CN102377726B - Timing synchronization method of OFDM (Orthogonal Frequency Division Multiplexing) system - Google Patents

Timing synchronization method of OFDM (Orthogonal Frequency Division Multiplexing) system Download PDF

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CN102377726B
CN102377726B CN201110406937.5A CN201110406937A CN102377726B CN 102377726 B CN102377726 B CN 102377726B CN 201110406937 A CN201110406937 A CN 201110406937A CN 102377726 B CN102377726 B CN 102377726B
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ofdm
timing synchronization
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training sequence
channel
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CN102377726A (en
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赵军辉
吕宸
李非
刘畅
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Beijing Jiaotong University
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Beijing Jiaotong University
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Abstract

The invention relates to a timing synchronization method of an OFDM (Orthogonal Frequency Division Multiplexing) system. The method comprises the following steps of: firstly, generating random sequences A at a transmitting end of the OFDM system, and outputting record sequences B in the direction reverse to the random sequences A, so as to constitute symmetrically-structured training sequences S=[A B]; secondly, establishing required OFDM baseband signals, adding the training sequences in front of data to be transmitted by adopting the sequences generated in the first step, and sending into a channel; and thirdly, carrying out timing synchronization of signals received by the transmitting end of the OFDM system in a time domain. According to the timing synchronization method provided by the invention, the accuracy and the stability of timing synchronization can be improved, and meanwhile, obvious increase in operand is not caused, and the method is suitable for a complex multi-path fading channel.

Description

The time synchronization method of ofdm system
Technical field
The present invention relates to communication technical field, be specifically related to the time synchronization method based on ofdm system.
Background technology
Modern Communication System extensively adopts orthogonal frequency division multiplexi (Orthogonal Frequency Division Multiplexing, OFDM) technology as one of physical layer standard.The basic principle of OFDM is that serial high speed circulation is become to low speed sub data flow, uses one group of mutually orthogonal subcarrier transmission data.The frequency spectrum of subchannel is quadrature and overlapped, so the OFDM availability of frequency spectrum is higher.
Because serial data is changed for low-speed parallel data, ofdm system can resist the multipath effectiveness of wireless channel effectively, but compares with single carrier communication, and it needs more accurate system synchronization.Time synchronized mistake not only can make the amplitude of signal and phase place produce distortion, also can cause producing and disturbing between each OFDM symbol, introduce ISI (Inter-Symbol Interference, intersymbol interference), destroy the integrality of symbol, thereby reduce the remarkable decline of systematic function.Traditional OFDM Timing Synchronization adds the training sequence of special construction conventionally before Frame, at receiving terminal, by the auto-correlation computation of input signal, gets the starting point that peak value obtains Timing Synchronization.This method can obtain good effect in the multi-path environment that non-multipath and the first footpath are most powerful path, but is not in the severe wireless channel environment of most powerful path in the first footpath, tends to cause delay regularly.Someone proposes the result of related operation to carry out read group total, and search maximum relevant and value is used as timing metric, and this method needs accurately to determine the size of search window, otherwise also can cause bad timing.
Summary of the invention
The object of the invention is to, the problem of moving after easily causing regularly for the existing Timing Synchronization technology of ofdm system under complicated multipath channel, a kind of time synchronization method of ofdm system is proposed, in order to improve the Stability and veracity of ofdm system Timing Synchronization under multipath channel.
Technical scheme of the present invention is:
The time synchronization method of ofdm system, the method comprises the steps:
1) at ofdm system transmitting terminal, produce random sequence A, by it, oppositely export records series B, form symmetrical structure training sequence S=[A B];
2) set up the OFDM baseband signal needing, the sequence that training sequence adopts step 1 to produce, is added on before data to be sent, and sends into channel;
3) signal that ofdm system receiving terminal receives carries out Timing Synchronization in time domain.
Further, described step 1) in, the OFDM symbol lengths that the concrete length of training sequence can be used as required determines.
Further, described step 2), OFDM baseband signal adopts multipath channel models.
Further, described step 3), Timing Synchronization can be specially following several step:
Step 101: while carrying out Timing Synchronization, the training sequence position of a certain symbol starts from receive ofdm signal, the data segment of respectively getting backward forward length and be L carries out related operation, and L is half of training sequence length in step 1; Utilize the energy that receives signal by the result normalization of related operation simultaneously, can obtain a computation measure M (t) like this to each sampled point, t is sampling number;
Relevant and the value V (t) of step 102: definition M (t), in formula, need to determine search window W, W should be about the maximum delay of channel;
Step 103, detects first trailing edge of V (t), and namely position regularly should be first and meets inequality V (t+1)-V (t)>=T hthe moment, T in formula hdetection threshold for definition, should be greater than interchannel noise average, and be less than multipath signal most powerful path peak value.
The invention has the advantages that, in OFDM receiving system Timing Synchronization, detect first trailing edge of timing metric rather than directly get peak value, can improve the Stability and veracity of Timing Synchronization, not bring obvious operand to promote simultaneously, and be applicable to complicated multidiameter fading channel.
Accompanying drawing explanation
Fig. 1 is general ofdm system link level model.
Fig. 2 is the data frame structure that the present invention adopts, and training sequence and data flow, consists of.
Fig. 3 is the concrete steps schematic diagram of Timing Synchronization of the present invention.
Fig. 4 utilizes computing cross-correlation to determine the design sketch of search box size.
Fig. 5 carries out the schematic diagram of summation operation to M (t).
Fig. 6 is the inventive method and the Contrast on effect of conventional method under complicated multi-path environment.
Embodiment
Below by reference to the accompanying drawings, to implementing example, elaborate.It should be noted that following explanation is only exemplary, is not construed as limiting the invention.
Fig. 1 is general purpose O FDM system architecture diagram, has comprised a basic module that complete ofdm system should possess.
Step 1: transmitting terminal is set ofdm system sub-carrier number 1024,128 of Cyclic Prefix, mapping mode is selected QPSK.Training sequence forms symmetrical structure by random sequence and its Inverse order sequence of one 128, is designated as T.Each frame data adds after training sequence, forms frame structure as shown in Figure 2, sends into channel and launches.
Step 2: signal, through multidiameter fading channel, can be subject to the interference effect of multipath effect and Gaussian noise.Wherein multipath channel number of path is 4, and the first footpath compares littlely with most powerful path energy, and the signal that receiving terminal receives is that 4 paths are subject to respectively after phase delay and power attenuation, and the result superposeing, is designated as r (t).
Step 3:OFDM system receiving terminal receives signal r (t), tackles it and carry out Time and Frequency Synchronization before demodulation, and the Timing Synchronization concrete grammar process that the present invention proposes as shown in Figure 3.After receiving signal, utilize formula carry out auto-correlation computation, obtain calculation of correlation M (t), wherein l is the length of training sequence.Utilize known training sequence T simultaneously and receive signal r (t) and carry out computing cross-correlation, the cross-correlation coefficient t calculating sthere is peak value, the Energy distribution that embodies multipath that can be rough in the sampled point that can distribute at multidiameter.Record last and be greater than threshold value T ht sworth position can be approximately equal to the maximum delay of channel, and choosing of threshold value can be determined according to actual conditions, is generally decided to be t s0.2 times of peak value.Fig. 4 has shown the effect that makes the channel maximum delay that obtains in this way.
The calculation window W that setting M (t) carries out summation operation is the channel maximum delay obtaining in upper step, determines after search window W, in W, M (t) is carried out to summation operation, as shown in Figure 5, obtains at search window
First trailing edge that detects V (t) in mouth W can obtain last Timing Synchronization result.Timing Synchronization result t timeshould be
First meets inequality V (t time+ 1)-V (t time)>=T hposition.
Experiment showed, and use the time synchronization method of the present invention's proposition to compare with conventional method, can obviously improve regularly Stability and veracity.Under identical multipath fading environments, by aforementioned concrete implementation step, conventional method and the inventive method regularly effect change as shown in Figure 6 with signal to noise ratio.Relatively can find, under this first unconspicuous complicated multi-path environment in footpath, conventional method timing results under less signal to noise ratio is insincere, and along with the increase of signal to noise ratio, the result of Timing Synchronization is close to most powerful path gradually, cannot accurately navigate to the first path position.The inventive method, after reaching signal to noise ratio requirement, can accurately navigate to the first footpath, and can not cause the mistake of Timing Synchronization.
Algorithm complex aspect, compares with conventional method, and the present invention has increased an add operation and fixing limited number of time subtraction on the basis of conventional method, the lifting that does not bring obvious amount of calculation, but can obtain good timing effect.
The above be only preferably embodiment of the present invention, but protection scope of the present invention is not limited to this.Be appreciated that protection scope of the present invention is as the criterion with the protection range of claim.

Claims (1)

  1. The time synchronization method of 1.OFDM system, is characterized in that, the method comprises the steps:
    1) at ofdm system transmitting terminal, produce random sequence A, by it, oppositely export records series B, form symmetrical structure training sequence S=[A B]; Described step 1), in, the OFDM symbol lengths that the concrete length of training sequence can be used as required determines;
    2) set up the OFDM baseband signal needing, the sequence that training sequence adopts step 1 to produce, is added on before data to be sent, and sends into channel; Described step 2) in, OFDM baseband signal adopts multipath channel models;
    3) signal that ofdm system receiving terminal receives carries out Timing Synchronization in time domain; Described step 3) in, Timing Synchronization can be specially following several step:
    Step 101: while carrying out Timing Synchronization, the training sequence position of a certain symbol starts from receive ofdm signal, the data segment of respectively getting backward forward length and be L carries out related operation, and L is half of training sequence length in step 1; Utilize the energy that receives signal by the result normalization of related operation simultaneously, can obtain a computation measure M (t) like this to each sampled point, t is sampling number;
    Relevant and the value V (t) of step 102: definition M (t), in formula, need to determine search window W, W should be about the maximum delay of channel;
    Step 103, detects first trailing edge of V (t), and namely position regularly should be first and meets inequality V (t+1)-V (t)>=T hthe moment, T in formula hdetection threshold for definition, should be greater than interchannel noise average, and be less than multipath signal most powerful path peak value, self adaptation definite threshold T h, utilize the training sequence T that receiving terminal is known and receive signal r and carry out computing cross-correlation, calculate cross-correlation coefficient t s, get threshold value T hfor t s0.2 times of peak value, records last and is greater than threshold value T ht sposition, the approximate maximum delay W as channel.
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CN103298100B (en) * 2013-06-19 2016-02-03 东南大学 For the time synchronization method of OFDM-WLAN system
KR101838415B1 (en) 2013-07-25 2018-04-26 엠파이어 테크놀로지 디벨롭먼트 엘엘씨 System for timing synchronization in an orthogonal frequency-division multiplexing (ofdm) system
CN104410597B (en) * 2014-11-20 2017-09-01 武汉邮电科学研究院 A kind of ofdm signal time-domain synchronizing method based on the synchronous header sequence of two-stage
EP3366068A1 (en) * 2015-10-23 2018-08-29 Telefonaktiebolaget LM Ericsson (PUBL) Apparatus and method for adaptive discovery signal measurement timing configuration
CN107969029B (en) * 2016-10-19 2021-06-29 华为技术有限公司 Awakening lead code generation method, synchronization method and device

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