CN102075486A - Synchronized method of orthogonal frequency division multiplexing (OFDM) system - Google Patents
Synchronized method of orthogonal frequency division multiplexing (OFDM) system Download PDFInfo
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Abstract
The invention provides a synchronized method of an orthogonal frequency division multiplexing (OFDM) system. The invention discloses an associated synchronized method of frame synchronization, carrier frequency synchronization and sampling clock synchronization. Carrier frequency offset estimation, channel estimation and fine estimation of symbol timing offset are performed again after sampling clock offset correction is finished, thereby avoiding influences of sampling clock offset on the carrier frequency offset estimation, the channel estimation and the fine estimation of the symbol timing offset, compensating phase deflection caused by the residual carrier frequency offset and the sampling clock offset simultaneously, greatly improving the synchronized performance, and meeting the demands of a high-order modulated system. In addition, due to aiming at a low time-varying channel, the channel estimation is required to be performed once only, thereby reliving the complexity of the system.
Description
Technical field
The invention belongs to communication technical field, relate to method for synchronous, relate in particular in the ofdm system, symbolization regularly synchronously, the method for synchronous of the ofdm system of carrier frequency synchronization and the synchronous three kinds of method of synchronization combined synchronization of sampling clock.
Background technology
OFDM(Orthogonal Frequency Division Multiplexing is an orthogonal frequency division multiplexi) system mainly comprise three homochronousness synchronously: timing synchronization (frame synchronization), carrier frequency synchronization, sampling clock is synchronous.In ofdm system, because path delay when receiving continuous data or burst frame, if the wrong starting position of having estimated symbol or frame then can cause the FFT window offset, is introduced data error.Transmitting terminal and receiving terminal oscillator frequency of oscillation do not match and cause the transmitting-receiving two-end carrier frequency not match, and introduce carrier frequency offset, can destroy the orthogonality between subcarrier, thereby cause inter-carrier interference (ICI).Because the transmitting-receiving two-end sampling clock is not quite identical, its sampling frequency deviation that causes can make the deflection of subcarrier generation phase place, introduces ICI simultaneously, cause the snr loss, also can cause slow symbol timing deviation in addition, it is inaccurate that FFT is windowed, and disturbs between created symbol (ISI).Therefore need carry out timing synchronization, carrier frequency synchronization and sampling clock are synchronous.For above three homochronousness, because the accuracy of The noise and algorithm itself can stay remaining estimated bias, these remaining deviations can impact ofdm system equally, particularly to having the ofdm system of high order modulation, influence more responsive.
Summary of the invention
The objective of the invention is provides a kind of timing synchronization (frame synchronization), carrier frequency synchronization, the combined synchronization method that sampling clock is synchronous at slow time varying channel environment and ofdm system with high order modulation.
In the method: be divided into synchronously thick and synchronously smart in the frame synchronization, thick find the general original position of frame by detecting thick synchronization decisions function greater than a certain threshold value synchronously, smart synchronously by in certain search window, finding the auto-correlation maximal peak point as smart synchronous points.Carrier frequency synchronization is according to disposable fractional part of frequency offset and the integer several times frequency deviation of estimating of the error range of carrier frequency, do frequency offset correction then, sampling clock carries out sample rate conversion to sampled data synchronously after estimating sample clock frequency deviation, sampled value after the conversion is done Frequency Synchronization and regularly synchronously smart again, last for residual carrier frequency offset, sample clock frequency deviation compensates by the mode of disposable phase compensation, have high synchronization accuracy, can fully satisfy the demand of ofdm system with high order modulation.
Provide a kind of frame synchronization in the technical scheme of the present invention, carrier frequency synchronization, the combined synchronization method that sampling clock is synchronous.Finish and done Nonlinear Transformation in Frequency Offset Estimation again after sampling clock deviation is proofreaied and correct, channel estimating, symbol is regularly synchronously smart, avoided sampling clock deviation to Nonlinear Transformation in Frequency Offset Estimation, the influence that channel estimating and symbol are regularly smart synchronous, compensate for residual carrier wave frequency deviation and the caused phase place deflection of sampling clock deviation simultaneously, promoted synchronization performance greatly, satisfy the demand of High Order Modulation System.In addition since at be slow time varying channel, channel estimating only need be carried out once, thereby has alleviated the complexity of system.
Below in conjunction with description of drawings and embodiment, the present invention is described in further detail.
Description of drawings
Fig. 1 is the training sequence structure figure that uses in the embodiment of the invention 1.
Fig. 2 is a combined synchronization flow chart of the present invention.
Embodiment
Embodiment 1, present embodiment provide method synchronous in a kind of ofdm system, be provided for realizing synchronously and the short training sequence and the long training sequence of channel estimation function at transmitting terminal, the structure of training sequence can be received these sequences successively at receiving terminal as shown in Figure 1.Need to insert in data symbol and be used for the discrete frequency domain pilot tone that phase place deflection is estimated, the position of scattered pilot is about 0 frequency symmetry, and uniformly-spaced.Processing procedure as shown in Figure 2, the representative of frame of broken lines and dotted line uses training sequence to handle among the figure, wherein the representative of thick dashed line frame and thick dashed line needs to handle twice, solid box and solid line representative receive data symbol and handle.Treatment step is as follows:
The first step: use preceding two short training sequence A to do timing coarse synchronization, do the time-delay auto-correlation according to preceding two short training sequences and calculate, utilize the periodicity of training sequence, time-delay D equals the cycle of training sequence, L is the length of training sequence, and r is the time-domain sampling value of received signal.Calculating parameter
,
, the decision function that timing coarse synchronization detects is
,
Done energy normalized, irrelevant with signal energy.It is 0.75 that threshold value is set, when
0.75 o'clock, the arrival of representative information frame.
Second step: do carrier frequency offset estimation with any 2L continuous sampling point among first three short training sequence A, establish the transmitting terminal time-domain signal and be
, the receiving terminal time-domain signal is
, under the situation that has frequency deviation, have
,
Be the normalization carrier frequency offset, N is an IFFT length, the definition intermediate variable
, the estimated value that obtains the normalization carrier frequency offset is:
The frequency deviation region of estimating is
According to the maximum magnitude of the carrier wave frequency deviation of particular system, we get D=N/4 here, so the carrier frequency offset that can estimate is
, can disposable integer frequency bias and the decimal frequency bias of estimating.Estimate after the carrier wave frequency deviation, use
Carrying out carrier wave frequency deviation to received signal corrects.
The 3rd step: use the 4th the short training sequence B and the B of local storage to do the synchronously smart of computing cross-correlation achieve frame,
, in certain search window, find
Getting the maximum place is smart synchronized sampling point place.
The 4th step: use first and second long training sequences to carry out channel estimating, earlier first and second training sequences are carried out FFT and transform to frequency domain, adopt the LS algorithm to carry out channel estimating then.Channel estimation value is got the average of two training sequence channel estimation values.
The 5th step: uses third and fourth long training sequence to carry out channel equalization, carry out sampling clock deviation then and estimate, sampled value use polynomial interopolation filter is carried out sample rate conversion after estimating sampling clock deviation.
The 6th step: repeated for the two the third and fourth steps, reject the influence of sampling clock deviation and obtain the carrier frequency offset value again, channel estimation value and smart synchronous points.
The 7th step: the processing that enters data symbol, the data symbol is carried out sampling clock deviation to be proofreaied and correct, carrier frequency transforms to frequency domain through FFT after proofreading and correct, carrying out phase place deflection by pilot tone estimates, estimate because residual sampling clock deviation, phase compensation is carried out in the phase place deflection that carrier frequency offset causes then.
Processing procedure finishes.
Concrete; if FFT length N=256 in the ofdm system; the protection gap length is 16; effectively the subcarrier number is 210; short training sequence A is 64 pseudo random sequence is passed through 64 IFFT generation through BPSK mapping back at frequency domain a time-domain training sequence; short training sequence B is 64 pseudo random sequence is passed through 64 IFFT generation through BPSK mapping back at frequency domain a time-domain training sequence; long training sequence T1; T2; T3; T4 is that 210 pseudo random sequences are modulated on 210 effective subcarriers after frequency domain is through the BPSK mapping, produces time domain sequences by 256 IFFT, adds that 16 point protections at interval.T1, T2, T3, the T4 value is identical.
Transmitting terminal according to Fig. 1 in the order of each training sequence send these training sequences successively, and in data symbol, insert 8 scattered pilots about 0 frequency symmetry, pilot tone adopts 8 pseudo random sequences through the BPSK mapping.
At receiving terminal, at first carry out the time domain timing coarse synchronization according to step 1, use preceding two short training sequence A to do timing coarse synchronization, time-delay D is 64, L is 64, calculating parameter
,
, the decision function that timing coarse synchronization detects is
, detect and work as
0.75 o'clock, the arrival of representative information frame.
According to step 2, do carrier frequency offset with any 128 the continuous sampling points among first three short training sequence A and estimate then, calculate intermediate variable
, the estimated value that obtains the normalization carrier frequency offset is:
, estimate after the carrier wave frequency deviation, use
The received signal of back is carried out carrier wave frequency deviation corrects.
Use the 4th the short training sequence B and the B of local storage to do the synchronously smart of computing cross-correlation achieve frame according to step 3 again, in the search window of L=128, find
Get the maximum place and be smart synchronized sampling point place.
Use T1 and T2 to transform to frequency domain according to step 4 then, adopt the LS algorithm to carry out channel estimating then with FFT.Channel estimation value is got T1, the average of T2 channel estimating.To T3, T4 carries out channel equalization according to step 5, carries out sampling clock deviation then and estimates, uses the polynomial interopolation filter to carry out sample rate conversion to sampled value after estimating sampling clock deviation.Repeating step two, three, four obtains the carrier frequency offset value again, channel estimation value and smart synchronous points.After desired parameters obtains and finishes the data symbol that receives is carried out sampling clock deviation and proofread and correct, carrier frequency is proofreaied and correct, and the FFT conversion after frequency domain carries out channel equalization, uses 8 pilot tone sign estimation to go out phase place deflection, carries out phase compensation then.
Claims (6)
1. the method for synchronous in the ofdm system, this method is at slow time varying channel environment and has the timing synchronization that comprises that the ofdm system of high order modulation provides, carrier frequency synchronization, the method of the combined synchronization that sampling clock is synchronous, it is characterized in that: be provided for realizing synchronously and the training sequence of channel estimation function at transmitting terminal, can receive these training sequences successively at receiving terminal; May further comprise the steps:
Steps A, to carry out frame slightly synchronous, and described is by detecting thick synchronization decisions function finds frame greater than a certain threshold value general original position synchronously slightly;
Step B, carry out that carrier frequency offset is estimated and correct;
The essence of step C, achieve frame is synchronous, and described essence is by finding the auto-correlation maximal peak point as smart synchronous points in certain search window synchronously;
Step D, carry out channel estimating;
Step e, the channel equalization of carrying out, sampling clock deviation is estimated and sample rate conversion;
Step F, repeating step B, step C, step D; Again obtain the carrier frequency offset value, channel estimation value and smart synchronous points;
Step G, carry out the processing of data symbol, the data symbol is carried out sampling clock deviation to be proofreaied and correct, carrier frequency transforms to frequency domain through FFT after proofreading and correct, carrying out phase place deflection by pilot tone estimates, estimate because residual sampling clock deviation, phase compensation is carried out in the phase place deflection that carrier frequency offset causes then.
2. the method for synchronous in a kind of ofdm system according to claim 1, it is characterized in that: described training sequence comprises 4 short training sequences and 4 long training sequences, and described 4 short training sequences comprise that time-domain training sequence (A) that 3 64 pseudo random sequence produces through the IFFT of BPSK mapping back by 64 at frequency domain and 1 pseudo random sequence that is at 64 pass through the time-domain training sequence (B) of 64 IFFT generation through BPSK mapping back at frequency domain; Described long training sequence is that 210 pseudo random sequences are modulated on 210 effective subcarriers after frequency domain is through the BPSK mapping, produces time domain sequences by 256 IFFT, adds 16 point protections long training sequence (T1, T2, T3, T4) at interval.
3. the method for synchronous in a kind of ofdm system according to claim 2 is characterized in that: in the described steps A by following carry out step by step thick synchronous:
Steps A 1, be calculated as follows first parameter (
) and second parameter (
);
In the formula: D is the cycle (T) that time-delay equals described short training sequence (A), and L is the length of described short training sequence (A), and r is the time-domain sampling value of received signal;
Steps A 2, the decision function that detects by following timing coarse synchronization, and to result of calculation
Carry out normalized:
And threshold value is set is 0.75, when
0.75 o'clock, the arrival of representative information frame realizes synchronously thick.
4. the method for synchronous in a kind of ofdm system according to claim 3 is characterized in that: among the step B:
Step B1, do carrier frequency offset with the individual continuous sampling point of the length (2L) of any twice short training sequence in first three short training sequence (A) in the described short training sequence (A) and estimate, according to the transmitting terminal time-domain signal (
) be calculated as follows the receiving terminal time-domain signal (
):
In the formula:
Be the normalization carrier frequency offset;
Step B2, press following formula definition intermediate variable (
):
Here N is an IFFT length;
Step B3, obtain the normalization carrier frequency offset estimated value (
) be:;
The frequency deviation region of step B4, estimation is
Step B5, estimate after the carrier wave frequency deviation, use
Carrying out carrier wave frequency deviation to received signal corrects.
5. the method for synchronous in a kind of ofdm system according to claim 4, it is characterized in that: among the step C: use the 4th short training sequence (B) in the described short training sequence (A) and the identical short training sequence (B) of local storage to do the synchronously smart of computing cross-correlation achieve frame
, in certain search window, find
Getting the maximum place is smart synchronized sampling point place.
6. the method for synchronous in a kind of ofdm system according to claim 4 is characterized in that: among the step D: use that first long training sequence (T1) and second long training sequence (T2) carry out channel estimating in the described long training sequence,
Step D1: first and second training sequences are carried out FFT transform to frequency domain,
Step D2, employing LS algorithm carry out channel estimating, and channel estimation value is got the average of two long training sequence channel estimation values in this step.
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