CN102088432B - Sampling frequency difference correction method and device of orthogonal frequency division multiplexing (OFDM) system - Google Patents

Abstract

The invention discloses a sampling frequency difference correction method and a sampling frequency difference correction device of an orthogonal frequency division multiplexing (OFDM) system. The correction method comprises the following steps of: firstly, performing interpolation filtering or extraction filtering on oversampling data of a receiving terminal through a Farrow structure filter; secondly, performing downsampling on the oversampling data on which sampling frequency difference correction is performed so as to obtain a baseband OFDM time domain symbol; thirdly, removing a guard interval from the OFDM time domain symbol and performing Fourier transformation so as to obtain an OFDM frequency domain symbol; fourthly, extracting continuous pilot frequency information from the OFDM frequency domain symbol and estimating a sampling frequency difference so as to obtain a sampling relative frequency difference estimated value; and lastly, performing proportional integration on the estimated value to obtain an integral value. Correspondingly, the correction device comprises the Farrow structure filter, a downsampling unit, a fast Fourier transform (FFT) unit, a sampling frequency difference estimation unit, a proportional integration unit and the like. Due to the method and the device, the sampling frequency difference can be effectively estimated and corrected in real time. The method and the device have the advantages of high accuracy, high stability and the like.

Description

A kind of bearing calibration of sampling frequency difference and device of ofdm system

Technical field

The present invention relates to the communications field, especially relate to a kind of bearing calibration of sampling frequency difference and device of ofdm system.

Background technology

OFDM (Orthogonal Frequency Division Multiplexing, OFDM) system is a kind of multi-carrier communications systems, it has used a plurality of subcarriers in channel width to send data, and have the tolerance degree of multidiameter expansion and frequency selective fading high, the characteristics such as the validity height that frequency spectrum is used and anti-interference are good, can be for the larger data of transmission rate, be with a wide range of applications, such as at present at CMMB (China Mobile Multimedia Broadcasting, China Mobile multimedia broadcasting) in system, be exactly to adopt OFDM technology as modulation-transmission technology.

In ofdm system, between symbol synchronization error and transmitter and receiver, the factor such as asynchronous of frequency will cause ISI (Inter-symbol Interference, intersymbol interference) and ICI (Inter-carrierInterference, between subcarrier, disturb), thus cause receptivity to decline.In actual application, because OFDM symbol period is longer, to sign synchronization, skew is not very sensitive, and ISI can well be suppressed.But, because the subcarrier spacing of OFDM symbol is smaller, system is very responsive to frequency departure, the existence of sampling frequency difference can cause the position of windowing of the OFDM symbol of receiving terminal to drift about, cause each subcarrier generation phase rotating of OFDM symbol, thereby cause generation and the increase of ICI, so frequency difference estimation and correction are that receiver carries out the synchronous top priority of processing to OFDM symbol.

At present; OFDM transmitting system conventionally adopts information to be sent is carried out to FFT (Fast FourierTransform; fast Fourier transform) after, through subcarrier, send information, and in effective code element front portion, add the protection interval that Cyclic Prefix forms, to reduce multi-path influence.At receiving terminal, conventionally can adopt sampling clock to realize simultaneously and receive the synchronous of OFDM symbol.But, the common crystal oscillator inferior quality used of sampling clock in the consumer receiving terminal of OFDM digit broadcasting system, can cause more serious sampling frequency difference, thereby affected accuracy and validity that OFDM symbol is further processed, therefore how sampling frequency deviation being carried out effectively proofreading and correct is a problem demanding prompt solution.

Summary of the invention

For the deficiencies in the prior art, the object of the invention is to provide a kind of bearing calibration and device of sampling frequency difference of ofdm system, it can be proofreaied and correct the sampling frequency difference of over-sampling data in real time effectively in time domain, is applicable to have the OFDM digital communication system of pilot tone.

To achieve these goals, the invention provides a kind of sampling frequency difference bearing calibration of ofdm system, comprise the following steps: the over-sampling data to receiving terminal are carried out filtering interpolation or filtering extraction; Filtered over-sampling data are carried out to time-frequency conversion and obtain OFDM frequency domain symbol; From OFDM frequency domain symbol, extract continuous pilot information, and the frequency difference estimation of sampling, relative frequency difference estimation value obtains sampling

and to the relative frequency difference estimation value of sampling

carry out proportional integral and obtain the relative frequency difference integrated value of current sampling ζ ', and with integrated value ζ ' guidance, over-sampling data are carried out to filtering interpolation or filtering extraction.

Correspondingly, the present invention also provides a kind of sampling frequency difference means for correcting of ofdm system, comprising: filter, and it carries out filtering interpolation or filtering extraction for the over-sampling data to receiving terminal; Time-frequency conversion unit, it obtains OFDM frequency domain symbol for filtered over-sampling data are carried out to time-frequency conversion; Sampling frequency difference estimation unit, it is for extracting continuous pilot information the frequency difference estimation of sampling to OFDM frequency domain symbol, and relative frequency difference estimation value obtains sampling; And pi element, it obtains the relative frequency difference integrated value of current sampling for the relative frequency difference estimation value of sampling being carried out to proportional integral, wherein, and filter, time-frequency conversion unit, sampling frequency difference estimation unit and the pi element loop that is in series.

The invention has the beneficial effects as follows, this sampling frequency difference bearing calibration utilization feeds back to Farrow Structure Filter to instruct it to carry out correspondingly interpolation processing or extraction processing to sampled data to the result of sampling frequency difference, thereby realize dynamically controlling in real time sampling frequency difference estimation and correction, and the sampling frequency difference that can effectively proofread and correct ofdm system, the computational accuracy of raising system.Use the means for correcting of this sampling frequency difference bearing calibration also to have simple in structure, the advantage such as stability is high, applied widely.

Accompanying drawing explanation

Fig. 1 is the flow chart of the sampling frequency difference bearing calibration of ofdm system according to the preferred embodiment of the invention;

Fig. 2 is the principles of signal processing figure of the sampling frequency difference bearing calibration of ofdm system according to the preferred embodiment of the invention;

Fig. 3 is the structure principle chart of Farrow Structure Filter in the sampling frequency difference bearing calibration of ofdm system according to the preferred embodiment of the invention;

Fig. 4 is the structural representation of the sampling frequency difference means for correcting of ofdm system according to the preferred embodiment of the invention;

Fig. 5 is the structure principle chart of the sampling frequency difference means for correcting of ofdm system according to the preferred embodiment of the invention;

Fig. 6 is the structure of time slot schematic diagram of ofdm system; And

Fig. 7 is the structural representation of interpolation filter in the sampling frequency difference means for correcting of ofdm system according to the preferred embodiment of the invention.

Embodiment

Below in conjunction with the drawings and specific embodiments, the present invention is described in further detail.

Bearing calibration provided by the invention and device are applicable to have the OFDM digital communication system of continuous pilot.Adopt this bearing calibration, first the over-sampling data of receiving terminal are carried out to filtering interpolation or filtering extraction; Then filtered over-sampling data are carried out to time-frequency conversion and obtain OFDM frequency domain symbol; Next from OFDM frequency domain symbol, extract continuous pilot information, and the frequency difference estimation of sampling, relative frequency difference estimation value obtains sampling afterwards to the relative frequency difference estimation value of sampling carry out proportional integral and obtain the relative frequency difference integrated value of current sampling ζ ', and with integrated value ζ ' guidance, over-sampling data are carried out to filtering interpolation or filtering extraction.

Fig. 1 shows according to the roughly flow process of the sampling frequency difference bearing calibration of ofdm system of the present invention, and its key step comprises:

S101: the over-sampling data to receiving terminal are carried out filtering interpolation or filtering extraction, and the operation of this filtering interpolation or filtering extraction is undertaken by Farrow Structure Filter.

Shown in Fig. 2 is the principles of signal processing of this sampling frequency difference bearing calibration, in order more clearly to understand the present invention, below in conjunction with Fig. 2, this bearing calibration is further described.

In step S101, adopt Farrow Structure Filter to carry out filtering interpolation or filtering extraction to the over-sampling data of receiving terminal, to proofread and correct sampling frequency difference.If the sampling rate of system transmitting terminal is T, the sampling rate of receiving terminal is T ', and definition integer sampling frequency difference is fractional sampling frequency difference is u _n=nT/T '-m _n, n is sample value sequence number,

expression rounds under z.If selected low pass filter, the effective bandwidth of this filter should meet the bandwidth requirement of ofdm system, can use piecewise polynomial $h\left(t\right)=h\left[(i+u_n)T^{\′}\right]=\underset{l=0}{\overset{N}{\mathrm{\Σ}}}{b}_l{\left(i\right)u}_n^l$ Express, the expression formula of Farrow Structure Filter is $y\left(n\right)=\underset{l=0}{\overset{N}{\mathrm{\Σ}}}{u}_n^l\underset{i=I_1}{\overset{l_2}{\mathrm{\Σ}}}{b}_{l}x(m_n-i),$ Fig. 3 shows the structural principle of Farrow Structure Filter.At numeric field, adopt Farrow Structure Filter to carry out filtering and process and there is simple in structure, stability and the advantage such as real-time performance is good, be a kind of use widely interpolation decimation filter.

S102: filtered over-sampling data are carried out to time-frequency conversion and obtain OFDM frequency domain symbol.

The step that filtered over-sampling data are carried out to time-frequency conversion comprises: because the A/D converter of CMMB receiving terminal carries out over-sampling to received signal with high sampling rate, therefore must carry out down-sampling and make signal meet baseband transmission requirement filtered over-sampling data, obtain baseband OFDM time-domain symbol; Then after OFDM time-domain symbol being removed to protection interval, carry out FFT, thereby obtain OFDM frequency domain symbol by time-frequency conversion.

S103: from OFDM frequency domain symbol, extract continuous pilot information, and the frequency difference estimation of sampling, relative frequency difference estimation value obtains sampling it mainly utilizes about 38 pairs of continuous pilot of zero-frequency point almost symmetry estimates to obtain sampling relative frequency difference estimation value

The continuous pilot of supposing a system distributes about zero-frequency point symmetry at the positive and negative frequency domain of frequency axis of OFDM symbol, k subcarrier z of l OFDM symbol of receiving terminal _{l, k}for continuous pilot, positive and negative frequency domain is called to right and left part, in left part, k ∈ [(K-1)/2,0), in right part, k ∈ (0, (K-1)/2], K is effective sub-carrier number of OFDM symbol.K the subcarrier by adjacent two symbols obtains:

X _l，k＝z _l，k·conjg(z _l-1，k)＝e ^{j2π(1+g)(Δf+ζ·k)}·|H _l，k| ²β ²+noise (1)

G=N wherein _g/ N, N _gfor OFDM symbol protection gap length, N is OFDM symbol total length, and β is the pilot energy factor, and Δ f is carrier beat.To the two-part X in left and right _{l, k}phase angle summation obtain:

By phase angle with

can be in the hope of the estimated value of sample frequency relative deviation ζ=(T '-T)/T:

Obtain the frequency difference estimation value of sampling

after, it is carried out to proportional integral, frequency difference integrated value according to ζ and u obtains sampling _krelation instruct Farrow Structure Filter that over-sampling value is carried out interpolation processing or extract processed.

S104: to the relative frequency difference estimation value of sampling

carry out proportional integral and obtain the relative frequency difference integrated value of current sampling ζ ', and with integrated value ζ ' guidance, over-sampling data are carried out to filtering interpolation or filtering extraction.

In step S104, the impulse response that proportional integral adopts is:

$H\left(z\right)=K_p+\frac{z^{-1}}{1-z^{-1}}{K}_i---\left(4\right)$

With integrated value ζ ', instruct the Specific Principles of filtering interpolation or filtering extraction to be: when ζ ' > 0, to instruct Farrow Structure Filter to insert an over-sampling value; When ζ ' < 0, instruct Farrow Structure Filter in over-sampling data sequence, to extract a sampled value.

Fig. 4 shows according to the means for correcting of the frequency difference of sampling in ofdm system of the present invention, and this device mainly comprises that post-processing unit and series connection become the Farrow Structure Filter 1 of a feedback control loop, time-frequency conversion unit 2, sampling frequency difference estimation unit 3 and pi element 4.As shown in Figure 5, time-frequency conversion unit 2 can also comprise downsampling unit and FFT unit to the concrete structure of this means for correcting.

According to a preferred embodiment of the present invention, the bandwidth of supposing CMMB system is 8MHz, and baseband sampling speed is 10MHz, and a frame comprises 40 time slots.Each time slot has 250,000 data, and as shown in Figure 6, it comprises a sender unit identification sequence, two synchronizing sequences and 53 OFDM symbols to structure of time slot, and each OFDM symbol has 3076 effective subcarriers, wherein containing 82 continuous pilot.

In means for correcting, Farrow Structure Filter 1 can be selected Subsection Conic Interpolation filter, and its over-sampling data to CMMB receiving terminal are carried out filtering interpolation, and the parting expression of Subsection Conic Interpolation filter is:

$y\left(t\right)=\underset{i=-2}{\overset{1}{\mathrm{\&Sigma;}}}{C}_{i}x(-1-i)$

C _-2＝αu ²-αu

(5)

C _-1＝-αu ²+(α+1)u

C ₀＝-αu ²+(α-1)u+1

C ₊₁＝αu ²-αu

α=0.5 wherein, as shown in Figure 7, the relative frequency difference integrated value of the sampling ζ ' being obtained by feedback control loop decides digitized current interpolation filter variable u to the Farrow structure of the Subsection Conic Interpolation filter corresponding with the parameter adopting in the present embodiment _n.

In downsampling unit, if CMMB receiving terminal A/D converter with the sample rate of 40MHz, sample to received signal, in the present embodiment, adopt 4 times of down-samplings so that baseband signal data y ' (n) speed be 10MHz.

In FFT unit, the base band time slot signal of 10MHz removes OFDM symbol protection interval after slot synchronization, obtains 53 OFDM symbols of each time slot, carries out Fourier transform and obtains frequency-domain OFDM symbol, and its formula adopting is:

$Y\left(k\right)=\mathrm{FFT}\left[y^{\&prime;}\left(n\right)\right]=\frac{\sqrt{2}}{128}{\mathrm{\&Sigma;}}_{n=0}^{4095}{y}^{\&prime;}\left(n\right)\&CenterDot;e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="H2009102414532E00031.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;nk}/4096},0\&le;k\&le;4095,---\left(6\right)$

Sampling frequency difference estimation unit 3 is mainly used in carrying out the extraction of continuous pilot and the estimation of sampling frequency difference.In CMMB system, OFDM symbol has 82 continuous pilot, and these 82 subcarrier number corresponding to continuous pilot are respectively:

0，22，78，92，168，174，244，274，278，344，382，424，426，496，500，564，608，650，688，712，740，772，846，848，932，942，950，980，1012，1066，1126，1158，1214，1244，1276，1280，1326，1378，1408，1508，1537，1538，1566，1666，1736，1748，1794，1798，1830，1860，1916，1948，2008，2062，2094，2124，2132，2142，2226，2228，2302，2334，2362，2386，2424，2466，2510，2574，2578，2648，2650，2692，2730，2796，2800，2830，2900，2906，2982，2996，3052，3075。

Wherein, 38 continuous pilot in 38 continuous pilot in left-half except the 1538th, 1736 and 3075 and right half part except the 0th, 1378 and 1537 are put almost symmetry about zero-frequency, if that is: the continuous pilot of left-half at zero-frequency point left side label is-k, always to have a label be that the pilot tone of k-1 is corresponding with it zero-frequency point the right, because this 38 couple amounts to 76 continuous pilot almost symmetries, so can be with they sample estimations of frequency difference.

Time slot of CMMB system has 53 OFDM symbols, can use the sample estimation of frequency difference of wherein adjacent 2≤L≤53 symbol at every turn, makes L=53 in the present embodiment, and each time slot obtains the estimated value of a sampling frequency difference.N=4096 in CMMB system, N _g=536, g=536/4096, K=3076.

From frequency-domain OFDM symbol, extract after the continuous pilot of 53 symbols in a time slot, by k subcarrier of adjacent l and two symbols of l-1, obtained:

X _l，k＝z _l，k·conjg(z _l-1，k)＝e ^{j2π(1+g)(Δf+ζ·k)}·|H _l，k| ²+noise (7)

In a time slot, try to achieve:

The frequency difference estimation value that finally obtains sampling is:

Sampling frequency difference estimation value to each time slot in pi element 4 is carried out proportional integral, the structure of proportional integral device as shown in Figure 5,0 < (K wherein _i-K _p) < < K _p< < 1.To the relative frequency difference of sampling

adopt the impulse response shown in formula (4) to carry out obtaining integrated value ζ ' after proportional integral, then according to ζ ', with the relation of un, interpolation Farrow filter is adjusted, particularly:

When ζ ' > 0, need to carry out interpolation to over-sampling data, $u=u_n=1+\frac{n(T-T^{\&prime;})}{T^{\&prime;}},$ U moves closer to 0, should insert Farrow Structure Filter and in over-sampling data sequence, insert a value when u≤0.

When ζ ' < 0, need to extract processing to over-sampling data, $u=u_n=\frac{n(T-T^{\&prime;})}{T^{\&prime;}},$ U moves closer to 1, and when u >=1, Farrow Structure Filter extracts and should extract a sampled value.

Disclosed is only the preferred embodiments of the present invention above, certainly can not limit the scope of the present invention with this.Be appreciated that the equivalent variations of doing according to the present invention's essence defined in the appended claims and scope, still belong to the scope that the present invention is contained.

Claims (8)

1. a sampling frequency difference bearing calibration for ofdm system, is characterized in that, comprises the following steps:

Over-sampling data to receiving terminal are carried out filtering interpolation or filtering extraction;

Filtered described over-sampling data are carried out to time-frequency conversion and obtain OFDM frequency domain symbol;

From described OFDM frequency domain symbol, extract continuous pilot information, and the frequency difference estimation of sampling, relative frequency difference estimation value obtains sampling

; And

To the relative frequency difference estimation value of described sampling carry out proportional integral and obtain the relative frequency difference integrated value of current sampling ζ ', and instruct described over-sampling data are carried out to filtering interpolation or filtering extraction with described integrated value ζ '.

2. sampling frequency difference according to claim 1 bearing calibration, is characterized in that, described filtering interpolation or filtering extraction adopt Farrow Structure Filter to carry out.

3. sampling frequency difference according to claim 1 bearing calibration, is characterized in that, the described step that filtered described over-sampling data are carried out to time-frequency conversion comprises:

Filtered described over-sampling data are carried out to down-sampling, obtain baseband OFDM time-domain symbol; And

After described OFDM time-domain symbol is removed to protection interval, carry out FFT, obtain OFDM frequency domain symbol.

4. sampling frequency difference according to claim 1 bearing calibration, it is characterized in that, describedly from described OFDM frequency domain symbol, extract continuous pilot information, and the step of the frequency difference estimation of sampling comprises utilizing and estimates to obtain the relative frequency difference estimation value of described sampling about the point-symmetric continuous pilot of zero-frequency

.

5. sampling frequency difference according to claim 2 bearing calibration, is characterized in that, the described integrated value ζ ' of described use instructs the step that described over-sampling data are carried out to filtering interpolation or filtering extraction to comprise:

When ζ ' > 0, instruct described Farrow Structure Filter to insert an over-sampling value; When ζ ' < 0, instruct described Farrow Structure Filter in described over-sampling data, to extract a sampled value.

6. a sampling frequency difference means for correcting for ofdm system, is characterized in that, comprising:

Filter, it carries out filtering interpolation or filtering extraction for the over-sampling data to receiving terminal;

Time-frequency conversion unit, it obtains OFDM frequency domain symbol for filtered described over-sampling data are carried out to time-frequency conversion;

Sampling frequency difference estimation unit, it is for extracting continuous pilot information the frequency difference estimation of sampling to described OFDM frequency domain symbol, and relative frequency difference estimation value obtains sampling; And

Pi element, it obtains the relative frequency difference integrated value of current sampling for the relative frequency difference estimation value of described sampling being carried out to proportional integral,

Wherein, described filter, described time-frequency conversion unit, described sampling frequency difference estimation unit and the described pi element loop that is in series.

7. sampling frequency difference means for correcting according to claim 6, is characterized in that, described filter is Farrow Structure Filter.

8. sampling frequency difference means for correcting according to claim 6, is characterized in that, described time-frequency conversion unit comprises:

Downsampling unit, it is for carrying out down-sampling to obtain baseband OFDM time-domain symbol to filtered described over-sampling data; And

FFT unit, it is for carrying out FFT to obtain OFDM frequency domain symbol to the described OFDM time-domain symbol of removing behind protection interval.

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