CN102571676B - Method for accurately estimating frame synchronization and frequency offset in orthogonal frequency division multiplexing (OFDM) system - Google Patents

Abstract

The invention discloses a method for accurately estimating frame synchronization and frequency offset in an orthogonal frequency division multiplexing (OFDM) system and relates to the field of OFDM. The method comprises the following steps of: fetching the OFDM data of a frame, performing average removal on each character, performing complex exponential modulation on each character, adding a pseudo random binary sequence (PRBS) frame header to the OFDM data and adding a complex exponential sequence to the PRBS frame header, and transmitting the OFDM data to a channel for transmitting; performing a fast Fourier transform (FFT) operation on the data in a receiving window, comparing the modulus square maximum of an FFT operation result with a setting threshold, obtaining a primary frequency offset estimator according to the position offset of the modulus square maximum, and compensating the primary frequency offset estimator; performing a cross correlation operation on the data in a new window and a local known PRBS, comparing the modulus square maximum of a cross correlation operation result with the setting threshold, obtaining the position of the frame header, dividing characters, and calculating the average of the characters; and performing the FFT operation on an average sequence, obtaining a secondary frequency offset estimator according to the position offset of the modulus square maximum of an FFT operation result, and compensating the secondary frequency offset estimator. According to the method, the accuracy positioning of the frame synchronization and the frequency offset compensation is realized by two-time frequency offset estimations and related operations, and the performance of the whole OFDM system is improved.

Description

The method that frame synchronization of OFDM system and frequency deviation are accurately estimated

Technical field

The present invention relates to OFDM (Orthogonal Frequency Division Multiplexing, OFDM) field, particularly relate to a kind of method that in ofdm system, frame synchronization and frequency deviation are accurately estimated.

Background technology

In ofdm system, frame synchronization is the starting point of all algorithms, first must determine that the original position of each frame data could carry out various follow-up algorithm smoothly accurately.Generally by PRBS (Pseudo-Random Binary Sequence, pseudo-random binary sequence) as frame head, because PRBS has good autocorrelation performance: its auto-correlation function R (τ) is when τ=0, and its mould has larger peak value; And during τ ≠ 0, its mould is almost nil.Therefore the position of frame head accurately can be judged by arranging thresholding.Be that (L is generally 2 to L with length ^l, l is integer) PRBS be example, when it is 0, export as-1-i; When it is 1, export as l+i; Form the synchronization timing frame head [a that length is L ₁, a ₂..., a _l], be L (being determined by transmission sequence synchronization frame head) in receiving terminal setting receive window length, its conjugate sequence with local frame head is done computing cross-correlation; When the frame head received aligns with local conjugate sequence, its cross-correlation output valve is:

$\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{a}_i\×a_i^{*}=\underset{i=1}{\overset{L}{\mathrm{\Σ}}}2=2L---\left(1.1\right)$

Get square judging of its mould under normal circumstances, therefore final output valve is 4L ², its value, much larger than the correlation under other conditions, therefore can determine the position of its frame head by setting thresholding.

But when coherent reception, the centre frequency and the local synchronization centre carrier frequency that send sequence carrier wave have certain difference, therefore cause producing certain frequency shift (FS) in the final signal recovered, frequency deviation shows as the sequence received in time domain can a linear phase difference in phase place, as: [a ₁e ^{j [2 π Δ f Δ t+ α]}, a ₂e ^{j [2 π Δ f (2 Δ t)+α]}..., a _ne ^{j [2 π Δ f (L Δ t)+α]}], wherein Δ f is frequency deviation, and Δ t is sample interval, and α is initial phase.Under the condition having frequency deviation, its Output rusults relevant to local conjugate sequence is:

$\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{a}_i\×{a_i^{*}e}^{j[2\mathrm{\π\Δf}\left(\mathrm{i\Δt}\right)+\mathrm{\α}]}=2e^{\mathrm{j\α}}\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{e}^{j2\mathrm{\π\Δfi\Δt}}=2e^{j(2\mathrm{\π\Δf\Δt}+\mathrm{\α})}\frac{1-e^{j2\mathrm{\πL}\frac{\mathrm{\Δf}}{\mathrm{fs}}}}{1-e^{j2\mathrm{\π}\frac{\mathrm{\Δf}}{\mathrm{fs}}}}---\left(1.2\right)$

Wherein fs is sample frequency, and when frequency difference is larger, the peak value of the mould of output square reduces even not determine, shown in Figure 1, therefore the method judged like this lost efficacy, and simultaneously because frequency difference itself also can produce harmful effect to the signal of subsequent treatment, therefore needed to be estimated and compensate.

Therefore the method for auto-correlation frame synchronization is applied, i.e. SC (Schmidl & Cox, timing and frequency Combined estimator) algorithm, and the make a start structure of frame head of the method is [a ₁, a ₂..., a _l, a ₁, a ₂..., a _l], and change receive window into 2L at receiving terminal, a front L data and a rear L data are done the computing cross-correlation of plural number, its result is:

$\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{a}_i{e}^{j[2\mathrm{\π\Δf}\left(\mathrm{i\Δt}\right)+\mathrm{\α}]}\×a_i^{*}{e}^{j[2\mathrm{\π\Δf}\left((-L-i)\mathrm{\Δt}\right)-\mathrm{\α}]}=2\underset{i=1}{\overset{L}{\mathrm{\Σ}}}{e}^{-j2\mathrm{\π\ΔfL\Δt}}=2{\mathrm{Ne}}^{-j2\mathrm{\π\ΔfL\Δt}}---\left(1.3\right)$

To its ask mould square, obtain also for 4L ²but owing to being the result of cross-correlation, when its front L data and a rear L data misplace on a small quantity, also have larger modulus value to export, therefore cause the cross-correlation modular curve of output to be one slowly to rise and the curve slowly declined, and when signal to noise ratio is not good, have a larger peak value platform, give and judge that frame synchronization accurate location brings difficulty.Need by autocorrelative result e for estimation frequency difference simultaneously ^{-j2 π Δ fL Δ t}calculate, but for FPGA (Field Programmable Gate Array, field programmable gate array) etc. actual treatment circuit, the normal complex calculations process of its L/mono-right and wrong is asked for plural number, and the restriction of sampling resolution causes its estimated accuracy also can not be very high, do not get rid of the effect of noise simultaneously in estimation procedure, will inevitably affect the accuracy that it is estimated, the frequency deviation obtained can not be compensated accurately finally to make it estimate.

Summary of the invention

The object of the invention is the deficiency in order to overcome above-mentioned background technology, a kind of method that frame synchronization of OFDM system and frequency deviation are accurately estimated is provided, complete the accurate location to frame synchronization and compensate of frequency deviation by twice frequency deviation estimation and related operation, thus improve the performance of whole ofdm system.

The method that frame synchronization of OFDM system provided by the invention and frequency deviation are accurately estimated, comprise the following steps: A, the orthogonal frequency division multiplex OFDM data of frame to be sent to be taken out, described OFDM data comprises some symbols, average value processing is gone to each symbol, and the average of one group of data that each symbol comprises is modulated; OFDM data is added pseudo-random binary sequence PRBS frame head, and before PRBS frame head, add the preceding paragraph complex exponential sequence, re-send to channel and transmit; B, set up movably receiving window, fast Fourier transform FFT computing is carried out to the data in this window, the mould of operation result square maximum and setting threshold value are compared, adjustment window makes complex exponential sequence enter window, skew according to mould square maximum value position and home position obtains a frequency deviation estimator, carries out a compensate of frequency deviation to reception data; C, set up new movably receiving window, the known PRBS sequence of data in new window and this locality is done computing cross-correlation, the mould of operation result square maximum and new settings threshold value are compared, adjustment new window to new window meets this threshold value, obtain PRBS initial point position, then divide each symbol and ask its average, completing the synchronous of frame; D, each symbol average of frame extracted and recovers modulation signal information, FFT computing being carried out to equal value sequence, obtains secondary frequency biased estimator according to the mould square maximum value position of operation result and the skew in home position, carrying out secondary frequency offset compensation to receiving data.

In technique scheme, each symbol in OFDM data described in steps A contains one group of identical data of quantity.

In technique scheme, adjusting window described in step B, to make complex exponential sequence enter the process of window as follows: if the mould of a FFT operation result square maximum is less than setting threshold value, then judge that complex exponential sequence does not enter receive window, after move this window until complex exponential sequence enters this window.

In technique scheme, adjusting new window to new window described in step C, to meet the process of this threshold value as follows: if judge, new window does not meet described new settings threshold value, then move new window afterwards until new window meets threshold value.

In technique scheme, further comprising the steps of after step D: to repeat step C, D, until obtain the accurate frame synchronization position and the frequency deviation estimator that meet system requirements.

Compared with prior art, advantage of the present invention is as follows:

The present invention is based on the characteristic of frequency deviation sequence frequency domain shift, respectively twice estimation is carried out to frequency deviation: rough estimate and thin estimation, accurately locate the original position of OFDM frame according to the correlation properties of PRBS sequence height simultaneously, finally reach fine compensation frequency deviation and locating frame synchronous, thus improve the performance of whole ofdm system, overcome original SC method to the peak value platform problem of frame synchronization and the problem lower to the precision of frequency deviation estimation.

Accompanying drawing explanation

When Fig. 1 is L=64 formula 1.2 result mould square with frequency ratio function relation figure.

Fig. 2 is the process chart of ofdm system transmitting terminal frame data in the embodiment of the present invention.

Fig. 3 is the process chart of ofdm system receiving terminal frame data in the embodiment of the present invention.

Embodiment

Below in conjunction with drawings and Examples, the present invention is described in further detail.

The frame synchronization of OFDM system that the embodiment of the present invention provides and the method that frequency deviation is accurately estimated, comprise the following steps:

A, the OFDM data of frame to be sent to be taken out, OFDM data comprises some symbols, each symbol contains one group of identical data of quantity, average value processing is gone to each symbol, and the average of one group of data that each symbol comprises is modulated, namely the average of one group of data that each symbol comprises is modulated; OFDM data is added PRBS frame head, and before PRBS frame head, add the preceding paragraph complex exponential sequence, namely complete the process of transmitting terminal data, be sent to channel and transmit.

B, data arrive receiving terminal: set up movably receiving window, FFT (Fast Fourier Transformation is carried out to the data in this window, fast Fourier transform) computing, to operation result delivery square, the mould of operation result square maximum and setting threshold value are compared, adjustment window makes complex exponential sequence enter window: if the mould of a FFT operation result square maximum is less than setting threshold value, then judge that complex exponential sequence does not enter receive window, after move this window until complex exponential sequence enters this window.Skew according to mould square maximum value position and home position obtains a frequency deviation estimator, carries out a compensate of frequency deviation according to a frequency deviation estimator to all data received.

C, set up new movably receiving window, the known PRBS sequence of data in new window and this locality is done computing cross-correlation, to operation result delivery square, the mould of operation result square maximum and new settings threshold value are compared, adjustment new window to new window meets this threshold value: if judge, new window does not meet described new settings threshold value, then move new window afterwards until new window meets threshold value, obtain PRBS initial point position, then divide each symbol and ask its average, completing the synchronous of frame.

D, determine frame head after, each symbol average (i.e. the average of each group of data) of frame is extracted and recovers modulation signal information, FFT computing is carried out to equal value sequence, to operation result delivery square, obtain secondary frequency biased estimator according to the mould square maximum value position of operation result and the skew in home position, according to secondary frequency biased estimator, secondary frequency offset compensation is carried out to the data of all receptions.Step C, D can also be repeated, until obtain the accurate frame synchronization position and the frequency deviation estimator that meet system requirements.

Shown in Figure 2, in the embodiment of the present invention, the handling process of ofdm system transmitting terminal frame data is as follows:

Step 101: the OFDM data of frame to be sent is taken out, suppose that an original frame OFDM data is Data01, these frame data comprise N1 symbol (i.e. N1 group data), and each symbol (namely often organizing data) comprises N2 data point (wherein N1, N2 is the integral number power of 2), amount to N1 × N2 point data, structure is as follows:

[A ₁, A ₂..., A _n1]=[a _1,1, a _1,2..., a _{1, N2}, a _2,1, a _2,2..., a _{2, N2}, a _n1,1, a _n1,2..., a _{n1, N2}], wherein A is symbol, and a is data point.

Step 102: remove average value processing according to each symbol of following formula to data Data01:

$\mathrm{step}1:{\mathrm{mean}}_i=\frac{1}{N2}\underset{j=1}{\overset{N2}{\mathrm{\Σ}}}{a}_{i,j}$

step2：b _i，j＝a _i，j-mean _i

I=1,2 ..., N1, j=1,2 ..., N2 (1.4), obtains data Data02.

Step 103: the average of one group of data that each symbol comprises is modulated, modulation signal this as:

$x\left(i\right)=e^{j2\mathrm{\π}\frac{i}{M1}},i=\mathrm{0,1,2},...N1-1---\left(1.5\right)$

Wherein N1=nM1, N1, n, M1 are the integral number power of 2.After the FFT change of N1 point being done to it according to the character of Fourier's change: X (k)=FFT [x (i)] (1.6)

According to Fourier's qualitative change, its export for upper projection, therefore, only have frequency n+1 to have to export and count S1 for the energy of signal x (i), it is 0 that other points export:

${\left|X\left(k\right)\right|}^2=\left\{\begin{array}{c}S1,k=n+1\\ 0,k\≠n+1\end{array}\right.---\left(1.7\right)$

To each symbol-modulated process be:

$B_i=A_i+x\left(i\right)\⇔b_{i,j}=a_{i,j}+x\left(i\right)$

i＝1，2，…，N1，j＝1，2，…，N2 (1.8)

Obtain data Data03 after completing modulation, as required the required symbol size of modulation is changed, to change the position of modulation signal frequency.

Step 104: add the PRBS sequence that length is L (L with in background technology) before data, obtain data Data04;

Step 105: add the preceding paragraph complex exponential sequence before the PRBS frame head of data Data04, setting its length is N (being generally the integral number power of 2), and its signal is:

$y\left(i\right)=e^{j2\mathrm{\π}\frac{i}{M}},i=\mathrm{1,2},...N---\left(1.9\right)$

Wherein N=mM, N, m, M are the integral number power of 2.As mentioned above, only have frequency m+1 to have to export and count S for the energy of signal y (i), it is 0 that other points export, and finally obtains the output data Data05 of transmitting terminal.

Data arrive receiving terminal after transmission, and shown in Figure 3, the handling process of receiving terminal frame data is as follows:

Step 201: first set up the movably receiving window that length is N (being determined by transmitting terminal complex exponential sequence length, the N with in step 105), FFT computing is done to the N point data of receive window, to operation result delivery square.

Step 202: the mould of operation result square maximum and setting threshold value are compared, judges whether mould square maximum is greater than setting threshold value, if so, then forwards step 204 to; Otherwise forward step 203 to.According to the character (as formula 1.7) of complex exponential sequence, after sequence enters receive window, its mould square can be equipped with an impact in certain bits, therefore can judge whether its complex exponential frame head enters receive window by setting threshold value.

Step 203: adjustment window makes complex exponential sequence enter window: if a mould square maximum is less than setting threshold value, then receive window is moved backward, return step 202, continue to judge whether mould square maximum is greater than setting threshold value, until complex exponential frame head enters receive window.

Step 204: the skew according to mould square maximum value position and home position obtains a frequency deviation estimator, carries out a compensate of frequency deviation according to a frequency deviation estimator to all data received.According to the character of Fourier's change, under the condition having frequency deviation, its mould square maximum position can change:

X(k)＝FFT[x(n)]

(1.10)

X(k-t)＝FFT[x(n)e ^j2πnt]

Therefore according to square maximum of its mould after FFT computing in receive window with the skew in home position, its frequency deviation estimator can be obtained.If now sample frequency is fs sample clock interval is Δ t, wherein fs Δ t=1; Now FFT length of window is N, therefore once estimates that frequency difference unit gap is Δ f ₁=fs/N, sets the threshold value maximum bias estimation amount that now judges as Δ n ₁(i.e. the conversion of maximum coordinate points position is integer), obtaining its frequency deviation estimator is:

ΔF ₁＝Δf ₁Δn ₁(1.11)

A compensate of frequency deviation is carried out to all Received signal strength: now residual frequency is to the maximum

Step 205: set up new movably receiving window, setting new receive window length is that L (is determined by transmitting terminal, L with in step 104) the known PRBS sequence of data in new window and this locality is done computing cross-correlation, to operation result delivery square.As shown in formula 1.2 and Fig. 1 when inherent spurious frequency deviation is much smaller than sample frequency, the correlation of its sequence does not affect substantially, therefore can pass through the position that the specified threshold judgment frame of new settings is synchronous.

Step 206: the mould of operation result square maximum and new settings threshold value are compared, judges whether mould square maximum is greater than new settings threshold value, if so, then forwards step 208 to; Otherwise forward step 207 to.

Step 207: adjustment new window to new window meets this threshold value: a mould square maximum is less than new settings threshold value if judge, namely new window does not meet new settings threshold value, then move new window afterwards, return step 206, continue to judge whether mould square maximum is greater than new settings threshold value, until new window meets threshold value.

Step 208: adjust new receive window to after new receive window meets this threshold value, obtain PRBS initial point position, thus determine frame header position, then divide each symbol and ask its average:

$R_i=\frac{1}{N2}\underset{j=1}{\overset{N2}{\mathrm{\Σ}}}{r}_{i,j}$

i＝1，2，…，N1，j＝1，2，…，N2 (1.12)

Wherein, r _{i, j}for each symbol after dividing receives data point, so far the synchronous working of frame completes.

Step 209: after determining frame head, each symbol average (i.e. the average of each group of data) of frame is extracted and recovers modulation signal information, FFT computing is carried out to equal value sequence, to operation result delivery square, and ask the position of mould square maximum, the skew according to mould square maximum value position and home position obtains secondary frequency biased estimator.

Setting residual frequency is Δ f, and original modulated signal is [b ₁, b ₂..., b _n1], due to inherent spurious frequency deviation and the impact be averaging, actual Received signal strength is as follows:

$\left\{\begin{array}{c}{r}_1=b_1(e^{j2\mathrm{\π\Δf\Δt}}+e^{j2\mathrm{\π\Δf}2\mathrm{\Δt}}+...+e^{j2\mathrm{\π\ΔfN}2\mathrm{\Δt}})\\ r_2=b_2(e^{j2\mathrm{\π\Δf\Δt}}+e^{j2\mathrm{\π\Δf}2\mathrm{\Δt}}+...+e^{j2\mathrm{\π\ΔfN}2\mathrm{\Δt}})e^{j2\mathrm{\π\ΔfN}2\mathrm{\Δt}}\\ .\\ .\\ .\\ r_{N1}=b_{N1}(e^{j2\mathrm{\π\Δf\Δt}}+e^{j2\mathrm{\π\Δf}2\mathrm{\Δt}}+...+e^{j2\mathrm{\π\ΔfN}2\mathrm{\Δt}})e^{j2\mathrm{\πfN}2(N2-1)\mathrm{\Δt}}\end{array}\right.---\left(1.13\right)$

Cumulative item is above the same does not affect FFT spectral characteristic, and therefore it is actual under frequently deceiving Δ f condition, and sample rate is the sampled data of fs/N2, estimated by unit frequency deviation Δ f ₂for fs/ (N1N2) is by FFT, set the threshold value maximum side-play amount that now judges as Δ n ₂, be integer, obtaining its secondary frequency biased estimator is:

ΔF ₂＝Δf ₂Δn ₂(1.14)

Now residual frequency is to the maximum according to secondary frequency biased estimator, secondary frequency offset compensation is carried out to the data of all receptions.

Step 205 ~ 209 can also be repeated, to obtain more accurate frame synchronization position and frequency deviation estimator if needed.

Obviously, those skilled in the art can carry out various change and modification to the present invention and not depart from the spirit and scope of the present invention.Like this, if these amendments of the present invention and modification belong within the scope of the claims in the present invention and equivalent technologies thereof, then the present invention is also intended to comprise these change and modification.The content be not described in detail in this specification belongs to the known prior art of professional and technical personnel in the field.

Claims (5)

1. the method accurately estimated of frame synchronization of OFDM system and frequency deviation, is characterized in that comprising the following steps:

A, the orthogonal frequency division multiplex OFDM data of frame to be sent taken out, described OFDM data comprises some symbols, removes average value processing to each symbol, and modulates the average of one group of data that each symbol comprises; OFDM data is added pseudo-random binary sequence PRBS frame head, and before PRBS frame head, add the preceding paragraph complex exponential sequence, re-send to channel and transmit;

B, set up movably receiving window, fast Fourier transform FFT computing is carried out to the data in this window, the mould of operation result square maximum and setting threshold value are compared, adjustment window makes complex exponential sequence enter window, skew according to mould square maximum value position and home position obtains a frequency deviation estimator, carries out a compensate of frequency deviation to reception data;

C, set up new movably receiving window, the known PRBS sequence of data in new window and this locality is done computing cross-correlation, the mould of operation result square maximum and new settings threshold value are compared, adjustment new window to new window meets this threshold value, obtain PRBS initial point position, then divide each symbol and ask its average, completing the synchronous of frame;

D, each symbol average of frame extracted and recovers modulation signal information, FFT computing being carried out to equal value sequence, obtains secondary frequency biased estimator according to the mould square maximum value position of operation result and the skew in home position, carrying out secondary frequency offset compensation to receiving data.

2. the method accurately estimated of frame synchronization of OFDM system as claimed in claim 1 and frequency deviation, is characterized in that: each symbol in OFDM data described in steps A is containing the identical one group of data of quantity.

3. the method accurately estimated of frame synchronization of OFDM system as claimed in claim 1 and frequency deviation, it is characterized in that: adjusting window described in step B, to make complex exponential sequence enter the process of window as follows: if the mould of a FFT operation result square maximum is less than setting threshold value, then judge that complex exponential sequence does not enter receive window, after move this window until complex exponential sequence enters this window.

4. the method accurately estimated of frame synchronization of OFDM system as claimed in claim 1 and frequency deviation, it is characterized in that: adjusting new window to new window described in step C, to meet the process of this threshold value as follows: if judge, new window does not meet described new settings threshold value, then move new window afterwards until new window meets threshold value.

5. the method that the frame synchronization of OFDM system according to any one of Claims 1-4 and frequency deviation are accurately estimated, it is characterized in that: further comprising the steps of after step D: repeat step C, D, until obtain the accurate frame synchronization position and the frequency deviation estimator that meet system requirements.