KR100594094B1 - Time and Frequency Synchronization Method for Reverse Link in Orthogonal Frequency Division Multiple Access Based Mobile Communication System - Google Patents

Abstract

According to the present invention, a time / frequency synchronization method in an orthogonal frequency division multiple access based communication system for synchronizing time / frequency of a base station and terminals by using a preamble signal transmitted by a plurality of terminals is a preamble at each terminal. Generating a signal, correcting and transmitting the preamble signal in time and frequency domain according to a time reference signal received from a base station; The base station separates the preamble signals received from the terminals, estimates time and frequency synchronization errors for each terminal using the preamble signals received from each terminal, and calculates the estimated time and frequency synchronization errors. Send to the terminal.

Quadrature Frequency Division Multiple Access, Time Synchronization, Frequency Synchronization, Preamble, Waveform Shaping Filter

Description

Time and Frequency Synchronization Method for Reverse Link in Orthogonal Frequency Division Multiple Access Based Mobile Communication System {UPLINK TIME AND FREQUENCY SYNCHRONIZATION METHOD FOR OFDMA BASED MOBILE COMMUNICATION SYSTEM}             

1 is a block diagram showing a preamble generating device of a user terminal according to a preferred embodiment of the present invention;

2 is a diagram illustrating a reverse preamble signal transmission process of an OFDMA system;

3 is a block diagram showing an apparatus for estimating time / frequency synchronization error of a base station receiver according to a preferred embodiment of the present invention;

4 is a graph showing a frequency response change by applying a waveform shaping filter according to a preferred embodiment of the present invention;

5A and 5B are graphs showing the results of comparing the leakage power according to the time synchronization error with and without the waveform shaping filter, respectively; And

6 is a graph showing the results of performance comparison experiments of the time / frequency synchronization estimation method according to the present invention and the conventional synchronization estimation method by observing the phase change after the Fourier transform.

The present invention relates to a mobile communication system, and more particularly, to a time and frequency synchronization method in an orthogonal frequency division multiple access (OFDMA) based mobile communication system.

In next generation mobile communication, high speed and high quality data transmission is required to support various multimedia services with higher quality. Recently, research on orthogonal frequency division multiple access has been actively conducted.

OFDM reduces performance degradation due to frequency selective fading channels that may be encountered in wideband transmission by dividing data into narrowband subcarriers orthogonal to each other. In addition, OFDM solves the problem of inter-symbol interference (ISI) due to multipath fading by inserting a guard interval.

Initial synchronization acquisition is very important in a communication system. Especially, a multicarrier-based communication system is sensitive to timing error, and therefore, a synchronization technique having excellent performance is required.

In general, in the OFDMA system, a synchronization technique using a guard interval or a time / frequency synchronization technique using a training symbol (preamble) in a system operating in a packet mode is applied. These techniques use the repetition characteristics of the received signal, and in a multi-user access method such as OFDMA, each user cannot be distinguished by the received signal alone.

For the time synchronization in the OFDMA environment, a method of estimating the time synchronization of each user by transmitting a multi-tone signal and observing the phase change after the Fourier transform (FFT) at the receiving side has been proposed. have. However, in most multi-carrier systems, the time synchronization estimation is performed on the time axis, and since the synchronization is estimated on the frequency axis to separate each user signal, a time error of each user may occur to prevent interference between users after the FFT. It is necessary to set a long enough cyclic guard interval as long as possible, and this long cyclic guard interval causes a reduction in the bandwidth efficiency of the system.

The present invention was devised to solve the above problems, and an object of the present invention is a time when a base station can perform synchronization with each user terminal while maintaining orthogonality between user terminals in a multi-carrier based multiple access system. And a frequency synchronization method.

Another object of the present invention is to provide a time and frequency synchronization method having excellent bandwidth efficiency because synchronization is performed without excessive cyclic guard intervals for maintaining orthogonality between user terminals.

로 정의되며, 여기서, K는 전체 부반송파의 수이고, k는 부반송파의 번호이고, N _f 는 한 개의 톤 집합에 포함된 부반송파의 개수인 것을 특징으로 한다. 상기 프리엠블 생성과정은 변조/매핑된 신호에 대해 역푸리에 변환을 수행하고 역푸리에 변환된 신호 병렬/직렬 변환하는 단계를 더욱 포함한다. 상기 프리엠블 생성과정은 병렬/직렬 변환된 신호에 대해 부대역 외부로의 방출 전력이 최소화 되도록 파형 성형 필터를 적용하는 것을 특징으로 한다.상기 파형 성형 필터는 레이즈드 코사인 (Raised Cosine: RC)필터인 것을 특징으로 한다. 상기 파형 성형 필터의 시간 응답 특성은 수학식 In order to achieve the above object, the time / frequency synchronization method according to the present invention generates a preamble signal in each terminal, and transmits by correcting the preamble signal in the time and frequency domain according to the time reference signal received from the base station and; The base station separates the preamble signals received from the terminals, estimates time and frequency synchronization errors for each terminal using the preamble signals received from each terminal, and calculates the estimated time and frequency synchronization errors. Characterized in that the transmission to the terminal. The preamble generation process includes a modulation / mapping step of selecting a plurality of adjacent subcarriers as subbands, modulating them into an arbitrary signal sequence, and filling zeros into the remaining subcarriers. The subband is expressed as:

Where K is the total number of subcarriers, k is the number of subcarriers, and N _f is the number of subcarriers included in one tone set. The preamble generation process further includes performing inverse Fourier transform on the modulated / mapped signal and performing inverse Fourier transform signal parallel / serial conversion. The preamble generation process is characterized by applying a waveform shaping filter to minimize the emission power to the outside of the subband for the parallel / serial conversion signal. The waveform shaping filter is a Raised Cosine (RC) filter It is characterized by that. The time response characteristic of the waveform shaping filter is

It is characterized by that. The time / frequency synchronization error estimation process is calculated by delaying the received preamble signal by N samples, multiplying the received preamble signal by a training signal of the corresponding terminal, and taking a moving sum. Taking a complex conjugate to a value, multiplying the training signal with the N sample delayed preamble signal to obtain a moving island, and taking the complex conjugate value and a moving island result for the N sample delayed preamble signal. The time synchronization error metric is calculated by multiplying the values, the maximum time synchronization error is detected from the time synchronization error metric, and the frequency synchronization error is detected using the maximum time synchronization error.

In another aspect of the present invention, in an orthogonal frequency division multiple access based communication system in which a base station estimates time / frequency error using a preamble signal received from a plurality of terminals, the terminal includes: a plurality of adjacent subcarriers Selected as a subband to modulate an arbitrary signal sequence and fills the rest of the subcarriers with zeros; A parallel / serial converter, and a waveform shaping filter for processing the output signal of the parallel / serial converter to minimize the amount of interference between different subbands for any time synchronization error. The waveform shaping filter is a Raised Cosine (RC) filter. The time response characteristic of the waveform shaping filter is expressed by Equation:

It is characterized by that. The base station includes a delayer for delaying a preamble signal of a specific user terminal separated from a received signal for N sample periods, a first mixer and a second mixer for multiplying each of the output signals of the preamble signal and the delayer with a training signal. A complex conjugate that takes a complex conjugate on the output of each of the first moving island unit and the second moving island unit and the first moving island unit to perform a moving sum operation on the outputs of the first mixer and the second mixer. A third mixer for calculating a plurality of time synchronization errors using the firearm, the output of the second moving island unit and the output of the complex conjugate, and detecting a maximum time synchronization error among the time synchronization errors calculated from the third mixer. And a calculator for detecting a frequency synchronization error using the maximum value detector and the maximum time synchronization error detected by the maximum value detector.

Hereinafter, a time / frequency synchronization method according to a preferred embodiment of the present invention with reference to the accompanying drawings.

1 is a view showing a preamble generating device of a user terminal according to an embodiment of the present invention.

As shown in FIG. 1, the preamble generating apparatus includes a tone selector 101 for selecting several adjacent subcarrier sets (subbands) and an inverse fast Fourier transform on output signals of the tone selector 101. An inverse Fourier transformer 103 for performing IFFT, a history / serial converter 105 for converting a parallel signal output from the inverse Fourier transformer 103 into a serial signal sequence, and a spectral shaping filter 107 It consists of.

The tone selector 101 selects subcarriers, modulates them into an arbitrary signal sequence { C _m }, and fills the remaining subcarriers with zeros. The arbitrary signal sequence is selected such that the final signal sequence after being converted by the inverse Fourier transformer 103 has a small Peak to Average Power Ratio (PAPR). This prevents signal leakage to other subbands when the Fourier transform is performed at exactly the same time synchronization time on the receiving side.

In a multipath fading environment, the time synchronization of each user is shifted from each other by factors such as distance from the base station, so it is difficult for the base station to completely separate signals of these other users using the FFT.

In order to separate the signal of each user terminal, the user must predict that a random time synchronization error will occur and set a longer cyclic protection interval to transmit a preamble signal, which causes excessive bandwidth waste.

In the present invention, the signal passing through the waveform shaping filter by using the waveform shaping filter instead of the cyclic protection interval becomes difficult to recover the original transmission signal { C _m } as it is due to the distortion of the magnetic signal in the subband, but instead the arbitrary In order to minimize the amount of interference between different subbands, the time synchronization error is selected.

In the Orthogonal Frequency Division Multiple Access (OFDMA) based communication system according to the present invention, a time-divided sync time slot is applied for a user who attempts initial access or a user who needs to reset. Each user terminal to be initially connected selects any one tone set among K subcarriers. In this case, the tone set may be expressed as Equation 1 below.

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Here, k represents the number of subcarriers, and N _f is the number of subcarriers included in one tone set. In order to minimize multi-user interference due to time synchronization and frequency synchronization error, an arbitrary tone set for the synchronization preamble is preferably defined using subcarriers adjacent to each other as shown in Equation 2, unlike the traffic signal sequence.

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In the case of such adjacent subcarriers, a single user terminal may simultaneously use two or more subbands to obtain frequency diversity gain. The training signal of the p-th user configured using the selected tone set may be represented by Equation 3 below.

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Here, g (n) denotes a discrete waveform shaping filter, and C _p (k) denotes a frequency axis signal sequence used for generating a training signal. The frequency signal sequence is chosen so that the peak-to-average power ratio (PAPR) of the training signal observed in the time domain is sufficiently small and has good autocorrelation characteristics.

In order to enable frequency synchronization estimation, the preamble of the p-th user is configured by the repetition signal sequence of the training signal and the N sample delayed training signal as shown in Equation 4 below.

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여기서, h_p(l)은 p번째 사용자의 다중경로 채널에 대한 임펄스 응답을, w(n)은 백색 가우시안 잡음을 나타내고,

와

는 각각 p번째 사용자의 주파수 오차와 시간동기 오차를 의미한다. 2 is a diagram illustrating a reverse preamble signal transmission process of an OFDMA system, in which a p th user terminal and a q th user terminal receive a time reference signal transmitted from a base station through a forward link and synchronize time and frequency with the reference signal. To match. Each user terminal transmits after correcting the preamble signal generated using the tone set of the tone itself by the set time and frequency synchronization. In this case, the time synchronization inaccuracy in the initial access of the user signal in the uplink is approximately twice the user transmission delay. Since the preamble signal transmitted from each user terminal includes different multipath channels, time, and frequency synchronization errors, the preamble signal received by the base station is represented by Equation 5 below.

Where h _p (l) represents the impulse response for the multipath channel of the p-th user, w (n) represents the white Gaussian noise,

Wow

Denotes the frequency error and time synchronization error of the p-th user, respectively.

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Using the preamble signal received in this way, the base station estimates the time and frequency synchronization error of each user.

3 is a diagram illustrating an apparatus for estimating time / frequency synchronization error of a base station receiver according to a preferred embodiment of the present invention.

As shown in FIG. 3, the apparatus for estimating the time / frequency synchronization error includes a delay 301 for delaying the received preamble signal y (n) by N samples, and training the p-th user on the preamble signal y (n). signal s _p (n) for multiplying a first mixer 302 and second mixer 303 for multiplying a p-th user of the training signal s _p (n) to an output signal of the delayer 301, the first mixer ( The first and second moving island units 304 and 305 for calculating moving sums of respective output signals output from the 302 and the second mixer 303, and the first moving island unit 304 of the first moving island unit 304. A complex conjugate 306 that takes a complex conjugate to an output signal, a third mixer 307 that generates a time-synchronizing metric that multiplies the output of the complex conjugate 306 by the output of the second moving island unit 305, A maximum value detector 308 and a maximum value detector 308 which detect a time point at which the time synchronization metric becomes the maximum and obtain a time synchronization error. By using the time offset obtained it consists of a calculator 309 to obtain the frequency synchronization error.

When N _m subbands are used for the training signal of the p-th user, the output of the correlator and the time synchronization metric for the corresponding user terminal may be expressed as in Equations 6 and 7 below.

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The time and frequency synchronization error of the p-th user terminal from the equation (7) is obtained as shown in equations (8) and (9), respectively.

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과 지연된 반복 신호열

이 시간축에서 서로 중첩되지 않는다면, 상관기의 출력에서의 사용자 단말 p의 신호 성분은 근사적으로 다음 수학식 10과 같이 표현된다.

Training signal train

And delayed repeat signal sequence

If they do not overlap each other on this time axis, the signal component of user terminal p at the output of the correlator is approximately expressed as in Equation 10 below.

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는 파형 성형 필터의 시간 및 주파수 거리에 따른 산란 함수를 의미하며, 상기 수학식 10에서는 B(Δ_t , Δ_f ) 의 Δ_t =0, Δ_f =ε_p 인 경우 이다. B(Δ_t , Δ_f )는 다음 수학식 11과 같이 정의된다.

Is when the mean scattering function of the time and the frequency distance of the waveform shaping filter, the equation (10) In _{(Δ t, Δ f) B} Δ t = 0, and the Δ _f = ε _p. B ( Δ _t , Δ _f ) is defined as in Equation 11 below.

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With this definition of _{B (Δ t, Δ f)} , interference caused by the user terminal q is simply expressed as equation (12).

Equation 12 shows the leakage power by the k th tone according to the time difference between the user terminals p and q.

Therefore, non-overlapping waveform shaping filters should be used to mitigate interference by other users. As such a waveform shaping filter, a preferred embodiment of the present invention uses a time-axis raised cosine (RC) filter.

The time response characteristic of the RC filter is expressed by Equation 13.

는 천이 시간의 길이를 나타낸다.

인 경우에는 성형 필터가 사용되지 않는 상태를 의미하고

인 필터 응답은 Hamming 윈도우로 잘 알려져 있다. here,

Represents the length of the transition time.

In the case of means that the shaping filter is not used

In filter response is well known as the Hamming window.

가 증가할수록 주파수 응답에서의 주부엽이 넓어지고 측부엽의 전력이 줄어드는 특징이 있기 때문에 다른 사용자의 간섭신호를 완화시킬 수 있는 특성을 가지게 된다. 따라서, 훈련 신호열은 파형 성형 필터를 통과함으로써 부채널간 간섭을 겪게 되지만 그 대신 부대역 외부로의 방출 전력이 감소하게 된다. Figure 4 is a graph showing the frequency response change by the waveform shaping filter according to a preferred embodiment of the present invention. As shown in Figure 4,

As is increased, the main lobe in the frequency response becomes wider and the power of the side lobe decreases, so the interference signal of other users can be mitigated. Thus, the training signal sequence experiences inter-channel interference by passing through the waveform shaping filter but instead reduces the emission power out of the subbands.

여기서,

은 사용자 단말 p의

번째 톤에서의 채널 주파수 응답을 나타낸다. 수학식 14에서 보는 바와 같이 전송된 주파수 신호성분

의 위상 상관관계에 의해 예민한 시간동기 메트릭을 얻을 수 있다. In order to evaluate the performance of the correlator using the training signal sequence shown in FIG. 3, a frequency selective channel is assumed. Assuming that the symbol length is sufficiently long compared to the channel response and the intersymbol interference is sufficiently small, the signal component in the metric can be approximated as shown in Equation 14 below.

here,

Of the user terminal p

Indicates the channel frequency response at the first tone. Frequency signal component transmitted as shown in equation (14)

The sensitive time synchronization metric can be obtained by the phase correlation of.

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5A and 5B are graphs comparing the leakage power according to the time synchronization error when the waveform shaping filter is not applied and the power shaping at the correlator output even when a time synchronization error occurs when the waveform shaping filter is applied. You can see that this is blocked. Therefore, by using such a waveform shaping filter, it is possible to obtain time synchronization while keeping the interference between subbands sufficiently small using a correlator on the time axis.

6 is a graph showing the results of performance comparison experiments of the time / frequency synchronization estimation method according to the present invention and the conventional synchronization estimation method by observing the phase change after the Fourier transform.

The performance test environment of the time / frequency synchronization method according to the present invention is as follows.

)의 범위 이내에서 발생하며, 시간 동기 오차는 한 심볼 구간 이내에서 균일한 분포를 가진다고 가정하였다.The total number of subcarriers (size of FFT) is 512 and 132 subcarriers at both ends corresponding to the outer boundary of the band among the total subcarriers are left unused for pulse shaping. Thus, the number of available tones is 380. Assume that you use 16 mutually exclusive tones of each user. The number of subbands available by this allocation method is 23. The channel environment is a Rayleigh fading channel environment with 64 taps, and the frequency error of each user is a fractional multiple (half of the subcarrier spacing:

), And it is assumed that time synchronization error has a uniform distribution within one symbol interval.

As shown in FIG. 6, it can be seen that the time / frequency synchronization estimation method according to the present invention has a significantly lower time synchronization error with respect to time / frequency signal-to-noise ratio (SNR) than the conventional synchronization estimation method.

As described above, the time and frequency synchronization method according to the present invention does not require excessive cyclic guard interval for maintaining orthogonality, thereby providing excellent bandwidth efficiency.

In addition, since the time-base correlator is used to estimate the synchronization error at the receiving end, it is possible to implement a robust estimator for the frequency error.

In addition, if bandwidth efficiency is kept constant, multiple subbands can be assigned to a single user, thereby improving frequency diversity performance.

Claims (12)

In an orthogonal frequency division multiple access based communication system for synchronizing time / frequency of a base station and terminals by using a preamble signal transmitted by a plurality of terminals, Generating, at each terminal, a preamble signal, correcting and transmitting the preamble signal in time and frequency domain according to a time reference signal received from a base station; The base station separates the preamble signals received from the terminals, estimates the time and frequency synchronization error for each terminal using the preamble signal received from each terminal, and estimates the estimated time and frequency synchronization error. Time / frequency synchronization method for transmitting to the terminal. The time / frequency synchronization method of claim 1, wherein the preamble generation process includes a modulation / mapping step of selecting a plurality of adjacent subcarriers as subbands, modulating them into an arbitrary signal sequence, and filling the remaining subcarriers with zeros. 3. The method of claim 2, wherein the subband is expressed as:

Wherein K is the total number of subcarriers, k is the number of subcarriers, and N _f is the number of subcarriers included in one tone set. 3. The time / frequency synchronization method of claim 2, wherein the preamble generation includes performing inverse Fourier transform on the modulated / mapped signal and performing parallel / serial conversion on the inverse Fourier transform signal. 5. The time / frequency synchronization method of claim 4, wherein the preamble generation process comprises applying a waveform shaping filter to minimize the emission power to the outside of the subband for the parallel / serial converted signal. 6. The method of claim 5, wherein the waveform shaping filter is a Raised Cosine (RC) filter. The time response characteristic of the waveform shaping filter is:

Time / frequency synchronization method characterized in that. The method of claim 1, wherein the time / frequency synchronization error estimation process is: Delaying the received preamble signal by N samples; Taking a complex conjugate to a value calculated by multiplying the received preamble signal by a training signal of a corresponding terminal and taking a moving sum; Multiplying the training signal by an N sample delayed preamble signal to obtain a moving island; Calculating a time synchronization error metric by multiplying the resultant result of the complex conjugate with a moving sum resultant value for the N sample delayed preamble signal; Detect a maximum time synchronization error in the time synchronization error metric; Time / frequency synchronization method characterized in that for detecting the frequency synchronization error using the maximum time synchronization error. In an orthogonal frequency division multiple access based communication system in which a base station estimates time / frequency error using a preamble signal received from a plurality of terminals, the terminal includes: A tone selector that selects a plurality of adjacent subcarriers as subbands, modulates them into an arbitrary signal sequence, and fills the remaining subcarriers with zeros; An inverse Fourier transformer for inverse Fourier transforming the output signal of the tone selector; A parallel / serial converter converting the output signal of the inverse Fourier transformer into a serial signal sequence; And And a waveform shaping filter for processing the output signal of the parallel / serial converter to minimize the amount of interference between different subbands for any time synchronization error. 10. The communication system of claim 9, wherein the waveform shaping filter is a Raised Cosine (RC) filter. 10. The method of claim 9, wherein the time response characteristic of the waveform shaping filter is:

A communication system, characterized in that. 10. The method of claim 9, wherein the base station is: A delayer for delaying a preamble signal of a specific user terminal separated from the received signal for N sample periods; A first mixer and a second mixer for multiplying each of the received preamble signal and the output of the delay by a training signal; A first moving island unit and a second moving island unit for performing a moving island operation on the outputs of the first mixer and the second mixer, respectively; A complex conjugate that takes a complex conjugate at an output of the first moving island unit; A third mixer configured to calculate a plurality of time synchronization errors by using the output of the second moving island unit and the output of the complex conjugate; A maximum value detector for detecting a maximum time synchronization error among the time synchronization errors calculated from the third mixer; And a calculator for detecting a frequency synchronization error using the maximum time synchronization error detected by the maximum detector.

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