阅读说明：本技术 一种短波通信极低信噪比下同步头捕获方法 (Method for capturing synchronization head under extremely low signal-to-noise ratio of short wave communication ) 是由 张凯 陈测库 李子墨 田杰 王小军 仇妙月 于 2020-06-30 设计创作，主要内容包括：本发明属于短波通信技术领域,公开了一种短波通信极低信噪比下同步头捕获方法,包括以下步骤：对接收序列以任意采样点进行截取,对截取的接收序列和本地序列分别进行等长的分段；将每一段接收序列与对应的一段本地序列进行相关并作FFT,确定FFT序列的幅值最大值,从而得到Q段幅值最大值；对Q段幅值最大值进行求和,将求和后的值作为截取的接收序列的峰值；遍历接收序列的每个采样点,得到每个采样点对应的峰值,从而得到峰值曲线,选择峰值曲线中峰值最大值对应的采样点作为整个接收序列的同步起点；该同步头捕获方法能准确的找到同步点,还能掌握收发两段频率偏移量的变化情况,能够在极低信噪比、大频偏情况下实现同步捕获。(The invention belongs to the technical field of short-wave communication, and discloses a method for capturing a synchronization head under an extremely low signal-to-noise ratio of short-wave communication, which comprises the following steps: intercepting a receiving sequence by using any sampling point, and respectively carrying out equal-length segmentation on the intercepted receiving sequence and a local sequence; correlating each segment of receiving sequence with a corresponding segment of local sequence and performing FFT (fast Fourier transform), and determining the maximum value of the amplitude of the FFT sequence so as to obtain the maximum value of the amplitude of the Q segment; summing the maximum value of the amplitude of the Q section, and taking the summed value as the peak value of the intercepted receiving sequence; traversing each sampling point of the receiving sequence to obtain a peak value corresponding to each sampling point so as to obtain a peak value curve, and selecting the sampling point corresponding to the maximum value of the peak value in the peak value curve as a synchronous starting point of the whole receiving sequence; the synchronous head capturing method can accurately find out a synchronous point, can master the change condition of the frequency offset of the receiving section and the transmitting section, and can realize synchronous capturing under the conditions of extremely low signal-to-noise ratio and large frequency offset.)

1. A synchronization head capturing method under a very low signal-to-noise ratio of short wave communication is characterized by comprising the following steps:

step 1, a synchronization information sequence to be sent of a transmitting end is modulated by binary phase shift keying to obtain a sending sequence X, the sending sequence X is transmitted by a Gaussian channel, and a receiving sequence R is obtained at a receiving end due to the fact that carrier frequencies of the transmitting end and the receiving end are not identical and a frequency error f (t) changing along with time exists; the sending sequence X is called a synchronization sequence at a transmitting end, and is called a local sequence at a receiving end;

step 2, intercepting the receiving sequence R by using any sampling point to obtain an intercepted receiving sequence; respectively carrying out equal-length segmentation on the intercepted receiving sequence and the local sequence to obtain a Q-segment local sequence X (X ═ X)₁,…,X_q,…,X_Q) And Q segment receive sequence R ═ (R)₁,…,R_q,…,R_Q) (ii) a Wherein Q is the number of segments;

step 3, receiving the sequence R according to each segment in the Q segments_qWith a corresponding local sequence X_qAfter correlation, calculating a corresponding sequence after fast Fourier transform, and determining the maximum value of the amplitude in the sequence after fast Fourier transform so as to obtain the maximum value of the amplitude of a Q section;

step 4, summing the maximum value of the amplitude of the Q section, and taking the summed value as the peak value of the intercepted receiving sequence;

and 5, traversing each sampling point of the receiving sequence R, repeating the steps 2-4 to obtain a peak value corresponding to each sampling point so as to obtain a peak value curve, and selecting the sampling point corresponding to the maximum value of the peak value in the peak value curve as the synchronous starting point of the whole receiving sequence R.

2. The short-wave communication synchronization head acquisition method under extremely low signal-to-noise ratio according to claim 1, wherein the step 1 is specifically: the transmission sequence is X ═ X (X)₁,…,x_n,…,x_N)，x_n± 1; if the transmission sequence is transmitted via a gaussian channel, then the receiving sequence is obtained at the receiving end as R ═ R (R)₁,…,r_n,…,r_N) (ii) a Wherein:

wherein the content of the first and second substances,representing frequency offset versus transmitted symbol x_nThe influence of (a);

3. The short wave communication synchronization head acquisition method under extremely low signal-to-noise ratio according to claim 1, wherein the step 3 comprises the following sub-steps:

substep 3.1, let index variable i equal to 1;

substep 3.2, obtaining the maximum value v of the amplitude value of the ith section based on the synchronous algorithm of fast Fourier transform_iAnd v_iCorresponding frequency offset f_i；

Substep 3.3, sequentially increasing the index variable i by 1, if i is less than or equal to Q, skipping to substep 3.2, otherwise, entering step 4;

after substeps 3.1, 3.2, the maximum value v of the amplitude of the Q section can be obtained₁,…,v_q,…v_Q。

4. The short-wave communication synchronization head acquisition method under extremely low signal-to-noise ratio according to claim 3, characterized in that in substep 3.2, the fast Fourier-based synchronization algorithm is specifically:

according to the received sequence R ═ (R)₁,…,r_n,…,r_N) And the local sequence X ═ X₁,…,x_n,…,x_N) Constructing quasi-sinusoidal signal sequences

Aligning the quasi-sinusoidal signal sequencePerforming N-point fast Fourier transform to obtain a sequence after fast Fourier transform, and searching an amplitude maximum value and a frequency offset corresponding to the amplitude maximum value from the sequence after fast Fourier transform; solving the maximum value v of the amplitude of the Q section in the same way₁,…,v_q,…v_QAnd corresponding frequency deviation f₁,…,f_q,…,f_Q。

5. The acquisition method of the short-wave communication synchronization head under the condition of extremely low signal-to-noise ratio according to claim 4, wherein in the step 4, the summation formula is as follows:

Technical Field

The invention relates to the technical field of short-wave communication, in particular to a synchronization head capturing method under an extremely low signal-to-noise ratio of short-wave communication.

Background

Short-wave communication refers to a radio communication technology with the wavelength of 10 meters to 100 meters and the frequency range of 3MHz to 30 MHz. The electric wave transmitted by short wave communication can reach the receiving end only by the reflection of the ionized layer, and the communication distance is long, which is the main means of remote communication. Despite the continuous emergence of new radio communication systems, the ancient and traditional communication method of short-wave communication is still receiving universal attention all over the world, and not only is it not eliminated, but also it is still developing rapidly. Because it has advantages not possessed by other communication systems: firstly, short wave is the only remote communication means which is not limited by network and relay, for example, in case of war or disaster, when satellite is attacked, the anti-destruction capability and autonomous communication capability of short wave are not comparable to those of other communication equipment; secondly, communication in remote areas such as mountainous areas, Gobi areas and oceans mainly depends on short waves; finally, the low communication costs also make shortwaves have a broad market.

In order to facilitate information (voice or image) transmission, a low-frequency signal carrying information is usually up-converted into a high-frequency signal at a transmitting end, and a high-frequency carrier is required in the process; through channel transmission, in order to extract useful information, a user needs to down-convert a received high-frequency signal to a low-frequency signal, and a high-frequency carrier with the same frequency as that of a transmitting end is needed in the process. However, due to factors such as manufacturing process, material and electrical characteristics of the components, carrier frequencies generated at the transmitting end and the receiving end cannot be completely the same, and an error always exists, which affects demodulation at the back end. The effect on the demodulator when the error is small is almost negligible; when the error is large, phase rotation occurs, which causes uncorrectable error, so that the performance of the communication system is rapidly deteriorated, even the synchronization head cannot be captured.

In order to perform subsequent operations such as equalization and demodulation on a signal, a receiver first needs to determine the starting position of useful information in a low-frequency signal (also referred to as synchronization estimation), and then can perform subsequent processing on the signal. However, in a poor communication environment and in the presence of frequency offset in both transmission and reception, synchronization acquisition becomes increasingly difficult.

The existing unequal error protection technical scheme is basically realized by adopting an autocorrelation algorithm and performing Fourier transform. For example, in the method and apparatus for signal synchronization disclosed in CN108270707A, a local differential sequence is used to perform correlation operation with a received differential sequence, and then Fast Fourier Transform (FFT) is performed, and frequency offset is calculated according to a correlation result to obtain a differential sequence in a received signal, so as to determine a synchronization position. In the signal synchronization method, apparatus, computer device, and storage medium of publication No. CN109818644A, the obtained signal is filtered by different filters to obtain a first correlation value and a second correlation value, then the maximum value of the first correlation value and the maximum value of the second correlation value are compared with a preset threshold, and when the comparison result is successfully matched, compensation synchronization is performed on the received signal. From the above, the prior art uses the good autocorrelation property of the sequence, and uses the correlation between the local sequence and the received signal to find the peak value and determine the synchronization start point in the received signal.

However, in the prior art, synchronization acquisition is mainly performed under the condition that no frequency offset or fixed frequency offset exists between the transmitting side and the receiving side, and is rarely performed in a harsh communication environment (extremely low signal-to-noise ratio) and under the condition that a variable frequency offset exists.

Disclosure of Invention

The method comprises the steps of segmenting a local synchronization sequence, correlating the segmented sequence in a received signal, summing correlation values of each segment, and finally taking the position with the maximum correlation value as a synchronization point.

In order to achieve the purpose, the invention is realized by adopting the following technical scheme.

A synchronization head capturing method under a very low signal-to-noise ratio of short wave communication comprises the following steps:

step 1, a synchronization information sequence to be sent of a transmitting end is modulated by binary phase shift keying to obtain a sending sequence X, the sending sequence X is transmitted by a Gaussian channel, and a receiving sequence R is obtained at a receiving end due to the fact that carrier frequencies of the transmitting end and the receiving end are not identical and a frequency error f (t) changing along with time exists; the sending sequence X is called a synchronization sequence at a transmitting end, and is called a local sequence at a receiving end;

step 2, intercepting the receiving sequence R by using any sampling point to obtain an intercepted receiving sequence; respectively carrying out equal-length segmentation on the intercepted receiving sequence and the local sequence to obtain a Q-segment local sequence X (X ═ X)₁,…,X_q,…,X_Q) And Q segment receive sequence R ═ (R)₁,…,R_q,…,R_Q) (ii) a Wherein Q is the number of segments;

step 3, receiving the sequence R according to each segment in the Q segments_qWith a corresponding local sequence X_qAfter correlation, calculating a corresponding sequence after fast Fourier transform, and determining the maximum value of the amplitude in the sequence after fast Fourier transform so as to obtain the maximum value of the amplitude of a Q section;

step 4, summing the maximum value of the amplitude of the Q section, and taking the summed value as the peak value of the intercepted receiving sequence;

and 5, traversing each sampling point of the receiving sequence R, repeating the steps 2-4 to obtain a peak value corresponding to each sampling point so as to obtain a peak value curve, and selecting the sampling point corresponding to the maximum value of the peak value in the peak value curve as the synchronous starting point of the whole receiving sequence R.

Further, step 1 specifically comprises: the transmission sequence is X ═ X (X)₁,…,x_n,…,x_N)，x_n± 1; if the transmission sequence is transmitted via a gaussian channel, then the receiving sequence is obtained at the receiving end as R ═ R (R)₁,…,r_n,…,r_N) (ii) a Wherein:

wherein the content of the first and second substances,

representing frequency offset versus transmitted symbol x_nThe influence of (a);indicating the magnitude of the additional phase of the nth symbol with a time interval of two symbolsSecond, R_symIs the symbol transmission rate; phi is a₀Is the initial phase of the sequence; i is an imaginary unit; w is a_nRepresenting noise versus transmitted symbol x_nInfluence of (a), w_nSubject to mean of 0 and variance of σ²Is normally distributed, two-dimensional noise sample values.

Further, step 3 comprises the following substeps:

substep 3.1, let index variable i equal to 1;

substep 3.2, obtaining the maximum value v of the amplitude value of the ith section based on the synchronous algorithm of fast Fourier transform_iAnd v_iCorresponding frequency offset f_i；

Substep 3.3, sequentially increasing the index variable i by 1, if i is less than or equal to Q, skipping to substep 3.2, otherwise, entering step 4;

after substeps 3.1, 3.2, the maximum value v of the amplitude of the Q section can be obtained₁,…,v_q,…v_Q。

Further, in the substep 3.2, the synchronization algorithm based on fast fourier transform specifically includes:

according to the received sequence R ═ (R)₁,…,r_n,…,r_N) And the local sequence X ═ X₁,…,x_n,…,x_N) Constructing quasi-sinusoidal signal sequences

Wherein

Aligning the quasi-sinusoidal signal sequencePerforming N-point fast Fourier transform to obtain a sequence after fast Fourier transform, and searching an amplitude maximum value and a frequency offset corresponding to the amplitude maximum value from the sequence after fast Fourier transform; solving the maximum value v of the amplitude of the Q section in the same way₁,…,v_q,…v_QAnd corresponding frequency deviation f₁,…,f_q,…,f_Q。

Further, in step 4, the summation formula is:

the summed value v_maxAs the peak of the truncated received sequence; constructing a frequency offset vector f ═ (f)₁,…,f_q,…,f_Q)。

Compared with the prior art, the invention has the beneficial effects that:

the invention aims to solve the problem that the synchronization head is difficult to capture in the extremely severe environment of short-wave communication and under the condition that frequency offsets exist at the receiving end and the transmitting end, and the frequency offsets of the receiving end and the transmitting end change along with time due to the relative motion of the two parties and the severe communication environment in the communication process, so that the search of the synchronization head in a received signal becomes increasingly difficult; after finding the information starting point, the receiving party can perform subsequent operations such as equalization, demodulation and the like on the received information. The method comprises the steps of segmenting a local synchronization sequence, correlating the segmented sequence in a received signal, performing fast Fourier transform to obtain each segment of correlation value (namely the maximum value of the amplitude), summing each segment of correlation value, and taking the position with the maximum correlation value after summation as a synchronization point; the invention not only finds out the synchronization point, but also can master the change condition of the frequency offset of the receiving and transmitting sections, can realize synchronous capture under the conditions of extremely low signal-to-noise ratio and large frequency offset, has simple method and is beneficial to engineering realization.

Drawings

The invention is described in further detail below with reference to the figures and specific embodiments.

FIG. 1a is a schematic diagram of changes in frequency offset and sign-added phase when both sides of a transceiver move at a relatively constant speed;

FIG. 1b is a schematic diagram of time-frequency offset and symbol-added phase change of relative uniform acceleration motion between a transmitter and a receiver;

FIG. 2 is a schematic diagram of the correlation and FFT of the truncated different signals with the local sequence;

FIG. 3 is a diagram of correlation and FFT results of signals intercepted at different starting points and a local sequence; wherein, the graph (a) is a graph of correlation and FFT result between the signal intercepted by taking the point A as the starting point and the local sequence; the graph (B) is a graph of correlation and FFT results of signals intercepted by taking the point B as a starting point and a local sequence;

FIG. 4 is a graph of a peak value of a synchronization operation performed across the entire received signal;

fig. 5 is a schematic diagram of the step acquisition peak.

Detailed Description

Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by those skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention.

A synchronization head capturing method under a very low signal-to-noise ratio of short wave communication comprises the following steps:

step 1, a synchronization information sequence to be sent of a transmitting end is modulated by binary phase shift keying to obtain a sending sequence, the sending sequence is transmitted by a Gaussian channel, and a receiving sequence is obtained at a receiving end due to the fact that carrier frequencies of the transmitting end and the receiving end are not identical and a frequency error f (t) changing along with time exists.

Specifically, the step 1 is as follows: assume that a synchronization information sequence to be transmitted is subjected to Binary Phase Shift Keying (BPSK) modulation to obtain a transmission sequence X ═ X₁,…,x_n,…,x_N)，x_n± 1. The symbol transmission rate (symbol rate) is R_symOne symbol/second with a two symbol time interval of

And second. The transmission is carried out through a Gaussian channel, and because carrier frequencies at the transmitting end and the receiving end are not identical, a frequency error f (t) which changes along with time exists. Then at the receiving end the receive sequence is R ═ (R)₁,…,r_n,…,r_N)。

Wherein:representing frequency offset versus transmitted symbol x_nThe influence of (2) is a multiplicative interference, which is a main factor of phase rotation generated by symbols,

indicating the magnitude of the phase added by the nth symbol; phi is a₀Is the initial phase of the sequence; i is a unit of an imaginary number,w_nrepresenting noise versus transmitted symbol x_nThe influence of (2) is an additive interference. w is a_nSubject to mean of 0 and variance of σ²Is normally distributed, two-dimensional noise sample values. For convenience of description, the sequence X is set to (X) at the transmitting end₁,…,x_n,…,x_N) Called sync sequence, the sequence X is (X) at the receiving end₁,…,x_n,…,x_N) Called local sequence。

Step 2, intercepting the receiving sequence by using any sampling point to obtain an intercepted receiving sequence; respectively carrying out equal-length segmentation on the intercepted receiving sequence and the local sequence to obtain a Q-segment local sequence X (X ═ X)₁,…,X_q,…,X_Q) And Q segment receive sequence R ═ (R)₁,…,R_q,…,R_Q) (ii) a Where Q is the number of segments.

Step 3, receiving the sequence R according to each segment in the Q segments_qWith a corresponding local sequence X_qAnd calculating a corresponding fast Fourier transformed sequence after correlation, and determining the maximum value of the amplitude in the fast Fourier transformed sequence so as to obtain the maximum value of the amplitude of the Q section.

Specifically, step 3 comprises the following substeps:

substep 3.1, let index variable i equal to 1;

substep 3.2, obtaining the maximum value v of the amplitude value of the ith section based on the synchronous algorithm of fast Fourier transform_iAnd v_iCorresponding frequency offset f_i(ii) a The number of points of the fast Fourier transform FFT is N;

substep 3.3, sequentially increasing the index variable i by 1, if i is less than or equal to Q, skipping to substep 3.2, otherwise, entering step 4;

after substeps 3.1, 3.2, the maximum value v of the amplitude of the Q section can be obtained₁,…,v_q,…v_Q。

And 4, summing the maximum value of the Q section amplitude value, and taking the summed value as the peak value of the intercepted receiving sequence.

Wherein, the concrete formula of summing is:the summed value v_maxAs the peak of the truncated received sequence; constructing a frequency offset vector f ═ (f)₁,…,f_q,…,f_Q)。

And 5, traversing each sampling point of the receiving sequence R, repeating the steps 2-4 to obtain a peak value corresponding to each sampling point so as to obtain a peak value curve, and selecting the sampling point corresponding to the maximum value of the peak value in the peak value curve as the synchronous starting point of the whole receiving sequence R.

The specific principle and the synchronization method of the synchronization algorithm based on the fast Fourier are as follows:

a. analyzing the characteristics of additional phases in different environments

1) When the two parties are relatively stationary (fixed frequency deviation)

In this case, the frequency offset is determined only by the components, and the frequency offset does not change with time, i.e., f (t) remains constant, and the additional phase increment △ phi between the two symbols before and after is phi_n-φ_n-1Also remains constant. The additional phase of each symbol due to frequency offset under ideal conditions (no noise interference) should be on a straight line.

Let f (t) be b a function of the variation of the frequency offset (in this case, the frequency offset is a fixed value), then the additional phase of the nth symbol of the received sequence R should beWherein, a₁＝2πbT_sym、a₀＝φ₀Is the initial phase of the sequence R. In this case, the line can be plotted by a linear function, and the linear analytic equation of the phase is that y is a₁n+a₀。

2) When the transmitter and the receiver move relatively (dynamic frequency deviation)

In this case, the frequency shift is determined by two factors, namely, the diversity of the communication environment and the Doppler shift f caused by the relative motion_d：

Wherein f is the carrier frequency of the transmitter, c is the speed of light, v is the relative movement speed of the transmitter and the receiver, and theta is the relative movement included angle. When the relative movement speeds of the two parts are constant, the Doppler frequency shift f is generated_dRemains constant when the additional phase of each symbol due to frequency offset is the sameAlso in a straight line. When the relative movement speeds of the two are not constant (for example, acceleration/deceleration movement), the doppler shift f can be seen from equation (2)_dThe time-varying phase increment varies linearly, so that the additional phase increment between the two symbols varies continuously, and the characteristic of a quadratic curve is presented.

Let f (t) kt + b be a function of the variation of the frequency offset (in this case, the frequency offset varies with time), the additional phase of the nth symbol of the received sequence R should be

Wherein

a₁＝2πbT_sym、a₀＝φ₀Is the initial phase of the received sequence R. In this case, the curve can be plotted by a quadratic function, and the curve of the phase can be solved by the equation of y ═ a₂n²+a₁n+a₀。

FIG. 1 shows the time-frequency offset and symbol-added phase change of the transceiver during relative uniform motion and uniform acceleration (the initial phase phi of the sequence in the figure)₀0). As shown in fig. 1(a), when the motion is uniform, the time is constant and the symbol phase is in a linear state; as shown in fig. 1(b), the time offset exhibits a linear characteristic and the symbol phase exhibits a quadratic curve when the acceleration is uniform.

In a practical communication environment, the ideal situation does not exist, and the two situations are usually superimposed, and the dynamic frequency offset does not necessarily follow linear change. However, in a short period of time, we can abstract it into the superposition of the two cases of fixed frequency offset and dynamic frequency offset. From the above, the frequency offset is embodied in the form of an additional phase in the signal, for example, the additional phase of the fixed frequency offset in the signal is changed by a linear curve; the additional phase of the dynamic frequency offset in the signal is changed in a quadratic curve. The invention aims to obtain an information starting point in the two frequency deviation superposition models, namely synchronous acquisition.

b principle of synchronization

Assuming that the received sequence R contains only fixed frequency offsets, the observation of equation (1) is due to x_nAs known, the variation of the phase can be intercepted by any sampling point in the receiving sequence R, so that the quasi-sinusoidal signal sequence after the phase is intercepted is obtainedWherein

Due to the fixed frequency deviation, therefore

It may be equivalent to a constant frequency sinusoidal signal superimposed with noise. To quasi-sinusoidal signal sequence after phase truncationFast Fourier Transform (FFT) is performed to obtain a sequence after the fast fourier Transform. If the quasi-sinusoidal signal sequence after phase truncation

If the sequence is a real synchronous sequence, a peak value appears in the sequence after fast Fourier transform, an intercept point corresponding to the peak value is a synchronous starting point of an intercepted signal, and a frequency corresponding to the peak value is a fixed frequency offset estimation value of the intercepted signal; otherwise, no peak appears after the transformation, and the schematic diagram is shown in fig. 2. The correlation and FFT results of the received signal and the local sequence, which are respectively cut with point a and point B as starting points, are shown in fig. 3.

As can be seen from fig. 3(a), when the truncated signal is not the true synchronization sequence, after FFT, all values are substantially the same, and no peak occurs. As shown in fig. 3(b), when the intercepted signal is a synchronization sequence, after FFT, there is an obvious peak, and the frequency corresponding to the peak is the frequency offset estimation value existing in the received signal; for example, a very distinct peak occurs at a frequency of 10Hz, indicating that point B is the sync head position of the received signal and that there is a frequency offset of 10 Hz. The FFT-based synchronization algorithm is given below:

it is known that: receiving sequence R ═ (R)₁,…,r_n,…,r_N) And the local sequence X ═ X₁,…,x_n,…,x_N) (ii) a The FFT parameter N.

The target is as follows: finding the peak value v_maxAnd a frequency offset f.

1. Based on the received sequence R ═ (R)₁,…,r_n,…,r_N) And the local sequence X ═ X₁,…,x_n,…,x_N) Constructing a quasi-sinusoidal signal sequence:

wherein

2. Aligning sinusoidal signal sequences

And performing N-point fast Fourier transform to obtain a sequence after fast Fourier transform, and searching the maximum amplitude value and the frequency corresponding to the maximum amplitude value from the sequence after fast Fourier transform.

Obviously, if a synchronization head is to be found in the whole signal, it is necessary to continuously intercept the received signal and execute the above process, and at the same time, record the peak value (maximum value of amplitude) after each FFT transformation, and finally, use the sampling point corresponding to the maximum point in the peak value as the starting point of the synchronization head. It should be noted that when the received signal is truncated at a point near point B as a starting point (when the truncated signal contains most of the synchronization sequence), and the above process is performed, a peak similar to that in fig. 3(B) may occur, but the peak is lower than that at the true synchronization point (point B). Fig. 4 shows a peak profile for a synchronization operation over the entire received signal.

c. Synchronization head capture algorithm under extremely low signal-to-noise ratio

According to the synchronization principle given by the b, the synchronization position can be determined in the received signal, and if the synchronization sequence is longer, the peak value is more obvious, and the synchronization sequence can work in a severe environment (low signal-to-noise ratio). However, considering the fact that there is a linear change in frequency offset, when the sequence is continuously increased, the frequency offset value at the beginning of the sequence is far from the frequency offset value at the end, and it can not be equivalent to a sinusoidal signal with constant frequency superimposed with noise. If the synchronization is performed using the above-described synchronization principle, no peak occurs, and thus, improvement thereof is required.

In order to avoid the phase difference between the frequency offset values of the starting point and the end point of the intercepted signal being too large, the intercepted received signal can be segmented again, and the received signal R of each segment can be segmented_qAnd a segmented local sequence X_qB, performing the synchronization process, wherein the number of segments Q and the FFT parameter N are included; and recording the maximum value v of each section of amplitude value after FFT_qThereby obtaining the maximum value of the amplitude of the Q section; the accumulated sum of the maximum values of the Q-section amplitudes is used as the peak value of the truncated received signal.

As shown in fig. 5, the local sequence is divided into 4 segments, and the received signal is also divided into 4 segments, for example, the FFT maximum value of segment 1 is v₁The peak value of the signal is cut off by taking A as a starting point and is v₁+v₂+v₃+v₄。

By the synchronous head capturing algorithm under the extremely low signal-to-noise ratio, the peak value corresponding to each sampling point is extracted by traversing the whole receiving signal under the extremely low signal-to-noise ratio, so that a peak value curve is obtained. And selecting a point corresponding to the maximum value in the peak value curve as a starting point of the synchronous head, and constructing a frequency offset vector f after the starting point of the synchronous head is determined, wherein the frequency offset vector f can roughly know the frequency offset change condition of the signal.

Although the present invention has been described in detail in this specification with reference to specific embodiments and illustrative embodiments, it will be apparent to those skilled in the art that modifications and improvements can be made thereto based on the present invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.