CN102098256A - OFDM time-domain integer frequency offset estimation realization method - Google Patents

Abstract

The invention discloses an orthogonal frequency division multiplexing (OFDM) time-domain integer frequency offset estimation realization method. The method comprises the following steps of: (1) considering a decimal fraction-multiple frequency offset of a time-domain training sequence of a time synchronization error in a compensation way; (2) performing integer frequency offset adjustment on a local training sequence by a coarse integer frequency offset point scanning method in a large span; (3) respectively correlating time-domain training sequences after sampling the decimal fraction-multiple frequency offset compensation repeatedly in a fixed time span with the local time-domain training sequence which is subjected to the integer-multiple frequency offset adjustment to determine the range of an integer frequency offset; (4) performing integer frequency offset adjustment on the local training sequence by a fine integer frequency offset point scanning method in the integer frequency offset range determined in the step (3); and (5) respectively correlating the time-domain training sequences after sampling the decimal fraction-multiple frequency offset compensation repeatedly in another fixed time span with the local time-domain training sequence which is subjected to the integer-multiple frequency offset adjustment to determine an accurate value of the integer frequency offset. By adopting a coarse integer frequency offset and fine integer frequency offset combined method, the correlation of the time-domain training sequence and the local known training sequence of a received signal can also be realized rapidly even in the situation of a time-domain synchronization error so as to provide an integer-multiple frequency offset result accurately.

Description

A kind of OFDM time domain is put in order the frequency offset estimating implementation method

Technical field

The present invention relates to the frequency deviation estimating method in a kind of communication field, relate in particular to the whole frequency offset estimating implementation method of a kind of OFDM time domain.

Background technology

OFDM (Orthogonal Frequency Division Multiplexing OFDM) technology has been used the orthogonal property of frequency, in case its orthogonality goes to pot, the error rate of system sharply increases, and this is a major defect of OFDM technology.The main cause that produces frequency deviation is exactly that radio circuit reception and delivery section centre frequency do not match.If the transmitting terminal centre carrier frequency is f _t, the receiving terminal centre carrier frequency is f _r, then source data x (n) at transmitting terminal is: $T\left(n\right)=x\left(n\right)e^{-\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="H2009102002509E00021.png,H2009102002509E00022.png"\; state="\{\{state\}\}">j</figure-callout>}2\mathrm{\&pi;}{f}_{t}t},$ At receiving terminal be $y\left(n\right)=T\left(n\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="H2009102002509E00021.png,H2009102002509E00022.png"\; state="\{\{state\}\}">e</figure-callout>}}^j=x\left(n\right){\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="H2009102002509E00021.png,H2009102002509E00022.png"\; state="\{\{state\}\}">e</figure-callout>}}^j,$ There is frequency deviation f in the data of receiving terminal after down-conversion as can be seen _r-f _t

Can see that from following formula along with the increase of sampled point accumulates gradually, equivalence is offset for linear phase in time domain, phase deviation that carrier wave frequency deviation caused.Disturb between rotation of average phase that frequency domain, carrier wave frequency deviation cause and subcarrier, when frequency deviation was Δ f, frequency-region signal can be expressed as:

$Y\left(k\right)=X_k{H}_k\left\{\frac{\sin \mathrm{\&pi;\&Delta;f}}{N\sin (\mathrm{\&pi;\&Delta;f}/N)}\right\}{e}^{\mathrm{j\&pi;\&Delta;f}(N-1)/N}$

$+\underset{\stackrel{l=-K}{l\&NotEqual;k}}{\overset{K}{\mathrm{\&Sigma;}}}{X}_k{H}_k\left\{\frac{\sin \mathrm{\&pi;\&Delta;f}}{N\sin (\mathrm{\&pi;}(l-k+\mathrm{\&Delta;f})/N)}\right\}{e}^{\mathrm{j\&pi;\&Delta;f}(N-1)/N}{e}^{-\mathrm{j\&pi;}(l-k)/N}$

$=Y^{\&prime;}\left(k\right)+Y_{\mathrm{ICI}}\left(k\right)$

Wherein, the signal that Y (k) receives for frequency domain, X _kBe the primary signal that sends, H _kBe the frequency domain response of k position of channel, N is a FFT length, and Y ' (k) and X _kLinear, Y _ICI(k) disturb between subcarrier.Along with the increase of carrier wave frequency deviation Δ f, function

Along with increase, disturb also along with increase between subcarrier.

In ofdm system, when carrier frequency offset takes place, partly be called integer frequency bias Δ f greater than the frequency deviation of subcarrier spacing _I, partly be called decimal frequency bias Δ f less than the frequency deviation of subcarrier spacing _fNeed the integer frequency bias Δ f that exists in the system of estimating in the OFDM receiver _IWith decimal frequency bias Δ f _f, by the frequency offset correction module data stream is proofreaied and correct then.

Because the system carrier frequency is higher, the subcarrier interbody spacer is less, small frequency deviation will cause the skew of several or dozens of carrier wave in the frequency domain after FFT (fast Fourier transform fast Fourier transform) modulation of OFDM symbol, so the frequency offset estimating of ofdm system will be divided into decimal frequency bias Δ f _fEstimate and integer frequency bias Δ f _IEstimate.Fractional part of frequency offset is estimated to utilize maximum likelihood method to finish usually.Integer frequency offset adopts whole frequency deviation estimating method of frequency domain or the whole frequency deviation estimating method of time domain usually.

1, the whole frequency deviation estimating method of frequency domain:

The integer Algorithm of Carrier Frequency Offset Estimation that is based on pilot tone commonly used in the whole frequency deviation estimating method of frequency domain.In frequency domain, can utilize the scattered pilot or the continuous pilot of insertion to carry out integer frequency bias Δ f _IEstimation, this algorithm are based on the maximum likelihood theory.Because decimal frequency bias is proofreaied and correct, the interference between the subcarrier is eliminated substantially.Originally the k ' th subcarrier is because integer frequency bias has been displaced to k ' th+ Δ f after the FFT demodulation process _ISet up departments the system in continuous pilot c _{K '}Its sub-carrier positions k ' ∈ C, the continuous pilot after demodulation that then receives moves to k ∈ C+ Δ f _IIf two continuous OFDM symbol l-1, l, carry out conjugation and be correlated with and can get:

$C_k=\underset{k\&Element;C+m}{\mathrm{\&Sigma;}}{z}_{l,k}\mathrm{conj}\left(z_{l-1,k}\right)$

$\widehat{\mathrm{\&Delta;}{f}_I}=\arg \underset{m}{\max }\left|C_k\right|$

Z wherein _{L, k}K subcarrier representing l OFDM symbol, m ∈ [Δ f _{I, max}, Δ f _{I, max},].

This method is subjected to the restriction of pilot tone and OFDM symbol quantity, estimates the correlation of pilot tone in a plurality of OFDM symbols, can improve the precision and the accuracy of estimation, otherwise if the number of pilot tone is less in the OFDM symbol, then the accuracy of Gu Jiing is relatively poor.In actual ofdm system, consider that for efficiency of transmission the number of pilot tone is always limited.Therefore, it is many that this method is fit to number of pilots, and the high ofdm system of pilot portion energy.

2, the whole frequency deviation estimating method of time domain

The whole frequency deviation estimating method that is based on time-domain training sequence commonly used in the whole frequency deviation estimating method of time domain.The correlation of time-domain training sequence and local known training sequence is finished the integer frequency offset estimation in this algorithm employing received signal.It is the rotation of signal phase angle that the influence of frequency deviation is reflected in the time domain, the rotation of the time-domain signal phase angle that different integer frequency offsets is corresponding different.Adopt all frequencies in the whole frequency offset estimation range, respectively local training sequence is carried out repeatedly whole frequency deviation adjustment.In time domain, utilize time-domain training sequence then, carry out related operation respectively with the known time-domain training sequence in the adjusted a plurality of this locality of frequency deviation, thereby find integer frequency offset.Whole frequency deviation estimating method based on time-domain training sequence is widely applicable, has the ofdm system of certain-length time-domain training sequence to use in the frame structure.But because the frequency deviation adjustment of local known time-domain training sequence has covered all frequencies in the whole frequency offset estimation range, the time of using the whole at every turn frequency offset estimating of this algorithm to need can be long.

In addition, this method requires the time-domain training sequence and the local known training sequence strict time of received signal synchronous, otherwise evaluated error is very big.For an OFDM receiving system, all there is certain error ± Δ T usually in the time synchronized point that obtains time-domain training sequence in the received signal _Sync, promptly the correct time synchronous points is positioned at [Δ T _Sync, Δ T _Sync] within the interval.In this case, when adopting the whole frequency deviation estimating method based on time-domain training sequence, need the original position of time-domain training sequence be scanned.The original position of time-domain training sequence that is received signal is at [Δ T _Sync, Δ T _Sync] the interval variation, obtain 2* Δ T _SyncIndividual different time-domain training sequence respectively with local known training sequence carry out relevant, thereby obtain integer frequency offset.Therefore, for time synchronization error ± Δ T _SyncBigger system, the time of using the whole at every turn frequency offset estimating of this algorithm to need can be longer.

Summary of the invention

Based on the problems referred to above, the present invention proposes the whole frequency offset estimating implementation method of a kind of OFDM time domain, can guarantee to exist under the situation of error in Domain Synchronous, the time-domain training sequence that also can realize received signal fast is relevant with local known training sequence, and the frequency deviation result of integral multiple accurately is provided.

The invention provides the whole frequency offset estimating implementation method of a kind of OFDM time domain, comprise following content:

(1) fractional part of frequency offset of the time-domain training sequence of time synchronization error is considered in compensation;

(2) employing is used thick integer frequency bias spot scan method than large span, and local training sequence is put in order the frequency deviation adjustment;

(3) adopt repeatedly take a sample time-domain training sequence after the fractional part of frequency offset compensation of a set time span, do relevantly with the adjusted local time-domain training sequence of integer frequency offset respectively, determine the scope of integer frequency bias;

(4) use thin integer frequency bias spot scan method in the scope of the integer frequency bias of in step (3), determining, local training sequence is put in order the frequency deviation adjustment;

(5) adopt repeatedly take a sample time-domain training sequence after the fractional part of frequency offset compensation of another set time span, do relevantly with the adjusted local time-domain training sequence of integer frequency offset respectively, determine the accurate numerical value of integer frequency bias.

The time-domain training sequence of considering time synchronization error in the step (1) is: if time-domain training sequence length is N, the estimated time-domain training sequence start time point of ofdm system is T, then is positioned at [T-Δ T in the receiving data stream _Sync, T+ Δ T _Sync+ N] the length of an interval degree is T+2* Δ T _SyncOne piece of data promptly consider the time-domain training sequence of time synchronization error by way of compensation.

Adopt the time-domain training sequence after a set time span sampling fractional part of frequency offset compensates to be in the step (3): to use thick integer frequency bias spot scan method, select the time-domain training sequence starting point after a set time span compensates from fractional part of frequency offset, initial sampled point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

In the step (5), adopt the time-domain training sequence after a set time span sampling fractional part of frequency offset compensates to be: to use thin integer frequency bias spot scan method to select a set time span, time-domain training sequence starting point after the fractional part of frequency offset compensation, initial sampled point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

The foregoing invention content adopts the method for thick integer frequency bias and thin integer frequency bias combination, simultaneously the time sweep span can be set flexibly.Even exist in Domain Synchronous under the situation of error, the time-domain training sequence that also can realize received signal fast is relevant with local known training sequence, and the frequency deviation result of integral multiple accurately is provided.

Description of drawings

A kind of OFDM time domain of Fig. 1 is put in order frequency offset estimating implementation method flow chart

The corresponding result schematic diagram of the whole thick integer frequency bias spot scan of the frequency offset estimating method of Fig. 2 OFDM time domain

The corresponding result schematic diagram of the whole thin integer frequency bias spot scan of the frequency offset estimating method of Fig. 3 OFDM time domain

Specific embodiments

Below in conjunction with each accompanying drawing content of the present invention is described in detail:

Fig. 1 is the whole frequency offset estimating implementation method flow chart of a kind of OFDM time domain provided by the present invention, and implementation step is:

(1) fractional part of frequency offset of the time-domain training sequence of time synchronization error is considered in compensation

If time-domain training sequence length is N, the estimated time-domain training sequence start time point of ofdm system is T, then is positioned at [T-Δ T in the receiving data stream _Sync, T+ Δ T _Sync+ N] the length of an interval degree is T+2* Δ T _SyncOne piece of data promptly consider the time-domain training sequence of time synchronization error by way of compensation.Be shown below:

r _I(k)＝r(k)·e ^-j2πεfk，k＝0，1，...，N+2*ΔT _sync

ε is the fractional part of frequency offset that estimates, and f is the synchronizing signal carrier spacing, and N is the length of time-domain training sequence, T _SyncBe half of time synchronization error scope.

(2) adopt than large span SFc, use thick integer frequency bias spot scan method, local training sequence is put in order the frequency deviation adjustment.Formula specific as follows:

sync _c，m(k)＝sync(k)·e ^-j2πmfk k＝0，1，...，N

Wherein, m=1, SFc, 2*SFc..., MAX, [1, MAX] is the integer frequency offset adjusting range.

(3) when using thick integer frequency bias spot scan method, for each whole frequency deviation point, select a set time span STc, time-domain training sequence starting point after the fractional part of frequency offset compensation, starting point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

r _n，I(k)＝r(k)·e ^-j2πεfk，k＝[T-ΔT _sync+n*STc，T-ΔT _sync+n*STc+N]

Wherein n represents the n time sampling.

Doing relevant, each relevant process with the adjusted local time-domain training sequence of integer frequency offset respectively then is shown below:

$\mathrm{Corr}\left(n\right)=\underset{k=0}{\overset{N}{\mathrm{\&Sigma;}}}{r}_{n,I}\left(k\right)\&CenterDot;{\mathrm{sync}}_{c,m}\left(k\right)$

The peak value of Corr (n) is as the correlated results of this whole frequency deviation point, and the m value of correlated results maximum correspondence is the central value M of integer frequency offset scope, thereby determines that the integer frequency offset scope is [M-SFc, M+SFc].

(4) in the scope of the integer frequency bias of in last step (3), determining, use thin integer frequency bias spot scan method, local training sequence is put in order the frequency deviation adjustment.Adopt span value 1, use thin integer frequency bias spot scan method, local training sequence is put in order the frequency deviation adjustment.Formula specific as follows:

Sync _{F, i}(k)=sync (k) e ^{-j2 π ifk}, k=0,1 ..., N, wherein, the i scope is [M-SFc, M+SFc] interval.

(5) when using thick integer frequency bias spot scan method, for each whole frequency deviation point, select a set time span STf, time-domain training sequence starting point after the fractional part of frequency offset compensation, initial sampled point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

r _{J, I}(k)=r (k) e ^{-j2 π ε fk}, k=[T-Δ T _Sync+ n*STf, T-Δ T _Sync+ n*STf+N], wherein j represents the j time sampling.

Doing relevant, each relevant process with the adjusted local time-domain training sequence of integer frequency offset respectively then is shown below:

$\mathrm{Corr}\left(j\right)=\underset{k=0}{\overset{N}{\mathrm{\&Sigma;}}}{r}_{j,I}\left(k\right)\&CenterDot;{\mathrm{sync}}_{f,m}\left(k\right)$

The peak value of Corr (j) is as the correlated results of this whole frequency deviation point.The i value of correlated results maximum correspondence and integer frequency offset swept frequency range central point poor is the accurate numerical value of estimated integer frequency bias.

Adopt a specific embodiment of above-mentioned specific embodiments, its parameter is selected as follows:

Awgn channel;

Signal to noise ratio Eb/N0=1dB;

Synchronous error Δ T _Sync=50;

Time-domain training sequence length N=2048;

Subcarrier spacing f=2.4kHz;

In esse little frequency deviation Δ f _f=0.5kHz;

In esse whole frequency deviation Δ f _I=28.8kHz (suitable 12 times subcarrier spacing);

Whole frequency offset estimation range is [0,200];

SFc=5 in the thick integer frequency bias spot scan method;

STc=10 in the thick integer frequency bias spot scan method;

STf=5 in the thin integer frequency bias spot scan method;

The correlated results of thick integer frequency bias spot scan method correspondence as shown in Figure 2, integer frequency point adjustment is spaced apart 5, the central value 86 of integer frequency offset scope as seen from the figure, thereby definite integer frequency offset scope is [81,91].

According to above-mentioned determined integer frequency offset scope [81,91], adopt thin integer frequency bias spot scan method to be correlated with, correlated results is as shown in Figure 3.Correlation peak point frequency is 89 among the figure, and integer frequency bias is 101-89=12 in this ofdm system so can estimate, and is consistent with actual value.

Claims (4)

1. an OFDM time domain is put in order the frequency offset estimating implementation method, comprises following content:

(1) fractional part of frequency offset of the time-domain training sequence of time synchronization error is considered in compensation;

(2) employing is used thick integer frequency bias spot scan method than large span, and local training sequence is put in order the frequency deviation adjustment;

(3) adopt repeatedly take a sample time-domain training sequence after the fractional part of frequency offset compensation of a set time span, do relevantly with the adjusted local time-domain training sequence of integer frequency offset respectively, determine the scope of integer frequency bias;

(4) use thin integer frequency bias spot scan method in the scope of the integer frequency bias of in step (3), determining, local training sequence is put in order the frequency deviation adjustment;

(5) adopt repeatedly take a sample time-domain training sequence after the fractional part of frequency offset compensation of another set time span, do relevantly with the adjusted local time-domain training sequence of integer frequency offset respectively, determine the accurate numerical value of integer frequency bias.

2. a kind of OFDM time domain as claimed in claim 1 is put in order the frequency offset estimating implementation method, it is characterized in that: the time-domain training sequence of described consideration time synchronization error is: time-domain training sequence length is N, the estimated time-domain training sequence start time point of ofdm system is T, is positioned at [T-Δ T in the receiving data stream _Sync, T+ Δ T _Sync+ N] the length of an interval degree is T+2* Δ T _SyncOne piece of data promptly consider the time-domain training sequence of time synchronization error by way of compensation.

3. a kind of OFDM time domain as claimed in claim 1 is put in order the frequency offset estimating implementation method, it is characterized in that: the time-domain training sequence after the compensation of a described set time of employing span sampling fractional part of frequency offset is: use thick integer frequency bias spot scan method to select a set time span, time-domain training sequence starting point after the fractional part of frequency offset compensation, initial sampled point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

4. a kind of OFDM time domain as claimed in claim 1 is put in order the frequency offset estimating implementation method, it is characterized in that: the time-domain training sequence after the compensation of a described set time of employing span sampling fractional part of frequency offset is: use thin integer frequency bias spot scan method to select a set time span, time-domain training sequence starting point after the fractional part of frequency offset compensation, initial sampled point is increased a time span at every turn, this sequence of sampling obtains a plurality of different time-domain training sequences.

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