KR20070098003A - Downlink symbol timing recovery apparatus and method of terminal receiver in orthogonal frequency division multiplexing system - Google Patents

Abstract

An apparatus and a method for recovering the downlink symbol timing of a terminal in an OFDM(Orthogonal Frequency Division Multiplexing) system are provided to prevent the generation of adjacent symbol interference by detecting a valid symbol starting point of a base station signal received at first. An apparatus for recovering the downlink symbol timing of a terminal in an OFDM system includes a preamble signal extracting device(701), a correlator block(703), and a window starting point selecting device(705). The preamble signal extracting device(701) extracts each preamble signal from a received signal of a serving base station and a plurality of adjacent base stations. The correlator block(703) calculates a correlation value at a time axis between the received signal and a preamble signal of the serving base station and the plurality of adjacent base stations. The window starting point selecting device(705) selects a first time point where the correlation value over a constant limit value among the correlation values calculated from the correlator block(703) is detected as a starting point of an FFT(Fast Fourier Transform) window.

Description

DOWNLINK SYMBOL TIMING RECOVERY APPARATUS AND METHOD OF TERMINAL RECEIVER IN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SYSTEM}

1 is a diagram showing the configuration of a typical OFDM system having a frequency reuse coefficient of 3;

2 is a diagram illustrating an example of allocating OFDM sub-broadbands having a frequency reuse factor of 3 into three groups and assigning them to each base station;

3 is a block diagram illustrating a configuration of a downlink symbol timing recovery apparatus in a terminal of a synchronous OFDM system using a frequency reuse factor 3 according to the related art;

4 is a diagram illustrating an example of determining a starting point of an FFT window when signals of a serving base station and a neighboring base station are received at the same time;

5 illustrates an example of determining a starting point of an FFT window when signals of a serving base station and a neighboring base station are received at different times;

FIG. 6 is a diagram illustrating an example of an interference effect of a neighbor base station signal on a subcarrier used by a serving base station in FIG. 5;

7 is a block diagram illustrating a downlink symbol timing recovery apparatus of a terminal in a synchronous OFDM system using a frequency reuse factor N according to an embodiment of the present invention;

8 is a flowchart illustrating a downlink symbol timing recovery method in an OFDM system according to an embodiment of the present invention;

FIG. 9 is a diagram illustrating an example in which signals from three base stations are received at different times by a terminal as a result of determining the starting point of the FFT window of FIG. 7;

10 is a graph illustrating a comparison of reception performance when a conventional symbol timing recovery method and a symbol timing recovery method according to an embodiment of the present invention are applied.

The present invention relates to an apparatus and method for restoring downlink symbol timing of a terminal in an orthogonal frequency division multiplexing (OFDM) system, and in particular, orthogonal frequency division using a frequency usage coefficient greater than one. An apparatus and method for recovering downlink symbol timing of a terminal in a multiplexing system.

The OFDM method is a transmission method of increasing data rate by converting serially input data streams into N parallel data streams and transmitting them on separate subcarriers. In this case, the subcarriers should be appropriately selected to maintain orthogonality. Since orthogonality of subcarriers can be transmitted by superimposing each subcarrier at the same time, the symbol period in each subcarrier can be made longer by the number of subcarriers than when data is sequentially transmitted using one carrier. Therefore, the OFDM scheme uses subcarriers with mutual orthogonality, which provides better bandwidth efficiency and longer symbol periods than conventional frequency division multiplexing (FDM), so that inter-symbol interference is less than that of a single carrier modulation scheme. "ISI") has a strong characteristic. When a signal is transmitted through a multipath channel, the received signal generates ISI by multipath. In particular, in high-speed data transmission, since the period of the symbol is smaller than the delay spread of the channel, the ISI becomes more severe. Therefore, a complicated reception technique is required to compensate for the distortion caused by the ISI and accurately recover the transmission signal. The OFDM scheme is a modulation scheme that can compensate for distortion in such a multipath channel.

On the other hand, when the OFDM transmission method is used for cellular mobile communication, wireless LAN, wireless mobile Internet, etc., not for broadcasting, a multiple access method for multiple users is required in the same manner as the single carrier transmission method. Orthogonal Frequency Division Multiple Access (hereinafter, referred to as "OFDMA") is used as an OFDM multiple access method. The OFDMA method allows each user to use some subchannels and some OFDM symbols among all subchannels and OFDM symbols, and allocates different numbers of subcarriers and OFDM symbols according to transmission rates required for each user. By doing so, resource distribution can be efficiently performed.

Hereinafter, unless otherwise mentioned, the OFDMA scheme will be referred to collectively as the OFDM scheme.

Meanwhile, a mobile data communication system adopting the OFDMA scheme is based on IEEE 802.16. For example, a frequency broadband reuse factor is used for the WiBro system, which is a synchronous OFDMA system based on IEEE802.16. 1 and 3 are used.

The frequency reuse coefficient is a parameter representing frequency usage efficiency in a cellular system and refers to a cell indicating how many cells the entire frequency band is divided into.

A synchronous OFDM system using the frequency reuse factor 1 allows each base station to use all of the available subcarriers. Therefore, the synchronous OFDM system using the frequency reuse factor 1 has an excellent advantage in terms of frequency efficiency. However, all subcarriers used by the serving base station overlap with the subcarriers of the neighboring base station, which causes interference with each other. There is a disadvantage working. However, the synchronous OFDM system using the frequency reuse factor 3 divides the entire available subcarriers into three groups so that three adjacent base stations use subcarriers of different groups.

Hereinafter, a downlink symbol timing synchronization method in a terminal of a synchronous OFDM system using a frequency reuse factor 3 will be described with reference to the accompanying drawings.

1 is a diagram illustrating a configuration of a general OFDM system having a frequency reuse factor of three. In the OFDM system of FIG. 1, since the frequency reuse factor is 3, the entire frequency band is allocated to three cells 101a, 103a, and 105a. The base station system in each cell is divided into a serving base station 101 and neighbor base stations 103 and 105. In addition, it is assumed that the terminal 100 is located at the boundary of each cell.

FIG. 2 is a diagram illustrating an example of allocating subcarriers c1 into three groups 201, 203, and 205 and assigning them to respective base stations in an OFDM system having a frequency reuse factor of 3. FIG. Assuming that the frequency reuse factor 3 is used as shown in the example of FIG. 2 and the transmission signals of all base stations are received by the terminal at the same time, the terminal 100 serves if the frequency bands used by each base station are different. When receiving a signal through a subcarrier used by the base station 101 is not interfered with the signals of the adjacent base stations (103, 105).

3 is a block diagram illustrating a configuration of a downlink symbol timing recovery apparatus in a terminal of a synchronous OFDM system using a frequency reuse coefficient 3 according to the related art.

For example, in the WiBro system, when each base station transmits a downlink signal, the base station transmits a unique preamble signal in the first OFDM symbol period within one frame. The terminal receives the preamble signal transmitted by the serving base station and performs symbol timing recovery.

The preamble extractor 301 of the conventional symbol timing recovery apparatus 300 of FIG. 3 extracts only the preamble signal of the serving base station from the received signal and transmits it to the correlator 303 to perform symbol timing recovery. The correlator 303 calculates a correlation in the time axis between the received signal and the preamble signal of the serving base station delivered from the preamble extractor 301 and outputs the correlation value. The window starting point selector 305 determines the starting point of the FFT window based on the correlation value so that ISI by symbols of neighboring base stations does not occur. The FFT processor 307 performs the fast Fourier transform according to the determined FFT window start point.

However, the FFT widow starting point determined by the conventional symbol timing recovery apparatus 300 of FIG. 3 may cause the following problem according to the reception timing of the signal transmitted from the serving base station and the neighboring base station.

4 illustrates an example of determining a starting point of an FFT window when signals of a serving base station and a neighboring base station are received at the same time. Reference numeral 401 of FIG. 4 is a cyclic prefix (CP) for periodically inserting a signal in an effective symbol interval in order to prevent orthogonality destruction that may be caused by delay of a subcarrier in a protection interval. Reference numeral 403 denotes OFDM data excluding CP, and reference numeral 405 denotes an FFT window section.

 4 is an ideal case, it is assumed that the transmission signals at all base stations are received at the same time by the terminal. In addition, in FIG. 4, for convenience of description, it is assumed that a signal transmitted from each base station is received through a single path to a receiver of the terminal. When the receiver of the conventional terminal performs downlink symbol timing recovery, the receiver determines the FFT window starting point so that ISI does not occur only for the signal of the serving base station. Ideally, in the synchronous OFDM system, since the transmission signals of all the base stations will be received at the receiver of the terminal at the same time, the signals of the subcarriers used by the serving base station as shown in FIG. Will not be interfered with. In reality, however, even in a synchronous OFDM system, a transmission signal from each base station may be received at a receiver of the terminal at different times, and FIG. 5 assumes such a case.

5 illustrates an example of determining a starting point of an FFT window when signals of a serving base station and a neighboring base station are received at different times. Reference numeral 501 of FIG. 5 is a CP for periodically inserting a signal in an effective symbol interval in order to prevent the destruction of orthogonality which may be caused by the delay of the subcarrier in the protection interval. Reference numeral 503 denotes OFDM data excluding CP, and reference numeral 505 denotes an FFT window section.

In FIG. 5, it is assumed that a signal transmitted from each base station is received through a single path to a receiver of the terminal. In addition, in FIG. 5, it is assumed that two adjacent base station signals are first received by 16 OFDM samples (1/8 of a cyclic prefix length) compared to the serving base station signal. In this case, when the receiver of the terminal uses the conventional downlink symbol timing recovery technique, the correlation is calculated only for the signal of the serving base station, so that the signal of the serving base station does not generate ISI, but the signal of the neighboring base station may generate ISI. .

FIG. 6 shows a case in which two adjacent base station signals are received at a length of 16 OFDM samples earlier than the serving base station signal, when the receiver of the terminal uses the conventional downlink symbol timing recovery scheme. An example of interference effects of neighboring base station signals is shown. When the neighboring base station signal arrives earlier than the serving base station signal as shown in FIG. 6, the subcarrier of the serving base station is subjected to the interference by the neighboring base station signal, and the interference affects the reception performance of the terminal. In particular, when the neighbor base station signal is received before the serving base station signal, the greater the reception time difference between the two signals, the greater the interference effect of the subcarriers of the serving base station from the neighbor base station signal, and thus, the reception performance deterioration of the terminal is increased.

Accordingly, even in a synchronous OFDMA system, even when a transmission signal from each base station is received at a different time, a method for improving reception performance of a terminal is required by performing accurate downlink symbol timing recovery.

The present invention provides a downlink symbol timing recovery apparatus and method for improving reception performance when a signal from each base station of a terminal is received at a different time in a synchronous OFDM system.

The present invention also provides an apparatus and method for downlink symbol timing recovery to prevent interference between adjacent symbols in a synchronous OFDM system having a frequency reuse factor greater than one. In an orthogonal frequency division multiplexing system according to the present invention, an apparatus for reconstructing downlink symbol timing of a terminal includes a preamble signal extractor for extracting preamble signals from received signals from a serving base station and a plurality of neighboring base stations, and the received signal and the received signal. A correlator block for calculating a correlation value on a time axis between a serving base station and preamble signals from a plurality of neighboring base stations, and an initial time point at which a correlation value exceeding a predetermined threshold value among correlation values calculated in the correlator block is detected. It includes a window starting point selector to select as the starting point.

In the orthogonal frequency division multiplexing system according to the present invention, a method for reconstructing downlink symbol timing of a user equipment includes extracting preamble signals from received signals from a serving base station and a plurality of neighboring base stations, and receiving the received signal and the serving base station. And calculating a correlation value on a time axis between preamble signals from a plurality of neighboring base stations, and an initial time point at which a correlation value exceeding a predetermined threshold value among correlation values calculated in the correlator block is detected as a start point of the FFT window. It is characterized by including the process of selection.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 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, the detailed description thereof will be omitted. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the specification.

First, in the present invention, when performing downlink symbol timing recovery of an MS in an OFDM system, the FFT window start point is determined so that not only the serving base station signal but also the ISI caused by the neighboring base station signal is not generated. It is possible to prevent interference from neighboring base station signals.

Hereinafter, a description will be given on the assumption that a frequency reuse coefficient N (N> 1) is used according to an embodiment of the present invention with reference to the drawings.

7 is a block diagram illustrating an apparatus for reconstructing downlink symbol timing of a terminal in an OFDM system using a frequency reuse factor N according to an embodiment of the present invention.

In FIG. 7, the preamble extractor 701 of the symbol timing recovery apparatus 700 according to an embodiment of the present invention extracts a preamble signal of a serving base station and a preamble signal of neighboring base stations from a received signal to perform symbol timing recovery. (703). The correlator block 703 includes a first correlator 703 ₁ that calculates a correlation value between the received signal and the first preamble signal P ₁ of the serving base station, and a second preamble signal P ₂ of the received signal and the first neighboring base station. A second correlator 703 ₂ for calculating a correlation value between the < RTI ID = _{0.0 >} ), < / RTI &_gt; do.

The correlator block 703 calculates a correlation value on a time axis between a received signal and a preamble signal received from a serving base station and neighboring base stations respectively transmitted from the preamble extractor 701 and stores the plurality of correlation values in a window starting point selector 705. ) For example, if the frequency reuse factor is 3, such as a WiBro system, the correlator block 703 has three correlators. The window start point selector 705 determines the start point of the FFT window from the first time point exceeding a predetermined threshold value among the correlation values of the time axis transmitted from the correlator block 703 so that the ISI by the symbol of the adjacent base station does not occur. The FFT processor 707 performs fast Fourier transform according to the determined FFT window start point.

That is, the symbol timing recovery apparatus 700 of FIG. 7 obtains a time axis correlation by using both a preamble signal of a serving base station and a preamble signal of an adjacent base station, and then displays an initial time point at which a correlation value exceeding a predetermined threshold is obtained. Decide on a starting point. This is to detect a valid symbol start point of the first received base station signal. Therefore, according to the configuration of FIG. 7, the starting point of the FFT window may be determined so that the ISI of the neighboring base station signal does not occur in the received signal transmitted from the serving base station.

8 is a flowchart illustrating a downlink symbol timing recovery method in an OFDM system according to an embodiment of the present invention. First, a receiver of a terminal may receive preamble signals transmitted by a serving base station and neighbor base stations through frame synchronization acquisition and cell search. In the following description, the neighbor base station index is defined as K, and its initial value is set to one.

That is, in step 801, the preamble extractor 701 receives and extracts the preamble signal of the serving base station and the preamble signals of all neighboring base stations. In step 803, the correlator block 703 calculates a time axis correlation value between the received signal and the preamble signal of the serving base station. In step 805, the correlator block 703 calculates a time axis correlation value between the received signal and the preamble signal transmitted by the K-th neighboring base station. Operation 805 is performed for all neighboring base stations through steps 807 and 809. That is, when the total number of neighboring base stations is M, the correlator block 703 performs the operation of step 805 by increasing K by 1 using a counter (not shown).

In operation 811, the window start point selector 705 determines the first time point at which a correlation value exceeding a predetermined threshold value among the correlation values calculated through operations 803 and 805 is determined as the FFT window start point.

FIG. 9 is a diagram illustrating a result of determining a starting point of an FFT window according to an exemplary embodiment of the present invention, which illustrates an example in which signals from three base stations are received at different times. In FIG. 9, reference numeral 901 denotes a CP for periodically inserting a signal in an effective symbol interval in order to prevent the destruction of orthogonality which may be caused by the delay of the subcarrier in the protection interval. Reference numeral 903 denotes OFDM data excluding CP, and reference numeral 905 denotes an FFT window section. According to the determination result of the starting point of the FFT window of FIG. 9, an effective symbol start point of the first received base station signal is detected to prevent the occurrence of ISI by signals of neighboring base stations in the received signal from the serving base station.

10 is a graph illustrating a comparison of reception performance when the symbol timing recovery method according to the exemplary embodiment of the present invention is applied. In FIG. 10, it is assumed that a terminal is located at a boundary of a cell and receives a transmission signal of the same strength from a serving base station and two neighboring base stations. In addition, it is assumed that carrier frequency recovery, channel estimation, and the like are perfectly performed, and that each subcarrier is applied with a quadrature phase shift keying (QPSK) modulation scheme.

In the case of performing the conventional downlink symbol timing recovery as shown in FIG. 10, in the ideal case, reference code c (in the case where all base station signals are received in the same signal to the receiver of the terminal, see FIG. 4) in actual case reference code a It can be seen that the reception performance of, b (refer to FIG. 5 when two adjacent base station signals are received by 8 or 16 OFDM sample lengths earlier than the serving base station signal) is degraded. On the contrary, in case of performing the downlink symbol timing restoration proposed by the present invention, it can be seen that the reception performance of the ideal case and the actual case is the same.

Therefore, when the neighbor base station signal is first received at the terminal compared to the serving base station signal, the greater the reception time difference between the neighbor base station signal and the serving base station signal, the greater the performance degradation when performing the conventional downlink symbol timing recovery. It can be seen that the reception performance gain obtained by applying symbol timing recovery is increased.

Meanwhile, in the detailed 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 should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

As described above, according to the present invention, considering the fact that a transmission signal from each base station of a terminal may be received at different times in an OFDM system using a frequency reuse factor greater than 1, the first received base station signal is effective. By detecting the symbol start point, it is possible to prevent the occurrence of neighbor symbol interference in the received signal for the serving base station and the neighbor base stations, thereby improving the reception performance of the terminal.

Claims (4)

An apparatus for recovering downlink symbol timing of a terminal in an orthogonal frequency division multiplexing system, A preamble signal extractor for extracting preamble signals from the received signals from the serving base station and the plurality of neighbor base stations, respectively; A correlator block for calculating a correlation value on a time axis between the received signal and the preamble signal from the serving base station and a plurality of neighboring base stations, respectively; And a window start point selector for selecting an initial time point at which a correlation value exceeding a predetermined threshold value is detected among the correlation values calculated in the correlator block as a start point of an FFT window. The method of claim 1, And the orthogonal frequency division multiplexing system uses a frequency reuse factor greater than one. A method for recovering downlink symbol timing of a terminal in an orthogonal frequency division multiplexing system, Extracting preamble signals from received signals from a serving base station and a plurality of neighboring base stations, respectively; Calculating a correlation value on a time axis between the received signal and the preamble signals from the serving base station and a plurality of neighboring base stations, respectively; And selecting, as a start point of an FFT window, an initial time point at which a correlation value exceeding a predetermined threshold value is detected among correlation values calculated in the correlator block. The method of claim 3, wherein And the orthogonal frequency division multiplexing system uses a frequency reuse factor greater than one.

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