CN100471094C - OFDM time and frequency synchronizing method capable of correcting long-range frequency deviation - Google Patents

Abstract

The invention provides a method of OFDM time frequency synchronization for correcting wide range frequency deviation, wherein the sending end dispensing the PN sequence replication according to code segment, then the sequences are repeated totally, constructing an exercise sequence, which is transmitted together with the OFDM primary data through spot-to-spot weighted superposition, the receiving end first obtains the time synchronism, then performs related calculus of differences to the received data with difference distance to be 1, the method by the invention can proceed evaluation and compensation in the bandwidth range of 1/2 OFDM system bandwidth.

Description

A kind of OFDM time, frequency synchronization method that can the correcting long-range frequency deviation

Technical field

The invention belongs to radio communication or wire communication field.

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: ADSL (Asymmetric Digital Subscriber Line), Wireless LAN is 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 anti-multipath, cochannel interference and impact noise ability, see document: Bingham, J.A.C., " Multicarrier modulation for data transmission:an idea whose time has come; " IEEE CommunicationsMagazine, Volume:28 Issue:5, May 1990.Page (s): 5-14; And document: Yun Hee Kim; Iickho Song; Hong Gil Kim; Taejoo Chang; Hyung Myung Kim, " Performance analysis of a coded OFDM system in time-varying multipathRayleigh fading channels; " Vehicular Technology, IEEE Transactions on, Volume:48 Issue:5, Sept.1999, Page (s): 1610-1615 is described.

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 and frequency offset in OFDMsystems; " Signal Processing, IEEE Transactions on, Volume:45 Issue:7, July 1997, and Page (s): 1800-1805 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) that receiving end is made a start relatively.

In the system that uses the OFDM technology, obtain after the time synchronized, can adopt the relevant method of calculating difference to come the calculated rate offset information, referring to document Moose, P.H., " A technique for orthogonal frequency division multiplexing frequencyoffset correction; " Communications, IEEE Transactionson, Volume:42 Issue:10, Oct.1994, Page (s): 2908-2914 is described.

The basic principle following (as shown in Figure 2) that difference is relevant: at first make a start and put into the identical training sequence in two ends in data, the distance between the identical data is d.Receiving end is estimated frequency deviation according to following computing formula with the difference correlation then:

$\widehat{\mathrm{\ϵ}}=\frac{1}{2\mathrm{\π}}{\tan }^{-1}\left(\underset{k=0}{\overset{m-1}{\mathrm{\Σ}}}{r}^{*}[k+\mathrm{\θ}]r[k+\mathrm{\θ}+d]\right)---\left(1\right)$

Wherein, θ represents the time synchronized point estimated, Expression estimated frequency shift value, r[k] be received signal, m represents the length of training sequence, and d represents the difference distance.

The selection of difference correlation distance d has a significant impact the estimated performance of frequency shift (FS).Difference is selected more for a short time apart from d, and then the frequency departure estimated ranges is just big more.Referring to document Marti i Puig, P.; Alvarez, J.S., " Coarse frequency estimation inOFDM packet oriented systems; " Acoustics, Speech, and Signal Processing, 2001.Proceedings.2001 IEEEInternational Conference on, 2001, Page (s): 2337-2340 vol.4 is described.

In the reality, usually requirement can be estimated and corrects the frequency deviation in as far as possible on a large scale.Therefore according to above-mentioned principle, need dwindle the difference distance as much as possible.Conventional solution is to make a start to place PN sequence (pseudo-noisesequences, pseudo random sequence) (as shown in Figure 3) continuously, dwindles difference apart from d by selecting as far as possible little PN sequence period.Because the PN sequence need provide certain spreading gain to realize antijamming capability, otherwise can have a strong impact on the performance of Frequency Synchronization, therefore need the cycle of PN sequence not select very for a short time, corresponding difference can not be very little apart from d, and this has just limited the scope that conventional method can be carried out frequency offset estimating.

Summary of the invention

The object of the present invention is to provide a kind of OFDM time, the method for Frequency Synchronization that can the correcting long-range frequency deviation, it comprises the training sequence setting of making a start and the respective handling of receiving end.

The realization of task of the present invention is earlier the PN sequence to be repeated to place by chip by making a start, and then the sequence that obtains is carried out integral body again and repeats, and the composing training sequence is gone out this training sequence and the point-to-point cum rights superposed and transmitted of OFDM initial data at last; Receiving end at first obtains time synchronized, and to carry out the difference distance be 1 difference related operation to receiving data then, in the scope of 1/2 ofdm system bandwidth frequency deviation is estimated at last and is compensated.

Innovation part of the present invention is making a start when repeating for the first time, the PN sequence is pressed chip, rather than the cycle of PN sequence repeats to place, carry out Frequency offset estimation and compensation at the corresponding difference correlation technique of receiving end utilization to receiving data then, thereby obtain the Frequency Synchronization of ofdm system.Here, the implication of chip is a data bit, and for example, it is exactly the number of chips of its initial data that the FFT of ofdm system counts.Obviously, this method overcome the restriction that the high order range of conventional method frequency shift (FS) is subjected to the PN sequence period, its frequency offset estimation range and PN sequence period are irrelevant, so difference can be contracted to theoretical minimum value 1 apart from d, and the related offset estimation range is 1/2 of an ofdm system bandwidth.

According to a kind of OFDM time, the method for Frequency Synchronization that can the correcting long-range frequency deviation of the present invention, it is characterized in that it comprises the following step:

One, makes a start

The step following (as shown in Figure 4) of making a start and handling to received signal:

1) selecting one period cycle is N _mM sequence m[k], k ∈ [0, N _m-1], this moment m[k] but value is plural form, i.e. m[k] ∈ 1+j ,-1-j} also can be real number form, i.e. m[k] ∈ 1 ,-1};

2) repeat (as shown in Figure 5) for the first time: with m[k] press the chip repetition, each chip repeats N _nInferior, generating a length is N _mN _nPN sequence m ¹[k], and k ∈ [0, N _mN _n-1], its mathematic(al) representation is as follows:

m ¹[k]＝m[k/N _n]k∈[0，N _mN _n-1] (2)

Wherein ,/expression divides exactly computing.

3) repeat (as shown in Figure 6) for the second time: above-mentioned sequence m ¹[k] complete number of repetition N in training sequence _RepBe N/ (N _mN _n) integer part, N is that the FFT of ofdm system counts; With above-mentioned PN sequence m ¹[k] integral body continuously repeats to place, and makes total counting be N, obtains training sequence t[k], its mathematic(al) representation is as follows:

t[k]＝m ¹[k?mod?N _mN _n]k∈[0，N-1] (3)

Wherein, mod represents modulo operation.

4) with training sequence t[k] carry out point-to-point cum rights stack back (as shown in Figure 7) with OFDM initial data (not comprising the Cyclic Prefix part) by following formula, obtain final emission data s[k] launch:

$\begin{array}{cc}s\left[k\right]=\sqrt{1-\mathrm{\ρ}}\·d\left[k\right]+\sqrt{\mathrm{\ρ}}\·t\left[k\right]& k\∈[0,N-1]\end{array}---\left(4\right)$

Wherein, ρ ∈ [0,1] is a weighted value, and its physical significance is the normalized value of the energy of training sequence with respect to emission data gross energy.

Two, receiving end

The step that receiving end is handled to received signal following (as shown in Figure 8):

1) according to the following formula calculating target function, to obtain time synchronized:

$\mathrm{\γ}[k,a]=\underset{i=0}{\overset{N_{\mathrm{rep}}}{\mathrm{\Σ}}}\underset{n=0}{\overset{N_m-1}{\mathrm{\Σ}}}\left[(\underset{m=0}{\overset{N_n-1}{\mathrm{\Σ}}}{m}^{*}[k-a]r[k-{\mathrm{nN}}_n-{\mathrm{lN}}_m{N}_n-m]\right)$ (5)

$\·{\left(\underset{m=0}{\overset{N_n-1}{\mathrm{\Σ}}}{m}^{*}[k-a]r[k-{\mathrm{nN}}_n-(l+1)N_m{N}_n-m]\right)}^{*}]$

Time synchronized point is for making | γ [k, a] | and the value of a of value maximum, a is counting of sliding of the local relatively PN sequence of the PN sequence of placing in the receiving sequence.

2) begin N from the training sequence starting point _mN _nN _RepThe receiving sequence of individual point is divided into N _RepSection is designated as:

r _i[k]i∈[0，N _rep-1] k∈[0，N _mN _n-1] (6)

3) to each r _i[k] asks relevant with local m sequence according to following formula, obtains N _nIndividual correlation

$\begin{array}{ccc}{\mathrm{Cor}}_n^i=\underset{k=0}{\overset{N_m-1}{\mathrm{\Σ}}}{m}^{*}\left[k\right]r_i[N_n\·k+n]& i\∈[0,N_{\mathrm{rep}}-1]& n\∈[0,N_n-1]\end{array}---\left(7\right)$

To every group of correlation

With two adjacent correlations

Grip altogether and multiply each other, add up, obtain each group and grip correlation altogether

$\begin{array}{cc}{\mathrm{Cor}}_{\mathrm{total}}^i=\underset{n=0}{\overset{N_n-2}{\mathrm{\Σ}}}[{\mathrm{Cor}}_n^i\·{\left({\mathrm{Cor}}_{n+1}^i\right)}^{*}]& i\∈[0,N_{\mathrm{rep}}-1]\end{array}---\left(8\right)$

Above-mentioned N _RepGrip correlation altogether for individual group

Add up, ask argument, adjust coefficient, obtain final frequency offset estimating value and be:

$\widehat{\mathrm{\&epsiv;}}=-\frac{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="C02134107E00111.png"\; state="\{\{state\}\}">N</figure-callout>}}{2\mathrm{\&pi;}}\&CenterDot;{\tan }^{-1}\left(\underset{i=0}{\overset{N_{\mathrm{rep}}-1}{\mathrm{\&Sigma;}}}{\mathrm{Cor}}_{\mathrm{total}}^i\right)---\left(9\right)$

Total computing formula is:

$\widehat{\mathrm{\&epsiv;}}=-\frac{\mathrm{<figure-callout\; id="2"\; label="N"\; filenames="C02134107E00111.png"\; state="\{\{state\}\}">N</figure-callout>}}{2\mathrm{\&pi;}}\&CenterDot;{\tan }^{-1}(\underset{i=0}{\overset{N_{\mathrm{rep}}-1}{\mathrm{\&Sigma;}}}\underset{n=0}{\overset{N_n-2}{\mathrm{\&Sigma;}}}\left[(\underset{k=0}{\overset{N_m-1}{\mathrm{\&Sigma;}}}{m}^{*}\left[k\right]r[\left(N_{n}k-{\mathrm{iN}}_m{N}_n\right)+n]\right)---\left(10\right)$

$\&CenterDot;{\left(\underset{k=0}{\overset{N_m-1}{\mathrm{\&Sigma;}}}{m}^{*}\left[k\right]r[\left(N_{n}k-{\mathrm{iN}}_m{N}_n\right)+n+1]\right)}^{*}])$

Wherein,

Expression estimated frequency shift value, r[k] be received signal, N is a FFT length, N _mBe PN sequence m[k] length, N _nBe PN sequence m[k] press the number of times that chip repeats, N _RepBe sequence m ¹The whole number of repetition of [k].

4) carry out the related offset compensation.

The foundation of this method for designing is:

1) because each PN sequence is identical, so the correlation between them can embody frequency shift (FS);

2) owing to make a start when the PN sequence repeated for the first time to repeat by chip, minimum interval in the training sequence between the identical data is 1, therefore reached the requirement of minimum differential distance in theory, receiving end comes deal with data can estimate the frequency deviation of maximum magnitude according to this difference distance;

3) owing to make a start when repeating for the first time, the PN sequence does not repeat to place according to whole length, so the difference distance is irrelevant with the length of PN sequence, so the selection of PN sequence period can be more flexible, if the PN sequence that selection cycle is long can improve antijamming capability;

4) the PN sequence m that obtains after will repeating by chip ¹[k] repeats to place N again _RepInferior purpose is the energy that adds up, and improves performance;

5), can improve the signal power of launching training sequence composition in the data by improving the ρ value at acquisition phase, and reduce the ρ value at tracking phase for assurance accurately realizes synchronously.Regulate ρ and can realize Best Times and Frequency Synchronization.When ρ=1, the emission data are entirely the training sequence signal; When ρ=0, the emission data are entirely the OFDM initial data.The selection of ρ value also is one of technology of this patent.

Theory analysis proves, adopts the described method of this patent the Frequency offset estimation scope of ofdm system can be reached maximum, and promptly 1/2 OFDM bandwidth has very strong practical value.

Description of drawings

Fig. 1 is general ofdm system block diagram

Among the figure, 11 is synchronization module;

Fig. 2 is the relevant basic principle figure of difference

It is 17 and 18 identical that training data 1～m among Fig. 2 constitutes, it is exactly any one known array among Fig. 3, owing to have only and just have correlation between corresponding data, thereby the difference distance can only be chosen as the distance between the corresponding data position, notice that the difference distance among Fig. 2 might not be the number m of training data, it also depends on the number of data points of being separated by between two training sequences;

Fig. 3 is the structure of training sequence in the conventional OFDM symbol

Training sequence is made of a plurality of identical PN sequences among the figure, n PN sequence 19,20,21,22 is identical, and they are to place continuously, be between adjacent two PN sequences continuously every, therefore, minimum range between the identical data is that the minimum differential distance is the cycle of PN sequence, and this shows that the difference distance is subjected to the restriction in the cycle of PN sequence;

Fig. 4 is the generation step of training sequence in the OFDM symbol of making a start of present specification

Among the figure, can find out and contain twice repetitive process in the treatment step of making a start, repeat for pressing chip for the first time, be for the second time whole the repetition;

Fig. 5 is the PN sequence m that obtains after making a start of present specification repeated for the first time ¹The structural representation of [k]

Among the figure, the PN sequence period is N _m, the PN sequence has repeated N by chip _nInferior;

The training sequence t[k that Fig. 6 obtains after repeating for the second time for making a start of present specification] structural representation

Among the figure, training sequence t[k] length be the FFT points N of ofdm system, PN sequence m ¹[k] is at training sequence t[k] in the complete N that repeated _RepInferior;

Fig. 7 sends out position view in the symbol for the training sequence of present specification at OFDM

Among the figure, training sequence and OFDM initial data are carried out point-to-point cum rights stack;

Fig. 8 is the enforcement block diagram of the receiving end data processing step of present specification

Embodiment

Below to provide a concrete OFDM configuration down, the performing step of this patent.Annotate: 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 PN sequence selection cycle is $N_m^{\&prime;}=127$ The m sequence, number of repetition N for the first time _n=2, number of repetition N for the second time then _Rep=4096/ (127*2)=16.Get ρ=0.5, the expression training sequence accounts for the energy of half in the emission data.

Make a start the m sequence is repeated 2 times by chip, obtain m ¹[k] is with m ¹[k] is whole to be repeated 17 times, clips the unnecessary data of afterbody, constitutes length and be 4096 training sequence t[k], according to formula 5 with training sequence t[k] carry out point-to-point cum rights stack with the OFDM initial data and constitute OFDM emission symbol and launch.

Receiving end at first obtains time synchronized according to formula 5, just finds the starting point of OFDM symbol, and the OFDM useful data is designated as d[i] i ∈ [0,4095]; Meanwhile also found the starting point of training sequence.Begin to estimate frequency deviation below:

(1) from the training sequence starting point, every N _mN _n=127*2=254 data are divided into one section, altogether N _Rep=16 sections, be designated as r ₀[k]～r ₁₅[k] k ∈ [0,253];

(2) relevant according to formula 7 with local m sequence work, obtain 32 correlations, be designated as

With ${{\mathrm{<figure-callout\; id="1"\; label="Cor"\; filenames="C02134107E00111.png,C02134107E00131.png"\; state="\{\{state\}\}">Cor</figure-callout>}}_1}^0~{{\mathrm{<figure-callout\; id="1"\; label="Cor"\; filenames="C02134107E00111.png,C02134107E00131.png"\; state="\{\{state\}\}">Cor</figure-callout>}}_1}^{15},$ That is:

$\begin{array}{ccc}{\mathrm{Cor}}_n^i=\underset{k=0}{\overset{127-i}{\mathrm{\&Sigma;}}}{m}^{*}\left[k\right]r_i[2\&CenterDot;k+n]& i\&Element;[0,15]& n\&Element;[0,1]\end{array}$

(3) obtain each group according to formula 8 and grip correlation altogether That is:

$\begin{array}{cc}{\mathrm{Cor}}_{\mathrm{total}}^i={\mathrm{Cor}}_n^i\&CenterDot;{\left({\mathrm{Cor}}_1^i\right)}^{*}& i\&Element;[0,N_{\mathrm{rep}}-1]\end{array}$

(4) carry out frequency offset estimating according to formula 9:

$\widehat{\mathrm{\&epsiv;}}=\frac{4096}{2\mathrm{\&pi;}}{\tan }^{-1}\left(\underset{k=0}{\overset{15}{\mathrm{\&Sigma;}}}{\mathrm{Cor}}_{\mathrm{total}}^i\right)$

(5) according to following formula data are carried out frequency compensation:

$\begin{array}{cc}d\left[i\right]=d\left[i\right]\&CenterDot;e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="C02134107E00111.png"\; state="\{\{state\}\}">j</figure-callout>}\frac{2\mathrm{\&pi;}\widehat{\mathrm{\&epsiv;}}}{4096}}& i=\mathrm{0,1},...,4095\end{array}$

Claims (2)

1, a kind of OFDM time, frequency synchronization method that can the correcting long-range frequency deviation, it is characterized in that making a start and earlier the PN sequence is repeated to place by chip, then the sequence that obtains being carried out again integral body repeats, the composing training sequence is gone out this training sequence and the point-to-point cum rights superposed and transmitted of OFDM initial data at last; Receiving end at first obtains time synchronized, and to carry out the difference distance be 1 difference related operation to receiving data then, in the scope of 1/2 ofdm system bandwidth frequency deviation is estimated at last and is compensated, and it is characterized in that:

The described step of making a start is:

It is N that step 1 is selected one period cycle _mM sequence m[k], k ∈ [0, N _m-1], this moment m[k] value is plural form, i.e. m[k] ∈ { 1+j ,-1-j};

Step 2 repeats for the first time: with m[k] press the chip repetition, each chip repeats N _nInferior, generating a length is N _mN _nPN sequence m ¹[k], and k ∈ [0, N _mN _n-1]; Its mathematic(al) representation is as follows:

m ¹[k]＝m[k/N _n]?k∈[0，N _mN _n-1]

Wherein ,/expression divides exactly computing;

Step 3 repeats for the second time: above-mentioned sequence m ¹[k] complete number of repetition N in training sequence _RepBe N/ (N _mN _n) integer part, N is that the FFT of ofdm system counts; With above-mentioned PN sequence m ¹[k] integral body continuously repeats to place, and makes total counting be N, obtains training sequence t[k]; Its mathematic(al) representation is as follows:

t[k]＝m ¹[kmodN _mN _n]?k∈[0，N-1]

Wherein, mod represents modulo operation;

Step 4 is with training sequence t[k] carry out point-to-point cum rights stack with the OFDM initial data that does not comprise Cyclic Prefix part by following formula after, obtain final emission data s[k] launch:

$\begin{array}{cc}s\left[k\right]=\sqrt{1-\mathrm{\&rho;}}\&CenterDot;d\left[k\right]+\sqrt{\mathrm{\&rho;}}\&CenterDot;t\left[k\right]& k\&Element;[0,N-1]\end{array}$

Wherein, ρ ∈ [0,1] is a weighted value;

Described receiving end step is:

Step 5 is according to the following formula calculating target function, to obtain time synchronized:

$\mathrm{\&gamma;}[k,a]=\underset{l=0}{\overset{N_{\mathrm{rep}}}{\mathrm{\&Sigma;}}}\underset{n=0}{\overset{N_m-1}{\mathrm{\&Sigma;}}}[\left(\underset{m=0}{\overset{N_n-1}{\mathrm{\&Sigma;}}}{m}^{*}[k-a]r[k-n{N}_n-{\mathrm{lN}}_m{N}_n-m]\right)$

$\&CenterDot;{\left(\underset{m=0}{\overset{N_n-1}{\mathrm{\&Sigma;}}}{m}^{*}[k-a]r[k-n{N}_n-(l+1)N_m{N}_n-m]\right)}^{*}]$

Time synchronized point is for making | γ [k, a] | and the value of a of value maximum, a is counting of sliding of the local relatively PN sequence of the PN sequence of placing in the receiving sequence;

Step 6 begins N from the training sequence starting point _mN _nN _RepThe receiving sequence of individual point is divided into N _RepSection is designated as:

r _i[k]i∈[0，N _rep-1]?k∈[0，N _mN _n-1]

Step ₇To each ri[k], ask relevant with local m sequence according to following formula, obtain N _nIndividual correlation

:

$\begin{array}{ccc}{\mathrm{Cor}}_n^i=\underset{k=0}{\overset{N_m-1}{\mathrm{\&Sigma;}}}{m}^{*}\left[k\right]r_i[N_n\&CenterDot;k+n]& i\&Element;[0,N_{\mathrm{rep}}-1]& n\&Element;[0,N_n-1]\end{array}$

Step 8 pair every group of correlation

With two adjacent correlations

Grip altogether and multiply each other, add up, obtain each group and grip correlation altogether

$\begin{array}{cc}{\mathrm{Cor}}_{\mathrm{total}}^i=\underset{n=0}{\overset{N_n-2}{\mathrm{\&Sigma;}}}[{\mathrm{Cor}}_n^i\&CenterDot;{\left({\mathrm{Cor}}_{n+1}^i\right)}^{*}]& i\&Element;[0,N_{\mathrm{rep}}-1]\end{array}$

Step 9 is above-mentioned N _RepGrip correlation altogether for individual group

Add up, ask argument, adjust coefficient, thereby draw final frequency offset estimating value:

$\widehat{\mathrm{\&epsiv;}}=-\frac{N}{2\mathrm{\&pi;}}\&CenterDot;{\tan }^{-1}\left(\underset{i=0}{\overset{N_{\mathrm{rep}}-1}{\mathrm{\&Sigma;}}}{\mathrm{Cor}}_{\mathrm{total}}^i\right)$

Step 10 is carried out the related offset compensation.

2, according to claim 1 described a kind of OFDM time, frequency synchronization method that can the correcting long-range frequency deviation, weighted value ρ when it is characterized in that described point-to-point cum rights stack is to different OFDM symbol differences, 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.

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