CN1259780C - New OFDM time, frequency synchronization method - Google Patents

Abstract

The present invention provides a new OFDM time and frequency synchronization method. Data on Ng points behind OFDM raw data and a training sequence are weighted and superposed to form guard spacing point to point on a sending terminal of an OFDM system, and OFDM raw data is kept unchanged. A receiving terminal of the OFDM system uses a merging algorithm, and the correlativity of data in the OFDM guard spacing and data of rear Ng points and the correlativity induced by the training sequence are comprehensively used for treating received data so as to realize the synchronization of time and frequency. The method of the present invention enables the OFDM system to have the advantages of good synchronization performance and small interference of the training sequence to the data.

Description

A kind of OFDM time, frequency synchronization method

Technical field

The invention belongs to radio communication or wire communication field, it is particularly related to the Time and Frequency Synchronization technology in the ofdm system.

Background technology

OFDM is owing to have the message transmission rate height, and the anti-multipath interference performance is strong, and the spectrum efficiency advantages of higher more and more comes into one's own.It successfully is used for wired, radio communication.As: among DAB (Digital Audio Broadcasting), DVB, EEE802.11a and the HyperLAN/2, in the IEEE802.16 that is formulating at present, also related to the OFDM technology in a large number.This new modulation technique of OFDM also can be used in the mobile communication system of a new generation.Use the OFDM technology will improve the transmission data rate and the spectrum efficiency of the third generation mobile communication system greatly, and have good ability of anti-multipath, cochannel interference and impact noise ability, see document: Bingham, J.AC., " Multicarrier modulation for data transmission:an idea whosetime has come, " IEEE Communications Magazine, Volume:28 Issue:5, May 1990, Page (s): 5-14; And document: YunHee Kim; Iickho Song; Hong Gil Kim; Taejoo Chang; Hyung Myung Kim, " Performance analysis of a coded OFDMsystem in time-varying multipath Rayleigh fading channels; " Vehicular Technology, IEEE Transactions on, Volume:48 Issue:5, Sept, 1999, Page (s): 1610-1615 is described.

OFDM is divided into time synchronized and Frequency Synchronization synchronously.The module 11 among Fig. 1 is seen in the position of synchronization module.The purpose of time synchronized is to find out the border of each OFDM symbol in the serial data stream of receiving; And the purpose of Frequency Synchronization is to obtain and correct the frequency shift (FS) of receiving end.One of weakness of OFDM technology be to the requirement of time and Frequency Synchronization particularly Frequency Synchronization require more much higher than single-carrier system.The general system of employing OFDM technology that requires is no more than 2% of its subcarrier spacing in the receiving terminal frequency shift (FS), sees document van de Beek, J.J.; Sandell, M.; Borjesson, P.O., " ML estimation of time andfrequency offset in OFDM systems; " Signal Processing, IEEE Transactions on, Volume:45 Issue:7, July 1997, and Page (s): 1800-1805 is described.One of simultaneous techniques of OFDM is to utilize protection redundancy at interval to carry out time and Frequency Synchronization, sees document van de Beek, J.J.; Sandell, M.; Borjesson, P.O., " ML estimation of time and frequency offset in OFDMsystems; " Signal Processing, IEEE Transactions on, Volume:45 Issue:7, July 1997, and Page (s): 1800-1805 is described.

In the OFDM technology, disturb for eliminating intersymbol interference and cochannel, generally before each OFDM symbol, add protection at interval.General 2 times or 4 times of requiring of protection gap length greater than channel impulse response length.Protection content at interval generally is the part of OFDM symbol.

The method for synchronous of conventional OFDM has two kinds:

1) utilizes protection interval and the intersymbol correlation of OFDM, can realize time and Frequency Synchronization.Referring to document vande Beek, J.J.; Sandell, M.; Borjesson, P.O., " ML estimation of time and frequency offset in OFDMsystems; " Signal Processing, IEEE Transactions on, Volume:45 Issue:7, July 1997, Page (s): 1800-1805.Place protection at interval according to the mode among Fig. 2.Because data 17 in the protection at interval and the back N in the OFDM useful data _g Individual data 19 are identical, and the difference correlation that is calculated as follows received signal in receiving end is to obtain time and Frequency Synchronization:

$P\left(\widehat{\mathrm{\θ}}\right)=\arg \underset{\mathrm{\θ}}{\max }\left(\underset{k=1}{\overset{N_g}{\mathrm{\Σ}}}{r}^{*}\right[k+\mathrm{\θ}\left]r\right[k+N+\mathrm{\θ}\left]\right)---\left(1\right)$

$\widehat{\mathrm{\ϵ}}=\frac{1}{2\mathrm{\π}}\·{\tan }^{-1}\left(P\left(\widehat{\mathrm{\θ}}\right)\right)---\left(2\right)$

Wherein,

The time synchronized point that expression is estimated,

Expression estimated frequency shift value, r[k] be received signal, N _g

Be the protection gap length, N is counting of an OFDM symbol sampling.

2) make a start and fill the OFDM symbol, following dual mode can be arranged: a) training sequence 20 is placed in the protection at interval of OFDM (as shown in Figure 3) with training sequence; B) training sequence 22 is placed on (as shown in Figure 4) before the protection at interval of OFDM.Receiving end is asked the relevant time synchronized of carrying out to received signal with known training sequence according to following formula:

$\mathrm{\γ}\left[a\right]=\underset{k=1}{\overset{N_t}{\mathrm{\Σ}}}r[k-a]\·t\left[k\right]---\left(3\right)$

Wherein, r[k] be received signal, N _tBe the length of training sequence, what a slided when being the alignment of the local relatively training sequence of receiving sequence counts t[k] be training sequence.

3) make a start training sequence is superimposed upon on the OFDM useful data, referring to document Tufvesson, F.; Edfors, O.; Faulkner, M., " Time and frequency synchronization for OFDM using PN-sequence preambles; " VehicularTechnology Conference, 1999.VTC 1999-Fall.IEEE VTS 50th, Volume:4,1999, Page (s): 2203-2207.

Place protection at interval according to the mode of Fig. 5, be about to training sequence 28 and 30 and be superimposed upon on OFDM useful data 29 and 31, move in 26 and 27 30 and 31 then, form protection at interval, receiving end is utilized two formula recited above to ask to be correlated with to carry out time synchronized.

Yet above-mentioned OFDM protection method for designing at interval all has shortcoming, and the shortcoming of method (1) is exactly that the correlation peak of receiving end is not obvious, and the scope of its frequency offset estimating has only 1/2 of ofdm system subcarrier spacing.The shortcoming of method (2) is to be time-division (perhaps frequency division) multiplexed form between training sequence and the OFDM initial data, causes the decline of data transmission efficiency; In addition, when the position of training sequence is shown in Fig. 3 the time, can reduce performance for estimating channel.The shortcoming of method (3) is that training sequence is excessive to the data interference, and the energy efficiency of emission data is lower.

Summary of the invention

The purpose of this invention is to provide a kind of method that is used for ofdm system time and Frequency Synchronization, make ofdm system have the advantage that net synchronization capability is good and training sequence is little to the interference of data.

Innovation part of the present invention is: 1) make a start and only place training sequence in the protection at interval of OFDM symbol; this training sequence is repeated repeatedly to constitute by a selected PN sequence continuously; receiving end is carried out relevant detection and processing to this sequence, realize OFDM synchronously.Because training sequence only is positioned in the protection at interval; therefore it is little to the laying method of the training sequence of the interference ratio routine of data; owing to be point-to-point cum rights stack between training sequence and the OFDM initial data, therefore do not cause the decline of data transmission efficiency simultaneously.2) back N in data in the receiving end comprehensive utilization OFDM protection at interval and the OFDM initial data _gThe correlation that correlation between the data of point and training sequence are introduced adopts blending algorithm to realize time and Frequency Synchronization, and performance is better than conventional method.

The present invention is a kind of OFDM time, frequency synchronization method, and back N with the OFDM initial data is characterized in that making a start _gThe point-to-point cum rights stack of data on the individual point and training sequence constitutes protection at interval, and the OFDM initial data remains unchanged; The receiving end of ofdm system adopts blending algorithm, back N in data in the comprehensive utilization OFDM protection at interval and the OFDM initial data _gThe correlation that correlation between the data of point and training sequence are introduced handles realization time and Frequency Synchronization to receiving data.

According to a kind of OFDM time of the present invention, frequency synchronization method, it is characterized in that it comprises the following step (as shown in Figure 6):

One, make a start:

1) selecting a length is N _PNThe PN sequence, be designated as PN[k], note the PN[k of this moment] value is plural form, i.e. m[k] ∈ { 1+j ,-1-j};

2) above-mentioned PN sequence is repeated continuously place repeatedly, clip the unnecessary data in back, structure is grown into N _gTraining sequence (as shown in Figure 7), be designated as t[k], its mathematic(al) representation is:

t[k]＝m[kmod?N _PN] k∈[0，N _g-1] (4)

3) as shown in Figure 6, with training sequence and OFDM initial data back N _gAfter the point-to-point cum rights stack of data on the individual point, be positioned in the protection at interval; The OFDM initial data is constant.So the mathematic(al) representation of OFDM emission data is as follows:

$s\left[k\right]=\left[\begin{array}{cc}\sqrt{1-\mathrm{\ρ}}\·d\left[k\right]+\sqrt{\mathrm{\ρ}}\·t\left[k\right]& k\∈[0,N_g-1]\\ d\left[k\right]& k\∈[N_g,N+N_g-1]\end{array}\right.---\left(5\right)$

Wherein, s[k] represent OFDM emission data, d[k] represent OFDM useful data, t[k] be training sequence, ρ represent weighted value, and its physical significance is in protecting the interval, and the energy of training sequence is with respect to the normalized value of launching the data energy.

Two, receiving end:

1) time is slightly synchronous: data 33 in the utilization protection at interval and the back N in the OFDM initial data _g Individual data 35 are identical, adopt conventional synchronization method (1), can find the probable ranges of time synchronized point according to formula (1).

2) time is smart synchronously: in above-mentioned scope, utilize the training sequence of stack in the protection at interval to find the exact position of time synchronized point, concrete grammar is as follows: establish L and be the full number of the PN sequence that contains in the training sequence, it is N that receiving end is provided with a size _PNThe detection window of L carries out shifting function to receiving data, asks relevant according to following formula then:

$\mathrm{\γ}[k,a]=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}[\left(\underset{n=0}{\overset{N_{\mathrm{PN}}-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-l{N}_{\mathrm{PN}}\left]\right)$

$\·{\left(\underset{n=0}{\overset{N_{\mathrm{PN}}-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-(l+1)N_{\mathrm{PN}}\left]\right)}^{*}]---\left(6\right)$

Wherein, r[k] be received signal, a is counting of the local relatively PN sequence slip of receiving sequence.

3) when γ [k, a] obtains peak value or surpasses certain thresholding (this thresholding can be set according to real system), promptly realize the synchronous of time.

4) frequency rough is synchronous: on the basis of above-mentioned time synchronized, at first utilize the training sequence of stack in the protection at interval, carry out the frequency deviation rough estimate according to following formula:

$\widehat{\mathrm{\ϵ}}=\frac{N}{2\mathrm{\π}\·N_{\mathrm{PN}}}\·\arg \left(\mathrm{\γ}\right[k,a\left]\right)---\left(7\right)$

5) the frequency essence synchronously: data 33 during utilization is protected at interval then and the back N in the OFDM initial data _g Individual data 35 are identical, adopt conventional synchronization method (1), can carry out according to formula (2) that frequency deviation is smart to be estimated.

6), make compensate of frequency deviation to receiving data according to top frequency offset estimating.

The foundation of this method for designing is:

1) because training sequence is known, therefore certain correlation is provided, therefore be easy to realize time and the Frequency Synchronization of OFDM;

2) compare with the method for placing training sequence in whole OFDM symbols, the method for only placing training sequence in the protection interval that the present invention adopts can reduce the interference of training sequence to initial data, has improved energy efficiency simultaneously;

3) owing in protection at interval, be point-to-point cum rights stack between training sequence and the OFDM initial data, therefore do not cause the decline of data transmission efficiency;

Weighted value ρ when 4) point-to-point cum rights superposes can be different to different OFDM symbols, but in same OFDM symbol, the value of ρ is constant.For guaranteeing accurately to realize synchronously, can improve the signal power of training sequence composition in the protection at interval by raising ρ value at acquisition phase, and reduce the ρ value at tracking phase.Regulate ρ and can realize Best Times and Frequency Synchronization.When ρ=1, all protection compartment signals replace with PN sequence signals; When ρ=0, protection spacer structure and GPF (General Protection False interval (foregoing method one, promptly protecting blank signal is that the part of OFDM useful signal is identical) structure is identical.The design of the selection of ρ value and PN sequence all is one of key technology of this protection spacer structure design.

5) receiving end is in that to utilize training sequence to carry out the time smart synchronously and frequency rough when synchronous, at first will receive data sementation does relevant with local PN sequence, and then carry out the difference correlation computations, this processing method can be offset the influence of channel impulse response, thereby improves net synchronization capability.

6) because the present invention has fully utilized back N in data 33 and the OFDM initial data in the OFDM protection at interval _gThese two carries out time and Frequency Synchronization (this is called blending algorithm) correlation that the correlation of the data 35 of point and training sequence are introduced, and therefore, performance is better than conventional method.

7) because receiving end is known to the training sequence signal, also can utilize it to carry out channel estimating or other purposes.

The present invention has following feature:

1, the data in the OFDM protection at interval are by OFDM useful signal back N _gData on the individual point and the point-to-point cum rights of training sequence are formed by stacking, and the OFDM initial data is constant;

Weighted value when 2, point-to-point cum rights superposes is ρ, and the ρ that different OFDM symbols is got can be different, but in same OFDM symbol, the value of ρ is constant.Come to be used for making between the synchronous reliability of estimating compromise by adjusting these weights to the validity and the training sequence of transmission;

3, be used for synchronous training sequence by a plurality of identical PN sequence of continuous placement or the sequence with further feature constitutes (as shown in Figure 7) with good correlation;

When 4, receiving end is carried out time and Frequency Synchronization, adopt blending algorithm, promptly fully utilized back N in data 33 and the OFDM initial data in the OFDM protection at interval _gThe correlation that the correlation of the data 35 of point and training sequence are introduced.

5, receiving end is in that to utilize training sequence to carry out the time smart synchronously and frequency rough when calculating synchronously, and it is relevant with local PN sequence work at first will to receive data sementation, and then carries out the processing method of difference correlation computations.

Description of drawings

Fig. 1 is general ofdm system block diagram

Among the figure, 1 is string and modular converter, and 2 is mapping block, 3 is the IFFT module, and 4 is parallel serial conversion module, and 5 for adding protection time slot module, 6 is the D/A low-pass filtering module, and 7 is the up-conversion module, and 8 is channel module, 9 is down conversion module, and 10 is the A/D low-pass filtering module, and 11 is synchronization module, 12 for going to protect the time slot module, and 13 is the FFT module, and 14 is the single order balance module, 15 is mapping block, and 16 are and the string module;

Fig. 2 is the structure of the OFDM symbol of the conventional method for synchronous of employing (1)

Among the figure, with the back N in the OFDM initial data _gThe data 19 of individual point copy to 17 places, front of OFDM useful data, and formation length is N _gProtection at interval, OFDM initial data 18,19 is constant, owing to can't guarantee the correlated performance of random data, that is to say that receiving end is used for the time synchronized search and the acuteness of the target function that calculates can't guarantee that therefore the time synchronized performance of this method is bad;

Fig. 3 is the structure of the OFDM symbol in the protection that training sequence is placed on OFDM at interval of adopting conventional method for synchronous (2)

Among the figure, the data 20 in the protection at interval of OFDM emission symbol do not contain any OFDM initial data for to be used for synchronous training sequence, and OFDM initial data 21 is constant;

Fig. 4 is the structure of the preceding OFDM symbol in the protection interval that training sequence 22 is placed on OFDM of adopting conventional method for synchronous (2)

Among the figure, elder generation is with the back N of OFDM initial data _gThe data 25 of individual point copy to 23 places, front of OFDM initial data, and then will be used for synchronous training sequence 22 and be placed on 23 fronts, other OFDM initial data 24,25 are constant, obviously, because training sequence has been monopolized a part of transmission time, has caused the decline of data transmission rate;

Fig. 5 is the structure of the OFDM symbol of the conventional method for synchronous of employing (3)

Among the figure, will be used for the back N that synchronous training sequence 30 is superimposed upon the OFDM initial data earlier _gOn the data 31 of individual point, then with the back N in the sequence that obtains _gThe data 30,31 of individual point copy to OFDM front 26,27, constitute protection at interval, because each OFDM initial data has all participated in superposeing with the point-to-point cum rights of training sequence data, the interference that therefore is subjected to training sequence is bigger;

Fig. 6 is the structure of the OFDM symbol of the method for synchronous of employing present specification

Among the figure, sequence 33 is the back N of OFDM initial data _gThe copy of the data 35 on the individual point, sequence 33 and training sequence 32 point-to-point cum rights stacks constitute protection at interval, OFDM initial data 34,35 is constant, wherein, the content of training sequence 32 is seen Fig. 7, because training sequence data only superposes with the OFDM initial data in protection at interval, therefore the OFDM initial data of other parts is not disturbed;

Fig. 7 is the internal structure of training sequence in the protection at interval of OFDM symbol of present specification

Among the figure, training sequence is placed continuously by certain PN sequence and is constituted, wherein, 36,37,38 ..., 39 be the copy of same PN sequence, 40 may be this complete PN sequence, also may be this PN sequence through truncation, and the length of the training sequence that finally obtains is N _g, the good correlation that the PN sequence has can guarantee the temporal frequency net synchronization capability of the described method of this patent;

Fig. 8 is the implementation step block diagram of making a start of present specification

Fig. 9 is the receiving end implementation step block diagram of present specification

Need to prove that in Figure of description, N is counting of FFT, N _gFor protection is counted T at interval _sBe the sampling interval.

Embodiment

Below to provide a concrete OFDM configuration down, the performing step of this patent.Need to prove: the parameter in the following example does not influence the generality of this patent.

If the useful symbol lengths of OFDM is N=4096, the protection gap length is N _g=512.Get ρ=0.5, the energy of training sequence accounts for 50% of gross energy in the expression protection at interval.The PN sequence selection is to be N in the cycle _PN=31 m sequence is designated as PN[i] i ∈ [0,30], then the number of repetition of m sequence is L=N in the training sequence _g/ N _PN=512/31=17.

Make a start the m sequence is repeated (last m sequence can by truncation) composing training sequence 17 times continuously according to formula 4, it is carried out point-to-point cum rights superposed and transmitted according to formula 5 and OFDM initial data go out.

It is slightly synchronous that receiving end is at first carried out the time according to formula 1, determines the Position Approximate of time synchronized point, and it is synchronously smart to carry out the time according to formula 6 then, computing time synchronous points the exact position, obtain time synchronized, that is:

$\mathrm{\γ}[k,a]=\underset{l=0}{\overset{16-1}{\mathrm{\Σ}}}[\left(\underset{n=0}{\overset{31-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-l{N}_{\mathrm{PN}}\left]\right)$

$\·{\left(\underset{n=0}{\overset{31-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-(l+1)N_{\mathrm{PN}}\left]\right)}^{*}]$

Suppose that when a is 18 γ [k, a] is that γ [k, 18] obtains peak value, then obtains time synchronized.

After obtaining time synchronized, at first carry out the frequency deviation rough estimate, that is: according to formula 7

$\widehat{\mathrm{\ϵ}}=\frac{4096}{2\mathrm{\π}\·31}\·\arg \left(r\right[k,18\left]\right)$

Then, according to the smart estimation of formula 2 beginning frequency deviations.At last, carry out compensate of frequency deviation, obtain Frequency Synchronization.

Claims (3)

1, a kind of OFDM time, frequency synchronization method, the treatment step of making a start of this method for synchronous is:

It is N that step 1 is selected a length _PNThe PN sequence, be designated as PN[k], the PN[k of this moment] value is plural form, i.e. m[k] ∈ { 1+j ,-1-j};

Step 2 repeats above-mentioned PN sequence to place repeatedly continuously, clips the unnecessary data in back, and structure is grown into N _gTraining sequence, be designated as t[k], its mathematic(al) representation is:

t[k]＝m[kmodN _PN] k∈[0，N _g-1]

Step 3 is with training sequence and OFDM initial data back N _gAfter the point-to-point cum rights stack of data on the individual point, be positioned in the protection at interval; The OFDM initial data is constant, so the mathematic(al) representation of OFDM emission data is as follows:

$s\left[k\right]=\left\{\begin{array}{cc}\sqrt{1-\mathrm{\ρ}}\·d\left[k\right]+\sqrt{\mathrm{\ρ}}\·t\left[k\right]& k\∈[0,N_g-1]\\ d\left[k\right]& k\∈[N_g,N+N_g-1]\end{array}\right.$

The step that described ofdm system receiving end is handled is:

Step 4 time is slightly synchronous: data (33) in the utilization protection at interval and the back N in the OFDM initial data _gIndividual data (35) are identical, in the difference correlation of receiving terminal calculating received signal, can realize the synchronously thick of time according to following formula;

$P\left(\widehat{\mathrm{\θ}}\right)=\arg \underset{\mathrm{\θ}}{\max }\left(\underset{k=1}{\overset{N_g}{\mathrm{\Σ}}}{r}^{*}\right[k+\mathrm{\θ}\left]r\right[k+N+\mathrm{\θ}\left]\right)$

Step 5 time is smart synchronously: in above-mentioned scope, utilize the training sequence of stack in the protection at interval to find the exact position of time synchronized point, concrete grammar is as follows: establish L and be the full number of the PN sequence that contains in the training sequence, it is N that receiving end is provided with a size _PNThe detection window of L carries out shifting function to receiving data, asks relevant according to following formula then:

$\mathrm{\γ}[k,a]=\underset{l=0}{\overset{L-1}{\mathrm{\Σ}}}[\left(\underset{n=0}{\overset{N_{\mathrm{PN}}-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-l{N}_{\mathrm{PN}}\left]\right)$

$\·{\left(\underset{n=0}{\overset{N_{\mathrm{PN}}-1}{\mathrm{\Σ}}}{\mathrm{PN}}^{*}\right[k-n-a\left]r\right[k-n-(l+1)N_{\mathrm{PN}}\left]\right)}^{*}]$

Step 6 is worked as r[k, a] when obtaining peak value or surpassing necessarily the threshold value of setting according to real system, promptly realize the time synchronously;

Step 7 frequency rough is synchronous: on the basis of above-mentioned time synchronized, at first utilize the training sequence of stack in the protection at interval, carry out the frequency deviation rough estimate according to following formula:

$\widehat{\mathrm{\ϵ}}=\frac{N}{2\mathrm{\π}\·N_{\mathrm{PN}}}\·\arg \left(\mathrm{\γ}\right[k,0\left]\right)$

Step 8 frequency is synchronously smart: data during utilization is protected at interval then and the back N in the OFDM initial data _gIndividual data are identical, adopt the method for synchronous of step 4, can carry out the smart estimation of frequency deviation according to following formula:

$\widehat{\mathrm{\ϵ}}=\frac{1}{2\mathrm{\π}}\·{\tan }^{-1}\left(P\left(\widehat{\mathrm{\θ}}\right)\right)$

Step 9 is made compensate of frequency deviation according to top frequency offset estimating to receiving data.

2, according to described a kind of OFDM time of claim 1, frequency synchronization method, it is characterized in that the weighted value ρ when described point-to-point cum rights superposes can be different to different OFDM symbols, but in same OFDM symbol, the value of ρ is constant: come to be used for making between the synchronous reliability of estimating compromise to the validity and the training sequence of transmission by adjusting these weights.

3, according to described a kind of OFDM time of claim 1, frequency synchronization method, it is characterized in that in the step of described ofdm system receiving end processing, receiving end is in that to utilize training sequence to carry out the time smart synchronously and frequency rough when calculating synchronously, it is relevant with local PN sequence work at first will to receive data sementation, and then carries out the processing method of difference correlation computations.

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