KR20080001998A - Apparatus and method for estimating frequency offset in uplink of orthogonal frequency division multiple access system - Google Patents

Abstract

An apparatus and a method for estimating a frequency offset in an uplink of an OFDMA(Orthogonal Frequency Division Multiple Access) system are provided to estimate a frequency offset efficiently without inducing power consumption and complexity of a base station receiver. An apparatus for estimating a frequency offset in an uplink of an OFDMA system includes an observation window setting unit(701), a fourier transform unit(702), a phase compensation unit(703), a subcarrier selector(704), and a frequency offset estimation unit(706). The observation window setting unit outputs at least two time domain signals having a predetermined time delay difference from a received signal through first and second observation windows having a different time delay. The fourier transform unit transforms at least two time domain signals to at least two frequency domain signals. The phase compensation unit compensates a phase change by a time delay difference between the first observation window and the second observation window. The subcarrier selector selects subcarriers of a specific user for estimating the frequency offset in at least two frequency domain signals of which a phase change is compensated. The frequency offset estimation unit calculates a phase difference of subcarriers having the same subcarrier index from the selected subcarriers. The frequency offset estimation unit estimates a frequency offset for a received signal of a specific user based on the phase difference.

Description

Apparatus and method for estimating frequency offset in uplink in orthogonal frequency division multiple access system {APPARATUS AND METHOD FOR ESTIMATING FREQUENCY OFFSET IN UPLINK OF ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS SYSTEM}

1 is a block diagram schematically illustrating an apparatus for estimating and compensating a frequency offset of a user in uplink of a typical OFDMA system.

FIG. 2 is a diagram schematically illustrating a structure of a block allocation method in a method of allocating a subcarrier signal of a user to a service band in a typical OFDMA system. FIG.

FIG. 3 is a diagram schematically illustrating a structure of an interleaving allocation method of a method of allocating a subcarrier signal of a user to a service band in a typical OFDMA system. FIG.

4 is a diagram illustrating a method of estimating a frequency offset by observing a change in phase inherent in an OFDMA symbol in a general OFDMA system.

FIG. 5 is a block diagram illustrating an apparatus for estimating user frequency offset in uplink of an OFDMA system according to the prior art. FIG.

6 is a block diagram conceptually illustrating a configuration of an apparatus for estimating a frequency offset in a receiver of an OFDMA system according to an embodiment of the present invention.

7 is a block diagram illustrating a configuration of an apparatus for estimating a frequency offset in a receiver of an OFDMA system according to an embodiment of the present invention.

FIG. 8 is a block diagram showing the configuration of a phase comparator among the components of the frequency offset estimation device shown in FIG. 7; FIG.

9 is a flowchart illustrating a frequency offset estimation method in a receiver of an OFDMA system according to an embodiment of the present invention.

10 and 11 are graphs showing the performance of the frequency offset estimation apparatus according to an embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

701: observation window setting unit 702: high-speed Fourier transducer

703: phase compensator 704: user subcarrier selector

704: phase difference comparator 705: phase detector

706 multiplier

The present invention relates to an apparatus and method for estimating a frequency offset of a user in an uplink of an orthogonal frequency division multiple access system (hereinafter referred to as OFDMA). Blind Frequency Offset Estimation for estimating the frequency offset experienced by a particular user's signal without any type of training sequence or repeating pattern of symbols.

The OFDMA system is currently in the spotlight as one of the multiple access systems capable of simultaneously serving several different users while supporting high frequency band usage efficiency and transmission rate. In the OFDMA system, the total frequency resources are allocated to different users. Each user uses multiple subcarriers, which overlap on the frequency axis, but maintain orthogonality with each other due to the nature of the Orthogonal Frequency Division Multiplexing (OFDM) signal. Like the OFDM system, the OFDMA system is resistant to narrowband noise and does not require an adaptive equalizer on the time base.

On the other hand, OFDMA systems are also very sensitive to synchronization problems, like OFDM systems, and especially when frequency offsets exist, it is difficult to maintain orthogonality between subcarriers, causing Inter Carrier Interference (ICI). There is a problem that the performance of the system is severely degraded by causing multiple access interference (hereinafter referred to as MAI). In addition, unlike the case of the OFDM system or the downlink such as a broadcast channel, in the uplink of the OFDMA system, the user's transmission signal has a different frequency offset, so it is much more difficult to measure and remove the frequency offset. It is a problem. The signal of a particular user trying to estimate the frequency offset must first be separated from the signals of other users prior to the operation of the frequency offset estimation algorithm. To this end, the conventional method uses a method of separating a received user's signal from the signals of different users using a filter in the time domain.

FIG. 1 is a block diagram schematically illustrating an apparatus for estimating and compensating a frequency offset experienced by a received signal in a receiver of a typical OFDMA system.

Referring to FIG. 1, an apparatus for estimating and compensating a frequency offset of a receiver includes an RF receiver 101 and a carrier frequency offset (CFO) estimator 102. The RF receiver 101 converts an RF signal received through the antenna ANT into a baseband signal by performing frequency down converting. The CFO estimator 102 analyzes a received OFDMA signal converted into a baseband signal to estimate a carrier frequency offset (CFO) of a specific user signal to estimate a frequency offset. The frequency offset of the specific user signal estimated by the CFO estimator 102 is transmitted to a specific user in the form of a control message through a downlink control channel to control the oscillator of the transmitting end of the user terminal. Alternatively, the estimated frequency offset of the specific user is used to compensate for the phase change caused by the frequency offset experienced by the specific user signal in the receiving end of the base station.

Hereinafter, a method of allocating subcarriers of users in a service band of a typical OFDMA system will be described with reference to FIGS. 2 and 3.

FIG. 2 illustrates a structure in which subcarriers of a specific user are allocated in a block form to a specific region of a service band in a typical OFDMA system.

That is, assuming that the total number of subcarriers in the OFDMA system is N and the number of users concurrently accessing the service is U, assuming that a uniform number of subcarriers is allocated to all users, The number of subcarriers is N / U. These N / U subcarriers are allocated in block form to occupy adjacent positions in the frequency domain. This type of subcarrier allocation may be relatively less affected by intercarrier interference (ICI) caused by frequency offset or user-to-user multiple access interference (MAI) caused by frequency offset. However, this subcarrier allocation scheme does not gain diversity gain on the frequency axis, and thus loses strong characteristics against narrow band interference, which is an advantage of OFDMA.

FIG. 3 is a diagram illustrating a structure in which subcarriers of a specific user are allocated in a form distributed in all areas of a service band in a general OFDMA system.

That is, assuming that the total number of subcarriers of the OFDMA system is N and the number of users concurrently connected to the service is U, it is assumed that a uniform number of subcarriers is allocated to all users. The number of subcarriers received is N / U. These N / U subcarriers are arranged in the form of interleaving with other users' subcarriers to be uniformly distributed in the entire frequency domain. This type of subcarrier allocation scheme can maximize diversity gain on the frequency axis, and thus has a strong characteristic against narrowband interference. However, this subcarrier allocation scheme has a disadvantage in that the structure of the base station receiver for estimating the frequency offset in the uplink of the OFDMA system is different because the frequency offset between users is different.

FIG. 4 is a view for explaining a method of estimating a frequency offset by observing a phase change in an OFDMA symbol in a receiver of a typical OFDMA system, and illustrates a structure of an OFDMA transmission signal of a specific user.

The method of estimating the frequency offset by observing the change of phase inherent in the OFDMA symbol of FIG. 4 uses the feature that the cyclic prefix (CP) has the same pattern as a specific part of the transmission symbol. It is a method of estimating the frequency offset on the time axis by calculating the correlation between them. For convenience, the number of subcarriers in the OFDMA system is set to N. Therefore, the length of an OFDM symbol excluding the CP is N samples and the length of the CP is shown as a cyclic prefix length.

Hereinafter, a conventional frequency offset estimation method will be described.

5 is a block diagram illustrating an apparatus for estimating a user frequency offset of a base station receiver in an uplink of an OFDMA system according to the prior art.

In FIG. 5, it is assumed that the total number of users served for convenience is U. FIG. Referring to FIG. 5, a frequency offset estimation apparatus for a user according to the related art includes filters 501-1 to 501-U, delayers 502-1 to 502-U, and multipliers 503-1 to 503-U. And accumulators 504-1 through 504-U, phase detectors 505-1 through 505-U, and multipliers 506-1 through 506-U. The frequency offset estimation apparatus according to the related art separates signals for each user using a plurality of filters on the time axis, and then estimates the frequency offset on the time axis using the CP part. The operation of each block is described in detail.

In FIG. 5, the filters 501-1 to 501 -U separate a signal of a specific user to estimate a frequency offset from baseband-converted uplink received signals. The delayers 502-1 to 502-U delay the user-specific signals separated by the filters 501-1 to 501-U for a specified value N, and then multiply this value by the multiplier 503-1. ~ 503-U). The multipliers 503-1 to 503-U multiply the conjugate signal of the delayed received signal by the currently received signal and output a correlation value between the received signals to the accumulators 504-1 to 504-U. That is, the phase difference between the CP portion and the same pattern of the transmission data in the received signal is compared and output to the accumulators 504-1 to 504-U. The accumulators 504-1 to 504-U accumulate correlation values between the received signals, and the phase detectors 505-1 to 505-U accumulate the output signals of the accumulators 504-1 to 504-U. Detect phase. The multipliers 506-1 to 506-U multiply the outputs of the phase detectors 505-1 to 505-U by 1 / (2 * circumference) to calculate a frequency offset.

As described above, the frequency offset estimation method of a user uplink signal of the prior art performs an operation of separating a signal of a specific user from overlapping signals of several users using a filter. This is a method suitable for a case in which a user-carrier allocation scheme described above with reference to FIGS. 2 and 3 uses a block allocation structure in which subcarriers of a specific user are located in an adjacent frequency band, as shown in FIG. 2. Are difficult to apply to the subcarrier interleaving allocation structure in which they are mixed in a frequency band.

Therefore, in order to obtain sufficient frequency diversity of the user's signal as described in FIG. 3, a more efficient frequency offset estimator suitable for an interleaving structure distributed and mixed with subcarriers of other users over the entire frequency band of the OFDMA system; A method is required.

The present invention proposed to solve the problems of the prior art as described above, the object of the present invention is to provide a frequency offset estimation apparatus and method for estimating a frequency offset of a user without consuming bandwidth in the uplink of the OFDMA system have.

Another object of the present invention is to provide a frequency offset estimation apparatus and method for estimating a frequency offset of a user without increasing hardware complexity of a receiver in an uplink of an OFDMA system.

Another object of the present invention is to provide an apparatus and method for estimating a user frequency offset suitable for a subcarrier interleaving allocation structure in which subcarriers of users are mixed in a frequency band in an uplink of an OFDMA system.

In order to achieve the above object, an apparatus for estimating a frequency offset in an uplink of an orthogonal frequency division multiple access (OFDMA) system according to an embodiment of the present invention is provided through a first observation window and a second observation window having different time delays. An observation window setting unit for outputting at least two time domain signals having a predetermined time delay difference from the received signal; A Fourier transformer converting the at least two time domain signals into at least two frequency domain signals; A phase compensator for compensating for a phase change caused by the time delay difference between the first and second observation windows; A subcarrier selector for selecting subcarriers of a specific user to estimate the frequency offset in the at least two frequency domain signals compensated for by the phase change; Frequency offset estimation means for obtaining a phase difference between subcarriers having the same subcarrier index from the selected subcarriers and estimating a frequency offset for the received signal of the specific user based on the phase difference.

In addition, the method for estimating the frequency offset in the uplink of an orthogonal frequency division multiple access (OFDMA) system according to an embodiment of the present invention for achieving the above object through a first observation window and a second observation window having different time delays. Setting at least two time domain signals having a predetermined time delay difference from the received signal; Converting the at least two time domain signals into at least two frequency domain signals; Compensating for a phase change caused by the time delay difference between the first and second observation windows; Selecting subcarriers of a particular user to estimate the frequency offset in the at least two frequency domain signals whose phase change is compensated for; Obtaining a phase difference between subcarriers having the same subcarrier index from the selected subcarriers; Estimating a frequency offset of the received signal of the specific user based on the phase difference.

The base station receiver of the OFDMA system according to the present invention having the above-described configuration can estimate a frequency offset of a specific user to estimate without consuming bandwidth, which is an important resource of the system. It does not increase the hardware complexity of the receiver frequency offset estimator, and in particular the frequency offset of a particular user without increasing the complexity of the hardware in a subcarrier interleaving assignment scheme that mixes and assigns subcarriers of users over the entire service frequency band to obtain frequency diversity. Efficient frequency offset estimation and control can be achieved without consuming bandwidth.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention.

6 is a block diagram conceptually illustrating a configuration of an apparatus for estimating a frequency offset in a receiver of an OFDMA system according to an embodiment of the present invention.

는 두 개의 관찰 윈도우(observation window 1, 2)간의 시간 지연을 나타내며, U는 상향 링크에 접속한 전체 사용자들의 수를 나타낸 것이다. 그리고

는 n번째 OFDMA 신호를 i번째 관찰 윈도우(i=1,2)를 통하여 출력한 후 푸리에 변환한 주파수 영역 부반송파 중 p 번째 부반송파를 나타내며, p∈{u}는 u 번째 사용자에 속하는 부반송파의 인덱스들을 나타낸 것이다.In the drawing of FIG. 6

Denotes the time delay between two observation windows 1 and 2, and U denotes the total number of users connected to the uplink. And

Denotes the pth subcarrier of the Fourier transformed frequency domain subcarriers after outputting the nth OFDMA signal through the i th observation window (i = 1,2), and p∈ {u} denotes indices of subcarriers belonging to the u th user. It is shown.

Referring to FIG. 6, in an uplink of an OFDMA system, a base station receiver down-converts a received user signal and then converts a baseband user signal into a predetermined time according to two preset observation windows 601 and 611. Output as two different time domain signals with delay differences. The two different time domain signals are transformed into OFDMA signals in two frequency domains through two Fast Fourier Transforms (FFTs) 602 and 612. In the OFDMA signals of the two frequency domains, subcarriers of different users separated from frequency bands are located. At this time, if only the subcarriers of a specific user are selected among the subcarriers included in the two Fourier transformed frequency domain OFDMA signals, only the user's signal for estimating the frequency offset is separated without requiring the user's distinction through the filter in the time domain. I can make it.

Comparing the phase difference between carriers located in the same frequency band among the two subcarrier signal sets of the selected specific user, the frequency offset experienced by the specific user signal in the uplink can be estimated. That is, multiplying the complex conjugate signal between the subcarriers of the signal output from the second observation window 2 611 having the same index as the Fourier transformed subcarrier of the signal output from the first observation window 601 results in the phase due to the frequency offset. Since changes can be observed, accumulating these values can estimate the frequency offset experienced by a particular user signal. In this case, as shown in FIG. 6, the relative phase shift according to the set time delay between the first and second observation windows 601 and 611 is phased with respect to the frequency domain signal corresponding to the second observation window 611. Compensation is made by the compensator 613.

7 is a block diagram showing the configuration of an apparatus for estimating a frequency offset in a receiver of an OFDMA system according to an embodiment of the present invention.

Referring to FIG. 7, an apparatus for estimating frequency offset according to an embodiment of the present invention receives two received signals in a time domain including CP and OFDMA received symbols and has a predetermined time delay difference according to two preset observation windows. An observation window setting unit 701 for separating into two different time domain signals, and a Fourier transform unit 702 for converting two time domain signals outputted through the observation window setting unit 701 into OFDMA signals in a frequency domain ), A phase compensator 703 for compensating for the phase change according to the time delay between the observation windows set in the two Fourier transformed frequency domain signals, and a specific user who is trying to estimate the frequency offset in the phase compensated two frequency domain signals. A subcarrier selector 704 that selects only subcarriers of the subcarrier and a subcarrier having the same subcarrier index in the selected set of subcarriers A phase difference comparator 705 for comparing the phase difference between the waves, a phase detector 706 for detecting the phase of the signal by receiving the output of the phase difference comparator 705, and a time difference between the observation windows at the output of the phase detector 706, OFDMA And a multiplier 707 for size conversion considering the length and circumference of the symbol.

인 경우도 있으므로, 이 경우를 제외하고는 모든 관찰 윈도우는 CP의 일부를 포함하게 된다.Here, one of the two observation windows includes at least a part of the CP, and both observation windows have the same length as the symbols of the OFDM except the CP. And out of the observation window

In some cases, except for this case, all observation windows include a part of the CP.

Hereinafter, the operation of the receiver of FIG. 7 to which the OFDMA system according to the embodiment of the present invention having the above configuration is applied will be described in detail.

In FIG. 7, the observation window setting unit 701 separates the received OFDMA symbol into two different time domain signals according to the set observation window and outputs the same to the fast Fourier transformer 702. The two time-domain signals output from the observation window setting unit 701 may be simply expressed in a matrix form as shown in Equation 1 below.

는 관찰 윈도우의 시간 지연에 따른 수신 신호의 위상차를 나타내며,

는 주파수 오프셋에 의해 발생하는 시간 영역 수신 신호의 위상 변화를 나타낸다. 또한 u는 주파수 오프셋을 추정하려는 u번째 사용자를 나타내며,

는 p번째 사용자의 전송 데이터의 채널 응답 성분을 나타내며, n은 수신된 n번째 OFDMA 신호를 나타내며,

는 추정하려는 주파수 오프셋 값이다. 또한, W는 하기의 <수학식 2>와 같은 N by N 역 푸리에 변환(Inverse Discrete Fourier Transform; 이하 IDFT라 함) 매트릭스를 나타낸다.here,

Represents the phase difference of the received signal according to the time delay of the observation window,

Denotes the phase change of the time-domain received signal caused by the frequency offset. U also represents the u-th user who wants to estimate the frequency offset,

Denotes a channel response component of the p-th user's transmission data, n denotes the received n-th OFDMA signal,

Is the frequency offset value to be estimated. W represents an N by N Inverse Discrete Fourier Transform (IDFT) matrix as shown in Equation 2 below.

은 m번째 요소가 a(m)인 1 by N 벡터를 나타낸다.In the above description of <Equation 1>

Denotes a 1 by N vector whose mth element is a (m).

에 의해 발생한 상기 <수학식 1>에 표현된

과

간의 위상의 천이

를 보상하여 상기 제1, 제2 관찰 윈도우(601, 611) 구간에 따른 2개의 N by 1 벡터의 주파수 영역 신호를 사용자 부반송파 선택기(704)로 출력한다. 사용자 부반송파 선택기(704)는 입력된 두 개의 N by 1 벡터의 주파수 영역 신호에서 주파수 오프셋을 추정하려는 사용자의 부반송파 성분만을 선택하여 위상차비교기(705)에 출력한다. 하기의 <수학식 3>은 상기 제1 관찰 윈도우(601)에 대응하는 부반송파 선택기(704)의 출력 신호이다. In FIG. 7, the Fourier transform unit 702 receives two outputs of the observation window setting unit 701 as inputs, performs Fourier transform, and then outputs them to the phase compensator 703. The phase compensator 703 has a time difference set between the first observation window 601 and the second observation window 611.

Expressed by Equation 1 generated by

and

Transition of phase of liver

By compensating for the frequency domain signals of two N by 1 vectors along the first and second observation windows 601 and 611, the user subcarrier selector 704 is output. The user subcarrier selector 704 selects only the subcarrier components of the user whose frequency offset is to be estimated from the input frequency domain signals of the two N by 1 vectors and outputs them to the phase difference comparator 705. Equation 3 below is an output signal of the subcarrier selector 704 corresponding to the first observation window 601.

는 푸리에 변환된 N by 1 벡터 성분중 사용자 u에게 할당된 부반송파 신호를 골라내는 함수이다. 즉, N 개의 부반송파 중 L(=N/U)개의 부반송파를 선택하기 위한 L by N 매트릭스로서 그 성분인

는 k 번째 부반송파가 사용자 u의 l 번째 성분이 될 때에만 1이고, 나머지는 0으로 이루어지게 된다.here

Is a function of selecting a subcarrier signal assigned to user u among Fourier transformed N by 1 vector components. That is, the L by N matrix for selecting L (= N / U) subcarriers among the N subcarriers,

Is 1 only when the k-th subcarrier becomes the l-th component of the user u, and the rest is 0.

를 나타낸다.Equation 4 below performs the phase compensation of the phase compensator 703 to the frequency domain signal of the second observation window 611 and passes the user subcarrier selector 704 to the frequency domain signal.

Indicates.

는 제1 관찰 윈도우(601)와 제2 관찰 윈도우(611) 출력간의 시간차에 의한 위상차이를 보상하는 부분이다. here

Is a portion that compensates for the phase difference due to the time difference between the output of the first observation window 601 and the second observation window 611.

에 의한 위상 변화 성분과 수신 신호가 겪은 주파수 오프셋에 의한 위상 변화 성분을 포함한다. 즉, 시간 영역 신호

는 시간 영역 신호

으로부터

와

의 위상 차이를 가지게 되므로 푸리에 변환된 주파수 영역 신호

도

로부터 같은 수준의 위상 차이를 갖는 다. 따라서 위상보상기(703)의 동작과 같이

에 대한 위상천이를 보상하면 주파수 오프셋에 의한 위상의 변화 성분만이 남게 된다.In more detail, the frequency domain signal of the Fourier transformed second observation window 611 has an additional phase difference from the signal from the Fourier transformed first observation window 601. This phase difference is the time difference between observation windows

It includes a phase change component by and a phase change component by the frequency offset experienced by the received signal. That is, time domain signal

Time domain signal

From

Wow

Fourier transformed frequency-domain signal

Degree

From the same level of phase difference. Therefore, as in the operation of the phase compensator 703

Compensating the phase shift with respect to leaves only the component of the phase change due to the frequency offset.

Therefore, by applying this, the frequency offset v of the user u can be obtained as shown in Equation 5 below.

과

사이의 위상의 차이를 비교하여 출력한다. 즉,

과

의 켤레 복소수와의 곱을 취해 두 신호간의 위상 차이인

를 구한 후 위상 검출기(706)으로 출력한다. 위상검출기(706)은 입력된 신호에서 위상을 검출하여 곱셈기 (707)로 출력하고, 곱셈기(707)은

를 곱한 후 최종적으로 사용자 u의 주파수 오프셋 v를 출력한 다.Equation 5 is implemented as follows. The phase comparator 705 is represented by Equations 3 and 4 above.

and

Compare the phase difference between the outputs. In other words,

and

Multiply by the complex conjugate of to determine the phase difference between the two signals,

Is obtained and output to the phase detector 706. The phase detector 706 detects a phase from the input signal and outputs the phase to the multiplier 707. The multiplier 707

Multiply by and finally output the frequency offset v of user u.

의 성분과

성분의 켤레 복소수와의 곱을 취한다. 이를

과

의 부반송파로 표현하면, 제1 관찰윈도우에 대응하는

과

의 켤레 곱을 취한 후 사용자 u에게 할당된 부반송파 성분들의 복소합을 취한다. 여기서 p는 부반송파의 인덱스로 p가 사용자 u에게 할당된 경우만 위상차 비교에 반영이 된다. FIG. 8 is a diagram showing the configuration of the phase comparator 705 shown in FIG. 7, and the detailed operation of the phase comparator 705 will be described with reference to FIG. 8. The phase comparator 705 corresponds to the first observation window.

With the ingredients of

Take the product of the complex conjugate of the component. This

and

In terms of the subcarrier of, corresponding to the first observation window

and

After taking the conjugate product of, we take the complex sum of the subcarrier components assigned to user u. Here, p is an index of the subcarrier and is reflected in the phase difference comparison only when p is assigned to the user u.

9 is a flowchart illustrating a frequency offset estimation method in a receiver of an OFDMA system according to an embodiment of the present invention, which illustrates a frequency offset estimation process in a receiver having the configuration of FIG. 7.

를 곱한 후(S907), 908 단계에서 최종적으로 특정 사용자 신호의 주파수 오프셋을 출력한다.In step 901 of FIG. 9, the observation window setting unit 701 sets first and second observation windows having a predetermined time difference to the received baseband signal of the uplink user. That is, the OFDM received symbols are separated into different time domains according to the set observation window. In step 902, the fast Fourier transform unit 702 converts two time-domain signals corresponding to the first and second observation windows into OFDMA signals in the frequency domain, and in step 903, the phase compensator 703 observes the two observations. Compensate for the phase shift due to the time difference between the windows. Subsequently, in step 904, the subcarrier selector 704 selects a subcarrier of a user who wants to estimate a frequency offset from two phase compensated frequency domain OFDMA signals. Next, in step 905, the phase comparator 705 compares a phase difference between subcarriers having the same subcarrier index between two frequency domain OFDMA signals corresponding to the first and second observation windows in which only a subcarrier of a corresponding user remains. 705 calculates the difference in phase by multiplying the subcarrier signal from the first observation window by the conjugate of the subcarrier signal from the second observation window. In operation 906, the phase detector 706 detects a phase shift due to an uplink frequency offset in a specific user signal based on a phase difference between subcarriers transmitted from the phase comparator 705. The receiver then replies with the phase value detected in step 907.

After multiplying by (S907), in step 908 finally outputs the frequency offset of the particular user signal.

10 and 11 are graphs showing the performance of the frequency offset estimation apparatus according to an embodiment of the present invention in a white Gaussian noise channel.

In the computer simulation showing the graph characteristics of FIG. 10, the number of subcarriers was 256, the length of the guard interval was 32 samples, and the number of simultaneous accessors was 8 people. Each user uses an interleaving structure in which 32 subcarriers are uniformly distributed in the service frequency band. The horizontal axis of the graph represents the signal-to-noise ratio (SNR) of the received signal, and the vertical axis represents the average squared error value (Nomalized MSE) of the estimated frequency offset. As shown in FIG. 10, the method proposed in the embodiment of the present invention showed a very good mean square error value when the frequency offset experienced by the user signal is not large, and the performance deteriorates as the frequency offset of the user increases.

FIG. 11 is a graph illustrating a performance change of the frequency offset estimating apparatus according to an embodiment of the present invention when the frequency synchronization of different users connected simultaneously is random. In FIG. 11, the horizontal axis of the graph indicates a user to be estimated. It represents the fluctuation of the frequency offset of other connectors except the one. That is, it is a graph showing the change of the performance of the proposed frequency offset estimator due to the multiple access interference of these users when the frequency offsets of other users vary randomly from -F to + F. As shown in FIG. 11, the frequency offset estimator of the proposed method showed a very good mean square error value, unless the signals of other users suffered from severe frequency offsets. see. As shown in FIG. 10 and FIG. 11, the proposed method shows that the user's signal for estimating the frequency offset does not generate a significant frequency offset error or that the signals of other users do not generate a serious frequency offset error. Unless otherwise, the mean squared error value is very small.

The proposed method of the present invention does not require any kind of pilot symbol or training sequence in order to estimate the frequency offset of a specific user in the uplink of an OFDMA system in which signals of several users have different frequency offsets. Therefore, there is a gain that can continuously estimate the frequency offset without consuming the bandwidth of the system. Therefore, the frequency offset estimation method proposed in the present invention can be very effectively applied to a situation in which the frequency offset tracking of a specific user is performed continuously.

According to the present invention made as described above, the frequency offset can be estimated without consuming bandwidth by estimating the frequency offset through two observation windows having a predetermined time difference of one symbol of the received OFDMA system scheme. In the embodiment, it was found that the base station receiver can achieve smooth frequency offset estimation and control without increasing the hardware complexity when estimating the frequency offset without the training sequence.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

The present invention described above does not require any kind of pilot symbol or training sequence in order to estimate the frequency offset of a particular user in the uplink of an OFDMA system in which signals of several users are connected simultaneously with different frequency offsets. There is a benefit in that the frequency offset can be estimated continuously without consuming bandwidth.

In addition, there is an effect that the frequency offset can be efficiently estimated without causing an increase in the complexity and power consumption of the base station receiver, which can be caused when estimating the frequency offset without transmitting pilot symbols or repetitive data.

The present invention also provides an apparatus and method for estimating a user frequency offset that can be flexibly applied to a subcarrier interleaving allocation structure in which subcarriers of users are mixed in a frequency band in an uplink of an OFDMA system.

Claims (7)

An apparatus for estimating frequency offset in the uplink of an orthogonal frequency division multiple access (OFDMA) system, An observation window setting unit configured to output at least two time domain signals having a predetermined time delay difference from a received signal through a first observation window and a second observation window having different time delays; A Fourier transformer for converting the at least two time domain signals into at least two frequency domain signals; A phase compensator configured to compensate for a phase change caused by the time delay difference between the first and second observation windows; A subcarrier selector for selecting subcarriers of a specific user to estimate the frequency offset in the at least two frequency domain signals whose phase change is compensated for; A frequency offset estimating means for obtaining a phase difference between subcarriers having the same subcarrier index from the selected subcarriers and estimating a frequency offset for the received signal of the specific user based on the phase difference. Estimation device. According to claim 1, One of the first and second observation windows includes a portion of a cyclic prefix (CP) included in an OFDM symbol of the received signal, and both the first and second observation windows are the same as the OFDM symbol except for the CP. Apparatus for estimating frequency offset in the uplink of the OFDMA system, characterized in that having a length. The method of claim 1, The Fourier transformed signal through the Fourier transform unit includes a phase change component caused by a time difference between the first and second observation windows and a phase change component caused by a frequency offset experienced by the received signal. Device for estimating frequency offset on a link. The method of claim 1, The frequency offset estimating means A phase difference comparator for comparing phase differences between subcarriers having the same subcarrier index;  A phase detector for detecting a phase shift of the signal of the specific user by the frequency offset based on the phase difference;  And a multiplier for outputting the frequency offset by multiplying the detected phase shift value by a predetermined compensation value. A method of estimating frequency offset in the uplink of an orthogonal frequency division multiple access (OFDMA) system, Setting at least two time domain signals having a predetermined time delay difference from the received signal through the first observation window and the second observation window having different time delays; Converting the at least two time domain signals into at least two frequency domain signals; Compensating for a phase change caused by the time delay difference between the first and second observation windows; Selecting subcarriers of a particular user to estimate the frequency offset in the at least two frequency domain signals with the phase change compensated; Obtaining a phase difference between subcarriers having the same subcarrier index from the selected subcarriers; Estimating a frequency offset for the received signal of the specific user based on the phase difference. The method of claim 5, One of the first and second observation windows includes a portion of a cyclic prefix (CP) included in an OFDM symbol of the received signal, and both the first and second observation windows are the same as the OFDM symbol except for the CP. A method of estimating frequency offset in the uplink of an OFDMA system, characterized in that it has a length. The method of claim 5, The Fourier transformed signal includes a phase change component caused by the time delay difference between the first and second observation windows and a phase change component caused by the frequency offset experienced by the received signal. Method of estimating frequency offset in.

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