CN101577580B - Frame synchronization method - Google Patents
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Abstract
The invention belongs to the technical field of digital communication, and in particular relates to a frame synchronization method adopting cross-correlation operation. The frame synchronization method based on sectional cross-correlation provided by the invention increases several arithmetic units used for solving squares of signal modulus; when large carrier frequency deviation exists, a path ofcorrelator is only needed as usual, and a plurality paths of cross-correlators are not needed for the cross-correlation operation on different frequency points; and moreover, after completion of the estimation and compensation on the carrier frequency deviation, the method still can utilize the same arithmetic units for frame synchronization, and accurately estimate a frame start position. Therefore, the sectional cross-correlation method provided by the invention greatly saves the consumption of hardware resources, and improves the utilization rate of the hardware resources.
Description
The technical field is as follows:
the invention belongs to the technical field of digital communication, and particularly relates to a frame synchronization method adopting cross-correlation operation, which can accurately estimate a frame starting position under the condition that a large carrier frequency deviation exists in a received signal.
Background art:
in a burst-mode communication system, since a receiving end cannot accurately predict a transmission time of each physical layer frame, it is necessary to estimate a start time of a received physical layer frame using partial information in a received signal. Most communication systems use correlation methods to perform cross-correlation or auto-correlation on a specific sequence in a received signal, and determine the starting time of a physical layer frame according to the position of a correlation peak. For example, in IEEE 802.11(Wireless Local Area Network, WLAN), short training symbols in the preamble sequence can be used for frame synchronization by correlation. When the correlation method is used for synchronization, a threshold is usually preset, and if the correlation result exceeds the threshold, the starting position of the current correlation window is considered as the starting position of the physical layer frame.
The self-correlation method is adopted for frame synchronization, and the influence of interference such as carrier frequency deviation on the correlation result can be effectively overcome. However, the autocorrelation curve of the frame synchronization sequence varies linearly with the sliding of the correlation window position, and the magnitude of the correlation value near the correlation peak is very close to the correlation peak. In the presence of various random noises, it is difficult to accurately estimate the starting time of the physical layer frame.
Compared with the autocorrelation method, the cross-correlation curve of the received frame synchronization sequence and the local sequence obtained by the cross-correlation method has the advantage of prominent correlation peak. When the initial position of the correlation window slides to the initial position of the frame synchronization sequence, a larger correlation value can be obtained; when the correlation window is slid to other positions, the obtained correlation value is far smaller than the correlation peak value. Therefore, the cross-correlation method is adopted for frame synchronization, so that the influence of various random noises can be effectively overcome, and the starting time of the physical layer frame can be estimated more accurately.
If the cross-correlation method is adopted for frame synchronization, a corresponding method needs to be designed to overcome the interference of carrier frequency deviation. When the receiving end initially accesses to the network, since the estimation of the parameters such as the carrier frequency deviation is not completed at this time, the frame synchronization needs to be performed under the condition that the received signal has a large carrier frequency deviation. Carrier frequency offset causes a deviation in the phase of the received signal that increases linearly with the number of samples. When the carrier frequency deviation is large, the variation range of the phase deviation caused by the carrier frequency deviation in the frame synchronization sequence can approach to or even exceed pi, so that the cross-correlation peak value of the received frame synchronization sequence and the local sequence is greatly reduced. Therefore, when the carrier frequency deviation is not compensated, it is difficult to set a threshold in advance to determine whether or not the start position of the physical layer frame is found, because the cross correlation peak value varies greatly with the magnitude of the carrier frequency deviation. The current common method is to preset the variation range of carrier frequency deviation, the receiving end performs cross-correlation operation on the received signals at different frequency points within the frequency range according to a certain step pitch, and selects the maximum correlation value in each frequency point to compare and judge with the threshold value, thereby realizing frame synchronization. However, this searching method needs to implement multiple sliding correlators, and only one sliding correlator needs to be reserved for frame synchronization after the estimation and compensation of carrier frequency offset are completed, which results in higher consumption of hardware resources and lower utilization rate.
The invention content is as follows:
the invention aims to provide a frame synchronization method based on cross correlation, which can accurately estimate the frame starting position under the condition of large carrier frequency deviation.
The purpose of the invention is realized by the following technical scheme:
a frame synchronization method based on cross correlation comprises the following steps:
a) the method comprises the steps that a sending end adds a frame synchronization sequence with the length of N into a frame header of a physical layer frame, and an autocorrelation curve of the sequence has the characteristic of prominent peak value; such as an m-sequence;
b) the receiving end sets a threshold value for judging whether the frame starting position is detected;
c) presetting the maximum carrier frequency deviation delta f existing in the systemmaxAccording to Δ fmaxDetermining the length Lmax(ii) a If the length of frame synchronization sequence exceeds LmaxWhen the carrier frequency deviation is Δ fmaxThe variation range of the phase deviation caused in the sequence exceeds thetamax；
According to LmaxDetermining that a condition L is satisfiedb≤LmaxSegment length L ofb(ii) a Taking a frame synchronization sequence used by a sending end as a local sequence, and dividing the local sequence into K segments; wherein K is not less than N/LbThe smallest integer of (a);
d) the receiving end performs the segmental cross correlation on the received signal sampling point sequence and the local sequence: dividing the received signal sampling point sequence in the correlation window into K sections, performing cross correlation on each section with a corresponding local sequence section, and accumulating correlation results of all the sections; wherein the correlation window is a general term referring to the range of all signal samples currently participating in the correlation operation;
e) the receiving end, energy normalization is carried out on the segmented cross-correlation result obtained in the step d): dividing the correlation result by the total energy of all received signal samples within the correlation window;
f) the receiving end compares and detects the energy normalization subsection cross-correlation result obtained in the step e) with the threshold set in the step b): if the energy normalization correlation result exceeds a set threshold, the initial position of the correlation window is the frame synchronization initial position; otherwise, continuously sliding the correlation window point by point, and performing segment cross-correlation operation until finding the frame synchronization initial position.
Further, in the above step c), the length L is determinedmaxThe method comprises the following steps:
let the signal bandwidth be fbandWhen the carrier frequency deviation is Δ fmaxThe phase deviation caused in the frame synchronization sequence with length L is 2 pi · Δ fmax·L/fbandAnd theta is less than or equal to thetamaxThen 2 π Δ fmax·L/fband≤θmaxThe following can be obtained:
Further, in the step c), the method for determining the number of segments K is as follows:
after segmenting the frame synchronization sequence, it is necessary to make the phase deviation caused by the carrier frequency deviation within each segment of sub-sequence not exceed θmaxTherefore, the number of segments K must satisfy K ≧ N/Lmax(ii) a In addition, segmenting the frame sync sequence destroys the correlation properties of the sequence, and the more segments, the worse the correlation properties of the sequence. Therefore, L can be made to beb＝LmaxAnd K is taken
Represents the smallest integer no less than the value in brackets.
Further, in the step d), a specific method for performing the segment cross-correlation between the received signal sample point sequence and the local sequence is as follows:
let the local sequence (i.e. the original frame synchronization sequence sent by the sending end) be St＝{st(1)，st(2)，…，st(N), the signal sample sequence in the receiving end correlation window is Rt＝{rt(1)，rt(2)，…，rt(N), where N is the length of the frame synchronization sequence, the correlation window length at the receiving end is also N. The segment length determined by step b) above is LbThen the number of segments isThe cross-correlation value of each segment is
…
The squares of the module values of the cross-correlation values of the segments are added to obtain the square of the module values of the cross-correlation values of the segments
|corr|2＝|corr(1)|2+|corr(2)|2+…+|corr(K)|2
For computational convenience, the square | corr |, of the modulus of the piecewise cross-correlation values may be directly computed2As a decision quantity, a comparison is made with the corresponding threshold value, thereby omitting the evolution.
The number of the segments divided by the signal sample point sequence is the same as that of the segments divided by the local sequence.
Further, in the step e), the energy normalization is to divide the cross-correlation result by the total energy of all the received signal samples in the correlation window, i.e. the energy-normalized segmented cross-correlation result is Dcorr＝|corr|2/Pr(ii) a Wherein,
is the total energy of the received signal sample sequence within the correlation window.
The invention has the technical effects that:
the frame synchronization method based on the piecewise cross correlation provided by the invention can still accurately estimate the initial position of the physical layer frame under the condition of large carrier frequency deviation. Compared with the conventional common method, the method only needs to realize one sliding correlator, thereby saving hardware resources and improving the utilization rate of the hardware resources.
Description of the drawings:
FIG. 1 is a flow chart of a prior art digital communication system;
fig. 2 is a block diagram of a frame synchronization module structure based on a cross-correlation method, where:
‘Z-1' denotes a register unit; '×' denotes a multiplier;
'+' denotes an adder; '/' denotes a divider;
‘|·|2' means an arithmetic unit for squaring a modulus value; ' > T? ' denotes a logic unit that compares with a threshold;
'Gate' indicates a Gate switch, and if the correlation result is greater than a threshold value, a signal is allowed to be output;
FIG. 3 is a block diagram of a frame synchronization module based on a cross-correlation method;
fig. 4 is a block diagram of a frame synchronization module structure based on a segment cross-correlation method, in which:
‘Z-1' denotes a register unit; '×' denotes a multiplier;
'+' denotes an adder; '/' denotes a divider;
‘|·|2' means an arithmetic unit for squaring a modulus value; ' > T? ' denotes a logic unit that compares with a threshold;
'Gate' indicates a Gate switch, and if the correlation result is greater than a threshold value, a signal is allowed to be output;
FIG. 5 illustrates the energy normalized correlation result without carrier frequency offset in the prior art;
FIG. 6 illustrates the energy normalized correlation results for the case of large carrier frequency offset in the prior art;
FIG. 7 shows the correlation results without energy normalization in the presence of large carrier frequency offset in the prior art;
FIG. 8 illustrates the energy normalized correlation results in the absence of carrier frequency offset according to the present invention;
fig. 9 shows the energy normalized correlation results in the presence of large carrier frequency offset according to the present invention.
The specific implementation mode is as follows:
the frame synchronization method based on the piecewise cross-correlation proposed by the present invention is described in detail below with reference to the accompanying drawings, but the present invention is not limited thereto.
The components of a prior art digital communication system are shown in fig. 1. The sending end, the data source module produces the binary data stream; the digital modulation module modulates the binary data stream to obtain a data symbol stream ds(n); frame synchronization sequence insertion module in ds(n) Pre-insertion frame synchronization sequence st(n) obtaining a transmission signal s (n). At the receiving end, the frame synchronization module detects the starting position of the physical layer frame by using the frame synchronization sequence in the received signal, thereby confirming whether the current received signal is the data symbol stream dr(n); digital demodulation module pair drAnd (n) demodulating to obtain a binary data stream and outputting the binary data stream.
The structure of the frame synchronization module based on the cross-correlation method in the existing system without considering the carrier frequency deviation is shown in fig. 2. In order to overcome the influence of large carrier frequency deviation on frame synchronization, a common frame synchronization module in the existing system is shown in fig. 3. The frame synchronization steps are as follows:
1. according to the maximum carrier frequency deviation Deltaf occurring in the systemmaxDetermining the step size Deltaf of the frequency sweep and the sweep range [ -Mafaf, Mafaf](ii) a Wherein M delta f is more than or equal to delta fmax：
2. The frame synchronization module modulates the received signal to each scanning frequency point, which must have one path modulatedThe residual carrier frequency deviation of the signal does not exceed Δ f, so that the phase deviation caused by the residual carrier frequency deviation in the sequence of signal samples within the correlation window does not vary by more than 2 π Δ f N/fband. Setting delta f can ensure that one path in the multi-path cross-correlation result in the figure 3 is less influenced by the carrier frequency deviation, and the correlation peak value is equivalent to that when the carrier frequency deviation does not exist;
3. in a cross correlator, the square of the module value of the cross correlation value of the received signal sample point sequence and the local sequence in a correlation window is obtained, and the square is divided by the total energy of the received signal sample point sequence in the correlation window to carry out normalization;
4. in the comparator, comparing the normalized cross-correlation results output by each path of cross-correlator, and selecting the maximum value as the final cross-correlation result;
5. in the detector, the cross-correlation result output by the comparator is compared with a threshold: if the value is larger than the threshold value, the initial position of the relevant window at the moment is the frame synchronization initial position; otherwise, continuously sliding the correlation window point by point, and performing segment cross-correlation operation until finding the frame synchronization initial position.
In fig. 3, each cross correlator has a structure similar to that of the frame synchronization module shown in fig. 2, except that the correlation result does not need to be compared with a threshold. Therefore, the hardware resource consumption of the frame synchronization module shown in fig. 3 is about 2M times that of the frame synchronization module shown in fig. 2. In addition, after the estimation and correction of the carrier frequency deviation are completed, the frequency scanning of the received signal is not needed during the frame synchronization, and only one cross correlator is actually needed to be reserved, so that other 2M-1 cross correlators are idle, and the utilization rate of hardware resources is low.
The invention improves the frame synchronization module in the existing system, and the structural block diagram of the improved frame synchronization module is shown in fig. 4. The frame synchronization steps of the present invention are as follows:
1. according to the maximum carrier frequency deviation delta f in the systemmaxDetermining to divide a frame sync sequenceSubsequence length L of each segment after segmentbAnd a number of segments K;
2. within the correlation window, a subsequence of received signal samples and a local subsequence { s } in each segmentt_1，st_2，…，st_KPerforming cross-correlation operation respectively, and solving the square of the modulus of the obtained cross-correlation value to obtain the cross-correlation result of each segment;
3. adding the cross-correlation results of all the segments to obtain a segment cross-correlation result of the received signal sample sequence and the local sequence in the correlation window;
4. performing energy normalization on the segmented cross-correlation result obtained in the step 3, namely dividing the energy normalization by the total energy of the received signal sample sequence in the correlation window;
5. comparing the energy normalization piecewise cross-correlation result obtained in the step 4 with a threshold value: if the value is larger than the threshold value, the initial position of the relevant window at the moment is the frame synchronization initial position; otherwise, continuously sliding the correlation window point by point, and performing segment cross-correlation operation until finding the frame synchronization initial position.
Compared with the existing frame synchronization module based on the cross-correlation method, the frame synchronization method based on the piecewise cross-correlation provided by the invention only adds some operation units for solving the square of the signal modulus. When there is a large carrier frequency deviation, only one correlator is still needed, and the frame synchronization module in fig. 3 does not need to perform cross correlation operation on different frequency points by using multiple cross correlators. Also, after the estimation and compensation of the carrier frequency offset are completed, the frame synchronization may be performed by using the frame synchronization module shown in fig. 4 as well. Therefore, the segmental cross-correlation method provided by the invention greatly saves the consumption of hardware resources and improves the utilization rate of the hardware resources.
Two embodiments are listed below to illustrate the influence of a large carrier frequency offset on the cross-correlation result, and the frame synchronization capability of the segmented autocorrelation method proposed by the present invention in the presence of a large carrier frequency offset.
Example 1
Let m-sequence of length 63 be used as frame synchronization sequence, whose generator polynomial is p (z) ═ z6+z5+1. Meanwhile, let the signal-to-noise ratio in the channel be 20 dB. In the absence of carrier frequency deviation, the cross-correlation curve of the frame synchronization module shown in fig. 2 around the frame start position is shown in fig. 5; in the presence of a size fbandThe cross-correlation curve of the frame synchronization module shown in fig. 2 around the start position of the frame is shown in fig. 6 for carrier frequency deviation of/64.
Comparing the peaks of the cross-correlation curves in fig. 5 and 6, it can be seen that the magnitude f is present compared to the case where there is no carrier frequency deviationbandIn the case of carrier frequency deviation of/64, accurate frame synchronization cannot be achieved by the existing cross-correlation method. It should be noted that the absence of the correlation peak in fig. 6 is due to the energy normalization of the correlation result, and fig. 7 is the cross-correlation result without energy normalization.
Example 2
In example 1, let θmaxPi/2, having:
is provided with Lb＝Lmax16, the number of segments
The subsequences of the four segments are 16, 16 and 15 in length, respectively. The cross-correlation value is reduced by the piecewise cross-correlation, but the initial state of the m sequence is reasonably set, so that the influence can be almost ignored. In this exampleLet each subsequence be st_1＝{1，-1，-1，-1，-1，1，1，-1，-1，-1，1，-1，1，-1，-1，1}，st_2＝{1，1，1，-1，1，-1，-1，-1，1，1，1，-1，-1，1，-1，-1}，st_3＝{1，-1，1，1，-1，1，1，1，-1，1，1，-1，-1，1，1，-1}，st_4＝{1，-1，1，-1，1，1，1，1，1，1，-1，-1，-1，-1，-1}。
In the absence of carrier frequency deviation, the cross-correlation curve of the frame synchronization module shown in fig. 4 around the frame start position is shown in fig. 8; exists with a size of fbandThe cross-correlation curve of the frame synchronization module shown in fig. 4 around the start position of the frame is shown in fig. 9 for carrier frequency deviation of/64. As can be seen from fig. 8, under the condition that there is no carrier frequency offset, the difference between the result of the cross-correlation by segmentation and the result of the cross-correlation in fig. 5 is not large, which indicates that the initial state of the frame synchronization sequence is selected reasonably, and the influence of the cross-correlation by segmentation on the correlation result can be effectively reduced. As can be seen from fig. 9, in the case of a large carrier frequency deviation, the frame synchronization method based on the piecewise cross-correlation can overcome the influence of the carrier frequency deviation on the cross-correlation result, and can still accurately estimate the frame start position.
Although specific embodiments of the invention have been disclosed for illustrative purposes and the accompanying drawings, which are included to provide a further understanding of the invention and are incorporated by reference, those skilled in the art will appreciate that: various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. The invention should not be limited to the preferred embodiments and drawings disclosed herein, but rather should be defined only by the scope of the appended claims.
Claims (5)
1. A frame synchronization method based on cross correlation comprises the following steps:
a) adding a frame synchronization sequence with the length of N into a frame header of a current physical layer frame at a sending end, wherein an autocorrelation curve of the sequence has the characteristic of prominent peak value;
b) the receiving end sets a threshold value for judging whether the frame synchronization position is detected;
c) the receiving end, according to the maximum carrier frequency deviation delta f existing in the systemmaxDetermining the length Lmax(ii) a From LmaxDetermining a satisfaction of Lb≤LmaxSegment length L ofb(ii) a Using the frame synchronization sequence of step a) as a local sequence according to LbDividing a local sequence into K sections;
d) at a receiving end, dividing a received signal sampling point sequence in a correlation window into K sections, and performing cross correlation on each section with a corresponding local sequence section respectively and accumulating cross correlation results of all the sections;
e) the receiving end is used for carrying out energy normalization on the segmented cross-correlation result;
f) and the receiving end compares and detects the energy normalization segmentation cross-correlation result with the set threshold value: if the energy normalization correlation result exceeds a set threshold, the initial position of the correlation window at the time is used as the frame synchronization initial position; otherwise, continuing sliding the relevant window point by point, and returning to the step d) until finding the frame synchronization starting position, thereby completing the frame synchronization.
2. The frame synchronization method based on cross-correlation as claimed in claim 1, wherein the maximum carrier frequency deviation Δ f existing in the system is set before step a)max。
3. A method for frame synchronization based on cross-correlation as claimed in claim 1 or 2, wherein L) in step c) ismaxThe determination method of (2) is as follows:
when the carrier frequency deviation is Δ fmaxThe amplitude of change of the phase shift caused in the frame synchronization sequence of length N is 2 pi · Δ fmax·N/fbandThen there is
Wherein theta is less than or equal to thetamax，
Represents the largest integer not exceeding the value in brackets; f. ofbandFor signal bandwidth,. DELTA.fmaxIs the maximum carrier frequency offset present in the system.
4. The frame synchronization method based on cross-correlation as claimed in claim 1, wherein K in step c) is not less than N/LmaxIs the smallest integer of (a).
5. The frame synchronization method based on cross-correlation as claimed in claim 1, wherein the number of the segments divided by the signal sample point sequence in the step d) is the same as the number of the segments divided by the local sequence.
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| CN110311747B (en) * | 2019-07-16 | 2020-09-01 | 京信通信系统（中国）有限公司 | Anti-large frequency offset frame synchronization method, device and system |
| CN111541515B (en) * | 2020-04-15 | 2023-02-28 | 北京中宸泓昌科技有限公司 | Burst frame header detection method and system based on cyclic sequence type lead code |
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