantization on the received signal, mapping the signal within different angle ranges to 8 phases respectively. Then, the quantized received signal is correlated with the local ZC sequence. In this embodiment, the correlation algorithm is as follows: the quantized received signal and the local ZC sequence are multiplied by their conjugates and then accumulated. The first correlation result yields N peaks, where N is the length of the small m-sequence.

[0047]

[0048] in This represents the i-th received signal, j is the sampling point, and L is the length of the spreading code.

[0049] After dedifferentiating the first correlation result (i.e., multiplying by the conjugate of a spreading code length L), it is then correlated again with the local m-sequence before the local difference. This involves taking N numbers at intervals L and multiplying them by the conjugate of the local m-sequence, then adding them together. The second correlation result will show a large peak. This peak is used as a threshold for capture, and the maximum value is searched for for timing. The dedifferentiating algorithm is as follows:

[0050]

[0051] Where L is the spreading code length and P is the upsampling factor.

[0052] The second relevant calculation method is as follows:

[0053]

[0054] Coarse Spectrum Estimation: A coarse spectrum estimation is performed using the preamble sequence. After timing is complete, N peaks from the first correlation result are extracted from the timing position and multiplied by the locally differentiated small m-sequence. An M-point FFT is performed on the result, and the modulus is taken to find the maximum value. The frequency offset is calculated using the maximum value and its corresponding index to estimate the first-level frequency offset. .

[0055]

[0056] Where b is the position with the largest modulus of the FFT output sequence, Fsymb is the symbol rate, L is the spreading code length, and M is the number of FFT points.

[0057] Spectrum estimation: Then, the pilot symbols before and after the two data symbols are extracted and autocorrelation is calculated to perform frequency offset estimation. The estimation results of the first-level frequency offset are used to correct the correlation results to obtain the total frequency offset.

[0058] Extract the preceding and following pilot symbols from the two OFDM data symbols, perform correlation, and then use the first-level frequency offset. The relevant results should be corrected.

[0059]

[0060] Where LL is the pilot length, Fd is the data symbol rate, and D is the interval between the two pilot segments.

[0061] The angles of the corrected correlation results are calculated to obtain the second-level fine-esti angle and the second-level frequency offset exact_esti.

[0062]

[0063]

[0064] Adding the first-level frequency offset to the second-level frequency offset yields the final total frequency offset:

[0065]

[0066] Convert the calculated final frequency offset total_offset_esti into Then multiply it by the data after timing is complete to obtain the data after frequency offset correction:

[0067]

[0068] Where x represents the data sampling point.

[0069] It is understood that those skilled in the art can make equivalent substitutions or modifications to the technical solution and inventive concept of the present invention, and all such substitutions or modifications should fall within the protection scope of the appended claims.

Claims

1. An OFDM synchronization method for high-speed environments, characterized in that: The transmitting end includes the following steps: The subcarrier spacing is determined based on the frequency offset estimation range and multipath delay, and the OFDM symbol length and effective subcarrier number are calculated. The length of the ZC sequence is determined based on the OFDM symbol length and effective subcarrier number, and the ZC sequence is subjected to inverse fast Fourier transform (IFFT) to generate a time-domain spreading code. The generated spreading code is used to spread the small m-sequence, generate a preamble sequence, and perform 8-phase quantization. The length of the ZC sequence and the length of the time-domain pilot symbol are determined based on the OFDM symbol length and the number of effective subcarriers. The ZC sequence is then subjected to an IFFT transform to obtain pilot symbols. Two cyclic prefixes of different lengths are added before the pilot symbols to generate the first pilot and the second pilot. The first and second pilots are then subjected to 8-phase quantization. The cyclic prefix length of the first pilot is the same as that of the pilot symbol, and the cyclic prefix length of the second pilot is half that of the pilot symbol. After scrambling, encoding, interleaving, modulation and OFDM modulation of the original transmitted data, data symbols are generated. A first pilot is inserted at the beginning of a data block, and a second pilot is inserted after every two data symbols. After framing, the data is transmitted.

2. The OFDM synchronization method under high-speed environment according to claim 1, characterized in that: The receiving end includes the following steps: After receiving the transmitted signal, the received signal is quantized into 8 phases, and the signals in different angle ranges are mapped to 8 phases respectively. Then, the quantized received signal is correlated with the local ZC sequence to complete the timing synchronization. After the timing is completed, a rough spectral estimate is performed using the preamble sequence; The pilot symbols before and after the two data symbols are extracted and autocorrelation is calculated to perform fine frequency offset estimation. The coarse frequency offset estimation results are then used to correct the correlation results to obtain the total frequency offset.

3. The OFDM synchronization method under high-speed environment according to claim 2, characterized in that: The method for correlating the quantized received signal with the local ZC sequence is to first perform a first correlation, that is, multiply the quantized received signal and the local ZC sequence by their conjugates and then sum them. The result of the first correlation is N peaks, where N is the length of the small m-sequence. After dedifferentiating the first correlation result, i.e., multiplying by the conjugate of the length L of the spread code, and then performing a second correlation with the small m-sequence before local differential, i.e., taking N numbers at each interval L and multiplying them by the conjugate of the local small m-sequence and then adding them; the second correlation result will have a large peak value, which is used to set a threshold for capture, and then searching for the maximum value for timing.

4. The OFDM synchronization method under high-speed environment according to claim 3, characterized in that: The coarse spectral estimation involves multiplying the N peaks from the first correlation result taken at the timing position with the locally differentiated small m-sequence, performing an M-point FFT on the result, taking the modulus to find the maximum value, and then calculating the frequency offset using the maximum value and its corresponding index to estimate the first-level frequency offset. : Where b is the position with the largest modulus of the FFT output sequence, Fsymb is the symbol rate, L is the spreading code length, and M is the number of FFT points.

5. The OFDM synchronization method under high-speed environment according to claim 4, characterized in that: Spectrum estimation involves correlating the pilot symbols before and after the data symbol, and then using the first-level frequency offset. Correct the relevant results; Where LL is the pilot length, Fd is the data symbol rate, and D is the interval between the two pilot segments; The angles of the corrected correlation results are calculated to obtain the second-level fine-esti angle and the second-level frequency offset exact_esti. Adding the first-level frequency offset to the second-level frequency offset yields the final total frequency offset: Convert the calculated final frequency offset total_offset_esti into Then multiply it by the data after timing is complete to obtain the data after frequency offset correction: Where x represents the data sampling point.

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