CN107426131B - Anti-frequency-deviation low-correlation search peak timing estimation method based on cyclic prefix - Google Patents
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Abstract
The invention discloses a frequency deviation resistant low correlation search peak timing estimation method based on cyclic prefix, which comprises the following steps: (1) initializing; (2) generating a sampling index which is the sum of the iterative sequence and the sampling sequence in the public time delay and guard interval; (3) obtaining a sample value at a sample index andis N apart from the sampling indexcSample values at sampling intervals; (4) taking an absolute value of the two sampling values in the step (3) and then making a difference, taking the absolute value of the value after the difference is made, and taking the value as a timing measurement curve value; (5) judging whether the timing measurement curve value is greater than the maximum difference value or not, if so, taking the timing measurement curve value as the maximum difference value, and estimating the symbol timing as the difference between the symbol length and the common time delay and the iteration times; and if the difference value is smaller than the maximum difference value, returning to the step (2). The invention does not need additional data symbols, has higher utilization rate of channel resources and solves the problem of timing synchronization under normal conditions.
Description
Technical Field
The invention relates to a cyclic prefix-based frequency deviation resisting low correlation search peak timing estimation method suitable for an OFDM system, and belongs to the technical field of signal sampling.
Background
The mobile communication mainly uses wireless electromagnetic wave as a carrier for information transmission, and the electromagnetic wave transmitted in a wireless channel mainly has two characteristics, namely multipath propagation and time dispersion. Normally, the received signals include mixed superposition of direct transmission signals, reflected signals, refracted signals and the like, and the signals with different time delays, fading and phases are generated by passing through wireless transmission paths with different distances. In this case, the phases of the waveforms obtained by the receiver are different from each other, and the mixed superposition of them causes the amplitude of the in-phase wave to be strengthened and the amplitude of the out-phase wave to be weakened, which is a main cause of multipath fading.
In a multipath fading channel, signal distortion is caused by inter-symbol interference (ISI), and in an OFDM system, data symbols are transmitted in parallel on subcarriers orthogonal to each other in the time domain, and a guard interval is added, so that the problem can be solved well. However, when the symbol timing is problematic, the resulting ISI and inter-carrier interference (ICI) may cause the finally received signal to be incorrectly demodulated, thereby causing the reliability of the OFDM transmission system to be affected.
Since one of the core technologies of the OFDM system is to use Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) operations in the modulation process at the transmitting end and the demodulation process at the receiving end, respectively, N is required to be performed during demodulation at the receiving endcThe FFT of the points, thus requiring accurate sampling of the received signal. The premise of accurate sampling is that the correct starting point of one OFDM symbol can be obtained. In general, symbol timing in the presence of frequency offset can be divided into three cases: precise timing, early timing, and late timing. Accurate timing is naturally required by the system; the timing advance causes a phase difference of the received signals, which can be eliminated by an equalizer, so the influence is small; the most common situation in practical systems is timing lag, which not only occurs intersymbol interference but also causes carrier interference, and the equalizer cannot solve the problem. In this case, so-called "symbol timing synchronization" is required to obtain accurate signal samples.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide an anti-frequency-offset low correlation search peak timing estimation method based on cyclic prefix, which is suitable for an OFDM system, does not need additional data symbols, has high channel resource utilization rate, is simple and easy to realize, and solves the timing synchronization problem under the normal condition.
In order to achieve the purpose, the invention is realized by the following technical scheme:
the invention relates to a frequency deviation resistant low correlation search peak timing estimation method based on cyclic prefix, which comprises the following steps:
(1) initializing;
(2) the sampling index is the sum of the iterative sequence and the sampling sequence in the public time delay and guard interval;
(3) setting the number of sampling values for FFT calculation to be NcObtaining the sampling value at the sampling index and being separated from the sampling index by NcSample values at sampling intervals;
(4) taking an absolute value of the two sampling values in the step (3) and then making a difference, taking the absolute value of the value after the difference is made, and taking the value as a timing measurement curve value, namely a symbol timing algorithm based on a difference value; is provided with two sliding windows M1And M2The length of the sliding window is the same as that of the cyclic prefix, and the sliding window can move left and right; when the cyclic prefix CP falls completely within the sliding window M1In the middle time, part of sampling data in the signal to be transmitted completely falls in the sliding window M2Through sliding window M1And M2Estimating a timing deviation according to the similarity; finding out the point which minimizes the difference in the sliding window to calculate the timing deviation, namely, the timing deviation is calculated based on the similarity of the sliding window;
(5) judging whether the timing measurement curve value is larger than the maximum difference value or not, if so, taking the timing measurement curve value as the maximum difference value, estimating the symbol timing, namely subtracting the common time delay from the symbol length and subtracting the iteration times, and ending the cycle; and if the difference value is smaller than the maximum difference value, returning to the step (2).
In the step (1), the initialized content is symbol timing estimation, public time delay and maximum difference, and the iteration times are set.
In step (3), signal sampling is performed before performing signal demodulation FFT calculation.
In step (3), the cyclic prefix and the signal to be transmitted are identical, and the sampling values of the two parts are separated by NcA sample value.
In the step (4), when the similarity of the two windows is maximum, the corresponding interpolation is minimum.
In the step (4), the calculation formula of the timing deviation is as follows:
symbol y in the above formulal[n+i]And yl[n+Nc]Is a received signal, where n, i is a unit of count, ylTo receive a signal, NcFor the number of sample values used for the FFT calculation,
representing an optimal estimate of the timing deviation, NGIs the guard interval length.
The anti-frequency-deviation low-correlation search peak timing estimation method based on the cyclic prefix is suitable for an OFDM system, and based on a symbol timing algorithm based on a difference value, a new optimization algorithm step based on the similarity of a sliding window is adopted, so that the problem that the lowest correlation peak is difficult to accurately find in the presence of frequency deviation is effectively solved, the method is suitable for a working scene in which the lowest correlation peak is difficult to find in the presence of frequency deviation, not only is a correlation peak curve smoothed, but also the calculation complexity is simplified. The invention is based on the characteristics of the symbol itself, does not need additional data symbol, therefore, the channel resource utilization rate is higher, and the invention is simple and easy to realize, and can solve the timing synchronization problem under the normal condition.
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FIG. 1(a) is a diagram of ideal FFT window start point timing positions;
FIG. 1(b) is a diagram of a better timing position of the starting point of the FFT window;
FIG. 1(c) is a diagram of poor FFT window start point timing positions;
FIG. 2 is a diagram of a cyclic prefix based timing offset estimation technique according to the present invention;
fig. 3 is a flowchart illustrating an implementation of a method for estimating a symbol timing offset based on a cyclic prefix according to the present invention;
fig. 4(a) is a diagram of a simulation result of conventional timing estimation when the timing offset ST0 is-3 and the carrier frequency offset CFO is 0;
fig. 4(b) is a diagram of a simulation result of conventional timing estimation when the timing offset ST0 is-3 and the carrier frequency offset CFO is 0.5;
fig. 4(c) is a diagram of a simulation result of conventional timing estimation when the timing offset ST0 is 2 and the carrier frequency offset CFO is 0;
fig. 4(d) is a diagram of a simulation result of conventional timing estimation when the timing offset ST0 is 2 and the carrier frequency offset CFO is 0.5;
fig. 5(a) is a simulation effect diagram of the timing estimation method of the present invention when the timing offset ST0 is-3 and the carrier frequency offset CFO is 0;
fig. 5(b) is a simulation effect diagram of the timing estimation method of the present invention when the timing deviation ST0 is-3 and the carrier frequency offset CFO is 0.5;
fig. 5(c) is a simulation effect diagram of the timing estimation method of the present invention when the timing offset ST0 is 2 and the carrier frequency offset CFO is 0;
fig. 5(d) is a graph showing simulation effects of the timing estimation method of the present invention when the timing offset ST0 is 2 and the carrier frequency offset CFO is 0.5.
Detailed Description
In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further described with the specific embodiments.
The embodiment of the invention mainly considers the influence of the timing deviation estimation in the multipath fading channel environment, and the method provided by the invention can accurately obtain the timing deviation through simulation verification, thereby providing theoretical guidance for the synchronization in the actual transmission process of signals.
The main purpose of timing synchronization is to select a relatively suitable window for each OFDM symbol for FFT computation to eliminate inter-symbol interference. Fig. 1(a) to fig. 1(b) show the timing results of three symbols of the OFDM system, and it can be found from the graph that:
the first case is the most ideal timing result, where it is exactly one complete OFDM symbol that is FFT calculated within the FFT window, which is what we want to achieve.
The second case is timing advance, which is not ideal timing, but is also a good timing result since no inter-symbol interference is introduced, only phase shift occurs and is within an acceptable range. The FFT window contains most of the OFDM symbol and includes a portion of the CP that is a partial copy of the tail of the current symbol. In this case, we can observe the signal in the frequency domainTo examine the effect of timing skew. For time domain discrete signal
An FFT operation, in which the symbol timing offset is used, can be used to obtain an expression of the signal in the frequency domain. The whole calculation process is as follows:
the above formula has the following relation:
it can be seen from the observation of equation (1) that orthogonality between subcarriers is not destroyed, i.e., well maintained. But the received signal produces a phase offset, in which case the phase offset caused by the timing offset can be compensated for by a simple frequency domain equalizer.
In the third case: at this time, the symbol timing is delayed, that is, in an FFT window, the signal participating in the FFT operation has a crosstalk symbol of the next data symbol in addition to the current symbol. At this time in the interval TsThe received signal may be represented as:
wherein N isGIs the guard interval length.
For the signal
Performing FFT operation to obtain a demodulation signal:
the above formula is as follows:
it can be found from equation (4) that the third term in the last row appears from the next OFDM symbol Xl+1[p]While the second term of the last row indicates that orthogonality between subcarriers has been destroyed, this illustrates that the third symbol synchronization (late synchronization) scenario causes both intersymbol interference and carrier interference.
From the above three cases, as long as there is a timing offset, even if the case is light, a phase offset will be generated, and an additional equalizer is needed to eliminate the offset; if the system is in a serious condition, intersymbol interference and intercarrier interference are generated, and the system cannot work normally, so that symbol timing needs to be synchronized to obtain timing deviation estimation.
The basic idea of the timing estimation algorithm of the present invention is based on the cyclic prefix of an OFDM system. As shown in fig. 2, signal sampling is performed before signal demodulation FFT calculation, and the number of sampling values used for FFT calculation is Nc. As can be seen from FIG. 2, the sample data in B and B' are identical, and the samples corresponding to the two parts are separated by NcA sample value. In fig. 2, there are two sliding windows M1And M2And the length of the sliding window is the same as that of the cyclic prefix, and the sliding window can move left and right. But only when the CP falls completely within the sliding window M1In the middle, the sampling data of the B' part can fall on the sliding window M completely2In this way, the window M can be slid1And M2The timing offset is estimated from the similarity of the signals. When the similarity of the two windows is maximum, the corresponding interpolation will be minimum. We can calculate the timing offset by simply finding the point within the sliding window that minimizes the difference. The timing offset is calculated as follows:
symbol in the above formula
Representing an optimal estimate of the timing deviation. Fig. 4(a) to 5(d) are graphs comparing simulation results of the timing estimation method of the present invention and the conventional method. In the simulation process, a common Rayleigh fading channel is selected as a transmission channel, and a symbol timing estimate is calculated by calculating the difference between sample values between two sample blocks.
As can be seen from simulation of fig. 4(a) to 4(d), when the symbol timing deviation (STO) is-3 or 2, but there is no Carrier Frequency Offset (CFO), i.e., CFO is 0, the actual timing symbol estimation point (black real point) coincides with the theoretical symbol timing estimation point (red empty point), so that the timing lowest point can be accurately found; however, once the system has a carrier frequency offset (e.g., CFO 0.5), no matter STO-3 or 2, the difference magnitude will no longer be V-shaped and the value at the actual symbol timing estimation point will no longer be a minimum, which obviously would bring great difficulty to the search for the timing estimation point.
As can be seen from the simulation of fig. 5(a) to 5(d), when the method of the present invention is used, the lowest point is very obvious no matter whether there is carrier frequency offset or not, and the amplitude value is relatively small, so that the timing estimation can be found relatively conveniently. Therefore, the method can accurately find the lowest point of the sliding correlation when the frequency offset exists, the simulation curve is smoother, and the calculation complexity is lower.
Referring to fig. 3, the following are specific implementation steps and procedures of the cyclic prefix-based anti-frequency offset low correlation search peak timing estimation algorithm of the OFDM system provided in the present invention:
(1) initializing symbol timing estimation, public time delay and maximum difference value, and setting iteration times;
(2) generating a sampling index which is the sum of the iterative sequence and the sampling sequence in the public time delay and guard interval;
(3) obtaining a sample value at a sample index and being spaced from the sample index by NcSample values at sampling intervals;
(4) taking an absolute value of the two sampling values in the step (3) and then making a difference, taking the absolute value of the value after the difference is made, and taking the value as a timing measurement curve value;
(5) if the timing measurement curve value is larger than the maximum difference value, taking the timing measurement curve value as the maximum difference value, estimating the symbol timing as the difference between the symbol length and the public time delay and the iteration times, and ending the cycle; if the difference is smaller than the maximum difference, repeating the steps (2), (3), (4) and (5).
The method can accurately find the lowest point of the sliding correlation when the frequency deviation exists, and the simulation curve is smoother and has lower calculation complexity.
The foregoing shows and describes the general principles and broad features of the present invention and advantages thereof. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (4)
1. A method for estimating the timing of an anti-frequency-offset low correlation search peak based on a cyclic prefix is characterized by comprising the following steps:
(1) initializing; the initialized content comprises symbol timing estimation, public time delay and a threshold value, and the iteration times are set;
(2) the sampling index is the sum of the iterative sequence and the sampling sequence in the public time delay and guard interval;
(3) setting the number of sampling values for FFT calculation to be NcObtaining a sample value at the sample index and spaced N apart from the sample indexcSample values at sampling intervals;
(4) taking an absolute value of the two sampling values in the step (3) and then making a difference, taking the absolute value of the value after the difference is made, and taking the value as a timing measurement curve value; is provided with two sliding windows M1And M2And the length of the sliding window is the same as that of the cyclic prefix; when the cyclic prefix CP falls completely within the sliding window M1In the middle time, part of sampling data in the signal to be transmitted completely falls in the sliding windowMouth M2Through sliding window M1And M2Estimating a timing deviation according to the similarity; finding the point in the sliding window which makes the difference value minimum to calculate the timing deviation;
in the step (4), the calculation formula of the timing deviation is as follows:
symbol y in the above formulal[n+i]And yl[n+Nc]Is a received signal, where n, i is a unit of count, ylTo receive a signal, NcFor the number of sample values used for the FFT calculation,
representing an optimal estimate of the timing deviation, NGIs the guard interval length;
(5) judging whether the timing measurement curve value is larger than a threshold value, if so, taking the timing measurement curve value as the threshold value, estimating the symbol timing, namely subtracting the common time delay from the symbol length, and simultaneously subtracting the iteration times, and ending the cycle; and if the threshold value is smaller than the threshold value, returning to the step (2).
2. The method of claim 1, wherein in step (3), signal sampling is performed before performing FFT computation.
3. The method of claim 1, wherein in step (3), the cyclic prefix and the portion of the signal to be transmitted are the same, and the sampled values of the two portions are separated by NcA sample value.
4. The method for peak timing estimation based on cyclic prefix anti-frequency offset low correlation search of claim 1, wherein in step (4), when the similarity between the values of the two windows is the largest, the corresponding difference reaches the minimum.
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