CN102130871A - Channel estimation method and device - Google Patents
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- CN102130871A CN102130871A CN201010022872XA CN201010022872A CN102130871A CN 102130871 A CN102130871 A CN 102130871A CN 201010022872X A CN201010022872X A CN 201010022872XA CN 201010022872 A CN201010022872 A CN 201010022872A CN 102130871 A CN102130871 A CN 102130871A
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Abstract
The invention discloses a channel estimation method which comprises the following steps of: performing channel response estimation on a discrete pilot sub-carrier in an orthogonal frequency division multiplexing symbol output after fast Fourier transform to obtain a channel response value of the discrete pilot sub-carrier; performing interpolation estimation in time direction according to the channel response value of the discrete pilot sub-carrier to obtain a time direction channel response value; transforming the time direction channel response value from a frequency domain to a time domain through fast Fourier transform to estimate the maximum multi-path time delay of a channel; and selecting a filter set corresponding to the maximum multi-path time delay from a preset filter group according to the maximum multi-path time delay and performing interpolation estimation in frequency direction according to the channel response value in the time direction to obtain the response value of the entire channel. The invention also provides a channel estimation device. By adopting the channel estimation method and device, the performance requirement on channel estimation can be met, and low complexity of a system can be realized.
Description
Technical field
The present invention relates to a kind of channel estimating apparatus that is applicable to the channel estimation methods of ofdm system and is applied to receiver.
Background technology
OFDM (OFDM:Orthogonal Frequency Division Multiplexing) is a kind of multi-transceiver technology, its cardinal principle is that channel is divided into several orthogonal sub-channels (Sub Channel), on each subchannel, use a subcarrier (Subcarrier) to modulate, and each subcarrier parallel transmission.Like this, although total channel be non-flat forms, have frequency selectivity, but each subchannel is a relatively flat, what carry out on each subchannel is narrow band transmission, and therefore signal bandwidth just can significantly reduce the phase mutual interference between the subchannel less than the respective bandwidth of channel; In addition, because the carrier wave of each subchannel is mutually orthogonal in ofdm system, so their frequency spectrum can be overlapped, so not only reduced the phase mutual interference between subcarrier, simultaneously improved the availability of frequency spectrum again, so for example be widely used in the digital video broadcasting communication systems such as (DVB:Digital Video Broadcasting).
The performance of wireless communication system mainly is subjected to the restriction of wireless channel, and the propagation path between the transmitter and receiver is very complicated, and the topography and geomorphology from simple line-of-sight propagation to the various complexity of experience is as the propagation of influences such as building, mountain range and forest.In addition, wireless channel is fixing unlike wire message way also can be predicted, but has very big randomness, causes amplitude, phase place and the frequency distortion of received signal, is difficult to analyze.In order to guarantee that communication system has good performance in wireless channel environment, must to the time multipath wireless channel that becomes estimate and this difficulty particularly under the high-speed mobile situation as far as possible accurately.
Because the raising of the performance with 2.3dB-3dB is compared in coherent demodulation with non-coherent demodulation, so generally adopt coherent demodulation technology at receiving terminal.Coherent demodulation need be used the information of channel, and the soft decoding of channel simultaneously also needs the information of channel, and therefore requiring has good channel estimating, performance for estimating channel directly to influence the result of demodulation sign indicating number.
For the channel estimation methods in the ofdm system, be broadly divided into auxiliary and blind Channel Estimation two classes of pilot tone (pilot), because the channel estimation methods superior performance that pilot tone is auxiliary, the channel estimation methods that has practical value at present generally all will be by known pilot frequency information.In ofdm system, usually adopt the channel estimation methods of pilot assisted modulation, promptly in data streams, insert pilot frequency information, behind the receiving terminal receiving data stream, extract pilot frequency information, by calculating the channel response on the pilot frequency locations, utilize the interpolation estimation approach to estimate other locational channel response again.But, in existing ofdm system, the design of channel estimator mainly contains two problems: the one, and the choosing of pilot frequency information is because the time-varying characteristics of wireless channel need receiver constantly channel to be followed the tracks of, so pilot frequency information also must constantly transmit; The 2nd, satisfy the requirement that existing lower complexity has good pilot tracking capability again, under definite pilot tone send mode and channel estimating criterion condition, seek best channel estimator structure.In actual design, the design of choosing with the optimum channel estimator of pilot frequency information is mutually related again usually, because the performance of channel estimator is relevant with the load mode of pilot frequency information.The load mode of pilot tone is determining the method and the performance of channel estimation technique.Two topmost parameters must considering when pilot frequency information is chosen are: maximum doppler frequency and maximum multipath time delay, and the former has determined minimum correlation time, the latter has determined minimum correlation bandwidth.
In digital video broadcast-terrestrial or digital video mobile broadcast (DVB-T/H) system, can adopt two kinds of pilot tone modes of continuous pilot and scattered pilot, as shown in Figure 1, continuous pilot just is meant the pilot frequency information that the same position at each frame all exists, each frame of information transmitted all is the same on the pilot sub-carrier, concrete numerical value is controlled by a pseudo random sequence (PRBS), as the K among the figure
MinAnd K
MaxSubcarrier.Continuous pilot is dispersed in some fixing sub-carrier positions according to certain rules, and 45/89/177 continuous pilot subcarrier is arranged respectively under the 2k/4k/8k pattern.And scattered pilot is meant the pilot sub-carrier according to the certain rule discrete distribution, the scattered pilot subcarrier of adjacent four OFDM symbols can not repeat, with four OFDM symbols is the cycle, and for the individual symbol of l (l=0～67) in the frame, the position of its scattered pilot belongs to subclass { k=K
Min+ 3 * (lmod4)+12p|pint, 0 of p, k ∈ [K
Min, K
Max], 131/262/524 scattered pilot subcarrier is arranged respectively under the 2k/4k/8k pattern.Wherein, as long as k does not exceed effective range, then p is an integer, gets all probable values more than or equal to zero.Continuous pilot is present in the stator carrier position of each OFDM symbol, be mainly used to carry out carrier frequency offset estimation and tracking, and therefore scattered pilot is used to carry out the channel estimating of frequency domain interpolation owing to have the very position of rule.The arrangement of this pilot tone in the DVB-T/H system has determined channel estimating to finish at frequency domain, could do channel estimating according to the scattered pilot subcarrier.The channel estimating of pilot tone can be divided into two steps.At first from the frequency-region signal that receives, extract the channel response value of pilot tone point, and because the pilot tone that sends is known, therefore can obtain the channel response value of pilot frequency locations, second step was to estimate to obtain channel estimating on other data subcarrier by interpolation, finished the equilibrium to signal then.
The complexity of channel estimating and performance are a pair of contradiction, how to design and realize satisfying the systematic function requirement, possess the channel estimator of low implementation complexity simultaneously, are difficult problems.In traditional pilot tone point channel estimating, generally all be based on least square (Least Square, LS) channel estimation method.LS estimates only to need a single-order equalizer just can estimate pilot tone point channel response, and its operand is only along with the pilot tone linearity increase of counting.But the LS algorithm is subjected to the influence disturbed between white Gaussian noise and subcarrier very big, and poor-performing when signal to noise ratio is low has limited the application of this kind method.Therefore, other has proposed a kind of based on least mean-square error (Minimum Mean Square Error, MMSE) or linear minimum mean-squared error (this method is for disturbing between subcarrier and white Gaussian noise has the good restraining effect for LineMMSE, LMMSE) channel estimation method.But it is the correlation of utilizing all N subcarrier that MMSE or LMMSE estimate, needs the matrix of a N x N to take advantage of, and its complexity depends on the number of system carrier, and complexity is higher, and the increase of counting along with computing, and its algorithm complex is the index multiplication and grows.
In addition, after obtaining the channel response value of pilot frequency locations, the channel response value of data subcarrier position just can obtain by the time-frequency two-dimensional filtering interpolation.Have nothing in common with each other in complexity, realizability and the aspect of performance that can reach based on the channel estimation technique of pilot tone interpolation, what wherein performance was best is two-dimentional Wiener filtering, utilize Weiner filter to carry out channel estimating, its performance is relevant with the exponent number of Weiner filter, exponent number is big more, its performance is superior more, but its computational complexity is also high more, so need compromise between performance and computational complexity.Because two-dimentional Weiner filter needs very big computational complexity, so proposed corresponding alternative method, as list of references " An Adaptive Two-Dimensional Channel Estimatorfor Wireless OFDM with Application to Mobile DVB-T " (Frieder Sanzi.JoachimSpeidel, IEEE on Broadcasting, VOL.46, NO.2, JUNE 2000), its alternative method that adopts is exactly to utilize the cascade of two one dimension Weiner filters to substitute original two-dimentional Weiner filter, reduces computational complexity under the prerequisite that does not reduce performance greatly.Yet, adopt Weiner filter (no matter be one dimension or two dimension), its filter factor is to obtain according to the auto-correlation function of channel and cross-correlation function, come the coefficient of real-time update Weiner filter in theory by the variation of following the tracks of channel, but the coefficient that calculates Weiner filter in fact in real time is very complicated, and it is higher that complexity still shows comparatively speaking.
Summary of the invention
The invention provides a kind of channel estimation methods and be applied to the channel estimating apparatus of receiver, reduce system complexity.
Embodiments of the present invention provide a kind of channel estimation methods, described channel estimation methods comprises the steps: that the scattered pilot subcarrier in the OFDM symbol of being exported after the fast fourier transform is carried out channel response to be estimated, obtains the channel response value of scattered pilot subcarrier; According to the channel response value of described scattered pilot subcarrier, obtain the time orientation channel response value in the enterprising row interpolation estimation of time orientation; With described time orientation channel response value by invert fast fourier transformation by frequency domain transform to time domain, estimate the maximum multipath time delay of channel; According to described maximum multipath time delay, in the group of filters that sets in advance, choose the bank of filters corresponding with described maximum multipath time delay; Adopt selected bank of filters,, estimate, obtain the response of whole channel at the enterprising row interpolation of frequency direction according to the channel response value of described time orientation.
Optionally, the interpolation of described time orientation estimates to be meant and utilizes that adjacent two scattered pilot subcarriers carry out interpolation on the time orientation, and the interpolation of finishing an OFDM symbol will be used a plurality of OFDM symbols in the front and back that comprise this OFDM symbol.
Optionally, when the included OFDM symbol of the described OFDM of finishing interpolation has different sequential, remove/apply sequential and regulate.
Optionally, the described maximum multipath time delay that estimates channel specifically comprises: described time orientation channel response value is carried out many times of interpolation and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then; With the smart OFDM symbol original position of determining synchronously of sequential is starting point, slide backward protection length at interval, every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time; With write down last the footpath with OFDM symbol original position separately time corresponding subtract each other, obtain maximum multipath time delay.Described many times of interpolation are meant three times of interpolation.
Optionally, after obtaining maximum multipath time delay,, choose one group of corresponding in described group of filters bank of filters according to described maximum multipath time delay and protection ratio size at interval.Described group of filters has four bank of filters, and each bank of filters has three filters respectively.
Another embodiment of the present invention provides a kind of channel estimating apparatus that is applied to receiver, it is characterized in that, described channel estimating apparatus comprises: the pilot tone estimation unit, be used for that the scattered pilot subcarrier in the OFDM symbol of being exported after the fast fourier transform is carried out channel response and estimate, obtain the channel response value of scattered pilot subcarrier; The time domain interpolation filter is used for the channel response value according to the scattered pilot subcarrier, estimates at the enterprising row interpolation of time orientation, obtains the time orientation channel response value; The time delay estimation unit, according to described time orientation channel response value by invert fast fourier transformation by frequency domain transform to time domain, estimate the maximum multipath time delay of channel; The frequency domain interpolation group of filters, have a plurality of bank of filters, and each bank of filters has a plurality of filters, be used for choosing and have the bank of filters corresponding with described maximum multipath time delay according to described maximum multipath time delay, and then according to the channel response value of described time orientation, estimate at the enterprising row interpolation of frequency direction, obtain the response of whole channel.
Optionally, the interpolation of described time orientation estimates to be meant and utilizes that adjacent two scattered pilot subcarriers carry out interpolation on the time orientation, and the interpolation of finishing an OFDM symbol will be used a plurality of OFDM symbols in the front and back that comprise this OFDM symbol.
Optionally, when the included OFDM symbol of the described OFDM of finishing interpolation has different sequential, remove/apply sequential and regulate.
Optionally, described pilot tone estimation unit is the least square estimation device, and described least square estimation device is the single-order equalizer.
Optionally, described time delay estimation unit estimates maximum multipath time delay and specifically comprises: described time orientation channel response value is carried out many times of interpolation and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then; With the smart OFDM symbol original position of determining synchronously of sequential is starting point, slide backward protection length at interval, every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time; With write down last the footpath with OFDM symbol original position separately time corresponding subtract each other, obtain maximum multipath time delay.Described many times of interpolation are meant three times of interpolation.
Optionally, after obtaining maximum multipath time delay,, choose one group of corresponding in described group of filters bank of filters according to described maximum multipath time delay and protection ratio size at interval.Described frequency domain interpolation group of filters has four bank of filters, and each bank of filters has three filters respectively.
Compared with prior art, technique scheme provides the bank of filters with adaptive-filtering coefficient, can on time orientation, adopt the linear interpolation estimation to obtain the time orientation channel response value by the channel response value of the scattered pilot subcarrier estimating based on channel response to obtain, and then in time domain, maximum multipath time delay is estimated in view of the above, choose adaptively with the corresponding filter of described maximum multipath time delay and at the enterprising row interpolation of frequency direction and estimate, finally obtain the response of whole channel, with respect to available technology adopting two dimension Weiner filter or utilize the channel estimation methods such as cascade of two one dimension Weiner filters, the present invention can not only satisfy the performance for estimating channel requirement, more the low complex degree of feasible system.
Description of drawings
Fig. 1 is the frame structure schematic diagram that generally has the OFDM symbol of pilot sub-carrier;
Fig. 2 is the basic procedure step of a kind of execution mode of channel estimation methods of the present invention;
Fig. 3 is the frame structure schematic diagram of estimating at the enterprising row interpolation of time orientation in one embodiment of the present invention;
Fig. 4 is the frame structure schematic diagram of estimating at the enterprising row interpolation of frequency direction in one embodiment of the present invention;
Fig. 5 is the detailed process step of the method for estimation of maximum multipath time delay among the step S25 shown in Figure 2;
Fig. 6 is the structural representation of a kind of execution mode of channel estimating apparatus of the present invention.
Embodiment
The technical program is to be applied in the receiving system of ground digital television broadcast (DVB) standard, particularly DVB-H standard, mainly be based on channel response and estimate that the channel response value of the scattered pilot subcarrier obtain in advance adopts the linear interpolation estimation to obtain the time orientation channel response value on time orientation, and in time domain, maximum multipath time delay is estimated, choose the interpolation of going forward side by side on the line frequency direction adaptively and estimate, obtain the response of whole channel with the corresponding filter of described maximum multipath time delay.Below in conjunction with drawings and Examples technical solution of the present invention is done detailed explanation.
Channel estimation methods as shown in Figure 2 in the embodiments of the invention, wherein, described channel signal includes data subcarrier and pilot sub-carrier by the modulation of OFDM technology in the described OFDM symbol, and described pilot sub-carrier comprises continuous pilot subcarrier and scattered pilot subcarrier.Before implementing channel estimating, receiver for example after receiving signal, carried out earlier carrier synchronization, sign synchronization, and clock synchronous etc. in interior Synchronous Processing, make receiving terminal obtain precise synchronization, so the present invention carry out channel estimating under the prerequisite that obtains precise synchronization.In addition, also carried out to received signal fast fourier transform (Fast Fourier Transform, FFT), described fast fourier transform is that received signal is converted to frequency domain by time domain, it has 2k, 4k and 8k, and k=1024 wherein is in three kinds of interior transmission modes.As shown in Figure 2, described channel estimation methods at first carries out step S21.
In step S21, the scattered pilot subcarrier in the OFDM symbol of being exported after the FFT conversion is carried out channel response estimate, obtain the channel response value of scattered pilot subcarrier.In the present embodiment, the length of described FFT comprises for example being 2k, 4k or 8k.Please cooperate and consult Fig. 1, as shown in Figure 1, in ofdm system, each frame comprises a plurality of OFDM symbols (for example the 0th, 1,2...66,67 OFDM symbols), wherein comprise for example being 1705 (2k patterns), 3409 (4k patterns) or the individual subcarrier of 6817 (8k patterns) in each OFDM symbol again, the scattered pilot subcarrier that is inserted is according to the certain rule discrete distribution.As shown in Figure 1, the interval N of described scattered pilot subcarrier on time orientation
t=4, and on frequency direction, be spaced apart N
f=12.In the present embodiment, it is least square estimation that described channel response is estimated, it specifically comprises: suppose that original received signal is
Wherein, the primary signal that x (n) sends for transmitting terminal, h (n) is a channel impulse response, and w (n) is an interchannel noise, in the present embodiment, w (n) is meant additive white Gaussian noise (AWGN).Like this, through after the FFT conversion, described received signal by spatial transform to frequency domain:
According to OFDM model shown in Figure 1, the frequency-region signal of scattered pilot subcarrier is on k the subcarrier of interior l the OFDM symbol of a frame OFDM: Y
L, k=X
L, kH
L, k+ W
L, k, because the pilot value X of described scattered pilot subcarrier
L, kBe known, therefore can obtain the channel response value of scattered pilot subcarrier
Need to prove, above-mentioned channel response estimates that being estimated as example with LS describes, because the operand that LS estimates is only along with pilot tone linear the increasing of counting, circuit design is comparatively simple, but it not is to be used for limiting the present invention that the LS in the present embodiment estimates, described channel response estimates it also can is to adopt other for example MMSE or LMMSE channel estimating.
In step S23, according to the channel response value of described scattered pilot subcarrier, estimate at the enterprising row interpolation of time orientation, obtain the time orientation channel response value.Interpolation on the described time orientation is estimated by adopting the linear interpolation method of estimation to carry out.Specifically, please cooperate and consult the 3rd figure, as shown in the figure, according to the channel response value of the scattered pilot subcarrier of gained among the step S21
Estimate at the enterprising row interpolation of time orientation, obtain being estimated as on the time orientation:
L≤m≤N
t-1, wherein, the scattered pilot spacing N on the time orientation
t=4;
The channel estimating of scattered pilot symbol on the express time l frequency k position;
Express time l+N
tThe channel estimating of scattered pilot symbol on the frequency k position.In present embodiment, frame structure according to OFDM, for each OFDM symbol, is 131* (12/3)=524 (2k pattern), 262* (12/3)=1048 (4k pattern), 524* (12/3)=2096 (8k pattern) by the scattered pilot subcarrier through the available quantity with subcarrier (comprising data subcarrier and pilot sub-carrier) of channel response value of time orientation linear interpolation, the channel response value of linear interpolation gained can be rearranged and arrange back with HT like this
L, j(0≤l≤67,0≤j≤523/1047/2095) expression.Then enter step S25.
In addition, as shown in Figure 3, in the present embodiment, because of the interval N of scattered pilot subcarrier on time orientation that is inserted
t=4, so estimating to be meant, the interpolation of described time orientation utilizes that adjacent two scattered pilot subcarriers carry out interpolation on the time orientation, specifically, from time orientation, the 0th label OFDM symbol is the same with the position of the scattered pilot subcarrier of the 4th label OFDM symbol, the 1st label OFDM symbol is the same with the position of the scattered pilot subcarrier of the 5th label OFDM symbol, by that analogy, 4 the OFDM symbols in every as can be seen interval, a scattered pilot subcarrier can appear in same position, as P12 among the figure and P11, P22 and P21.Suppose that present OFDM symbol to the 3rd label carries out the channel estimating on the time orientation, then the P1 point need be used the information of P11 and 2 scattered pilot subcarriers of P12, and the P2 point need be used the information of P22 and 2 scattered pilot subcarriers of P21.Wherein the P12 point is positioned on the OFDM symbol of the 0th label, the P11 point is positioned on the OFDM symbol of the 4th label, the P21 point is positioned on the OFDM symbol of the 6th label, the P22 point is positioned on the OFDM symbol of the 2nd label, and the information of two scattered pilot subcarriers that the P3 point is used lays respectively on the OFDM symbol of the 1st label and the 5th label.As seen utilize the channel response of the OFDM symbol of scattered pilot subcarrier direction estimated time the 3rd label to use 3 OFDM symbols before it and the scattered pilot subcarrier in 3 OFDM symbols afterwards thereof, the OFDM symbol that comprises estimative the 3rd label like this, the interpolation of finishing this OFDM symbol need altogether to estimate the scattered pilot subcarrier information of 7 OFDM symbols.
It should be noted that the described channel estimation methods of the embodiment of the invention is to carry out under the prerequisite that obtains precise synchronization, in fact, present embodiment is to have utilized the smart synchronous result of sequential.In the smart Synchronous Processing of sequential, every frame need carry out a sequential adjusting during owing to system design, and all OFDM symbols have identical sequential in the frame, and have different sequential between the different frames.When therefore in described step S23, carrying out the interpolation estimation on the time orientation, be to belong to different frames and when having different sequential in 7 OFDM symbols using before and after the time domain interpolation, need remove/apply sequential to described each OFDM symbol and regulate, so that the sequential unanimity of each related OFDM symbol.Specifically, suppose the P1 point on the 3rd label OFDM symbol to be estimated in desire on the time orientation, and the 0th label OFDM symbol at the pilot sub-carrier P12 place of this P1 point participation estimation is to belong to two frames with different sequential respectively with the 4th label at pilot sub-carrier P11 place relatively, P12 has passed through the sequential adjusting, and P11 does not regulate through sequential.If the 3rd label OFDM symbol at P1 point place and the 0th label OFDM symbol at P12 place belong in the same frame, be that the P1 point has also passed through the sequential adjusting, that pilot sub-carrier P11 that does not regulate through sequential that then participates in estimating must apply the influence that sequential is regulated; Otherwise, if the 3rd label OFDM symbol at P1 point place and the 4th label OFDM symbol at P11 place belong in the same frame, be that the P1 point is not regulated through sequential yet, that pilot sub-carrier P12 that has regulated through sequential that then participates in estimating must remove the influence that sequential is regulated.Subcarrier in described like this each OFDM symbol keeps consistency, and guarantees the accuracy that interpolation is estimated.
In step S25, the time orientation channel response value that interpolation on the described time orientation is estimated the back gained by the IFFT conversion by frequency domain transform to time domain, estimate the maximum multipath time delay τ of channel
MaxIn the present embodiment, the length of described IFFT is to comprise 2k, 4k or 8k, wherein k=1024.Described maximum multipath time delay τ
MaxBe resultant with last pairing time difference in footpath according to detected first footpath in IFFT length, its concrete method of estimation can be seen shown in Figure 5 in addition.Then enter step S27.
In step S27, according to resulting maximum multipath time delay τ
Max, in the group of filters that sets in advance, choose and described maximum multipath time delay τ
MaxCorresponding bank of filters.Have a plurality of bank of filters in the described group of filters, and have a plurality of filters in each bank of filters.Described filter is a low pass filter and have corresponding filter factor for example.In the present embodiment, scope division that can maximum multipath time delay is possible is four time delay sections, and each time delay section is corresponding to one group of bank of filters, so described group of filters has four groups of bank of filters, corresponding to the maximum multipath time delay τ that estimates
MaxSpecifically; for example can be divided into four time delay sections 0～0.25,0.25～0.5,0.5～0.75, reach 0.75～1.0 with protection ratio at interval according to the possible scope of maximum multipath time delay, each time delay section corresponds respectively to bank of filters 0,1,2, reaches 3.Described protection is relevant with FFT/IFFT length at interval, and FFT/IFFT length can be 2K, 4K or 8K, and protection length at interval can be 1/32,1/16,1/8,1/4 of FFT/IFFT length.That is to say, when in step S23, obtaining maximum multipath time delay τ
MaxAfter, can be according to described maximum multipath time delay τ
MaxWith protection ratio size at interval, judge described maximum multipath time delay τ
MaxBe fall within divide in the scope of which time delay section, according to judged result, choose and affiliated time delay section corresponding that the group bank of filters, wherein, the bandwidth of selected filter is the maximum multipath time delay τ that is slightly larger than gained
MaxSimultaneously, because each filtering will draw three values, and corresponding each value needs a filter, therefore each group bank of filters has three filters, whole filter group has 12 filters altogether like this, and in the present embodiment, each filter is 12 rank filters in the group of filters.This shows, the bank of filters of passing through to be provided with adaptive-filtering coefficient, can choose corresponding filter according to the estimated maximum multipath time delay that obtains of real-time detection adaptively, can overcome available technology adopting Weiner filter (no matter being two-dimentional Weiner filter or one dimension Weiner filter) relatively because of requiring to learn that the channel correlation properties cause circuit to realize complexity and estimated performance instability, perhaps carry out real-time tracking to the variation of channel and easily cause problems such as circuit complexity.Then enter step S29.
In step S29, according to described time orientation channel response value, adopt the bank of filters of selected correspondence, estimate at the enterprising row interpolation of frequency direction, obtain the response of whole channel.Specifically, please cooperate and consult the 4th figure, as shown in the figure, according to selected three 12 rank filters corresponding filter factor COE1 (i), COE2 (i) and COE3 (i), i=0-11 estimates at the enterprising row interpolation of frequency direction, obtains being estimated as on the frequency direction:
Wherein,
Expression frequency direction time l, the channel estimating of the locational pilot sub-carrier symbol of frequency k, HT
L, jThe result of time orientation channel estimating before being.Like this, estimate that by the two-dimensional interpolation on time orientation and the frequency direction can obtain all positions (comprising data subcarrier and pilot sub-carrier) exactly is the channel response value of whole channel.And, utilize in the prior art two-dimentional Weiner filter or the cascade of two one dimension Weiner filter phases to carry out interpolation and estimate easily to cause the higher disappearance of algorithm complex, the present invention adopts has the bank of filters that self adaptation is adjusted filter factor, can not only satisfy the performance for estimating channel requirement, more the low complex degree of feasible system.
The described multidiameter delay of above-mentioned steps S25 is because therefore the transmission path difference of signal each paths component experience in transmission course has different time delays.Maximum multipath time delay is that first reaches the time difference between the signal component of the signal component of receiving terminal and last arrival, and the accurate estimation of maximum multipath time delay can be the follow-up filter of choosing suitable filter factor reliable judgment basis is provided.The maximum multipath time delay that present embodiment provided is estimated as shown in Figure 5.As shown in Figure 5, step S51 is at first carried out in described estimation.
At step S51, described time orientation channel response value is carried out many times of interpolation and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then.In fact, before carrying out invert fast fourier transformation, needing to carry out many times of interpolation to described time orientation channel response value earlier, is that the distributing position according to the scattered pilot subcarrier adopts three times of interpolation in the present embodiment.As shown in Figure 3, because the time orientation channel response value of step S23 gained is to have the scattered pilot subcarrier in the OFDM symbol to be in interpolation estimated result on the time orientation, the frame structure of OFDM symbol, what the scattered pilot subcarrier occurs on frequency direction is spaced apart three, just the scattered pilot subcarrier can appear every three Frequency points promptly, as the K=3 among Fig. 3,6,9..., so for satisfying the length requirement of IFFT, gained time orientation channel response value need be carried out three times of interpolation in advance before carrying out the IFFT conversion.Described average energy is meant that the sample value energy phase adduction work of channel impulse response in IFFT length is average and obtains that described IFFT length is corresponding with described FFT length, comprises 2k, 4k or 8k, wherein k=1024.Then carry out step S53.
In step S53; with the smart original position of determining synchronously of sequential is starting point; slide backward protection length at interval; every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time.Wherein, the smart original position of determining synchronously of described sequential is with regard to first footpath of multidiameter delay.Then carry out step S55.
In step S55, with last footpath of being write down and the smart original position of determining synchronously of sequential separately time corresponding subtract each other, obtain maximum multipath time delay τ
MaxDescribed maximum multipath time delay τ
MaxBe less than described protection interval G
L.Because the present invention is under the smart synchronous prerequisite of sequential, obtained maximum multipath time delay according to interpolation results estimated on the described time orientation, can be the follow-up filter of choosing suitable corresponding filter factor, implementation method is simple relatively, effectively reduces the complexity of system.
Corresponding to above-mentioned channel estimation methods, the present invention provides a kind of channel estimating apparatus that is applied to receiver on the other hand, as shown in Figure 6, the OFDM symbol that receiver described in the embodiment of the invention receives includes data subcarrier and pilot sub-carrier, comprise lock unit 100, fast Fourier transform unit 120, and invert fast fourier transformation unit 140, described lock unit 100 is used to carry out carrier synchronization, sign synchronization, and clock waits synchronously in interior Synchronous Processing, described FFT unit 120 is used for signal is transformed into frequency-region signal by time-domain signal, and IFFT unit 140 is used for signal is transformed into time-domain signal by frequency-region signal.Described channel estimating apparatus comprises: pilot tone estimation unit 61, time domain interpolation filter 62, time delay estimation unit 63, frequency domain interpolation group of filters 64.
Pilot tone estimation unit 61 is that the scattered pilot subcarrier in the OFDM symbol that is used for FFT unit 120 is exported carries out the channel response estimation, obtains the channel response value of scattered pilot subcarrier.The least square estimation device that described pilot tone estimation unit 61 is for example single-order equalizers, its operand is linear the increasing along with the scattered pilot number of subcarriers only, and described scattered pilot subcarrier is known, so the computing of the channel response value of described scattered pilot subcarrier is relatively simple.
Time domain interpolation filter 62 is the channel response value that are used for according to the scattered pilot subcarrier of described pilot tone estimation unit 61 gained, estimates at the enterprising row interpolation of time orientation, obtains the time orientation channel response value.Described time domain interpolation filter 62 is low pass filters for example, can be in the enterprising line linearity interpolation of time orientation.
Time delay estimation unit 63 be the time orientation channel response value exported according to IFFT unit 140 by invert fast fourier transformation by frequency domain transform to time domain, estimate the maximum multipath time delay of channel.Concrete method of estimation comprises: described time orientation channel response value is carried out many times of interpolation (distributing position according to the scattered pilot subcarrier adopts three times of interpolation in the present embodiment) and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then; With the smart original position of determining synchronously of sequential is starting point, slide backward protection length at interval, every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time; With last footpath of being write down and the smart original position of determining synchronously of sequential separately time corresponding subtract each other, obtain maximum multipath time delay.
Frequency domain interpolation group of filters 64, have a plurality of bank of filters, and have a plurality of filters in each bank of filters, be used for choosing the filter that has with the corresponding filter factor of described maximum multipath time delay according to described maximum multipath time delay, and then according to the described channel response value that obtains in the enterprising row interpolation estimation of time orientation, estimate at the enterprising row interpolation of frequency direction, obtain the response of whole channel.Described filter is a low pass filter and have corresponding filter factor for example.In the present embodiment, described frequency domain interpolation group of filters 64 has four bank of filters 640,641,642,643, corresponds respectively to the maximum multipath time delay of four different delay sections.In addition, because in the present embodiment according to the distributing position of scattered pilot subcarrier, each filtering will draw three values, so each bank of filters has three filters respectively, be that bank of filters 640 has filter 640a, 640b, reaches 640c, bank of filters 641 has filter 641a, 641b, reaches 641c, and the rest may be inferred, and described frequency domain interpolation group of filters 64 has 12 filters altogether.Like this, according to estimated multipath delay maximum τ
MaxSize, judge described maximum multipath time delay τ
MaxBe to fall in the scope of which time delay section of being divided, so just can choose that group bank of filters of corresponding time delay section, wherein, the bandwidth of selected bank of filters is the maximum multipath time delay τ that is slightly larger than gained
MaxBecause, described frequency domain interpolation group of filters 64 has the bank of filters 640,641,642,643 of different filter factors, so can choose corresponding bank of filters adaptively according to the estimated maximum multipath time delay that obtains of real-time detection, can overcome available technology adopting Weiner filter (no matter being two-dimentional Weiner filter or one dimension Weiner filter) relatively and cause the high complexity of system, perhaps real-time tracking be carried out in the variation of channel and easily cause problems such as circuit complexity.
In sum, channel estimation methods of the present invention and channel estimating apparatus, in advance the scattered pilot subcarrier being carried out channel response estimates, obtain the channel response value of scattered pilot subcarrier, then estimate to obtain the time orientation channel response value at the enterprising row interpolation of time orientation, it is estimated maximum multipath time delay by invert fast fourier transformation, choose adaptively with the corresponding filter of described maximum multipath time delay and at the enterprising row interpolation of frequency direction and estimate, finally obtain the response of whole channel, can not only satisfy the performance for estimating channel requirement, more the low complex degree of feasible system.Can overcome and utilize two-dimentional Weiner filter or the cascade of two one dimension Weiner filter phases to carry out the problem that interpolation estimates easily to increase system complexity in the prior art, perhaps real-time tracking be carried out in the variation of channel and easily cause shortcomings such as circuit complexity.And, utilized the smart synchronous result of sequential, can obtain accurate maximum channel time delay.
In addition, be that what to adopt is comparatively simple LS method of estimation carrying out when interpolation is estimated in the embodiment of the invention, MMSE or LMMSE channel estimating relatively, operand is less and system complexity is lower; Selected filter more can filter because LS estimates the outer part of the band of additional noise during follow-up frequency domain interpolation, has effectively remedied the deficiency of LS estimated performance.
Though the present invention with preferred embodiment openly as above; but it is not to be used for limiting the present invention; any those skilled in the art without departing from the spirit and scope of the present invention; can make possible change and modification, so protection scope of the present invention should be as the criterion with the scope that claim of the present invention was defined.
Claims (17)
1. a channel estimation methods is characterized in that, described channel estimation methods comprises the steps:
Scattered pilot subcarrier in the OFDM symbol of being exported after the fast fourier transform is carried out channel response estimate, obtain the channel response value of scattered pilot subcarrier;
According to the channel response value of described scattered pilot subcarrier, estimate at the enterprising row interpolation of time orientation, obtain the time orientation channel response value;
With described time orientation channel response value by invert fast fourier transformation by frequency domain transform to time domain, estimate the maximum multipath time delay of channel;
According to described maximum multipath time delay, in the group of filters that sets in advance, choose the bank of filters corresponding with described maximum multipath time delay;
According to the channel response value of described time orientation, adopt selected bank of filters, estimate at the enterprising row interpolation of frequency direction, obtain the response of whole channel.
2. channel estimation methods according to claim 1, it is characterized in that, the interpolation of described time orientation estimates to be meant and utilizes that adjacent two scattered pilot subcarriers carry out interpolation on the time orientation, and the interpolation of finishing an OFDM symbol will be used a plurality of OFDM symbols in the front and back that comprise this OFDM symbol.
3. channel estimation methods according to claim 2 is characterized in that, when the included OFDM symbol of the described OFDM of finishing interpolation has different sequential, removes/applies sequential and regulate.
4. channel estimation methods according to claim 1 is characterized in that, the described maximum multipath time delay that estimates channel specifically comprises:
Described time orientation channel response value is carried out many times of interpolation and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then;
With the smart original position of determining synchronously of sequential is starting point, slide backward protection length at interval, every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time;
With last footpath of being write down and the smart original position of determining synchronously of sequential separately time corresponding subtract each other, obtain maximum multipath time delay.
5. channel estimation methods according to claim 4 is characterized in that, described many times of interpolation are meant three times of interpolation.
6. channel estimation methods according to claim 4 is characterized in that, after obtaining maximum multipath time delay, according to described maximum multipath time delay and protection ratio size at interval, chooses one group of corresponding in described group of filters bank of filters.
7. channel estimation methods according to claim 6 is characterized in that, described group of filters has four bank of filters, and each bank of filters has three filters respectively.
8. a channel estimating apparatus that is applied to receiver is characterized in that, described channel estimating apparatus comprises:
The pilot tone estimation unit is used for that the scattered pilot subcarrier in the OFDM symbol of being exported after the fast fourier transform is carried out channel response and estimates, obtains the channel response value of scattered pilot subcarrier;
The time domain interpolation filter is used for the channel response value according to the scattered pilot subcarrier, estimates at the enterprising row interpolation of time orientation, obtains the time orientation channel response value;
The time delay estimation unit, according to described time orientation channel response value by invert fast fourier transformation by frequency domain transform to time domain, estimate the maximum multipath time delay of channel;
The frequency domain interpolation group of filters, have a plurality of bank of filters, and each bank of filters has a plurality of filters, be used for choosing and have corresponding filter filtering group with described maximum multipath time delay according to described maximum multipath time delay, and then according to the channel response value of described time orientation, estimate at the enterprising row interpolation of frequency direction, obtain the response of whole channel.
9. channel estimating apparatus according to claim 8, it is characterized in that, the interpolation of time orientation estimates to be meant and utilizes that adjacent two scattered pilot subcarriers carry out interpolation on the time orientation, and the interpolation of finishing an OFDM symbol will be used a plurality of OFDM symbols in the front and back that comprise this OFDM symbol.
10. channel estimating apparatus according to claim 9 is characterized in that, when the included OFDM symbol of the described OFDM of finishing interpolation has different sequential, removes/applies sequential and regulate.
11. channel estimating apparatus according to claim 8 is characterized in that, described pilot tone estimation unit is the least square estimation device.
12. channel estimating apparatus according to claim 11 is characterized in that, described least square estimation device is the single-order equalizer.
13. channel estimating apparatus according to claim 8 is characterized in that, described time domain interpolation adopts linear interpolation.
14. channel estimating apparatus according to claim 8 is characterized in that, described time delay estimation unit estimates maximum multipath time delay and specifically comprises:
Described time orientation channel response value is carried out many times of interpolation and invert fast fourier transformation, obtain the channel impulse response in the time domain, calculate the average energy of the channel impulse response in the invert fast fourier transformation length then;
With the smart original position of determining synchronously of sequential is starting point, slide backward protection length at interval, every interior detection and write down in the described channel impulse response sample value energy greater than the last point of described average energy, described last point is last footpath of multidiameter delay during this time;
With last footpath of being write down and the smart original position of determining synchronously of sequential separately time corresponding subtract each other, obtain maximum multipath time delay.
15. channel estimating apparatus according to claim 14 is characterized in that, described many times of interpolation are meant three times of interpolation.
16. channel estimating apparatus according to claim 14 is characterized in that, after obtaining maximum multipath time delay, according to described maximum multipath time delay and protection ratio size at interval, chooses one group of corresponding in described group of filters bank of filters.
17. channel estimating apparatus according to claim 16 is characterized in that, described frequency domain interpolation group of filters has four bank of filters, and each bank of filters has three filters respectively.
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