KR20120059243A - Method and device for estimation frequency offset in the OFDM system - Google Patents

Abstract

PURPOSE: A method for estimating frequency offsets simplified in an OFDM(Orthogonal Frequency Division Multiplexing) system and a receiver thereof are provided to efficiently estimate frequency offsets by using a simplified near ML estimation function. CONSTITUTION: A receiver receives two OFDM signals from a transmitter(S301). A frequency offset estimation module of the receiver calculates a coefficient and a constant defined by using a received signal vector to generate a CFO(Carrier Frequency Offset) and a SFO(Sampling Frequency Offset) estimated according to a preset near ML estimation function(S302). The frequency offset estimation module calculates the CFO estimated by using a received signal vector and the coefficient and the constant calculated in a pre-stage(S303). The frequency offset estimation module calculates the SFO estimated by using the coefficient and the constant(S304).

Description

Simplified Frequency Offset Estimation Method and Receiver in Orthogonal Coded Frequency Division Multiplex Communication System

The present invention relates to a frequency offset estimation method and a receiver capable of frequency offset estimation in an orthogonal coded frequency division multiplexing method. To propose a frequency offset estimation method and receiver.

Orthogonal Frequency Division Multiplexing (hereinafter, referred to as 'OFDM') is a communication method for transmitting time signals using orthogonality of frequencies, and is currently used in digital video broadcasting (DVB) and wireless LAN. By using the OFDM scheme, it is possible to stably receive additional data services even at high speeds and is considered as a core technology of the next generation mobile communication system.

In OFDM system, since there are a large number of subcarriers in the entire transmission band and orthogonality is established between different subcarriers, a signal is generated in the frequency domain and an inverse discrete fourier transform (IDFT) is used to transmit the time domain. After creating a transmission to the channel.

The receiver removes the signal of the guard interval from the signal received through the channel, performs a Discrete Fourier Transform (DFT), converts the signal back into the frequency domain, and restores the transmitted data symbol.

In this case, if the DFT is not performed in the proper DFT section, that is, the portion that is not damaged by the channel delay in the guard section, inter-symbol interference or inter-carrier interference occurs and the desired received signal cannot be separated. A synchronization process, a detection of a pilot for channel estimation, a frame synchronization process of finding a boundary between frames for proper detection of data, and the like are performed. In addition, once the synchronization with the OFDM signal is synchronized, a cell identification process for confirming in which cell the signal is transmitted is also required.

Orthogonal Coded Frequency Division Multiplexing (Coded OFDM) is a proposed scheme for higher transmission rates, especially in frequency selective environments where multiple paths exist. However, this method is characterized by being sensitive to the carrier frequency offset, which is mainly caused by different carrier frequencies between the local oscillator of the transmitter and the receiver. Such carrier frequency offset may cause intercarrier interference in frequency division multiplexing, which may be a significant performance reduction factor in terms of system performance.

In the frequency division multiplexing system, various ICI cancellation methods have been proposed in the past. However, when applying the ICI cancellation method proposed in the prior art, there is a problem that the loss of frequency efficiency.

As another method, a method of estimating an offset using a frequency and time difference between a transmitter and a receiver in a frequency division multiplexing system has been proposed.

The present invention is to overcome the problems of frequency offset estimation of the prior art as described above, and proposes a scheme for estimating the offset using the frequency and sampling time difference between the transmitter and the receiver in the OFDM scheme.

Specifically, the present invention combines a carrier frequency offset generated by a frequency difference between a transmitter and a receiver and a sampling frequency offset issued by a clock time difference sampling between the transmitter and the receiver. We propose a method of estimating by

Technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

Frequency offset estimation method in an orthogonal coded frequency division multiplexing communication system according to an embodiment of the present invention for solving the above problems is applied to a carrier index of a received signal received from a transmitter and the received signal. Calculating a specific parameter used for frequency offset estimation based on the fast fourier transform (FFT) transform length and the cyclic prefix length; Estimating a carrier frequency offset using the received signal and the specific parameter; And estimating a sampling frequency offset using the specific parameter and the estimated carrier frequency offset.

The specific parameter according to an embodiment of the present invention may include a first parameter a ₁ and a second parameter b ₁ defined as in Equation 1 below.

&Quot; (1) "

In Equation 1, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, N represents an FFT length, and N _g represents a cyclic prefix length. K denotes the subcarrier index of the received signal which is an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1}. .

The carrier frequency offset estimation step according to an embodiment of the present invention may be defined based on the received signal and the specific parameter.

<Equation 2>

는 추정된 반송파 주파수 오프셋을 나타내고, R₀(k) 및 R₁(k)는 상기 수신신호에 포함되는 제1 수신 신호 및 제2 수신신호를 나타내고, a₁ 및 b₁은 상기 특정 파라미터에 포함되는 제1 파라미터 및 제2 파라미터를 나타내고, N은 FFT 길이를 나타내고, N_g는 cyclic prefix 길이를 나타내고, k는 M={-K/2, ..., -1, 0, 1, ..., K/2-1}로 정의되는 M집합의 원소인 상기 수신신호의 서브 반송파 인덱스를 나타낸다.In Equation 2,

Represents an estimated carrier frequency offset, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, and a ₁ and b ₁ are included in the specific parameter. Represents a first parameter and a second parameter, N represents an FFT length, N _g represents a cyclic prefix length, and k represents M = {-K / 2, ..., -1, 0, 1, .. , Subcarrier index of the received signal which is an element of the M set defined by K / 2-1}.

The sampling frequency offset estimating step according to an embodiment of the present invention may use Equation 3 defined based on the estimated carrier frequency offset and the specific parameter.

&Quot; (3) &quot;

은 추정된 샘플링 주파수 오프셋을 나타내고,

는 추정된 반송파 주파수 오프셋을 나타내고, a₁ 및 b₁은 상기 특정 파라미터에 포함되는 제1 파라미터 및 제2 파라미터를 나타낸다.In Equation 3,

Represents the estimated sampling frequency offset,

Denotes the estimated carrier frequency offset, and a ₁ and b ₁ denote the first parameter and the second parameter included in the specific parameter.

Preferably, the first received signal R ₀ and the second received signal R ₁ may be defined as in Equation 4 below.

&Quot; (4) &quot;

으로

,

을 이용하고, X_m은 상기 통신 시스템의 송신기로부터 전송된 노운 심볼을 나타내고, V_m은 노이즈 신호를 나타낸다.In Equation 4,

to

,

X _m denotes a known symbol transmitted from a transmitter of the communication system, and V _m denotes a noise signal.

According to another aspect of the present invention, a receiver capable of frequency offset estimation in an orthogonal coded frequency division multiplexing communication system includes a receiving module for receiving a radio signal transmitted from a transmitter and the And a frequency offset estimating module for estimating a frequency offset between a signal transmitted from a transmitter and a signal received through the receiving module, wherein the frequency offset estimating module comprises: a carrier index of the received signal; Calculate a specific parameter used for frequency offset estimation based on a fast fourier transform (FFT) transform length and a cyclic prefix length applied to the received signal, and use the calculated specific parameter to calculate a carrier frequency offset and Sampling frequency offset Can be estimated.

The frequency offset estimation module according to an embodiment of the present invention may calculate the first parameter a ₁ and the second parameter b ₁ included in the specific parameter by using Equation 5 below.

<Equation 5>

In Equation 5, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, N represents an FFT length, and N _g represents a cyclic prefix length. K denotes the subcarrier index of the received signal which is an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1}. .

The frequency offset estimation module according to an embodiment of the present invention may derive the estimated carrier frequency offset by using Equation 6 below.

<Equation 6>

는 추정된 반송파 주파수 오프셋을 나타내고, R₀(k) 및 R₁(k)는 상기 수신신호에 포함되는 제1 수신 신호 및 제2 수신신호를 나타내고, a₁ 및 b₁은 상기 특정 파라미터에 포함되는 제1 파라미터 및 제2 파라미터를 나타내고, N은 FFT 길이를 나타내고, N_g는 cyclic prefix 길이를 나타내고, k는 M={-K/2, ..., -1, 0, 1, ..., K/2-1}로 정의되는 M집합의 원소인 상기 수신신호의 서브 반송파 인덱스를 나타낸다.In Equation 6,

Represents an estimated carrier frequency offset, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, and a ₁ and b ₁ are included in the specific parameter. Represents a first parameter and a second parameter, N represents an FFT length, N _g represents a cyclic prefix length, and k represents M = {-K / 2, ..., -1, 0, 1, .. , Subcarrier index of the received signal which is an element of the M set defined by K / 2-1}.

The frequency offset estimation module according to an embodiment of the present invention estimates the sampling frequency offset by using the specific parameter and the estimated carrier frequency offset, but the estimated sampling frequency offset by using Equation 7 below. Can be derived.

<Equation 7>

은 추정된 샘플링 주파수 오프셋을 나타내고,

는 추정된 반송파 주파수 오프셋을 나타내고, a₁ 및 b₁은 상기 특정 파라미터에 포함되는 제1 파라미터 및 제2 파라미터를 나타낸다.In Equation 7,

Represents the estimated sampling frequency offset,

Denotes the estimated carrier frequency offset, and a ₁ and b ₁ denote the first parameter and the second parameter included in the specific parameter.

The above embodiments are only some of the preferred embodiments of the present invention, and various embodiments reflecting the technical features of the present invention are based on the detailed description of the present invention described below by those skilled in the art. Can be derived and understood.

According to the present invention, the accuracy and reliability of the frequency offset estimation method can be improved by using the frequency and sampling time difference between the transmitter and the receiver in the OFDM method.

In addition, according to the present invention, frequency offset estimation can be efficiently performed using a simplified near ML estimation function.

Technical problems to be achieved in the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above will be clearly understood by those skilled in the art from the following description. Could be.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, included as part of the detailed description in order to provide a thorough understanding of the present invention, provide examples of the present invention and together with the description, describe the technical idea of the present invention.
1 shows an example of a preamble structure related to the present invention.
2 is a block diagram illustrating an example of a transceiver of an orthogonal coding frequency division multiplexing scheme for frequency offset estimation according to an embodiment of the present invention.
3 is a diagram illustrating an example of using a near ML estimation method to perform frequency offset estimation according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is capable of various modifications and various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. In the following description of the present invention, if it is determined that the detailed description of the related known technology may obscure the gist of the present invention, the detailed description thereof will be omitted.

Terms such as 'first' and 'second' may be used to describe various components, but the components are not limited by the terms, and the terms are only used to distinguish one component from another component. Used.

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description, together with the accompanying drawings, is intended to illustrate exemplary embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The following detailed description includes specific details in order to provide a thorough understanding of the present invention. However, one of ordinary skill in the art appreciates that the present invention may be practiced without these specific details.

The ODFM system has a problem that synchronization errors may occur. In particular, a Carrier Frequency Offset (CFO) caused by the frequency difference between the transmitter and the receiver and a Sampling Frequency Offset (SFO) caused by the clock time difference between the transmitter and the receiver generate interference. And degrade the OFDM system performance.

In order to solve this problem, the present invention proposes a method of combining and estimating the CFO and SFO.

First, the preamble used for frequency offset estimation will be briefly described. In an OFDM system, a preamble consists of a training symbol previously promised before data is transmitted, and is used for timing synchronization, carrier frequency offset estimation, sampling frequency offset estimation, and channel estimation.

1 shows an example of a preamble structure related to the present invention.

Referring to FIG. 1, the preamble may be divided into a first training symbol and a second training symbol.

The first training symbol consists of two OFDM symbols while the second training symbol consists of one OFDM symbol. The second training symbol may be represented by Equation 1 by transmitting 0 through an odd subcarrier for transmitting data through an even subcarrier.

In Equation 1, X [k] represents the training sequence that is sent to the k-th sub-carrier in the m-th OFDM symbol on a second training symbol, d _k represents the k th sub-carrier for the pilot. That is, the second training symbol transmits pilot on the even subcarrier and 0 on the odd subcarrier.

The present invention proposes a frequency offset estimation method for combining and estimating the CFO and the SFO for the first training symbol and the second training symbol, and for simplicity of explanation, in the following embodiment of the present invention, the frequency offset for the second training symbol is provided. The estimation method will be described.

2 is a block diagram illustrating an example of a transceiver of an orthogonal coding frequency division multiplexing scheme for frequency offset estimation according to an embodiment of the present invention.

Referring to FIG. 2, the transmitter 200 block includes a serial to parallel (S / P) converter 201 and an inverse fast fourier transform (IFFT) for converting two known symbols from a serial signal to a parallel signal. Transmitter 202, Parallel to Serial (P / S) converter 203, Digital to Analog (D / A) converter 204 for converting digital signals to analog signals, RF local oscillator 205 and transmitting signals And a transmitting module 206 for the same.

In addition, the receiver 300 block includes a receiving module 301 for receiving a signal transmitted from the transmitter 200, an RF local oscillator 302 for performing frequency synthesis on the received signal, and converting an analog signal into a digital signal. Analog to Digital (A / D) (303), Serial to Parallel (S / P) Converter 304, Fast Fourier Transform (FFT) Converter 305, Parallel to Serial (P / S) Converter 306 And a frequency offset estimation module 307 for performing estimation of the frequency offset for the received signal.

Referring to FIG. 2, the transmitter 200 transmits two known symbols to the receiver 300 through each converter included in the transmitter. Known symbol X _m (k) is a value obtained independently from complex constellatio such as QPSK, and consists of a transmission vector in parallel form defined in Equation 2 in a serial signal via S / P converter 201.

는 이후 IFFT 변환기(202)를 거쳐서 시간 도메인 전송 벡터로 구성되고, 다시 P/S 변환기(203)를 거쳐서 직렬 신호로 변환된다. Transmission Vector of Equation 2

Is then composed of a time domain transmission vector via an IFFT converter 202 and then converted to a serial signal via a P / S converter 203.

시간마다 디지털 신호로 샘플링된다. 샘플링된 전송 벡터는 D/A 컨버터(204)에서 아날로그 신호로 변환되고, 변환된 아날로그 신호를 수학식 3과 같다.The transmission vector passing through the P / S converter 203 is determined by the sampling period T of the transmitter in the D / A converter 204.

It is sampled as a digital signal every hour. The sampled transmission vector is converted into an analog signal in the D / A converter 204, and the converted analog signal is expressed by Equation 3 below.

In Equation 3, m denotes an OFDM symbol index, k denotes an OFDM subcarrier index, N denotes an FFT length, N _g denotes a cyclic prefix length, and u (t) denotes a unit. Represents a step function. Also, the OFDM subcarrier index k is defined as {-K / 2, ..., K / 2-1}.

는 RF 국부 발진기(205)로부터 공급되는 주파수 합성을 위한 LO 주파수와 합성된다. 합성된 노운 심볼

는 수학식 4와 같이 송신기의 반송파 주파수(f_c)를 이용한 업 컨버전(up-conversion) 형태로 구성된다.Known symbol converted to an analog signal in Equation 3

Is synthesized with the LO frequency for frequency synthesis supplied from the RF local oscillator 205. Synthetic Known Symbol

As shown in Equation 4, is configured in the form of up-conversion (up-conversion) using the carrier frequency (f _c ) of the transmitter.

은

의 in-phase 성분을 나타내고,

는

의 구적법(quadrature) 성분을 나타낸다.In Equation 4,

silver

Represents the in-phase component of,

Is

The quadrature component of.

As such, the up-converted knockdown signal x (t) is transmitted to the receiver 300 using the multipath channel through the transmission module 206.

The signal received through the receiving module 301 of the receiver 300 is in the form of an additive white Gaussian noise (AWGN) noise (white noise) added to the transmission signal of the modified form through the multipath channel as shown in Equation 4 above. It may be received in the form of Equation 5.

은 다중 경로 채널로 정의되고, α_l 은 real값의 attenuation factor를 나타내고, τ_l은 딜레이 성분을 나타내고, w(t)는 수신신호에 포함되는 노이즈를 나타낸다.In Equation 5

Is defined as a multipath channel, α _l represents an attenuation factor of a real value, τ _l represents a delay component, and w (t) represents noise included in a received signal.

The receiver may perform down-conversion on the received signal using the carrier frequency f _c ′ of the receiver and convert the received signal as shown in Equation 6 below.

은 수신신호에 포함된 노이즈에 RF 국부발진기(302)로부터 인가된 LO 주파수 성분(

)이 합성된 형태를 나타낸다.In Equation 6,

Is the LO frequency component applied from the RF local oscillator 302 to the noise contained in the received signal.

) Represents the synthesized form.

시간마다 discrete한 디지털 신호로 변환된다. 하기 수학식 7은 시간 도메인에서의 샘플링된 수신 신호를 나타내는 것이다.The down-converted received signal of Equation 6 is the sampling period T 'of the receiver in the A / D converter 303.

Each time it is converted into a discrete digital signal. Equation 7 shows a sampled received signal in the time domain.

으로 정의되고,

으로 정의된다.In Equation 7,

Defined as

Is defined.

와 같은 벡터 형태고 구성된다. 이를 FFT 변환기(305)에서 FFR 변환을 수행하면 하기 수학식 8과 같은 주파수 도메인에서의 수신 벡터 신호로 나타낼 수 있다.The sampled received signal of Equation 7 is converted from a serial signal to a parallel signal form in the S / P converter 304,

Consists of a vector such as When the FFR transform is performed in the FFT converter 305, it may be represented as a received vector signal in the frequency domain as shown in Equation (8).

The received vector signal is converted into a serial received signal in the frequency domain as shown in Equation 9 through the P / S converter 306.

는

의 주파수 응답을 나타낸다. In Equation 9, k is defined as an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1},

Is

Represents the frequency response.

In addition, intercarrier interference (ICI) noise may be defined as in Equation 10.

는 본 발명의 실시예에 따른 주파수 오프셋 추정 함수를 이용하여 정의되며, 구체적으로는 반송파 주파수 오프셋(CFO)과 샘플링 주파수 오프셋(SFO)의 함수를 이용한다. In Equations 9 and 10

Is defined using a frequency offset estimation function according to an embodiment of the present invention, and specifically, a function of a carrier frequency offset (CFO) and a sampling frequency offset (SFO) is used.

Equation 11 below defines the CFO (ε) and SFO (η) estimated by the frequency offset estimation module 306 of the receiver 300 in the present invention. In the embodiment of the present invention, the carrier frequency and the sampling period of the transmitter 200 and the receiver 300 for the Joint Maximum-Likelihood function (hereinafter referred to as the joint ML function) for CFO (ε) and SFO (η). Define CFO (ε) and SFO (η) using.

로 송신기(200)의 반송파 주파수(f_c)와 수신기(300)의 반송파 주파수(f_c')의 차로 정의된다. 또한, 상기 수학식 10에서 T는 송신기(200)의 샘플링 주기를 나타내고, T'는 수신기(300)의 샘플링 주기를 나타낸다.Δf in Equation 11

This is defined as the difference between the carrier frequency f _c of the transmitter 200 and the carrier frequency f _c ′ of the receiver 300. In addition, in Equation 10, T represents a sampling period of the transmitter 200, and T 'represents a sampling period of the receiver 300.

는 하기 수학식 12와 같이 정의된다.Function using CFO (ε) and SFO (η) defined in Equation 11 above

Is defined as in Equation 12 below.

If the received signal in the frequency domain represented by Equation 8 is expressed in a vector form, it can be expressed as Equation 13 below.

) 벡터 및 ICI 벡터(

)의 합인

으로 나타낼 수 있다. 여기서,

은 영점 평균 및 코베리언스 행렬(covariance matrix)의 가우시안 분산 벡터인

로 추정할 수 있다.In Equation 13, a vector related to noise is noise received in a process of receiving a signal from the transmitter 200.

) And ICI vectors (

Sum of)

It can be represented as here,

Is the Gaussian variance vector of the zero mean and covariance matrix

It can be estimated as

) 및 SFO(

)을 도출하기 위한 함수이다.Thereafter, the frequency offset estimation module 307 of the receiver 300 may use a joint ML function for frequency offset estimation using two OFDM known symbols obtained as shown in Equation (13). Equation 14 below is a CFO estimated using CFO (ε) and SFO (η).

) And SFO (

) Is a function for deriving

은 수신신호 벡터이다. 송신기(200)로부터 동일한 2개의 노운 심볼(

)이 전송된다고 가정하면 수신신호 벡터(

)는 하기 수학식 15와 같이 나타낼 수 있다. In Equation 14

Is the received signal vector. The same two known symbols from the transmitter 200 (

) Is transmitted, the received signal vector (

) May be expressed as in Equation 15 below.

및 N은 수학식 16과 같이 정의될 수 있다.In Equation 15

And N may be defined as in Equation 16.

으로 가정할 수 있다. 이에 따라, 상기 수학식 14는 하기 수학식 17과 같이 간단하게 유도할 수 있다.In Equation 14 CFO (ε) and SFO (η) does not give information on the first received signal vector (R _o),

Can be assumed. Accordingly, Equation 14 may be simply derived as in Equation 17 below.

에 대한 최대값으로 나타낼 수 있다. likelihood function

의 평균은

으로, 코베리언스는

인 가우시안 벡터로 가정하면 likelihood function

는 수학식 18과 같이 도출할 수 있다.CFO (ε) and SFO (η) estimated in Equation 17 are likelihood functions

It can be expressed as the maximum value for. likelihood function

The average of

By, Coberians

Assuming a Gaussian vector that is likelihood function

Can be derived as in Equation 18.

으로 정의되므로, 이를 이용하여 상기 수학식 17을 벡터의 정의를 이용하여 계산하면, 본 발명의 실시예에 따른 주파수 오프셋 추정 모듈(307)에서 주파수 오프셋 추정을 위해 이용하는 조인트 ML 함수의 최종 형태는 하기 수학식 19와 같이 정의할 수 있다.In Equation 18

Since the equation 17 is calculated using the definition of the vector, the final form of the joint ML function used for the frequency offset estimation in the frequency offset estimation module 307 according to the embodiment of the present invention is as follows. It may be defined as in Equation 19.

) 및 추정된 SFO(

)를 도출할 수 있다. 2차원 서치 과정은 일정 범위 내 있는 CFO와 SFO 값들을 상기 수학식 19에 대입하여 계산한 값의 최소값을 구하는 방식이다.The frequency offset estimation module 307 is an example of an estimation method. The CFO estimated by a two-dimensional search process of CFO (ε) and SFO (η) in the joint ML function defined in Equation 19 above. (

) And estimated SFO (

) Can be derived. In the two-dimensional search process, a minimum value of a value calculated by substituting CFO and SFO values within a predetermined range into Equation 19 is obtained.

Alternatively, unlike the method of substituting all CFOs and SFOs within a predetermined range and viewing the estimated CFOs and SFOs as the estimated values, a simplified near ML estimation method may be used according to an embodiment of the present invention.

The near ML estimation method according to an embodiment of the present invention uses a simplified near ML estimation function by canceling constant portions not related to CFO (ε) and SFO (η) in the frequency offset estimation function described above.

Equation 20 is a near ML estimation function according to an embodiment of the present invention, in which Equation 19 eliminates constant portions not directly related to CFO (ε) and SFO (η).

를 하기 수학식 21과 같이 Taylor series expansion의 2차항 수식으로 근사화할 수 있다.Herein, when CFO (ε) and SFO (η) are sufficiently small,

It can be approximated by the quadratic formula of Taylor series expansion as in Equation 21 below.

으로 근사화할 수 있고, 이에 따라 상기 수학식 21은 하기 수학식 22와 같이 간소화될 수 있다.In Equation 21, when CFO (ε) and SFO (η) are sufficiently small

It can be approximated to, and accordingly, Equation 21 can be simplified as shown in Equation 22 below.

함수에 대입하면,

함수는 하기 수학식 23과 같이 나타낼 수 있다.Equation 22 to Equation 20

If you assign it to a function,

The function can be expressed as in Equation 23 below.

에 따라

함수를 최대화하기 위해, SFO(η)에 대한 2차 함수인

함수가

를 만족하는 때의 SFO(η)를 구할 수 있다. 이에 따라 추정된 SFO(

)는 하기 수학식 24와 같이 CFO(ε)에 대한 함수로 나타낼 수 있다.Frequency offset estimation function of Equation 20

Depending on the

To maximize the function, the quadratic function for SFO (η)

Function

The SFO (η) at the time of satisfying can be obtained. The estimated SFO (

) Can be expressed as a function of CFO (ε) as shown in Equation 24 below.

)를 구성하는 계수 a₁ 및 상수 b₁은 하기 수학식 25와 같이 정의될 수 있다.The estimated SFO, which can be expressed as a function of CFO (ε),

The coefficient a ₁ and the constant b ₁ constituting) may be defined as in Equation 25 below.

As shown in Equation 25, the coefficient a ₁ and the constant b ₁ may be expressed as a function of the reception signal vector, the OFDM subcarrier index k, N representing the FFT length, and N _g representing the cyclic prefix length. The index k is an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1}.

)를 도출할 수 있다. 이 과정에서, 주파수 오프셋 추정 모듈(307)은 송신기(200)로부터 수신한 수신신호(R₀, R₁)를 이용하여 상기 수학식 25와 같이 정의되는 상기 수학식 24의 계수 a₁ 및 상수 b₁을 구할 수 있다. That is, the frequency offset estimation module 307 is estimated using Equation 24 using an arbitrary CFO (ε) value to derive a frequency offset value through a near ML estimation function as shown in Equation 20 above. SFO (

) Can be derived. In this process, the frequency offset estimation module 307 uses the received signals R ₀ and R ₁ received from the transmitter 200 to determine the coefficient a ₁ and the constant b of Equation 24 defined as in Equation 25. ₁ can be obtained.

)에 관한 함수인 상기 수학식 23에 대입하여 CFO(ε)에 대한 함수로 정리하면,

함수는 하기 수학식 26과 같이

함수로 변환된다.Accordingly, the estimated SFO (

Substituting into Equation 23, which is a function of

The function is as shown in Equation 26

Converted to a function.

함수를 최대화하기 위해,

를 만족하는 CFO(ε)를 구할 수 있다. A quadratic function for the CFO (ε) summed up as in Equation 26 according to the frequency offset estimation function in Equation 20

To maximize the function,

The CFO (ε) satisfying can be obtained.

)는 하기 수학식 27을 통해 도출할 수 있다.Accordingly, the estimated CFO by near ML estimation according to an embodiment of the present invention

) Can be derived from Equation 27 below.

)값을 도출될할 수 있다. 그리고, 상기 수학식 27을 통해 도출된 추정된 CFO(

) 값을

으로 가정하여 상기 수학식 24에 대입하면, 하기 수학식 28과 같이 추정된 CFO(

)에 대한 함수로 정리되는 추정된 SFO(

)를 구할 수 있다. That is, the frequency offset estimation module 307 may estimate the estimated CFO defined by Equation 27 from the received signals R ₀ and R ₁ according to a preset near ML estimation function.

) Value can be derived. And, the estimated CFO (derived through Equation 27)

Value

Substituting the above equation (24), the estimated CFO (Equation 28)

Estimated SFO (), which is summarized as a function of

) Can be obtained.

) 및 추정된 SFO(

)를 구할 수 있다.Accordingly, the frequency offset estimation module 307 according to an embodiment of the present invention uses the estimated CFO (Simplified Estimation Function) as shown in Equation 28 derived from Equation 20, which is a near ML estimation function.

) And estimated SFO (

) Can be obtained.

3 is a diagram illustrating an example of using a near ML estimation method to perform frequency offset estimation according to an embodiment of the present invention.

In detail, the frequency offset estimation module 307 of the receiver 300 describes a process of estimating the frequency offset using the received signal received from the transmitter 200.

Referring to FIG. 3, the receiver 300 receives two OFDM signals from the transmitter 200 (S301).

) 및 추정된 SFO(

)를 도출하기 위해, 수신 신호 벡터(R₀. R₁)를 이용하여 상기 수학식 25와 같이 정의되는 계수 a₁과 상수 b₁을 계산한다(S302).The frequency offset estimation module 307 of the receiver calculates the estimated CFO (

) And estimated SFO (

), A coefficient a ₁ and a constant b ₁ defined as in Equation 25 are calculated using the received signal vector R _0. R ₁ (S302).

)를 계산한다(S303). 추정된 CFO(

)를 계산하기 위한 함수로는 상기 수학식 27을 이용한다.In addition, the frequency offset estimation module 307 calculates the estimated CFO using the received signal vector R _0. R ₁ and the coefficient a ₁ and the constant b ₁ calculated in the previous step.

) Is calculated (S303). Estimated CFO (

) Is used as a function to calculate

)가 도출되면, 주파수 오프셋 추정 모듈(307)은 단계 S302에서 구한 계수 a₁과 상수 b₁ 을 이용하여 상기 수학식 28과 같이 정의되는 추정된 SFO(

)를 계산한다(S304).Estimated CFO (

Is derived, the frequency offset estimation module 307 calculates the coefficient a ₁ and the constant b ₁ obtained in step S302. The estimated SFO defined by Equation 28 using

) Is calculated (S304).

The foregoing description is merely illustrative of the technical idea of the present invention, and various changes and modifications may be made by those skilled in the art without departing from the essential characteristics of the present invention. Therefore, the embodiments described in the present invention are not intended to limit the technical idea of the present invention but to explain, and the technical idea of the present invention is not limited by these embodiments. The scope of protection of the present invention should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.

Claims (10)

A frequency offset estimation method in a orthogonal coding frequency division multiplex communication system,
Calculating a specific parameter used for frequency offset estimation based on a carrier index of a received signal received from a transmitter, a fast fourier transform (FFT) transform length and a cyclic prefix length applied to the received signal;
Estimating a carrier frequency offset using the received signal and the specific parameter; And
Estimating a sampling frequency offset using the specific parameter and the estimated carrier frequency offset. The method of claim 1,
The specific parameter includes a first parameter (a ₁ ) and a second parameter (b ₁ ) defined as in Equation 1 below.
<Equation 1>

In Equation 1, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, N represents an FFT length, and N _g represents a cyclic prefix length. K denotes the subcarrier index of the received signal which is an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1}. . The method of claim 1,
The carrier frequency offset estimating step,
Using Equation 2 defined based on the received signal and the specific parameter.
&Quot; (2) &quot;

In Equation 2,

Represents an estimated carrier frequency offset, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, and a ₁ and b ₁ are included in the specific parameter. Represents a first parameter and a second parameter, N represents an FFT length, N _g represents a cyclic prefix length, and k represents M = {-K / 2, ..., -1, 0, 1, .. , Subcarrier index of the received signal which is an element of the M set defined by K / 2-1}. The method of claim 1,
The sampling frequency offset estimating step,
And using Equation 3 defined based on the estimated carrier frequency offset and the specific parameter.
<Equation 3>

In Equation 3,

Represents the estimated sampling frequency offset,

Denotes the estimated carrier frequency offset, and a ₁ and b ₁ denote the first parameter and the second parameter included in the specific parameter. The method of claim 2,
The first received signal (R ₀ ) and the second received signal (R ₁ ) are defined as in Equation 4, frequency offset estimation method.
<Equation 4>

In Equation 4,

to

,

X _m denotes a known symbol transmitted from a transmitter of the communication system, and V _m denotes a noise signal. A receiver capable of frequency offset estimation in an orthogonal coding frequency division multiplex communication system,
A receiving module for receiving a radio signal transmitted from a transmitter;
A frequency offset estimation module for estimating a frequency offset between a signal transmitted from the transmitter and a signal received through the receiving module,
The frequency offset estimation module,
Calculate a specific parameter used for frequency offset estimation based on a carrier index of the received signal, a fast fourier transform (FFT) transform length and a cyclic prefix length applied to the received signal, and use the calculated specific parameter And estimating a carrier frequency offset and a sampling frequency offset. The method of claim 6,
The frequency offset estimation module calculates a first parameter (a ₁ ) and a second parameter (b ₁ ) included in the specific parameter by using Equation 5 below.
<Equation 5>

In Equation 5, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, N represents an FFT length, and N _g represents a cyclic prefix length. K denotes the subcarrier index of the received signal which is an element of the M set defined by M = {-K / 2, ..., -1, 0, 1, ..., K / 2-1}. . The method of claim 6,
The frequency offset estimation module derives the estimated carrier frequency offset using Equation 6 below.
&Quot; (6) &quot;

In Equation 6,

Represents an estimated carrier frequency offset, R ₀ (k) and R ₁ (k) represent a first received signal and a second received signal included in the received signal, and a ₁ and b ₁ are included in the specific parameter. Represents a first parameter and a second parameter, N represents an FFT length, N _g represents a cyclic prefix length, and k represents M = {-K / 2, ..., -1, 0, 1, .. , Subcarrier index of the received signal which is an element of the M set defined by K / 2-1}. The method of claim 6,
The frequency offset estimation module estimates the sampling frequency offset using the specific parameter and the estimated carrier frequency offset,
A receiver for deriving the estimated sampling frequency offset using a preset equation (7).
&Quot; (7) &quot;

In Equation 7,

Represents the estimated sampling frequency offset,

Denotes the estimated carrier frequency offset, and a ₁ and b ₁ denote the first parameter and the second parameter included in the specific parameter. The method of claim 7, wherein
The first received signal (R ₀ ) and the second received signal (R ₁ ) are defined as in Equation 8 below.
<Equation 8>

In Equation 8,

to

,

X _m denotes a known symbol transmitted from a transmitter of the communication system, and V _m denotes a noise signal.

Images