CN101989968A - Frequency offset estimation method and device - Google Patents
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Abstract
The invention relates to the field of communication technology and discloses a frequency offset estimation method and a frequency offset estimation device. The method comprises the following steps of: removing a circulating prefix of a received signal, and transforming the received signal of which the circulating prefix is removed from time domain to frequency domain; acquiring received data at a reference signal position; carrying out LS channel estimation and noise estimation on the received data; according to the noise estimation result and the LS channel estimation result, calculating channel estimation of different orthogonal frequency division multiplexing (OFDM) symbols; carrying out frequency offset estimation by using phase difference among channel estimation of the different OFDM symbols to obtain the frequency offset estimation result of the current subframe. Due to the adoption of the invention, the accuracy of the frequency offset estimation result can be improved.
Description
Technical field
The present invention relates to the communication technology, be specifically related to a kind of frequency deviation estimating method and device.
Background technology
The frequency deviation of mobile communication system is normally owing to there are differences between the frequency of local oscillator and the carrier frequency and to cause.If frequency deviation is bigger, terminal and base station can't be synchronous, can't finish receiving course.And to OFDM (Orthogonal Frequency Division Multiplexing, OFDM) system, because frequency deviation can be destroyed the orthogonality between each subcarrier, between subcarrier, introduce and disturb, make the mis-behave of system, so ofdm system is more responsive to the deviation ratio other system.Therefore be necessary frequency deviation is effectively measured and estimated, calibrate and follow the tracks of according to measured value then, as far as possible little to guarantee the deviation between local oscillator frequencies and the carrier frequency.
One of modal frequency deviation estimating method is to utilize Cyclic Prefix (CP, Cyclic Prefix) to carry out frequency offset estimating in the ofdm system, and this method is wider owing to not needing extra pilot frequency sequence to use.
The frequency offset estimation procedure of prior art as shown in Figure 1.
At first, the data that receive path receives are carried out frequency offset estimating, particularly, the data of utilizing CP in the OFDM symbol data and symbol tail end are simple relations that repeat, promptly difference between the two mainly is the skew influence that is caused by frequency deviation, obtains the frequency deviation of this sample divided by the time difference and all samples of getting is on average obtained the estimation of frequency deviation with skew.Then, carry out the calibration of time domain frequency deviation, removal CP, FFT processing such as (fast Fourier transforms).
This method is a kind ofly to carry out the method for frequency offset estimating in time domain, and complexity is lower, and if to estimate frequency deviation bigger, can calibrate immediately in time domain, thereby reduce the influence of frequency deviation subsequent operation.But there is certain limitation in this method, mainly is because CP can be subjected to the multipath of last OFDM symbol to be disturbed, thereby causes the frequency offset estimating error bigger.
In addition, also having a kind of method commonly used in the prior art is to carry out frequency offset estimating at frequency domain, and the phase difference that promptly utilizes frequency deviation to produce on different OFDM symbols carries out frequency offset estimating.Disclosed the most frequently used method is LS (least square) channel estimating that at first obtains all reference signal places at frequency domain at present, estimate frequency deviation according to the phase difference value of the LS channel estimating at reference signal place on the different OFDM symbols then, this method estimation performance is much more accurate than the time domain method of estimation.But because the channel estimating of LS is affected by noise bigger, therefore the frequency deviation estimation performance is affected by noise also bigger when signal to noise ratio is low, and the accuracy of estimation can descend.
Summary of the invention
The embodiment of the invention provides a kind of frequency deviation estimating method and device, to improve the accuracy of frequency offset estimation result.
For this reason, the embodiment of the invention provides following technical scheme:
A kind of frequency deviation estimating method comprises:
Remove the Cyclic Prefix in the received signal, and the received signal that will remove behind the Cyclic Prefix transforms from the time domain to frequency domain;
Obtain the locational reception data of reference signal;
Described reception data are carried out least square channel estimating and Noise Estimation respectively;
According to Noise Estimation result and least square channel estimation results, calculate the channel estimating of different OFDM symbols;
Utilize the phase difference between the Minimum Mean Square Error channel estimating of different OFDM symbols to carry out frequency offset estimating, obtain the frequency offset estimation result of current subframe.
Alternatively, described according to the Noise Estimation result to the least square channel estimation results, the channel estimating of calculating different OFDM symbols comprises:
According to the Noise Estimation result least square channel estimation results is carried out Minimum Mean Square Error filtering, obtain containing in the OFDM symbol of reference signal the Minimum Mean Square Error channel estimating of data on the reference signal position; Perhaps
According to the Noise Estimation result least square channel estimation results is carried out interpolation, obtain containing the channel estimating of all or part Resource Unit on the OFDM symbol of reference signal.
Preferably, describedly received signal transformed from the time domain to frequency domain comprise:
Utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
Preferably, described method also comprises: the frequency offset estimation result to the frequency offset estimation result of current subframe and a plurality of subframes before is weighted on average, with the frequency offset estimation result of mean value as current subframe.
Preferably, described method also comprises:
Having many to send and/or during reception antenna, to described many send and/or reception antenna on the frequency offset estimation result of current subframe be weighted on average, with the frequency offset estimation result of mean value as current subframe.
A kind of frequency deviation estimation device comprises:
Remove the unit, be used for removing the Cyclic Prefix of received signal;
Converter unit is used for the received signal behind the removal Cyclic Prefix is transformed from the time domain to frequency domain;
Extraction unit is used to obtain the locational reception data of reference signal;
Channel estimating unit is used for described reception data are carried out the least square channel estimating;
Noise estimation unit is used for described reception data are carried out Noise Estimation;
Computing unit is used for calculating the channel estimating of different OFDM symbols according to Noise Estimation result and least square channel estimation results;
Frequency deviation estimating unit is used to utilize the phase difference between the different OFDM symbol upper signal channels estimations to carry out frequency offset estimating.
Alternatively, described computing unit comprises:
The filtering subelement is used for according to the Noise Estimation result least square channel estimation results being carried out Minimum Mean Square Error filtering, obtains containing in the OFDM symbol of reference signal the Minimum Mean Square Error channel estimating of data on the reference signal position; Perhaps
The interpolation subelement is used for according to the Noise Estimation result least square channel estimation results being carried out interpolation, obtains containing the channel estimating of all Resource Units on the OFDM symbol of reference signal.
Preferably, described converter unit specifically is used to utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
Preferably, described device also comprises:
The smoothing processing unit is used for the frequency offset estimation result of the frequency offset estimation result of current subframe and a plurality of subframes before is weighted on average, with the frequency offset estimation result of mean value as current subframe.
Preferably, described smoothing processing unit, also be used for having many to send and/or during reception antenna, to described many send and/or reception antenna on the frequency offset estimation result of current subframe be weighted on average, with the frequency offset estimation result of mean value as current subframe.
Frequency deviation estimating method that the embodiment of the invention provides and device, by the Cyclic Prefix in the removal received signal, and the received signal that will remove behind the Cyclic Prefix transforms from the time domain to frequency domain; Obtain the locational reception data of reference signal; Described reception data are carried out least square channel estimating and Noise Estimation respectively; According to Noise Estimation result and least square channel estimation results, calculate the channel estimating of different OFDM symbols; Utilize the phase difference between the channel estimating of different OFDM symbols to carry out frequency offset estimating, obtain the frequency offset estimation result of current subframe.Because time span is a plurality of OFDM symbols, so estimated value is relatively accurate.
Description of drawings
Fig. 1 is a kind of flow chart that carries out frequency offset estimating in the prior art in time domain;
Fig. 2 be in the LTE system reference signal at the distribution schematic diagram of time-frequency domain;
Fig. 3 is the flow chart of embodiment of the invention frequency deviation estimating method;
Fig. 4 is a kind of specific implementation flow chart of embodiment of the invention frequency deviation estimating method;
Fig. 5 is the another kind of specific implementation flow chart of embodiment of the invention frequency deviation estimating method;
Fig. 6 is a RS place initial channel estimation schematic diagram in the embodiment of the invention;
Fig. 7 is the result of calculation schematic diagram of three kinds of different frequency deviation estimating methods;
Fig. 8 is a kind of structural representation of embodiment of the invention frequency deviation estimation device;
Fig. 9 is the another kind of structural representation of embodiment of the invention frequency deviation estimation device.
Embodiment
In order to make those skilled in the art person understand the scheme of the embodiment of the invention better, the embodiment of the invention is described in further detail below in conjunction with drawings and embodiments.
Embodiment of the invention frequency deviation estimating method and device for the channel estimating in the advanced line frequency of received signal territory, utilize the phase difference between the different OFDM symbol upper signal channels estimations to carry out frequency offset estimating then.
In order to be illustrated more clearly in the embodiment of the invention, at first ofdm system is carried out simple declaration below.
In the ofdm system, carry out channel estimating in the discrete reference signal (RS) of inserting of time-frequency domain usually.With LTE (Long Term Evolution, Long Term Evolution) system is example, reference signal in the distribution of time-frequency domain as shown in Figure 2, the square that has filling pattern is corresponding RS signal.Be to simplify, the time orientation length (during common CP) of a subframe of only having drawn among the figure, frequency domain direction 12 subcarriers that only drawn, and only represented RS distribution situation on the transmitting antenna port.
The position that has the expression reference signal place of filling pattern among the figure, in the actual conditions different cell reference signals is had different side-play amounts on frequency domain, different transmitting antenna ports is had different distributions, but the relative position between each reference signal substantially to figure in similar, what all adopt is this discrete reference signal distribution mode.
Received signal to all Resource Units is numbered, and is designated as r
K, l, wherein k represents the index of subcarrier, l represents the index of OFDM symbol.A subframe is divided into two time slots (slot), i.e. slot0 and slot1 again.(k l) can determine the position of arbitrary Resource Unit, and the set of supposing all RS position coordinateses is S by coordinate.
Below in conjunction with the distribution legend of above-mentioned reference signal the embodiment of the invention is elaborated at time-frequency domain.
As shown in Figure 3, be the flow chart of embodiment of the invention frequency deviation estimating method, may further comprise the steps:
Described LS channel estimating is to remove the RS value of last correspondence position transmission with the receive channel of RS, such as, assumed position is that (k, l) ((k, l) ∈ S), the data that the RS place sends are d
K, l, the data that the RS place receives are r
K, l, then the LS channel estimating at RS place is as follows:
Described reception data are carried out Noise Estimation, obtain noise variance σ
2, noise estimation method can adopt noise estimation method of the prior art.
Step 304 according to Noise Estimation result and LS channel estimation results, is calculated the channel estimating of different OFDM symbols.
Step 305 utilizes the phase difference between the channel estimating of different OFDM symbols to carry out frequency offset estimating, obtains the frequency offset estimation result of current subframe.
In above-mentioned steps 304,, when calculating the channel estimating of different OFDM symbols, multiple implementation method can be arranged according to Noise Estimation result and LS channel estimation results.Such as, can carry out MMSE (Minimum Mean Square Error) filtering to the LS channel estimation results according to the Noise Estimation result, obtain containing in the OFDM symbol of reference signal the MMSE channel estimating of data on the reference signal position; Perhaps the LS channel estimation results is carried out interpolation, obtain containing the channel estimating of all or part Resource Unit on the OFDM symbol of reference signal, below this is elaborated respectively according to the Noise Estimation result.
As shown in Figure 4, be a kind of specific implementation flow chart of embodiment of the invention frequency deviation estimating method, may further comprise the steps:
Step 401 is removed the Cyclic Prefix in the received signal, and the received signal that will remove behind the Cyclic Prefix transforms from the time domain to frequency domain, such as, can utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
Step 402 is obtained the locational reception data of reference signal.
Step 403 is carried out LS channel estimating and Noise Estimation respectively to described reception data.
Step 404 is carried out MMSE filtering according to the Noise Estimation result to the LS channel estimation results, obtains containing in the OFDM symbol of reference signal the MMSE channel estimating of data on the reference signal position.
The cross-correlation matrix of supposing all RS places in the same OFDM symbol is A, and then the process of frequency domain MMSE filtering can be expressed as:
Wherein, E (|| x||
2) expression received signal average power,
The vector of representing the LS channel estimating composition at RS place in l the OFDM symbol,
K represents the sum of RS in the OFDM symbol, []
TThe transposition of expression vector.
The vector of representing the MMSE channel estimating composition at RS place in l the OFDM symbol.
Said process carries out respectively each OFDM symbol.
The generation of cross-correlation matrix R can adopt prior art to carry out approximate calculation, repeats no more here.
Step 405 utilizes the phase difference between the channel estimating of different OFDM symbols to carry out frequency offset estimating, obtains the frequency offset estimation result of current subframe.
Particularly, can utilize the phase deviation between the MMSE channel estimating at RS place on the different OFDM symbols to carry out frequency offset estimating, there is the phase difference that causes by non-frequency deviation reason owing to the channel estimating on the different sub carrier itself, therefore can only use the phase difference between the channel estimating at different OFDM symbol same sub-carrier place when calculating phase difference, shown in the arrow as shown in Figure 2, be exactly to utilize the phase difference between the MMSE channel estimating at the RS place that subcarrier number is identical in k=7 and two OFDM symbols of k=0 to carry out frequency offset estimating in Fig. 2, and the phase difference between the MMSE channel estimating at the RS place that subcarrier number is identical in k=11 and two OFDM symbols of k=4 carry out frequency offset estimating.
Can meet the phase difference that the Resource Unit of above condition calculates to all and average, to improve estimated accuracy.Can calculate frequency deviation after estimating phase difference θ:
Wherein Δ t represents to be used for to estimate the time difference between two OFDM symbols of phase difference, can be 0.5 millisecond for present embodiment Δ t.
As shown in Figure 5, be the another kind of specific implementation flow chart of embodiment of the invention frequency deviation estimating method, may further comprise the steps:
Step 501 is removed the Cyclic Prefix in the received signal, and the received signal that will remove behind the Cyclic Prefix transforms from the time domain to frequency domain, such as, can utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
Step 502 is obtained the locational reception data of reference signal.
Step 503 is carried out LS channel estimating and Noise Estimation respectively to described reception data.
Step 504 is carried out interpolation according to the Noise Estimation result to the LS channel estimation results, obtains containing the channel estimating of all or part Resource Unit on the OFDM symbol of reference signal.
Particularly, can adopt the MMSE interpolation method, can also adopt lagrange-interpolation or transform domain interpolation method, this embodiment of the invention is not done qualification.
The cross-correlation matrix of supposing interior all subcarriers of same OFDM symbol and all RS places is R, and the cross-correlation matrix at all RS places is A in the same OFDM symbol, and then the process of frequency domain MMSE interpolation can be expressed as:
Wherein, E (|| x||
2) expression received signal average power,
The vector of representing the LS channel estimating composition at RS place in l the OFDM symbol,
K represents the sum of RS in the OFDM symbol, []
TThe transposition of expression vector.
Represent the vector that all MMSE of subcarrier place channel estimating of l OFDM symbol are formed, N represents the total number of sub-carriers in the symbol.
This process is carried out respectively each OFDM symbol.The generation of cross-correlation matrix R can adopt prior art to carry out approximate calculation, no longer gives unnecessary details here.
Step 505 utilizes the phase difference between the channel estimating of different OFDM symbols to carry out frequency offset estimating, obtains the frequency offset estimation result of current subframe.
Particularly, can utilize the phase deviation between the MMSE channel estimating at RS place on the different OFDM symbols to carry out frequency offset estimating, there is the phase difference that causes by non-frequency deviation reason owing to the channel estimating on the different sub carrier itself, therefore can only use the phase difference between the channel estimating at different OFDM symbol same sub-carrier place when calculating phase difference, since this moment by step 504 interpolation obtain containing the channel estimating of all subcarriers in the OFDM symbol of RS, therefore can use the phase difference between the OFDM symbol of two adjacent band RS this moment, shown in the arrow as shown in Figure 6, it in Fig. 6 the phase difference that utilizes between the MMSE channel estimating at the Resource Unit place that subcarrier number is identical in k=4 and two OFDM symbols of k=0, and k=11 and two OFDM symbols of k=7, the Resource Unit that meets above-mentioned requirements is to being that one of them is the Resource Unit at RS place, as r
0,0And r
0,4, r
3,7And r
3,11, Resource Unit that also can right and wrong RS place is as r
1,0And r
1,4
Can the Resource Unit that all or part meets above condition be averaged the phase difference that calculates, to improve estimated accuracy.
Can calculate frequency deviation after estimating phase difference θ
Wherein Δ t represents to be used for to estimate the time difference between two OFDM symbols of phase difference, is the time difference between adjacent two OFDM symbols that contain RS for present embodiment Δ t.
In order further to improve the accuracy of frequency offset estimation result, in the various embodiments described above, also can further may further comprise the steps:
Frequency offset estimation result to the frequency offset estimation result of current subframe and a plurality of subframes before is weighted on average, with the frequency offset estimation result of mean value as current subframe.
If system has adopted MIMO (Multiple Input Multiple Output, multiple-input and multiple-output) technology, when many transmissions and/or reception antenna are promptly arranged, can also the frequency offset estimation result of the current subframe on described many transmissions and/or the reception antenna be weighted on average, with the frequency offset estimation result of mean value as current subframe.
Embodiment of the invention frequency deviation estimating method for the channel estimating in the advanced line frequency of received signal territory, utilizes the phase difference between the different OFDM symbol upper signal channels estimations to carry out frequency offset estimating then.Because when utilizing the embodiment of the invention to carry out frequency offset estimating, time span is a plurality of OFDM symbols, therefore can improve the accuracy of estimated value.
Certainly, in embodiments of the present invention, the method for channel estimating is not limited in the step 404 of above-mentioned Fig. 4 and this dual mode of step 504 shown in Figure 5, also can adopt some other modes.
Can further prove by The Realization of Simulation, utilize the method for the embodiment of the invention can improve the accuracy of frequency offset estimation result.
As shown in Figure 7, be the result of calculation schematic diagram of three kinds of different frequency deviation estimating methods:
Test condition is as follows: channel, ETU70; Frequency deviation (FreOffset)=500Hz; Signal to noise ratio (snr)=-5dB.
Icon td represents that the prior art time domain uses CP to carry out estimation approach, and fd1 represents that the prior art frequency domain uses LS to carry out the method for channel estimating, and fd2 represents the frequency offset estimation result that the method according to the embodiment of the invention shown in Figure 4 obtains.
By test shown in Figure 7 as seen, compared with prior art, utilize the method for the embodiment of the invention, can improve the accuracy of frequency offset estimation result effectively.
One of ordinary skill in the art will appreciate that all or part of step that realizes in the foregoing description method is to instruct relevant hardware to finish by program, described program can be stored in the computer read/write memory medium, described storage medium, as: ROM/RAM, magnetic disc, CD etc.
The embodiment of the invention also provides a kind of frequency deviation estimation device, as shown in Figure 8, is a kind of structural representation of this frequency deviation estimation device.
In this embodiment, described device comprises:
Remove unit 801, be used for removing the Cyclic Prefix of received signal;
Frequency deviation estimating unit 807 is used to utilize that the phase difference between the MMSE channel estimating carries out frequency offset estimating on the different OFDM symbols.
In embodiments of the present invention, described computing unit can have multiple implementation, specifically can comprise: filtering subelement or interpolation subelement (not shown), wherein, described filtering subelement, be used for the LS channel estimation results being carried out MMSE filtering, obtain containing in the OFDM symbol of reference signal the MMSE channel estimating of data on the reference signal position according to the Noise Estimation result; Described interpolation subelement is used for according to the Noise Estimation result LS channel estimation results being carried out interpolation, obtains containing the channel estimating of all Resource Units on the OFDM symbol of reference signal.
The realization flow that utilizes the embodiment of the invention to carry out frequency offset estimating can not repeat them here with reference to the description in the embodiment of the invention frequency deviation estimating method of front.
Embodiment of the invention frequency deviation estimation device for the channel estimating in the advanced line frequency of received signal territory, utilizes the phase difference between the different OFDM symbol upper signal channels estimations to carry out frequency offset estimating then.Because when utilizing the embodiment of the invention to carry out frequency offset estimating, time span is a plurality of OFDM symbols, therefore can improve the accuracy of estimated value.
As shown in Figure 9, be the another kind of structural representation of embodiment of the invention frequency deviation estimation device.
Be with difference embodiment illustrated in fig. 8, in this embodiment, also comprise:
If system adopts the MIMO technology, promptly there are many to send and/or reception antenna, then described smoothing processing unit 808, also can be further used for the frequency offset estimation result of the current subframe on described many transmissions and/or the reception antenna is weighted on average, with the frequency offset estimation result of mean value as current subframe.Thereby can further improve the accuracy of frequency offset estimating.
More than the embodiment of the invention is described in detail, used embodiment herein the present invention set forth, the explanation of above embodiment just is used for help understanding method and apparatus of the present invention; Simultaneously, for one of ordinary skill in the art, according to thought of the present invention, the part that all can change in specific embodiments and applications, in sum, this description should not be construed as limitation of the present invention.
Claims (10)
1. a frequency deviation estimating method is characterized in that, comprising:
Remove the Cyclic Prefix in the received signal, and the received signal that will remove behind the Cyclic Prefix transforms from the time domain to frequency domain;
Obtain the locational reception data of reference signal;
Described reception data are carried out least square channel estimating and Noise Estimation respectively;
According to Noise Estimation result and least square channel estimation results, calculate the channel estimating of different OFDM symbols;
Utilize the phase difference between the Minimum Mean Square Error channel estimating of different OFDM symbols to carry out frequency offset estimating, obtain the frequency offset estimation result of current subframe.
2. method according to claim 1 is characterized in that, described according to Noise Estimation result and least square channel estimation results, the channel estimating of calculating different OFDM symbols comprises:
According to the Noise Estimation result least square channel estimation results is carried out Minimum Mean Square Error filtering, obtain containing in the OFDM symbol of reference signal the Minimum Mean Square Error channel estimating of data on the reference signal position; Perhaps
According to the Noise Estimation result least square channel estimation results is carried out interpolation, obtain containing the channel estimating of all or part Resource Unit on the OFDM symbol of reference signal.
3. method according to claim 1 is characterized in that, describedly received signal is transformed from the time domain to frequency domain comprises:
Utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
4. according to each described method of claim 1 to 3, it is characterized in that described method also comprises:
Frequency offset estimation result to the frequency offset estimation result of current subframe and a plurality of subframes before is weighted on average, with the frequency offset estimation result of mean value as current subframe.
5. method according to claim 4 is characterized in that, described method also comprises:
Having many to send and/or during reception antenna, to described many send and/or reception antenna on the frequency offset estimation result of current subframe be weighted on average, with the frequency offset estimation result of mean value as current subframe.
6. a frequency deviation estimation device is characterized in that, comprising:
Remove the unit, be used for removing the Cyclic Prefix of received signal;
Converter unit is used for the received signal behind the removal Cyclic Prefix is transformed from the time domain to frequency domain;
Extraction unit is used to obtain the locational reception data of reference signal;
Channel estimating unit is used for described reception data are carried out the least square channel estimating;
Noise estimation unit is used for described reception data are carried out Noise Estimation;
Computing unit is used for calculating the channel estimating of different OFDM symbols according to Noise Estimation result and LS channel estimation results;
Frequency deviation estimating unit is used to utilize the phase difference between the different OFDM symbol upper signal channels estimations to carry out frequency offset estimating.
7. device according to claim 6 is characterized in that, described computing unit comprises:
The filtering subelement is used for according to the Noise Estimation result LS channel estimation results being carried out Minimum Mean Square Error filtering, obtains containing in the OFDM symbol of reference signal the Minimum Mean Square Error channel estimating of data on the reference signal position; Perhaps
The interpolation subelement is used for according to the Noise Estimation result LS channel estimation results being carried out interpolation, obtains containing the channel estimating of all Resource Units on the OFDM symbol of reference signal.
8. device according to claim 6 is characterized in that,
Described converter unit specifically is used to utilize fast Fourier transform that received signal is transformed from the time domain to frequency domain.
9. according to each described device of claim 6 to 9, it is characterized in that described device also comprises:
The smoothing processing unit is used for the frequency offset estimation result of the frequency offset estimation result of current subframe and a plurality of subframes before is weighted on average, with the frequency offset estimation result of mean value as current subframe.
10. device according to claim 9 is characterized in that,
Described smoothing processing unit also is used for having many to send and/or during reception antenna, to described many send and/or reception antenna on the frequency offset estimation result of current subframe be weighted on average, with the frequency offset estimation result of mean value as current subframe.
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