CN113612711A - Frequency offset estimation method of short burst modulation signal under low signal-to-noise ratio - Google Patents
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Abstract
The invention discloses a frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio, which relates to the field of wireless communication and solves the estimation precision of the modulation signal under the low signal-to-noise ratio, and the technical scheme is as follows: step one, acquiring a frame header position according to a local sequence and a received signal; intercepting a signal length according to the frame header position, and acquiring a signal symbol stream before frequency offset compensation according to the signal length; step three, de-rotating the signal symbol stream to obtain a symbol stream before rotation; fourthly, performing fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream; fifthly, performing fast Fourier transform of an L point on the fourth power spectrum to obtain a frequency deviation estimation value; on the premise of low signal-to-noise ratio and short burst, the obtained frequency offset value has small estimation variance and high estimation precision.
Description
Technical Field
The invention relates to the field of wireless communication, in particular to a frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio.
Background
Synchronization is an important factor affecting the performance of wireless communications, and correct coherent demodulation of a signal can only be achieved if both the frequency and phase of the local carrier remain synchronized with the received signal. In a burst communication system, signals are bursty and transient, and synchronization needs to be rapidly achieved after the burst signals are successfully acquired. The shorter the burst length, the more important the fast synchronization method appears. The pi/4-CQPSK commonly adopted in burst communication is a linear digital modulation technology developed on the basis of QPSK, and has the characteristics of small peak-to-average ratio, strong multipath resistance and easy realization. In a short burst communication system using CQPSK modulation, especially under the condition of low signal-to-noise ratio caused by complex wireless environment, how to perform frequency offset estimation after signal acquisition is a problem worthy of study.
Existing frequency offset estimation methods can be divided into two categories: (1) time domain method: extracting parameters through the correlation operation of time domain signals; (2) the frequency domain method comprises the following steps: the spectrum is searched to find a peak whose position corresponds to the estimated value of the frequency offset.
The time domain method is usually a data-aided method based on the maximum likelihood theory, needs to occupy additional aided resources, and generally cannot give consideration to the estimation precision, the estimation range and the signal-to-noise ratio threshold. The frequency domain method is usually based on Discrete Fourier Transform (DFT), and Fast Fourier Transform (FFT) can be used to replace DFT, so that the operation speed is increased, and the method is more suitable for burst communication, especially for short burst systems. However, both the time domain method and the frequency domain method rarely consider the situation of short burst CQPSK modulation signals, and with the development of wireless communication technology, data traffic is increasing, burst communication is becoming more common, and the demand for a high-precision fast frequency offset estimation method is becoming more urgent.
Disclosure of Invention
The invention aims to provide a frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio, which is used for solving the problems of large frequency offset estimation variance and low estimation precision of a CQPSK modulation signal under the condition of the low signal-to-noise ratio in the prior art, and achieves the purposes of small estimation variance and high estimation precision of an obtained frequency offset value aiming at the CQPSK modulation signal on the premise of the low signal-to-noise ratio.
The technical purpose of the invention is realized by the following technical scheme:
a frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio comprises the following steps: performing correlation operation on a local sequence and a received signal to acquire a frame header position;
intercepting the signal length according to the frame header position, and acquiring a signal symbol stream before frequency offset compensation according to the signal length;
step three, de-rotating the signal symbol stream to obtain a symbol stream before rotation;
fourthly, performing fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream;
and fifthly, performing fast Fourier transform of an L point on the fourth power spectrum to obtain a frequency deviation estimation value.
By adopting the technical scheme, in the prior art, the modulation signal is submerged under the condition of low signal-to-noise ratio, the frequency offset can cause the signal to rotate, and the correct demodulation can possibly be caused, so that the mean square error of the modulation signal is large, and the estimation accuracy is low. The invention takes the maximum value obtained after the signal is subjected to two times of correlation operations as the position of the frame header, and takes the position of the frame header as the initial position to obtain the complete signal symbol stream, because the complete signal can be subjected to the operation of signal rotation in the channel transmission process, the rotated signal needs to be de-rotated back to the state of not being rotated, then each signal in the de-rotated symbol stream is subjected to the fourth power operation, the obtained fourth power spectrum has an obvious peak value, and finally the fourth power spectrum is subjected to the fast Fourier transform of an L point to obtain the frequency deviation estimation value.
Further, the first step specifically includes: performing cross-correlation operation on the local sequence and a received signal to obtain a calculation result; and carrying out autocorrelation operation on the calculation result, wherein the position with the maximum result of the autocorrelation operation is the position of the frame header.
Further, the second step specifically includes: setting the number of intercepted signal length symbols as N, and the signal symbol flow before frequency offset compensation as Scap＝{c1,c2,…,cN}。
Further, the third step specifically includes: performing de-rotation operation on the signal symbol stream to obtain a symbol stream of frequency offset estimation
Wherein,
indicating that the rotated CQPSK-modulated signal is de-rotated.
Further, the fourth step specifically includes: the derotated symbol stream is subjected to a fourth power operation to obtain a fourth power spectrum of the symbol stream
Further, the fifth step specifically includes: performing fast Fourier transform of L point on the fourth power spectrum, and obtaining the maximum value of the modulus of the result as nmax＝max|Sfour|；
Obtaining an estimated value of frequency deviation according to the maximum value
Wherein f issRepresenting the sampling rate, nmaxThe maximum value of the modulus of the result of the fast fourier transform is represented.
Further, the modulation signal is a pi/4-CQPSK modulation signal.
An apparatus for frequency offset estimation, the apparatus being configured to perform the method, the apparatus comprising:
the correlation operation module is used for acquiring the position of the frame header according to the local sequence and the received signal;
the signal intercepting module is used for intercepting the signal length according to the frame header position and acquiring a signal symbol stream before frequency offset compensation according to the signal length;
a de-rotation module, configured to de-rotate the signal symbol stream to obtain a symbol stream before rotation;
the operation module is used for carrying out fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream;
and the frequency offset calculation module is used for performing L-point fast Fourier transform on the fourth power spectrum to obtain a frequency offset estimation value.
A computer-readable storage medium, on which a computer program is stored, which program, when being executed by a processor, carries out the steps of the method.
An electronic device comprising a memory and a processor, a computer program being stored in the memory and being executable on the processor, the processor implementing the steps of the method when executing the program.
Compared with the prior art, the invention has the following beneficial effects:
the invention relates to a frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio, which comprises the steps of carrying out correlation operation twice on a pilot frequency, wherein the peak value of a correlation peak is the initial position of the signal, thus obtaining a complete signal, carrying out de-rotation on the signal, calculating the fourth power of the signal, carrying out fast Fourier transform to obtain a frequency offset estimation value, and measuring the estimation accuracy of the frequency offset estimation calculation method by calculating the mean square error of normalized frequency offset.
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The accompanying drawings, which are included to provide a further understanding of the embodiments of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principles of the invention. In the drawings:
FIG. 1 illustrates a flow chart of a method of frequency offset estimation in accordance with the present invention;
fig. 2 shows a constellation diagram of a burst signal after CQPSK modulation according to an embodiment;
FIG. 3 illustrates a burst signal structure diagram according to one embodiment;
FIG. 4 illustrates a channel output signal at a normalized frequency offset of 0 and a signal-to-noise ratio of 0dB in accordance with one embodiment;
FIG. 5 illustrates a channel output signal at a normalized frequency offset of 0 and a signal-to-noise ratio of 30dB in accordance with one embodiment;
FIG. 6 illustrates a channel output signal at a normalized frequency offset of 0.01 and a signal-to-noise ratio of 30dB in accordance with one embodiment;
FIG. 7 shows a fourth power spectrum at a normalized frequency offset of 0.01 and a signal-to-noise ratio of 0dB, in accordance with one embodiment of the present invention;
FIG. 8 shows a fourth power spectrum at a normalized frequency offset of 0.01 and a signal-to-noise ratio of 5dB, according to one embodiment of the invention;
FIG. 9 shows a fourth power spectrum at a normalized frequency offset of 0.01 and a signal-to-noise ratio of 10dB, in accordance with one embodiment of the present invention;
fig. 10 shows a comparison of the effect of mean square error in the estimation range of the frequency offset estimation method according to the present invention and the L & R method of the prior art.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to examples and accompanying drawings, and the exemplary embodiments and descriptions thereof are only used for explaining the present invention and are not meant to limit the present invention.
It will be understood that when an element is referred to as being "secured to" or "disposed on" another element, it can be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly or indirectly connected to the other element.
It will be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like, as used herein, refer to an orientation or positional relationship indicated in the drawings that is solely for the purpose of facilitating the description and simplifying the description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and is therefore not to be construed as limiting the invention.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
Example one
According to an embodiment of the present invention, a method for estimating frequency offset of a short burst modulation signal with a low signal-to-noise ratio is provided, and pi/4-CQPSK modulation signal is taken as an example and described below. In the embodiment, the phase characteristics of the pi/4-CQPSK modulation signal are considered, two times of correlation operation is carried out on the pilot frequency, and the peak value of the correlation peak is the initial position of the signal, so that the complete signal is obtained. And de-rotating the signal, calculating the fourth power of the signal, performing fast Fourier transform on the result of the fourth power operation to obtain a frequency deviation estimation value, and measuring the estimation precision of a frequency deviation algorithm by calculating the mean square error of the normalized frequency deviation. Specifically, referring to fig. 1, this embodiment includes the following steps:
step S110, acquiring a frame header position according to the local sequence and the received signal;
step S120, intercepting the signal length according to the position of the frame header, and acquiring a signal symbol stream before frequency offset compensation according to the signal length;
step S130, de-rotating the signal symbol stream to obtain the symbol stream before rotation;
step S140, the symbol stream is operated to the fourth power to obtain the fourth power spectrum of the symbol stream
And S150, performing fast Fourier transform of an L point on the fourth power spectrum to obtain a frequency deviation estimation value.
The local sequence in step S110 refers to an original pilot symbol stream, and the received signal in step S110 is a transmitted signal obtained by adding noise, frequency offset, and time delay in channel transmission.
Preferably, step S110 specifically includes: performing cross-correlation operation on the local sequence and the received signal to obtain a calculation result; and performing autocorrelation operation on the calculation result, wherein the position with the maximum result of the autocorrelation operation is the position of the frame header.
Preferably, step S120 specifically includes: setting the number of intercepted signal length symbols as N and the signal symbol flow before frequency offset compensation as Scap＝{c1,c2,…,cN}。
Preferably, step S130 specifically includes: performing de-rotation operation on the signal symbol stream to obtain a symbol stream of frequency offset estimation
Wherein,
indicating that the rotated CQPSK-modulated signal is de-rotated. D in the formula of the present embodiment1,d2,…,dNThe number of symbol streams is N, and each symbol is rotated, so that the symbol stream is rotated to a state before the rotation.
Preferably, step S140 specifically includes: the derotated symbol stream is subjected to a fourth power operation to obtain a fourth power spectrum of the symbol stream
Preferably, step S150 specifically includes: performing fast Fourier transform of L point on the fourth power spectrum, and obtaining the maximum value of the modulus of the result as nmax＝max|Sfour|；
Obtaining an estimated value of frequency deviation according to the maximum value
Wherein f issRepresenting the sampling rate, nmaxThe maximum value of the modulus of the result of the fast fourier transform is represented.
Preferably, the modulated signal is a pi/4-CQPSK modulated signal.
A first embodiment further provides a frequency offset estimation apparatus, where the apparatus is configured to execute the method, and the apparatus includes:
the correlation operation module is used for acquiring the position of the frame header according to the local sequence and the received signal;
the signal intercepting module is used for intercepting the signal length according to the frame header position and acquiring the signal symbol stream before frequency offset compensation according to the signal length;
the de-rotation module is used for de-rotating the signal symbol stream to obtain the symbol stream before rotation;
the operation module is used for performing fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream;
and the frequency offset calculation module is used for performing L-point fast Fourier transform on the fourth power spectrum to obtain a frequency offset estimation value.
The present embodiment also provides a computer-readable storage medium, on which a computer program is stored, wherein the program, when executed by a processor, implements the steps of the method.
Specifically, the computer-readable storage medium may be a flash memory, a hard disk, a multimedia card, a card type memory (e.g., SD or DX memory, etc.), a Random Access Memory (RAM), a Static Random Access Memory (SRAM), a Read Only Memory (ROM), an Electrically Erasable Programmable Read Only Memory (EEPROM), a Programmable Read Only Memory (PROM), a magnetic memory, a magnetic disk, an optical disk, a server, etc.
The present embodiment further provides an electronic device, which includes a memory and a processor, where the memory stores a computer program capable of running on the processor, and the processor implements the steps of the method when executing the program.
Example two
In order to further verify the beneficial effects of the present invention, in the second embodiment, a pi/4-CQPSK modulated signal is used to perform a simulation experiment based on the first embodiment, from burst signal generation to shaping filtering to channel filtering to matched filtering, and the frequency offset estimation algorithm is used for the matched filtered signal to obtain a frequency offset estimation result, and the result is analyzed.
The burst signal generation process: the bit data with the length of 150 bits is randomly generated, pi/4-CQPSK modulation is adopted, namely QPSK modulation is firstly carried out, then rotation is carried out, the rotation angle is related to the serial number in the signal and is a multiple of pi/4, the obtained constellation diagram is shown in figure 2, 8 phases are totally obtained, the maximum phase jump is 3 pi/4 every time, and the QPSK is improved.
As shown in fig. 3, the pilot is modulated by BPSK, and is divided into three segments and dispersedly inserted into the signal, so as to obtain the structure of the burst signal.
After the signal is shaped and filtered, adding noise and frequency offset to the signal, and performing matched filtering to obtain an object of frequency offset estimation, where the shaped filtering and the matched filtering both use root raised cosine filters, and roll-off factors of the root raised cosine filters are both 0.35, as shown in fig. 4 and 5, the result is obtained after the signal is transmitted through channels when the signal-to-noise ratio is 0dB and 30dB respectively under the condition that the normalized frequency offset is 0, as shown in fig. 6, the result is obtained after the signal is transmitted through channels when the normalized frequency offset is 0.01 under the condition that the signal-to-noise ratio is 30dB, it is seen that the signal with a low signal-to-noise ratio can be submerged, the frequency offset can be submerged, and correct demodulation can not be caused.
The frequency offset estimation method provided by the invention is applied: and performing correlation operation on the pilot frequency twice, wherein the peak value of a correlation peak is the initial position of the signal, so as to obtain a complete signal, performing de-rotation on the signal, calculating the power of the signal to the fourth power, performing fast Fourier transform, obtaining a frequency offset estimation value, and measuring the estimation accuracy of a frequency offset algorithm by calculating the mean square error of normalized frequency offset.
In order to obtain the estimation accuracy performance of the frequency offset algorithm provided by the invention, noise under different signal-to-noise ratios is added to repeat the operations of generating, shaping filtering, channel and matched filtering from burst signals, under the condition of low signal-to-noise ratio, for example, when the signal-to-noise ratios are respectively 0dB, 5dB and 10dB, the frequency spectrums of signals after signal derotation and fourth power operation are shown in fig. 7, 8 and 9, and even when the signal-to-noise ratio is 0dB, the frequency offset estimation algorithm provided by the invention enables the fourth power spectrogram to have obvious peak values. The relationship between the estimation accuracy and the signal-to-noise ratio obtained by simulating 1000 times under the condition that the normalized frequency offset is 0.001 is shown in fig. 10, two curves are shown in fig. 10, wherein the curve positioned on the upper part and smoother is the curve of the L & R algorithm in the prior art, the curve positioned on the upper part and steeper is the curve of the invention when the signal-to-noise ratio is less than 1dB, and as can be seen from fig. 10, when the signal-to-noise ratio is more than 1dB, compared with the traditional L & R algorithm in the time domain method, the mean square error of the normalized frequency offset is smaller, so the estimation accuracy of the algorithm provided by the invention is higher.
In summary, on the premise of a low signal-to-noise ratio and a short burst, for the CQPSK modulated signal, the estimation variance of the estimated frequency offset value is small in this embodiment, and the estimation accuracy is high enough.
The method of the present invention can be applied to a wireless communication system, such as a satellite mobile communication system, etc., for performing frequency offset estimation at a receiver side, or can also be applied to a test apparatus for performing frequency offset estimation on a modulated signal.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are merely exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention, and any modifications, equivalent substitutions, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A frequency offset estimation method of a short burst modulation signal under a low signal-to-noise ratio is characterized by comprising the following steps:
performing correlation operation on a local sequence and a received signal to acquire a frame header position;
intercepting a signal length according to the frame header position, and acquiring a signal symbol stream before frequency offset compensation according to the signal length;
step three, de-rotating the signal symbol stream to obtain a symbol stream before rotation;
fourthly, performing fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream;
and fifthly, performing fast Fourier transform of an L point on the fourth power spectrum to obtain a frequency deviation estimation value.
2. The method of claim 1, wherein the first step specifically comprises: performing cross-correlation operation on the local sequence and a received signal to obtain a calculation result; and carrying out autocorrelation operation on the calculation result, wherein the position with the maximum result of the autocorrelation operation is the position of the frame header.
3. The method for estimating frequency offset of a short burst modulation signal with low snr according to claim 1, wherein the second step specifically includes: setting the number of intercepted signal length symbols as N, and the signal symbol flow before frequency offset compensation as Scap＝{c1,c2,…,cN}。
4. The method for estimating frequency offset of a short burst modulation signal with low snr according to claim 1, wherein the third step specifically includes: performing de-rotation operation on the signal symbol stream to obtain a symbol stream of frequency offset estimation
Wherein,
indicating that the rotated CQPSK-modulated signal is de-rotated.
5. The method for estimating frequency offset of a short burst modulation signal with low snr according to claim 1, wherein the fourth step specifically includes: the derotated symbol stream is subjected to a fourth power operation to obtain a fourth power spectrum of the symbol stream
6. The method for estimating frequency offset of a short burst modulation signal with low snr according to claim 1, wherein the step five specifically includes: performing fast Fourier transform of L point on the fourth power spectrum, and obtaining the maximum value of the modulus of the result as nmax＝max|Sfour|；
Obtaining an estimated value of frequency deviation according to the maximum value
Wherein f issRepresenting the sampling rate, nmaxThe maximum value of the modulus of the result of the fast fourier transform is represented.
7. The method of claim 1, wherein the modulation signal is a pi/4-CQPSK modulation signal.
8. An apparatus for frequency offset estimation, the apparatus configured to perform the method, the apparatus comprising:
the correlation operation module is used for acquiring the position of the frame header according to the local sequence and the received signal;
the signal intercepting module is used for intercepting the signal length according to the frame header position and acquiring a signal symbol stream before frequency offset compensation according to the signal length;
a de-rotation module, configured to de-rotate the signal symbol stream to obtain a symbol stream before rotation;
the operation module is used for carrying out fourth power operation on the symbol stream to obtain a fourth power spectrum of the symbol stream;
and the frequency offset calculation module is used for performing L-point fast Fourier transform on the fourth power spectrum to obtain a frequency offset estimation value.
9. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out the steps of the method according to any one of claims 1 to 7.
10. An electronic device comprising a memory and a processor, on which a computer program is stored which is executable on the processor, characterized in that the steps of the method of any of claims 1 to 7 are implemented when the processor executes the program.
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