CN100542160C - Realize the synchronous method and apparatus of downlink frame - Google Patents

Abstract

The present invention relates to the orthogonal frequency division multiplex OFDM technology, provide especially and realize the synchronous method and apparatus of downlink frame in a kind of ofdm system under the TDD mode, to solve the synchronous computing complicated problems of downlink frame in the prior art.The autocorrelation that first kind of scheme of the present invention introduced according to the Cyclic Prefix of each OFDM symbol is located the original position of each OFDM symbol, on utilizing then/destructiveness that change-over time between the downlink frame on time domain the cycle is appearred in OFDM symbol original position at interval orients the first symbol of each frame, the last definite and relevant lead code in sub-district or sector according to the cross correlation of modulating data in the first symbol and lead code again, thereby finish Cell searching and downlink frame synchronizing process, reduced computational complexity.Second kind of scheme of the present invention directly oriented the leading symbol of downlink frame according to the OFDM symbol numbers of known downlink frame, simplified Cell searching and downlink frame synchronizing process more.

Description

Realize the synchronous method and apparatus of downlink frame

Technical field

(Orthogonal Frequency Division Multiplexing, OFDM) technology realize the synchronous method and apparatus of downlink frame under the TDD mode in particularly a kind of ofdm system to the present invention relates to OFDM.

Background technology

With OFDM (Orthogonal Frequency Division Multiplexing, OFDM) for the multi-carrier transmission technology of representative owing to have good anti-frequency selective fading performance and higher advantages such as band efficiency, be subjected to application more and more widely in recent years.Not only be applied to video broadcast system (Digital Audio Broadcasting, DAB), digital video broadcast terrestrial (Terrain Digital VideoBroadcasting, DVB-T) and WLAN (wireless local area network) (Wireless Local Area Network, WLAN) in, more become the key technology of back 3 g mobile communication system.OFDM (OrthogonalFrequency Division Multiple Access, OFDMA) being a kind of new access mode that is modulated to the basis with OFDM, is by provide the method for part available subcarrier to realize that the multi-user inserts for each user.

Ofdm system is by the data flow string of input and be transformed in several parallel subchannels, and the OFDM data symbol cycle is expanded as the several times of initial data OFDM symbol period like this, expands thereby can effectively resist multidiameter delay.Increase protection by original position and can further resist ISI (Inter-Symbol Interference at interval at each OFDM symbol; intersymbol interference); this protection is a kind of recursive copying at interval; before soon last some sampling points of each OFDM symbol will copy to the OFDM symbol; form CP (Cyclic Prefix, Cyclic Prefix).As shown in Figure 1, Fig. 1 is an OFDM symbolic construction schematic diagram, and wherein, valid data are modulated after the counting of IFFT (Inverse Fast Fourier Transformation, anti-fast fourier transform) in frequency domain and the set of corresponding subcarrier.1024 points for example, N _FftExpression valid data length, N _FftConstitute a complete OFDM symbol together with CP.

In radio communication, can use 3 kinds of different duplexing load modes: FDD (FrequencyDivision Duplex, Frequency Division Duplexing (FDD)), TDD (Time Division Duplex, time division duplex) and (SDD (Space Division Duplex, Space Division Duplex).Wherein the uplink and downlink link of TDD mode takies identical frequency band, but alternately changes transmission direction in time.For the system that has adopted TDD, need one period change-over time at interval between the uplink downlink emission, as shown in Figure 2, be called transmission/reception transmission intercal (Transmit/Receive Transition Gap the change-over time that downlink frame is transformed into uplink frame at interval, TTG), be called at interval the change-over time that uplink frame is transformed into downlink frame reception/transmission transmission intercal (Receive/TransmitTransition Gap, RTG).

Adopt the wireless access system of sub-district (Cell) system usually the coverage to be divided into several adjacent sub-districts, (Base Station BS) finishes its signal and covers with a base station in each sub-district.(Mobile Station MS) after start, at first seeks a suitable base station by a specific search procedure and realizes the access service of oneself portable terminal in the network, and this process is called Cell searching.MS carries out after the Cell searching, will obtain the down-going synchronous information of this cell base station and specific identification number thereof, with the important parameter as the follow-up access procedure of realization.

OFDMA system (the follow-up explanation of the present invention is that example is analyzed with this OFDMA wireless access system of IEEE802.16e) is when using, general each microzonation is divided into several sectors (being generally 3), adopt several (for example 32) different identification numbers, be lead code come mark several (accordingly, 32) adjacent sub-district, this lead code is generally the PN sequence.When adopting the OFDMA transmission means, the subcarrier of downlink OFDMA transmission OFDM symbol is divided into several (common 3, corresponding with sector number) different set, and different subcarrier set are corresponding to different sectors.Above-mentioned lead code is gathered in order to modulate corresponding OFDMA subcarrier in the leading symbol (being the first symbol of each downlink frame) of each downlink frame, thereby determines different sub-districts and sector by different lead codes and different sub-carrier positions thereof.In IEEE 802.16e agreement, stipulate to constitute one group of pseudorandom (PN) sequence of lead code, had good autocorrelation performance and their cross correlation preferably between this PN sequence.

In the system of 802.16e agreement regulation, the frequency-domain structure of lead code is to exist two to be 0 subcarrier between adjacent two subcarriers of modulating lead code (known PN sequence).The corresponding preamble carrier (Preamble Cartier Set) of lead code under each sector (Segment), the difference of different preamble carrier are to modulate the starting point difference of the subcarrier of lead code.The constructive method of preamble carrier is as shown in Equation (1):

n＝Guard_left+segment_offset+3k (1)

Wherein: n is the position that preamble carrier is concentrated each subcarrier, and Guard_left is a left side protection number of sub carrier wave; Segment_offset=1,2,3, represent the sheet biasing, its three kinds of values are corresponding segment0, segment1 and segment2 respectively; K=0,1,2 ... 283, respectively corresponding 87 o'clock to 1024 o'clock fast Fourier transform (FFT) the OFDM symbol.In ofdm system, whole frequency band is divided into a plurality of subcarriers, a frequency in the corresponding frequency band of each subcarrier, the position of so-called subcarrier just is meant the position of subcarrier frequency points corresponding, the frequency that the different frequent points position is corresponding different, the gap size between the different sub carrier position can quantize with the difference on the frequency between the different frequent points of different sub carrier correspondence.

Fig. 3 is the lead code frequency-domain structure schematic diagram under three different Segment in the 802.16e agreement.Three different Segment are respectively among Fig. 3: Segment0, Segment1 and Segment2.The preamble carrier of these three the different Segment correspondences all by formula method of (1) description constitutes; at 1024 FFT (Fast Fourier Transformation; fast fourier transform) in the OFDM character position; left side protection number of sub carrier wave is 86, and right protection number of sub carrier wave also is 86.

Wherein, the preamble carrier of Segment0 correspondence is concentrated, and the start position of subcarrier is since 87, and finishes 936 position, and wherein the subcarrier of non-zero is represented with filled arrows upwards, is that zero subcarrier is represented with round dot; The preamble carrier of Segment1 correspondence is concentrated, and the start position of subcarrier is since 88, and finishes 937 position, and wherein the subcarrier of non-zero is represented with the vertical line of upper end band closed square, is that zero subcarrier is represented with round dot; The preamble carrier of Segment2 correspondence is concentrated, and the start position of subcarrier is since 89, and finishes 938 position, and wherein the subcarrier of non-zero is represented with the vertical line of upper end band empty circles, is that zero subcarrier is represented with round dot; Concentrate in the preamble carrier of this three Segment, all exist two to be zero subcarrier between per two subcarriers of having modulated known PN sequence.

The preamble carrier that lead code under the visible different Segment shown in Figure 3 is corresponding different, the difference of different preamble carrier is to modulate the starting point difference of the subcarrier of lead code, in Fig. 3, the subcarrier start position of the preamble carrier modulation PN sequence of Segment0 and Segment1 correspondence differs 1 point, and the subcarrier start position of the preamble carrier modulation PN sequence of Segment1 and Segment2 correspondence differs 1 point; And, concentrate in same preamble carrier, all exist two to be 0 subcarrier between per two subcarriers of having modulated known PN sequence.

The MS of OFDMA system must carry out cell search process when initially starting shooting and switching, realize that cell ID obtains with the downlink frame timing information to obtain, promptly obtain the downlink frame start position information after, just can finish follow-up access procedure.The main difficult point of this process is: the original position (promptly carrying the original position of the downlink frame leading symbol of cell identity information) of the unknown downlink frame of the firm starting up's fashion of MS, and because the modulating data of downlink frame leading symbol is uncertain, and be subjected to the influence of carrier wave frequency deviation, channel distortions and decline and other adjacent cell signal, therefore be difficult to its position be detected with common method.

Prior art one

In the ofdm system that adopts lead code,, adopt the preamble structure that has repeat property on the time domain usually in order to introduce more redundant information.Because the correlation that repetitive structure is introduced is so can utilize this correlation to finish functions such as Time and Frequency Synchronization.

Utilize the approximate repeat property of the lead code of every frame frame head among the IEEE 802.16e to carry out the regularly synchronous and mark correcting frequency deviation of frame.Obtaining regularly sync bit and carrying out utilizing the cross correlation of lead code behind the correcting frequency deviation on frequency domain, carry out related operation in all lead code set, that of correlation peak maximum is the lead code that searches.Correlation function is formula (2) for example:

$\mathrm{Corr}\left(i\right)=\frac{\underset{k=0}{\overset{M-1}{\mathrm{\Σ}}}R\left(k\right)\·P_i^{*}\left(k\right)}{\sqrt{\underset{k=0}{\overset{M-1}{\mathrm{\Σ}}}{\left|R\left(k\right)\right|}^2\·\underset{k=0}{\overset{M-1}{\mathrm{\Σ}}}{\left|P_i\left(k\right)\right|}^2}}---\left(2\right)$

Wherein: | Corr (i) | be correlation, P _iRepresent i code word in the known preamble set, M represents preamble length, the lead code that the R representative receives.As | Corr (i) | i lead code of correspondence is the lead code that searches when obtaining maximum, equally in actual applications, needs to set certain thresholding, as | Corr (i) | maximum during greater than thresholding, just think to search lead code.

If but the mobile subscriber is in cell edge, when inserting, it can receive superposed signal from the lead code that belongs to different Segment of a plurality of sub-districts.Three lead code superposed signals that for example belong to Segment0, Segment1, Segment2 respectively, the frequency-domain structure of superposed signal as shown in Figure 4, wherein, the preamble carrier of this superposed signal correspondence is concentrated, the start position of subcarrier is since 87, and finish, and there is not the characteristic of zero insertion between each subcarrier 938 position.Like this, the approximate repeat property on the time domain no longer keeps, and causes utilizing the repeat property correlation to carry out the regularly synchronous method of frame and loses efficacy, thereby can't form tangible correlation peak, omission occurs.Can not obtain under the situation regularly of frame more accurately this, the follow-up method based on carrying out Cell searching on the frequency domain also can't search corresponding lead code.Like this, the mobile subscriber who is in three sub-district intersections can't search corresponding Segment and ID_Cell information, and the user just can't insert.

Prior art two

Because the Cyclic Prefix in the ofdm system is the copy of each OFDM symbol last part data, so in each OFDM symbol, all exist because the autocorrelation that the repeat property of Cyclic Prefix is introduced.Consult shown in Figure 1ly, set the sampling window of a slip data are sampled, the width of this sliding window equals the width of CP, the sampled point of whenever being separated by, and a N adjusts the distance _FftThe data that sample in two windows of length are carried out auto-correlation computation, if find greater than the correlation peak of setting the first threshold value value, think that then the data in two sampling windows of this correlation peak correspondence lay respectively at the Cyclic Prefix part and the end repeating part of an OFDM symbol, realize the location of OFDM symbol original position with this.

Synchronous, original position at each OFDM symbol of orienting, carrying out the FFT computing is converted on the frequency domain, on frequency domain, utilize the cross correlation of lead code, each lead code in the data of modulating in this OFDM symbol and the lead code set is carried out the correlation computing, when the correlation peak that exists greater than second threshold value, think that the original position of this OFDM symbol of orienting is the downlink frame original position, the data of corresponding modulating in this OFDM symbol are the lead code that searches, then Cell searching success; In having traveled through set after all lead codes, when not having the correlation peak greater than second thresholding, think that current OFDM symbol is not the first symbol of downlink frame, but common OFDM data symbol, continue to utilize the repeat property of Cyclic Prefix this moment, search for the original position of next OFDM symbol, so continue, up to searching correct downlink frame original position and lead code.If continuous sampling still do not find correct downlink frame original position and lead code after the maximum frame number length of setting, then synchronizing process failure.

Since the repeat property by Cyclic Prefix obtains be the OFDM symbol regularly, all to be transformed on the frequency domain in each OFDM symbol original position, and each OFDM symbol original position all will be carried out related operation and be searched for lead code in all lead code set, computation complexity is very high.

Summary of the invention

The invention provides and realize the synchronous method and apparatus of downlink frame in a kind of ofdm system under the TDD mode, to solve the synchronous computing complicated problems of downlink frame in the prior art.

For solving the problems of the technologies described above, the invention provides following technical scheme:

Realize the synchronous method of downlink frame in first kind of ofdm system under the TDD mode, comprise the steps:

According to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orient the original position of each OFDM symbol;

Go out the first symbol of each frame according to positioned at intervals change-over time that whether comprises uplink frame and downlink frame between the adjacent OFDM symbol original position, concrete grammar is, spacing between each a current OFDM symbol of synchronous monitoring and the adjacent last OFDM symbol original position, wherein: when the spacing between a current OFDM symbol and the adjacent last OFDM symbol original position was approximately equal to an OFDM symbol lengths, this current OFDM symbol was the non-first symbol of a uplink frame or downlink frame; Otherwise this current OFDM symbol is the first symbol of a uplink frame or downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Determine the leading symbol and the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the first symbol and known preamble.

Wherein, the described autocorrelation of introducing according to the OFDM symbol cyclic prefix method of locating each OFDM symbol original position specifically comprises the steps:

Set slip sampling window sample frame data, the length of this sampling window and the equal in length of Cyclic Prefix;

Calculate the autocorrelation value of two sections correlated sampling data respectively, these correlated sampling data are to be that an OFDM symbol lengths deducts two segment datas of sampling and obtaining in two sampling windows of a circulating prefix-length in spacing respectively;

When the autocorrelation value of calculating when setting first threshold value, after in one group of correlation calculating, the correlation peak that search is wherein maximum, according in two sections correlated sampling data of this correlation peak correspondence the last period data sampling window locate the original position of current OFDM symbol.

In the described method, elder generation is according to the synchronous monitoring result of described spacing, abandon wherein spacing with an adjacent last OFDM symbol original position less than all OFDM symbols of setting first spacing after, carry out the location of first symbol again, this first spacing deducts the length of a Cyclic Prefix less than an OFDM symbol lengths.

Wherein, described cross correlation according to data of modulating in the first symbol and known preamble determines that the method for downlink frame lead code specifically comprises:

Correlation during data of modulating in the first symbol of frequency-domain calculations and known preamble are gathered respectively between each lead code, and:

Judge whether to occur correlation peak greater than second threshold value, if, this first symbol leading symbol that is downlink frame then, the lead code of this correlation peak correspondence is the lead code of downlink frame; Otherwise, first OFDM symbol that this first symbol is a uplink frame.

Described method also comprises: if in the computing cross-correlation to the first symbol of setting number determined continuously and known preamble, described correlation peak all do not occur, then finish this downlink frame synchronizing process.

Realize that above-mentioned first kind is realized the synchronous device of downlink frame under the TDD mode in ofdm system, comprising:

Sampling unit, the data in the sampling OFDM symbol;

Memory cell, storage comprise the lead code set of all known preamble;

First positioning unit connects described sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Second positioning unit, connect described first positioning unit, orient the original position of each OFDM symbol according to first positioning unit, go out the first symbol of each frame again according to positioned at intervals change-over time that whether comprises uplink frame and downlink frame between the adjacent OFDM symbol original position, if the spacing between a current OFDM symbol and the adjacent last OFDM symbol original position is approximately equal to an OFDM symbol lengths, this current OFDM symbol is the non-first symbol of a uplink frame or downlink frame, otherwise, this current OFDM symbol is the first symbol of a uplink frame or downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Confirmation unit, connect the described memory cell and second positioning unit respectively, orient the first symbol of each frame according to second positioning unit, determine the leading symbol and the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the first symbol and known preamble again.

Realize the synchronous method of downlink frame in second kind of ofdm system provided by the invention under the TDD mode, the OFDM symbol numbers that comprises in the described downlink frame is a fixed value, and the OFDM symbol numbers that comprises in the uplink frame is not equal to this fixed value, and described method comprises the steps:

According to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orient the original position of each OFDM symbol; And

Spacing between the synchronous monitoring two adjacent OFDM symbol original position, search adjacent OFDM symbol original position spacing and all be approximately equal to the OFDM symbol numbers of the continuous appearance of an OFDM symbol lengths, when described number equals described fixed value, continuous first OFDM symbol of an OFDM symbol leading symbol that is downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Determine the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the downlink frame leading symbol and known preamble.

Realize realizing the synchronous device of downlink frame in above-mentioned second kind of ofdm system under the TDD mode, comprising:

Sampling unit, the data in the sampling OFDM symbol;

Memory cell, storage comprise the lead code set of all known preamble;

Positioning unit connects sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Monitoring means, connect positioning unit, orient the original position of each OFDM symbol according to positioning unit, spacing between the synchronous monitoring two adjacent OFDM symbol original position again, search adjacent OFDM symbol original position spacing and all be approximately equal to the OFDM symbol numbers of the continuous appearance of an OFDM symbol lengths, when described number equals described fixed value, continuous first OFDM symbol of an OFDM symbol leading symbol that is downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Confirmation unit, connect memory cell and monitoring means respectively, the leading symbol of the downlink frame of orienting according to monitoring means is determined the leading symbol and the lead code of downlink frame and is realized that downlink frame is synchronous according to the cross correlation of data of modulating in this leading symbol and known preamble again.

Beneficial effect of the present invention is as follows:

First kind of the present invention is based on the periodic frame synchornization method of time domain, the autocorrelation of introducing according to the Cyclic Prefix of each OFDM symbol is located the original position of each OFDM symbol, on utilizing then/destructiveness that change-over time between the downlink frame on time domain the cycle is appearred in OFDM symbol original position at interval orients the first symbol of each frame, the last definite and relevant lead code in sub-district or sector according to the cross correlation of modulating data in the first symbol and lead code again, thus Cell searching and downlink frame synchronizing process finished.Technical solution of the present invention selectively in some OFDM symbol original position, is carried out the Cell searching on the frequency domain, but not all carries out Cell searching on the frequency domain in each OFDM symbol original position, thereby reduces the computation complexity in the initial synchronization;

Second kind of the present invention is based on the periodic frame synchornization method of time domain, still on having utilized/destructiveness that change-over time between the downlink frame on time domain the cycle is appearred in OFDM symbol original position at interval orients the first symbol of each frame, and go out the leading symbol of downlink frame according to the OFDM symbol numbers synchronized positioning of known downlink frame, simplified Cell searching and downlink frame synchronizing process more.

Description of drawings

Fig. 1 is an OFDM symbolic construction schematic diagram;

Fig. 2 be in the TDD system on/change-over time of the generation in when conversion in downlink frame transmission time schematic diagram at interval;

Fig. 3 is the lead code frequency-domain structure schematic diagram under three different Segment in the 802.16e agreement;

Fig. 4 is the frequency-domain structure schematic diagram of the lead code superposed signal under three different Segment shown in Figure 3;

Fig. 5 is the main schematic flow sheet of first method of the present invention;

Fig. 6 is for sampling to data and progressively calculate the main schematic flow sheet that autocorrelation is determined the original position of OFDM symbol by setting slip sampling window that a width equals CP;

Fig. 7 determines the main schematic flow sheet of downlink frame original position for the method for the invention;

Fig. 8 judges by the data of modulating in the leading symbol and the correlation of lead code whether first symbol is the main schematic flow sheet of the leading symbol of downlink frame;

Fig. 9 is the primary structure schematic diagram of a kind of device of realization method shown in Figure 5;

Figure 10 is the main schematic flow sheet of second method of the present invention;

Figure 11 is the primary structure schematic diagram of a kind of device of realization method shown in Figure 10.

Embodiment

As shown in Figure 5, first kind of synchronous method of achieve frame in the OFDMA system that adopts the TDD mode provided by the invention, the major technique design comprises following three steps:

S100, according to the autocorrelation that the Cyclic Prefix characteristic of each OFDM symbol on time domain introduced, orient the original position of OFDM symbol;

Because the Cyclic Prefix of OFDM symbol has the head and the tail repeat property, can utilize this repeat property like this, the line correlation computing of going forward side by side of sampling received signal, because therefore the auto-correlation computation value maximum of carrying out between the part that repeat at CP part and OFDM symbol end provides by seeking the location that correlation peak is realized OFDM symbol original position.In common ofdm system, every frame (comprising uplink frame and downlink frame) all is made up of the OFDM symbol of plurality of continuous, every like this starting point place at each OFDM symbol, utilize above-mentioned auto-correlation computation can both obtain corresponding peaks, promptly obtain each OFDM symbol original position by the autocorrelation that utilizes CP.

S200, according to whether comprise between the adjacent OFDM symbol original position/interval T TG/RTG change-over time (RTG or TTG) between the downlink frame (uplink frame or downlink frame) orients the first symbol of each frame;

Because on each/downlink frame in, the OFDM symbol is to occur continuously, the length of an OFDM symbol so should be separated by between the original position of two adjacent OFDM symbol, just on one/downlink frame in, the OFDM symbol original position of orienting has the characteristic that the cycle occurs.

In the TDD system, because there is RTG/TTG in uplink and downlink interframe, when running into RTG/TTG, the gap that appears between former and later two OFDM symbol original positions of RTG/TTG is that an OFDM symbol lengths adds a RTG/TTG size.So it is the cyclophysis of an OFDM symbol lengths that RTG and TTG can destroy the spacing that OFDM symbol original position described above presents, be gap between current OFDM symbol original position and the last OFDM symbol original position greater than an OFDM symbol lengths, can claim that the OFDM symbol original position after the RTG/TTG is a breakdown point.The first symbol that occurs after each breakdown point is on one/the first symbol of downlink frame.

S300, determine the first symbol of downlink frame according to the cross correlation of data of modulating in the first symbol and lead code, i.e. leading symbol, and finish Cell searching synchronously and downlink frame is synchronous.

In IEEE 802.16e OFDMA system and since have only downlink frame begin have lead code, so need further distinguish by the Cell searching on the frequency domain/original position of downlink frame.In the OFDM of breakdown point symbol original position, carrying out the FFT computing is converted on the frequency domain, on frequency domain, utilize the cross correlation of lead code, carry out computing cross-correlation with each lead code in the lead code set, the reasonable threshold of correlated results and setting is compared, under the situation of existence, just think that this OFDM symbol original position is the downlink frame original position greater than the correlation peak of thresholding, the lead code of corresponding modulating in this OFDM symbol is the lead code that searches, the Cell searching success; All lead codes in having traveled through set, when not having the correlation peak greater than thresholding, think that then current OFDM symbol original position is not the downlink frame original position, but the original position of uplink frame, continue this moment the OFDM symbol original position of the next breakdown point that searches is carried out above-mentioned computing cross-correlation, up to searching correct downlink frame original position and lead code.

If still do not find correct downlink frame original position and lead code after having searched for some frame lengths continuously, then synchronization failure.

Among the step S100, frame data are sampled progressively calculate autocorrelation by setting slip sampling window that a width equates with the CP width to determine the original position of OFDM symbol, wherein, sampling step length is adjacent two distances between counting in the OFDM symbol, the corresponding total number of sample points that comprises equates with the sum of counting that comprises in the OFDM symbol lengths, 1024 sampled points for example, sampling window sampled point that at every turn slides carries out continuous sampling, respectively at a distance of a N _FftTwo segment datas of adopting in two sampling windows of length (consulting shown in Figure 1) constitute one group of correlated sampling data, and every section sampled data length is CP, respectively each group correlated sampling data are carried out the original position that related operation is determined current OFDM symbol.

As shown in Figure 6, current OFDM symbol original position determine comprise the steps:

S101, obtain one group of correlated sampling data;

S102, calculate the correlation of these group correlated sampling data;

Correlation function such as formula (3):

$M\left(d\right)=\left|\frac{\underset{n=1}{\overset{\mathrm{CP}}{\mathrm{\Σ}}}r(n+d)r^{*}(n+d+\mathrm{Nfft})}{0.5*(\underset{n=1}{\overset{\mathrm{CP}}{\mathrm{\Σ}}}{\left|r(n+d)\right|}^2+\underset{n=1}{\overset{\mathrm{CP}}{\mathrm{\Σ}}}{\left|r(n+d+\mathrm{Nfft})\right|}^2)}\right|---\left(3\right)$

Wherein: r is a received signal; N is the sequence number of sampled point in the sampling window, and its span is the integer of 1～CP; The original position of correspondence when d constantly slides for first sampling window, r* represents the conjugated signal of received signal.

S103, judge that correlation is whether greater than first threshold value of setting, if then continue S104; Otherwise execution in step 105 is returned step S102 after obtaining adjacent next group correlated sampling data;

S104, in one group of correlation of subsequent calculations the maximum correlation peak of search, in two sampling windows of one group of correlated sampling data of this correlation peak correspondence, previous sampling window should be the original position of current OFDM symbol mutually, and a back sampling window should be the end position of current OFDM symbol mutually.

The search procedure of correlation peak is: the correlation that continues several groups of correlated sampling data of calculated for subsequent, until correlation peak occurring, therefore two sampling windows of one group of sampled data of this correlation peak correspondence are respectively the CP and the tail data of current OFDM symbol, utilize the sampling window of this group sampled data can orient the original position of this OFDM symbol.

Step S101-S104 circulation is carried out, can determine the original position of each OFDM symbol like this, since one complete up/original position of all OFDM symbols of comprising in the downlink frame should be that periodic spacings occurs, desired pitch between the two adjacent OFDM symbol original position is an OFDM symbol lengths, in the TDD mode, when up/downlink frame is changed, the RTG/TTG that occurs has destroyed should the cycle (consulting shown in Figure 1), the first symbol that destroyed back of each cycle occurs all should be the first symbol of a up/downlink frame mutually.

Consider when the mobile subscriber is in cell edge, can receive superposed signal as shown in Figure 4 when it inserts from the lead code that belongs to different Segment of a plurality of sub-districts.Though in the TDD system, requiring between BS is Time and Frequency Synchronization, because that factor such as propagation delay time can make that the signal of each sub-district arrives mobile subscriber's time order and function is variant.In this case, when utilizing the circulating prefix structure of CP to carry out OFDM symbol location, a more than correlation peak greater than first threshold value may appear in the OFDM symbol lengths, correspondingly orient a more than first symbol, like this, can't determine according to step S200 which is a breakdown point, promptly goes up/the first symbol of downlink frame, then cause the Cell searching failure.

Appearance for fear of above-mentioned situation, spacing between two OFDM symbols of synchronous monitoring original position, for current OFDM symbol original position, at first whether the spacing between a judgement and the adjacent last OFDM symbol original position is less than first spacing of setting, this first spacing should be slightly less than Nfft length (consulting shown in Figure 1), promptly be slightly less than an OFDM symbol lengths and deduct the length of a Cyclic Prefix, if then illustrate on a current OFDM symbol and the adjacent last OFDM symbol different base station signal/downlink frame OFDM symbol, abandon current OFDM symbol.

In the reality, owing to be subjected to the influence of noise and channel, the OFDM symbol start bit that obtains by the repetitive cycling prefix of utilizing CP is equipped with certain ambiguity, OFDM symbol original position of obtaining and real original position have certain deviation, so when carrying out the searching of breakdown point, judge that OFDM symbol original position satisfies periodic condition needs appropriateness and relaxes, as long as before and after the absolute value of difference between spacing and the OFDM symbol lengths between the original position of OFDM symbol of twice acquisition in threshold value necessarily, just think and satisfy periodic characteristic that this threshold size depends on that minimum among RTG/TTG value.Otherwise just can't distinguish breakdown point, can consider usually less than 1/2 CP length, for example 1/4 CP length.Otherwise, the difference that spacing between the original position of a current OFDM symbol original position and an adjacent last OFDM symbol deducts an OFDM length is during greater than this setting threshold, think that then current OFDM symbol has destroyed periodic characteristic, be on one/the first symbol of downlink frame.Accordingly, consider the influence of noise and signal, aforesaid first spacing also should deduct on the basis of a CP length at an OFDM symbol lengths, deducts a threshold value again, and for example 1/4 CP length promptly is equivalent to deduct 5/4 CP length.

Concrete steps by each OFDM symbol original position of synchronous recording (can only write down an adjacent last OFDM symbol original position) inquiry breakdown point comprise as shown in Figure 7:

S201, determine the original position of current OFDM symbol;

S202, judge that whether spacing is greater than first spacing between the original position of a current OFDM symbol original position and an adjacent last OFDM symbol, if then continue, otherwise execution in step S206 abandons current OFDM symbol;

S203, judge that whether the spacing between the original position of a current OFDM symbol original position and an adjacent last OFDM symbol deducts the difference of an OFDM length greater than preset threshold, if step S204 then, this OFDM symbol is the first symbol of last/downlink frame; Otherwise step S205, this OFDM symbol be not on/the first symbol of downlink frame;

For the first symbol that finds out, judge this first symbol whether be downlink frame first symbol concrete grammar as shown in Figure 8, comprise the steps:

S301, this first symbol is done the FFT computing to frequency domain;

Carry out Cell searching on frequency domain, concrete search procedure is:

S302, from lead code set, obtain a lead code to be measured;

The data of modulating in S303, the current OFDM symbol of calculating and the correlation of this lead code;

S304, judge that correlation is whether greater than second threshold value of setting, if then continue S305; Otherwise execution in step S311 returns step S303 after residue is obtained a lead code to be measured the lead code again;

S305, judge whether to exist correlation peak, if step S306 then, this is searched for successfully, and this OFDM symbol is the leading symbol of a downlink frame, and the lead code to be measured of correlation peak correspondence is the lead code of the sub-district (or sector) that searches; Otherwise step S307, this searches for failure, and this OFDM symbol is the first symbol of a uplink frame.

Among S308, the step S307, after the search failure each time, carry out the S308 step and judge the first total number of symbols of search continuously, be whether the frame sum reaches the setting number, if then execution in step S309 finishes synchronizing process, otherwise execution in step S310 obtains next first symbol, and returns step S302 continuation utilization and carry out Cell searching.

The determination methods of correlation peak is for continuing to calculate the correlation of the residue lead code correspondence in the lead code set, if correlation peak, then this is searched for successfully, carrying out the sub-district according to the lead code that searches out inserts, and synchronous according to the first symbol realization downlink frame of this lead code of modulation, this synchronizing process finishes; If after having calculated the correlation of all lead code correspondences, correlation peak does not appear, and then this search failure is searched for according to the first symbol of the next one again.

If the hunting zone has surpassed the some first symbol of setting continuously, correlation peak does not still appear in promptly some frames, and then Cell searching failure finishes this synchronizing process.

Said method is based on the regularly synchronous method of the periodic frame of time domain, the autocorrelation of introducing according to the Cyclic Prefix of each OFDM symbol is located the original position of each OFDM symbol, on utilizing then/destructiveness that change-over time between the downlink frame on time domain the cycle is appearred in OFDM symbol original position at interval orients the first symbol of each frame, the last definite and relevant lead code in sub-district or sector according to the cross correlation of modulating data in the first symbol and lead code again, thus Cell searching and downlink frame synchronizing process finished.Technical solution of the present invention selectively in some OFDM symbol original position, is carried out the Cell searching on the frequency domain, but not all carries out Cell searching on the frequency domain in each OFDM symbol original position, thereby reduces the computation complexity in the initial synchronization.

As shown in Figure 9, realize in the above-mentioned ofdm system that the device 100 of downlink frame method for synchronous comprises under the TDD mode:

Sampling unit 101, the data in the sampling OFDM symbol;

Memory cell 102, storage comprise the lead code set of all known preamble;

First positioning unit 103 connects sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Second positioning unit 104, connect first positioning unit 103, orient the original position of each OFDM symbol according to first positioning unit 103, go out the first symbol of each frame again according to positioned at intervals change-over time that whether comprises uplink frame and downlink frame between the adjacent OFDM symbol original position;

Confirmation unit 105, connect the memory cell 102 and second positioning unit 104 respectively, orient the first symbol of each frame according to second positioning unit 104, determine the leading symbol and the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the first symbol and known preamble again.

Concrete computational methods no longer repeat as previously mentioned here.

Based on identical technical conceive, the present invention also provides a kind of method of determining leading symbol according to the OFDM symbol numbers that comprises in the downlink frame, if the OFDM symbol numbers difference that comprises in the uplink and downlink frame, and during the contained OFDM number of symbols of known downlink frame, judge that number that the OFDM symbol original position that satisfies periodic spacings continuously occurs determines the original position of downlink frame.Suppose that downlink frame comprises N OFDM symbol, when then occurring N adjacent OFDM symbol original position spacing continuously and being the OFDM symbol of an OFDM symbol lengths, the first symbol of this N OFDM symbol is the leading symbol of a downlink frame.

This method and preceding method in fact all are by searching the leading symbol that breakdown point is determined downlink frame, if promptly comprise N OFDM symbol between two breakdown points, and this N OFDM symbol that the OFDM symbol all is a downlink frame then.

As shown in figure 10, this method comprises the steps:

S100 ', according to the autocorrelation that the Cyclic Prefix characteristic of each OFDM symbol on time domain introduced, orient the original position of OFDM symbol;

Concrete grammar is with step S100.

Spacing between S200 ', the synchronous monitoring two adjacent OFDM symbol original position, searching adjacent OFDM symbol original position spacing all is a N continuous OFDM symbol of an OFDM symbol lengths, the first symbol of this N continuous OFDM symbol is the leading symbol of downlink frame, and wherein N is the OFDM symbol numbers that comprises in the downlink frame;

The lead code of adjusting in S300 ', the cross correlation affirmation downlink frame leading symbol according to data of modulating in the leading symbol and lead code is finished Cell searching and downlink frame is synchronous.

Detailed process is identical with step S300.

As shown in figure 11, realize in the above-mentioned ofdm system that the device 200 of downlink frame method for synchronous comprises under the TDD mode:

Sampling unit 201, the data in the sampling OFDM symbol;

Memory cell 202, storage comprise the lead code set of all known preamble;

Positioning unit 203 connects sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Monitoring means 204, connect positioning unit 203, orient the original position of each OFDM symbol according to positioning unit 203, spacing between the synchronous monitoring two adjacent OFDM symbol original position again, searching adjacent OFDM symbol original position spacing all is a N continuous OFDM symbol of an OFDM symbol lengths, the first symbol of this N continuous OFDM symbol is the leading symbol of downlink frame, and wherein N is the OFDM symbol numbers that comprises in the downlink frame;

Confirmation unit 205, connect memory cell 202 and monitoring means 204 respectively, the leading symbol of the downlink frame of orienting according to monitoring means 204 is determined the leading symbol and the lead code of downlink frame and is realized that downlink frame is synchronous according to the cross correlation of data of modulating in this leading symbol and known preamble again.

Concrete computational methods no longer repeat as previously mentioned here.

Said method of the present invention can be widely used in the OFDMA system or other adopts in the communication system of similar system, is not limited in the system of existing 802.16e.

Obviously, those skilled in the art can carry out various changes and modification to the present invention and not break away from the spirit and scope of the present invention.Like this, if of the present invention these are revised and modification belongs within the scope of claim of the present invention and equivalent technologies thereof, then the present invention also is intended to comprise these changes and modification interior.

Claims (11)

1, realizes the synchronous method of downlink frame in a kind of ofdm system under the TDD mode, it is characterized in that, comprise the steps:

According to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orient the original position of each OFDM symbol;

Go out the first symbol of each frame according to positioned at intervals change-over time that whether comprises uplink frame and downlink frame between the adjacent OFDM symbol original position, concrete grammar is, spacing between each a current OFDM symbol of synchronous monitoring and the adjacent last OFDM symbol original position, wherein: when the spacing between a current OFDM symbol and the adjacent last OFDM symbol original position was approximately equal to an OFDM symbol lengths, this current OFDM symbol was the non-first symbol of a uplink frame or downlink frame; Otherwise this current OFDM symbol is the first symbol of a uplink frame or downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Determine the leading symbol and the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the first symbol and known preamble.

2, the method for claim 1 is characterized in that, the method that the described autocorrelation of introducing according to the OFDM symbol cyclic prefix is located each OFDM symbol original position specifically comprises the steps:

Set slip sampling window sample frame data, the length of this sampling window and the equal in length of Cyclic Prefix;

Calculate the autocorrelation value of two sections correlated sampling data respectively, these correlated sampling data are to be that an OFDM symbol lengths deducts two segment datas of sampling and obtaining in two sampling windows of a circulating prefix-length in spacing respectively;

When the autocorrelation value of calculating when setting first threshold value, after in one group of correlation calculating, the correlation peak that search is wherein maximum, according in two sections correlated sampling data of this correlation peak correspondence the last period data sampling window locate the original position of current OFDM symbol.

3, the method for claim 1, it is characterized in that, elder generation is according to the synchronous monitoring result of described spacing, abandon wherein spacing with an adjacent last OFDM symbol original position less than all OFDM symbols of setting first spacing after, carry out the location of first symbol again, this first spacing deducts the length of a Cyclic Prefix less than an OFDM symbol lengths.

4, the method for claim 1 is characterized in that, described cross correlation according to data of modulating in the first symbol and known preamble determines that the method for downlink frame lead code specifically comprises:

Correlation during data of modulating in the first symbol of frequency-domain calculations and known preamble are gathered respectively between each lead code, and:

Judge whether to occur correlation peak greater than second threshold value, if, this first symbol leading symbol that is downlink frame then, the lead code of this correlation peak correspondence is the lead code of downlink frame; Otherwise, first OFDM symbol that this first symbol is a uplink frame.

5, method as claimed in claim 4, it is characterized in that, described method also comprises: if in the computing cross-correlation to the first symbol of setting number determined continuously and known preamble, described correlation peak all do not occur, then finish this downlink frame synchronizing process.

6, a kind of synchronous device of downlink frame of realizing under the TDD mode in ofdm system comprises:

Sampling unit, the data in the sampling OFDM symbol;

Memory cell, storage comprise the lead code set of all known preamble;

It is characterized in that described device also comprises:

First positioning unit connects described sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Second positioning unit, connect described first positioning unit, orient the original position of each OFDM symbol according to first positioning unit, go out the first symbol of each frame again according to positioned at intervals change-over time that whether comprises uplink frame and downlink frame between the adjacent OFDM symbol original position, if the spacing between a current OFDM symbol and the adjacent last OFDM symbol original position is approximately equal to an OFDM symbol lengths, this current OFDM symbol is the non-first symbol of a uplink frame or downlink frame, otherwise, this current OFDM symbol is the first symbol of a uplink frame or downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Confirmation unit, connect the described memory cell and second positioning unit respectively, orient the first symbol of each frame according to second positioning unit, determine the leading symbol and the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the first symbol and known preamble again.

7, realize the synchronous method of downlink frame in a kind of ofdm system under the TDD mode, the OFDM symbol numbers that comprises in the described downlink frame is a fixed value, and the OFDM symbol numbers that comprises in the uplink frame is not equal to this fixed value, and described method comprises the steps:

According to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orient the original position of each OFDM symbol; And

Spacing between the synchronous monitoring two adjacent OFDM symbol original position, search adjacent OFDM symbol original position spacing and all be approximately equal to the OFDM symbol numbers of the continuous appearance of an OFDM symbol lengths, when described number equals described fixed value, continuous first OFDM symbol of an OFDM symbol leading symbol that is downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Determine the lead code of downlink frame and realize that downlink frame is synchronous according to the cross correlation of data of modulating in the downlink frame leading symbol and known preamble.

8, method as claimed in claim 7 is characterized in that, the method that the described autocorrelation of introducing according to the Cyclic Prefix of each OFDM symbol is located OFDM symbol original position specifically comprises the steps:

Set slip sampling window sample frame data, the length of this sampling window and the equal in length of Cyclic Prefix;

Calculate the autocorrelation value of two sections correlated sampling data respectively, these correlated sampling data are for being the sampled data that obtains in the sampling window of the OFDM symbol lengths length that deducts a Cyclic Prefix two spacings respectively;

When the autocorrelation value of calculating when setting first threshold value, after in one group of correlation calculating, the correlation peak that search is wherein maximum, according in two sections correlated sampling data of this correlation peak correspondence the last period data sampling window locate the original position of current OFDM symbol.

9, method as claimed in claim 7 is characterized in that, described cross correlation according to data of modulating in the leading symbol and known preamble determines that the method for the lead code of downlink frame specifically comprises:

Correlation during data of modulating in the frequency-domain calculations leading symbol and known preamble are gathered respectively between each lead code, and:

Search is greater than the lead code of the correlation peak correspondence of second threshold value, and this lead code is the lead code of downlink frame.

10, method as claimed in claim 9, it is characterized in that, described method also comprises: if in the computing cross-correlation to the setting number leading symbol determined continuously and known preamble, described correlation peak all do not occur, then finish this downlink frame synchronizing process.

11, realize the synchronous device of downlink frame in a kind of ofdm system under the TDD mode, the OFDM symbol numbers that comprises in the described downlink frame is a fixed value, and the OFDM symbol numbers that comprises in the uplink frame is not equal to this fixed value, and described device comprises:

Sampling unit, the data in the sampling OFDM symbol;

Memory cell, storage comprise the lead code set of all known preamble;

It is characterized in that described device also comprises:

Positioning unit connects sampling unit, according to the autocorrelation that the Cyclic Prefix in each OFDM symbol is introduced, orients the original position of each OFDM symbol;

Monitoring means, connect positioning unit, orient the original position of each OFDM symbol according to positioning unit, spacing between the synchronous monitoring two adjacent OFDM symbol original position again, search adjacent OFDM symbol original position spacing and all be approximately equal to the OFDM symbol numbers of the continuous appearance of an OFDM symbol lengths, when described number equals described fixed value, continuous first OFDM symbol of an OFDM symbol leading symbol that is downlink frame, described be approximately equal to be meant the difference between described spacing and the OFDM symbol lengths absolute value less than setting threshold;

Confirmation unit, connect memory cell and monitoring means respectively, the leading symbol of the downlink frame of orienting according to monitoring means is determined the leading symbol and the lead code of downlink frame and is realized that downlink frame is synchronous according to the cross correlation of data of modulating in this leading symbol and known preamble again.

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