KR20070087449A - A preamble structure and a syncronous method for frequency offset correction in ofdm-fdma/cdma/tdma system - Google Patents

Abstract

A preamble structure and a synchronizing method for improving frequency offset correction in an OFDM(Orthogonal Frequency Division Multiplexing)-FDMA(Orthogonal Frequency Division Multiple Access)/CDMA(Code Division Multiple Access)/TDMA(Time Division Multiple Access) system are provided to perform integer frequency offset correction at a time domain and more exact symbol timing correction by using the symmetric and repetitive preamble structure. A preamble is repeated four times by first to fourth sample groups in a time domain and has a symmetric structure. In the second and fourth sample groups, samples having the same number as the first and third sample groups are arranged in reverse order of an order that samples of the first and third sample groups are received. Integer frequency offset correction can be possible.

Description

A PREAMBLE STRUCTURE AND A SYNCRONOUS METHOD FOR FREQUENCY OFFSET CORRECTION IN OFDM-FDMA / CDMA / TDMA SYSTEM}

1 is a diagram illustrating a preamble structure of a conventional 802.16a / d / e and WiBro system.

2 is a diagram illustrating the structure of an integer frequency offset and a cell searcher in a frequency domain using a conventional preamble.

3 is a diagram illustrating a structure in a time domain of a preamble according to the present invention.

4 is a diagram illustrating a real value in the time domain of a preamble according to the present invention.

5 illustrates imaginary values in the time domain of a preamble according to the present invention.

6 is a hardware configuration diagram for frequency offset estimation using a preamble according to the present invention.

7 is a diagram illustrating a comparison of a frequency offset estimation range of a conventional preamble and a preamble according to the present invention.

8 illustrates timing metric M _(d) of symbol timing estimation using a preamble in accordance with the present invention.

9 illustrates timing metric A _(d) of symbol timing estimation using a preamble in accordance with the present invention.

10 illustrates frequency offset estimation performance of a conventional preamble.

11 illustrates frequency offset estimation performance of a preamble according to the present invention.

<Explanation of symbols for main parts of the drawings>

110: autocorrelator 111: delay unit

112: complex number 113: inverter

114: multiplier 120: moving average

130: phase detector

The present invention relates to a preamble structure and a synchronization method for synchronizing a mobile communication system. In particular, the present invention relates to an OFDM- scheme that enables integer frequency offset estimation and more accurate symbol timing estimation in a time domain using a mutually symmetrical and repeated preamble structure. A preamble structure and a synchronization method for improving frequency offset estimation performance in an FDMA / CDMA / TDMA system.

In the 4th Generation (hereinafter referred to as '4G') communication system, a transmission rate of about 100 Mbps and various quality of service (hereinafter referred to as 'QoS') services are provided to users. Active research is underway to provide.

Currently, 3rd Generation (hereinafter, referred to as '3G') communication system generally supports a transmission rate of about 384 Kbps in an outdoor channel environment having a relatively poor channel environment, and even in an indoor channel environment having a relatively good channel environment. It supports up to 2Mbps transfer rate.

Meanwhile, wireless local area network (LAN) systems and wireless metropolitan area network (MAN) systems generally support transmission rates of 20 Mbps to 50 Mbps.

Therefore, in the current 4G communication system, a new communication system is developed in a form of guaranteeing mobility and QoS in a wireless LAN system and a wireless MAN system that guarantee a relatively high transmission speed to provide a high-speed service to be provided in the 4G communication system. There is a lot of research going on to support it.

Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) schemes are considered as a solution.

OFDM is a type of multicarrier modulation that provides excellent performance in multipath and mobile reception environments. For this reason, it is attracting attention as a modulation method suitable for terrestrial digital TV and digital voice broadcasting.

The existing IEEE 802.11 WLAN uses 2.4GHz Industrial, Scientific, and Medical (ISM) using Direct Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS), and Infrared (IR). It supported a transmission rate of 2Mbps in the band.

However, these standards cannot meet the increasing demand for higher data rates. In 1999, new physical layer standards for IEEE 802.11a and IEEE 802.11b were finalized.

IEEE 802.11b supports a transmission rate of 11 Mbps using CCK (Complementary Code Keying), which extends the existing DSSS method in the 2.4 GHz band, and is now widely commercialized.

Meanwhile, IEEE 802.11a adopts an OFDM modulation scheme to overcome the limitation of the DSSS scheme in the unlicensed national information infrastructure (U-NII) unlicensed band of 5 GHz and to obtain a higher data rate.

For error correction, a convolutional coder with code rates 1/2, 2/3, and 3/4 and a 1/2 Viterbi decoder are used to decode convolutional codes. Binary Phase Shift Keying (BPSK) is used for subcarrier modulation. Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (16-QAM), and 64-QAM.

In such an OFDM system, in order to accurately demodulate a transmission signal, frequency offset and symbol synchronization should be considered. If the symbol start point is not properly found, Inter-symbol Interference (ISI) occurs and the transmission signal cannot be restored correctly.

In general, a start point of a symbol is found by using correlation values between received signal sequences, and a signal sequence of a specific preamble is used to obtain a correlation value.

The preamble is a signal used to synchronize transmission timing between two or more systems in network communication. Appropriate timing ensures that all systems correctly interpret the beginning of information delivery.

The preamble is typically used by connecting the short preamble required to perform coarse synchronization and the long preamble required to perform fine frequency synchronization.

1 shows a preamble structure of WiBro and 802.16 d / e systems.

The downlink preamble of the portable Internet system based on OFDMA is used for initial time synchronization, frequency offset estimation, and cell search, and has a high iteration three times in the time domain after passing through the IFFT. The subcarriers constituting the preamble are transmitted after a specific PN (Pseudorandom Noise) code is modulated by BPSK and has a characteristic of repeating Np (1/3 of N _FFT where Np = FFT size) in the time domain.

In the preamble structure currently supported by the 802.16 and WiBro standards, the integer frequency offset cannot be estimated during the frequency offset estimation process.

See also P.Moose, "A technique for orthogonal frequency division multiplexing frequency offset correction," IEEE Trans. Comm., Vol. 42, pp. 2908-2914, Oct. In 1994., we proposed a method for estimating prime frequency offset using guard interval of OFDM system.

Therefore, a separate process of estimating an integer frequency offset is required, which is performed in the cell search process.

2 shows the structure of an integer frequency offset and a cell searcher in a frequency domain using a conventional preamble.

The cell searcher has 2N + 1 cross-correlators when the integer frequency offset to be estimated is N to estimate the integer frequency offset.

That is, each of the cross-correlators 20 receives the preambles of the received frequency domain from 0 to 1 with respect to a signal through which the input signal (Pream_AFFT), which is a signal subjected to FFT with respect to the received signal, has passed through the serial / parallel converter 10. The correlation is shifted to ± N.

The peak detector 30 compares the maximum value of each cross-correlator 20 and outputs the cell-index (Cell_IDX) and the information (int_CFO) of the cross-correlator 20 having the maximum value, which are not shown. The frequency offset estimator may estimate the frequency offset.

That is, the receiver estimates the cell identification and the integer offset by measuring the cross correlation value between the preamble in the received frequency domain and a known preamble signal stored in the preamble register 40.

Assuming that the symbol timing is obtained within the guard interval, the received signal undergoes the following phase rotation in the frequency domain due to timing error in the correct FFT timing.

......(수학식 1)

(Equation 1)

In Equation 1, P (k) is a preamble signal, and Δt is a timing error.

Therefore, the receiver obtains the cross-correlation value for each cell for each cross-correlator 20 as shown in Equation 2 so that the cross-correlation value can be measured regardless of the phase rotation caused by the timing error. By comparing the output values, the index of the maximum value is determined as the cell index, and the cross-correlator 20 from which the maximum value is output is estimated as an integer multiple frequency offset.

.....(수학식2)

..... (Equation 2)

는 c번째 셀 코드를 사용하는 프리앰블 신호를 의미한다.In equation (2)

Denotes a preamble signal using the c-th cell code.

As described above, the frequency offset estimation according to the conventional preamplifier structure has a problem of increasing complexity when implementing the system to estimate the frequency offset of integer multiples.

The present invention has been made in view of the above, and an object of the present invention is to provide a mutually symmetrical and repeating preamble structure capable of estimating integer frequency offset in the time domain.

Another object of the present invention is the frequency offset estimation performance in an OFDM-FDMA / CDMA / TDMA system that allows the estimation of integer frequency offset and more accurate symbol timing in the time domain using a mutually symmetrical and repeated preamp structure The present invention provides a synchronization method for improvement.

In the preamble structure according to the present invention for achieving the object of the present invention, in the preamble structure of a mobile communication system, the preamble is repeated four times by the first sample group to the fourth sample group in the time domain, the symmetric structure Wherein the second sample group and the fourth sample group has the same number of samples as the first and third sample group are arranged in the reverse order of the received order of the samples of the first and third sample group integer frequency offset It is characterized in that the estimation is possible.

Preferably, the number of samples of the first to fourth sample groups is 1/4 of the size of the FFT.

에 의해 주파수 옵셋을 추정하여 주파수 옵셋 추정 범위를 증대시키는 것을 특징으로 한다. A synchronization method for improving frequency offset estimation performance in an OFDM-FDMA / CDMA / TDMA system for achieving the above object is repeated a predetermined time by the first to fourth sample groups in the time domain and has a symmetrical structure. The second sample group and the fourth sample group may have a preamble structure in which the same number of samples as the first and third sample groups are arranged in the reverse order of the received samples of the first and third sample groups. In a method of estimating and synchronizing a frequency offset through a frequency offset estimator in a system, d is a sample index of a received signal, l is a correlation length index (0 to N _FFT / 4), and N _FFT : FFT size ( Size),

It is characterized in that to increase the frequency offset estimation range by estimating the frequency offset.

In addition, the synchronization method for improving the frequency offset estimation performance in the OFDM-FDMA / CDMA / TDMA system of the present invention is repeated a predetermined time by the first to fourth sample group in the time domain, and has a symmetrical structure, The second sample group and the fourth sample group have a preamble structure in which the same number of samples as the first and third sample groups are arranged in a reverse order to the received order of the samples of the first and third sample groups. A method of estimating and synchronizing a symbol timing offset through a symbol timing estimator in

When r _(d) is the received signal of the preamble and N is N _FFT (FFT Size),

,

이며, 타이밍 메트릭 M_pro(d)는

일 때, 상기 프리앰블이 N_FFT/8을 주기로 임펄스 형태를 갖는 특성을 바탕으로 상기 타이밍 메트릭 M_pro(d)에 대한 이동 평균을 취하여 심볼 타이밍 옵셋을 추정하는 것을 특징으로 한다.

,

The timing metric M _pro (d) is

In this case, a symbol timing offset is estimated by taking a moving average of the timing metric M _pro (d) based on a characteristic in which the preamble has an impulse shape with N _FFT / 8.

에 의해 얻어지며, 초기 심볼 타이밍은 상기 타이밍 메트릭이 최대가 되는 지점에 N_FFT/2만큼 빼주고, 예상되는 채널의 딜레이 확장(delay spread)만큼 추가로 빼준 위치로 설정하는 것이 바람직하다.The moving average is

The initial symbol timing is preferably set to a position where the initial metric timing is subtracted by N _FFT / 2 at the point where the timing metric becomes the maximum, and further subtracted by the delay spread of the expected channel.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following examples are merely to illustrate the present invention is not limited to the contents of the present invention.

3 shows the structure of the preamble in the time domain according to the present invention.

As shown, the preamble has a structure in which the same number of sample groups A, -A, A, -A are repeated four times after the cyclic prefix (hereinafter, referred to as CP) in the time domain.

Each sample group has a number of samples having a length of N _FFT / 4, which is a value obtained by dividing N _FFT , which is the size of _FFT by 4, -A in the reverse order of receiving the same number of samples as A sample group. Means an array of sample groups. In FIG. 3, CP has 128 samples, and A and -A have 256 samples.

......(수학식 3)This preamble will be examined in detail through the following equation.

(Equation 3)

In Equation 3, A is composed of B and C, and C is a form of symmetry of B and a negative sign attached only to the real part of the signal.

4 and 5 show the real and imaginary values of the preamble according to the present invention in the time domain, respectively. As described in Equation 3, it can be seen that the preamble according to the present invention is repeated and symmetric in the time domain.

6 shows a hardware configuration for estimating frequency offset using a preamble according to the present invention.

Frequency offset estimation using the preamble of the present invention is auto-correlated in the auto-correlator 110 using the characteristics of the preamble repeatedly transmitted with a time interval of N _FFT / 4.

That is, the auto-correlator 110 delays the signal received from the delay unit 111 by one quarter of the symbol size N _FFT , and the empty complex samples of the delayed signal are output through the complex unit 112. The inverted signal of the received signal inverted through the inverter 113 and the output of the complex unit 112 are multiplied by the multiplier 114 to autocorrelate.

A moving average is taken by the moving averager 120 from the output of the auto-correlator 110, and the phase detector 130 detects a phase of the moving average of the autocorrelation value, and from this, a frequency offset weight not shown. The government will estimate the frequency offset.

The estimated frequency offset is expressed by Equation 4.

.....(수학식 4)

..... (Equation 4)

Wherein, d: sample index of the received signal, l: Any (Corellation) length index _{(0 ~ N FFT / 4)} , N FFT: a FFT size (Size), with respect to the sample (d) of the input signal l Correlate with increasing (0 ~ N _FFT / 4) and add them together.

Figure 7 shows the frequency offset estimation range of the conventional preamble and the preamble of the present invention.

Frequency offset estimation of an OFDM signal in the time domain uses a repeated characteristic of the signal, and the process is as follows. If the transmitted signal is s _n , the complex baseband representation of the band pass signal y _n is expressed by Equation 5.

......(수학식 5)

(Equation 5)

In Equation 5, f _tx is a transmitter carrier frequency and T _s is a sampling period.

The received signal r _n ignoring noise after the receiver demodulates the signal at the receiver carrier frequency f _tx is as follows.

......(수학식 6)

(Equation 6)

In Equation 6, Δf = f _tx -f _rx , which is a difference between a transmitter and a receiver carrier frequency.

The delay between samples of the repeated signal is referred to as D, and the parameter z can be expressed as follows.

......(수학식 7)

(Equation 7)

  The last value calculated in Equation 7 is a sum of complex variables having a value proportional to a frequency offset. The frequency offset is estimated by Equation 8.

.....(수학식 8)

..... (Equation 8)

In the above equation, the range of z is Rz denotes a received signal.

Therefore, the maximum estimable frequency offset is defined as in Equation 9.

......(수학식 9)

(Equation 9)

The preamble provided by the conventional general standard has a structure that is repeated three times in the time domain as shown in FIG.

However, since the FFT has a butterfly structure, it is a square of 2, so it is not repeated three times in the time domain.

Therefore, the frequency offset estimation in the time domain should use the repetitive characteristics of the guard interval of the OFDM signal. As such, when the frequency offset is estimated using the guard interval, D = N _FFT in Equation 9, and the range of the estimated frequency offset is limited to within ± 0.5 of the subcarrier frequency interval.

As shown in FIG. 3, since the preamble of the present invention has a structure that is repeated four times in the time domain, D = N _FFT , and when substituted into Equation 9, the range of the estimated frequency offset is ± 2.0. The frequency offset estimation range is four times larger than in the case of using the preamble, and the frequency offset estimation process in the separate frequency domain is not required. Therefore, it is possible to greatly reduce the complexity and operation process in the actual hardware implementation.

8 is a diagram illustrating a timing metric M _(d) of symbol timing estimation using a preamble according to the present invention. Although not shown, the symbol timing estimation in the present invention is a symbol timing estimation unit of a receiver of a mobile communication system. Of course it is done in.

Timing offset estimation using the preamble of the present invention does not use the conventional Schmidl & Cox method, and performs a cross-correlation method using Equation 10 using symmetrically repeated characteristics of the preamble.

.....(수학식 10)

..... (Equation 10)

In Equation 10, r _(d) is a received signal of a preamble according to the present invention, and N is N _FFT .

Equation 10 is normalized to the power of the received signal (R _(d) ), the power of the received signal is shown in Equation 11.

.....(수학식 11)

..... (Equation 11)

Equation 10 is normalized to Equation 11 to generate the timing metric M _(d) , which is the same as Equation 12.

......(수학식 12)

(Equation 12)

9 shows the newly generated metric A _(d) by taking the moving average of the timing metric M _(d) .

It can be seen that the generated timing metric M _(d) has an impulse shape unlike conventional timing metrics.

This is because the symmetrical nature of the preamble was used when calculating cross correlation. However, the calculated timing metric M _(d) are ahead of the point by the guard interval length than the mid-point and the mid-point of the signal, so therefore has a maximum value at two points, is correct in such a manner as to detect a maximum value of M _(d) The timing acquisition becomes difficult.

의 길이 즉, N_FFT/8을 주기로 임펄스 형태를 갖고 있는데, 이러한 특성을 이용하여 수학식 13과 같은 이동 평균을 취하도록 한다. This problem can be solved by taking a moving average. The timing metric M _(d) is

The impulse shape has a length of N _FFT / 8, and the moving average is obtained by using this property.

......(수학식 13)

(Equation 13)

Referring to FIG. 9, it can be seen that the timing metric has an impulse shape every N _FFT / 8, and has a maximum value at the center of the symbol, that is, the N _FFT / 2 point.

In the ideal case where there is no influence of the channel or interference signal, the frame start position and initial symbol timing are obtained at the center of the symbol, that is, N _FFT / 2 point, so the FFT window position is added to the acquired position by N _FFT / 2 length.

However, due to the delay spread of the channel or the influence of interfering signals, the timing metric is more likely to be obtained later than it would be ideal.

Therefore, the initial symbol timing is subtracted by N _FFT / 2 at the point where the timing metric becomes the maximum, and is set to a position where the expected delay delay of the channel is further subtracted.

10 is a diagram illustrating frequency offset estimation performance of a conventional preamble.

If the applied frequency offset is 0.2 or 0.4, the AFC (Automatic Frequency Control) operates normally and converges to a certain value, and then efficiently removes the frequency offset of the VCTCXO.However, if the frequency offset is 0.8 or 1.3, the frequency offset can be estimated. It can be seen that AFC does not operate normally because it is out of range and converges to integer frequency offset 1.

This is because the frequency offset estimation range of the conventional preamble is ± 0.5. The frequency offset generated in this way can be eliminated through the frequency offset estimation process in the frequency domain, but the frequency offset becomes larger in the course of convergence to integer frequency offset 1, resulting in performance degradation of the traffic channel.

11 illustrates frequency offset estimation performance of a preamble according to the present invention. It can be seen that the frequency offset of the VCTCXO is efficiently removed by converging to a constant value regardless of the applied frequency offset. This is because the frequency offset estimation range of the preamble of the present invention is ± 2.0.

As described above, although described with reference to a preferred embodiment of the present invention, those skilled in the art various modifications of the present invention without departing from the spirit and scope of the invention described in the claims below Or it may be modified.

As described above, the present invention proposes a preamble structure in which a pattern is repeated four times in the time domain after passing an Inverse Fast Fourier Transform (IFFT), and thus the frequency offset estimation range of the existing IEEE802.16a / d / e and WiBro It is a four-fold improvement from ± 0.5 to ± 2.0 of the system, and more accurate symbol timing can be obtained by generating impulse timing metrics using the mutually symmetrical characteristics of the preamble.

In addition, if the preamble structure having a pattern repeated four times is applied to an OFDM-FDMA / CDMA / TDMA system, the integer frequency offset estimation process is shifted from the time domain to the frequency domain, thereby reducing complexity in implementing the system and mutual symmetry of the preamble. By using the initial symbol timing estimation method using the characteristic, the accurate timing can be obtained to improve the performance of the traffic channel.

Claims (6)

In the preamble structure of a mobile communication system, The preamble is repeated four times by the first to fourth sample groups in the time domain, and has a symmetrical structure, wherein the second sample group and the fourth sample group are the same number as the first and third sample groups. The preamble for improving the frequency offset estimation performance of the OFDM-FDMA / CDMA / TDMA system, characterized in that the samples are arranged in the reverse order of the received order of the samples of the first and third sample group rescue. The preamble structure for improving frequency offset estimation performance of an OFDM-FDMA / CDMA / TDMA system according to claim 1, wherein the number of samples of the first to fourth sample groups is 1/4 of an FFT size. . The sample is repeated a predetermined time by the first to fourth sample groups in the time domain, and has a symmetrical structure, wherein the second sample group and the fourth sample group include the same number of samples as the first and third sample groups. A method of estimating and synchronizing a frequency offset through a frequency offset estimator in a mobile communication system having a preamble structure arranged in a reverse order to a received order of samples of the first and third sample groups, d is the sample index of the received signal, l is the correlation length index (0 ~ N _FFT / 4), N _FFT : FFT size (Size),

And estimating the frequency offset to increase the frequency offset estimation range. The synchronization method for improving the frequency offset estimation performance of an OFDM-FDMA / CDMA / TDMA system. The sample is repeated a predetermined time by the first to fourth sample groups in the time domain, and has a symmetrical structure, wherein the second sample group and the fourth sample group include the same number of samples as the first and third sample groups. A method of estimating and synchronizing a symbol timing offset through a symbol timing estimator in a mobile communication system having a preamble structure arranged in a reverse order to a received order of samples of a first and third sample groups, When r _(d) is the received signal of the preamble and N is N _FFT (FFT Size),

,

The timing metric M _pro (d) is

In this case, OFDM-FDMA / CDMA / characterized in that to estimate the symbol timing offset by taking a moving average for the timing metric M _pro (d) based on the characteristic that the preamble has an impulse shape with a period of N _FFT / 8 Synchronization Method for Improving Frequency Offset Estimation in TDMA Systems. The method of claim 4, wherein the moving average is

A synchronization method for improving the frequency offset estimation performance of the OFDM-FDMA / CDMA / TDMA system characterized in that obtained by. The OFDM symbol according to claim 4, wherein the initial symbol timing is set to a position where the timing metric is subtracted by N _FFT / 2 at the point where the timing metric becomes the maximum, and further subtracted by the delay spread of the expected channel. Synchronization Method for Improving Frequency Offset Estimation in FDMA / CDMA / TDMA Systems.

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