CN100534081C - Time synchronism method for multi-input multi-output crossing frequency division multiplexing system - Google Patents

Abstract

The invention relates to a time synchronizing method of multi-input multi-output orthogonal frequency division multiplexing system. Wherein, it comprises: first based on the received training sequence of system, processing rough time synchronization; from the rough synchronous position, and from the baseband signal, cutting the training sequence used in frequency bias evaluation to evaluate the frequency bias, and compensate, then relating the corrected training sequence with local training sequence, to obtain the reference energy En whose average value of all receiving antennas is noise; calculating the average energy E1 between judging position and next position relative value; calculating the average value E2 of all positions inside the whole sliding window after the judging position; comparing ratios between E1 and E2 and between E1 and E2, to find the first multi-path position. The invention has delay distributions on SNTR and power, irrelative to the structure of training sequence; therefore, its synchronous property is affected little by the random element.

Description

Method for synchronizing time in a kind of multi-input multi-output-orthogonal frequency division multiplexing system

Technical field

The present invention relates to a kind of method for synchronizing time that is used for multiple-input and multiple-output-OFDM (hereinafter to be referred as MIMO-OFDM) system, belong to technical field of wireless communication.

Background technology

Time synchronization position is identical on each antenna in the MIMO-OFDM system, carry out time synchronized again after can be with the received signal on each antenna average, the time synchronization problem of MIMO-OFDM system has just become the stationary problem of general ofdm system at this moment, but because channel estimation errors can be introduced interference in the MIMO-OFDM system between antenna, and timing error can influence channel estimating performance, therefore this will cause the serious decline of systematic function, can not be simply the timing method of conventional single-antenna-OFDM (hereinafter to be referred as SISO-OFDM) system be expanded in the MIMO-OFDM system.The timing method of introducing traditional ofdm system below earlier describes reason again in detail.

Generally in ofdm system, all use Cyclic Prefix (hereinafter to be referred as CP) to reduce intersymbol interference (hereinafter to be referred as ISI), because making, the existence of CP exists between a synchronization zone in the ofdm system, can not cause ISI when sync bit drops in this interval, sync bit drops on outside this interval will introduce ISI.Time synchronized algorithm in the ofdm system can be divided into two classes: a class is called front and back method for synchronous correlation time, the another kind of method for synchronizing time that is called based on channel estimating.

Before and after correlation time method for synchronous be relevant carrying out synchronously before and after adopting by the characteristics of utilizing the synchronizing sequence cycle to repeat; Method for synchronizing time based on channel estimating is the power delay profile information that obtains channel by channel estimating, selects sync bit then.Front and back synchronized algorithm correlation time is not subjected to the influence of frequency deviation, the time synchronized when being fit to frequency deviation is arranged, and the very little suitable real-time operation of amount of calculation.When between the synchronization zone when big system bigger to the tolerance of synchronous error, the front and back related algorithm generally can satisfy the synchronization accuracy requirement; But system is less to the tolerance of synchronous error when less between the synchronization zone or signal to noise ratio are hanged down, and this algorithm generally is difficult to satisfy system requirements.To seek the position of channel article one multipath based on the method for synchronizing time of channel estimating,, therefore generally channel response be added an associated window and seek and make the original position of window of window self-energy maximum because real system can not always guarantee that article one footpath energy is very big.Simultaneously, also require the time domain channel energy of this sync bit correspondence enough strong, promptly greater than channel certain thresholding of the strongest path energy for fear of being synchronized on the noise.The prerequisite of this method for synchronous is the dull decline of the power delay profile of supposition channel, and this method performance is subjected to the influence of channel energy random distribution bigger, when the energy in article one footpath and energy when the ratio dynamic range in strong path is very big, be difficult to find a suitable thresholding to obtain the performance of being satisfied with.Even article one the footpath is little with the ratio dynamic range of the strongest path energy, the net synchronization capability of this method also has relation with the power delay profile of multipath.

People often combine use with above-mentioned two kinds of methods in traditional SISO-OFDM system, time synchronized is decomposed into synchronously thick and synchronously smart, the thick relevant method in front and back of using synchronously finds general sync bit, and the smart channel estimation results adjustment sync bit that utilizes on thick synchronous basis synchronously obtains smart synchronized result.

Because the data transfer rate of MIMO-OFDM system is generally higher, system improves the requirement of net synchronization capability, and a method for synchronous to the SISO-OFDM system carries out the existing synchronous error of simple extension will become the system for restricting bottleneck of performance.And to transmit and receive the antenna number in the MIMO-OFDM system be variable, carries the also not decoding of data of system parameter message when carrying out time synchronized, therefore can't determine the quantity of transmitting antenna, also can't determine the length and the structure of training sequence.Because in the MIMO-OFDM system, the power delay profile that receives the data equivalent channel is relevant with the structure of training sequence (during as employing cyclic shift training sequence, utilize the cyclic shift of the character training sequence of convolution equivalence to be the cyclic shift of channel, equivalent channel will extend), if therefore do not know the structure of training sequence, will make the width of window to determine, can't use the timing method of in the SISO-OFDM system, seeking the energy maximal window, therefore need to propose new timing method.

Summary of the invention

The objective of the invention is to propose the method for synchronizing time in a kind of multi-input multi-output-orthogonal frequency division multiplexing system, in the MIMO-OFDM system, power delay profile is had robustness, to improve the time synchronized performance.

Method for synchronizing time in a kind of multi-input multi-output-orthogonal frequency division multiplexing system that the present invention proposes comprises following each step:

(1) receives the radiofrequency signal that sends in the short-range wireless networking, carry out sampling with period T after the down-frequency conversion, obtain the base-band digital received signal, the base-band digital received signal is carried out thick time synchronized, obtain thick time synchronized position N _Coarse

(2) begun by above-mentioned thick sync bit, the training sequence that intercepting is used for frequency offset estimating from baseband signal carries out frequency offset estimating, and training sequence is carried out compensate of frequency deviation, obtains the training sequence behind the correcting frequency deviation, is designated as x _j(n), represent n received signal constantly on j the reception antenna, the cycle of this training sequence is N;

(3) on each antenna, get N before the thick sync bit _nIndividual, training sequence behind the above-mentioned correcting frequency deviation and corresponding local training sequence are carried out relevant, obtain N _nIndividual correlation is with the N on all reception antennas _nIndividual correlation averages, and the mean value that obtains is as the reference energy E of noise _n

(4) the training sequence x behind the calculating correcting frequency deviation _j(n) with the correlation of local training sequence:

$\mathrm{\ξ}\left(n\right)=\underset{l=0}{\overset{L_f-1}{\mathrm{\Σ}}}\underset{j=1}{\overset{N_R}{\mathrm{\Σ}}}{\left|\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{x}_j^{*}(n+i+l)p\left(i\right)\right|}^2,$

P in the following formula (n) is local training sequence, and N is the training sequence x behind the correcting frequency deviation _j(n) cycle, L _fLong for sliding window, * represents conjugation;

(5) on each reception antenna, calculate above-mentioned correlation, and the correlation of each antenna same position is averaged;

(6) get above-mentioned thick time synchronized position N _CoarseAfter the position for being the judgement position of article one multi-path location in the channel whether, calculate the average energy of this judgement position and the next position correlation, obtain first ENERGY E ₁

(7) calculating begins L in the whole sliding window from above-mentioned judgement position _fThe average energy of individual position correlation obtains second ENERGY E ₂

(8) to above-mentioned E ₁Ratio and E with En ₁With E ₂Ratio judge, if E ₁With E _nRatio set thresholding η greater than first ₁, and E ₁With E ₂Ratio set thresholding η greater than second ₂, then found article one multi-path location, and carried out step (9), otherwise decision bit postpone has been moved one, repeating step (6)～(8);

(9) with above-mentioned E ₁With E ₂Ratio and the 3rd set thresholding η ₃Compare, if E ₁With E ₂Ratio set thresholding η greater than the 3rd ₁, then with 1 of article one multi-path location reach, otherwise it is constant to keep article one multi-path location.

In the above-mentioned synchronized algorithm, the base-band digital received signal is carried out thick time synchronized obtain thick time synchronized position N _CoarseMethod,, may further comprise the steps:

(1) gets that length is L in the base-band digital received signal _c+ N _sReceiving sequence, and according to the following formula ratio calculated:

$\mathrm{\ρ}=\frac{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{x}_i^{*}\left(n\right)x_i(n+N_s)\right|}^2}{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{\left|x_i\left(n\right)\right|}^2\right|}^2},$

In the following formula, N _RBe the reception antenna number, N _sBe before and after the baseband signal when relevant the institute interval count L _cThe thick synchronous sliding window that is setting is long, x _i(n) be n data in this sliding window that receives on i the reception antenna;

(2) to size and the predefined decision threshold ρ of above-mentioned ratio ρ ₀Judge, if ρ 〉=ρ ₀, then sliding window moves one backward, recomputates ratio ρ, up to ρ＜ρ ₀In time, stop to slide, and shift count is since 0 counting, and the number of times that moves behind the accumulative total sliding window when stopping to slide is designated as N _Back

(3) according to above-mentioned N _Back, obtain thick sync bit N _Coarse=N _Back-L _c+ N _s

Method for synchronizing time in the multi-input multi-output-orthogonal frequency division multiplexing system that the present invention proposes is divided into time synchronized synchronously thick and synchronously smart.At first undertaken synchronously thick by common front and back correlation technique.Synchronous with the relevant essence of carrying out of received training sequence then with local training sequence, determine smart sync bit by triple Threshold detection, in view of infeasible by the method for seeking the energy maximal window, changing the position of seeking article one footpath into is smart sync bit.The following judgement position of mentioning is whether will adjudicate be the position in article one footpath, first ENERGY E ₁Be judgement position and the next position correlation and, second ENERGY E ₂Be in the energy window that begins from this judgement position correlation and, judge this point position article one if the ratio of the former with the latter surpasses certain thresholding directly.Setting the thresholding effect for wherein three is respectively: first sets thresholding η ₁Be used to reduce the probability that is synchronized to noise, second sets thresholding η ₂Be used to guarantee to adjudicate position E1 and reach E ₂Certain percentage, the 3rd sets thresholding η ₃Then smart sync bit is further finely tuned and obtain sync bit more accurately.This method has the following advantages:

(1) the present invention is by being provided with η ₁, η ₂Make and SNR and power delay profile are had robustness based on the time synchronized algorithm of seeking article one path position;

(2) structure-irrelevant of timing method of the present invention and training sequence;

(3) the reference thresholding of the present invention by will judging article one footpath becomes energy in the whole energy window from the strongest multipath energy, makes net synchronization capability be subjected to stochastic factor less;

(4) the present invention makes full use of the characteristics that exist in the ofdm system between the synchronization zone, by thresholding η is set ₃Reduce the synchronous influence of mistake, to improve the system synchronization performance to systematic function.

Description of drawings

Fig. 1 is the FB(flow block) of the inventive method.

Embodiment

Method for synchronizing time in a kind of multi-input multi-output-orthogonal frequency division multiplexing system that the present invention proposes, its FB(flow block) as shown in Figure 1, at first receive the radiofrequency signal that sends in the short-range wireless networking, carry out sampling with period T after the down-frequency conversion, obtain the base-band digital received signal, the base-band digital received signal is carried out thick time synchronized, obtain thick time synchronized position N _CoarseBegun by thick sync bit, the training sequence that intercepting is used for frequency offset estimating from baseband signal carries out frequency offset estimating, and training sequence is carried out compensate of frequency deviation, obtains the training sequence behind the correcting frequency deviation, is designated as x _j(n), represent n received signal constantly on j the reception antenna, the cycle of this training sequence is N; On each antenna, get the N before the thick sync bit _nIndividual, training sequence behind the above-mentioned correcting frequency deviation and corresponding local training sequence are carried out relevant, obtain N _nIndividual correlation is with the N on all reception antennas _nIndividual correlation averages, and the mean value that obtains is as the reference energy E of noise _nTraining sequence x behind the calculating correcting frequency deviation _j(n) with the correlation of local training sequence:

$\mathrm{\ξ}\left(n\right)=\underset{l=0}{\overset{L_f-1}{\mathrm{\Σ}}}\underset{j=1}{\overset{N_R}{\mathrm{\Σ}}}{\left|\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{x}_j^{*}(n+i+l)p\left(i\right)\right|}^2,$ P in the following formula (n) is local training sequence, and N is the training sequence x behind the correcting frequency deviation _j(n) cycle, L _fLong for sliding window, * represents conjugation; On each reception antenna, calculate above-mentioned correlation, and the correlation of each antenna same position is averaged; Get above-mentioned thick time synchronized position N _CoarseAfter the position for being the judgement position of article one multi-path location in the channel whether, calculate the average energy of this judgement position and the next position correlation, obtain first ENERGY E ₁Calculating begins the average energy of Lf position correlation in the whole sliding window from above-mentioned judgement position, obtain second ENERGY E ₂To above-mentioned E ₁With E _nRatio and E ₁With E ₂Ratio judge, if E ₁With E _nRatio set thresholding η greater than first ₁, and E ₁With E ₂Ratio set thresholding η greater than second ₂, then found article one multi-path location, otherwise decision bit postpone has been moved one, repeat abovementioned steps; With above-mentioned E ₁With E ₂Ratio and the 3rd set thresholding η ₃Compare, if E ₁With E ₂Ratio set thresholding η greater than the 3rd ₁, then with 1 of article one multi-path location reach, otherwise it is constant to keep article one multi-path location.

In the above-mentioned synchronized algorithm, the base-band digital received signal is carried out thick time synchronized obtain thick time synchronized position N _CoarseMethod, get at first that length is L in the base-band digital received signal _c+ N _sReceiving sequence, and according to the following formula ratio calculated: $\mathrm{\ρ}=\frac{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{x}_i^{*}\left(n\right)x_i(n+N_s)\right|}^2}{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{\left|x_i\left(n\right)\right|}^2\right|}^2},$ In the following formula, N _RBe the reception antenna number, N _sBe before and after the baseband signal when relevant the institute interval count L _cThe thick synchronous sliding window that is setting is long, x _i(n) be n data in this sliding window that receives on i the reception antenna; Size and predefined decision threshold ρ to above-mentioned ratio ρ ₀Judge, if ρ 〉=ρ ₀, then sliding window moves one backward, recomputates ratio ρ, up to ρ＜ρ ₀In time, stop to slide, and shift count is since 0 counting, and the number of times that moves behind the accumulative total sliding window when stopping to slide is designated as N _BackAccording to above-mentioned N _Back, obtain thick sync bit N _Coarse=N _Back-L _c+ N _s

Receive (N with 44 below _R=4, N _T=4) the MIMO-OFDM system is an example, introduces method of the present invention in detail.Training sequence structure on each transmitting antenna of this system is identical, all is made of two parts: first is to be that 16 short sequence constitutes in 10 identical cycles, is called short code; Second portion is to be made of two 64 training sequence, and the front is added with 32 CP, is called long code.

Receive the radiofrequency signal that sends in the short-range wireless networking, carry out sampling with period T after the down-frequency conversion, obtain the base-band digital received signal.The thick timing training sequence cycle is N _sL in the intercept baseband digital received signal _c+ N _sLong receiving sequence, rule of thumb desirable thick timing correlation window length L _c, ratio calculated,

$\mathrm{\ρ}=\frac{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{x}_i^{*}\left(n\right)x_i(n+N_s)\right|}^2}{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{\left|x_i\left(n\right)\right|}^2\right|}^2},$

In the following formula, N _RBe the reception antenna number, N _sBe before and after the baseband signal when relevant the institute interval count L _cThe thick synchronous sliding window that is setting is long, x _i(n) be n data in this sliding window that receives on i the reception antenna, * represents conjugation.

Size and predefined decision threshold ρ to above-mentioned ratio ρ ₀Judge, if ρ 〉=ρ ₀, then sliding window moves one backward, recomputates ratio ρ, up to ρ＜ρ ₀In time, stop to slide, and shift count is since 0 counting, and the number of times that moves behind the accumulative total sliding window when stopping to slide is designated as N _BackIn order to make the end position that slightly is no more than short code synchronously, thick sync bit is adjusted, obtain thick sync bit N _Coarse=N _Back-L _c+ N _s

Begun by above-mentioned thick sync bit, the training sequence that intercepting is used for frequency offset estimating from baseband signal carries out frequency offset estimating, and training sequence is carried out compensate of frequency deviation, obtains the training sequence behind the correcting frequency deviation, is designated as x _j(n), represent n received signal constantly on j the reception antenna, the cycle of this training sequence is N.

On each antenna, get the N before the thick sync bit _nIndividual, training sequence behind the above-mentioned correcting frequency deviation and corresponding local training sequence are carried out relevant, obtain N _nIndividual correlation is with the N on all reception antennas _nIndividual correlation averages, and the mean value that obtains is as the reference energy E of noise _n

Training sequence x behind the calculating correcting frequency deviation _j(n) with the correlation of local training sequence:

$\mathrm{\ξ}\left(n\right)=\underset{l=0}{\overset{L_f-1}{\mathrm{\Σ}}}\underset{j=1}{\overset{N_R}{\mathrm{\Σ}}}{\left|\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{x}_j^{*}(n+i+l)p\left(i\right)\right|}^2,$

P in the following formula (n) is local training sequence, and N is the training sequence x behind the correcting frequency deviation _j(n) cycle, L _fLong for sliding window, * represents conjugation.

Training sequence x behind the double counting correcting frequency deviation _j(n), on each reception antenna, calculate above-mentioned correlation, and the correlation of each antenna same position is averaged with the correlation of local training sequence.Get above-mentioned thick time synchronized position N _CoarseAfter the position for being the judgement position of article one multi-path location in the channel whether, calculate the average energy of this judgement position and the next position correlation, obtain first ENERGY E ₁Calculating begins L in the whole sliding window from above-mentioned judgement position _fThe average energy of individual position correlation obtains second ENERGY E ₂To above-mentioned E ₁With E _nRatio and E ₁With E ₂Ratio judge, if E ₁With E _nRatio set thresholding η greater than first ₁, and E ₁With E ₂Ratio set thresholding η greater than second ₂, then found article one multi-path location, otherwise decision bit postpone moved one, double counting E ₁With E ₂, and carry out E ₁With E _nRatio and E ₁With E ₂The judgement of ratio.With above-mentioned E ₁With E ₂Ratio and the 3rd set thresholding η ₃Compare, if E ₁With E ₂Ratio set thresholding η greater than the 3rd ₁, then with 1 of article one multi-path location reach, otherwise it is constant to keep article one multi-path location, has so just obtained last smart sync bit.

Claims (1)

1, the method for synchronizing time in a kind of multi-input multi-output-orthogonal frequency division multiplexing system is characterized in that this method comprises following each step:

(1) receives the radiofrequency signal that sends in the short-range wireless networking, carry out sampling with period T after the down-frequency conversion, obtain the base-band digital received signal, the base-band digital received signal is carried out thick time synchronized, obtain thick time synchronized position N _Coarse, the method for thick time synchronized may further comprise the steps:

(a) get that length is L in the base-band digital received signal _c+ N _sReceiving sequence, and according to the following formula ratio calculated:

$\mathrm{\ρ}=\frac{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{x}_i^{*}\left(n\right)x_i(n+N_s)\right|}^2}{{\left|\underset{i=1}{\overset{N_R}{\mathrm{\Σ}}}\underset{n=1}{\overset{L_c}{\mathrm{\Σ}}}{\left|x_i\left(n\right)\right|}^2\right|}^2}$

In the following formula, N _RBe the reception antenna number, N _sBe before and after the baseband signal when relevant the institute interval count L _cThe thick synchronous sliding window that is setting is long, x _i(n) be n data in this sliding window that receives on i the reception antenna;

(b) to size and the predefined decision threshold ρ of above-mentioned ratio ρ ₀Judge, if ρ 〉=ρ ₀, then sliding window moves one backward, recomputates ratio ρ, up to ρ＜ρ ₀In time, stop to slide, and shift count is since 0 counting, and the number of times that moves behind the accumulative total sliding window when stopping to slide is designated as N _Back

(c) according to above-mentioned N _Back, obtain thick sync bit N _Coarse=N _Back-L _c+ N _s

(2) begun by above-mentioned thick sync bit, the training sequence that intercepting is used for frequency offset estimating from baseband signal carries out frequency offset estimating, and training sequence is carried out compensate of frequency deviation, obtains the training sequence behind the correcting frequency deviation, is designated as x _j(n), represent n received signal constantly on j the reception antenna, the cycle of this training sequence is N;

(3) on each antenna, get N before the thick sync bit _nIndividual, training sequence behind the above-mentioned correcting frequency deviation and corresponding local training sequence are carried out relevant, obtain N _nIndividual correlation is with the N on all reception antennas _nIndividual correlation averages, and the mean value that obtains is as the reference energy E of noise _n

(4) the training sequence x behind the calculating correcting frequency deviation _j(n) with the correlation of local training sequence:

$\mathrm{\ξ}\left(n\right)=\underset{l=0}{\overset{L_f-1}{\mathrm{\Σ}}}\underset{j=1}{\overset{N_R}{\mathrm{\Σ}}}{\left|\underset{i=1}{\overset{N}{\mathrm{\Σ}}}{x}_j^{*}(n+i+l)p\left(i\right)\right|}^2,$

P in the following formula (n) is local training sequence, and N is the training sequence x behind the correcting frequency deviation _j(n) cycle, L _fFor sliding window long;

(5) on each reception antenna, calculate above-mentioned correlation, and the correlation of each antenna same position is averaged;

(6) get above-mentioned thick time synchronized position N _CoarseAfter the position for being the judgement position of article one multi-path location in the channel whether, calculate the average energy of this judgement position and the next position correlation, obtain first ENERGY E ₁

(7) calculating begins L in the whole sliding window from above-mentioned judgement position _fThe average energy of individual position correlation obtains second ENERGY E ₂

(8) to above-mentioned E ₁With E _nRatio and E ₁With E ₂Ratio judge, if E ₁With E _nRatio set thresholding η greater than first ₁, and E ₁With E ₂Ratio set thresholding η greater than second ₂, then found article one multi-path location, and carried out step (9), otherwise decision bit postpone has been moved one, repeating step (6)～(8);

(9) with above-mentioned E ₁With E ₂Ratio and the 3rd set thresholding η ₃Compare, if E ₁With E ₂Ratio set thresholding η greater than the 3rd ₁, then with 1 of article one multi-path location reach, otherwise it is constant to keep article one multi-path location.

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