KR100929704B1 - Timing synchronization method for wireless communication system - Google Patents

Abstract

PURPOSE: A time synchronization method for a wireless communication system is provided to reduce a time synchronization error and perform a high speed operation by reducing an amount of calculation for obtaining the time synchronization. CONSTITUTION: An autocorrelation function of a receiving signal is produced(S10). The produced autocorrelation function is added(S12). The plane of the added autocorrelation function is removed(S14). The selected maximum value is set as the low precise time synchronization estimation value(S16). A precise time synchronization function value is produced using the precise time synchronization function using the conjugate complex number symmetrical characteristic of a legacy long training symbol by a preamble format(S18). A search section window is set based on the low precise time synchronization estimation value. The time synchronization is obtained by searching for the maximum value of the precise time synchronization function value in the search section window(S20).

Description

Timing synchronization method for wireless communication system

The present invention relates to a time synchronization method for a wireless communication system, and more particularly, HT (High Throughput) in accordance with the IEEE 802.11n draft standard of a next generation wireless LAN system based on Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM). When operating in mixed mode, the present invention relates to a time synchronization method for a wireless communication system capable of high-speed processing and reducing time synchronization error by reducing the amount of computation for time synchronization acquisition.

Standard standards for wireless networks include the Institute of Electrical and Electronics Engineers (IEEE) 802.11.

For example, IEEE802.11a / g is a standard for wireless LANs and employs an Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme, which is one of the multicarrier schemes. The IEEE802.11a / g specification supports communication speeds up to 54Mbps, but higher data rates are required as the demand for large amounts of data increases.

Accordingly, in order to improve communication speed, standardization of IEEE802.11n, which is an extension standard of IEEE802.11a / g, is in progress. IEEE802.11n applies Orthogonal Frequency Division Multiplexing (OFDM) modulation to a Multiple-Input Multiple-Output (MIMO) system to ensure high data rates. have. The MIMO system includes a plurality of antenna elements at both the transmitter side and the receiver side, and by using a plurality of spatial multiplexed spatial streams, high data rate and bandwidth efficiency can be obtained in a multipath fading radio channel environment. In addition, IEEE802.11n realizes high throughput (HT) transmission by differentiating modulation and coding schemes (MCS) such as modulation schemes and coding schemes from IEEE802.11a / g.

However, when applying a new structure of a preamble having orthogonality to the MIMO-OFDM time synchronization to the IEEE 802.11n standard, it is not possible to provide compatibility with the existing IEEE 802.11 a / g wireless LAN equipment. Accordingly, the IEEE 802.11n draft standard defines three preambles used in each mode by providing three modes for compatibility with existing equipment. Modes proposed by the IEEE 802.11n draft standard include a non-HT mode (legacy mode) such as a preamble of a conventional wireless LAN, a HT mixed mode in which a legacy preamble and a high throughput preamble are combined, and high data. It is configured with HT Greenfield mode for data rate.

Here, in the HT mixed mode, the packet header includes a preamble (hereinafter referred to as legacy preamble) having the same format as IEEE802.11a / g, and a preamble specific to IEEE802.11n (hereinafter, HT preamble) is used in combination to enable communication with a communication terminal conforming to the IEEE802.11a / g standard.

In the MIMO system, on the other hand, there is a problem that an unintended beam is formed when the same or similar signals are transmitted through different spatial streams. Accordingly, in IEEE802.11n, a transmitter transmits a signal with a cyclic shift through each transmission antenna. Accordingly, when operating in the HT mixed mode, the Cyclic Shift value between the transmit antenna and the legacy preamble portion of the transport packet and the HT preamble portion is defined.

Therefore, in the HT mixed mode, since the cyclic shift is applied to transmit the preamble shifted in each transmitting antenna, the cross-correlation based time synchronization scheme used in the existing wireless LAN system is used in the receiving antenna. Since a large number of peak values are detected, there is a problem in that the synchronization failure probability increases and the system performance decreases during time synchronization.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and when operating in HT (High Throughput) mixed mode according to the IEEE 802.11n draft standard of the MIMO-OFDM-based next generation wireless LAN system, the amount of computation for time synchronization is reduced. It is an object of the present invention to provide a time synchronization method for a wireless communication system capable of high speed processing and reducing time synchronization error.

A time synchronization method for a wireless communication system according to the present invention for achieving the above object is, by using a short training symbol according to the preamble format of the HT mixed mode of the IEEE 802.11n draft standard (Legacy Short Training Symbol) of the received signal Calculating an autocorrelation function; Summing the calculated autocorrelation functions; Removing the planar surface of the summed autocorrelation function and setting the selected maximum value as a low precision time synchronization estimate value; Calculating a precision time synchronization function value using a precision time synchronization function using a conjugate complex symmetry characteristic of a legacy long training symbol according to the preamble format; Setting a search section window based on the low precision time synchronization estimate value; And searching for the maximum value of the fine time synchronization function value in the search section window to obtain time synchronization.

Here, in calculating the autocorrelation function of the received signal, it is possible to use the following equation.

Where R is the autocorrelation function, r is the received signal, n is the index of the autocorrelation function, L is the length of the short training symbol, Ls is the symbol delay time, * is the conjugate complex, and j is the index of the receiving antenna. .

In addition, the summing of the autocorrelation functions may use the following equation.

Where M is the total number of receive antennas.

And, after removing the flat surface of the summed autocorrelation function and setting the selected maximum value to the low precision time synchronization estimate value, removing the flat surface of the summed autocorrelation function using the following equation: Time synchronization method for a wireless communication system, characterized in that.

Where A (n) is a low precision time synchronization function and Wd is the window size.

In addition, the step of setting the selected maximum value to the low precision time synchronization estimate value after removing the flat surface of the summed autocorrelation function, wherein the maximum value is selected according to the following equation so that the value of the T _coarse point becomes the maximum value. It is possible to include the step selected.

Here, the step of calculating the precision time synchronization function value using the precision time synchronization function using the conjugate complex symmetry characteristic of the long training symbol according to the preamble format may be performed using the following equation. It is possible to be a step of calculating a function value.

Where C is the precision time synchronization function and N is the long training symbol length.

을 이용하여 산출하는 단계이고, 여기서, Wf는 저정밀시간동기 함수를 통해서 얻을 수 있는 최대값 지점의 검색구간 윈도우인 것이 가능하다.In addition, the step of obtaining the time synchronization by searching for the maximum value of the precision time synchronization function value in the search section window may be performed by the following equation.

It is a step of calculating using the equation, where Wf may be a search section window of the maximum value point that can be obtained through the low precision time synchronization function.

As described above, in the time synchronization method for a wireless communication system of the present invention, a low time synchronization is obtained based on autocorrelation and a precision time synchronization is calculated using a conjugate complex symmetry characteristic of a long training symbol. have.

Accordingly, it is possible to reduce the amount of computation for performing synchronization and to perform accurate time synchronization.

Hereinafter, a time synchronization method for a wireless communication system according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. However, in describing the present invention, when it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, a detailed description thereof will be omitted.

1 is a structure diagram of a representation layer protocol data unit (PPDU) frame format in the IEEE 802.11n standard to which a time synchronization method for a wireless communication system according to the present invention is applied, and illustrates a PPDU frame format used in HT mixed mode. .

The PPDU frame format used in the HT mixed mode includes a legacy preamble and a preamble and data of the HT format.

The legacy preamble includes a legacy short training field (L-STF) used for packet discovery, a legacy long training field (L-LTF) used for carrier frequency offset and time synchronization, and a legacy terminal. It consists of a L-SIG (Legacy SIGNAL Field) 5 in which a value for spoof is written.

After the legacy preamble, the HT preamble and HT format data 20 are added. The HT preamble includes an HT-SIG (HT SIGNAL Field) 11 in which information required for analyzing an HT format such as an MCS applied to a PHY payload (PSDU) or a data length of a payload is described. HT Short Training Field (HT-STF) (13) consisting of symbols for training to improve AGC (Auto Gain Control), and training symbol for channel estimation at the receiver side. Field) 15.

In the case of transmitting / receiving a PPDU frame format having such a configuration using the MIMO method, the receiver side needs to estimate a channel for each receiving antenna to obtain a channel matrix. For this reason, the transmitter side transmits HT-LTF symbols in time division from each transmitting antenna, and the number of HT-LTF symbols is equal to or larger than the number of spatial streams.

In addition, there is a problem that an unintended beam is formed when transmitting the same or similar signal through different spatial streams. Accordingly, in IEEE802.11n, a transmitter transmits a signal by applying a time shift (Cyclic Shift) to each transmitting antenna, and a Cyclic Shift value between the legacy preamble portion and the transmitting antenna is defined for each of the HT format portions. .

When operating in HT mixed mode according to the IEEE 802.11n draft standard of the MIMO-OFDM-based next-generation wireless LAN system having such a configuration, a time synchronization method according to the present invention will be described in detail.

2 is a flowchart of a time synchronization method for a wireless communication system according to the present invention. The time synchronization method of the present invention obtains time synchronization based on an autocorrelation method using a HT mixed mode preamble according to the IEEE 802.11n draft standard, and the calculation process for obtaining time synchronization is a part for low precision time synchronization estimation. And part for precision time synchronization estimation.

As shown in FIG. 2, in the present invention, for time synchronization, first, an autocorrelation function of a received signal is calculated (S10). Thus, the output value of the autocorrelation function at the receiving antenna j may be calculated through Equation 1 below.

Where R is the autocorrelation function, r is the received signal, n is the index of the autocorrelation function, L is the length of the Legacy Short Training Symbol (eg 16), and Ls is the symbol delay time (eg , 16), * is the conjugate complex, and j is the index of the receiving antenna.

When the autocorrelation function is calculated, all the output values of the autocorrelation function at each receiving antenna are added to improve performance (S12). When this is expressed as an equation, Equation 2 is as follows.

Where M is the total number of receive antennas.

When the addition of the autocorrelation function is completed, the flat surface of the sum of the autocorrelation functions is removed using Equation 3 below (S14).

Where A (n) is a low precision time synchronization function and Wd is the window size.

Accordingly, approximating symbol timing may be obtained by substituting the maximum value of the result value from which the flat surface is removed through Equation 3 into Equation 4 (S16).

Steps S10 to S16 are low-precision time synchronization estimation processes, which can be represented as shown in FIG. 3. As shown in FIG. 3, the present invention removes the flat surface of the sum of autocorrelation functions using a sliding window in the low precision time synchronization estimation process, and the low precision time synchronization estimation value has a maximum value at the T _coarse point. . However, it is impossible to find accurate time synchronization because the value estimated as the T _coarse point has an error with a large variance at a low signal to noise ratio (SNR).

Thus, accurate time synchronization can be obtained through the following precise time synchronization estimation. In the precision time synchronization estimation process, the conjugate complex symmetry characteristic of the legacy long training symbol of the IEEE 802.11n HT-mixed mode preamble is used. Among the 160 long training symbol structures in the HT-mixed mode, 128 sample structures except for 32 samples of the guard interval are shown in [Equation 5] and have a complex conjugate symmetric structure as shown in [Equation 6].

Where y is a long training symbol.

Thus, the present invention calculates the precise time synchronization using the precise time synchronization estimation function of Equation 7 using the conjugate complex symmetric structure characteristic of the long training symbol (S18).

Where C is a precision time synchronization function and N is a long training symbol length (e.g. 64).

The precise time synchronization function C (n) according to Equation 7 has four maximum values in the long training symbol interval. These four maximums appear at the beginning of the first and second long training symbols and at the center of each long training symbol.

Thus, the maximum value of the precision time synchronization function is searched using the search interval window set based on the low precision time synchronization estimation value (S20). When this is expressed as an equation, Equation 8 is obtained.

Here, Wf is a search section window of the maximum value point that can be obtained through the low precision time synchronization function, and the window size can be adjusted. In practice, since the received signal at antenna j is represented as the sum of the transmitted signals having different cyclic shift values at each transmit antenna, the precision time synchronization function is calculated as the average value of the transmitted signals, so the estimated value of the fine time synchronization is accurate and long training. It does not indicate the starting point of the symbol. However, the precision time synchronization function, which consists of the average value of the transmission signal, has only one maximum value, and since the receiver already knows how much cyclic shift is performed in each transmission antenna, it is possible to estimate an accurate starting point of a long training symbol.

As described above, the time synchronization method for a wireless communication system according to the present invention is a long training by solving the problem of having a flat surface at the maximum value point which is a problem of the time synchronization technique based on autocorrelation through low precision time synchronization estimation. After estimating the approximate starting position of the symbol, the precise timing synchronization estimates the starting point of the long training symbol. Accordingly, the time synchronization error that may be caused by the cyclic shift in the cross-correlation technique used in the conventional time synchronization estimation may be reduced, and may have one maximum value. In addition, the conjugate complex symmetry structure used in the precision time synchronization estimation process of the present invention can reduce the complex multiplication number from 64 to 32 required in the conventional cross-correlation method, thereby significantly reducing the computation amount for synchronization acquisition. have.

4 is a graph illustrating simulation results of a time synchronization method for a wireless communication system according to an exemplary embodiment of the present invention.

Assuming that the simulated carrier frequency offset has already been compensated, the multipath channel model is set to TGn Sync channel model 'D' which is a typical office environment with 50 ns delay spread and SNR = 14 dB ^] . In addition, the number of transmission and reception antennas was set to two, and the cyclic shift was set to 0ns in TX1 and -200ns in TX2.

As a result of simulating the time synchronization method of the present invention under these conditions, as shown in the graph of FIG. 4, it is shown that the precise time synchronization structure has only one maximum value. In the proposed structure, the conjugate complex symmetry of 64 long training symbols is used, and thus the maximum value is relatively larger than that of the short training symbol.

As described above, the present invention uses a HT mixed mode preamble in the IEEE 802.11n draft standard based on MIMO-OFDM, and estimates low precision time synchronization using a short training symbol. Precise time synchronization using the conjugate complex symmetry of the Legacy Long Training Symbol is estimated. In other words, it is possible to reduce the amount of computation of the precision time synchronization function by estimating the start position of a long training symbol through the low precision time synchronization estimation process. It is possible to reduce the time synchronization error that can be caused by the technique using cross correlation. In addition, through the time synchronization function using the conjugate complex symmetric structure of long training symbols, the amount of computation can be greatly reduced than in the conventional cross-correlation function.

Although the present invention has been described in detail with reference to exemplary embodiments above, those skilled in the art to which the present invention pertains can make various modifications to the above-described embodiments without departing from the scope of the present invention. I will understand. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the claims below but also by the equivalents of the claims.

1 is a structural diagram of a representation layer protocol data unit (PPDU) frame format in the IEEE 802.11n standard to which a time synchronization method for a wireless communication system according to the present invention is applied;

2 is a flowchart of a time synchronization method for a wireless communication system according to the present invention;

3 is a processing state diagram of a time synchronization method for a wireless communication system according to the present invention;

4 is a graph illustrating simulation results of a time synchronization method for a wireless communication system according to an exemplary embodiment of the present invention.

*** Explanation of symbols for the main parts of the drawing ***

 1: L-STF (Legacy Short Training Field)

 3: L-LTF (Legacy Long Training Field)

 5: L-SIG (Legacy SIGNAL Field)

11: HT-SIG (HT SIGNAL Field)

13: HT-STF (HT Short Training Field)

15: HT Long Training Field (HT-LTF)

20: data

Claims (7)

Calculating an autocorrelation function of a received signal using a short training symbol according to a preamble format of the HT mixed mode of the IEEE 802.11n draft standard; Summing the calculated autocorrelation functions; Removing the planar surface of the summed autocorrelation function and setting the selected maximum value as a low precision time synchronization estimate value; Calculating a precision time synchronization function value using a precision time synchronization function using a conjugate complex symmetry characteristic of a legacy long training symbol according to the preamble format; Setting a search section window based on the low precision time synchronization estimate value; And searching for the maximum value of the precision time synchronization function value in the search section window to obtain time synchronization. The method of claim 1, Computing the autocorrelation function of the received signal, A time synchronization method for a wireless communication system characterized by using the following equation.

Where R is the autocorrelation function, r is the received signal, n is the index of the autocorrelation function, L is the length of the short training symbol, Ls is the symbol delay time, * is the conjugate complex, and j is the index of the receiving antenna. . The method of claim 2, Summing the autocorrelation functions, A time synchronization method for a wireless communication system characterized by using the following equation.

Where M is the total number of receive antennas. The method of claim 3, wherein After removing the flat surface of the summed autocorrelation function and setting the selected maximum value to the low precision time synchronization estimate, And removing a flat surface of the summed autocorrelation function using the following equation.

Where A (n) is a low precision time synchronization function and Wd is the window size. The method of claim 4, wherein After removing the flat surface of the summed autocorrelation function and setting the selected maximum value to the low precision time synchronization estimate, And selecting the maximum value by selecting the maximum value according to the following equation and selecting the maximum value of the T _coarse point as the maximum value.

The method of claim 1, Computing a precision time synchronization function value using a precision time synchronization function using a conjugate complex symmetry characteristic of a long training symbol according to the preamble format, A time synchronization method for a wireless communication system, characterized in that the step of calculating the precision time synchronization function value using the following equation.

Where C (n) is a precision time synchronization function, N is a long training symbol length, r is a received signal, and n is an index of autocorrelation function. The method of claim 6, The step of obtaining the time synchronization by searching for the maximum value of the precision time synchronization function value in the search section window, Equation

Calculating a maximum value of the precision time synchronization function value by using the wireless communication system, wherein Wf is a search section window of the maximum value point obtained through the low precision time synchronization function. Time synchronization method.

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