CN113132287A - Synchronous detection method and device in OFDM system - Google Patents
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Abstract
The invention relates to a synchronous detection method and a synchronous detection device in an OFDM system, and belongs to the technical field of OFDM communication. The invention divides the synchronous detection into two processes of coarse synchronization and fine synchronization, firstly, the received baseband data signal and the segment correlation value of the local pilot frequency sequence and the power value of the received signal are utilized to carry out coarse synchronization so as to obtain a coarse synchronization position; and then, starting from the coarse synchronization position, carrying out frequency domain transformation and conjugate correlation processing on the received signal, calculating a fine synchronization judgment value according to a frequency domain conjugate correlation result, and carrying out fine synchronization judgment based on the fine synchronization judgment value, wherein the position meeting the fine synchronization judgment is the synchronization bit to be detected. Through the two processes, the invention can realize accurate frame synchronization under the conditions of bad channel conditions, large frequency offset, strong noise interference and deep fading, has the characteristics of high synchronization accuracy, small synchronization time delay, low realization complexity and low hardware resource overhead, and has high application value in an OFDM system.
Description
Technical Field
The invention relates to a synchronous detection method and a synchronous detection device in an OFDM system, and belongs to the technical field of OFDM communication.
Background
Synchronization is one of the key technologies of an Orthogonal Frequency Division Multiplexing (OFDM) system receiver, and the synchronization accuracy directly affects the performance of the receiver. Without precise synchronization there is no accurate reception of the signal. In a power line carrier communication system, data information is transmitted in burst frames, and a communication node is mainly in a receiving state. Synchronization needs to be able to detect the arrival of a data frame in time and accurately indicate the frame boundary position, so that subsequent data signals can be correctly received and decoded to obtain final received data.
In the prior art, there are two main methods for synchronizing specific sequences: the other is that the receiver performs time domain delay autocorrelation processing according to a time domain sequence of a synchronization signal in a received signal, determines the initial position of a data frame according to the time when a correlation result exceeds a preset threshold, and determines that no data frame arrives if no signal exceeding the preset threshold is detected. The method has stronger resistance to frequency offset interference, but the autocorrelation curve of the received signal obtained by the traditional time domain autocorrelation method often changes smoothly near the peak value, and the frame starting position is difficult to be accurately detected under the condition of larger Gaussian noise. To overcome this drawback, a complex synchronization sequence needs to be designed or the autocorrelation result needs to be subjected to a second correlation process. And the other method is to perform cross-correlation calculation on the received signal and the local sequence of the receiving end and perform synchronization judgment according to the cross-correlation peak value. The method has the advantage of prominent peak value of the correlation curve, and can better resist the interference of Gaussian noise. But the correlation peak obtained by the cross-correlation method is very susceptible to the influence of frequency offset. In addition, when the received signal power is large in floating and the AGC is not adjusted to the optimal state, the amplitude of the correlation peak is large in variation, a suitable universal threshold cannot be found, and missing detection and false detection are prone to occur, so that the synchronization performance of the system is reduced.
Disclosure of Invention
The invention aims to provide a synchronization detection method and a synchronization detection device in an OFDM system, which are used for solving the problems of high calculation complexity and low synchronization precision in the current synchronization process.
The present invention provides a method for detecting synchronization in an OFDM system to solve the above technical problem, the method comprising the steps of:
1) receiving a baseband sampling time domain signal to obtain a receiving sequence, and calculating a segment correlation value of the receiving sequence and a local preamble OFDM symbol sequence and a power value of the receiving sequence;
2) calculating a coarse synchronization judgment value according to the obtained correlation value and the power value, comparing the coarse synchronization judgment value with a preset peak threshold T, and performing peak side lobe search in a preset search window;
3) performing coarse synchronization judgment according to the peak side lobe search result to determine a coarse synchronization position;
4) from the coarse synchronization position, performing time-frequency transformation on the received two adjacent preamble OFDM symbol time domain data to obtain corresponding frequency domain data;
5) calculating the conjugate correlation accumulated value and the phase of the frequency domain data of two adjacent pilot symbols according to the frequency domain transformation result;
6) and calculating a fine synchronization judgment value according to the obtained conjugate correlation accumulated value and the phase, comparing the fine synchronization judgment value with a preset fine synchronization judgment condition, and performing fine synchronization judgment according to a comparison result.
The invention also provides a synchronization detection device in the OFDM system, which comprises a processor and a memory, wherein the processor executes a computer program stored by the memory to realize the synchronization detection method in the OFDM system.
The invention divides the synchronous detection into two processes of coarse synchronization and fine synchronization, firstly, the received baseband data signal and the segment correlation value of the local pilot frequency sequence and the power value of the received signal are utilized to carry out coarse synchronization so as to obtain a coarse synchronization position; and then, starting from the coarse synchronization position, carrying out frequency domain transformation and conjugate correlation processing on the received signal, calculating a fine synchronization judgment value according to a frequency domain conjugate correlation result, and carrying out fine synchronization judgment based on the fine synchronization judgment value, wherein the position meeting the fine synchronization judgment is the synchronization position to be detected. Through the two processes, the invention can realize accurate frame synchronization under the conditions of bad channel conditions, large frequency offset, strong noise interference and deep fading, has the characteristics of high synchronization accuracy, small synchronization time delay, low realization complexity and low hardware resource overhead, and has high application value in an OFDM system.
Further, the calculation formula adopted by the segment correlation value in step 1) is as follows:
l is the number of sampling points of a preamble symbol; n is the serial number of the receiving sampling point, and n is 0,1, 2.; p is the number of segments of the segment in the sliding window; n is a radical ofpFor the length of each of the segments it is,
sync (i) is a local preamble sequence, i is a data index in the p-th segment, i is 0,1,2, …, Np-1，sync*(i) The conjugate of sync (i), and r (i) the received baseband sampled signal.
Further, the calculation formula adopted by the coarse synchronization judgment value in the step 2) is as follows:
q (n) is a coarse synchronization judgment value, C1(n) is a piecewise cross-correlation value, E1And (n) is the power value of the received sequence r (n).
Further, in order to accurately perform coarse synchronization judgment, the search process of the peak sidelobe in step 2) is as follows:
A. comparing the coarse synchronization judgment value with a preset peak value threshold, and recording the tempPos of the position of the current point and the corresponding detection value tempPeak when the coarse synchronization judgment value is larger than the preset peak value threshold;
B. if the current point detects the first peak value, recording the peak position peakPos [0] ═ tempPos, the peak value peakValue [0] ═ tempPeak, and the peak count peakNum ═ 1; if the effective peak value exists, comparing tempPos of the current point with the stored position peakPos [ peakNum-1] of the previous peak value point;
C. when tempPos-peakPos [ peakNum-1]]＜LsIf tempPeak > peakValue [ peakNum-1]]Then tempPos and tempPeak are substituted for peakPos [ peakNum-1], respectively]And peakValue [ peakNum-1]]Otherwise, when tempPos-peakPos [ peakNum-1]]When L, record peak position peakPos [ peakNum]tempPos, peak value peakvale [ peakNum ]]The peak count peakNum value is accumulated by 1; wherein L issThe size of a peak side lobe search window is LsL/2+ 1; l is the number of sample points of one preamble OFDM symbol.
Further, the coarse synchronization decision process in step 3) is: and if the current peak count peakNum reaches the expected value and the position interval of two adjacent stored peak values is L, the coarse synchronization is successful.
Further, the calculation formula of the conjugate correlation accumulated value of the frequency domain data of two adjacent preamble symbols in the step 5) is as follows:
wherein N issetFor the number of sets of active subcarriers on each preamble OFDM symbol,
for the number of sub-carriers contained in each set, R (l-1, k) and R (l, k) are frequency domain data of two adjacent leading OFDM symbols, l is a leading OFDM symbol index, k is a sub-carrier index on each leading OFDM symbol, (. DEG)*The conjugate of the calculated value in the parenthesis is shown.
Further, the fine synchronization decision process in step 6) is as follows:
a. calculating the absolute value of the difference value of the conjugate correlation accumulated values of two adjacent leading symbol frequency domain data, and recording the absolute value as deltaPhi1[ i ];
b. calculating the absolute value of the difference value of the sum of every two conjugate correlation accumulated values, and recording the absolute value as deltaPhi2[ i ];
c. calculating the absolute value mean value of the three continuous conjugate correlation accumulated values, and recording as meanDeltaphi;
d. if any one of the following conditions is met, judging that the fine synchronization is successful, otherwise, failing to perform the fine synchronization;
condition 1: deltaPhi1[ i ] > Thld 1;
condition 2: deltaPhi2[ i ] > Thld1 and meanDeltaPhi < Thld 2;
condition 3: deltaPhi2[ i ] > Thld1 and
meandlPhi is more than or equal to Thld2 and abs (PrmDeltaPhi [ i ]) < Thld 3;
with Thld1, Thld2, and Thld3 being preset threshold values.
The invention also provides a synchronous detection device in the OFDM system, which comprises a coarse synchronization module, a time-frequency conversion module and a fine synchronization module;
the coarse synchronization module comprises a correlation detection submodule, a peak value search submodule and a coarse synchronization judgment submodule, wherein the correlation detection submodule is used for calculating a segmental cross-correlation value C of a received baseband sampling signal and a locally stored leading OFDM symbol1(n) and the energy value E of the received baseband sampled signal in a sliding window1(n); the peak search submodule is used for processing a result C according to the correlation detection submodule1(n) and E1(n) carrying out peak judgment and peak side lobe search with a preset peak threshold value T; the coarse synchronization judgment submodule is used for comparing the peak search result with a preset coarse synchronization judgment condition to obtain a coarse synchronization position;
the time-frequency conversion module is used for carrying out time-frequency conversion on the received time-domain data of two adjacent leading OFDM symbols from the coarse synchronization position and converting the time-domain data into corresponding frequency-domain data;
the fine synchronization module comprises a conjugate correlation submodule and a fine synchronization judgment submodule, and the conjugate correlation submodule is used for calculating conjugate correlation accumulated values and phases of two adjacent leading symbol frequency domain data; and the fine synchronization judgment submodule is used for calculating a fine synchronization judgment value and comparing the fine synchronization judgment value with a preset threshold, and outputting a frame synchronization success indication and a fine synchronization position, namely a frame boundary position, if a preset fine synchronization judgment condition is met.
The invention divides the synchronous detection into two processes of coarse synchronization and fine synchronization, firstly, the received baseband data signal and the segment correlation value of the local pilot frequency sequence and the power value of the received signal are utilized to carry out coarse synchronization so as to obtain a coarse synchronization position; and then, starting from the coarse synchronization position, carrying out frequency domain transformation and conjugate correlation processing on the received signal, calculating a fine synchronization judgment value according to a frequency domain conjugate correlation result, and carrying out fine synchronization judgment based on the fine synchronization judgment value, wherein the position meeting the fine synchronization judgment is the synchronization bit to be detected. Through the two processes, the invention can realize accurate frame synchronization under the conditions of bad channel conditions, large frequency offset, strong noise interference and deep fading, has the characteristics of high synchronization accuracy, small synchronization time delay, low realization complexity and low hardware resource overhead, and has high application value in an OFDM system.
Further, the peak search submodule performs a peak side lobe search as follows:
A. comparing the coarse synchronization judgment value with a preset peak value threshold, and recording the tempPos of the position of the current point and the corresponding detection value tempPeak when the coarse synchronization judgment value is larger than the preset peak value threshold;
B. if the current point detects the first peak value, recording the peak position peakPos [0] ═ tempPos, the peak value peakValue [0] ═ tempPeak, and the peak count peakNum ═ 1; if the effective peak value exists, comparing tempPos of the current point with the stored position peakPos [ peakNum-1] of the previous peak value point;
C. when tempPos-peakPos [ peakNum-1]]＜LsIf tempPeak > peakValue [ peakNum-1]]Then tempPos and tempPeak are substituted for peakPos [ peakNum-1], respectively]And peakValue [ peakNum-1]]Otherwise, when tempPos-peakPos [ peakNum-1]]When the number is equal to L,the peak position peakPos [ peakNum ] is recorded]tempPos, peak value peakvale [ peakNum ]]The peak count peakNum value is accumulated by 1; wherein L issThe size of a peak side lobe search window is LsL/2+ 1; l is the number of sample points of one preamble OFDM symbol.
Drawings
Fig. 1 is a diagram illustrating a preamble frame format in an OFDM system according to an embodiment of the present invention;
FIG. 2 is a flow chart of a synchronization detection method in the OFDM system of the present invention;
FIG. 3 is a schematic diagram of the structure of the synchronization detecting apparatus in the OFDM system of the present invention;
FIG. 4 is a schematic diagram of a coarse synchronization module in the synchronization detecting apparatus according to the present invention;
FIG. 5 is a schematic diagram of a fine synchronization module in the synchronous detection apparatus according to the present invention.
Detailed Description
The following further describes embodiments of the present invention with reference to the drawings.
Method embodiment
In the OFDM system, timing synchronization is required to demodulate data correctly. The synchronization accuracy directly affects the performance of the receiver. In a High-speed Power Line Communication (HPLC) system based on OFDM modulation, data information is transmitted in burst frames, and the Communication nodes are mainly in a receiving state. The HPLC receiver synchronization process requires frame synchronization and symbol synchronization, i.e. frame arrival detection is accomplished and the start position of useful data information is determined. If the Symbol synchronization point deviation is large, the FFT window position may be inaccurate, and serious Inter-Symbol Interference (ISI) may be generated, thereby deteriorating the system performance. There is therefore a need to improve the reliability of timing synchronization.
The preamble designed in the HPLC protocol has good correlation and can be used for timing synchronization. Power line communication channels are complex. The synchronization algorithm needs to be able to overcome the effects of large random noise disturbances. In addition, in the OFDM system, synchronization is generally performed before frequency offset estimation and compensation, and therefore, synchronization needs to be resistant to the influence of frequency offset. The leader sequence in HPLC is known to the receiving end, the received baseband digital signal and the locally known leader symbol are subjected to segmented cross correlation, and normalization is performed based on the energy of the received signal, so that the influence of random noise and frequency offset can be well resisted. When the received preamble is exactly aligned with the local preamble symbol in the sliding window portion, a spike occurs. Finding the peak point of the cross-correlation can determine the boundary position of the preamble symbol in a frame.
Generally, at the receiver side, a received signal is adjusted and stabilized by AGC and then sent to a digital baseband demodulator for signal demodulation. AGC adjustment in high speed power line carrier communication systems is also based on the received preamble sequence. When the channel condition is worse, the number of effective preamble symbols received by the system and used for subsequent timing synchronization may be less when the AGC adjustment is stable, and therefore, the method for determining frame timing synchronization by cross-correlation peak counting has low effectiveness.
The invention provides a synchronous detection method of an OFDM system, aiming at solving the problems that in the prior art, under the conditions of severe channel conditions and complex environment, the synchronous estimation position is seriously influenced by frequency deviation and strong noise interference, and can be seriously deviated from a correct position, so that the synchronous calculation complexity is high and the synchronous precision is low. The coarse synchronization position indicates the start position of the symbol and the fine synchronization position indicates the start position of the data frame. Taking the HPLC protocol as an example, the first 13 OFDM symbols of each physical frame are preamble symbols, which include 10.5 SYNCP symbols and 2.5 SYNCM symbols, where SYNCM is-SYNCP. Each SYNCP symbol datum is repeated. The coarse synchronization position indicates the starting position, i.e., the symbol boundary position, at which the current receiving side receives the preamble symbol. However, the specific number of preamble symbols is unknown, and the fine synchronization is based on the coarse synchronization position, and further gives the position of the SYNCM symbol, i.e. the frame boundary position, i.e. the final synchronization position, by using the characteristic that the phases of SYNCM and SYNCP differ by pi.
The method comprises the steps of firstly, calculating a subsection cross-correlation value of a received baseband data signal and a local pilot frequency sequence and a power value of the received signal, calculating a coarse synchronization judgment value based on the calculation result of the two, comparing the obtained coarse synchronization judgment value with a preset peak threshold, searching a peak sidelobe, performing coarse synchronization judgment according to the peak search result, and determining a coarse synchronization position, namely a symbol boundary position. And a second step of receiving the preamble OFDM symbol data according to the symbol boundary position obtained in the first step, performing FFT (fast Fourier transform), converting the symbol data into a frequency domain, performing conjugate correlation processing, calculating a fine synchronization judgment value according to a frequency domain conjugate correlation result, comparing the fine synchronization judgment value with a preset threshold, and outputting a frame synchronization indication and a frame synchronization position, namely a frame boundary position. The specific implementation flow of the method is shown in fig. 2, and the specific process is as follows.
1. Receiving the baseband sampling time domain signal to obtain a receiving sequence, and calculating the segment correlation value of the receiving sequence and the local leader sequence and the power value of the receiving sequence.
In the high-speed power line carrier communication system, the preamble sequence frame format is shown in fig. 1, and includes 10.5 SYNCP symbols and 2.5 SYNCM symbols, where SYNCM is-SYNCP, and each preamble SYNCP time domain point (FFT length) L is 1024. Where the first 0.5 SYNCPs of the preamble are the second half of the SYNCPs and the last 0.5 SYNCMs are the first half of the SYNCMs.
The preamble designed in the high-speed power line carrier communication protocol has good correlation and can be used for synchronous detection. For the receiving end, the preamble is known, and the received baseband sampling data r and the locally known preamble symbol sequence sync can be subjected to segment cross-correlation calculation to obtain a cross-correlation value C1(n) simultaneously calculating the received signal energy value E within the sliding window1(n), the specific calculation formula is as follows:
here, L is the number of sampling points of one preamble OFDM symbol, such as: l1024, n is the serial number of the receiving sampling point, and n 0,1, 2. P is the number of segments of the segment in the sliding window, NpFor the length of each of the segments it is,
i is the index of the data in the sliding window, and i is more than 0 and less than L. sync*(i) For the conjugate of sync (i), sync (i) is the local preamble sequence, i is the data index in the p-th segment, i is 0,1,2, …, Np-1, r (i) is the received baseband sampled signal.
2. And calculating a coarse synchronization judgment value according to the obtained correlation value and the power value, comparing the coarse synchronization judgment value with a preset peak threshold T, and searching a peak side lobe in a preset search window.
Taking the ratio of the correlation value to the power value as a coarse synchronization judgment value, wherein a specific calculation formula is as follows:
and comparing the coarse synchronization judgment value Q (n) with a preset peak value threshold T, and recording the tempPos of the position of the current point and the corresponding detection value tempPeak when Q (n) is equal to or more than T, wherein the tempPeak is Q (n). T is a peak threshold, which can be determined by simulation, where T is taken to be 0.02.
And searching peak side lobes according to the detected peak value. If the current point detects the first peak value, the peak position peakPos [0] is recorded]tempPos, peak value 0]tempPeak, peak count peakNum 1; if the effective peak value exists, the tempPos of the current point and the stored position peakPos of the previous peak value point are compared [ peakNum-1]]And (6) comparing. When tempPos-peakPos [ peakNum-1]]＜LsIf tempPeak > peakValue [ peakNum-1]]Then tempPos and tempPeak are substituted for peakPos [ peakNum-1], respectively]And peakValue [ peakNum-1]]Setting peakPos [ peakNum-1]]＝tempPos，peakValue[peakNum-1]Peak count pea ═ tempPeakThe value of kNum is unchanged; otherwise, when tempPos-peakPos [ peakNum-1]]When L, record peak position peakPos [ peakNum]tempPos, peak value peakvale [ peakNum ]]The peak count peakNum value is incremented by 1. Wherein L issThe size of the search window is the peak side lobe, and can be LsL/2+1, where L is preferreds513; l is the number of samples of one preamble OFDM symbol, where L is 1024.
3. And performing coarse synchronization judgment according to the peak sidelobe search result to determine a coarse synchronization position.
The current peak count peakNum reaches the expected value NpeakAnd the interval between two adjacent peak values is L, the synchronization is judged to be successful, otherwise, the synchronization is judged to be failed. Here, 2. ltoreq.NpeakAnd L is less than or equal to 12, the number of sampling points of one preamble OFDM symbol is taken as L-1024.
4. And starting from the coarse synchronization position, selecting two adjacent preamble OFDM symbol time domain data, and performing time-frequency transformation on the two adjacent preamble OFDM symbol time domain data to obtain corresponding frequency domain data. In the received time domain signal, two adjacent preamble OFDM symbols are taken at a time from the coarse synchronization position. For example, assuming that the input data is r (n), n is the data index, n is 0,1,2, … …, and the coarse synchronization position is L0, each time two adjacent preamble OFDM symbol data are taken as: r (L0+ m × L + i) and r (L0+ (m +1) × L + i), where m denotes the mth fetch, L is the length of one OFDM symbol, i is the data index in one OFDM symbol, and i is 0,1,2, …, L-1.
The time-frequency transformation of the invention adopts FFT (fast Fourier transform) to obtain corresponding frequency domain data R (l-1, k) and R (l, k), wherein l is a leading OFDM symbol index, and l is 1,2preamble，NpreambleFor an effective number of preamble OFDM symbols, N may be takenpreamble12. k is the subcarrier index on each leading OFDM symbol, and the value range of k is determined by the communication frequency band.
5. And calculating the conjugate correlation accumulated value and the phase of the frequency domain data of two adjacent pilot symbols.
The invention respectively marks the calculated conjugate correlation accumulated value and the phase thereof as PrmSymSum [ l ] and PrmDeltaPhi [ l ], and the calculation formula is as follows:
PrmDeltaPhi[l]＝angle(PrmSymSum[l])
here, NsetFor the number of sets of active subcarriers on each preamble OFDM symbol,
is the number of subcarriers contained in each set.
6. And calculating a fine synchronization judgment value according to the obtained conjugate correlation accumulated value and the phase, comparing the fine synchronization judgment value with a preset fine synchronization judgment condition, and performing fine synchronization judgment according to a comparison result.
Respectively calculating the absolute value of the difference value of the conjugate correlation accumulated values of two adjacent leading symbol frequency domain data, and marking as deltaPhi1[ i ], the absolute value of the difference value of the sum of every two conjugate correlation accumulated values is marked as deltaPhi2[ i ], and the absolute value mean value of three continuous conjugate correlation accumulated values is marked as meanDeltaphi, wherein the calculation method comprises the following steps:
deltaPhi1[i]＝abs(PrmDeltaPhi[i]-PrmDeltaPhi[i-1])
if (deltaPhi1[ i ] > Thld1), or (deltaPhi2[ i ] > Thld1) and ((meanDeltaPhi < Thld2) or (meanDeltaPhi ≧ Thld2 and abs (prmddeltaphi [ i ] < Thld3)), the fine synchronization is determined to be successful, otherwise the fine synchronization is determined to be failed.
Apparatus example 1
The apparatus proposed by the present embodiment comprises a processor, a memory, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method of the above method embodiment when executing the computer program. That is, the method in the above method embodiments should be understood that the flow of the synchronization detection method in the OFDM system may be implemented by computer program instructions. These computer program instructions may be provided to a processor such that execution of the instructions by the processor results in the implementation of the functions specified in the method flow described above.
The processor referred to in this embodiment refers to a processing device such as a microprocessor MCU or a programmable logic device FPGA; the memory referred to in this embodiment includes a physical device for storing information, and generally, information is digitized and then stored in a medium using an electric, magnetic, optical, or the like. For example: various memories for storing information by using an electric energy mode, such as RAM, ROM and the like; various memories for storing information by magnetic energy, such as hard disk, floppy disk, magnetic tape, magnetic core memory, bubble memory, and U disk; various types of memory, CD or DVD, that store information optically. Of course, there are other ways of memory, such as quantum memory, graphene memory, and so forth.
The apparatus comprising the memory, the processor and the computer program is realized by the processor executing corresponding program instructions in the computer, and the processor can be loaded with various operating systems, such as windows operating system, linux system, android, iOS system, and the like. As other embodiments, the device can further comprise a display, and the display is used for displaying the synchronous detection result for the reference of workers.
Apparatus example 2
As shown in fig. 3, the apparatus for implementing the synchronization detection method of the OFDM system includes: the device comprises a coarse synchronization module, a time frequency conversion module and a fine synchronization module.
The coarse synchronization module includes a correlation detection sub-module, a peak search sub-module and a coarse synchronization decision sub-module, as shown in fig. 4. The correlation detection submodule is used for calculating a segmental cross-correlation value C of the received preamble symbol and the local storage sequence1(n) sliding with the received preamble symbolEnergy value E in a moving window1(n); a peak search submodule for searching for a peak according to the processing result C of the correlation detection submodule1(n) and E1(n) carrying out peak judgment and peak side lobe search with a preset peak threshold value T; and the coarse synchronization judgment submodule is used for comparing the peak value search result with a preset coarse synchronization judgment condition and outputting a coarse synchronization state indication: if the peak value searching result meets the condition that the current peak value count peakNum reaches the expected value NpeakAnd the interval between two adjacent peak values is L, judging that the coarse synchronization is successful, and outputting a coarse synchronization success indication and a coarse synchronization position; otherwise, outputting a coarse synchronization failure indication. Here, 2. ltoreq.NpeakAnd L is less than or equal to 12, the number of sampling points of one preamble OFDM symbol is taken as L-1024.
And the time-frequency conversion module is used for carrying out FFT (fast Fourier transform) conversion and converting the time domain data into frequency domain data.
The fine synchronization module includes a conjugate correlation sub-module and a fine synchronization decision sub-module, as shown in fig. 5. The conjugate correlation submodule is used for calculating conjugate correlation accumulated values and phases of two adjacent pilot symbol frequency domain data; and the fine synchronization judgment submodule is used for calculating a fine synchronization judgment value, comparing the fine synchronization judgment value with a preset threshold, and outputting a frame synchronization success indication and a fine synchronization position, namely a frame boundary position, if a preset fine synchronization judgment condition is met. Otherwise, outputting a synchronization failure indication.
The specific implementation processes of the modules are already described in detail in the method embodiment, and are not described herein again.
Claims (10)
1. A method for detecting synchronization in an OFDM system, the method comprising:
1) receiving a baseband sampling time domain signal to obtain a receiving sequence, and calculating a segment correlation value of the receiving sequence and a local preamble OFDM symbol sequence and a power value of the receiving sequence;
2) calculating a coarse synchronization judgment value according to the obtained correlation value and the power value, comparing the coarse synchronization judgment value with a preset peak threshold T, and performing peak side lobe search in a preset search window;
3) performing coarse synchronization judgment according to the peak side lobe search result to determine a coarse synchronization position;
4) from the coarse synchronization position, performing time-frequency transformation on the received two adjacent preamble OFDM symbol time domain data to obtain corresponding frequency domain data;
5) calculating the conjugate correlation accumulated value and the phase of the frequency domain data of two adjacent pilot symbols according to the frequency domain transformation result;
6) and calculating a fine synchronization judgment value according to the obtained conjugate correlation accumulated value and the phase, comparing the fine synchronization judgment value with a preset fine synchronization judgment condition, and performing fine synchronization judgment according to a comparison result.
2. The method of claim 1, wherein the segment correlation value in step 1) is calculated by the following formula:
l is the number of sampling points of a preamble symbol; n is the serial number of the receiving sampling point, and n is 0,1, 2.; p is the number of segments of the segment in the sliding window; n is a radical ofpFor the length of each of the segments it is,
sync (i) is a local preamble sequence, i is a data index in the p-th segment, i is 0,1,2, …, Np-1，sync*(i) The conjugate of sync (i), and r (i) the received baseband sampled signal.
3. The method according to claim 1, wherein the coarse synchronization decision value in step 2) is calculated by the following formula:
q (n) is a coarse synchronization judgment value, C1(n) is a piecewise cross-correlation value, E1And (n) is the power value of the received sequence r (n).
4. The method of claim 3, wherein the searching procedure of the peak sidelobe in step 2) is as follows:
A. comparing the coarse synchronization judgment value with a preset peak value threshold, and recording the tempPos of the position of the current point and the corresponding detection value tempPeak when the coarse synchronization judgment value is larger than the preset peak value threshold;
B. if the current point detects the first peak value, recording the peak position peakPos [0] ═ tempPos, the peak value peakValue [0] ═ tempPeak, and the peak count peakNum ═ 1; if the effective peak value exists, comparing tempPos of the current point with the stored position peakPos [ peakNum-1] of the previous peak value point;
C. when tempPos-peakPos [ peakNum-1]]＜LsWhen, if
tempPeak＞peakValue[peakNum-1]Then tempPos and tempPeak are substituted for peakPos [ peakNum-1], respectively]And peakValue [ peakNum-1]]Otherwise, when tempPos-peakPos [ peakNum-1]]When L, record peak position peakPos [ peakNum]tempPos, peak value peakvale [ peakNum ]]The peak count peakNum value is accumulated by 1; wherein L issThe size of a peak side lobe search window is LsL/2+ 1; l is the number of sample points of one preamble OFDM symbol.
5. The method for detecting synchronization in an OFDM system according to claim 4, wherein the coarse synchronization decision process in step 3) is: and if the current peak count peakNum reaches the expected value and the position interval of two adjacent stored peak values is L, the coarse synchronization is successful.
6. The method as claimed in claim 1, wherein the calculation formula of the conjugate correlation accumulation value of the frequency domain data of two adjacent preamble symbols in step 5) is:
wherein N issetFor the number of sets of active subcarriers on each preamble OFDM symbol,
for the number of sub-carriers contained in each set, R (l-1, k) and R (l, k) are frequency domain data of two adjacent leading OFDM symbols, l is a leading OFDM symbol index, k is a sub-carrier index on each leading OFDM symbol, (. DEG)*The conjugate of the calculated value in the parenthesis is shown.
7. The method for detecting synchronization in an OFDM system according to claim 1, wherein the fine synchronization decision process in step 6) is as follows:
a. calculating the absolute value of the difference value of the conjugate correlation accumulated values of two adjacent leading symbol frequency domain data, and recording the absolute value as deltaPhi1[ i ];
b. calculating the absolute value of the difference value of the sum of every two conjugate correlation accumulated values, and recording the absolute value as deltaPhi2[ i ];
c. calculating the absolute value mean value of the three continuous conjugate correlation accumulated values, and recording as meanDeltaphi;
d. if any one of the following conditions is met, judging that the fine synchronization is successful, otherwise, failing to perform the fine synchronization;
condition 1: deltaPhi1[ i ] > Thld 1;
condition 2: deltaPhi2[ i ] > Thld1 and meanDeltaPhi < Thld 2;
condition 3: deltaPhi2[ i ] > Thld1 and
meandlPhi is more than or equal to Thld2 and abs (PrmDeltaPhi [ i ]) < Thld 3;
with Thld1, Thld2, and Thld3 being preset threshold values.
8. A synchronization detection arrangement in an OFDM system, characterized in that the detection arrangement comprises a processor and a memory, said processor executing a computer program stored by said memory for implementing a synchronization detection method in an OFDM system as claimed in any of the claims 1-7 above.
9. The synchronous detection device in an OFDM system is characterized by comprising a coarse synchronization module, a time-frequency conversion module and a fine synchronization module;
the coarse synchronization module comprises a correlation detection submodule, a peak value search submodule and a coarse synchronization judgment submodule, wherein the correlation detection submodule is used for calculating a segmental cross-correlation value C of a received baseband sampling signal and a locally stored leading OFDM symbol1(n) and the energy value E of the received baseband sampled signal in a sliding window1(n); the peak search submodule is used for processing a result C according to the correlation detection submodule1(n) and E1(n) carrying out peak judgment and peak side lobe search with a preset peak threshold value T; the coarse synchronization judgment submodule is used for comparing the peak search result with a preset coarse synchronization judgment condition to obtain a coarse synchronization position;
the time-frequency conversion module is used for carrying out time-frequency conversion on the received time-domain data of two adjacent leading OFDM symbols from the coarse synchronization position and converting the time-domain data into corresponding frequency-domain data;
the fine synchronization module comprises a conjugate correlation submodule and a fine synchronization judgment submodule, and the conjugate correlation submodule is used for calculating conjugate correlation accumulated values and phases of two adjacent leading symbol frequency domain data; and the fine synchronization judgment submodule is used for calculating a fine synchronization judgment value and comparing the fine synchronization judgment value with a preset threshold, and outputting a frame synchronization success indication and a fine synchronization position, namely a frame boundary position, if a preset fine synchronization judgment condition is met.
10. The apparatus of claim 9, wherein the peak search submodule performs peak sidelobe search as follows:
A. comparing the coarse synchronization judgment value with a preset peak value threshold, and recording the tempPos of the position of the current point and the corresponding detection value tempPeak when the coarse synchronization judgment value is larger than the preset peak value threshold;
B. if the current point detects the first peak value, recording the peak position peakPos [0] ═ tempPos, the peak value peakValue [0] ═ tempPeak, and the peak count peakNum ═ 1; if the effective peak value exists, comparing tempPos of the current point with the stored position peakPos [ peakNum-1] of the previous peak value point;
C. when tempPos-peakPos [ peakNum-1]]＜LsIf tempPeak > peakValue [ peakNum-1]]Then tempPos and tempPeak are substituted for peakPos [ peakNum-1], respectively]And peakValue [ peakNum-1]]Otherwise, when tempPos-peakPos [ peakNum-1]]When L, record peak position peakPos [ peakNum]tempPos, peak value peakvale [ peakNum ]]The peak count peakNum value is accumulated by 1; wherein L issThe size of a peak side lobe search window is LsL/2+ 1; l is the number of sample points of one preamble OFDM symbol.
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