CN100574509C

CN100574509C - A kind of improved method for synchronizing time

Info

Publication number: CN100574509C
Application number: CNB2007100245140A
Authority: CN
Inventors: 徐大专; 王磊
Original assignee: Nanjing University of Aeronautics and Astronautics
Current assignee: Nanjing University of Aeronautics and Astronautics
Priority date: 2007-06-21
Filing date: 2007-06-21
Publication date: 2009-12-23
Anticipated expiration: 2027-06-21
Also published as: CN101094517A

Abstract

The present invention relates to the time synchronization problem of communication technical field.The present invention is directed to the influence of noise in the signal and frequency deviation to net synchronization capability, proposition is designed to the short segment of a plurality of length with synchronizing sequence, and received signal is carried out related operation with each segment respectively, with consequent many correlation curve delayed additions, find relevant peaks, finish synchronously.This method can overcome well noise and fixedly frequency deviation to the influence of net synchronization capability.Simultaneously, at the structure of this method, designed the method for practicable simplification operand.

Description

A kind of improved method for synchronizing time

Technical field

The present invention relates to wireless communication field, particularly the time synchronization problem in wireless communication field.

Background technology

The time synchronized module is used for finding out the original position of useful signal in communication system, is the first step of system's reliability service.The channel quality of wireless channel is relatively poor relatively, not only exists serious multipath fading phenomenon and also has bigger noise jamming, and typical in the short wave communication channel, signal to noise ratio can be hanged down-3dB.Because the carrier frequency of transmitter and receiver can not be in full accord or be had Doppler frequency shift, cause existence frequency shift (FS) to a certain degree in the received signal after the mixing in addition.When utilizing local sequence synchronous, frequency shift (FS) is very big to the influence of net synchronization capability.

Method for synchronizing time commonly used mainly contains two classes:

(1) utilize the correlation of received signal self to carry out synchronously.Send the very strong sequence of two sections repetitions or correlation earlier as training sequence before sending signal, receiving terminal is relevant before and after input signal is done, and seeks relevant peaks.Such as ofdm system, can utilize the Cyclic Prefix of OFDM symbol and the repeatability of symbol itself to do synchronously.This homochronousness algorithm has been owing to utilized correlation before and after the signal self, offset fixedly frequency deviation to the influence of correlation.But all passed through channel owing to participate in two segment signals of related operation itself, a large amount of noise that superposeed is so under the relatively poor situation of channel quality, particularly in the remote-wireless channel, synchronization performance is poor.And because the existence of part correlation, the correlation curve that this method is tried to achieve is slowly to change, and gives to seek accurately relevant peaks and bring difficulty.

(2) utilize received signal and the correlation that is stored in local signal to carry out synchronously.Send one section specific training sequence earlier before sending signal, and sequence is stored in this locality, receiving terminal is made related operation with the training sequence of received signal and local storage, obtains the position of relevant peaks.This method is not because local sequence has superimposed noise, so even under the situation of low signal-to-noise ratio, can obtain bigger relevant peaks yet.But the method is to fixedly frequency deviation is very responsive.Frequency deviation can make the phase difference of received signal and local sequence along with the time becomes greatly gradually, thereby causes the correlation of synchronous head integral body sharply to descend, and when the excursion of phase difference reached 2 π, correlation disappeared substantially.

Summary of the invention

The present invention is directed to the pluses and minuses of the 2nd class methods, proposed a kind of improved time synchronized algorithm, and optimize at the operand of this algorithm.

Synchronous head partly is divided into original one section synchronizing sequence N isometric segment.It is thereby that received signal is done relevantly to find common relevant peaks to determine sync bit then with the short local sequence of N section respectively that receiving terminal is realized synchronous principle.Because received signal is to make related operation with local sequence, so still can overcome the influence of bigger noise jamming to net synchronization capability.Simultaneously every section local sequence length is all very short, so fixedly the excursion of the phase deviation that causes in every section of frequency deviation can be very not big, needs only excursion less than π, and every segment data and local sequence still can keep bigger correlation.

If but directly implement according to the method, received signal will be made the N correlations with local sequence, and operand can be quite big.Consider that doing relevant with the local sequence of N section all is same receiving sequence, so the employing shortcut calculation.Way is as follows:

At first store the frequency domain data of local sequence and the energy value of every segment data at receiving terminal.Be respectively that the local sequence of M is got conjugation and reversed with the N segment length, mend L point zero then and do to preserve after the L+M point FFT computing.Wherein counter-rotating is for related operation equivalence is become convolution algorithm, and zero padding is to replace the requirement of linear convolution to sequence length in order to satisfy with circular convolution.Obtain the energy value of every section local sequence then, be stored in receiving terminal, be used for participating in the normalization computing.These work can be finished in advance, so do not need the consumption systems amount of calculation in when operation.

After receiving terminal receives signal, earlier signal is pressed the segmentation of L point.According to the thought of overlap-save method, with each section L point data and the last period the end the M point data make L+M point FFT (guarantee that L+M is 2 power, first segment data is mended M zero in front) together.The result of FFT is multiplied each other with the N section frequency domain data of this locality respectively, at last multiplied result is remake IFFT, M point before giving up just can be obtained the correlation that N segment length is L.

Ask then input data each point square, obtain the corresponding energy of importing data of each correlation with the method for recursion, will import the data energy and local sequence energy is asked geometric average, draw and be used for normalized energy value.Divided by energy, just obtain normalized correlation with the correlation of trying to achieve previously.

Ask the mean value of N section normalization correlation again, because N section synchronizing sequence sends successively, so peak value also occurred successively when received signal was relevant with the local sequence work of each section, so with different the counting of each section normalization correlation time-delay, the 1st section normalization correlation time-delay M* (N-1) point, the 2nd section normalization correlation time-delay M* (N-2) point, and the like, with asking average after the normalization correlation addition after the time-delay, just can obtain final normalization correlation.

Set thresholding at last, final normalization correlation is carried out peak value detect, find out the position of peak value, and then obtain the original position of synchronous head, the deadline is synchronous.

N wherein, M, L is positive integer, and establishing local sequence length overall is D, and then N must aliquot D, and M equals D/N, and L+M is that 2 power is to satisfy the requirement of FFT computing.

The invention has the advantages that: 1, this synchronized algorithm becomes the short segment of a lot of length with local sequences Design, both kept with local sequence do relevant can antimierophonic advantage, solved the sharply problem of decline of the whole correlation that draws because of frequency deviation again.2, for the related operation part of input signal and N segment data, the present invention is translated into 1 FFT computing and N IFFT computing, so just can utilize unified FFT module to improve the synchronous efficiency of system greatly.For the normalization computing of correlation, the present invention calculates earlier import the data each point square, utilize the method for recursion to calculate the interior energy of the corresponding sliding window of each reference point then, can avoid double counting to energy value.

Description of drawings

Fig. 1 is basic principle figure of the present invention

Fig. 2 is a concrete enforcement block diagram of the present invention

Fig. 3 be do before and after the received signal relevant method and emulation comparison diagram of the present invention (signal to noise ratio :-3dB)

Fig. 4 is received signal and relevant method and the emulation comparison diagram (frequency deviation: 80Hz) of the present invention of local work

Fig. 5 is a simulation result figure of the present invention (signal to noise ratio :-3dB frequency deviation: 80Hz)

Specific implementation method

The present invention is verified at an ofdm system that is used for short wave communication, provides the parameter concrete configuration of system synchronization part below, and sets forth specific implementation step of the present invention.

The synchronous head parameter:

Sample rate: 40.96KHz

Signal bandwidth: 12KHz

Every section synchronizing sequence length: 256 points

Synchronizing sequence hop count: 8 sections

The receiving sequence segmentation is counted: 1792 points

After at first the counter-rotating of 8 sections 256 synchronizing sequences being got conjugation zero padding becoming remake FFT at 2048 then, be stored in the local reception end, store the quadratic sum of local each section of sequence simultaneously.

Synchronizing process is divided into following steps:

Step 1: receiving sequence is divided into one section according to 1792, every segment data again with the last period data 256 at end form 2048 point sequences (the 1st segment data front zero padding).

Step 2: 2048 point sequences after the segmentation are done to multiply each other with 8 sections local sequences respectively behind the FFT, again multiplied result is made IFFT respectively, cast out front 256 points, can obtain 8 sections 1792 correlation curve.

Step 3: calculate every of input data square, obtain the interior energy value of the corresponding sliding window of every spot correlation value with the method for recursion, obtain the energy value of correlation correspondence with the energy value of the local sequence evolution that multiplies each other respectively again.Correlation curve with the energy value normalization of trying to achieve, is obtained normalized correlation.

Step 4:, then N section normalization correlation delayed addition is asked average with the storage of each section normalization correlation.Normalization correlation time-delay 256* (8-1) point of the 1st section synchronizing sequence correspondence, normalization correlation time-delay 256* (8-2) point of the 2nd section synchronizing sequence correspondence, and the like, just obtain final normalization correlation curve after averaging.

Step 5: setting sync correlation peak thresholding is 0.16, seeks peak point, finishes synchronously.

Provide simulation result below:

Fig. 5 is simulation result figure.Simulated conditions is signal to noise ratio-3dB, two footpaths, and time-delay 2ms, Doppler expands 1Hz, frequency deviation 80Hz.Simulation result shows, under the very poor situation of channel condition, still can obtain bigger relevant peaks.

Claims

1, a kind of method for synchronizing time of radio communication comprises the steps:

(1), local sequence is divided into N isometric segment, every little segment length is M, carry out L+M point FFT computing to generate the frequency domain data of N segment after respectively L individual zero being mended in every segment conjugation counter-rotating again, receiving terminal is stored the energy value of these frequency domain datas and every segment in advance, and establishing local sequence length overall is D, and N is a positive integer, and N aliquot D, M equals D/N, and L is a positive integer also, and guarantees that L+M is 2 power;

(2), according to overlap-save method, carry out segmentation to receiving data by the L point earlier, every section L point data and the last period the end the M point data carry out L+M point FFT computing together, wherein M individual zero is mended in the front of first segment data, carry out the IFFT computing more respectively after the result of computing multiplied each other with the frequency domain data of N segment of this locality storage respectively, give up front M point, can obtain the correlation of N segment length for L;

(3), ask receive the data each point square, obtain the corresponding energy that receives data of each correlation with the method for recursion, with the geometrical mean of each section correlation in the step (2), thereby obtain the normalized correlation of N section divided by reception data energy and local sequence energy;

(4), the addition again of delaying time respectively of the N section normalization correlation in the step (3) is asked on average, find out its peak value, obtain the original position of signal with the method that peak value detects.

2, method for synchronizing time according to claim 1, it is characterized in that local sequence is divided into N isometric segment, the normalization correlation of N the isometric segment that calculating reception data and local sequence are divided into, obtain N section normalization correlation, ask average after again N section normalization correlation being delayed time respectively, find the original position of signal according to the peak value of the normalization correlation curve after average.

3, method for synchronizing time according to claim 1, combine the thought of overlap-save method in the fast convolution when it is characterized in that calculating the isometric segment normalization correlation of N of receiving that data and local sequence be divided into, the computing of N correlations is converted into 1 FFT computing and N IFFT computing.