CN101047686A - Receiving symbolic synchronous method for WiMAX system - Google Patents

Abstract

This invention relates to a synchronization method for WiMAX system, which fully uses the known and auto-correlation characteristics of a leading sequence to structure related timing judgment functions against different lengths of circular prefix length to adjust the judgment threshold dynamically based on the value of the timing judgment function to compute the timing point so as to determine the initial position of the WiMAX frame.

Description

A kind of receiving symbolic synchronous method that is used for the WiMAX system

Technical field

The present invention relates to wireless communication technology field, relate in particular to the synchronous of WiMAX system.

Background technology

At the problem of 3G (Third Generation) Moblie (3G) to the 4th third-generation mobile communication (4G) transition, industry has proposed some different schemes.With IEEE802.16 (wireless MAN) is that WiMAX (micro-wave access global inter communication) system of standard wherein obtains of extensive concern.This system is its physical layer implementation pattern with OFDM (OFDM) technology, and the bandwidth up to 20MHz can be provided, and fully satisfies the demand of wideband transmit.More than 100 WiMAX experimental network built together in the whole world at present.The telecom operators of China have also carried out positive tracking and preparation to WiMAX.

Occupy important status synchronously in communication system, the WiMAX system is no exception.Because the OFDM symbol is made of a plurality of sub-carrier signal stacks, utilizes orthogonality to distinguish between each subcarrier, therefore guarantees that this orthogonality is most important for the WiMAX system, and the accurate prerequisite of guaranteeing orthogonality synchronously just.The WiMAX system synchronization relates generally to the problem of two aspects: frequency shift (FS) and symbol are regularly.Frequency shift (FS) causes received signal to be offset in frequency domain, if frequency shift (FS) is n (n is an integer) times of subcarrier spacing, though still can keep quadrature between the subcarrier, but the frequency sampling value has been offset the position of n subcarrier, causes the error rate that is mapped in the data symbol in the OFDM frequency spectrum up to 0.5.If carrier shift is not the integral multiple of subcarrier spacing, then between subcarrier, will there be " leakage " of energy, cause the orthogonality between the subcarrier to be destroyed, disturb thereby between subcarrier, introduce, make the bit error rate performance of system worsen.In multi-path environment, the WiMAX system needs to determine best symbol regularly in order to obtain best systematic function.Symbol deviation regularly can cause the phase deviation between sampled value.The deviation of these phase places can be added in the phase noise that has existed, and reduces systematic function.In these external a lot of method for synchronous, timing synchronization becomes the prerequisite of Frequency offset estimation.

Present method for synchronous mainly contains based on training sequence, based on Cyclic Prefix with based on three kinds of pilot tones.Document [Timothy M.Schmidl and Donald C.Cox.Robust Frequency and TimingSynchronization for OFDM.IEEE Transactions on communications, Vol.45, No.12, December 1997.] a kind of OFDM method for synchronous of classics proposed, this method can provide stronger symbol timing and Frequency offset estimation, wherein the performance of frequency offset estimating can satisfy system requirements, but the peak value of its timing curve presents a platform, makes precision regularly reduce.

Document [Jie-Ping Xu; Bo-Ran Guan.An efficient timing synchronizationscheme for OFDM system in IEEE 802.16d.Volume 1,4-7 Dec.2005 Page (s): 3pp.Digital Object Identifier 10.1109/APMC.2005.] utilize the intellectual of training sequence that sliding correlation method is improved, this method can realize accurate symbol regularly under signal to noise ratio is positive situation, but the variance of this method timing error under negative state of signal-to-noise is bigger and this method only is applicable to the Cyclic Prefix of single length.

Summary of the invention

The present invention is directed to the above-mentioned deficiency in the WiMAX system synchronization in the prior art, propose a kind of receiving synchronous method of new WiMAX system.This method is applicable to the Cyclic Prefix of all lengths, can realize timing synchronization, the timing accuracy height, and have higher operation efficiency.The present invention by constructing new decision function, provides a kind of targeting sequencing that utilizes, and then finishes the synchronous reception of system under the situation of reindexing structure not.

The technical scheme that the present invention solves its technical problem employing is to determine circulating prefix-length L according to the multidiameter of channel to the subscriber station parameter initialization time _CPSelect corresponding timing decision function m (d) according to circulating prefix-length, call regularly decision function algoritic module, and, calculate the regularly value of decision function by the burst that receives; Carrying out timing calculates synchronously, value according to decision function m (d) is dynamically adjusted decision threshold, the timing decision function is done the slip related operation, the relatively value of decision function and regularly decision threshold, sequence number is defined as regularly point when the value of decision function is pairing above the timing decision threshold, call the original position that respective formula is determined the WiMAX frame according to the timing point that obtains, the reception of finishing the WiMAX system is synchronous.

Circulating prefix-length L _CPSatisfy following relation with timing decision function m (d):

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, $m\left(d\right)=\frac{\left|\underset{n=0}{\overset{P-1}{\mathrm{\Σ}}}(\left(r^{*}(d+n)p_n\right)\·\left(r^{*}(d+n+4P)p_n\right))\right|}{\underset{n=0}{\overset{P-1}{\mathrm{\Σ}}}{\left|p_n\right|}^4}$

When $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, $m\left(d\right)=\frac{\left|\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}(\left(r^{*}(d+n)l_n\right)\&CenterDot;\left(r^{*}(d+n+3P)l_n\right))\right|}{\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|l_n\right|}^4};$

The original position F of WiMAX frame puts d with timing _TSatisfy following relation:

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, F=d _T, when $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, F=d _T-L _CP

Receiving synchronous method compares in the present invention and the prior art, and the variance of timing error is less under the low signal-to-noise ratio situation, and timing stability is better, and the synchronization accuracy height has less operand.Because the Cyclic Prefix of different length can be selected by system, constructed different timing decision functions to satisfy the timing demand under the various Cyclic Prefix, even under the situation of big multipath fading, still can satisfy the requirement of WiMAX system.

Description of drawings

Figure 1 shows that the flow chart that the present invention is regularly synchronous

Figure 2 shows that the targeting sequencing structure chart

Figure 3 shows that the regularly computing schematic diagram of decision function.

Wherein (a) is the computing schematic diagram at formula (1), (b) is the computing schematic diagram at formula (2).

Embodiment

At drawings and the specific embodiments enforcement of the present invention is described further below.Figure 1 shows that the flow chart that the present invention is regularly synchronous, at first, determine circulating prefix-length the subscriber station parameter initialization; According to circulating prefix-length L _CPSelect regularly decision function, and calculate the regularly value of decision function m (d) according to the timing decision function of selecting by the burst that receives; Carry out timing computing synchronously, dynamically adjust decision threshold, calculate regularly point,, then determine the original position of WiMAX frame according to the timing point if can determine regularly point according to the value of timing decision function m (d).Done regularly after the judgement according to decision threshold,, whether found regularly point, all will return regularly synchronously operation stages and carry out the timing synchronization decisions of next round to detect the due in of next frame regardless of the result of judgement regularly.

WiMAX is that unit sends data with the frame in physical layer, and each frame is since a targeting sequencing.First OFDM symbol (comprising Cyclic Prefix) of hereinafter arranging in the targeting sequencing (Preamble) of WiMAX is a leading symbol.As shown in Figure 2, targeting sequencing adds respectively that by two OFDM symbols Cyclic Prefix (CP) forms, and these two OFDM symbols calculate by the pseudorandom complex sequences on the known frequency domain.Therefore under the prerequisite of determining circulating prefix-length, targeting sequencing is known to receiving-transmitting sides.First OFDM symbol of targeting sequencing only has value on sequence number is the subcarrier of 4 multiple, the waveform after through IFFT (invert fast fourier transformation) computing on its time domain is made up of the data segment of 4 repetitions, and each repeated segments has 64 sampled points.Second OFDM symbol only has value on the subcarrier of even number sequence number, this causes on time domain this symbol to be made up of two repeated segments, and each repeated segments has 128 sampled points.

1. by to the subscriber station parameter initialization, determine circulating prefix-length

When the each access network of subscriber station, the base station can send relevant information to it, and the parameter that comprises circulating prefix-length is carried out initialization.Circulating prefix-length is optionally in the WiMAX system, and the sub-carrier number that supposing the system adopts is N, then circulating prefix-length L _CPFollowing four kinds of selections can be arranged:

Adopt any circulating prefix-length generally can determine according to the multidiameter of channel, and definite when subscriber station (SS) inserts the WiMAX system initialization.Multidiameter can roughly be estimated according to the received signal error rate, as long as guarantee the maximum (sampled point) of circulating prefix-length greater than multidiameter, just can avoid intersymbol interference (ISI).Circulating prefix-length all is known to receiving-transmitting sides in addition, and remains unchanged in whole communication process.If circulating prefix-length changes, then subscriber station need carry out initialization again.

2. according to circulating prefix-length L _CPThe corresponding regularly decision function of structure

Usually wireless communication system is synchronous, all adopts training sequence and local reference signal to make the slip related operation and realizes.The WiMAX system provides pseudorandom targeting sequencing to be used for synchronously, and this targeting sequencing defines in the IEEE802.16 standard, is known for this targeting sequencing of receiving-transmitting sides therefore.According to the thinking of conventional method, adopt targeting sequencing as local reference signal, when targeting sequencing that receives and local reference signal were made the slip related operation, owing to the autocorrelation performance of targeting sequencing, peak value can appear in related operation.The moment that the contrast peak value occurs can obtain the targeting sequencing due in, because the WiMAX frame is begun by targeting sequencing, and just can obtain the initial moment of WiMAX frame this moment accordingly, finishes synchronously again.

But in actual applications, Chang Gui method is for WiMAX system and inapplicable.This is because the targeting sequencing of WiMAX has structural repeatability, as shown in Figure 2.If make the slip related operation with whole targeting sequencing as local reference signal this moment, the bigger several times peak value of amplitude also can appear in the peak value both sides of slip related operation when the WiMAX frame arrives, and these minor peaks can influence synchronized result.

In order to eliminate the influence of these minor peaks, the present invention only adopts the part of first OFDM symbol of targeting sequencing (being leading symbol) as local reference signal, designs suitable computing interval span simultaneously and carries out synchronous computing.

Be subjected to channel interference signals sequence r (k) if receiving terminal receives, this burst includes leading symbol; Any one repeated segments of leading symbol is sequence p _n, length is P sampled point; Be located at p _nIn remove that remainder is l after the part identical with Cyclic Prefix _n, its length is L sampled point.P wherein _nWith l _nAll be in the receiving terminal known sequences.

Become the reference sequences p of the computing that is used to slide at the subscriber station adnation _nOr l _n,

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, adopt formula (1) to determine decision function m (d):

$m\left(d\right)=\frac{\left|\underset{n=0}{\overset{P-1}{\mathrm{\&Sigma;}}}(\left(r^{*}(d+n)p_n\right)\&CenterDot;\left(r^{*}(d+n+4P)p_n\right))\right|}{\underset{n=0}{\overset{P-1}{\mathrm{\&Sigma;}}}{\left|p_n\right|}^4}---\left(1\right)$

When $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, adopt formula (2) to determine decision function m (d):

$m\left(d\right)=\frac{\left|\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}(\left(r^{*}(d+n)l_n\right)\&CenterDot;\left(r^{*}(d+n+3P)l_n\right))\right|}{\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|l_n\right|}^4}---\left(2\right)$

R in formula (1), (2) ^*(), expression is made conjugate operation to sequence r (), and d and n represent the sampling point timings of sequence, and d gets positive integer.With formula (1) be example interpretation algorithms: r (d+n) expression received signal from d to the sequence the d+P-1 sampled point, this sequence is asked after the conjugation and p _nCorrespondence multiplies each other and obtains r ^*(d+n) p _n, in like manner will do above-mentioned computing to the sequence the d+5P-1 point from d+4P, can be expressed as r ^*(d+n+4P) p _nWith r ^*(d+n) p _nAnd r ^*(d+n+4P) p _nSubstitution formula (1) computing can obtain the subsynchronous operation result at sequential d place.For example when d=1, r ^*(d+n) p _nExpression is done the 1st to P sampled point of received signal behind the conjugate operation and p _nThe sequence correspondence multiplies each other; r ^*(d+n+4P) p _nExpression is done 1+4P to a 5P sampled point of received signal behind the conjugate operation and p _nThe sequence correspondence multiplies each other.Increase the value of d successively, calculate the synchronous operation result under the different sequential d, finish the slip computing.

For different circulating prefix-lengths, different decision functions is adopted in this synchronous computing.The point that synchronous computing is obtained peak value is defined as regularly puts d _TWhen $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, adopt two p _nSequence is as local reference signal, and its principle is to guarantee two p at timing point place _nSequence is made related operation with corresponding sequence respectively and is all obtained minor peaks, and this moment, the result of computing made the height that product can strengthen peak value greatly respectively.On the other hand, at other some places, has only a p at most _nThe related operation of sequence is obtained minor peaks, and another p _nSequence is made related operation can only obtain pseudo random sequence, and the result of computing made the product minor peaks and will be eliminated respectively this moment.The result of whole computing will only stay next peak value, and this peak value is a sharp-pointed and very high spike of amplitude, help synchronous detecting.Desire to achieve the above object, must suitably set two p _nComputing between sequence is span at interval, the structure of contrast leading symbol, and should select 4 times of p this moment _nLength, shown in Fig. 3 (a).

When $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, because the length of Cyclic Prefix is less than p _nLength, if this moment carry out computing with the method for first kind of situation, can not obtain desirable synchronized result.To second kind of situation, adopt two l _nSequence is as local reference signal, and should adopt 3 times ps in order to follow the design principle identical with first kind of situation this moment _nLength as the computing span, shown in Fig. 3 (b).

Regularly point has following relation with the original position of WiMAX frame:

Regularly put d _TWith the starting point F of WiMAX frame relation is arranged, when $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, determined timing point is the original position F of WiMAX frame, when $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, be the original position F of WiMAX frame after regularly the position of point deducts circulating prefix-length, be formulated as follows:

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, F=d _T(3)

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, F=d _T-L _CP(4)

Can obtain the flow process of the synchronous computing of receiving terminal according to above synchronized algorithm analysis: at first determine its decision function according to the different choice corresponding formulas of circulating prefix-length, call regularly decision function algoritic module, then with the burst r (k) and the sequence p that receive _nOr l _nThe selected decision function of substitution carries out computing, obtains the regularly value of decision function m (d).

3. dynamically adjust decision threshold according to the value of decision function m (d).

Because channel constantly changes with environment, so the decline of signal also constantly changes thereupon.The way of setting the fixed sentence thresholding in this case is no longer reliable.This method is dynamically adjusted decision threshold according to the value of decision function m (d) and is carried out the timing synchronization decisions.Call regularly decision function algoritic module, calculate the regularly value of decision function, determine regularly decision threshold, in sampled point d, appoint and get 1 d _c, calculate d _cThe decision threshold at place is at first got d _cSome sampled points that front and back quantity equates (the computing span is not less than leading symbol length before and after general), and calculate the mean value of each self-corresponding decision function m (d), this mean value be multiply by a coefficient k (empirical value that coefficient k draws for test, desirable k=3.5), be regularly decision threshold D.

4. the timing decision function is done the slip related operation, calculate the regularly currency of decision function, its value and timing decision threshold D relatively carry out the timing synchronization decisions and determine regularly to put d _T

Be illustrated in figure 3 as regularly the computing schematic diagram of decision function, formula (1) and (2) are got the different Temporal Sampling point d computing of sliding respectively, the maximum of m in the calculating process (d) function curve, the timing point d under the circulating prefix-length separately that distinguished correspondence _T, receiver is to the data that receive direction and the training sequence p according to arrow _nOr l _nDo the slip computing.When the slip computing proceeds to diagram d _TDuring the position, decision function m (d) is to peaking.d _TShown in the position be regularly point, i.e. m (d _T) be regularly the maximum of decision function m (d).The value of observation m (d) is carried out synchronization decisions to determine regularly point, and when it surpassed regularly decision threshold, the corresponding time sequence number d of this value was timing and puts d _T, because WiMAX adopts the frame structure of random length, so behind the timing point of determining present frame, also will proceed the synchronization decisions computing to detect the due in of next frame.

5. determine the original position of WiMAX frame

The timing point d that obtains by the slip related operation _TThe original position of WiMAX frame just can be determined in the above-mentioned formula of substitution (3) or (4), to determine the due in of next frame, has also promptly finished receiving synchronously.

With an instantiation enforcement of the present invention is described below.In this example, supposing the system adopts N=256 sub-carrier number, and circulating prefix-length is

Circulating prefix-length remains unchanged in communication process.

When the each access network of subscriber station, the base station can send relevant information to it, and the parameter that comprises circulating prefix-length is carried out initialization.This moment, receiving-transmitting sides determined to use length to be Cyclic Prefix.Subscriber station is according to the length of Cyclic Prefix, selects formula (1) to determine decision function and carries out symbol regularly, and this moment is at subscriber station adnation become to be used to the to slide reference sequences p of computing _nAccording to the decision function sequence computing of sliding to the received signal, to define the time of advent with signal frame.

Dynamically update decision threshold, each is participated in the regularly sampled point of computing synchronously, to its calculating decision threshold separately.Get the length of a leading symbol, desirable 320 is decision threshold computing span in this example.Calculating sampling point d at first _cDecision function m (d _c), wait for d then _c(d is finished in the slip computing of follow-up 160 sampled points _c160 slip computing is prior to d before _cPoint is finished), ask d at last _cBefore and after the average of totally 320 each self-corresponding m (d), with this average multiplication by constants k (as k=3.5) as d _cThe decision threshold of point.Determine the original position of frame according to decision threshold, with sampled point d _cValue bring formula (1) into and calculate the decision function value, if some d _cCorresponding m (d) is more than or equal to the corresponding decision threshold of this point, and then this point is just as regularly putting d _T,, confirm that this puts the original position of WiMAX frame just according to formula (3).

Because this method utilizes method that two sections random sequences multiply each other to come the influence of attenuating noise and minor peaks, compares with additive method, the variance of timing error is less under the low signal-to-noise ratio situation, and timing stability is better, the synchronization accuracy height; In each sample point, because the denominator of decision function need not double counting, molecule has only at most simultaneously

Point participates in computing.Therefore adopt this method to have less operand; Because the Cyclic Prefix of different length can be selected by system, the different timing decision function of this method respective design is to satisfy the timing demand under the various Cyclic Prefix.

Claims (4)

1, the receiving symbolic synchronous method of a kind of WiMAX system is characterized in that this method may further comprise the steps: determine circulating prefix-length L according to the multidiameter of channel _CPConstruct corresponding timing decision function m (d) according to circulating prefix-length, call regularly decision function algoritic module, calculate the regularly value of decision function; Value according to the timing decision function is dynamically adjusted decision threshold, and relatively regularly the currency and the decision threshold of decision function calculate regularly point, determine the original position of WiMAX frame thus.

2, receiving symbolic synchronous method according to claim 1 is characterized in that, described circulating prefix-length L _CPSatisfy following relation with timing decision function m (d):

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, $m\left(d\right)=\frac{\left|\underset{n=0}{\overset{P-1}{\mathrm{\&Sigma;}}}(\left(r^{*}(d+n)p_n\right)\&CenterDot;\left(r^{*}(d+n+4P)p_n\right))\right|}{\underset{n=0}{\overset{P-1}{\mathrm{\&Sigma;}}}{\left|p_n\right|}^4}$

When $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, $m\left(d\right)=\frac{\left|\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}(\left(r^{*}(d+n)l_n\right)\&CenterDot;\left(r^{*}(d+n+3P)l_n\right))\right|}{\underset{n=0}{\overset{L-1}{\mathrm{\&Sigma;}}}{\left|l_n\right|}^4}.$

3, receiving symbolic synchronous method according to claim 1, it is characterized in that, calculate the step of regularly putting and specifically comprise, the timing decision function is done the slip related operation, calculate the regularly currency of decision function, sequence number d was for regularly putting d when its value was pairing above timing decision threshold D _T

4, receiving synchronous method according to claim 1 is characterized in that, the original position F of WiMAX frame puts d with timing _TSatisfy following relation:

When $L_{\mathrm{CP}}=\frac{1}{4}N$ The time, F=d _T, when $L_{\mathrm{CP}}<\frac{1}{4}N$ The time, F=d _T-L _CP, wherein, N is the sub-carrier number of system.

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