CN1855904A - Symbol timing method based on OFDM system - Google Patents

Abstract

A symbol timing method based on OFDM system that uses the correlative capability of the synchronal training symbol to estimate the symbol timing. It includes: A) parallel input the received synchronal training symbol to some correlators and the corresponding local reference list correlation; B) inputs correlation peak value that is tested by every correlator to the compatator, get the max value after the comparison, sends the correlator output result that is corresponded by the max correlation value to the delayer and the multiplier for the delay conjugate multiply disposal of the correlation result; C) inputs the correlative delay conjugate multiply result to the compatator for the searching test of the peak value. The symbol timing method is not only insensitive for the big frequency deviation of the carrier wave, but also independent of the channel estimate capability.

Description

A kind of signal-timing method based on ofdm system

Technical field

The present invention relates to orthogonal frequency division multiplexing OFDM (Orthogonal Frequency Division Multiplexing) technology, belong to areas of information technology.

Background technology

Orthogonal frequency division multiplex OFDM (Orthogonal Frequency Division Multiplexing) technology conception can be traced back to the end of the fifties of eighties of last century, is mainly used in the military communication.Because the complex structure of this system, early stage application is restricted.Up to the mid-80, along with the development of Digital Signal Processing and large scale integrated circuit, the orthogonal frequency division multiplex OFDM technology is just moved towards practicability.Nowadays, orthogonal frequency division multiplex OFDM is at Asymmetrical Digital Subscriber Line ADSL, Very-high-speed Digital Subscriber Line road VDSL, digital audio broadcasting DAB, obtained successful application among digital video broadcasting DVB and the WLAN (wireless local area network) WLAN, and be deep into gradually in the wideband transmit of wireless channel.

The principle of orthogonal frequency division multiplex OFDM technology is exactly that frequency domain channel is divided into many mutually orthogonal subchannels, on then that data flow is the parallel carrier wave that is modulated to each subchannel.Because the subcarrier of each subchannel is mutually orthogonal, frequency spectrum overlaps mutually, so improved the availability of frequency spectrum of system greatly.Simultaneously, the subcarrier of each channel can be designed to the coherence bandwidth less than channel, so just original wideband frequency selective channel has been divided into the flat fading channel of a plurality of arrowbands, has reduced greatly because the intersymbol interference that the frequency selective fading of channel is brought.So the orthogonal frequency division multiplex OFDM technology has the ability of inborn anti-wireless channel multipath fading, be specially adapted to the high speed data transfer in the frequency selective fading channels.Simultaneously disturb (ICI) in order to overcome to disturb between the intersymbol interference (ISI) that causes and subchannel by channel multi-path, Cyclic Prefix has been introduced in front at the OFDM symbol, and it is the foremost that is put into the OFDM symbol after a part of data of OFDM symbol afterbody are duplicated.The length of Cyclic Prefix is greater than the maximum delay spread length of channel, and the multipath copy of previous symbol has all been dropped in the Cyclic Prefix scope of a back symbol.Like this, receiving terminal when removing Cyclic Prefix, has also just removed intersymbol interference after receiving data.And because the adding of Cyclic Prefix makes original linear convolution channel become the circular convolution channel, thereby after receiving terminal process FFT, channel only is equivalent to the signal times upper signal channel fading factor on each subcarrier sending the influence of signal.So channel equalization can directly adopt simple single tap equalization technology.But the efficiency of transmission of ofdm system has been sacrificed in the adding of Cyclic Prefix, is exactly the redundant information of OFDM symbol because itself carries.So it is the length of Cyclic Prefix can not be long arbitrarily, general as long as expand greater than the maximum delay of channel.

OFDM symbol purpose regularly is exactly the accurate moment of estimating OFDM sign-on and end.Symbol result regularly will determine the original position of FFT conversion window.Because when symbol through after the different propagation delay times of channel, what receive at receiving terminal is exactly the cumulative signal with different delay symbol.In theory, regularly will timely lock exactly in the initial moment of symbol that arrives receiver at first for the optimal symbol of ofdm system.Like this, receiving terminal is after the protection of removing the OFDM symbol at interval, and the useful OFDM symbolic component of extraction is not exist to disturb (ICI) between intersymbol interference (ISI) and subchannel, and promptly the OFDM symbol energy of regularly collecting thus reaches maximum.But actual situation is difficult to accomplish fully accurate sign synchronization, simultaneously, owing to inserted the Cyclic Prefix protection at interval between the OFDM symbol, makes ofdm system comparatively loose to symbol requirement regularly.This has just alleviated symbol pressure regularly to a certain extent.Because as long as be engraved in during OFDM timing synchronization initial in the certain limit in the Cyclic Prefix (greater than the maximum delay extended area of channel), because the periodicity that Cyclic Prefix introduces for the OFDM symbol, after the useful data process FFT conversion of the OFDM symbol that extracts, except meeting produces linear phase place deflection, be can not cause to disturb (ICI) between intersymbol interference (ISI) and subchannel, and this linear phase is easy to eliminate in channel equalization.And having only the border that has exceeded the OFDM symbol when symbol FFT conversion window regularly, ISI and ICI just can be caused in the amplitude that has perhaps fallen into the OFDM symbol interval of roll-offing.Simultaneously, can select arbitrarily in the certain limit of Cyclic Prefix though the OFDM symbol is regularly put, the variation regularly of any symbol all can increase the sensitivity of ofdm system to the time delay expansion.Especially when the recycling-guard of ofdm system length at interval near the maximum multipath delay spread of channel the time, symbol regularly allows the scope that changes just very little, and at this moment, ofdm system will be very high to symbol requirement regularly.So, need reduce the error of timing synchronization as far as possible in order to reduce this negative effect.Particularly in the channel of Rayleigh fading,, need to determine best symbol regularly in order to obtain best systematic function.

The symbol of ofdm system method regularly can roughly be divided into data auxiliary type and non-data auxiliary type.Based on the symbol timing algorithm of data auxiliary type generally need be in transmission signals known training symbol or the pilot signal of the insertion in cycle.In the prior art: (1) Schimdl and Cox propose to adopt at transmitting terminal the training symbol of repetitive structure, receiver by the auto-correlation peak value to the training symbol that receives detect estimate symbol regularly (seeing also: T.M.Schmidle and D.C.Cox; " Robust Frequency and Timing Synchronization for OFDM " IEEE Trans.Commun.Vol.45; No.12; DEC; 1997, pp:1613-1621).Though this signal-timing method to symbol carrier frequency deviation very robust, is realized also very simple.But because there is the repeatability of Cyclic Prefix in the OFDM symbol, platform effect can appear in symbol autocorrelator trace regularly, causes symbol very mistake regularly; (2) for symbol timing accurately, Tufvesson proposes to utilize the strong autocorrelation of PN sequence to come sharp-pointed symbol timing detection peak value (to see also: F.Tufvesson, O.Edfors, and M.Faulkner, " Time and frequency synchronization for ofdm using pn-sequence preamble ", Proc.VTC ' 99, vol.4, pp, 2203-2207,1999); But the frequency shift (FS) meeting of receiving symbol destroys the correlation of the PN sequence that receives, and causes the regularly obvious decline of detection peak.In addition, it is mentioned utilize training symbol cross-correlation peak value estimate symbol technology regularly to be based on all that peak-peak detects or peak value adds up and detection.If the expansion of the time delay of rayleigh fading channel is very big, then this symbol regularly just probably very big deviation can occur, because based on peak-peak detect or peak value adds up and detection often the symbol of OFDM is timely locked on the strongest path of channel received power, and actual desirable symbol regularly is timely lock on first footpath of channel.So symbol this moment deviation regularly just distributes relevant with the received power of actual channel; (3) usually in order to improve the regularly precision under multidiameter fading channel of OFDN symbol, symbol regularly need come the thick symbol of further fine estimation error regularly (to see also: B.Yang by channel estimating, K.B.Letaief, R.S.Cheng and Z.Cao; " Timing Recovery for OFDM Transmission "; IEEE Joural on selected areas in communication; Vol.18; No.11; Novemeber, 2000), but this realization has just increased symbol complexity regularly; and under the very big situation of this external thick symbol timing error, performance for estimating channel also has damage; (4) adopt the Cyclic Prefix of OFDM symbol to finish symbol regularly usually based on non-data householder method, Van de Beek proposed a kind of maximum likelihood associating estimate symbol based on Cyclic Prefix regularly and the method for frequency shift (FS) (see also: J.J van de Beek, M.Sandell and P.O.B  rjesson; " ML estimation of time and frequency offset in OFDM systems; " IEEE Trans. Signal Processing, vol.45, pp1800-1805, July 1997), though this method is that this method is not suitable for using in the multipath dispersive channel near optimum under Gaussian channel.

Summary of the invention

The objective of the invention is to provide in based on ofdm system a kind of more accurate signal-timing method, this signal-timing method is not only insensitive to big carrier wave frequency deviation, nor depends on performance for estimating channel.

A kind of signal-timing method based on ofdm system adopts the correlated performance of synchronous training symbol to come estimate symbol regularly.

Employing has the synchronous training symbol of sequence conduct of strong autocorrelation and cross correlation to obtain sharp-pointed correlation peak; For example adopt the PN sequence of PN sequence and process FFT conversion.

At receiving terminal, receiver is that estimate symbol is regularly after carrying out cross-correlation by the synchronous training symbol that will receive and the local reference sequences that produces.

Comprise:

Being input in a plurality of correlators that the synchronous training symbol that A. will receive is parallel is relevant with corresponding local reference sequences;

B. the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum, the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively makes the delay conjugate multiplication of correlated results and handle;

C. the result of the delay conjugate multiplication of cross-correlation is input to and carries out peak value searching in the comparator and detect.The C step specifically comprises:

A. find peak-peak earlier, respectively with the amplitude V of peak-peak _MaxWith corresponding address D _MaxRecord counter C1, among the C2;

B. continue peak-peak search forward successively, judge whether to exist amplitude greater than KV in locking _Max, peak value; If exist, refresh counter C2 immediately then is with corresponding address D ₁Be written among the counter C2; If there is no, the amplitude of then judging this test point again whether greater than

If meet the demands, then with the address D of this peak point ₂Be written among the counter C2, and finish searching and detecting immediately, the D that obtains ₂The position that timely locks exactly; Wherein K satisfies 0＜K＜1, often gets K=0.5,0.6 in the general reality;

C. repeating step b reaches L up to the length of searching for, and then stops search, and the value of resulting counter C2 is with regard to is-symbol estimated value regularly; Wherein L can change according to the difference of channel, can determine optimum value by emulation testing in the reality, and general L the most very much not can surpass the length of the Cyclic Prefix of OFDM symbol.

For example, orthogonal frequency division multiplex OFDM system adopts the 10M system bandwidth, and the number of subcarrier is 256, and the bandwidth of subchannel is 44.53KHz; If actual carrier frequency shift is 140KHz to the maximum, then the maximum frequency range that need estimate of frequency acquisition is (4,4); Training symbol adopts the version of the regulation of IEEE 802.16d synchronously; Simulated channel can adopt SUI-1 and SUI-4 model; (SUI-1 and SUI-4 model are the test channel of recommending use among the IEEE802.16d, and wherein SUI-1 is the Rice channel model, and SUI-4 is the rayleigh fading channel model.)

Specifically comprise:

1) being input in a plurality of correlators that the synchronous training symbol that will receive is parallel is relevant with corresponding local reference sequences; Wherein local reference sequences is:

$p_i^{\′}\left(m\right)=p_s\left(m\right)\·e^{-j2\mathrm{\π}{f}_{i}m/256},m=\mathrm{0,2}..63,$

F wherein _iSearch rate when being exactly frequency acquisition, f _i∈ (4 ,-2,0,2,4), i=1,2 ... 5,,

P (n)=[p _s(m), p _s(m), p _s(m), p _s(m), p _s(m)], i.e. p _s(m) be the inner Short Training symbolic blocks of synchronous training symbol p (n), generally the PN sequence that is changed by PN sequence or process FFT constitutes.

2) the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum; And the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively make the delay conjugate multiplication of correlated results and handle, the length of delayer is 64;

3) result of the delay conjugate multiplication of cross-correlation is input to carries out peak value searching in the comparator and detect.

Description of drawings

Fig. 1 is the general time domain structural representation of synchronous training symbol;

Fig. 2 is the symbol realization block diagram regularly of an embodiment of the present invention;

Fig. 3 is a kind of internal structure schematic diagram of embodiment parallel duplex correlator bank.

Embodiment

See also accompanying drawing 1-3, signal-timing method of the present invention, especially based in the ofdm system under rayleigh fading channel, rely on the correlated performance of synchronous training symbol to come estimate symbol regularly.In order to obtain sharp-pointed correlation peak, employing has the PN sequence of strong autocorrelation or the synchronous training symbol of PN sequence conduct of process FFT conversion usually.At receiving terminal, receiver is that estimate symbol is regularly after carrying out cross-correlation by the synchronous training symbol that will receive and the local reference sequences that produces.This signal-timing method is particularly suitable for being applied in the transmission system under the bursty data pattern, because this symbol regularly just can reach more accurate timing accuracy in time domain.Symbol regularly need be by channel estimating like this, and accurate symbol regularly can further improve performance for estimating channel.

On the one hand, the deflection phase interference that the correlated performance of sequence can be caused by carrier frequency shift influences: when carrier frequency shift is bigger, as the frequency shift (FS) of carrier wave is that the sub-carrier frequencies of several integral multiples is at interval the time, during the local training symbol cross-correlation that produces of synchronous training symbol that directly adopt to receive and receiving terminal, because there is bigger phase deviation in two sequences, so the relevant of sequence is incoherent, peak value after will making sequence relevant like this becomes not obvious, especially under fading channel, the synchronizing sequence cross-correlation peak value under incoherent is to be difficult to detect.So in order to overcome carrier wave frequency deviation to symbol influence regularly, receiver needs in advance carrier wave frequency deviation to be caught, and reduces to receive the phase deviation of synchronous training symbol and local reference sequences as far as possible.On the other hand, the cross-correlation summit of sequence diffuses into a plurality of spikes because of the disperse of multipath channel, and distribute relevant with the amplitude size with the received power of channel the relative time delay of spike.Be exactly the relative time delay of channel multi-path the relative time delay of spike, and the amplitude size of spike also is directly proportional with channel multi-path received power at the moment.When symbol regularly adopts when detecting based on peak-peak, symbol timing is exactly to be locked on the most powerful path of receive channel.If there is visual route in channel, then the most powerful path of channel is exactly first footpath of receive channel, the symbol of this moment is regularly best, if but there is not visual route in channel, be channel each the footpath received power all be Rayleigh fading, then the most powerful path of channel received power may not be first the footpath and on other path.As seen, regularly will exist error based on the symbol that peak-peak detects this moment, and the size of error is exactly the size in relative time delay of most powerful path, if this time delay size exceeds the timing protection interval of Cyclic Prefix, then will produce ISI and ICI.So each relative time delay for rayleigh fading channel and channel multi-path is all very big, then the symbol that detects based on peak-peak regularly performance will worsen, need to adopt the method for thresholding search to detect the corresponding time of channel first footpath for this reason and go up correlation peak.Simultaneously, in order to reduce the influence of noise jamming, adopted the treatment technology that postpones conjugate multiplication further to improve the accurate performance of detection again to cross-correlation peak value.

In the actual ofdm system, symbol regularly generally is that the generation frame-grab is after synchronous training symbol is caught.The main realization device of signal-timing method of the present invention is a correlator, comparator, and delayer and multiplier, introduce symbol implementation procedure regularly below in conjunction with concrete realization device:

1) being input in a plurality of correlators that the synchronous training symbol that will receive is parallel is relevant with corresponding local reference sequences.Local reference sequences is the synchronous training symbol that passes through phase place deflection in advance.Wherein, synchronously training symbol is made of PN sequence or the PN sequence of advancing the FFT conversion.The phase place of deflection has predefined search rate and produces, and these search rates mainly are in order to estimate initial carrier wave frequency deviation.As long as the scope of search rate can cover the maximum magnitude of practical carrier frequency skew, then can in these search rates, find that Frequency point near the actual carrier frequency deviation.Consequent so local reference sequences is the strongest with the correlation that receives synchronous training symbol.Simultaneously, synchronously training symbol self possesses very strong autocorrelation performance again, so no matter the frequency shift (FS) of carrier wave is much, can find No. one correlator in the multichannel correlator of receiving terminal at least is to detect correlation peak.This moment, the peak value detection of correlator inside was based on the peak-peak detection.

2) the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum.And the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively make the delay conjugate multiplication of correlated results and handle, be i.e. s (n)=c (n) c ^*(n-M), the time M that wherein postpones is exactly the cycle time of synchronous training symbol.Because training symbol is periodic synchronously, and local reference sequences is based on the Short Training piece that constitutes the periodic synchronous training symbol.Present periodically so the cross correlation results of training symbol and reference sequences also can be similar to, but because noise is at random, so the result of cross-correlation is only periodically more obvious on each peak point, and the length in cycle is exactly M.And almost be at random on other aspect.So after this self-dalay conjugate multiplication processing, can further reduce random noise to the interference that peak value detects, make that the peak value of cross-correlation is distincter when detecting, be convenient to searching and detecting.

3) result of the delay conjugate multiplication of cross-correlation is input to carries out peak value searching in the comparator and detect.Concrete search step is as follows:

A) find peak-peak earlier, respectively with the amplitude V of peak-peak _MaxWith corresponding address D _MaxRecord counter C1, among the C2.

B) continue to search for forward successively, judge whether to exist amplitude greater than KV in the peak-peak of locking _MaxPeak value, if exist, refresh counter C2 immediately then is with corresponding address D ₁Be written among the counter C2.If there is no, the amplitude of then judging this test point again whether greater than If meet the demands, then with the address D of this peak point ₂Be written among the counter C2, and finish searching and detecting immediately.The D that obtains ₂The position that timely locks exactly.

C) repeating step 6), reach L up to the length of search, then stop search.The value of counter C2 is with regard to is-symbol estimated value regularly.

Below just a concrete application process of the present invention is described in conjunction with concrete example.

Suppose that orthogonal frequency division multiplex OFDM system adopts the 10M system bandwidth, the number of subcarrier is 256, and then the bandwidth of subchannel is 44.53KHz.If actual carrier frequency shift is 140KHz to the maximum, then the maximum frequency range that need estimate of frequency acquisition is (4,4).Training symbol adopts the version of the regulation of IEEE 802.16d synchronously.Simulated channel adopts SUI-1 and SUI-4 model.

Concrete symbol is step regularly:

1) being input in a plurality of correlators that the synchronous training symbol that will receive is parallel is relevant with corresponding local reference sequences.Wherein local reference sequences is:

$p_i^{\′}\left(m\right)=p_s\left(m\right)\·e^{-j2\mathrm{\π}{f}_{i}m/256},m=\mathrm{0,2}..63,$

F wherein _iSearch rate when being exactly frequency acquisition, f _i∈ (4 ,-2,0,2,4), i=1,2 ... .5.

P (n)=[p _s(m), p _s(m), p _s(m), p _s(m), p _s(m)], i.e. p _s(m) be the inner Short Training symbolic blocks of synchronous training symbol p (n).

2) the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum.And the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively make the delay conjugate multiplication of correlated results and handle, the length of delayer is 64.

The result of the delay conjugate multiplication of cross-correlation is input to carries out peak value searching in the comparator and detect.(the concrete as preceding explanation of doing, this does not give unnecessary details.)

Claims (8)

1, a kind of signal-timing method based on ofdm system is characterized in that adopting the correlated performance of synchronous training symbol to come estimate symbol regularly.

2, the signal-timing method based on ofdm system according to claim 1 is characterized in that adopting the synchronous training symbol of sequence conduct with strong autocorrelation and cross correlation to obtain sharp-pointed correlation peak.

3, the signal-timing method based on ofdm system according to claim 2 is characterized in that: at receiving terminal, receiver is that estimate symbol is regularly after carrying out cross-correlation by the synchronous training symbol that will receive and the local reference sequences that produces.

4, the signal-timing method based on ofdm system according to claim 3 is characterized in that: wherein local reference sequences is:

$p_i^{\′}\left(m\right)=p_s\left(m\right)\·e^{-j2\mathrm{\π}{f}_{i}m/256},$ m＝0，2..63，

F wherein _iSearch rate when being exactly frequency acquisition, f _i∈ (4 ,-2,0,2,4), i=1,2 ... 5.

P (n)=[p _s(m), p _s(m), p _s(m), p _s(m), p _s(m)], i.e. p _s(m) be the inner Short Training symbolic blocks of synchronous training symbol p (n).

5, the signal-timing method based on ofdm system according to claim 1 is characterized in that adopting following steps:

Being input in a plurality of correlators that the synchronous training symbol that A. will receive is parallel is relevant with corresponding local reference sequences;

B. the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum, the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively makes the delay conjugate multiplication of correlated results and handle;

C. the result of the delay conjugate multiplication of cross-correlation is input to and carries out peak value searching in the comparator and detect.

6, the signal-timing method based on ofdm system according to claim 5, it is characterized in that: the C step specifically comprises:

A. find peak-peak earlier, respectively with the amplitude V of peak-peak _MaxWith corresponding address D _MaxRecord counter C1, among the C2;

B. continue peak-peak search forward successively, judge whether to exist amplitude greater than KV in locking _MaxPeak value; If exist, refresh counter C2 immediately then is with corresponding address D ₁Be written among the counter C2; If there is no, the amplitude of then judging this test point again whether greater than If meet the demands, then with the address D of this peak point ₂Be written among the counter C2, and finish searching and detecting immediately, the D that obtains ₂The position that timely locks exactly;

C. repeating step b reaches L up to the length of searching for, and then stops search, and the value of resulting counter C2 is with regard to is-symbol estimated value regularly.

7, the signal-timing method based on ofdm system according to claim 6, it is characterized in that: wherein K satisfies condition: 0＜K＜1; L can change according to the difference of channel, can determine optimum value by emulation testing in the reality.

8, the signal-timing method based on ofdm system according to claim 1 is characterized in that: orthogonal frequency division multiplex OFDM system adopts the 10M system bandwidth, and the number of subcarrier is 256, and the bandwidth of subchannel is 44.53KHz; If actual carrier frequency shift is 140KHz to the maximum, then the maximum frequency range that need estimate of frequency acquisition is (4,4); Training symbol adopts the version of the regulation of IEEE 802.16d synchronously; Specifically comprise:

1) being input in a plurality of correlators that the synchronous training symbol that will receive is parallel is relevant with corresponding local reference sequences; Wherein local reference sequences is:

$p_i^{\′}\left(m\right)=p_s\left(m\right)\·e^{-j2\mathrm{\π}{f}_{i}m/256},$ m＝0，2..63，

F wherein _iSearch rate when being exactly frequency acquisition, f _i∈ (4 ,-2,0,2,4), i=1,2 ... .5.

P (n)=[p _s(m), p _s(m), p _s(m), p _s(m), p _s(m)], i.e. p _s(m) be the inner Short Training symbolic blocks of synchronous training symbol p (n);

2) the detected correlation peak of the correlator on each road is input to compare in the comparator after, obtain maximum; And the output result of the correlator of maximal correlation peak value correspondence is delivered to delayer and multiplier respectively make the delay conjugate multiplication of correlated results and handle, the length of delayer is 64;

4) result of the delay conjugate multiplication of cross-correlation is input to carries out peak value searching in the comparator and detect.

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