KR101508463B1 - Method and receiver for estimating ofdm frequency offset based on partial periodogram - Google Patents

Abstract

A method of estimating an OFDM frequency offset based on a partial period diagram according to the present invention includes performing envelope equalized processing (EEP) on a received signal of an OFDM (a), (b) Estimating the offset; and (c) iteratively estimating the precision offset using the initial offset and the partial period diagram. And (d) iteratively estimating a residual offset to estimate a possible offset after the precision offset estimation. This makes it possible to estimate a reliable frequency offset with a low computational complexity.

Description

METHOD AND RECEIVER FOR ESTIMATING OF FREQUENCY OFFSET BASED ON PARTIAL PERIODOGRAM < RTI ID = 0.0 > [0001] < / RTI &

The present invention relates to a frequency offset method using a partial periodic table in an orthogonal frequency division multiplexing (OFDM) system and a receiver using the method.

Many studies have been made to estimate the frequency offset of an OFDM based system. PH Moose has proposed a frequency offset estimation method using two OFDM signals and a maximum likelihood theory that are repeated ("A technique for orthogonal frequency division multiplexing frequency offset correction," IEEE Trans. Commun., Vol. 2908-2914, Oct. 1994).

TM Schmidl and DC Cox proposed a frequency offset estimation technique with performance similar to Moose but with wider estimation range using training symbols with repetitive structure in one OFDM signal ("Robust frequency and timing synchronization for OFDM , IEEE Trans. Commun., Vol. 45, no. 12, pp. 1613-1621, Dec. 1997.).

M. Morelli and U. Mengali proposed a frequency offset estimation technique with similar performance and wider estimation range using Cox's technique using training symbols having multiple repetitive structures in one OFDM signal ("Improved frequency offset estimator for OFDM applications, "IEEE Commun. Lett., vol.3, no.3, pp. 75-77, Mar. 1999.).

Recently, G. Ren, Y. Chang, H. Zhang, and H. Zhang proposed a technique that can be applied to arbitrary training symbols and has similar estimation performance and estimation range as those of the training symbol-based techniques IEEE Trans. Vehicle Technol., vol. 56, no. 4, pp. 1892-1895, July 2007).

The method proposed by P. H. Moose has a disadvantage that the frequency offset estimation range is too small, and the technique proposed by T. M. Schmidl and the technique proposed by M. Morelli necessarily have training symbols. In order to overcome this drawback, blind based frequency offset estimation has been proposed, but this technique has a problem that estimation performance itself is not good.

The present invention provides an offset estimation method that can be applied to arbitrary training symbols using a partial period diagram, but has a low computational complexity.

The present invention aims to provide an OFDM receiver using an offset estimation method with a low computation complexity based on a partial period diagram.

The solution of the present invention is not limited to those mentioned above, and other solutions not mentioned can be clearly understood by those skilled in the art from the following description.

A method for estimating an OFDM frequency offset based on a partial periodic table according to the present invention includes performing an envelope equalization (EEP) on an OFDM reception signal, repeatedly estimating an initial offset for the envelope- And estimating the precision offset using the initial offset and the partial period diagram.

)를 이용하여 추정된다.The precision offset can be expressed in the form of a partial periodic table (

).

Here, N is the size of IFFT, α is a period in which a frequency offset estimation performed range, y _m is the received signal, j is an imaginary, z is the frequency value.

인 관계를 이용하여

식을 통해 반복적으로 추정된다.The precision offset

Using the relationship

It is estimated repeatedly through the equation.

는 부분 주기도표 연산,

이고,

,

,

,

는 초기 오프셋 추정값이다.

Is a partial periodic chart operation,

ego,

,

,

,

Is an initial offset estimate.

The OFDM frequency offset estimation method based on the partial period diagram according to the present invention may further include estimating a residual frequency offset.

The residual frequency offset can be iteratively estimated using the partial period diagram in the following equation.

이고,

는 초기 오프셋 추정값,

는 정밀 오프셋 추정값,

,

,

는 부분 주기도표이다.
here,

ego,

Is an initial offset estimate,

Precision offset estimate,

,

,

Is a partial period diagram.

An OFDM receiver for estimating a frequency offset based on a partial period diagram according to the present invention includes an RF receiver for receiving a received signal, an EPP performing unit for performing an envelope equalization process on a received signal received by the RF receiver, An initial offset estimator for estimating an initial offset of the received signal and a precision offset estimator for estimating a precision offset repeatedly using an initial offset estimated by the initial offset estimator and a partial periodic table.

식으로 상기 정밀 오프셋을 반복적으로 추정한다.The precision offset estimator

Lt; RTI ID = 0.0 > repetitively < / RTI >

이고,

,

,

,

는 초기 오프셋 추정값이다.here,

ego,

,

,

,

Is an initial offset estimate.

And the residual frequency offset estimator may repeatedly estimate the residual frequency offset using the initial offset estimate, the precision offset estimate, and the partial period diagram in the following manner .

이고,

는 초기 오프셋 추정값,

는 정밀 오프셋 추정값,

,

이다.

ego,

Is an initial offset estimate,

Precision offset estimate,

,

to be.

The offset estimation method and receiver according to the present invention can estimate a reliable frequency offset with a low computational complexity as compared with the conventional estimation method. The present invention can thereby achieve improved wireless communication quality with low hardware complexity.

The effects of the present invention are not limited to those mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the following description.

1 is a flowchart illustrating a sequence of an OFDM frequency offset estimation method based on a partial period diagram according to an exemplary embodiment of the present invention.
2 is a block diagram schematically illustrating an exemplary configuration of an OFDM receiver for estimating a frequency offset based on a partial period diagram.
3 is a graph showing the total amount of computation according to the value of alpha when N = 64 in the OFDM frequency offset estimation method based on the partial period diagram.
4 is a graph showing a mean square error according to? In an OFDM frequency offset estimation method based on a partial period diagram.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

The terms first, second, A, B, etc., may be used to describe various components, but the components are not limited by the terms, but may be used to distinguish one component from another . For example, without departing from the scope of the present invention, the first component may be referred to as a second component, and similarly, the second component may also be referred to as a first component. And / or < / RTI > includes any combination of a plurality of related listed items or any of a plurality of related listed items.

As used herein, the singular " include "should be understood to include a plurality of representations unless the context clearly dictates otherwise, and the terms" comprises & , Parts or combinations thereof, and does not preclude the presence or addition of one or more other features, integers, steps, components, components, or combinations thereof.

Before describing the drawings in detail, it is to be clarified that the division of constituent parts in this specification is merely a division by main functions of each constituent part. That is, two or more constituent parts to be described below may be combined into one constituent part, or one constituent part may be divided into two or more functions according to functions that are more subdivided. In addition, each of the constituent units described below may additionally perform some or all of the functions of other constituent units in addition to the main functions of the constituent units themselves, and that some of the main functions, And may be carried out in a dedicated manner. Therefore, the existence of each of the components described in this specification will be interpreted as a function, and for this reason, the configuration of the components according to the OFDM receiver 100 for estimating the frequency offset based on the partial period diagram of the present invention, It is clear that it can be different from Fig. 2 within the scope of achieving the object of the invention.

First, an OFDM signal will be briefly described and the present invention will be described in detail.

In the time domain, an OFDM signal is obtained by performing an inverse fast Fourier transform (IFFT) on data generated through a phase shift keying (PSK) or a quadrature amplitude modulation (QAM) Can be expressed by the following equation (1).

Where N denotes a size of IFFT, X _k is the k-th data modulated in a PSK or QAM method. In the transmitter, a guard interval (GI) longer than the maximum delay time of the channel is inserted between the OFDM signals in order to eliminate interference between OFDM signals caused by the influence of the channel. In this case, the GI is used to guarantee orthogonality between subcarriers And is configured in the same manner as the rear part of the OFDM signal. When the time synchronization is completely performed, the nth sample of the GI-canceled received signal can be expressed by Equation (2) below.

은 길이 L인 채널의

번째 복소 임펄스 응답 (impulse response) 계수,

는 부반송파 간격으로 정규화된 주파수 오프셋이며,

은 평균 0, 분산

을 갖는 복소 덧셈꼴 정규 잡음 (additive white Gaussian noise: AWGN) 샘플이다. 신호대 잡음비는 (signal to noise ratio: SNR)

로 정의되며,

이다.
here

Of the channel of length L

The second complex impulse response coefficient,

Is a frequency offset normalized to a subcarrier interval,

Mean 0, variance

Is an additive white Gaussian noise (AWGN) sample. The signal-to-noise ratio (SNR)

Lt; / RTI >

to be.

1 is a flowchart illustrating a sequence of an OFDM frequency offset estimation method based on a partial period diagram according to an exemplary embodiment of the present invention.

A method of estimating an OFDM frequency offset based on a partial period diagram according to the present invention includes performing envelope equalized processing (EEP) on a received signal of an OFDM (a), (b) Estimating the offset; and (c) iteratively estimating the precision offset using the initial offset and the partial period diagram. And (d) estimating a residual offset to estimate a possible offset after precision offset estimation.

EEP is defined as Equation 3 below.

는 복소수

의 켤레 복소수를 의미한다. EEP 처리된 수신 신호는 아래의 수학식 4와 같이 나타낼 수 있다.
here,

Complex number

Lt; / RTI > complex number. The received signal subjected to EEP processing can be expressed by Equation (4) below.

은 길이 L인 채널의

번째 복소 임펄스 응답 (impulse response) 계수,

는 부반송파 간격으로 정규화된 주파수 오프셋이며,

은 평균 0, 분산

을 갖는 복소 덧셈꼴 정규 잡음 (additive white Gaussian noise: AWGN) 샘플이다. here

Of the channel of length L

The second complex impulse response coefficient,

Is a frequency offset normalized to a subcarrier interval,

Mean 0, variance

Is an additive white Gaussian noise (AWGN) sample.

는 중심 극한 정리에 의해 평균 0인 정규 확률 변수로 근사화될 수 있다.

Can be approximated to a normal random variable with an average of 0 by the central limit theorem.

중

부분을 추정하는 것이다.

에 대한 추정은 아래의 수학식 5와 같이 표현될 수 있다.
After EEP processing, the received signal is converted to a simple complex tone signal, which means that the frequency offset estimate results in a single complex tone estimate. The initial offset estimate is determined by the frequency offset < RTI ID = 0.0 >

medium

Quot;

Can be expressed as Equation (5) below. &Quot; (5) "

는 주파수 오프셋 추정치

을 찾기 위한 시험 값이며 α는 N보다 작은 2의 거듭제곱이다. α를 어떻게 설정하느냐에 따라 검사 횟수를 줄일 수 있으며, 이는 더욱 낮은 연산 복잡도를 가짐을 의미한다.here,

Lt; RTI ID = 0.0 >

And α is a power of 2 less than N. The number of tests can be reduced depending on how α is set, which means that it has lower computational complexity.

는 아래의 수학식 6과 같이 나타낼 수 있다.
Partial period diagram

Can be expressed by Equation (6) below.

Here, N is the size of IFFT, α is a period in which a frequency offset estimation performed range, y _m is the received signal, j is an imaginary, z is the frequency value.

의 간격은 α이므로, 종래 기법에서의 1 간격으로 시험 값을 사용하는 주기도표에 비해 연산량이 낮다. 잡음이 없다면 대략적인 주파수 오프셋 추정 값은

- α <

≤

범위의

에서 항상 가장 큰 값을 갖는다. 따라서 두 번째 추정 과정은

에 대해 추정 가능하도록 설계되어야 한다.

The calculation amount is lower than that of the periodic table using the test value at one interval in the conventional technique. Without noise, the approximate frequency offset estimate is

-?

≤

Range

The largest value is always. Therefore, the second estimation process

In order to be able to estimate.

및

는 테일러 시리즈에 의해 각각 아래의 수학식 7 및 8과 같이 근사화된다.
Assuming no noise and a single path channel

And

Are approximated by the Taylor series as shown in Equations (7) and (8) below, respectively.

이다. 상기 수학식 7 및 8을 이용하면 다음과 같은 수학식 9의 관계를 이끌어 낼 수 있다.
here

to be. Using Equations (7) and (8), the following Equation (9) can be derived.

이기 때문에,

인 경우에는 상기 수학식 9는 아래의 수학식 10과 같다.
here

Therefore,

Equation (9) is given by Equation (10) below.

Using this relationship, the precision offset can be estimated as shown in Equation (11) below.

이며,

,

,

이다.

Lt;

,

,

to be.

가 0 또는 α에 가까울 경우

와

중 하나는 SNR이 0에 가까워지므로 정밀 주파수 오프셋의 추정 성능은 떨어지며, 추정 이후, 나머지 주파수 오프셋이 존재하게 된다. The precise frequency offset estimation step performs an iterative operation of i times, which is intended to compensate for the drop in performance due to the low computation amount obtained in the partial period diagram.

Is close to 0 or?

Wow

The estimation performance of the precision frequency offset is degraded because the SNR approaches zero, and thereafter, the remaining frequency offset exists after the estimation.

와

은 어느 정도 적당한 SNR을 가지므로, 이들을 이용하여 잔여 주파수 오프셋을 추정한다.

와

를 이용하면 아래의 수학식 12와 같은 관계를 이끌어 낼 수 있다.
In this case

Wow

Lt; RTI ID = 0.0 &gt; SNR &lt; / RTI &gt;

Wow

, The following relationship (12) can be derived.

,

,

일 때 상기 수학식 12을 정리하면

이 된다. 이를 이용하여 잔여 오프셋 추정을 아래의 수학식 13과 같이 표현할 수 있다.
here,

,

,

(12) &lt; / RTI &gt;

. The residual offset estimation can be expressed as Equation (13) below.

이다.

to be.

In the remaining frequency offset estimation step, the larger the value of α is, the larger the amount of operation is, but the larger the value of α in the frequency offset step is, the larger the amount of operation is greatly decreased.

Another aspect of the present invention is an OFDM receiver 100 in which the frequency offset estimation method described above operates. 2 is a block diagram schematically illustrating a configuration according to an exemplary embodiment of an OFDM receiver 100 for estimating a frequency offset based on a partial period diagram.

An OFDM receiver 100 for estimating a frequency offset based on a partial periodic table according to the present invention includes an RF receiver 110 for receiving a received signal, an EPP processor 110 for performing envelope equalization on a received signal received by the RF receiver 110, An initial offset estimator 130 for estimating an initial offset of a received signal processed by the EPP performing unit 120 and an initial offset estimated by the initial offset estimator, And a precision offset estimating unit (140) for estimating the precision offset. Further, the residual offset estimation unit 150 may estimate the residual offset occurring after the precision offset estimation. Although not shown, a controller for controlling the operation of the initial frequency offset estimator 130, the precision offset estimator 140, and the residual offset estimator 150 may also be included.

The RF receiving unit 110 converts the OFDM signal received through the antenna into a baseband signal and outputs the baseband signal to an ADC (Main Log-to-DigitaL Converter). The ADC device converts a baseband signal from the RF receiving unit 110 into a digital signal . ADCs, and the like are not shown in the drawings.

The FFT unit 160 performs Fast Fourier Transform (FFT) using the corrected timing offset to convert the OFDM signal in the time domain into the OFDM symbol in the frequency domain.

The fast Fourier transformed input signal symbol is reconstructed into a data symbol, which is a signal transmitted through the data symbol reconstructing unit 170. To this end, the data symbol reconstructing unit 170 of the OFDM receiver 100 decodes a data source (Not shown), a Parallel to Serial (not shown), and an ADC (Analogue Digital Converter) (not shown), and these configurations may be configured to be out of the scope of the present invention And the detailed description thereof will be omitted.

The operations performed in each configuration are the same as those described in the OFDM frequency offset estimation method based on the partial period diagram described above. Therefore, detailed description will be omitted.

For example, the received signal processed by the EPP performing unit 120 is expressed by Equation (4), the initial offset estimating unit 130 estimates an initial offset using Equation (5), and the precision offset estimating unit The precision offset is repeatedly estimated by Equation (11). Also, the residual offset estimator 150 repeatedly estimates the residual offset according to Equation (13).

Furthermore, if the SNR value is known in advance, the offset estimation performance can be improved by setting the alpha value to the optimum value.

Now, experimental results for verifying the effect of the present invention will be described.

The amount of computation can be compared through the number of additions, multiplications, and divisions required for the technique to operate. Table 1 below shows the addition, multiplication, and division required for each step.

As shown in Table 1, it can be seen that the number of addition and multiplication required in the step of estimating the offset is reduced as? Increases. Although the number of additions and multiplications required in the detailed frequency offset estimation step and the remaining offset estimation step increases as? Increases, considering the large number, which is usually larger than 64, which is mainly used in the OFDM system, the total number of additions and multiplications Decreases sharply as α increases. The number of divisions increases as α increases, but is a very small change compared to the addition and multiplication. That is, the total amount of computation of the proposed method decreases as α increases.

3 is a graph showing the total amount of computation according to the value of alpha when N = 64 in the OFDM frequency offset estimation method based on the partial period diagram. As α increases, the amount of computation decreases as a whole.

4 is a graph showing a mean square error according to? In an OFDM frequency offset estimation method based on a partial period diagram.

번째 임펄스 응답의 전력

은

처럼 이 커짐에 따라 지수적으로 감소하게 설정하였다. 도플러 대역폭은 (Doppler bandwidth) 0.0017로 설정하였으며 이는 이동 속도가 120km/h인 상황을 고려한 것이다.The channel model uses a 4-path Rayleigh fading channel. In the Rayleigh fading channel, each channel response has a time delay of 0, 2, 4, and 6 samples.

Power of the ith impulse response

silver

As the size of the population increases. The Doppler bandwidth is set to 0.0017, which takes into consideration the situation where the traveling speed is 120 km / h.

As shown in FIG. 4, the MSE performance gradually decreases as the value increases. This is because the partial period chart value is formed by adding the values of the values obtained through the absolute value calculation, thereby increasing the noise level. However, this increase in noise does not have a significant effect on MSE performance. No matter how large the SNR is, the MSE performance will not be affected if the SNR exceeds 15dB. Therefore, it can be seen that it is optimal for the frequency offset estimation of the IEEE 802.11a signal. If the SNR is known, the frequency offset can be reliably estimated with low computational complexity.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. It will be understood that variations and specific embodiments which may occur to those skilled in the art are included within the scope of the present invention.

100: OFDM receiver
110: RF receiving unit 120: EPP performing unit
130: initial offset estimator 140: precision offset estimator
150: Residual offset estimation unit 160: FFT unit
170: Data symbol restoration unit

Claims (11)

A method for estimating an OFDM frequency offset using a periodic table,
Performing an envelope equalization (EEP) on an OFDM received signal;
Repeatedly estimating an initial offset for the envelope-equalized received signal;
Estimating a precision offset using the initial offset and the partial period diagram; And
Estimating a residual frequency offset,
Wherein the residual frequency offset is estimated repetitively using the partial period diagram as: &lt; EMI ID = 17.1 &gt;

(here,

ego,

Is an initial offset estimate,

Precision offset estimate,

,

,

Is a partial period diagram) The method according to claim 1,
The envelope equalization

&Lt; / RTI &gt;

Complex number

OFDM FREQUENCY OFFSET ESTIMATION METHOD BASED ON A CIRCUIT DIAGRAM. The method according to claim 1,
Wherein the envelope equalized received signal y _n 'is expressed by the following equation:

(here,

Of the channel of length L

The second complex impulse response coefficient,

Is a frequency offset normalized at subcarrier spacing,

Mean 0 and variance

Lt; RTI ID = 0.0 &gt; normal &lt; / RTI &gt; noise sample,

being) The method according to claim 1,
Wherein the initial offset is estimated by the following equation: &lt; EMI ID = 17.0 &gt;

(here,

Lt; RTI ID = 0.0 &gt;

And α is a power of 2 less than N) delete The method according to claim 1,
The precision offset

A method of estimating an OFDM frequency offset based on a partial periodic table, which is repeatedly estimated using the following equation.

(

Is a partial periodic chart operation,

ego,

,

,

,

Is an initial offset estimation value) In an OFDM receiver for estimating a frequency offset,
An RF receiver for receiving a received signal;
An EPP performing unit for performing an envelope equalization process on a received signal received by the RF receiving unit;
An initial offset estimator for estimating an initial offset of a received signal processed by the EPP performing unit;
A precision offset estimator for repeatedly estimating a precision offset using an initial offset estimated by the initial offset estimator and a partial periodic table; And
And a residual offset estimator estimating a residual offset occurring after the precision offset estimation,
Wherein the residual frequency offset estimator estimates a frequency offset based on a partial periodic table that repeatedly estimates a residual frequency offset using the initial offset estimate, the precision offset estimate, and the partial period diagram.

(

ego,

Is an initial offset estimate,

Precision offset estimate,

,

) 8. The method of claim 7,
And the received signal processed by the EPP performing unit estimates a frequency offset based on a partial periodic table expressed by the following equation.

(here,

Of the channel of length L

The second complex impulse response coefficient,

Is a frequency offset normalized at subcarrier spacing,

Mean 0 and variance

Lt; RTI ID = 0.0 &gt; normal &lt; / RTI &gt; noise sample,

being) 8. The method of claim 7,
Wherein the initial offset estimator estimates a frequency offset based on a partial periodic table that estimates an initial offset using the following equation.

(here,

Lt; RTI ID = 0.0 &gt;

And α is a power of 2 less than N) delete 8. The method of claim 7,
The precision offset estimator

And estimating a frequency offset based on a partial periodic table in which the precise offset is repeatedly estimated.
(here,

ego,

,

,

,

Is an initial offset estimation value)

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