CN102075475B - Combined tracking method for timing deviation and sampling frequency offset - Google Patents
Combined tracking method for timing deviation and sampling frequency offset Download PDFInfo
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Abstract
The present invention discloses a combined tracking method for timing deviation and sampling frequency offset, which is suitable for a digital mobile multimedia broadcasting system. The method comprises the steps of: according to channel impulse response information obtained by estimation, firstly estimating timing deviation for OFDM symbols and determining timing adjusting information for the next time slot; and then estimating sampling frequency offset according to the position information of the strongest path in the channel impulse response and the timing adjusting information for the last time slot, thereby realizing the combined adjusting of the timing deviation and the sampling frequency offset. The combined tracking method of the invention can not only track a symbol timing error brought by multipath distribution characteristic of a transmission channel, but also track and correct the sampling frequency offset brought by the inconsistency between two local oscillators for transmitting and receiving terminals; stable operation can also be realized even in the time division working mode of a receiver and the receiver can stably work by flexibly changing the estimation size of the sampling frequency offset according to the length of the time division working time of the receiver.
Description
Technical field
The present invention relates to the transmission of wireless signals field, particularly relate to a kind of timing offset of Techniques in Digital Mobile Multimedia Broadcasting system and combined tracking method of sampling frequency offset of being applicable to.
Background technology
To OFDM (Orthogonal Frequency Division Multiplexing, OFDM) system, Timing Synchronization and sample frequency all are one of its core technology synchronously.Because ofdm system has been introduced Cyclic Prefix as the protection interval, so ofdm system is comparatively loose to the requirement of Timing Synchronization.Only in the scope of Cyclic Prefix, change as long as guarantee the original position of FFT (fast Fourier transform) computing window, just can not disturb (ISI) between created symbol.But in wireless propagation environment, because the existence of scattering object on every side, wireless signal can arrive receiver through a plurality of propagation paths usually.Can there be obvious time delay in a plurality of signal copies for receiver receives between them usually.So wireless channel all is a multipath channel usually, and according to the conversion of receiver receiving position and the conversion of communication environments on every side, obvious conversion also can occur in corresponding multipath channel.The multidiameter delay characteristic of channel has increased ofdm system time delay expansion sensitivity, and only first signal copy of guaranteed transmitting signal and last signal copy all drop in the Cyclic Prefix, and ofdm system just can be evaded the impact of intersymbol interference.In order to reduce the impact of this negative factor as far as possible, therefore in the Wireless OFDM System of reality, need to reduce the error of Timing Synchronization as far as possible.Common way is to introduce the timing offset track loop to follow the tracks of in the multipath channel conversion trend corresponding to the first footpath of first signal copy in receiver.
And sampling frequency offset is present in all digital communication systems, and the main cause that produces sampling frequency offset is that transmitter and receiver local oscillator between the two exists certain frequency departure and phase deviation.Concerning the OFDM receiver, iff there being the sampling phase deviation, only can cause the phase rotating of each sub-carrier signal, the OFDM receiver utilizes channel estimating can compensate the impact of this phase rotating, therefore can not cause too much influence to signal to noise ratio.And if also have simultaneously sampling frequency deviation, concerning digital system, will show as on the time domain in identical duration, bring increase or the minimizing of sampled point, and the increase of this sampled point or reduce can the progressively accumulation along with the variation of time.This variation can bring the FFT of system window initial time to drift about on the one hand, disturbs between created symbol; Can destroy on the other hand the orthogonality between subcarrier, cause inter-carrier interference (ICI, Inter Carrier Interference).
The method of at present more general estimation sampling frequency offset is to utilize frequency domain training sequence to obtain the domain channel response of channel, then the conjugation multiplication of twice domain channel response before and after utilizing, after the unfavorable response of removing propagation channel, ask sampling frequency deviation according to residual phase deviation information.This method is applicable to adopt the ofdm system of time-frequency two-dimensional pilot structure, but needs to increase the channel frequency domain response information that certain storage resources is stored a moment.
Summary of the invention
The technical problem to be solved in the present invention provides a kind of timing offset of Techniques in Digital Mobile Multimedia Broadcasting system and combined tracking method of sampling frequency offset of being applicable to, when the signal timing mistake that can cause distributing because of propagation channel Multipath distribution characteristic is followed the tracks of, the sampling frequency deviation of the inconsistent generation of transmitting-receiving two-end local oscillator is followed the tracks of and rectified a deviation, even also can steady operation under receiver time-division mode of operation, and can change neatly according to the length of operating time receiver time-division the estimation size of sampling frequency offset, make the receiver steady operation.
For solving the problems of the technologies described above, the combined tracking method of timing offset of the present invention and sampling frequency offset comprises the steps:
Step 2, performance number is asked in pointwise to the channel impulse response sequence, and searches maximum performance number and corresponding sampling point position, and this performance number and sampling point position are respectively the power P of most powerful path in the channel impulse response
MaxAnd the position I of most powerful path
Max(n), n is used for the sign current time slots constantly;
Step 3, according to average power P
ePower P with most powerful path
MaxObtain the multipath detection threshold ξ of current time slots
1, ξ
1=max (k
1P
e, P
Max/ k
2); Desirable average power P
eK
1Doubly size and most powerful path power P
MaxK
2Higher value in/one size is the multipath detection threshold; Wherein, k
1And k
2Delay character according to propagation channel is configured, and need satisfy k
1And k
2It all is the positive number greater than 1;
Step 4, according to multipath detection threshold ξ
1In the channel impulse response sequence, carry out effective diameter identification, namely select all power greater than multipath detection threshold ξ
1Effective diameter; Find out first path position I in the current time slots Multipath distribution according to described effective diameter
f(n), thus determine the most powerful path time delay τ of current time slots
Max(n), τ
Max(n)=I
Max(n)-I
f(n), I
Max(n) be the position of the most powerful path in the step 2;
Step 5, the first path position I that estimates according to last effective time slot signal
f(n-1) and current time slots estimate the first path position I obtain
f(n), obtain the estimated value D of current time slots timing offset
e=I
f(n)-I
f(n-1), this estimated value being fed back to the timing offset adjustment loop was rectified a deviation to timing offset before next effective time slot signal arrives; The position of estimating simultaneously the most powerful path of next effective time slot signal is I '
Max(n+1)=I
Max(n)+D
e
The position I ' of step 6, the most powerful path estimated according to last effective time slot signal
Max(n-1) and current time slots estimate to obtain most powerful path position I
Max(n), estimate to obtain both sampling point deviation K=I '
Max(n-1)-I
Max(n);
Step 7, according to interval duration T between the twice effective time slot signal in front and back
uWith the sampling point deviation K that estimates, obtain the estimated value γ of sampling frequency offset=K/T
u, feed back this estimated value to digital sample module sample frequency rectified a deviation.
The present invention is directed to Chinese Digital mobile multimedia broadcast system (China Mobile MultimediaBroadcasting, CMMB) time-multiplexed physical frame structure, in conjunction with Timing Synchronization technology and sample frequency simultaneous techniques, realize the estimation of uniting to timing offset and sampling frequency offset.The present invention can be when the signal timing mistake cause be followed the tracks of because propagation channel Multipath distribution characteristic distributes, and the sampling frequency deviation of the inconsistent generation of transmitting-receiving two-end local oscillator is followed the tracks of and rectified a deviation.The method is specially adapted to this broadcast system with time-division mode of operation of CMMB.Method of the present invention is different with general ofdm system, a plurality of business are with the identical frequency resource of the mode time division multiplexing of time slot in the CMMB system, receiver can only engrave the reception transmitted signal when the time slot at current business place, and constantly close receiver at other, thereby reach the purpose of receiver economize on electricity, so the reception signal that receiver receives not is continuous; The present invention can be for this discontinuous reception signal, changes the estimation size of sampling frequency offset according to the flexible in size ground of interval duration between the useful signal, makes the receiver steady operation.
Description of drawings
The present invention is further detailed explanation below in conjunction with accompanying drawing and embodiment:
Fig. 1 is CMMB physical layer frame structure schematic diagram;
Fig. 2 is method one embodiment schematic flow sheet of the present invention.
Embodiment
Referring to shown in Figure 1, according to the protocol definition of CMMB, its physical layer signal is take physical frame as unit, and frame length is 1 second.Each physical frame signal is divided into again 40 time slots, and each time slot duration is 25 milliseconds.The beginning that is fixed on each time slot signal sends two known synchronizing signals, is used for the bearer service data after the synchronizing signal.Each business can take a plurality of time slots and send business datum, takies identical frequency spectrum resource in the mode of time-division and sends broadcasting service.This time-multiplexed physical frame definition is conducive to receiver and adopts time slot handoff technique (Timing Slice) to choose neatly the time slot signal that will receive in a plurality of time slot signals, keeps mourning in silence at other time slot signals, to reduce the power consumption of receiver.But increased simultaneously the requirement to timing offset tracking module and sampling frequency offset tracking module, estimated timing offset and sampling frequency offset so that receiver must also can be stable in flexible and changeable reception signal combination, and in time rectified a deviation.Referring to shown in Figure 2, set forth in one embodiment of this invention implementation process of the present invention as an example of CMMB example, specifically comprise the steps:
Step 2, degree of rectificating are the power P of most powerful path in 2048 the channel impulse response
MaxAnd the position I of most powerful path
Max(n).For example, performance number is asked in pointwise to the channel impulse response sequence, and searches wherein maximum performance number as the power P of most powerful path
Max, and corresponding sampling point position is the position I of most powerful path
Max(n), n is used for the sign current time slots constantly.
Step 3, according to average power P
ePower P with most powerful path
MaxObtain the multipath detection threshold ξ of current time slots
1,
ξ
1＝max(k
1P
e，P
max/k
2)。
Step 4, according to multipath detection threshold ξ
1In the channel impulse response sequence, carry out effective diameter identification, namely select all power greater than multipath detection threshold ξ
1Effective diameter.Find out first path position I in the current time slots Multipath distribution according to effective diameter
f(n), thus determine the most powerful path time delay τ of current time slots
Max(n)=I
Max(n)-I
f(n).
Step 5, the first path position I that estimates according to last effective time slot signal
f(n-1) and current time slots estimate the first path position I obtain
f(n), obtain the estimated value D of current time slots timing offset
e=I
f(n)-I
f(n-1), this estimated value being fed back to the timing offset adjustment loop was rectified a deviation to timing offset before next effective time slot signal arrives; Simultaneously, the position of estimating the most powerful path of next effective time slot signal is I '
Max(n+1)=I
Max(n)+D
e
The position I ' of step 6, the most powerful path estimated according to last effective time slot signal
Max(n-1) and current time slots estimate to obtain most powerful path position I
Max(n), obtain both sampling point deviation K=I '
Max(n-1)-I
Max(n).
Step 7, according to interval duration T between the twice effective time slot signal in front and back
uWith the sampling point deviation K that estimates, obtain the estimated value γ of sampling frequency offset=K/T
u, feed back this estimated value to digital sample module sample frequency rectified a deviation.
Here interval duration T between twice effective time slot signal before and after
uSpan from 25 milliseconds to 1 second, minimum change of scale is 25 milliseconds.If former and later two time slots all are effectively to receive signal, the interval duration is 25 milliseconds between former and later two effective time slot signals so, the sampling frequency deviation of the corresponding big or small 40*K of K sampling point deviation then, and unit is hertz.If only comprise the useful signal of a time slot in 1 second the physical frame of duration, the interval duration between former and later two effective time slots is 1 second so, and then K the corresponding size of sampling point deviation is the sampling frequency deviation of K, and unit is hertz.This shows that even under the time slot mode of operation, receiver also can change the estimation size of sampling frequency offset neatly according to the variation of interval duration between effective time slot signal, reaches the purpose of steady operation.
In described step 6, if the sampling point deviation K that estimates to obtain is greater than a certain given threshold k
1The time, can think that a same time slot signal of current time slots signal compares, great changes have occured in most powerful path, can force to set K=0 this moment, even to guarantee that the sampling frequency offset loop also keeps steady operation under varying Channels.
More than by embodiment the present invention is had been described in detail, but these are not to be construed as limiting the invention.In the situation that does not break away from the principle of the invention; those skilled in the art also can make many distortion and improvement; for example; although embodiments of the present invention are specifically set forth in top description as an example of CMMB example; but thought of the present invention is not limited in CMMB; can be applicable to the ofdm system that other have training sequence, these also should be considered as protection scope of the present invention.
Claims (2)
1. the combined tracking method of a timing offset and sampling frequency offset is characterized in that, comprises the steps:
Step 1, in the current time slots signal, obtain the channel impulse response sequence that length is N according to the synchronizing signal that comprises, and ask the average power P of channel impulse response sequence
e
Step 2, ask the power P of most powerful path in the channel impulse response
MaxAnd the position I of most powerful path
Max(n), n is used for the sign current time slots constantly;
Step 3, according to average power P
ePower P with most powerful path
MaxObtain the multipath detection threshold ξ of current time slots
1, ξ
1=max (k
1P
e, P
Max/ k
2); Wherein, k
1And k
2Delay character according to propagation channel is configured, and need satisfy k
1And k
2It all is the positive number greater than 1;
Step 4, according to multipath detection threshold ξ
1In the channel impulse response sequence, carry out effective diameter identification, namely select all power greater than multipath detection threshold ξ
1Effective diameter; Find out first path position I in the current time slots Multipath distribution according to described effective diameter
f(n), thus determine the most powerful path time delay τ of current time slots
Max(n)=I
Max(n)-I
f(n);
Step 5, the first path position I that estimates according to last effective time slot signal
f(n-1) and current time slots estimate the first path position I obtain
f(n), obtain the estimated value D of current time slots timing offset
e=I
f(n)-I
f(n-1), this estimated value being fed back to the timing offset adjustment loop was rectified a deviation to timing offset before next effective time slot signal arrives; The position of estimating simultaneously the most powerful path of next effective time slot signal is I '
Max(n+1)=I
Max(n)+D
e
The position I ' of step 6, the most powerful path estimated according to last effective time slot signal
Max(n-1) and current time slots estimate to obtain most powerful path position I
Max(n), estimate to obtain both sampling point deviation K=I '
Max(n-1)-I
Max(n);
Step 7, according to interval duration T between the twice effective time slot signal in front and back
uWith the sampling point deviation K that estimates, obtain the estimated value γ of sampling frequency offset=K/T
u, feed back this estimated value to digital sample module sample frequency rectified a deviation.
2. method according to claim 1 is characterized in that: the sampling point deviation K that estimates to obtain when step 6 is greater than a certain given threshold k
1The time, think that then a same time slot signal of current time slots signal compares most powerful path great changes have occured, force to set K=0 this moment, even to guarantee that the sampling frequency offset loop also keeps steady operation under varying Channels.
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| CN102685875B (en) * | 2012-05-03 | 2015-07-15 | 京信通信系统（中国）有限公司 | Method and device for timing synchronization adjustment between base stations |
| CN104735011B (en) * | 2013-12-24 | 2019-01-29 | 江苏卓胜微电子股份有限公司 | Apparatus and method for robust estimation of sampling frequency deviation in complex channel environment |
| CN105897304B (en) * | 2016-05-31 | 2018-02-06 | 西安空间无线电技术研究所 | A kind of fast synchronization method of frequency-hopping communication system |
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| CN101534270A (en) * | 2008-03-10 | 2009-09-16 | 卓胜微电子（上海）有限公司 | Carrier frequency offset estimation and tracking method based on signal channel estimation |
| CN101534426A (en) * | 2008-03-11 | 2009-09-16 | 卓胜微电子（上海）有限公司 | National standard timing tracking method of digital television terrestrial broadcasting |
| CN101534184A (en) * | 2008-03-11 | 2009-09-16 | 卓胜微电子（上海）有限公司 | Sampling frequency offset estimation method |
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| CN101534270A (en) * | 2008-03-10 | 2009-09-16 | 卓胜微电子（上海）有限公司 | Carrier frequency offset estimation and tracking method based on signal channel estimation |
| CN101534426A (en) * | 2008-03-11 | 2009-09-16 | 卓胜微电子（上海）有限公司 | National standard timing tracking method of digital television terrestrial broadcasting |
| CN101534184A (en) * | 2008-03-11 | 2009-09-16 | 卓胜微电子（上海）有限公司 | Sampling frequency offset estimation method |
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Address after: 214072 Jiangsu province Binhu District of Wuxi City Liyuan Development Zone 530 No. 1 building twelve room 1203 Patentee after: Jiangsu Zhuo Sheng microelectronics Limited by Share Ltd Address before: 214072 Jiangsu province Binhu District of Wuxi City Liyuan Development Zone 530 No. 1 building twelve room 1203 Patentee before: JIANGSU MAXSCEND TECHNOLOGY CO., LTD. |
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