CN102377726A - Timing synchronization method of OFDM (Orthogonal Frequency Division Multiplexing) system - Google Patents

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 time synchronization method based on ofdm system.

Background technology

Modern Communication System extensively adopts orthogonal frequency division multiplexi, and (Orthogonal Frequency Division Multiplexing, OFDM) technology is as one of physical layer standard.The basic principle of OFDM is that the serial high speed circulation is become the low speed sub data flow, uses one group of mutually orthogonal subcarrier transmission data.The frequency spectrum of subchannel is a quadrature and overlapped, so the OFDM availability of frequency spectrum is higher.

Because serial data is changed for the 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.The time synchronized mistake not only can make the amplitude of signal and phase place produce distortion; Also can cause producing between each OFDM symbol and disturb, introduce ISI (Inter-Symbol Interference, intersymbol interference); Destroy the integrality of symbol, thereby reduce the remarkable decline of systematic function.Traditional OFDM regularly adds the training sequence of special construction synchronously usually before Frame, get peak value at receiving terminal through the auto-correlation computation of input signal and obtain regularly synchronous starting point.This method for can obtaining good effect in the multi-path environment of most powerful path, but is not in the abominable wireless channel environment of most powerful path in first footpath in non-multipath and first footpath, tends to cause delay regularly.Someone proposes the result of related operation is carried out anded, and search maximum relevant and value is used as regularly tolerance, and this method needs accurately to confirm the size of search window, otherwise also can cause bad timing.

Summary of the invention

The objective of the invention is to; The problem of moving after being prone to cause regularly to the existing regularly simultaneous techniques of ofdm system under the complicated multipath channel; A kind of time synchronization method of ofdm system is proposed, in order to improve regularly synchronous accuracy and the stability of ofdm system under the multipath channel.

Technical scheme of the present invention is:

The time synchronization method of ofdm system, this method comprises the steps:

1) produces random sequence A at the ofdm system transmitting terminal,, form symmetrical structure training sequence S=[A B] by its reverse output records series B;

2) the OFDM baseband signal that needs of foundation, the sequence that training sequence adopts step 1 to produce, be added on data to be sent before, and send into channel;

3) signal of ofdm system receiving terminal reception carries out timing synchronously in time domain.

Further, in the said step 1), the OFDM symbol lengths decision that the concrete length of training sequence can be used as required.

Further, the OFDM baseband signal adopts multipath channel models said step 2).

Further, regularly can be specially following several steps synchronously in the said step 3):

Step 101: carry out timing synchronously the time, the training sequence position of a certain symbol begins from receive ofdm signal, and the data segment of respectively getting length forward backward and be L carries out related operation, and L is the half the of training sequence length in the step 1; Utilize the as a result normalization of the energy of reception signal with related operation simultaneously, can obtain a computation measure M (t) to each sampled point like this, t is a sampling number;

Relevant and the value V (t) of step 102: definition M (t); In

formula; Need to confirm search window W, W should be about the maximum delay of channel;

Step 103 detects first trailing edge of V (t), and just regularly position should be first and satisfies inequality V (t+1)-V (t)>=T _hThe moment, T in the formula _hBe the detection threshold of definition, should be greater than the interchannel noise average, and less than multipath signal most powerful path peak value.

The invention has the advantages that; In the OFDM receiving system is regularly synchronous, detect regularly first trailing edge of tolerance rather than directly get peak value; Can improve regularly synchronous accuracy and stability, not bring tangible operand to promote simultaneously, and be applicable to complicated multidiameter fading channel.

Description of drawings

Fig. 1 is general ofdm system link level model.

Fig. 2 is the data frame structure that the present invention adopts, and is made up of training sequence and data flow.

Fig. 3 is the regularly synchronous concrete steps sketch map of the present invention.

Fig. 4 utilizes computing cross-correlation to confirm the design sketch of search window size.

Fig. 5 is the sketch map that M (t) is carried out summation operation.

Fig. 6 is the inventive method and the effect comparison of conventional method under complicated multi-path environment.

Embodiment

Hereinafter combines accompanying drawing, elaborates to implementing example.It should be noted that following explanation only is exemplary, is not to be construed as limiting the invention.

Fig. 1 is a 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, and mapping mode is selected QPSK.Training sequence constitutes symmetrical structure by one 128 random sequence with its reverse sequence, is designated as T.Each frame data constitutes frame structure as shown in Figure 2 after adding training sequence, sends into channel and launches.

Step 2: signal can receive the interference effect of multipath effect and Gaussian noise through multidiameter fading channel.Wherein the multipath channel number of path is 4, and first footpath compares lessly with the most powerful path energy, and the signal that receiving terminal receives is after 4 paths receive phase delay and power attenuation respectively, and the result who superposes is designated as r (t).

Step 3:OFDM system receiving terminal receives signal r (t), before demodulation, tackles it and carries out Time and Frequency Synchronization, and the synchronous concrete grammar process of timing that the present invention proposes is as shown in Figure 3.Utilize formula

to carry out auto-correlation computation after receiving signal; Obtain calculation of correlation M (t), wherein

L is the length of training sequence.Utilize known training sequence T and reception signal r (t) to carry out computing cross-correlation simultaneously,

The cross-correlation coefficient t that calculates _sCan the Energy distribution that embodies multipath that can be rough peak value appear, at the sampled point that multidiameter distributes.Write down last 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 confirmed according to actual conditions, generally is 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 that obtains in the last step; After confirming search window W; In W, M (t) is carried out summation operation; As shown in Figure 5, obtain

in search window

First trailing edge that detects V (t) in the mouth W can get timing synchronized result to the end.Timing synchronized result t _TimeShould be

First satisfies inequality V (t _Time+ 1)-V (t _Time)>=T _hThe position.

Experiment showed, and use the time synchronization method of the present invention's proposition to compare, can obviously improve regularly accuracy and stability with conventional method.Under identical multipath fading environments, through aforementioned practical 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 the unconspicuous complicated multi-path environment in this first footpath, conventional method timing results under less signal to noise ratio is insincere, and along with the increase of signal to noise ratio; Regularly synchronous result gradually to most powerful path near, can't accurately navigate to first path position.The inventive method can accurately navigate to first footpath, and can not cause regularly synchronous mistake after reaching the signal to noise ratio requirement.

The algorithm complex aspect is compared 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 tangible amount of calculation, but can obtain good timing effect.

The above be merely the preferable embodiment of the present invention, but protection scope of the present invention is not limited thereto.Be appreciated that protection scope of the present invention is as the criterion with the protection range of claim.

Claims (4)

1.OFDM the time synchronization method of system is characterized in that, this method comprises the steps:

1) produces random sequence A at the ofdm system transmitting terminal,, form symmetrical structure training sequence S=[A B] by its reverse output records series B;

2) the OFDM baseband signal that needs of foundation, the sequence that training sequence adopts step 1 to produce, be added on data to be sent before, and send into channel;

3) signal of ofdm system receiving terminal reception carries out timing synchronously in time domain.

2. the time synchronization method of ofdm system according to claim 1 is characterized in that, in the said step 1), and the OFDM symbol lengths decision that the concrete length of training sequence can be used as required.

3. the time synchronization method of ofdm system according to claim 1 is characterized in that, said step 2) middle OFDM baseband signal employing multipath channel models.

4. the time synchronization method of ofdm system according to claim 1 is characterized in that, regularly can be specially following several steps synchronously in the said step 3):

Step 101: carry out timing synchronously the time, the training sequence position of a certain symbol begins from receive ofdm signal, and the data segment of respectively getting length forward backward and be L carries out related operation, and L is the half the of training sequence length in the step 1; Utilize the as a result normalization of the energy of reception signal with related operation simultaneously, can obtain a computation measure M (t) to each sampled point like this, t is a sampling number;

Relevant and the value V (t) of step 102: definition M (t); In

formula; Need to confirm search window W, W should be about the maximum delay of channel;

Step 103 detects first trailing edge of V (t), and just regularly position should be first and satisfies inequality V (t+1)-V (t)>=T _hThe moment, T in the formula _hBe the detection threshold of definition, should be greater than the interchannel noise average, and less than multipath signal most powerful path peak value.

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