Patent Publication Number: US-8125976-B2

Title: Apparatus for generating down link signal, and method and apparatus for cell search in cellular system

Description:
TECHNICAL FIELD 
     The present invention relates to a method and apparatus for generating a downlink signal in a cellular system, and more particularly, to a method of searching a downlink cell in an orthogonal frequency division multiplexing (OFDM)-based cellular system. 
     BACKGROUND ART 
     In a cellular system, for initial synchronization, a terminal should acquire timing synchronization and frequency synchronization on the basis of signals transmitted from a base station, and perform a cell search. After the initial synchronization, the terminal should track the timing and frequency, and perform the timing and frequency synchronization between adjacent cells and the cell search in order for handover. 
     In a synchronous cellular system, all base stations can perform frame synchronization using common time information from an external system. However, a cellular system that has been developed by 3GPP (3rd generation partnership project) is an asynchronous system in which the frame timings of all base stations are independent. The asynchronous cellular system needs to perform a cell search process, unlike the synchronous cellular system. 
     Therefore, a method of acquiring synchronization using a separate preamble and searching a cell has been proposed. However, the method cannot be applied to a system without the preamble. In addition, a method of acquiring synchronization and searching a cell using pilot symbols disposed at start and end points of a sub-frame has been proposed. However, the method has a problem in that a large number of pilots should be used. 
     DISCLOSURE 
     Technical Problem 
     The present invention has been made in an effort to provide a cell searching method and apparatus that are capable of forming a plurality of synchronization channels in one frame to effectively acquire synchronization and search a cell in an OFDM-based cellular system. 
     Technical Solution 
     In order to achieve the object, according to an exemplary embodiment of the present invention, there is provided an apparatus for generating a downlink signal in an orthogonal frequency division multiplexing (OFDM)-based cellular system. The downlink signal generating apparatus includes a pattern generator and a time-frequency mapping unit. The pattern generator generates synchronization patterns for a plurality of synchronization blocks forming one frame of the downlink signal, and the synchronization blocks each have a continuous series of sub-frames. The synchronization pattern includes a cell group number and information on a start point of the frame. The time-frequency mapping unit maps the synchronization patterns to a time-frequency domain to generate the downlink signal. 
     According to another exemplary embodiment of the present invention, there is provided an apparatus for searching a cell including a terminal in an orthogonal frequency division multiplexing (OFDM)-based cellular system. The cell searching apparatus includes a receiver and first to third estimators. The receiver receives one frame of synchronization blocks. Each of the synchronization blocks has a plurality of adjacent sub-frames, and a plurality of OFDM symbols of the synchronization block each have a synchronization pattern that is composed of a combination of a cell group identification code for identifying a cell group and a frame synchronization identification code for indicating a frame start point. The combination of the cell group identification code and the frame synchronization identification code is referred to as a combination of codes. The first estimator estimates a start point of the synchronization block from the synchronization pattern. The second estimator estimates the frame start point and a cell group number of the cell group to which the cell including the terminal belongs, using the start point of the synchronization block. The third estimator estimates a cell number of the cell including the terminal, using a cell identification scrambling code included in a pilot symbol of the frame. 
     According to still another exemplary embodiment of the invention, there is provided a method of searching a cell including a terminal in an orthogonal frequency division multiplexing (OFDM)-based cellular system. First, a downlink frame including a plurality of synchronization blocks, each having a synchronization pattern that is composed of a combination of a cell group identification code for identifying a cell group including the terminal and a frame synchronization identification code for indicating a start portion of the frame (a combination of codes), is received, and a start point of the synchronization block is estimated in the received downlink frame. Then, a cell group number and frame synchronization are acquired from the estimated start point of the synchronization block and the synchronization pattern, and a cell number is acquired from a cell identification scrambling code included in the downlink frame. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating an apparatus for generating a downlink signal in a cellular system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a diagram illustrating the configuration of a downlink frame of the cellular system according to the exemplary embodiment of the present invention. 
         FIG. 3  is a diagram illustrating the detailed configuration of the downlink frame shown in  FIG. 2 . 
         FIG. 4  is a diagram illustrating a signal waveform obtained by converting the downlink frame shown in  FIG. 3  into a time domain. 
         FIG. 5  is a diagram illustrating the bandwidth scalability of the downlink frame according to the exemplary embodiment of the present invention. 
         FIG. 6  is a diagram illustrating the bandwidth scalability of a downlink frame according to another exemplary embodiment of the present invention. 
         FIG. 7  is a block diagram schematically illustrating a cell searching apparatus according to an exemplary embodiment of the present invention. 
         FIG. 8  is a flowchart illustrating a cell searching method according to an exemplary embodiment of the present invention. 
         FIG. 9  is a block diagram schematically illustrating the configuration of a synchronization estimator according to an exemplary embodiment of the present invention. 
         FIG. 10  is a diagram illustrating a method of allocating a cell group identification code and a frame synchronization identification code according to an exemplary embodiment of the present invention. 
         FIG. 11  is a diagram illustrating a method of allocating a cell group identification code and a frame synchronization identification code according to another exemplary embodiment of the present invention. 
         FIG. 12  is a block diagram schematically illustrating the configuration of a cell group estimator according to an exemplary embodiment of the present invention. 
     
    
    
     MODE FOR INVENTION 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. However, the present invention is not limited to the following exemplary embodiments, but various modifications and changes of the invention can be made. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     It will be understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Hereinafter, a method and apparatus for generating a downlink signal and a method and apparatus for searching a cell in a cellular system according to exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram schematically illustrating an apparatus for generating a downlink signal in a cellular system according to an exemplary embodiment of the present invention, and  FIG. 2  is a diagram illustrating a downlink frame structure of a cellular system according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 1 , a downlink signal generating apparatus  100  according to an exemplary embodiment of the present invention includes a pattern generator  110 , a code generator  120 , a time-frequency mapping unit  130 , an OFDM transmitter  141 , and a transmitting antenna  142 , and is provided in a base station (not shown) of the cellular system. As shown in  FIG. 2 , the downlink signal generated by the downlink signal generating apparatus  100  according to the exemplary embodiment of the present invention includes a plurality of synchronization blocks  210 , and each of the synchronization blocks  210  includes a plurality of sub-frames  220 . Information for identifying a cell group and information for estimating frame synchronization are allocated to first symbol durations  230   a  and  230   b  of each synchronization block  210 . In addition, different frame synchronization identification codes are allocated to the synchronization blocks  210 . 
     The pattern generator  110  generates a synchronization pattern and a pilot pattern of the downlink signal using a set of orthogonal codes indicating cell number information, cell group information, and information for identifying frame synchronization. The pattern generator  110  allocates a series of orthogonal codes to a cell group number for identifying a cell group, and uses the series of orthogonal codes to recognize a frame start point. Hereinafter, for better comprehension and ease of description, the orthogonal codes allocated to the cell group numbers are referred to as “cell group identification codes,” and the orthogonal codes used to recognize the frame start points are referred to as “frame synchronization identification codes.” The pattern generator  120  matches the cell group identification codes with the frame synchronization identification codes to generate a set of codes, and allocates the set of codes to a frequency domain of a synchronization channel symbol duration of the downlink signal to generate a synchronization pattern of the downlink signal. The pattern generator  110  allocates to a pilot channel symbol duration a unique scrambling code that is allocated to each cell in order to encode a common pilot symbol and a data symbol in the cellular system, thereby generating a pilot pattern of the downlink signal. 
     The code generator  120  generates orthogonal code sets that are used as the cell group identification codes and the frame synchronization identification codes, and transmits the generated orthogonal code sets to the pattern generator  110 . Then, the pattern generator  110  uses the orthogonal code sets to generate a synchronization pattern and a pilot pattern. 
     The time-frequency mapping unit  130  maps data to a time-frequency domain, using the synchronization pattern information and the pilot pattern information generated by the pattern generator  110 , and frame structure information and transmission traffic data that are transmitted from the outside, to form a frame of downlink signals (reference numeral  200  in  FIG. 2 ). 
     Then, the OFDM transmitter  141  receives the downlink signal from the time-frequency mapping unit  130 , and transmits the signal through the transmitting antenna  142 . 
     Referring to  FIG. 2 , one frame  200  of downlink signals in a cellular system according to an exemplary embodiment of the present invention is composed of Nsync synchronization blocks  210 , and each of the synchronization blocks  210  includes Nsub sub-frames  220 . An OFDM symbol duration  230   a  of the downlink signal uses Nt subcarriers each having a frequency range of Δf. Pilot symbol durations  240   a  to  240   e , each having pilot data therein, are formed in the headers of the sub-frames  220  forming one synchronization block  210 . A first sub-frame of the synchronization block  210  is provided with synchronization symbol durations  230   a  and  230   b  each having data including a cell group identification code and a frame synchronization identification code arranged therein. The synchronization symbol durations  230   a  and  230   b  may be disposed in a first OFDM symbol duration of the first sub-frame or the last OFDM symbol duration of the first sub-frame. Each of the synchronization symbol durations  230   a  and  230   b  is divided into two frequency bands  250  and  260  in the frequency domain, and each of the frequency bands  250  and  260  has the cell group identification code and the synchronization identification code inserted therein. As shown in  FIG. 2 , the pattern generator  110  does not form a synchronization pattern in the entire frequency domain of each of the symbol durations  230   a  and  230   b , but allocates codes to only a central portion of the frequency bandwidth except a DC subcarrier to form the synchronization pattern in the central portion. In a 3GPP system, the downlink frame  200  includes 20 sub-frames  220 , and one sub-frame  220  corresponds to a time of 0.5 msec. In the case of unicast transmission, one sub-frame  220  includes 7 OFDM symbol durations, and in the case of multicast transmission, one sub-frame  220  includes 6 OFDM symbol durations. In the downlink frame of the 3GPP system, as an example, the synchronization block  210  may include 5 sub-frames  220 . In this case, one frame includes four synchronization channel symbol durations. 
     Next, the generation of the synchronization pattern and the pilot pattern by the pattern generator  110  shown in  FIG. 1  will be described in detail with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a diagram illustrating the OFDM symbols in the synchronization channel symbol duration in which the synchronization pattern is formed, and  FIG. 4  is a diagram illustrating a signal waveform when the synchronization channel symbol duration shown in  FIG. 3  is converted into a time domain. 
     As shown in  FIG. 3 , the pattern generator  110  divides a predetermined bandwidth into a frequency band  250  for inserting the cell group identification code and a frequency band  260  for inserting the frame synchronization identification code on the basis of a central subcarrier in the entire frequency bandwidth of the channel symbol duration  230   a , and sequentially inserts orthogonal codes into the divided frequency bands to form the synchronization pattern. 
     The pattern generator  110  allocates to the frequency bands  250  and  260  the orthogonal codes in two independent orthogonal code sets transmitted from the code generator  120 . Referring to  FIG. 3 , the pattern generator  110  allocates an 
     
         
         
           
             orthogonal code set of 
             and an orthogonal code set of 
             to the frequency band  250  for identifying a cell group and the frequency band  260  for identifying frame synchronization to form the synchronization pattern, respectively. In this case, “k” indicates a cell group number, “u” indicates a frame synchronization identification code number, “NG” indicates the length of the cell group identification code, and “NF” indicates the length of the frame synchronization identification code. The pattern generator  110  according to the exemplary embodiment of the present invention may use GCL (generalized chirp-like) codes as the cell group identification code and the frame synchronization identification code, and these codes can be expressed by the following Equations 1 and 2: 
           
         
       
    
     
       
         
           
             
               
                 
                   
                     
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     The orthogonal codes expressed by Equations 1 and Equation 2 are allocated to the positions shown in  FIG. 3  to generate the synchronization pattern. That is, the pattern generator  110  does not sequentially allocate the orthogonal codes obtained by Equations 1 and 2 to adjacent subcarriers, but allocates even-numbered subcarriers or odd-numbered subcarriers in the frequency bands  250  and  260 . Subcarriers between the subcarriers having the orthogonal codes allocated thereto are used as nulling subcarriers to which no sequence is allocated. Therefore, the subcarriers including the nulling carriers that are arranged in the synchronization channel symbol duration for forming the pattern occupy substantially 2*[(NG+NF)+NB] (hereinafter, referred to as NS) subcarrier bands. In this case, “NB” indicates the number of subcarriers in a guard band. 
     When the synchronization pattern is converted into a time domain, the signal waveform shown in  FIG. 4  is obtained.  FIG. 4  shows the signal waveform of the OFDM symbol except a cyclic prefix. As can be seen from  FIG. 4 , two repeated patterns are generated in the time domain due to two kinds of inserted orthogonal codes. 
     As shown in  FIG. 3 , the downlink signal generating apparatus  100  according to the exemplary embodiment of the present invention forms a synchronization pattern such that one nulling subcarrier exists between the subcarriers to which sequences are allocated over the frequency domain of the synchronization channel symbol duration in which the cell group identification code and the synchronization identification code are allocated, thereby generating signals. Therefore, the generated signal has the repeated pattern shown in  FIG. 4 , and a terminal having received the downlink frame acquires initial symbol synchronization and estimates a frequency offset, using the signal pattern shown in  FIG. 4 . 
     The lengths NG and NF of the cell group identification code and the synchronization identification code inserted into each of the synchronization channel symbol durations of the downlink frame may be different from each other, and information on the lengths of these identification codes and information on the synchronization patterns thereof are shared by a terminal and a base station. 
     The terminal having received the downlink frame  200  having the synchronization pattern shown in  FIG. 3  demodulates the two frequency bands  250  and  260  for each synchronization block to obtain information on the cell group number and the frame start point, which makes it possible to rapidly and effectively search the cells. In addition, the frequency domain of the channel symbol duration is divided into two frequency bands, and the same sequence or different types of sequences are allocated to the two divided frequency bands, which makes it possible to prevent the lowering of a correlation performance due to the selective fading of frequencies. 
     In the exemplary embodiment of the present invention, the cell group identification code is inserted before the frame synchronization identification code on a frequency axis of the synchronization channel symbol duration, but the invention is not limited thereto. For example, the cell group identification code may be inserted after the frame synchronization identification code to form the synchronization pattern. Further, in the exemplary embodiment of the present invention, the same type of orthogonal code is used as the cell group identification code and the frame synchronization identification code, but the invention is not limited thereto. For example, different types of orthogonal codes may be used as the cell group identification code and the frame synchronization identification code. In this case, general orthogonal codes, such as a Hadamard code, a KAZAC code, a gold code, a Golay code, and a pseudo-noise (PN) code, may be used as the identification codes. 
       FIG. 5  is a diagram illustrating the bandwidth scalability of a downlink frame according to an exemplary embodiment of the present invention, and  FIG. 6  is a diagram illustrating the bandwidth scalability of a downlink frame according to another exemplary embodiment of the present invention. 
       FIGS. 5 and 6  show the comparison between the bandwidth of the synchronization channel symbol duration shown in  FIG. 3  with the entire bandwidth supported by the cellular system. As shown in  FIGS. 2 and 3 , the downlink signal generating apparatus  100  according to the exemplary embodiment of the present invention inserts orthogonal codes into the center of the frequency bandwidth to generate a synchronization pattern. In the cellular system, since the terminals have different supportable bandwidths according to their levels, it is possible to support the bandwidth scalability of the terminals through the frame structure.  FIG. 5  shows a synchronization pattern allocated to a 1.25 MHz band within the frequency bandwidth. Traffic data cannot be allocated to an OFDM symbol without a synchronization pattern in the channel symbol duration, and transmitted thereto.  FIG. 6  shows a synchronization pattern allocated to a 1.25 MHz band or a 5 MHz band within the frequency bandwidth. A terminal supporting a 5 MHz band or more can receive all synchronization patterns transmitted, but terminals supporting a 1.25 MHz band and a 2.5 MHz band can receive some synchronization patterns that are arranged in the center of the frequency bandwidth. According to the exemplary embodiment of the present invention, it is possible to extract the cell group number and information on the synchronization start point from the downlink frame using only some of the received synchronization patterns, and thus support the bandwidth scalability. 
     Next, a method of allowing a terminal to search a cell using the downlink signal will be described in detail below with reference to  FIGS. 7 and 8 . 
       FIG. 7  is a block diagram schematically illustrating a cell searching apparatus according to an exemplary embodiment of the present invention, and  FIG. 8  is a flowchart illustrating a cell searching method according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , a cell searching apparatus  400  according to an exemplary embodiment of the present invention includes a receiver  410 , a symbol synchronization estimator  420 , a Fourier transformer  430 , a cell group estimator  440 , and a cell number estimator  450 . The Fourier transformer  430  can perform fast Fourier transform (FFT). 
     As shown in  FIG. 8 , the receiver  410  receives signals transmitted from a base station. The symbol synchronization estimator  420  filters the received signal within the bandwidth allocated to a synchronization channel, removes a guard interval, performs differential correlation to acquire symbol synchronization or sub-frame synchronization, and estimates a frequency offset (S 110 ). Then, the Fourier transformer  430  performs Fourier transform on the received signal on the basis of the symbol synchronization estimated by the symbol synchronization estimator  420  (S 120 ). The cell group estimator  440  estimates a frame start point from the sequence of the synchronization channel symbol duration included in the received signal that has been subjected to Fourier transform, acquires frame synchronization, and estimates the cell group number (S 130 ). The cell number estimator  440  estimates the cell number using scrambling code information included in the pilot symbol duration (S 140 ). 
     Next, the acquisition of sub-frame synchronization and the estimation of a frequency offset by the symbol synchronization estimator  420  will be described in detail with reference to  FIG. 9 . 
       FIG. 9  is a block diagram schematically illustrating the structure of the symbol synchronization estimator  420  according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , the symbol synchronization estimator  420  according to the exemplary embodiment of the present invention includes a filter  421 , a delay unit  422 , a correlator  423 , a power detector  424 , a comparator  425 , and a frequency offset detector  426 . 
     The symbol synchronization estimator  420  estimates sub-frame synchronization and frequency offset from a received signal having the time domain signal waveform shown in  FIG. 4  in the synchronization channel symbol duration. The symbol synchronization estimator  420  may estimate the last OFDM symbol duration of the sub-frame where the synchronization pattern is formed and a frequency offset in the last OFDM symbol duration. 
     The filter  421  filters the time domain signal within a bandwidth allocated to the synchronization channel and removes a guard interval to extract signals y(n+l) in NS subcarrier bands, which are central subcarrier bands, in which the synchronization patterns are formed in the entire frequency band corresponding to the synchronization channel symbol duration. The filter  421  can perform bandpass filtering. The length of the signal y(n+l) output from the filter  421  corresponds to NS. 
     The delay unit  422  delays the filtered signal y(n+l) by a time corresponding to half the effective symbol length NS. The correlator  423  performs differential correlation on the input signal y(n+l) and an output signal y(n+l+Ns/2) of the delay unit  422  in a sample duration corresponding to half the effective symbol length. The differential correction performed by the correlator  423  can be expressed by Equation 3 given below: 
     
       
         
           
             
               
                 
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     The power detector  424  having received the correlation result Y calculated by Equation 3 calculates a differential correlation value of the received signal, that is, the power of the received signal. The comparator  425  selects the time when the power detector  424  outputs a maximum value by Equation 4 given below, and sets the selected time as an initial symbol synchronization time. 
     
       
         
           
             
               
                 
                   
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     The frequency offset detector  426  estimates an initial frequency offset. 
     In this exemplary embodiment of the present invention, differential correlation is performed on only the time domain signals corresponding to one synchronization channel symbol duration to detect the initial symbol synchronization and the frequency offset, but the invention is not limited thereto. For example, the time domain signals in a different synchronization channel symbol duration in one downlink frame may be accumulated, and the differential correlation may be performed on the accumulated signals. In addition, in order to improve an estimating performance, data obtained from synchronization patterns of a plurality of frames may be accumulated, and the differential correlation may be performed on the accumulated data. 
     The Fourier transformer  430  performs Fourier transform on the received signal on the basis of sub-frame synchronization estimated by the symbol synchronization estimator  420 . 
     The estimation of the frame synchronization and the cell group number by the cell group estimator  440  from the synchronization pattern of the signal that has been subjected to Fourier transform will be described in detail below with reference to  FIGS. 10 to 12 . First, referring to  FIGS. 10 and 11 , a method of generating the synchronization pattern of the downlink frame and estimating the cell group number and frame synchronization from the generated synchronization pattern will be described with reference to  FIG. 12 . 
       FIGS. 10 and 11  are diagrams illustrating a method of allocating the synchronization pattern shown in  FIG. 3 . The downlink generating apparatus according to the exemplary embodiment of the present invention combines a cell group identification code C(k) with a frame synchronization identification code C(u) to generate a synchronization pattern.  FIGS. 10 and 11  show combinations of the cell group identification codes and the frame synchronization identification codes in the form of (k, u) (A in  FIG. 10  and A′ in  FIG. 11 ). In  FIGS. 10 and 11 , it is assumed that a frame  200  of downlink signals includes 4 synchronization blocks  210 . 
       FIG. 10  shows a synchronization pattern generated by combining orthogonal codes using only common frame synchronization identification codes C( 1 ), C( 2 ), C( 3 ), and C( 4 ) to all cell groups in the cellular system. In  FIG. 10 , cell No.  1  to cell No.  4  form cell group No.  1 , cell No.  5  to cell No.  8  form cell group No.  2 , and cell No.  9  to cell No.  12  form cell group No.  3 .  FIG. 10  shows a combination of codes when C(k) (k is cell group number, k=1, 2, 3, . . . ) is used as the cell group identification code. When the synchronization pattern is formed as shown in  FIG. 10 , the same frame synchronization identification code is transmitted from all cells. Therefore, it is possible to obtain a macro diversity gain. That is, the terminal having received the downlink frame performs correlation on a synchronization channel symbol duration to detect a frame synchronization identification code, in order to acquire the frame synchronization. In this case, since the same code is used for all cells, a correlation characteristic is improved, and thus a frame synchronization acquiring performance can be improved. In this case, the number of cell groups that can be divided may be set to be equal to the length of the code that is set to identify the cell groups, and the length of the frame synchronization identification code may be smaller than the length of the cell group identification code due to the diversity gain. 
       FIG. 11  shows the formation of a frame  200  of downlink signals using a combination of codes that is formed by allocating different frame synchronization identification codes to the cell groups. In this case, the number of frame synchronization identification codes that are available in the cellular system is equal to the length of the codes. When the synchronization pattern is formed as shown in  FIG. 11 , the number of combinations of the cell group numbers and the frame synchronization identification codes increases since various frame synchronization identification codes are used. Therefore, as compared with the synchronization pattern shown in  FIG. 10 , it is possible to increase the number of cell groups that can be identified. 
     A base station and terminals share information on the combination of codes according to the exemplary embodiment of the present invention, and the terminals use the information to search cells. 
       FIG. 12  is a block diagram schematically illustrating the cell group estimator  440  according to the exemplary embodiment of the present invention. 
     As shown in  FIG. 12 , the cell group estimator  440  according to the exemplary embodiment of the present invention includes a code storage unit  441 , a correlator  442 , an inverse Fourier transformer  443 , and a comparator  444 . 
     The code storage unit  441  stores orthogonal codes that are used as the cell group identification codes and the frame synchronization identification codes allocated to the synchronization channel symbol duration, and also stores information on the combination of codes forming the synchronization pattern. Meanwhile, when information on the cell including a terminal therein and peripheral cells (information on the cell number and the cell group) is known beforehand (that is, when the terminal is busy or in a standby state), the code storage unit  441  can extract a candidate combination of codes, and use the extracted combination of codes to search cells. 
     The correlator  442  receives the signals in the synchronization channel symbol duration that have been subjected to Fourier transform, and multiplies the signals having been subjected to Fourier transform by the conjugates of the orthogonal codes included in a combination of codes that are stored in the code storage unit  441 . 
     That is, when the correlator  442  sequentially performs a conjugate operation on sequences in the synchronization channel section of the received downlink frame over the frequency domain, an operation for identifying a cell group and an operation for estimating frame synchronization are sequentially performed, which makes it possible to shorten the time to search cells. 
     The inverse Fourier transformer  443  performs inverse Fourier transform on a cell group identifying band and a frame synchronization identifying band among the signals output from the correlator  442  to generate time domain signals. In this case, the inverse Fourier transformer  443  may perform inverse fast Fourier transform (IFFT). The comparator  444  selects the maximum value from the time domain signals output from the inverse Fourier transformer  443 , and extracts information on a combination of codes having the maximum value from the code storage unit  441 , thereby identifying the cell group number and the frame synchronization. As can be seen from  FIG. 10 , as an example, when information on a combination of codes extracted by the comparator  444  is ( 1 ,  2 ), the current cell belongs to the cell group No.  1 , and the terminal starts estimating the frame synchronization in the second synchronization block of the downlink frame. In this way, it is possible to estimate a frame start point. 
     Finally, the terminal estimates the cell number using scrambling information included in the pilot symbol duration. Since the terminal knows the cell group information, the terminal estimates the cell number on the basis of the scramble information of the cells belonging to the corresponding cell group. In this case, a general estimating method, such as a method of using the sum of powers of a set of subcarriers of the pilot symbol, may be used to estimate the cell number. 
     In this exemplary embodiment of the present invention, the cell number is estimated from the scrambling information of the pilot symbol duration, but the invention is not limited thereto. For example, the cell number may be estimated by using symbols in a common channel section including system information of a base station. 
     In addition, in this exemplary embodiment of the present invention, the cell group identification code is allocated to the synchronization pattern, but the invention is not limited thereto. Instead of the cell group identification code, a cell identification code may be allocated to one of two bands of the synchronization symbol duration to generate a downlink frame. In this case, the estimation of the cell number using the scramble code may be used to verify cell number information obtained from the synchronization pattern. 
     The constituent elements according to the exemplary embodiment of the present invention may be implemented by at least one hardware component composed of a programmable logic element, such as a DSP (digital signal process) processor, a controller, an ASIC (application specific integrated circuit), or a FPGA (field programmable gate array), other electronic devices, or a combination thereof. In addition, at least a portion of the function or procedure according to the exemplary embodiment of the present invention may be executed by software, and the software may be recorded on a recording medium. Further, the constituent elements, the function, and the procedure according to the exemplary embodiment of the present invention may be implemented by a combination of hardware and software. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
     As described above, according to the exemplary embodiment of the present invention, it is possible to use a plurality of synchronization patterns formed in one frame to search a cell group and to estimate frame synchronization. In addition, it is possible to use the synchronization patterns to estimate sub-frame synchronization.