KR100605977B1 - Cell searching apparatus and method of mobile station in code division multiple access communication system - Google Patents

Abstract

A cell search method of a mobile station of a code division multiple access communication system having at least one threshold value for selecting a codeword and performing synchronization with a base station in three steps, the first synchronization channel (P-SCH) signal The process of searching for slot synchronization by searching for, searching of the first stage, searching of the second sync channel signal, detecting a correlation value for the received signal, and a codeword in which the detected correlation value exceeds a set threshold The frame synchronization is performed by using the scrambler, and the base station group corresponding to the codeword is determined. After performing the two-stage discovery process, the signal of the common channel is correlated using the scrambling code corresponding to the base station group. And a process of selecting a spreading code.

cell search, hand-off, sync channel,

Description

Cell searching apparatus and method of mobile station in code division multiple access communication system {APPARATUS AND METHOD FOR SEARCHING CELL OF MOBILE STATION IN CDMA COMMUNICATION SYSTEM}             

1 illustrates a structure of a synchronization channel used for cell search in an asynchronous W-CDMA system.

2 illustrates another structure of a synchronization channel used for cell search in an asynchronous W-CDMA system.

3A is a diagram for explaining a case where a handoff target base station exists in a neighbor list, and FIG. 3B is a diagram for explaining a case where a handoff target base station does not exist in a neighbor list.

4 is a diagram showing the configuration of a receiving apparatus of a mobile station for selecting a codeword of a base station for handoff in an asynchronous code division multiple access communication system according to an embodiment of the present invention;

5 is a diagram for explaining an example of detecting a codeword of a second synchronous channel in an asynchronous code division multiple access communication system;

6A and 6B are diagrams for comparing and explaining a procedure for performing three-step synchronization after performing two-step synchronization in accordance with the prior art and the embodiment of the present invention in an asynchronous code division multiple access communication system.

7 is a flowchart illustrating a process of performing a three-step synchronization procedure after a mobile station performs a two-step synchronization procedure in a code division multiple access communication system according to an embodiment of the present invention.

The present invention relates to a synchronous device and method of a code division multiple access communication system, and more particularly, to a synchronous device and method of an asynchronous code division multiple access communication system.

The 3GPP W-CDMA communication system (3rd generation partnership project wideband code division multiple access communication system), which is currently the next generation mobile communication, is composed of an asynchronous base station system, and a different base station scrambling code for each base station for asynchronous operation between the respective base stations. scrambling code) is used. Assuming 512 cells, 512 different scrambling codes are allocated to distinguish the 512 base stations.

In the base station communication system of the asynchronous mode as described above, in order for the mobile station to make a successful call, it is necessary to find out the base station code of the base station signal that is most strongly entering the mobile station. However, when performing a cell search of an asynchronous base station system with such codes, it is very difficult for the mobile station to check the phase of all the possible codes. Therefore, since it takes a considerable time to perform the cell search in the above manner, the method for the mobile station to search by applying a general cell search algorithm is inefficient. So the proposed method is a multi-level cell search algorithm. This method divides 512 cells into 32 groups and divides each group into 16 cells. A synchronization channel is used to facilitate the cell search, which uses a first synchronization channel and a second synchronization channel.

FIG. 1 illustrates a structure of a sync channel (SCH) used for cell search in the asynchronous W-CDMA system.

Referring to FIG. 1, 1-1 is a primary sync channel (P-SCH), 1-3 is a second sync channel (S-SCH), and 1-5 is a common channel. (common channel). The common channel may use a broadcasting channel or a pilot channel. Here, it is assumed that the common channel is a common pilot channel. In addition, one frame is composed of 16 slots. In this case, the first synchronization channel and the second synchronization channel are transmitted by the length of the N1 chip at the beginning of each slot, and orthogonality is maintained between the two channels of the first and second synchronization channels, as shown in FIG. They are sent overlapping each other. A different PN scrambling code is used for each base station in the common pilot channel, and the period of the PN scrambling code is equal to the length of one frame.

In the asynchronous code division multiple access communication system (hereinafter, referred to as W-CDMA communication system) having the channel structure as described above, only one frame length of gold codes of 2 ¹⁸ -1 periods is used as different PN scrambling codes. Use only M (= 512) of the total possible gold codes. As shown in FIG. 1, the common pilot channel is not transmitted in the portion in which the first synchronization channel and the second synchronization channel are transmitted, but may be transmitted only in other portions.

A sync code is used for the sync channel, and the sync code is generated as follows. It is created by modulo operation between Hadamad sequence and Hierarchical sequence. The hierarchical sequence y is generated as follows using sequences x ₁ and x ₂ having lengths n ₁ and n ₂ , respectively.

y (i) = x ₂ (i mod n ₂ ) + x ₁ (i div n ₁ ) for i = 0. . . (n ₁ * n ₂ ) -1

Sequences x ₁ and x ₂ each select a sequence of length 16 as follows.

x ₁ = <0,0,1,1,0,1,0,1,1,1,1,1,0,0,0,1>

x ₂ = <0,0,1,1,1,1,0,1,0,0,1,0,0,0,1,0>

Therefore, by performing modulo operation on the Hadamard sequence hn (i) (i = 0 ... 255) and y (i) of length 256, a sync code is generated as follows.

c_sc_n = <hn (0) + y (0), hn (1) + y (1), hn (2) + y (2), ..., hn (255) + y (255)>

The first sync code c _p and the second sync code c ₁ , ..., c ₁₇ can be assigned as follows.

c_p = c_sc_0

c_l = c_sc_i ~~~~ (i = 1, .... 17)

The above definitions of c _P , c ₁ , ..., c ₁₇ are for the purpose of using a 32 point fast Hadamard transform.

The '0' th c _p is repeatedly sent in every slot of only 256 chips which are 1 / 10th of a slot. The sync code used for the first sync channel is the same for all cells. The first synchronization channel is used by the mobile station to find the slot timing of the received signal. A comma free code was introduced when the second synchronization channel was sent by the transmitter. The comma free code consists of 32 codewords, one codeword consists of 16 symbols, and is repeatedly transmitted every frame. However, 16 symbol values are not directly transmitted, but are mapped and transmitted to the sync code. As shown in FIG. 1, an i-th sync code corresponding to a symbol value i is transmitted for each slot. The 32 codewords of the comma-free code distinguish 32 groups, and the characteristic of this code is that the cyclic shift of each codeword is unique. Therefore, the information on the code group and the frame synchronization can be obtained by using the second synchronization channel. Here, the frame synchronization means synchronization with a timing or phase within one period of the scrambling spreading code of the spread spectrum system. However, in the current W-CDMA system, since one period of the spreading code and the frame length are 10ms, this is called frame synchronization.

Finally, the correlation value of the spreading code of the base station is obtained to distinguish the base station spreading code used in the base station. In this case, when obtaining a correlation value for the spreading code of the base station, a pilot channel, a broadcasting channel (BCH), etc., which are forward common channels, may be used as described above. In the conventional W-CDMA system, a pilot symbol is transmitted in a time division multiplexing (TDM) mode on a broadcasting channel. However, in the recent standardization integration process, a forward common pilot is transmitted in the form of a code division multiplexing (CDM). It is becoming. 1 illustrates an example in which a forward common pilot channel is transmitted in a CDM form and transmission of a pilot channel is stopped at the time when the sync code is transmitted.

In the embodiment of FIG. 2, when a forward common pilot channel is transmitted in the form of CDM, the pilot channel is continuously transmitted without being interrupted even when the synchronization channels are transmitted.

In the common pilot channel, pilot symbols and data may be multiplexed and transmitted in a time division manner in each slot (current W-CDMA structure), and a channel for transmitting data may be transmitted separately. In this case, the channel frame for transmitting the data must match the boundary with the common pilot channel frame.

As described above, the W-CDMA communication system performs synchronization in three steps. In the first step, a synchronization of a time slot of 0.625 ms is obtained using the first synchronization channel P-SCH, and in the second step, frame synchronization is obtained by identifying a frame synchronization and a scrambling code group. In the final three steps, after frame synchronization is performed, the common channel is searched to find a spreading code currently allocated to the base station among 16 possible code candidates in the code group. On the other hand, in the handoff situation, since the mobile station knows in advance information about the code being used for the neighbor base station from the neighbor list, faster synchronization can be realized.

However, the above-described base station asynchronous system does not use the base station sequence offset, so there is a possibility of collision between SCHs of neighboring base stations. 3A and 3B illustrate a situation in which sync channel SCHs collide between base stations. In this case, there may be two situations in which a neighboring base station in collision as shown in FIG. 3A is included in the neighbor list, and two base stations in which a collision base station is not included in the neighbor list as shown in FIG. 3B. Here, BTS1 denotes a base station to which a mobile station currently belongs, and BTS2 denotes a base station that caused an SCH collision.

Using 5MHz (4.096Mcps chip rate) in current W-CDMA systems, a time slot of 0.625ms is 2560 chips long. Therefore, this possibility of collision cannot be ignored even more. In particular, if the clock frequency of the base station is elaborate, a situation may occur for a long time between collisions between SCHs. In this situation, it is expected that the value of the decision variable of the two-stage operation for the SCH of the base station that caused the collision will be relatively large. If only the largest decision variable is selected in the base station identification process, as in the conventional two-stage search method, the signals of other base stations except for the corresponding base station in the collision situation are ignored. However, if the mobile station is in a handoff situation for a conflicting base station, i.e. if the conflicting base station is a target base station, it is inefficient to ignore this signal, and this search method may result in poor handoff performance. have.

Accordingly, an object of the present invention is to provide an apparatus and method for selecting a codeword of a base station for handoff by a mobile station in an asynchronous code division multiple access communication system.

Another object of the present invention is to provide an apparatus and method for selecting a handoff target base station regardless of collision of a synchronization channel between a current base station and a handoff target base station in an asynchronous code division multiple access communication system.

It is still another object of the present invention to provide handoff performance in a 3GPP W-CDMA system based on asynchronous operation between base stations in preparation for a situation where a SCH of a base station to which a mobile station belongs currently collides with a SCH of a target base station of handoff. It is to provide an apparatus and method that can be improved.

In an embodiment of the present invention for achieving the above object, a cell search of a mobile station of a code division multiple access communication system having at least one threshold for selecting a codeword and performing synchronization with the base station in three steps The method includes searching for a slot sync by searching for a first sync channel (P-SCH) signal, searching for the first sync channel, searching for a second sync channel signal, and detecting a correlation value for the received signal. And searching for frame synchronization using the codewords whose detected correlation value exceeds a set threshold and determining a base station group corresponding to the codeword, and after performing the step 2 discovery process, corresponding to the base station group After the signal of the common channel is correlated using a scrambling code, a spreading code having a maximum value is selected.

Hereinafter, the configuration and operation of the present invention will be described in detail with reference to the accompanying drawings.

In the following description specific details of code group selection for base station code identification, such as codewords, cyclic shifts for codewords, decision variables, slots, etc., are presented to provide a more general understanding of the invention. It will be apparent to one of ordinary skill in the art that the present invention may be readily practiced without these specific details and also by their modifications. The present invention is applicable to the initial cell search when idle handoff, soft handoff, hard handoff, when the mobile terminal is powered on, and when the terminal needs cell search.

As described above, the synchronization between the base station and the mobile station in the W-CDMA communication system uses a multi-stage synchronization method. In the embodiment of the present invention, a three-stage synchronization method is assumed. Referring to the three-stage synchronization search method between the base station and the mobile station, if the mobile station performs slot synchronization by searching for the P-SCH received in step 1, the mobile station can distinguish the SCH interval. Of course, since the time slot synchronization may not be performed properly due to a malfunction of the first stage operation, the SCH period is tentatively determined. Since the synchronization method according to the embodiment of the present invention relates to the synchronization search in two steps, it is assumed in the following description that the correct slot synchronization is acquired in the first step for convenience, and the SCH interval determined in the first step is temporarily described below. Called SCH interval.

After acquiring the timing information of the slot in the first step, the mobile station stores a correlation result for a code that can be transmitted as an S-SCH whenever a signal of a section temporarily determined to be an S-SCH is received in the second step. . When K SCHs are used for the two-stage operation, if a correlation result for K SCHs is collected, 32 comma-free codewords and 16 codewords cyclically shifted for each codeword are used. Corresponding correlation values are extracted and added, thus having a total of 512 (= 32 * 16) decision variables.

After acquiring the timeslot timing information in step 1 of the base station identification process, the mobile station stores a correlation result for a code that can be transmitted as an S-SCH whenever a signal of a section temporarily determined to be S-SCH is received in the second step operation. do. If K (e.g., 16) SCHs are used for the two-stage operation, if correlation results are collected for K (e.g., 16) SCHs, 32 comma-free codewords and cyclic shifts for each codeword By extracting and adding correlation values corresponding to the 16 shifted codewords, a total of 512 (= 32 * 16) decision variables are obtained. In the second step, scrambling code group information and frame synchronization information are obtained by selecting the one having the largest value among them. In the embodiment of the present invention, in addition to the codeword having the maximum decision variable value in step 2, a codeword that satisfies the set threshold condition is selected. When a handoff situation occurs, it is possible to select a desired codeword in a situation where an SCH collision occurs between a current base station and a target base station to be handed off.

After acquiring frame synchronization in step 2 of the base station identification process, the mobile station searches for a common channel to find a spreading code used by the base station.

4 shows a configuration of a receiver of a mobile station for identifying a code group of the base station according to an embodiment of the present invention in a W-CDMA communication system.

Referring to FIG. 4, when the first stage operation of the base station code identification process is completed, the controller 420 which performs the determination variable calculation and control function operates the SCH spreading code generator 431 of the code generator 430 every time the SCH interval is received. Let's do it. The code generator 430 includes an SCH spreading code generator 431 for generating an SCH spreading code and a scrambling code generator 432 for generating a scrambling code. Then, the SCH spreading code generator 431 generates SCH codes that can be generated under the control of the controller 420. Then, a correlator bank 410, which receives signals transmitted from the base station, correlates the received signal with the SCH spreading code and outputs the results to the controller 420. Then, the control unit 420 counts the number of SCHs used in step 2, and stores these results until the K (eg, 16) SCH intervals are received (ie, K (eg, 16) slots). Store in variable memory 450.

When the correlation values for the K (eg, 16) SCHs are detected in the same manner as described above, the controller 420 refers to the codeword memory 440 to each alphabet of the codeword from the correlation result values stored in the decision variable memory 450. Corresponding correlation values are extracted, and the extracted correlation values are asynchronously accumulated (noncoherent integration). Here, since the decision variables are 32 codewords and 16 cyclic shifts for each codeword, the decision variables have a total of 512 decision variables (ie, asynchronous cumulative results).

FIG. 5 is a diagram illustrating an example of decision variables that may appear in a situation where a SCH collision occurs between base stations in a W-CDMA communication system. In FIG. 5, BTS1 represents a base station to which a mobile station currently belongs, and BTS2 represents a base station that collides with the BTS1. FIG. 5 illustrates, for example, magnitudes of correlation values of BTS1 and BTS2 among 512 decision variables.

Referring to FIG. 5, among the 512 decision variables, a decision variable for a codeword of BTS2, which is a base station that caused a collision with a decision variable of a codeword of BTS1, which is a base station to which the current mobile station belongs, is particularly large. It is very likely to have, and the remaining decision variables may have large and small values depending on the channel environment.

6A and 6B are diagrams for describing a method of selecting a codeword with respect to the determination variables detected in step 2. 6A illustrates an example in which a three-step operation is performed on a value having the largest determination variable among the determination variables detected in step 2, and FIG. 6B illustrates a threshold value set among the determination variables detected in step 2 above. An example of performing a three-step operation is also shown for the values. Here, the operation in step 3 is to despread the possible spreading codes for the code group and frame synchronization information of the codeword selected in step 2. In step 3, since the spreading code is unique to each base station, even if a sync channel transmitted from different base stations collides, the spreading code can be despread without a problem.

Referring to FIG. 6A, a three-step operation is performed only on the largest determination variable value among the determination variables. However, there is a possibility that the codeword detection method as described above cannot detect the signal of the BTS2 serving as the target base station in the handoff situation. That is, when the decision variable value of the base station BTS1 to which the current mobile station belongs is a maximum value such as 611, and the decision variable value of the target base station BTS2 to which the collision has occurred is smaller than the base station BTS1 as shown in 615, the handoff performance is deteriorated. Let's do it. Therefore, in the embodiment of the present invention, the codeword is detected in the same manner as in FIG. 6B during handoff.

Referring to FIG. 6B, as in FIG. 6A, step 3 is performed on a codeword corresponding to a maximum decision variable such as 652, and step 3 is also performed on decision variables exceeding a preset threshold. In the embodiment of the present invention, the first threshold value θ ₁ and the second threshold value θ ₂ (θ _{1 ≧} θ ₂ ) are used in advance. Here, the first threshold value θ ₁ has a value that is relatively smaller than the maximum determination variable value, but is a reference value for operating step 3 with respect to the determination variable value having a correlation value of a predetermined size or more. The second threshold value θ ₂ has a value less than or equal to the first threshold value θ ₁ , but becomes a reference value for operating step 3 with respect to the determination variable value of the base station registered in the neighbor list with a poor channel environment. Therefore, the two thresholds, the second threshold value θ ₂ and the first threshold value θ ₁ (θ ₁ ≧ θ ₂ ) are set to improve the ability to detect codewords at handoff. In the embodiment of the present invention, an example of using two thresholds is described, but three or more thresholds may be used.

Referring to FIG. 6B, since the correlation between the neighboring base station that caused the collision is likely to have a high value, if the value of the decision variable exceeds the upper threshold value θ ₁ after performing the two-stage operation, the mobile station Unconditionally examines the code group corresponding to the current judgment variable in the next three steps. On the other hand, despite the case of the target base station of the handoff, if the channel situation is not good enough to exceed the upper threshold value θ ₁ , it may be omitted from the candidate of the target base station for performing the handoff. In the embodiment of the present invention, the lower threshold value θ ₂ is set to prevent such a situation. In this case, the codeword to be inspected is compared with a neighbor base station list, and as a result of the comparison, only the corresponding codeword is connected to the three-stage operation. Prevent false alarms caused by settings.

As described above, the synchronization method between the base station and the mobile station according to the embodiment of the present invention is for codeword selection in step 2 in the handoff situation of the base station code identification process. In order to understand the synchronization method as described above, the three-step operation will be described as follows. However, the following method is not absolute, and various modifications are possible by the designer who designs the receiver of the mobile station.

As described above, step 3 of the base station code identification process correlates the codes belonging to the code group with the code group information and the frame synchronization information to find the code having the highest energy (ie, the most likely). The process of choosing.

In the operation of step 3, the determination of the spreading code used in one group may be confirmed through forward common channels such as a pilot channel or a broadcasting channel. According to an embodiment of the present invention, the mobile station can select codewords that meet the threshold conditions in addition to the codeword with the maximum decision variable value. That is, when performing a three-step synchronous operation according to an embodiment of the present invention, one or more codewords are selected, and information of a code group and frame synchronization information corresponding to each of them is present. Therefore, in step 3, the code candidates (16 code candidates per codeword) belonging to the stored codewords are adjusted using cyclic shift information to adjust the phase (frame synchronization) of the code. Each of these is correlated in correlator bank 410. Here, the correlations for the candidates can be correlated in parallel or in series, and can also be sequentially correlated in a hybrid form. When correlation is made for all the codeword candidates, the control unit 420 identifies the base station by selecting a code having the largest correlation energy among them.

7A and 7B are flowcharts illustrating the procedures of operations performed in steps 2 and 3 when a mobile station performs a synchronization function with a base station. When the operation of step 1 of the base station code identification process is completed, a process of selecting a codeword is started by the procedure of FIG. 7.

Referring to FIG. 7A, first, a control unit 420 that performs a decision variable calculation and control function accesses a codeword for controlling correlation value detection in a codeword memory 440 in step 711. Run the spreading code generator 431. The SCH spreading code generator 431 generates SCH codes that can be generated, and the correlator bank 410 that receives the signals transmitted from the base station correlates the received signal with the SCH spreading code and outputs the results to the controller 420. In step 713, the control unit 420 extracts a correlation value that is a determination variable output from the correlator bank 410 and accumulates the correlation value in the correlation variable memory 450 in an asynchronous manner.

Thereafter, the controller 713 stores the codeword and the cyclic shift value by comparing the determination variable value with preset thresholds, and stores the codeword and the cyclic shift value having the maximum energy value among them. In detail, the operation as described above is _first checked in step 713 that the correlation value exceeds the upper threshold θ ₁ . If the upper threshold value θ ₁ is exceeded, a codeword having a corresponding correlation value and a cyclic shift value are stored in step 715. In addition, when the correlation value is smaller than the upper threshold value θ ₁ in step 713, it is checked whether the lower threshold value θ ₂ is exceeded in step 717. In this case, when the lower threshold value θ ₂ is exceeded, it is checked in step 719 whether the code word corresponding to the correlation value exists in the neighbor base station list. If the codeword exists in the neighboring base station list, the codeword and the cyclic shift value are stored in step 715. However, even if the correlation value detected in the above processes is smaller than the lower threshold value θ ₂ or has a correlation value between the upper threshold value θ ₁ and the lower threshold value θ ₂ , the corresponding codeword and the codeword are not registered in the neighboring base station list. Ignore cyclic shift values. In step 721, the controller 420 compares the current correlation value with the maximum correlation value up to the previous state and checks whether the maximum correlation value is changed. In this case, when the maximum value is changed, the codeword having the maximum correlation value and the cyclic shift value are updated and stored.

After analyzing the magnitude of the correlation value extracted by the above process, if the corresponding correlation value exceeds the set threshold, the corresponding codeword and the cyclic shift value are stored, and in step 723, the cyclic shift values of the corresponding codeword are stored. Examine whether the inspection is completed. That is, as shown in FIG. 5, when the cyclic shift value of one codeword is 16, the above operation is repeated 16 times when a specific codeword is selected. Therefore, if the check for all the cyclic shifts is not completed in step 723, the process proceeds to step 725, and the current codeword is cyclically shifted once, and then the process returns to step 713 and steps 713 to 721. Repeat this.

However, when the check for all the cyclic shifts is finished in step 723, it is checked whether the check for all the codewords is finished in step 727. That is, in the case where 32 codewords are used as shown in FIG. 5, if specific codewords are selected, steps 713 to 725 are repeated. Therefore, if the checking of all codewords is not finished in step 727, the process proceeds to step 729 by increasing the codeword number by one, and then returns to step 711 to select a changed codeword. The operation of step 721 is repeated. When the check for all codewords is finished in step 727, the process proceeds to step 751 to perform a three-step synchronous operation.

Therefore, when the codeword selection process is performed, the number of SCHs used in step 2 is counted and stored in the correlation variable memory 450 storing these results until K SCH intervals (ie, K slots) are received. do. When the correlation values for the K SCHs are detected in the same manner as described above, the controller 420 refers to the codeword memory 440 to obtain a correlation value corresponding to each alphabet of the codeword from the correlation result values stored in the decision variable memory 450. Extract and correlate the extracted correlations asynchronously (noncoherent integration). In this case, the decision variables have 32 codewords and 16 cyclic shifts for each codeword, and thus have a total of 512 decision variables (ie, asynchronous cumulative results). And comparing the 512 decision variables with the first threshold value θ ₁ and the second threshold value θ ₂ and having a codeword greater than the first threshold value θ ₁ or greater than the second threshold value θ ₂ and registered in the neighboring base station list 1. In this case, the codeword and the cyclic shift value are stored.

When the codeword is selected while performing the two-step operation as described above, the three-step operation is performed while the operation shown in FIG. 7B is performed. In a three-step operation, a possible spreading code is despread for the code group and frame offset information indicated by the selected codeword to test it. Referring to FIG. 7B, in step 751, the controller 420 correlates scrambling candidates belonging to a codeword (code group) using the stored codeword and the cyclic shift value. In step 753, the spreading code allocated to the current forward common channel is searched among 16 possible spreading code candidates in the code group. Therefore, in step 753, the code having the largest value among the correlation values for the scrambling code candidates belonging to the selected codeword is selected and determined as the spreading code of the base station.

Therefore, in the embodiment of the present invention, while performing the two-stage operation of the mobile station, it is possible to detect a codeword having a decision variable for a codeword of another base station causing a collision in addition to the codeword having the largest decision variable in the base station identification process. In addition, when the second threshold is satisfied even when the channel environment is poor and does not have the first threshold, the codeword is detected when the codeword is registered in the neighbor list. Therefore, in the handoff situation, it is possible to detect the codeword of the base station to which the handoff target is collided with the decision variable of the base station to which the current mobile station belongs.

As described above, in the 3GPP W-CDMA communication system, the detection rate of the codeword can be increased in a situation where the SCH channel collides with the base stations, and the handoff performance of the mobile station can be improved by performing fast base station identification. There is this.

Claims (4)

A cell search method of a mobile station in a code division multiple access communication system having at least one threshold for selecting a codeword and performing synchronization with a base station in three steps, the method comprising: A first step of searching for slot synchronization by searching for a first sync channel (P-SCH) signal;  After detecting a correlation value for the received signal by searching for a second synchronization channel (S-SCH) signal, the frame correlation is found using codewords whose detected correlation value exceeds a predetermined threshold. A two-step discovery process for determining a corresponding base station group, And performing a three step search process of correlating a signal of a common channel using a scrambling code corresponding to the base station group after the second step search process and selecting a spreading code having a maximum value. A cell search method of a mobile station in a code division multiple access communication system having at least one threshold for selecting a codeword and performing synchronization with a base station in three steps, the method comprising: A first step of searching for slot synchronization by searching for a first sync channel (P-SCH) signal;  A base station group according to codewords corresponding to a correlation value exceeding a first predetermined threshold among the correlation values after detecting a second synchronization channel (S-SCH) signal to detect a correlation value for a received signal And a second step searching process of determining a base station group according to codewords corresponding to a correlation value smaller than the first threshold and exceeding a predetermined second threshold;  A third step searching process for correlating a signal of a common channel using a scrambling code corresponding to the base station groups after performing the second step searching process, and then selecting a spreading code having a maximum value Way. 3. The method of claim 2, wherein the third step search is performed when a group of base stations according to codewords smaller than the first threshold and corresponding to a correlation value exceeding the second threshold is included in the neighbor list of the mobile station. Cell navigation method characterized in that A codeword generator for generating codewords, A correlator for correlating a signal transmitted from a base station to the generated codeword to generate a correlation value; A controller for comparing the correlation value with a preset threshold and detecting codewords and cyclic shift values having a correlation value exceeding the threshold value; A scrambling code generator for generating a scrambling code according to the detected codeword; And a means for correlating the scrambling code with a signal of a common channel transmitted from the base station to determine a spreading code of the base station.

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