WO2010139234A1

WO2010139234A1 - Method and device for estimating maximum doppler frequency offset

Info

Publication number: WO2010139234A1
Application number: PCT/CN2010/072836
Authority: WO
Inventors: 姚扬中; 曹南山; 李强; 邱宁; 张涛; 游月意
Original assignee: 中兴通讯股份有限公司
Priority date: 2009-06-03
Filing date: 2010-05-17
Publication date: 2010-12-09
Also published as: CN101909024B; CN101909024A

Abstract

A method and device for estimating the maximum Doppler frequency offset is disclosed. The method comprises: calculating a channel estimation on the basis of a received signal; calculating the amplitude spectra of the channel estimation, and filtering the amplitude spectra; collapsing and averaging the filtered amplitude spectra, and calculating the detection threshold on the basis of the collapsed and averaged amplitude spectra; calculating and outputting the maximum Doppler frequency offset on the basis of the detection threshold. The method solves the problem of poor estimation value stability and poor estimation precision in the algorithm for obtaining multiple-frequency offset information, thereby improving stability and achieving high estimation precision.

Description

TECHNICAL FIELD The present invention relates to the field of communications, and in particular to a method and apparatus for estimating a maximum Doppler shift. BACKGROUND OF THE INVENTION At present, Orthogonal Frequency Division Multiplexing (OFDM) has been widely applied to various wireless communication systems, for example, Digital Video Broadcasting (DVB), Wireless Office i or Wireless Local Area Network (WLAN), China Mobile Multimedia Broadcasting (CMMB), etc. These systems are required to work effectively in a variety of channel environments, for example, to accommodate various speed ranges of mobile stations, various carrier frequencies, various time delays, and Signal Noise Ratio (SNR). Therefore, it is very important to predetermine the channel quality and the speed of change. As the channel environment changes continuously, it is necessary to be able to select system parameters adaptively. In wireless mobile communication, due to the high-speed movement of the transmitter or receiver, Doppler frequency spreading will occur, which will affect the correct reception of the signal. In many adaptive link transmission systems, the system also needs to obtain the channel. Parameter information related to Doppler shift. For example, the two-dimensional Wiener filtering channel estimation method of an OFDM system needs to know the Doppler frequency offset of the channel. The pilot is filtered by a low pass filter having the same bandwidth as the maximum Doppler shift, which can be improved and the required transmit power is relatively low. In the related art, various algorithms proposed for acquiring multi-frequency offset information include: autocorrelation method, level cross rate method, diversity exchange rate method, etc., wherein the autocorrelation method is affected by channel noise, and needs to be accurately estimated in advance. The signal-to-noise ratio of the channel, and the autocorrelation method has poor performance at low SNR. In fact, the autocorrelation function of noise is a δ function. When the autocorrelation method estimates the signal energy, it is necessary to know the noise level to remove the estimated deviation caused by noise, even in the case of accurately knowing the noise signal-to-noise ratio. When the noise ratio is used, the stability of the estimated value is poor; while the level crossover rate method and the diversity exchange rate method can only roughly estimate the Doppler frequency offset value, and the estimation accuracy is low. The stability or estimation accuracy of the estimated value of the algorithm for obtaining multi-frequency offset information in the related art Awkward questions, no effective solutions have been proposed yet. SUMMARY OF THE INVENTION The present invention has been made in view of the problem of poor stability or low estimation accuracy of an algorithm for obtaining multi-frequency offset information in the related art. To this end, the main object of the present invention is to provide an improved maximum Doppler shift. Estimate the program to address at least one of the above issues. In order to achieve the above object, according to an aspect of the present invention, an estimation method of maximum Doppler shift is provided. The method for estimating the maximum Doppler shift according to the present invention includes: calculating a channel estimate based on the received signal; calculating an amplitude spectrum of the channel estimate, and filtering the amplitude spectrum; and performing a folding average on the filtered amplitude spectrum, according to the folding average The amplitude spectrum calculates the detection threshold; the maximum Doppler frequency offset is calculated and output according to the detection threshold. Preferably, the amplitude spectrum is filtered by the following formula: ∞(:) = Z_ _i<n<i ; ( _mo d(A: - _w , M )/^), where is the filter coefficient, mod(' M) is a modulo operation on M, M is the number of points of the fast Fourier transform FFT transform, 0 < k < M. Preferably, the filtered amplitude spectrum is subjected to a folding average by the following formula: aifik) = (t _m (k ) +

Wherein, ¾ _m W amplitude spectrum after the filtering, mod (-,) M for the modulo operation, Q≤k <^. Preferably, calculating the detection threshold according to the amplitude spectrum after the folding average comprises: calculating the detection threshold t/?r ^thr = Y _wx -a + N-(l- ), wherein, "is a constant less than 1, and _max is a folding The maximum value of the averaged amplitude spectrum is the average of the partial spectra in the amplitude spectrum after the folding average. Preferably, calculating the maximum Doppler frequency offset according to the detection threshold includes: searching for thr according to the following formula, 0≤k< } , where the number of points of the FFT transform; root, where T is the channel estimate

The time interval of the sample. Preferably, before calculating the maximum Doppler frequency offset/ _d according to the maximum index, the method further comprises: linearly interpolating the maximum index according to the following formula to obtain a corrected maximum index to be calculated according to the corrected maximum index _kedge Maximum Doppler frequency offset f _d '. dge = dge + ( ^ ^Γ _ ^half (J ¹ edge ) ) / ( ^half (^edge + _ ^half (^edge ) ) ° Preferably, the maximum The estimation method of the Pule frequency offset further includes: estimating a noise level, the estimated noise level comprising: calculating a maximum value of the amplitude spectrum after the folding average: iL _x = ^^ft W} , where (^ is the folded average Amplitude spectrum; Calculate the average of the partial spectrum of the amplitude spectrum after the folding average: Calculate the noise level as: . = /iL.

Preferably, the above method further comprises: calculating and updating the filter coefficients by the noise level and the maximum Doppler frequency offset. Preferably, calculating and updating the filter coefficients according to the noise level and the maximum Doppler frequency offset comprises: determining the noise level. Whether it is less than the set threshold TH, if yes, set the filter coefficient to the unit impulse function; otherwise, set the Gaussian filter with the σ value of the filter coefficient as σ = ^ < ΜΓ), and normalize the Gaussian filter. Preferably, after calculating and updating the filter coefficients according to the noise level and the maximum Doppler frequency offset, the method further includes: filtering the amplitude spectrum according to the updated filter coefficients. In order to achieve the above object, according to another aspect of the present invention, an apparatus for estimating a maximum Doppler shift is provided. The apparatus for estimating the maximum Doppler frequency offset according to the present invention includes: a first calculating module, configured to calculate a channel estimate according to the received signal; a second calculating module, configured to calculate an amplitude spectrum of the channel estimate obtained by the first calculating module; a module, configured to filter an amplitude spectrum obtained by the second calculation module; a processing module, configured to perform a folding average on the amplitude spectrum filtered by the filtering module, and calculate a detection threshold according to the amplitude spectrum after the folding average; The maximum Doppler shift is calculated and output based on the detection threshold. Preferably, the apparatus further includes: an estimation module, configured to estimate a noise level; a third calculation module, configured to calculate a filter coefficient according to a noise level and a maximum Doppler frequency offset; and an update module, configured to obtain by using a third calculation module The filter coefficients are updated. Through the invention, the amplitude spectrum of the channel estimation is calculated, the filtered amplitude spectrum is folded and averaged, the noise level is estimated, the detection threshold is calculated, the maximum Doppler frequency offset is calculated and output, and the multi-frequency offset information is obtained in the related art. The problem that the stability of the estimated value of the algorithm is poor or the estimation accuracy is low, thereby improving the stability and obtaining a high estimation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are set to illustrate,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, In the drawings: FIG. 1 is a flowchart of a method for estimating a maximum Doppler frequency offset according to an embodiment of the present invention; FIG. 2 is a physical layer of a CMMB system according to one of application environments according to an embodiment of the present invention; FIG. 3 is a schematic diagram of the structure of the OFDM symbol in FIG. 2; FIG. 4 is a schematic diagram of the effective subcarriers of the OFDM symbol allocated to the data subcarrier, the scattered pilot, the continuous pilot, and the allocation manner in the embodiment of the present invention; 5 is a schematic diagram of a maximum Doppler frequency offset estimation structure according to an embodiment of the present invention; FIG. 6 is a structural block diagram of an apparatus for estimating a maximum Doppler frequency offset according to an embodiment of the present invention; A preferred block diagram of an apparatus for estimating the maximum Doppler shift of an embodiment. The embodiment of the present invention provides a Doppler frequency offset acquisition method, which uses the frequency domain received signal dispersion method, in consideration of the problem that the stability of the estimated value of the algorithm for obtaining the multi-frequency offset information is poor or the estimation accuracy is low. The pilot is used to obtain the Doppler frequency offset, and the Doppler frequency offset is estimated based on the spectral method to detect the spectral width; the spectrum is filtered according to the possible size of the Doppler value, and then The spectrum is averaged in half, and the averaged spectrum is obtained by detecting and filtering, so that the algorithm can achieve high stability in a low SNR environment. The Doppler frequency offset estimation does not need to obtain the channel signal-to-noise ratio. The noise estimation of the Doppler frequency offset is small, and high estimation accuracy can be obtained. It overcomes the shortcomings of Doppler frequency offset estimation in OFDM systems. It should be noted that the embodiments in the present application and the features in the embodiments may be combined with each other without conflict. The invention will be described in detail below with reference to the drawings in conjunction with the embodiments. Method Embodiments In accordance with an embodiment of the present invention, an estimation method for maximum Doppler shift is provided. 1 is a flow chart of a method for estimating a maximum Doppler shift according to an embodiment of the present invention. As shown in FIG. 1, the method includes the following steps: step 102 to step 4: 106: Step 102, according to receiving The signal calculates the channel estimate. Step 104: Calculate an amplitude spectrum of the channel estimate, and filter the amplitude spectrum. Step 106: Perform a folding average on the filtered amplitude spectrum, and calculate a detection threshold according to the amplitude spectrum after the folding average; Step 108: Calculate and output a maximum Doppler frequency offset according to the detection threshold. It should be noted that the detection threshold is selected as = } ^?皿. "+ .(1 _ ") , where α is a constant with a preset less than 1, and f _max is the maximum value of the amplitude spectrum after the folding average, which is the folding average The average of the partial spectral values in the latter amplitude spectrum is generally selected as the average of the latter part of the spectrum of the amplitude spectrum after the folding average. When calculating the maximum Doppler frequency offset, first, the maximum index for finding that the spectral value is greater than the threshold is

The detection threshold, M is an FFT (Fast Fourier Transform) point number; preferably, in order to improve the resolution, the above-mentioned maximum index (edge point) may be linearly interpolated according to the following formula, so as to be based on the corrected maximum index Calculate the maximum Doppler frequency offset f _d K _dxe = K _dxe +

+ l) -Y _half (k _ed ), then, output Doppler frequency offset estimation Denominated / _d = L Shang, where, T is the channel estimation interval preclude comp.

Μ Τ Thereafter, the noise level can also be estimated based on the amplitude spectrum after the folding average, the filter coefficients are calculated according to the estimated noise level and the maximum Doppler frequency offset, and the filter coefficients are updated to adjust the amplitude according to the updated filter coefficients. The spectrum is filtered. Returning to step 102, the above steps are repeated. Specifically, it is determined whether the ratio of N to f _max is less than a preset threshold ΤΗ, and if so, the filter coefficient is set to a unit impulse function; otherwise, the filter coefficient is set to a Gaussian filter with a σ value of σ = · < ΜΓ), and Normalized Gaussian filter. The Gaussian filter coefficient is / ? _g O) = exp(- ² /(2 ² )) , 0 <| Λ |<3σ; the coefficient is normalized to h(k = h _g (k) H h _g (k With this embodiment, a method for estimating Doppler frequency offset in a mobile communication system is provided, which can obtain higher estimation accuracy, and the estimation accuracy is not affected by the channel estimation result. The implementation process of the embodiment of the present invention is described in detail.

There are many types of OFDM systems, and the implementation of the specific embodiments of the present invention is different for different kinds of OFDM systems. Hereinafter, the case where the present invention is applied to an OFDM system such as CMMB will be described. 2 is a schematic diagram of a frame structure of a physical layer of a CMMB system according to an application environment according to an embodiment of the present invention. As shown in FIG. 2, in a frame structure of a physical layer in a CMMB system, 1 second is equally divided into 40. Each time slot (slot 0 to time slot 39), each time slot 25 ms, each time slot consists of 1 beacon and 53 OFDM symbols (OFDM symbol 0 to OFDM symbol 52). 3 is a schematic diagram showing the structure of the OFDM symbol in FIG. 2. As shown in FIG. 3, the OFDM symbol is composed of a cyclic prefix (CP) and an OFDM data body. The OFDM data length is 409.6 μ _δ , the loop length ( r _cp ) is 51.2 μ _δ , and the OFDM symbol length ( ) is 460.8 μ s. 4 is a schematic diagram of effective subcarrier allocation of OFDM symbols in the embodiment of the present invention as data subcarriers, discrete pilots, and continuous pilots, and as shown in FIG. 4, showing effective subcarrier allocation of OFDM symbols. For data subcarriers, discrete pilots, and continuous pilots. 5 is a schematic diagram of a maximum Doppler frequency offset estimation structure according to an embodiment of the present invention. As shown in FIG. 5, based on the frame structure, the Doppler frequency offset acquisition method in the embodiment of the present invention includes the following step 4. Gather: Step 1: Calculate the channel estimate from the received signal. Specifically, the pilot subcarrier receiving signal is extracted, that is, the continuous pilot signal that receives the frequency domain signal in the time slot is extracted, and the continuous pilot signal actually received by the CMMB system is a fixed pilot signal, and the received continuous pilot signal is actually received. That is, the channel response of the pilot point, and each pilot subcarrier corresponds to a set of channel estimation samples. Step 2: Calculate the amplitude spectrum of the channel estimate, that is, calculate the amplitude spectrum of the pilot signal. An M-point FFT transform is performed on the extracted pilot signal with respect to the time direction. Each successive pilot subcarrier corresponds to a set of FFT transform data, and the transform data is subjected to absolute value processing to obtain an amplitude spectrum. The amplitude spectrum of different contiguous pilots is added and averaged to obtain a time slot amplitude spectrum, which is denoted as {^( }.^^. Step 3: Filter the time slot amplitude spectrum. Filter the time slot amplitude spectrum, It can effectively suppress the amplitude spectrum noise glitch, make the spectrum smooth and easy to detect. Suppose the coefficient of the filter is recorded as {/^) _≤ ^, right. _{When ≤ t < M} for filtering, the boundary value is cyclically extended, and then filtered according to the following formula: (k) = ∑ _ _{L ≤ n ≤ L} Y (mod (k - nM)) k) 0 ≤ k < M where is the filter coefficient, mod(.,M) is the modulo operation for M, and M is the number of points of the FFT transformation. Step 4: Perform a folding average on the filtered amplitude spectrum, estimate the noise level, calculate a detection threshold, detect a maximum Doppler frequency offset, and output a Doppler frequency offset estimation value. By averaging the amplitude spectrum, it is possible to reduce the probability of false detection of the edge of the amplitude spectrum, and at the same time, the level of the noise is obtained by the amplitude spectrum, and the detection threshold is adaptively selected by the noise. The amplitude spectrum folding average formula is as follows:

Yna _f (k) = (Y _sm (k) +

Where ( ^k ) is the filtered amplitude spectrum, and mod(', M) is the modulo operation for M, 0 ≤ A < . Calculate the maximum value of the folded average amplitude spectrum:

Where _alf (k) is the amplitude spectrum after folding average. Calculate the average of the partial frequency values of the folded average amplitude spectrum:

Calculate the ratio to L, which is recorded as:

N _a =NIY _m and the ratio is used. To measure the noise level and keep it for use in the next step. It should be noted that this should be. The step of measuring the noise level may also be performed before or after any step before the calculation of the filter coefficients after the amplitude spectrum is folded and averaged, such as after the calculation of the output maximum Doppler shift step, which is not limited by the present invention. Detection threshold _{calculated: thr = Y max -a + N} - (\ - a) wherein factor "is not larger than a predetermined positive number, for example, select" = ^05. Detect edge point position:

>thr,0≤k< ₄

The ^^ obtained is an integer value. Linear interpolation of functions to improve edge point resolution - repair value: thr-Y _half (k _edge )

k _d -k _d +-

Yhalf (^edge + 1) - ^half (^edge ) Calculate the maximum Doppler frequency offset using the following formula: ― edge 1

d _ M — T where T is the duration of one OFDM symbol; then, the maximum Doppler shift / _{d is} output. Step 5: Calculate the filter coefficients and update the filter coefficients according to the estimated noise level and the maximum Doppler shift in step 4. Go back to step 1, repeat the above steps. It is determined whether the noise level j ₀ is greater than a predetermined threshold TH, if. If it is greater than the predetermined threshold TH, no filtering is required, that is, the coefficient is set to /z(0) = l, h(k) = 0 , where k≠0; otherwise, the filter coefficient is set to the Gaussian filter coefficient . The Gaussian filter parameter ^σ size is determined by the following formula: a = j3-(f _d MT) where, constant /? A positive value less than 1 can be set in advance. The Gaussian filter coefficients are:

' k ² ,

0<|Λ|<3σ and normalize the Gaussian filter coefficients Return to step 4 for 1 and repeat step 4 for 1 to 5. It should be noted that the Gaussian filter is selected in step 5, and other types of window functions may be selected. Similarly, the width of the window function is selected according to the last Doppler estimation value. Apparatus Embodiment In accordance with an embodiment of the present invention, an apparatus for estimating the maximum Doppler shift is provided. 6 is a structural block diagram of an apparatus for estimating a maximum Doppler frequency offset according to an embodiment of the present invention. As shown in FIG. 6, the apparatus includes: a first calculating module 2, a second calculating module 4, a filtering module 6, and a processing module. 8. Calculate the output module 10. The above structure will be described below. a first calculation module 2, configured to calculate a channel estimation according to the received signal; a second calculation module 4, connected to the first calculation module 2, configured to calculate an amplitude spectrum of the channel estimation obtained by the first calculation module 2; The second calculation module 4 is configured to filter the amplitude spectrum obtained by the second calculation module 4; the processing module 8 is connected to the filtering module 6 for performing the folding average of the filtered amplitude spectrum of the filtering module 6, according to the folding average The subsequent amplitude spectrum calculates the detection threshold; the calculation output module 10 is connected to the processing module 8 for calculating and outputting the maximum Doppler frequency offset according to the detection threshold. FIG. 7 is a block diagram showing a preferred structure of an apparatus for estimating a maximum Doppler frequency offset according to an embodiment of the present invention. As shown in FIG. 7, the apparatus further includes: an estimating module 76 for estimating a noise level; a module 72, configured to calculate a filter coefficient according to a noise level and a maximum Doppler frequency offset; an update module 74, coupled to the third calculation module 72, for updating the filter coefficients obtained by the third calculation module 72 for filtering Module 6 performs filtering processing based on the updated filter coefficients. In summary, according to the above embodiment of the present invention, a method for detecting a Doppler U-shaped spectral width is provided, and a filter that adaptively selects a parameter according to a spectral width and a noise level is used for filtering, and a spectral folding average processing is performed. And adaptively selecting the detection threshold according to the noise level, so that the noise is small, and the operation can be performed under a low SNR channel, and the Doppler frequency offset estimation accuracy is high. Obviously, those skilled in the art will appreciate that the various modules or steps of the present invention described above can be implemented with a general purpose computing device, which can be centralized on a single computing device, or distributed. Alternatively, on a network of computing devices, they may be implemented by program code executable by the computing device, such that they may be stored in the storage device by the computing device, or they may be separately Each integrated circuit module, or a plurality of modules or steps thereof, are fabricated as a single integrated circuit module. Thus, the invention is not limited to any specific combination of hardware and software. The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes can be made to the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the scope of the present invention are intended to be included within the scope of the present invention.

Claims

Claims A method for estimating the maximum Doppler shift, which is characterized by:

Calculating a channel estimate based on the received signal;

Calculating an amplitude spectrum of the channel estimation, and filtering the amplitude spectrum; performing a folding average on the filtered amplitude spectrum, and calculating a detection threshold according to the amplitude spectrum after the folding average;

The maximum Doppler shift is calculated and output according to the detection threshold. The method according to claim 1, wherein the amplitude spectrum is filtered by the following formula:

Y _sm (k) = ∑ _ _L ≤ n _{≤ L} Y(moA(k - n, M))h(k), where h(J() is the filter coefficient, mod(', M) is the pair M Performing a modulo operation, M is the number of points of the fast Fourier transform FFT, Q < k < M. The method according to claim 1, characterized in that the filtered amplitude spectrum is subjected to a folding average by the following formula: if ik) = (Y _sm (k) + Y _sm (mod(M - k )))/2 , where t^) is the filtered amplitude spectrum, mod(', M) is a modulo operation on M, ≤ k<^. The method according to claim 1, wherein the calculating the detection threshold according to the amplitude spectrum after the folding average comprises: calculating the detection threshold /^^^^^^^, wherein "is a constant less than 1 And 7 _max is a maximum value of the amplitude spectrum after the folding average, and is an average value of a partial spectrum in the amplitude spectrum after the folding average. The method according to claim 1, wherein, according to the detection threshold Calculating the maximum Doppler frequency offset includes: According to the formula k _ecke = max{k; >thr,0≤k< }, find the most spectral value greater than the threshold.

a large index, wherein, for the amplitude spectrum after folding the average, t/?r is the detection threshold,

M is the number of points of the FFT; the maximum Doppler frequency offset = . is calculated according to the maximum index, where T is the sampling time interval of the channel estimation. The method according to claim 5, wherein before calculating the maximum Doppler frequency offset according to the maximum cable I k _edge , the method further comprises: according to the formula ^ + l) - Y _half ( k _edge ))

The maximum index is linearly interpolated, and the corrected maximum index is calculated according to the corrected maximum index to calculate the maximum Doppler frequency offset _fd . The method according to claim 1, further comprising: estimating a noise level; said estimating the noise level comprising: calculating a maximum value of the amplitude spectrum after said folding average: ) L = ^^ft (W}, Wherein, the amplitude spectrum after the folding is averaged;

Calculating the average of the partial spectrum of the amplitude spectrum after the folding average:

Calculate the noise level as . The method according to claim 7, wherein the method further comprises:

The filter coefficients are calculated and updated based on the noise level and the maximum Doppler shift. The method according to claim 8, wherein calculating and updating the filter coefficients based on the noise level and the maximum Doppler frequency offset comprises: determining the noise level. Whether it is less than the set threshold TH, and if so, setting the filter coefficient to a unit impulse function; otherwise, setting the σ value of the filter coefficient to a Gaussian filter of σ = β·(f _d MT), and The Gaussian filter is normalized.

10. The method of claim 8, wherein after calculating and updating the filter coefficients based on the noise level and the maximum Doppler shift, the method further comprises: according to the updated The filter coefficients filter the amplitude spectrum.

11. An apparatus for estimating a maximum Doppler frequency offset, comprising:

a first calculating module, configured to calculate a channel estimate according to the received signal;

a second calculating module, configured to calculate an amplitude spectrum of the channel estimation obtained by the first calculating module;

a filtering module, configured to filter the amplitude spectrum obtained by the second computing module, and a processing module, configured to perform a folding average on the amplitude spectrum filtered by the filtering module, and calculate the amplitude spectrum according to the folded average Detection threshold

And calculating an output module, configured to calculate and output the maximum Doppler frequency offset according to the detection threshold.

12. The device according to claim 11, further comprising:

An estimation module for estimating a noise level;

a third calculating module, configured to calculate a filter coefficient according to the noise level and the maximum Doppler frequency offset;

And an update module, configured to update using the filter coefficients obtained by the third calculation module.