KR20080042421A - Preamble symbol in ofdm system and method for designing the preamble symbol and method for acquiring timing/frequency synchronization - Google Patents

Abstract

A preamble symbol in an OFDM(Orthogonal Frequency Division Multiplexing) system, a method for designing the preamble symbol, and a method for acquiring timing/frequency synchronization are provided to generate various preamble patterns by differentiating short repetition patterns within long repetition patterns. A method of designing a preamble symbol includes the steps of: generating long repetition patterns of a length of P repeated K times in a temporal area by assigning preamble sequences to sub carriers in a frequency area and inverse-fast-Fourier-transforming of the preamble sequences(S21); generating short repetition patterns within the long repetition patterns by generating the short repetition patterns of a length L repeated M times in the temporal area and the short repetition patterns to a partial section within the long repetition pattern(S22); transforming a valid preamble symbol formed by the long repetition patterns and the short repetition patterns into a frequency area signal value by fast-Fourier transforming of the valid preamble symbol(S23); setting sub carriers in a protection area to 0 among the transformed frequency area signal values(S24); quantizing the frequency area signal(S25); and transforming the quantized signal into a temporal area signal by inverse-fast-Fourier transforming of the quantized signal(S26).

Description

Preamble symbol in OFDM system and method for designing the preamble symbol and method for acquiring timing / frequency synchronization

1 is a diagram illustrating a conventional preamble structure for symbol timing estimation.

2 is a flowchart illustrating a process of designing a preamble according to an embodiment of the present invention.

3 is a block diagram illustrating a preamble penalty structure according to an embodiment of the present invention.

4, 5 and 6 are block diagrams illustrating a preamble symbol structure according to another embodiment of the present invention.

FIG. 7 is a diagram illustrating magnitudes of frequency domain signal values after fast Fourier transforming a preamble symbol and magnitudes of frequency domain signal values after a zero padding process and a quantization process according to an embodiment of the present invention.

FIG. 8 is a diagram illustrating constellations of frequency domain signal values after performing fast Fourier transform on a preamble symbol according to an embodiment of the present invention and undergoing a quantization process.

FIG. 9 is a flowchart illustrating a process of timing / frequency synchronization and cell searching by a mobile station using a preamble symbol according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

S21: Create a long repeating pattern S22: Create a short repeating pattern

S23: FFT S24: Zero padding

S25: Quantization S26: IFFT

31: cyclic prefix 32: valid preamble symbol

The present invention relates to a preamble design for extending a carrier frequency offset estimation range in an OFDM system, and a timing / frequency synchronization and a base station search method using the preamble.

In a conventional multiple access data communication method for effectively using a limited frequency band, Frequency Division Multiple Access (FDMA) for transmitting signals by dividing a frequency band, a plurality of times in a data frame Time Division Multiple Access (TDMA), which allows multiple users to share the same physical channel by assigning time slots and appropriately distributing these time slots to users, allowing simultaneous signals from multiple users on the same frequency band. There is Code Division Multiple Access (CDMA) which allows each user to use orthogonal codes to transmit and separate these signals.

In addition, there is Orthogonal Frequency Division Multiplexing (OFDM), which is a multicarrier modulation method using a plurality of subcarriers (sub-carriers) having an orthogonal relationship. The orthogonal frequency division multiplexing may be regarded as a kind of frequency division multiplexing (FDM) method for transmitting data by dividing a frequency band to be used into several small frequency bands (subchannels). In other words, by parallelizing a series of data sequences to be transmitted by the number of subcarriers and modulating subcarriers corresponding to each subchannel with each parallel data, the overall data rate is a symbol in each subchannel while maintaining the original rate. The period is lengthened by the number of subchannels. Since digitally modulated data is allocated to each subcarrier (subcarrier) by serial-to-parallel conversion, much more carriers can be multiplexed than in frequency division multiplexing, thereby increasing frequency utilization efficiency. The orthogonal frequency division multiplexing uses duplexing using TDD or FDD.

This orthogonal frequency division multiplexing technique is a method in which multiple users simultaneously transmit signals through different subcarriers, and is proposed as one of the next-generation broadband wireless multiple access schemes due to the frequency selective fading phenomenon and the strong resistance to narrowband interference. And a simple single tap equalizer to compensate for channel distortion. In addition, the severe inter symbol interference (ISI) problem can be easily solved by using a cyclic prefix (CP).

However, the advantage of the orthogonal frequency division multiplexing is possible only when the orthogonality between subcarriers is maintained, and when the orthogonality is broken, inter-channel interference (ICI) is generated to degrade system performance. Therefore, in the orthogonal frequency division multiplexing system, the importance of time and frequency synchronization at the receiving end is greatly emphasized. Therefore, in the downlink, each mobile station performs frequency and time synchronization and initial cell search through the preamble transmitted by the base station. This process enables the mobile station to know its base station or sector and form a link for service. have. Therefore, a preamble signal having a repetitive characteristic is transmitted to the first half of the packet for signal synchronization, and the receiver performs the signal synchronization using the preamble signal.

In order to demodulate a symbol received as the preamble through a Fast Fourier Transform (FFT), the starting position of the symbol must be known correctly. The process of finding the starting position of a symbol is called symbol synchronization, and it is divided into a method of using auto correlation and cross correlation and a method of using a cyclic prefix. Cox algorithm is used as a representative symbol synchronization method.

1 illustrates a conventional preamble structure for symbol timing estimation presented by a Cox algorithm or the like.

Based on orthogonal frequency division multiplexing, the downlink preamble is used for initial synchronization, frequency offset, and cell search, and after the inverse fast fourier transform (IFFT) is passed, as shown in FIG. Repeated and symmetrical structure By using a preamble (hereinafter referred to as 'cox preamble') that follows the Cox algorithm, carrier frequency offset estimation can be performed by obtaining a phase difference of a correlation value between repetitive patterns in a time domain.

However, when one Cox preamble is used, the carrier frequency offset estimation range is limited within the subcarrier frequency interval, and it is difficult to estimate a carrier frequency offset larger than the subcarrier frequency interval.

The present invention has been made to solve the above problems, and it is an object of the present invention to propose a new preamble design method capable of estimating a wide range of carrier frequency offset using only one preamble even without using an additional preamble (or bandwidth). do. Another object of the present invention is to propose a method for timing / frequency synchronization acquisition and base station discovery using the proposed preamble.

In order to achieve the above object, a preamble symbol in an OFDM system of the present invention includes a cyclic prefix inserted in front of a valid OFDM symbol in a time domain, and a plurality of short repetitions within the long repeating patterns. Each pattern includes a valid preamble symbol that is repeatedly formed.

The effective preamble symbol is assigned to a preamble sequence to subcarriers in a frequency domain and inverse fast Fourier transform to form long repeating patterns of length P repeated K times in the time domain, and short length L repeated M times in the time domain. A short repeating pattern is formed in the long repeating pattern by generating a repeating pattern and copying it to a partial section in each long repeating pattern.

In addition, the present invention provides a long repetitive pattern generation process of assigning a preamble sequence to subcarriers in a frequency domain and performing inverse fast Fourier transform to generate a long repetitive pattern of length P repeated K times in the time domain, and M times in the time domain. A short repeating pattern generation process of generating a short repeating pattern in the long repeating pattern by generating short repeating patterns having a repeating length L and copying it to some sections within each long repeating pattern, and the long repeating pattern and short repeating pattern Converting the formed effective preamble symbol into a frequency domain signal value by performing fast Fourier transform, a zero padding process of filling subcarriers of a protection region with a '0' value among the converted frequency domain signal values to prevent inter-channel distortion; Quantizing the frequency domain signal and inverse fast Fourier Ring includes the process of converting into a time-domain signal.

The preamble of the OFDM system includes a cyclic prefix inserted before the effective OFDM symbol in the time domain, and a valid preamble symbol in which long repeating patterns are formed and a plurality of short repeating patterns are repeatedly formed in the long repeating patterns. A method for acquiring timing and frequency synchronization using a symbol and searching for a base station, comprising: a timing synchronization acquiring process of acquiring timing synchronization by calculating cross correlation between repetition patterns in a received preamble signal in a time domain; A carrier frequency offset estimation process for estimating a carrier frequency offset by obtaining a phase difference of cross-correlation values between repetitive patterns in a received preamble signal, and a fast Fourier transform of a received preamble signal after timing and frequency synchronization and a base station classification sequence Cross-correlation with the fields Comprising a base station search to perform a base station search by calculating.

Hereinafter, the detailed description of the preferred embodiments of the present invention will be described with reference to the accompanying drawings. In the following description of the reference numerals to the components of the drawings it should be noted that the same reference numerals as possible even if displayed on different drawings.

In addition, it is assumed that one preamble symbol in the preamble structure described in the embodiment of the present invention has a length of a cyclic prefix 'G', and assumes that a length of fast Fourier transform has a 'N'. Shall be.

2 is a flowchart illustrating a process of designing a preamble according to an embodiment of the present invention.

In order to design the preamble having the symbol of FIG. 3 according to the present invention, the preamble sequence is assigned to subcarriers in the frequency domain, and an inverse fast Fourier transform is performed to generate a repeating pattern of length P repeated K times in the time domain. Long repetition pattern generation process (S21), short repetition pattern generation process of generating a short repetition pattern of length L repeated M times in the time domain and copying to some intervals in each long repetition pattern to generate one effective preamble symbol (S22), a process of converting the generated valid preamble symbol into a frequency domain signal value by taking a fast Fourier transform (S23), and filling a signal value corresponding to subcarriers of a protection region with zeros among the converted frequency domain signal values. A zero padding process (S24), a step of quantizing the frequency domain signal (S25), an inverse fast Fourier transform of the quantized signal Taking consists of a process of generating a preamble (S27) by converting (S26) into a time-domain signal.

The long repeating pattern generation (S21) is designed to have a long repeating pattern of length P (P = N / K) repeated K times in the time domain. In detail, in a OFDM system using a preamble, a time generated by assigning symbols C _i (0 ≦ _i ≦ Q−1) corresponding to a preamble sequence of length Q to subcarriers having a K interval and then taking an inverse fast Fourier transform An example of the area signal s (l) is as follows.

[Equation 1]

In the above, N _used represents the number of effective subcarriers actually used except for the protection region. The time domain signal s (l) has a long repeating pattern of length P (P = N / K) repeated K times in the time domain.

After generating a long repeating pattern (S21), short repeating patterns are generated in the generated K long repeating patterns (S22). As a method for making M short repeating patterns in the generated K long repeating patterns, L OFDM samples in the long repeating pattern may be copied and used as follows.

[Equation 2]

In the above, a represents the starting position of the short repeating pattern in the long repeating pattern.

As described above, when the long repetition pattern generation S21 and the short repetition pattern generation S22 are performed as the preamble signal in the time domain, as a result, the long repetition patterns are repeatedly formed in the time domain, and a plurality of long repetition patterns are formed in the long repetition patterns. An effective preamble symbol is formed in which short repeating patterns of L are each formed repeatedly.

After the long repetition pattern generation S21 and the short repetition pattern generation S22 are performed, a fast Fourier transform of the preamble signal in the time domain may be performed to obtain a preamble signal in the frequency domain (S23). .

By artificially generating a short repetition pattern in the preamble signal in the time domain, some intervals in the long repetition pattern are changed to different values from the original values, so the preamble signal in the frequency domain is not only subcarriers in the K interval but also subcarriers in the protection domain. It will also appear in the field. Therefore, in order to prevent the subcarriers in the protection region from being used, a zero padding process (S24) is performed to make corresponding signal values '0'. The protection region is a frequency region that is not used to prevent inter-channel distortion, and makes a signal value of the protection region '0'.

In addition, unlike the signal values corresponding to the original preamble sequence, the preamble signal values of the K interval are expressed in an infinite number of bits and thus undergo a quantization process (S25) to express these values in a finite number of bits. The preamble signal allocated to the subcarriers of the quantized K interval is again subjected to inverse fast Fourier transform (S26) and converted into a final time domain preamble signal (S27).

3 is a block diagram illustrating a preamble symbol structure according to an embodiment of the present invention.

The structure of the preamble symbol 30 generated after the long repetition pattern generation process S21 and the short repetition pattern generation process S22 in FIG. 2 is illustrated in FIG. 3, wherein the preamble symbol is a valid OFDM in the time domain. And a cyclic prefix 31 inserted before the symbol, and a valid preamble symbol 32 in which long repeating patterns are repeatedly formed in the time domain, and a plurality of short repeating patterns are repeatedly formed in the long repeating patterns. .

)번 반복된다. 여기서,

는

보다 작은 최대 크기의 정수 값을 나타낸다. 시간 영역에서 유효 OFDM 심벌 앞에 순환 프리픽스를 둔다. The effective preamble symbol 32 is that the number of long repeating patterns 33 of length P is K = N / P, and the short repeating pattern 34 of length L is from any position in the long repeating pattern 33. M (M: positive integer,

Repeat) times. here,

Is

Represents the smaller maximum integer value. Put a cyclic prefix before the effective OFDM symbol in the time domain.

4, 5 and 6 are block diagrams illustrating a preamble symbol structure according to another embodiment of the present invention.

Another embodiment of the preamble symbol 30 generated through the long repeating pattern generation process S21 and the short repeating pattern generation process S22 in FIG. 2 is illustrated in FIGS. 4, 5, and 6. And the short repetition pattern are repeated twice each, and the short repetition pattern copies L OFDM samples from the rightmost position of each long repetition pattern by 2L, and pastes them to the right from the rightmost L position. Is generated.

As shown in the preamble symbol example of FIGS. 4 and 5, when the length of the short repetition pattern length is l ≦ L ≦ G, a short repetition pattern is generated even within the cyclic prefix, and the repetition pattern may be used to estimate the integer frequency offset. In particular, when the range of the short repeating pattern length is G / 2 ≤ L ≤ G as shown in the example of the preamble symbol shown in FIG. 5, not all short repeating patterns are repeated within the cyclic prefix, and only the portion corresponding to L '= GL is repeated. do.

On the other hand, if the short repetition pattern length is longer than the length of the cyclic prefix as in the preamble symbol example of FIG. 6, no separate repetition pattern is generated in the cyclic prefix.

When generating a short repetition pattern as in the preamble symbol example of FIGS. 4 and 5 and 6, the estimation range of the carrier frequency offset is within ± N / 2L.

7 is a diagram illustrating a magnitude of frequency domain signal values after a fast Fourier transform of a preamble symbol according to an embodiment of the present invention, and a magnitude of frequency domain signal values after a zero padding process (S24) and a quantization process (S25). to be.

In the example of FIG. 7, the structure of the preamble symbol example of FIG. 4 is assumed, and a preamble symbol composed of two long repeating patterns having a length of 512 and two short repeating patterns having a length of 20 is assumed. The size of fast Fourier transform is assumed to be 1024, and the number of effective subcarriers is assumed to be 864. In the quantization process S25, the number of quantization bits is assumed to be 4 bits. 7 (a) is a diagram illustrating the magnitude of the frequency domain signal values after the fast Fourier transform process (S23), Figure 7 (b) is a frequency after the quantization process (S25) The figure illustrates the magnitude of the area signal values.

8 is a diagram illustrating the constellation of frequency domain signal values after the fast Fourier transform of the preamble symbol of FIG. 4 after undergoing quantization (S25). The number of quantization bits is assumed to be 4 bits.

On the other hand, by generating a short repeating pattern in the long repeating pattern as shown in Figures 4, 5, 6, by dividing the entire base station identification IDs used to make the base station search in two steps, each base station ID group It is possible to assign a unique sequence of base station group to a sequence, and also assign a unique short length sequence to base station IDs belonging to each group. In this case, samples generated by taking the fast fast Fourier transform of the division sequence of the group to which the given base station distinguished ID belongs are used for a long repetition pattern, and samples generated by performing the fast fast Fourier transform on the short length sequence for the given base station distinguished ID in the group Can be used in short repeating patterns.

Accordingly, a description will be given of a process of timing / frequency synchronization and cell searching by a mobile station using a preamble symbol having a short repetition pattern in the long repetition pattern proposed by the present invention.

FIG. 9 is a flowchart illustrating a process of timing / frequency synchronization and cell searching by a mobile station using a preamble symbol according to an embodiment of the present invention.

In order to achieve timing / frequency synchronization and cell search according to an embodiment of the present invention, a process of acquiring timing synchronization by calculating cross-correlation between repetitive patterns in a received preamble signal in a time domain (S91), and then obtaining timing synchronization Estimating a carrier frequency offset by calculating a phase difference of cross correlation values between repetitive patterns in the received preamble signal (S92), and calculating cross-correlation between the received preamble signal and base station division sequences after acquiring timing / frequency synchronization. A process of performing a search is included (S93).

In the timing synchronization acquiring process S91, cross-correlation between repetitive patterns in the received preamble signal is calculated in the time domain, and a frame and symbol timing synchronization are obtained by detecting a moment when an absolute value exceeds a predetermined threshold. An example of timing metric calculation using cross correlation between long repetition patterns in a received preamble signal is as follows.

[Equation 3]

Where r [l] represents a time domain sample value of the received preamble symbol and d represents a variable corresponding to a symbol timing error. From the timing metric, the instant of exceeding a predetermined threshold may be detected to estimate the symbol timing d.

The carrier frequency offset estimating process (S92) includes estimating the fractional frequency offset using long repetition patterns in the reception preamble and estimating the integer frequency offset using the short repetition patterns.

The fractional frequency offset estimation value [epsilon] is obtained by estimating the phase rotation value by calculating the cross correlation between the long repeating patterns as follows.

[Equation 4]

Where r [l] represents a time-domain sample value of the OFDM symbol and B (k) represents a cross correlation value between the k-th long repeating pattern and the (k + 1) th long repeating pattern.

In the carrier frequency offset estimation step, the integer frequency offset estimation may be performed after the fractional frequency offset estimation is completed to compensate for the fractional frequency offset of the received preamble signal.

The integer frequency offset estimation value δ is obtained by estimating the phase rotation value between short repeating patterns as follows.

[Equation 5]

Where a represents the position of the short repeating pattern in the long repeating pattern.

In the carrier frequency offset estimation process (S92), the fractional frequency offset estimation and the integer frequency offset estimation may be performed simultaneously before the fractional frequency offset of the received preamble signal is compensated.

In the fractional frequency offset estimation process (S92), in order to improve the fractional frequency offset estimation performance, the fractional frequency offset estimation may be performed for several frames, and the result values may be combined and used. One method of combining the fractional frequency offset estimation for several frames may be to add the cross correlation values calculated for several frames and then estimate the phase rotation value.

[Equation 6]

In the above, B _i (k) represents the cross correlation value between the k th long repeating pattern and the (k + 1) th long repeating pattern during the i th frame and F represents the number of frames required for the fractional frequency offset estimation combination. Alternatively, a method of averaging the fractional frequency offset values estimated through Equation 4 for every F frames may be used.

In the integer frequency offset estimation process (S92), in order to improve the integer frequency offset estimation performance, integer frequency offset estimation may be performed for several frames, and the result values may be combined and used.

The base station search process (S93) extracts Q signal values R [i] from subcarriers used for the assignment of the preamble sequence by performing fast Fourier transform on the received preamble signal as follows. The cross-correlation of is calculated to select the sequence having the largest absolute value Y [q] as the base station and the sequence whose absolute value exceeds a certain threshold as the adjacent base station.

[Equation 7]

In the above, q represents an index of the base station division sequence and C _{q , i} (i = 0, 1, ..., Q-1) represents the q-th base station division sequence.

In the base station search step, instead of calculating the cross-correlation of the entire length of the sequence, the sequence is divided by a predetermined length U to obtain a noncoherent combining result value, which is given as an absolute value or a sum of squares of absolute values of the cross-correlation values for each. You can also calculate The following is an example of an asynchronous join calculation.

[Equation 8]

이며

는

보다 큰 최소 크기의 정수 값을 나타낸다. From above

And

Is

Represents an integer value of a greater minimum size.

In another method of searching for a base station according to the present invention, when using a long repetition pattern and a short repetition pattern designed to make the base station search in two stages, the base station group classification sequences that can use the value obtained by fast Fourier transforming the received preamble signal Cross-correlation is calculated and the sequence having the largest absolute value is selected as the base station group, and the groups whose absolute value exceeds a predetermined threshold are selected as the groups to which the adjacent base station belongs. After selecting the base station groups and performing fast Fourier transform by taking only the samples corresponding to the short repetition pattern among the received preamble signals, the cross sequence is calculated by using the available sequences included in the base station group and having the largest absolute value. Is selected as a corresponding base station, and a sequence whose absolute value exceeds a predetermined threshold is selected as an adjacent base station.

Even in the case of using the two-step base station discovery method, a noncoherent combining result value may be used when calculating cross-correlation.

In the foregoing description of the present invention, specific embodiments have been described, but various modifications may be made without departing from the scope of the present invention. Therefore, the scope of the present invention is not to be determined by the embodiments described above, but will be apparent in the claims as well as equivalent scope.

As described above, the preamble structure according to the present invention can be applied to estimate a wide range of carrier frequency offset using only one preamble without using an additional preamble or bandwidth.

The preamble design method according to the present invention can be applied to generate various preamble patterns by different short repetition patterns in long repetition patterns.

In the method of generating a long repeating pattern and a short repeating pattern according to the present invention, a base station discovery process may be performed in two steps by using samples generated by taking a fast fast Fourier transform of a short length sequence in a short repeating pattern.

By performing the base station discovery process in two steps, it is possible to significantly reduce the base station discovery computation while maintaining the same number of available base station IDs as in the existing Cox preamble.

Claims (38)

A cyclic prefix inserted before a valid OFDM symbol to prevent interference between adjacent symbols in the time domain; A valid preamble symbol in which long repeating patterns are repeatedly formed in the time domain and a plurality of short repeating patterns are formed repeatedly in the long repeating patterns. Preamble symbol in the OFDM system comprising a. The method of claim 1, wherein the effective preamble symbol is assigned to the subcarriers in the frequency domain and inverse fast Fourier transform to form long repetitive patterns of length P repeated K times in the time domain and M times in the time domain. A preamble symbol in an OFDM system in which a short repetition pattern is formed in each of the long repetition patterns by generating a short repetition pattern having a repeating length L and copying the partial repetition pattern in each of the long repetition patterns. 3. The valid preamble symbol of claim 2, wherein the long repeating pattern and the short repeating pattern are repeated twice each, and the short repeating pattern copies L OFDM samples from a position 2L from the rightmost side of each long repeating pattern. The preamble symbol in the OFDM system, characterized in that the effective preamble symbol formed by pasting from the rightmost position L by the right side. The preamble symbol of claim 1, wherein the long repetition pattern is a long repetition pattern of a length P repeated K times in the time domain by allocating a preamble sequence only to subcarriers having a K interval in the frequency domain. The preamble symbol of claim 1, wherein the short repetition pattern is formed by generating a short repetition pattern of length L repeated M times in a time domain, and copying the short repetition pattern to a partial interval within each long repetition pattern. 6. The preamble symbol of claim 5, wherein the short repetition pattern is formed by copying L samples successively from any position within a long repetition pattern and copying them to an adjacent portion. 6. The preamble symbol according to claim 5, wherein the preamble symbol is formed by taking a short-length sequence of inverse fast Fourier transforms and using samples generated in a short repetition pattern and copying them into the long repetition pattern. The preamble symbol of claim 1, wherein the protection region of the preamble symbol in the frequency domain is filled with a value of '0' to prevent interchannel distortion. A long repetition pattern generation process of assigning a preamble sequence to subcarriers in a frequency domain and inverse fast Fourier transform to generate a long repetition pattern of length P repeated K times in the time domain, A short repetition pattern generation process of generating a short repetition pattern in each of the long repetition patterns by generating a short repetition pattern having a length L repeated M times in the time domain and copying the partial repetition pattern in each long repetition pattern; Converting an effective preamble symbol formed of the long repeating pattern and the short repeating pattern into a frequency domain signal value by performing fast Fourier transform; A zero padding process of filling the subcarriers of the protection region with a value of '0' among the converted frequency domain signal values; Quantizing the frequency domain signal; Converting the quantized signal into an inverse fast Fourier transform into a time domain signal Preamble design method in an OFDM system comprising a. 10. The method of claim 9, wherein the long repetition pattern generation process comprises: assigning a predetermined preamble sequence only to subcarriers having a K interval in the frequency domain to generate a long repetition pattern of length P repeated K times in the time domain. Preamble design method. 11. The method of claim 10, wherein a long repetition pattern is generated using an arbitrary sequence as the preamble sequence. The method of claim 10, wherein symbols corresponding to a preamble sequence having a length Q are C _i , a time domain signal is S (l), and the number of effective subcarriers actually used except for a protection region is N _used .

A preamble design method in an OFDM system for generating a long repeating pattern of length P repeated K times in the time domain by the following equation. 10. The method of claim 9, wherein the generated long repetition pattern divides all base station identification IDs into a plurality of base station ID groups, assigns a unique base station group division sequence to each group, and reverses the division sequence of a group to which a given base station ID belongs. A preamble design method in an OFDM system for allocating samples generated by fast Fourier transform. The method of claim 9, wherein the short repetition pattern generation process generates a short repetition pattern of length L repeated M times in a time domain, and copies the short repetition pattern to a partial interval within each long repetition pattern. . 15. The method of claim 14, wherein generating a short repeating pattern of length L and copying it to some intervals in each long repeating pattern includes: OFDM copying L samples consecutively from any position in the long repeating pattern and pasting them into adjacent portions. How to design preambles in your system. 15. The method of claim 14, wherein generating a short repeating pattern of length L and copying it to a portion of each long repeating pattern uses a short repeating pattern using samples generated by inverse fast Fourier transform of a short length sequence. A preamble design method in an OFDM system that copies within a repeating pattern. 15. The method of claim 14, wherein generating a short repeating pattern of length L and copying it to some intervals in each long repeating pattern comprises: M number of short repeating patterns in the long repeating pattern, M, number of samples copied in the long repeating pattern. L, when the starting position of the short repeating pattern in the long repeating pattern is a,

A method of designing a preamble in an OFDM system copied by the following equation. 10. The method according to claim 9, wherein the generated short repetition pattern divides all base station identification IDs into a plurality of base station ID groups, and assigns a unique base station group classification sequence to each group, and also unique to base station IDs belonging to each group. A method of designing a preamble in an OFDM system that allocates one short length sequence and inverse fast Fourier transforms a short sequence of base station IDs within a given group, thereby generating a short repeating pattern and copying the short sequence. 10. The method of claim 9, wherein the quantization process performs b-bit quantization of frequency domain signals after the zero padding process. Using a preamble symbol of an OFDM system including a cyclic prefix inserted in front of the effective OFDM symbol in the time domain, and a valid preamble symbol in which long repeating patterns are formed and a plurality of short repeating patterns are repeatedly formed in the long repeating patterns. In the method for acquiring timing and frequency synchronization and searching for a base station, A timing synchronization acquisition process of obtaining a timing synchronization by calculating cross correlation between repetition patterns in the received preamble signal in a time domain; A carrier frequency offset estimation process of estimating a carrier frequency offset by obtaining a phase difference of cross-correlation values between repetitive patterns in a received preamble signal after timing synchronization is obtained; A base station discovery process for performing a base station search by calculating a cross-correlation between a value obtained by fast Fourier transforming a received preamble signal after timing and frequency synchronization and base station division sequences. Timing / frequency synchronization and base station discovery method in an OFDM system comprising a. 21. The method of claim 20, wherein the timing synchronization acquiring step calculates cross-correlation between repetitive patterns in the received preamble signal in the time domain and detects a moment when an absolute value exceeds a predetermined threshold to obtain frame and symbol timing synchronization. Timing / frequency synchronization and base station discovery method in an OFDM system. 22. The method of claim 21, wherein detecting an instant that exceeds a certain threshold is that r [l] represents a time domain sample value of a received OFDM symbol, and d represents a variable corresponding to a symbol timing error.

A timing / frequency synchronization and base station discovery method in an OFDM system for estimating symbol timing d by detecting a moment exceeding a predetermined threshold from a timing metric. The method of claim 20, wherein the carrier frequency offset estimation process, Estimating the fractional frequency offset using long repeating patterns in the received preamble, Process of estimating integer frequency offset using short repetition patterns Timing / frequency synchronization and base station discovery method in an OFDM system further comprising. 24. The method of claim 23, wherein the integer frequency offset estimation is performed after the fractional frequency offset estimation is completed to compensate for the fractional frequency offset of a received preamble signal. 24. The method of claim 23, wherein the fractional frequency offset estimation and the integer frequency offset estimation are performed simultaneously before the fractional frequency offset of the received preamble signal is compensated. 24. The method of claim 23, wherein the fractional frequency offset estimation is performed for several frames to combine the results to improve fractional frequency offset estimation performance in an OFDM system. 24. The method of claim 23, wherein the integer frequency offset estimation is performed for several frames to combine the results to improve integer frequency offset estimation performance in an OFDM system. 26. The timing / OFDM method of claim 24 or 25, wherein the fractional frequency offset estimation and the integer frequency offset estimation are performed for several frames, and the results are combined to improve the fractional and integer frequency offset estimation performance. Frequency synchronization and base station discovery method. 24. The method of claim 23, wherein estimating the fractional frequency offset using the long repeating patterns comprises: estimating the fractional frequency offset estimate with ε, time-domain sample value of the OFDM symbol, r [l], k-th long repeating pattern, and (k When the cross correlation value between +1) th long repeating pattern is B (k),

,

A method of timing / frequency synchronization and base station discovery in an OFDM system for estimating a phase rotation value by calculating cross correlation between long repetitive patterns according to Equation. 24. The method of claim 23, wherein estimating the fractional frequency offset using the long repeat pattern, the cross-correlation value between the i-th frame for a k-th long repeat pattern and (k + 1) th long repeating pattern B _i (k) If the number of frames required to combine the fractional frequency offset estimate is F,

A method for timing / frequency synchronization and base station discovery in an OFDM system that performs a fractional frequency offset estimation for several frames by using the following equation and combines the result values. The method of claim 23, wherein estimating an integer frequency offset using short repetition patterns, When the integer frequency offset estimation value is δ and the position of the short repeating pattern in the long repeating pattern is a,

A method for timing / frequency synchronization and base station discovery in an OFDM system for estimating phase rotation values between short repetition patterns by Equation 4 below. 24. The method of claim 23, wherein estimating integer frequency offset using short repetition patterns comprises performing integer frequency offset estimation for several frames and combining the resulting values. 21. The method of claim 20, wherein the searching for the base station comprises calculating cross-correlation with the available base station classification sequences using the fast Fourier transform of the received preamble signal, selecting the sequence having the largest absolute value as the corresponding base station, and the absolute value thereof. A method for timing / frequency synchronization and base station discovery in an OFDM system that selects sequences above this threshold as adjacent base stations. The method of claim 33, wherein the Q signal values are R [i], the absolute value is Y [q], the q th base station division sequence C _{q , i} , and the index of the base station division sequence is q.

A method for timing / frequency synchronization and base station discovery in an OFDM system in which the sequence whose absolute value exceeds a predetermined threshold is selected as an adjacent base station by Equation 2 below. 21. The OFDM system of claim 20, wherein a fast Fourier transform of the received preamble signal and a base station distinguished sequence are divided by a predetermined length to calculate an asynchronous combined result given as an absolute value of a cross-correlation value or a sum of squares of absolute values for each. Timing / Frequency Synchronization and Base Station Discovery Method 21. The method of claim 20, wherein the base station search method calculates a cross-correlation with the available base station group division sequences and the highest sequence value using the fast Fourier transform of the received preamble signal in order to perform the base station search in two stages. A method for timing / frequency synchronization and base station discovery in an OFDM system in which is selected as a corresponding base station group and selects groups whose absolute value exceeds a predetermined threshold as groups belonging to a neighboring base station. 37. The method of claim 36, after selecting the corresponding base station groups and performing fast fast Fourier transform by taking only samples corresponding to a short repetition pattern among the received preamble signals, the cross-correlation with the available sequences included in the corresponding base station group is calculated and A method for timing / frequency synchronization and base station discovery in an OFDM system in which a sequence having the largest absolute value is selected as a corresponding base station and a sequence whose absolute value exceeds a predetermined threshold is selected as an adjacent base station. 38. The method of claim 36 or 37, wherein a timing / frequency synchronization and base station discovery in an OFDM system uses asynchronous combining results in cross-correlation calculations.

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