CN110224968B - Frame timing synchronization method and device in OFDM communication system - Google Patents
Frame timing synchronization method and device in OFDM communication system Download PDFInfo
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Abstract
The invention provides a frame timing synchronization method and device in OFDM communication system, the method of the invention, first of all calculates the sliding cross correlation value of the received baseband data signal and local reference preamble symbol sequence and the power value of the received signal in the sliding window, then carries on the delay correlation accumulation to the obtained sliding cross correlation value and the power value of the received signal in the sliding window, calculates the threshold value based on the delay correlation accumulation result of the two, the power value of the local preamble symbol sequence and the preset preamble sequence OFDM symbol total number in a frame, compares the obtained threshold value with the preset peak value threshold and carries on the peak value side lobe search, then carries on the synchronization judgment according to the peak value search result, outputs the frame timing synchronization indication, the method has low calculation complexity, is convenient for hardware realization, can be accurately synchronized under the condition that the effective OFDM symbol number of the preamble sequence is less, the method is suitable for various complex power line channel environments.
Description
Technical Field
The invention relates to the technical field of OFDM wireless communication/power line carrier communication, in particular to a frame timing synchronization method and a frame timing synchronization device in an OFDM communication system.
Background
Orthogonal Frequency Division Multiplexing (OFDM) technology, as a multi-carrier modulation technology, has the advantages of strong anti-multipath interference capability, high spectrum utilization rate, and the like. OFDM techniques also have some drawbacks, one of which is the sensitivity to synchronization errors. In the OFDM system, parallel data is modulated onto subcarriers by inverse fourier transform at a transmitting end, and data is demodulated from the subcarriers by fourier transform at a receiving end, so that the determination of OFDM symbol positions at the receiving end affects system performance.
In the OFDM system, since a Cyclic Prefix (CP) is inserted between OFDM symbols, the requirement for timing estimation at a receiving end is reduced, but when a timing estimation point falls outside the CP, inter-symbol interference (ISI) still occurs, and meanwhile, in a multipath fading environment, a timing offset increases the sensitivity of the OFDM system to multipath delay spread, so that the OFDM system also needs more accurate timing synchronization in order to obtain optimal system performance.
The timing synchronization method in the conventional OFDM system mainly has three types: the first is a pilot-based timing synchronization method, i.e. using pilot data inserted in the frequency domain when the transmitter OFDM signal is formed for synchronization estimation; the second method is a timing synchronization method based on cyclic prefix, namely, the synchronization calculation is carried out by utilizing the characteristic that data which is used as the cyclic prefix after a part of OFDM symbols is copied to the front of the symbols is completely the same as the second half signal in the OFDM symbols; the third is a timing synchronization method based on training sequences, i.e. a transmitting end adds a specific sequence before data to be transmitted, and a receiving end performs synchronization estimation by using the characteristics of the sequence itself. A timing synchronization method based on a training sequence is adopted in a high-speed power line carrier communication system. The high-speed power line carrier communication standard adopts 13 preamble symbols as training sequences, and the 13 preamble symbols have very good correlation characteristics. The symbol timing synchronization is carried out by utilizing the characteristic, and the precision is higher. The timing synchronization algorithm based on the training symbols mainly comprises a time delay autocorrelation algorithm and a cross-correlation algorithm based on a local sequence. The delayed autocorrelation algorithm has a section of peak value platform, which is not favorable for accurate timing synchronization. The traditional local sequence cross-correlation-based method has higher precision under high signal-to-noise ratio, but has lower synchronization precision under low signal-to-noise ratio.
In addition, in the OFDM communication system, the time domain waveform of the OFDM symbol has the characteristics of a large dynamic range and a high peak-to-average ratio, and the power line channel attenuates a signal seriously, and the amplitude of a signal received by a receiver is small, and generally needs to be adjusted by a reliable AGC first to ensure the stability of the signal power input to the digital baseband demodulator, thereby reducing the influence of quantization noise or signal clipping on the reception performance. 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 effective preamble OFDM symbol number which can be further used for subsequent frame timing synchronization and channel estimation after being adjusted and stabilized by AGC will be reduced, which will reduce the accuracy of frame timing synchronization.
Disclosure of Invention
The invention provides a frame timing synchronization method and device in an OFDM communication system, aiming at solving the problems that the prior art is low in synchronization precision and high in calculation complexity in the aspect of frame timing synchronization and is difficult to obtain accurate synchronization under the condition that the number of effective OFDM symbols of a received leader sequence is small.
A method for frame timing synchronization in an OFDM communication system, comprising:
the first step is as follows: the method comprises the steps that a baseband sampling receiving sequence r of an OFDM communication system passes through a correlation detection window C and an energy detection window P, namely, the same baseband sampling receiving sequence r is subjected to local cross-correlation calculation and energy value calculation, the two calculations can be executed in parallel, wherein the correlation detection window C and the energy detection window P are used for calculating the latest sampling receiving sequence r data of L points along with the input data of each sampling point, L is configurable and is selected according to the number of FFT points of the OFDM system, the correlation detection window C adopts a sliding window method and calculates the cross-correlation between the data sequence in the sliding window and a locally stored synchronous sequence, and the output result of the correlation detection window C is recorded as C1(n) for the correlation detection window C, a correlation value C can be calculated for each input sample point1Thus C1(n) is a sequence of a set of data; calculating the power value of the data sequence in the window P by the energy detection window P, and recording the output result of the energy detection window P as E1(n) for the energy detection window P, each input sampling point can be calculated to obtain an energy value E1Thus E1(n) is a sequence of a set of data; here, n is the serial number of the input data in the sliding window, namely the number of the input data in the sliding window, and 0 < n < L; l is the window length of the sliding window;
the second step is that: correlating the results C of the detection window C1(n) performing delay correlation accumulation to obtain C2(n) detecting the result E of the window P1(n) performing delay correlation accumulation to obtain E2(n)；
The third step: according to the result C of delay correlation accumulation processing of each two sliding windows2(n) and E2(n) and local preamble symbol sequence power value EsAnd the number N of OFDM symbols in which the preamble sequence is complete in a frameCalculating a frame timing synchronization judgment value Q (n), wherein each time means that each sampling point needs to be calculated, and the interval between adjacent times is a sampling time interval;
the fourth step: comparing the frame timing synchronization judgment value Q (n) with a preset peak threshold T, and performing peak sidelobe search in a preset search window, wherein the length of the peak search window is set as LsPoint;
the fifth step: comparing the peak value search result with a preset frame timing synchronization judgment condition, and outputting a frame timing synchronization state indication: if the peak value searching result meets the preset frame timing synchronization judgment condition, judging that the frame timing synchronization is successful, and outputting a signal output success state indication in the frame timing synchronization state; otherwise, outputting a failure state indication by the frame timing synchronization state output signal.
Further, the frame timing synchronization method in the OFDM communication system, wherein the first step comprises the steps of:
step 1-1: the correlation detection window C calculates the cross-correlation value between the received baseband digital signal sequence r and the locally stored signature sequence s (k), and the calculation method is as follows:
k is a data sequence number of the receiving sequence and the local synchronous sequence in the cross-correlation calculation process;
m is the serial number of the OFDM symbol counted in the calculation process;
s (k) is a sign sequence of the local preamble sequence SYNC, that is, s (k) is a sign indication of the local preamble sequence SYNC data, where when the current preamble sequence data is non-negative, corresponding s (k) is 1, and when the preamble data is negative, corresponding s (k) is-1;
step 1-2: the energy detection window P calculates a power value of the received baseband digital signal sequence r in the window P, and the calculation method is as follows:
wherein, L is the size of the related detection window C and the energy detection window P, the value is the number of sampling points of a leading OFDM symbol, n is more than 0 and less than L, M is more than 0 and less than M, and M is the number of complete and effective leading OFDM symbols in the current receiving frame.
Further, in the cross-correlation detection method in step 1-1, wherein s (k) of the locally stored signature sequence is a symbol sequence of the local preamble sequence SYNC, that is, s (k) is a sign indication of the local preamble sequence SYNC data, and when the current preamble sequence SYNC data is a non-negative number, that is, SYNC (k) is greater than or equal to 0, corresponding s (k) is 1; when the preamble sequence SYNC data is negative, that is, SYNC (k) < 0, s (k) ═ 1, SYNC (k) is k-th data of the local preamble sequence SYNC; the SYNC may be a SYNC or a SYNC symbol, where the SYNC is a forward sequence of SYNC, i.e., is equivalent to a SYNC sequence, and the SYNC is a reverse sequence of the SYNC sequence, and k is 0.
Further, the frame timing synchronization method in the OFDM communication system, wherein the second step comprises the steps of:
step 2-1: output result C to correlation detection window C1(n) performing delay correlation accumulation processing calculation, wherein the formula is as follows:
wherein winmaxParkSign is the current C input to the delay-dependent accumulation window1The sign of the maximum value of the (n + m · L) sequence indicates that: when in use
Then winMaxPeakSign is-1; when in use
Then winMaxPeakSign is 1; if the peak sign flip flag peakSignFlipflag is 0 and the current sequence C is1(n+m1L) corresponding winMaxParkSign value to the preceding sequence C1(n+m0L) corresponding to winMaxPeakSign, and sets peak signnflipflag to 1, where 0 < n < L, and 0 < m0＜M,0＜m1＜M,m1＝m0+1；m0、m1The same meaning as m as above means the number of OFDM symbol, m1Number of current OFDM symbol, m0A sequence number representing a previous OFDM symbol; the peak sign inversion flag peakSignFleplag is used to indicate the current sequence C1(n+m1L) with the preceding sequence C1(n+m0L), and if the winMaxPeakSign values of the two are the same, peak signnflipflag is 0, otherwise peak signnflipflag is 1. The initial value of peakSignFllag is set to 0;
step 2-2: output result E to energy detection window P1(n) performing delay correlation accumulation processing calculation by the following method:
where M is the number of complete and valid preamble OFDM symbols in the current received frame.
Further, the frame timing synchronization method in the OFDM communication system, wherein the third step includes accumulating a result C according to the delay correlation of each two sliding windows2(n) and E2(n) and local preamble symbol power value EsAnd the number N of the complete OFDM symbols of the leader sequence in a frame, and calculating a frame timing synchronization judgment value Q (N), wherein the formula is as follows:
wherein E issFor local preamble symbol orderThe power values of the columns are,
the symbol sequence s (k) of the local preamble sequence is a known fixed sequence; n is the number of leading OFDM symbols stipulated in the OFDM-based communication standard; m is the number of complete and effective preamble OFDM symbols actually received by a receiver system frame timing synchronization module, and M is less than or equal to N.
Further, the frame timing synchronization method in the OFDM communication system, wherein the fourth step includes the steps of:
step 4-1: comparing the frame timing synchronization judgment value Q (n) with a preset peak value threshold T, and recording the position tempPos of the current sampling point and a corresponding detection value tempPeak when Q (n) is equal to or more than T, wherein the tempPeak is Q (n);
step 4-2: searching peak side lobes; 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 ]]Comparing: when tempPos-peakPos [ peakNum-1 ]]＜LsIf tempPeak > peakValue [ peakNum-1 ]]Then, peakPos [ peakNum-1 ] is set]＝tempPos，peakValue[peakNum-1]The peak count peakNum value is unchanged; 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 issFor peak sidelobe search window size, Ls<L; l is the number of sampling points of a preamble OFDM symbol, the number of sampling points being the same as the symbol length.
Further, the frame timing synchronization method in the OFDM communication system, wherein the frame timing synchronization decision process in the fifth step is: when the peak sign inversion mark peakSignFlipflag is detected to be 1 and the peak count peakNum is not 0, the frame timing synchronization is judged to be successful, and a frame timing synchronization state output signal outputs a synchronization success state indication.
Further, the frame timing synchronization method in the OFDM communication system, wherein the frame timing synchronization decision process in the fifth step is: when the effective peak value exists, the peak sign reversal mark peaksignfllag is 0 and no new effective peak value is detected after the L point is exceeded, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication.
Further, the frame timing synchronization method in the OFDM communication system, wherein the frame timing synchronization decision process in the fifth step is: if the current peak count peakNum reaches a preset expected value, judging that the frame timing synchronization is successful, and outputting a synchronization success state indication by a frame timing synchronization state output signal; assuming that the number of symbols of the forward preamble SYNCP is X, a preset expected value of the peak count may be set to X + 1.
A frame timing synchronization apparatus in an OFDM communication system, wherein the apparatus comprises: a correlation detection module, a delay accumulation module, a peak search module and a synchronization decision module, wherein,
the correlation detection module is used for calculating a sliding cross-correlation value C of the received preamble symbol and the local storage sequence1(n) energy value E in sliding window with received preamble symbol1(n)；
The delay accumulation module is used for calculating the sliding cross-correlation value C1(n) delayed correlation accumulation result C2(n) and calculating the sliding window received signal energy value E1(n) delayed correlation accumulation processing result E2(n)；
The peak searching module is used for accumulating a processing result C according to the delay correlation2(n) and E2(n) locally storing the power value E of the symbol sequencesCarrying out peak judgment and peak sidelobe search on the total number N of complete OFDM symbols of a leader sequence in a frame and a preset peak threshold value T;
the synchronization decision module is configured to compare the peak search result with a preset frame timing synchronization decision condition, and output a frame timing synchronization status indication: if the peak value searching result meets the preset frame timing synchronization judgment condition, judging that the frame timing synchronization is successful, and outputting a signal output success state indication in the frame timing synchronization state; otherwise, outputting a signal output failure state indication in a frame timing synchronization state;
the correlation detection module is specifically configured to pass a received baseband digital signal sequence r through a correlation detection window C and an energy detection window P, where the correlation detection window C calculates a cross-correlation value between the received baseband digital signal sequence r and a locally stored signature sequence s, and the calculation method is as follows:
the energy detection window P calculates the energy value of the received baseband digital signal sequence r in the window P, and the calculation method is as follows:
wherein, L is the length of the correlation detection window C and the energy detection window P, and is the number of sampling points of a leading OFDM symbol, n is more than 0 and less than L, M is more than 0 and less than M, and M is the number of complete and effective leading OFDM symbols in the current receiving frame;
the delay accumulation module specifically includes:
and calculating the delay correlation accumulation processing of the sliding cross correlation value, wherein the method comprises the following steps:
wherein winmaxParkSign is the current C input to the delay-dependent accumulation window1The sign of the maximum value of the (n + m.L) sequence, i.e., when
Then winMaxPeakSign is-1; when in use
Then winMaxPeakSign is 1; if the peak sign flip flag peakSignFlipflag is 0 and the current sequence C is1(n+m1L) corresponding winMaxParkSign value to the preceding sequence C1(n+m0L) is opposite, peak signfalipflag is set to 1. Here, 0 < n < L, 0 < m0＜M,0＜m1＜M,m1＝m0+1, peak sign flip flag peakSignFlilpFlag is used to indicate the current sequence C1(n+m1L) with the preceding sequence C1(n+m0L), and if the winMaxPeakSign values of the two are the same, the peaksignnflipflag is 0, otherwise the peaksignnflipflag is 1; the initial value of peaksignfllag is set to 0);
and calculating the delay correlation accumulation processing of the sliding window energy value, wherein the method comprises the following steps:
where M is the number of complete and valid preamble OFDM symbols in the current received frame;
the peak search module specifically comprises a peak decision module and a side lobe search module, wherein,
the peak value judging module firstly calculates a frame timing synchronization judging value, and the formula is as follows:
wherein E issFor the power value corresponding to the local preamble symbol sequence,
the symbol sequence s (k) of the native preamble sequence is a known fixed sequence, thus EsCan be calculated and stored in advance; n is the number of OFDM symbols with complete leader sequence in a frame;
comparing the frame timing 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 the frame timing synchronization judgment value Q (n) is equal to or more than T, wherein the tempPeak is Q (n);
the side lobe searching module carries out peak side lobe searching, if the current point detects that the current point is 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, peakPos [ peakNum-1 ] is set]＝tempPos，peakValue[peakNum-1]The peak count peakNum value 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 accumulated by 1; wherein L issThe size of the peak side lobe search window can be determined by simulation; l is the number of sampling points of a leading OFDM symbol;
the input of the synchronization judgment module is a peak value search output result, and the output is a frame timing synchronization state output signal, when a peak value symbol turning mark peakSignFlipflag is detected to be 1 and a peak value count peakNum is not 0, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication;
the synchronization decision module may further be configured to: the input of the frame timing synchronization state output signal is a peak value search output result, the output is a frame timing synchronization state output signal, when an effective peak value exists, a peak value symbol turning mark peakSignFliplig is 0 and a new effective peak value is not detected any more when the effective peak value exceeds an L point, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication;
the synchronization decision module may further be configured to: the input is a peak value search output result, the output is a frame timing synchronization state output signal, if the current peak value count peakNum reaches an expected value, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication.
The invention provides a frame timing synchronization method and a device in an OFDM communication system, which firstly calculate the sliding cross-correlation value of a received baseband data signal and a local reference preamble symbol sequence and the power value of the received signal in a sliding window, then respectively carry out delay correlation accumulation on the obtained sliding cross-correlation value and the power value of the received signal in the sliding window, calculate a frame timing synchronization judgment value based on the delay correlation accumulation results of the two, the power value of the local preamble symbol sequence and the total number of complete OFDM symbols of the preamble sequence in a frame, compare the obtained frame timing synchronization judgment value with a preset peak threshold and carry out peak sidelobe search, then carry out synchronization judgment according to the peak search result and output a frame timing synchronization state indication. Compared with the prior art, the frame timing synchronization scheme has low calculation complexity, is convenient for hardware realization, can realize accurate synchronization under the condition of less effective OFDM symbols of the received preamble sequence, and is suitable for various complex power line channel environments.
Drawings
FIG. 1 is a diagram illustrating a preamble frame format according to an embodiment of the present invention;
FIG. 2 is a block diagram of a frame timing synchronization calculation method according to an embodiment of the present invention;
FIG. 3 is a flow chart of a frame timing synchronization method according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a frame timing synchronization apparatus according to an embodiment of the present invention;
fig. 5 is a flow chart of searching for a frame timing synchronization peak according to an embodiment of the present invention.
Detailed Description
The following detailed description of embodiments of the invention refers to the accompanying drawings.
In a high-speed power line carrier communication system, a preamble frame format is shown in fig. 1. The preamble sequence contains 10.5 SYNCP symbols and 2.5 SYNCM symbols. SYNCM ═ SYNCP. Each preamble SYNCP time domain point number (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.
In OFDM systemsTiming synchronization typically includes frame synchronization and symbol timing synchronization. While the frame symbol format in a high-speed power line carrier communication system based on OFDM modulation is fixed, therefore, frame synchronization is equivalent to symbol timing synchronization. The preamble designed in the high-speed power line carrier communication protocol has good correlation and can be used for frame timing synchronization. For the receiving end, the preamble is known, and the received baseband data and the locally known SYNCP symbol sequence can be subjected to sliding cross-correlation calculation to obtain a cross-correlation value C1(n) simultaneously calculating the received signal energy value E within the sliding window1(n) and then correlating the sliding cross correlation value C1(n) and sliding window received signal energy value E1(n) performing delay correlation accumulation to obtain C2(n) and E2(n) of (a). Based on C2(n)、E2(n) power value E of symbol sequence of local preamble sequence SYNCPsAnd carrying out threshold value calculation on the total number N of OFDM symbols of the leader sequence in a preset frame to obtain Q (N). Comparing the threshold value Q (n) with a preset peak value threshold T, performing peak side lobe search in a preset search window, and finally performing timing synchronization judgment according to a peak value search result; the peak threshold T is an empirical value obtained according to a simulation technique; a block diagram of a frame timing synchronization calculation method according to an embodiment of the present invention is shown in fig. 2.
Fig. 3 is a flow chart of a frame timing synchronization method according to an embodiment of the invention. The specific process is as follows:
the first step is as follows: a received baseband digital signal sequence r passes through two sliding windows C and P, wherein the sizes of the sliding windows C and P are both L, the value of L is the number of points of one preamble OFDM symbol, i.e. L is 1024,
the sliding window C calculates the cross-correlation value between the received baseband digital signal sequence r and the locally stored sequence s, as shown in the following formula (1):
here, the locally stored sequence s (k) is a symbol sequence of the local preamble sequence SYNCP, that is, when SYNCP (k) < 0, s (k) ═ 1; when SYNCP (k) is greater than or equal to 0, s (k) is 1. Wherein k is 0.
The sliding window P calculates the energy value of the received baseband digital signal sequence r in the window P, and the calculation method is shown as the following formula (2):
here, L is the size of the sliding windows C and P, and here, the FFT point number of one preamble OFDM symbol is taken, that is, L is 1024, 0 < n < L, 0 < M, and M is the effective symbol number of the preamble OFDM symbol in the current received frame, and the specific value thereof is determined by the actual situation.
The second step is that: cross-correlating the sliding cross-correlation value C1(n) and sliding window received signal energy value E1(n) performing delay correlation accumulation to obtain C2(n) and E2(n) a step of, specifically,
output result C to correlation detection window C1(n) performing a delay correlation accumulation calculation by the following method:
wherein winmaxParkSign is the current C input to the delay-dependent accumulation window1(n) sign of maximum value of sequence, i.e. when
Then winMaxPeakSign is-1; when in use
Then winMaxPeakSign is 1. If the peak sign flip flag peaksignfllag is 0 and the winMaxPeakSign value of the current window is opposite to that of the previous window, setting peaksignfllag to 1; the peak sign inversion flag peakSignFleplag is used to indicate the current sequence C1(n+m1L) with the preceding sequence C1(n+m0L) value of WinMaxPakSignAnd if the two winmaxpeaksigns have the same value, the peaksignnflipflag is equal to 0, otherwise, the peaksignnflipflag is equal to 1. The initial value of peaksignfllag is set to 0.
Output result E to energy detection window P1(n) performing a delay correlation accumulation calculation by the following method:
here, M is the effective symbol number of the preamble OFDM symbol in the current received frame, and the specific value thereof depends on the actual situation.
The third step: based on the C2(n)、E2(n) power value E of symbol sequence of local preamble sequence SYNCPsAnd the total number N of OFDM symbols of the leader sequence in a preset frame, and calculating a threshold value, wherein the formula is as follows:
wherein E issThe power value of the symbol sequence for the local preamble sequence SYNCP,
the symbol sequence s (k) of the native preamble sequence is a known fixed sequence, thus EsCan be pre-calculated and stored. N is the total number of complete OFDM symbols of the preamble sequence in one frame, i.e., N is 12.
The fourth step: comparing the threshold value q (n) with a preset peak threshold T, and recording the tempPos of the current point and the corresponding detection value tempPeak when q (n) ≧ T is satisfied, where tempPeak is q (n). T is a peak threshold, which can be determined by simulation, where T is taken to be 0.02. Otherwise, the frame timing synchronization judgment is carried out.
The fifth step: 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]Peak value ═ tempPeak, peak valueThe number peakNum is 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, peakPos [ peakNum-1 ] is set]＝tempPos，peakValue[peakNum-1]The peak count peakNum value 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 for peak side lobe can be determined by simulation, where L can be takens250, namely the number of time domain points corresponding to the maximum multipath time delay of 10 us; l is the number of FFT points of one preamble OFDM symbol, where L is 1024.
The fifth step: judging whether the peak value search output result meets a preset frame timing synchronization judgment condition, preferably, the method may include one or a combination of the following methods:
the method comprises the following steps: when the peak sign inversion mark peakSignFlipflag is detected to be 1 and the peak count peakNum is not 0, judging that the frame timing synchronization is successful and outputting a frame timing synchronization success indication;
the second method comprises the following steps: and when the effective peak value exists, the peak sign reversal mark peakSignFlipflag is 0 and a new effective peak value is not detected after the L point is exceeded, judging that the frame timing synchronization is successful, and outputting a frame timing synchronization success indication.
The third method comprises the following steps: and if the current peak count peakNum reaches the expected value, judging that the frame timing synchronization is successful, and outputting a frame timing synchronization success indication.
As shown in fig. 4, the apparatus for implementing the frame timing synchronization method in the OFDM communication system includes: a correlation detection module, a delay accumulation module, a peak search module and a synchronization decision module, wherein,
the correlation detection module is used for calculating a sliding cross-correlation value C of the received preamble symbol and the local storage sequence1(n) energy value E in sliding window with received preamble symbol1(n)；
The delay accumulation module is usedIn calculating the sliding cross-correlation value C1(n) delay correlation accumulated value C2(n) and calculating the sliding window received signal energy value E1(n) delay correlation accumulation value E2(n)；
The peak value searching module is used for searching the accumulated value C according to the delay correlation2(n) and E2(n) locally storing the power value E of the symbol sequencesCarrying out peak judgment and peak sidelobe search on the total number N of OFDM symbols of a leader sequence in a preset frame and a preset peak threshold value T;
as shown in fig. 5, the peak search module further includes a peak decision module and a side lobe search module.
And the synchronization judging module is used for comparing the peak value searching result with a preset frame timing synchronization judging condition, judging that the frame timing synchronization is successful if the peak value searching result meets the preset frame timing synchronization judging condition, and outputting a frame timing synchronization success indication.
The invention can realize accurate timing synchronization under the condition of less effective OFDM symbol number of the received leader sequence, has the characteristics of high timing synchronization precision and low realization complexity, and has very high application value in an OFDM system.
The principle of the technical scheme is specifically explained as follows:
in OFDM systems, timing synchronization typically includes frame synchronization and symbol timing synchronization. While the frame symbol format in a high-speed power line carrier communication system based on OFDM modulation is fixed, therefore, frame synchronization is equivalent to symbol timing synchronization. The preamble designed in the high-speed power line carrier communication protocol has good correlation and can be used for frame timing synchronization. The preamble symbol designed in the protocol is known to the receiving end, so that the received baseband digital signal can be cross-correlated with the locally known preamble symbol, and when the received preamble data is exactly aligned with the local preamble SYNCP symbol in the sliding window portion, a peak occurs. Finding the peak point of the cross correlation can determine the end position of the preamble symbol in a frame, thereby realizing the frame timing synchronization.
Due to noise interference, amplitude attenuation of received data signals occurs, peak value variation of cross-correlation values is large, a timing threshold is not well determined, and timing is inaccurate. In addition, under the condition of high signal-to-noise ratio, the peak value of the cross-correlation value is very obvious, and the synchronization precision is higher; under the condition of low signal-to-noise ratio, the difference between the peak value of the cross correlation value and the side lobe is not large, and the synchronization precision is low.
In order to solve the problem, the result of the first cross-correlation can be accumulated by delay correlation, and the peak value obtained is more stable than the peak value of the first cross-correlation due to the mutual cancellation and the relative inhibition of noise.
The preamble in the high-speed power line carrier communication protocol includes 10.5 SYNCP symbols and 2.5 SYNCM symbols, where SYNCM is-SYNCP. 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 OFDM symbols of the preamble sequence that can be used for subsequent frame timing synchronization and channel estimation after being adjusted and stabilized by AGC will be less. It is possible to determine whether the sync symbol or the SYNCM currently subjected to the delay correlation accumulation is present, based on the symbol of the maximum value of the sequence currently input to the delay correlation accumulation window, and set the peak symbol inversion flag. Finally, the start position of the valid data can be found by finding the end point of the last preamble symbol through peak symbol inversion or peak counting.
When the power line channel condition is bad and the signal distortion is serious, the peak sidelobe may become large and exceed the initially set peak threshold, which affects the peak counting. By searching for side lobes, L, beside the peaksEven if a plurality of values in the vicinity of the point exceed the threshold value, the peak count is increased by only 1. Ideally, one peak occurs every L points. By searching L around side lobesThe point method eliminates the interference of side lobes and greatly improves the accuracy of peak counting.
The frame timing synchronization scheme of the invention has low calculation complexity, is convenient for hardware realization, can realize accurate synchronization under the condition of less effective OFDM symbols of the received preamble sequence, and is suitable for various complex power line channel environments.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (5)
1. A method for frame timing synchronization in an OFDM communication system, comprising:
the first step is as follows: the method comprises the steps that a baseband sampling receiving sequence r of an OFDM communication system passes through a correlation detection window C and an energy detection window P, namely, the same baseband sampling receiving sequence r is subjected to local cross-correlation calculation and energy value calculation, the two calculations can be executed in parallel, wherein the correlation detection window C and the energy detection window P are used for calculating the latest sampling receiving sequence r data of L points along with the input data of each sampling point, L is configurable and is selected according to the number of FFT points of the OFDM system, the correlation detection window C adopts a sliding window method and calculates the cross-correlation between the data sequence in the sliding window and a locally stored synchronous sequence, and the output result of the correlation detection window C is recorded as C1(n) for the correlation detection window C, a correlation value C can be calculated for each input sample point1Thus C1(n) is a sequence of a set of data; calculating the power value of the data sequence in the window P by the energy detection window P, and recording the output result of the energy detection window P as E1(n) for the energy detection window P, each input sampling point can be calculated to obtain an energy value E1Thus E1(n) is a sequence of a set of data; here, n is the serial number of the input data in the sliding window, namely the number of the input data in the sliding window, and 0 < n < L; l is the window length of the sliding window;
the first step comprises the following steps:
step 1-1: the correlation detection window C calculates the cross-correlation value between the received baseband digital signal sequence r and the locally stored signature sequence s (k), and the calculation method is as follows:
k is a data sequence number of the receiving sequence and the local synchronous sequence in the cross-correlation calculation process;
m is the serial number of the OFDM symbol counted in the calculation process;
the locally stored signature sequence s (k) is a symbol sequence of the local preamble sequence SYNC, that is, s (k) is a sign indication of the local preamble sequence SYNC data, and when the current preamble sequence SYNC data is a non-negative number, that is, SYNC (k) is greater than or equal to 0, corresponding s (k) is 1; when the preamble sequence SYNC data is negative, that is, SYNC (k) < 0, s (k) ═ 1, SYNC (k) is k-th data of the local preamble sequence SYNC; the SYNC may be a SYNC or a SYNC symbol, where the SYNC is a forward sequence of SYNC, i.e., is equivalent to a SYNC sequence, and the SYNC is a reverse sequence of the SYNC sequence, and k is 0.
Step 1-2: the energy detection window P calculates a power value of the received baseband digital signal sequence r in the window P, and the calculation method is as follows:
wherein, L is the size of the related detection window C and the energy detection window P, the value is the number of sampling points of a leading OFDM symbol, n is more than 0 and less than L, M is more than 0 and less than M, and M is the number of complete and effective leading OFDM symbols in the current receiving frame;
the second step is that: correlating the results C of the detection window C1(n) performing delay correlation accumulation to obtain C2(n) detecting the result E of the window P1(n) performing delay correlation accumulation to obtain E2(n)；
The second step comprises the following steps:
step 2-1: to correlationOutput result C of detection window C1(n) performing delay correlation accumulation processing calculation, wherein the formula is as follows:
wherein winmaxParkSign is the current C input to the delay-dependent accumulation window1The sign of the maximum value of the (n + m · L) sequence indicates that: when in use
Then winMaxPeakSign is-1; when in use
Then winMaxPeakSign is 1; if the peak sign flip flag peakSignFlipflag is 0 and the current sequence C is1(n+m1L) corresponding winMaxParkSign value to the preceding sequence C1(n+m0L) corresponding to winMaxPeakSign, and sets peak signnflipflag to 1, where 0 < n < L, and 0 < m0＜M,0＜m1＜M,m1＝m0+1；m0、m1The same meaning as m as above means the number of OFDM symbol, m1Number of current OFDM symbol, m0A sequence number representing a previous OFDM symbol; the peak sign inversion flag peakSignFleplag is used to indicate the current sequence C1(n+m1L) with the preceding sequence C1(n+m0L), and if the winMaxPeakSign values of the two are the same, the peaksignnflipflag is 0, otherwise the peaksignnflipflag is 1; the initial value of peakSignFllag is set to 0;
step 2-2: output result E to energy detection window P1(n) performing delay correlation accumulation processing calculation by the following method:
where M is the number of complete and valid preamble OFDM symbols in the current received frame;
the third step: including accumulating the processed results C according to the delay correlation of two sliding windows at a time2(n) and E2(n) and local preamble symbol power value EsAnd the number N of the complete OFDM symbols of the leader sequence in a frame, and calculating a frame timing synchronization judgment value Q (N), wherein the formula is as follows:
each time means that each sampling point needs to be calculated, and the interval between adjacent times is a sampling time interval;
Esfor the power value of the local preamble symbol sequence,
the symbol sequence s (k) of the local preamble sequence is a known fixed sequence; n is the number of leading OFDM symbols stipulated in the OFDM-based communication standard; m is the number of complete and effective preamble OFDM symbols actually received by a receiver system frame timing synchronization module, and M is less than or equal to N;
the fourth step: comparing the frame timing synchronization judgment value Q (n) with a preset peak threshold T, and performing peak sidelobe search in a preset search window, wherein the length of the peak search window is set as LsPoint;
the fourth step comprises the following steps:
step 4-1: comparing the frame timing synchronization judgment value Q (n) with a preset peak value threshold T, and recording the position tempPos of the current sampling point and a corresponding detection value tempPeak when Q (n) is equal to or more than T, wherein the tempPeak is Q (n);
step 4-2: searching peak side lobes; 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 there is already a valid peak, the tempPos of the current point is compared with the stored previous onePeakPos [ peakNum-1 ] position of peak point]Comparing: when tempPos-peakPos [ peakNum-1 ]]＜LsIf tempPeak > peakValue [ peakNum-1 ]]Then, peakPos [ peakNum-1 ] is set]＝tempPos，peakValue[peakNum-1]The peak count peakNum value is unchanged; 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 issFor peak sidelobe search window size, Ls<L; l is the number of sampling points of a leading OFDM symbol, and the significance of the number of sampling points is the same as the length of the symbol;
the fifth step: comparing the peak value search result with a preset frame timing synchronization judgment condition, and outputting a frame timing synchronization state indication: if the peak value searching result meets the preset frame timing synchronization judgment condition, judging that the frame timing synchronization is successful, and outputting a signal output success state indication in the frame timing synchronization state; otherwise, outputting a failure state indication by the frame timing synchronization state output signal.
2. The method of claim 1, wherein the frame timing synchronization decision process in the fifth step is: when the peak sign inversion mark peakSignFlipflag is detected to be 1 and the peak count peakNum is not 0, the frame timing synchronization is judged to be successful, and a frame timing synchronization state output signal outputs a synchronization success state indication.
3. The method of claim 1, wherein the frame timing synchronization decision process in the fifth step is: when the effective peak value exists, the peak sign reversal mark peaksignfllag is 0 and no new effective peak value is detected after the L point is exceeded, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication.
4. The method of claim 1, wherein the frame timing synchronization decision process in the fifth step is: if the current peak count peakNum reaches a preset expected value, judging that the frame timing synchronization is successful, and outputting a synchronization success state indication by a frame timing synchronization state output signal; assuming that the number of symbols of the forward preamble SYNCP is X, a preset expected value of the peak count may be set to X + 1.
5. An apparatus for frame timing synchronization in an OFDM communication system, the apparatus comprising: a correlation detection module, a delay accumulation module, a peak search module and a synchronization decision module, wherein,
the correlation detection module is used for calculating a sliding cross-correlation value C of the received preamble symbol and the local storage sequence1(n) energy value E in sliding window with received preamble symbol1(n)；
The delay accumulation module is used for calculating the sliding cross-correlation value C1(n) delayed correlation accumulation result C2(n) and calculating the sliding window received signal energy value E1(n) delayed correlation accumulation processing result E2(n)；
The peak searching module is used for accumulating a processing result C according to the delay correlation2(n) and E2(n) locally storing the power value E of the symbol sequencesCarrying out peak judgment and peak sidelobe search on the total number N of complete OFDM symbols of a leader sequence in a frame and a preset peak threshold value T;
the synchronization decision module is configured to compare the peak search result with a preset frame timing synchronization decision condition, and output a frame timing synchronization status indication: if the peak value searching result meets the preset frame timing synchronization judgment condition, judging that the frame timing synchronization is successful, and outputting a signal output success state indication in the frame timing synchronization state; otherwise, outputting a signal output failure state indication in a frame timing synchronization state;
the correlation detection module is specifically configured to pass a received baseband digital signal sequence r through a correlation detection window C and an energy detection window P, where the correlation detection window C calculates a cross-correlation value between the received baseband digital signal sequence r and a locally stored signature sequence s, and the calculation method is as follows:
the energy detection window P calculates the energy value of the received baseband digital signal sequence r in the window P, and the calculation method is as follows:
wherein, L is the length of the correlation detection window C and the energy detection window P, and is the number of sampling points of a leading OFDM symbol, n is more than 0 and less than L, M is more than 0 and less than M, and M is the number of complete and effective leading OFDM symbols in the current receiving frame;
the delay accumulation module specifically includes:
and calculating the delay correlation accumulation processing of the sliding cross correlation value, wherein the method comprises the following steps:
wherein winmaxParkSign is the current C input to the delay-dependent accumulation window1The sign of the maximum value of the (n + m.L) sequence, i.e., when
Then winMaxPeakSign is-1; when in use
Then winMaxPeakSign is 1; if the peak sign flip flag peakSignFlipflag is 0 and the current sequence C is1(n+m1L) corresponding winMaxParkSign value to the preceding sequence C1(n+m0L) if the winMaxPeakSign value corresponding to the target is opposite, setting peak signnflipflag to 1; here, 0 < n < L, 0 < m0＜M,0＜m1＜M,m1＝m0+1, peak sign flip flag peakSignFlilpFlag is used to indicate the current sequence C1(n+m1L) with the preceding sequence C1(n+m0L), and if the winMaxPeakSign values of the two are the same, the peaksignnflipflag is 0, otherwise the peaksignnflipflag is 1; the initial value of peaksignfllag is set to 0);
and calculating the delay correlation accumulation processing of the sliding window energy value, wherein the method comprises the following steps:
where M is the number of complete and valid preamble OFDM symbols in the current received frame;
the peak search module specifically comprises a peak decision module and a side lobe search module, wherein,
the peak value judging module firstly calculates a frame timing synchronization judging value, and the formula is as follows:
wherein E issFor the power value corresponding to the local preamble symbol sequence,
the symbol sequence s (k) of the native preamble sequence is a known fixed sequence, thus EsCan be calculated and stored in advance; n is the number of OFDM symbols with complete leader sequence in a frame;
comparing the frame timing 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 the frame timing synchronization judgment value Q (n) is equal to or more than T, wherein the tempPeak is Q (n);
and the side lobe searching module carries out peak side lobe searching: 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 ]]Comparing; when tempPos-peakPos [ peakNum-1 ]]＜LsIf tempPeak > peakValue [ peakNum-1 ]]Then, peakPos [ peakNum-1 ] is set]＝tempPos，peakValue[peakNum-1]The peak count peakNum value 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 accumulated by 1; wherein L issThe size of the peak side lobe search window can be determined by simulation; l is the number of sampling points of a leading OFDM symbol;
the input of the synchronization judgment module is a peak value search output result, and the output is a frame timing synchronization state output signal, when a peak value symbol turning mark peakSignFlipflag is detected to be 1 and a peak value count peakNum is not 0, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication;
the input of the synchronization judgment module is a peak value search output result, the output is a frame timing synchronization state output signal, when an effective peak value exists, a peak value symbol turning mark peaksignfllipflag is 0 and a new effective peak value is not detected any more when the effective peak value exceeds an L point, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication;
the input of the synchronization judgment module is a peak value search output result, and the output is a frame timing synchronization state output signal, if the current peak value count peakNum reaches an expected value, the frame timing synchronization is judged to be successful, and the frame timing synchronization state output signal outputs a synchronization success state indication.
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