KR101002865B1 - Cell Reselection Method in Dual Mode Mobile Telecommunication - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a dual mode mobile terminal that supports different Radio Access Technology (RAT), and more particularly relates to cell reselection between RATs.

2. The technical problem to be solved by the invention

An object of the present invention is to provide a cell reselection method in a dual mode mobile terminal in which a cell reselection is given by giving priority to a mode preferred by a user in a mobile terminal operating in a dual mode.

3. Summary of the Solution of the Invention

A cell reselection method in a dual mode mobile communication system, the method comprising: a first step of determining a PLMN (Public Land Mobile Network) preferred by a mobile terminal; A second step of measuring a power of a received signal from cells adjacent to a registered location of the mobile terminal; Ranking the adjacent cells according to the measured power of the received signal; Classifying the ranked cells according to the preferred PLMN; And a fifth step of camping on one of the ranked cells classified according to PLMN in the fourth step.

4. Important uses of the invention

The present invention is used in a dual mode mobile communication system.

Dual Mode, RAT, PLMN, Cell Reselection

Description

Cell reselection method in dual mode mobile communication system {Cell Reselection Method in Dual Mode Mobile Telecommunication}             

1 is a diagram illustrating a mode of a mobile station in an idle state in a system combining a GSM network and a UMTS network.

2 is a flowchart illustrating a method for reselecting a cell of a mobile station in a system in which a GSM network and a UMTS network are combined according to the related art.

3 is a flowchart illustrating a method for cell reselection of a mobile station in a system in which a GSM network and a UMTS network are combined according to the present invention.

4A through 4B are detailed flowcharts of an embodiment of a method for cell reselection of a mobile station in a dual mode system according to the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to dual mode mobile terminals supporting different radio access technologies (RATs), and more particularly, to cell reselection between RATs.

Global System for Mobile Communication (GSM) is a digital mobile phone system classified as a 2'nd Generation (2G) cellular system and is widely used in Europe and elsewhere. GPRS (General Packet Radio Services), developed based on GSM, is a more advanced packet-based mobile communication service, and is intended to provide high-quality, high-quality voice and multimedia services. It is an evolutionary step toward mobile communication systems, especially UMTS (Universal Mobile Telecommunication System).

The UMTS is based on the GSM communication standard, but uses wide-band Code Division Multiple Access (CDMA) technology, and enables packet-based text, digitized voice or video whether mobile phone or computer users are anywhere in the world. And a service capable of transmitting multimedia data at a high speed of 2Mbps or more. The UMTS uses the concept of a virtual connection called a packet-switched connection using a packet protocol such as the Internet Protocol (IP), and is always accessible to any other end point in the network.

Mobile communication technology will evolve from 2G (2'nd Generation) cellular systems, especially GSM / GPRS to 3G (3G) mobile communication systems, especially UMTS. But the transition from 2G to 3G will be gradual over the years, for quite some time. This has led to the need for network operators to provide users with mobile terminals that can work on both technologies, GSM / GPRS and UMTS. This necessity is the main reason for the development of dual mode user equipment or dual mode mobile terminal supporting GSM / GPRS and UMTS simultaneously. Moreover, when mobile subscribers move from one place to another across a world of different countries with different RATs deployed in their respective radio networks, a multi-mode mobile terminal capable of operating in these networks. Will reduce the need for Thus, for 3G technology to be successful, mobile terminals need to be able to operate in one or more wireless technologies. In the pre-implementation stage of UMTS, users can use dual-mode mobile terminals that support both GSM / GPRS and UMTS simultaneously, enabling users to switch to currently available technologies such as GSM 900 or GSM 1800 even in areas where UMTS is not available. RAT service can be provided.

In a GSM system (Global System for Mobile Telecommunications), each base station transmits a signal on a channel of a so-called BCCH channel (broadcast control channel) whose frequency varies from neighboring base stations. Mobile stations measure the strengths of the received BCCH signals and determine which cell is most advantageous with respect to the quality of the radio connection based on the strengths. The base stations also send information to the mobile stations about the BCCH frequencies used in the neighboring cells so that the mobile stations know which frequencies to listen for in order to find the BCCH transmissions of the neighboring cells. In each cell, the BCCH channel transmission also contains information about how the mobile stations can make so-called random access requests in the cell to establish a call connection. Such prior art arrangements include second generation digital cellular wireless systems, such as GSM and its extensions, the DCS1800 (Digital Communications System at 18000 MHz), IS-54 (provisional standard 54) and PDC. Ideal for (Personal Digital Cellular).

However, in third generation digital cellular systems it has been proposed that the quality of service provided to mobile stations by cells varies substantially from cell to cell. UMTS (Universal Mobile Telecommunications System) has been proposed for the third generation of systems. In this third generation system, the cells are divided into pico-, nano-, micro-, and macro cells, depending on size and characteristics, for example, and the data transfer speed can be used as an example of quality of service. have. Therefore, picocells have the highest data transmission speed, and macrocells have the lowest data transmission speed. The cells may partially or completely overlap and there may be other kinds of mobile stations, so that all mobile stations will not be able to take advantage of the quality of service provided by all cells.

As shown in FIG. 1, a mode diagram of a mobile station in an idle state in a system combining a GSM network and a UMTS network is as shown in FIG. 1.

1 is a diagram illustrating a mode of a mobile station in an idle state in a system combining a GSM network and a UMTS network.

Mobile stations in mobile communication systems always tend to select any base station that moves or camps in the cell.                         

As such, the purpose of the UE camping on the cell in idle mode is to first allow the UE to receive system information from the Public Land Mobile Network (PLMN). Secondly, the wireless connection can be established by initially accessing the network to the control channel of the camped cell when registering or establishing a wireless connection. And, thirdly, if the PLMN receives a call for a registered UE, the UE knows the registration area of the cell where it is camped and sends a “paging” message for the UE to the control channel of all cells of the registration area. will be. Accordingly, the UE can receive the paging message delivered to the control channel of the cell of the registration area and respond to the control channel. And finally, it becomes possible for the UE to receive the cell broadcast service. That is, the UE camps on a base station of an adjacent cell and receives a service from the base station.

However, due to its mobility, the UE changes in a cell or changes between cells according to movement even in an idle state. Accordingly, reselection of the cells is required.

The mode of the mobile station in the idle state normally operates with PLMN selection / reselection 11, cell selection / reselection 12 and location registration 13. Referring to FIG. 1 for a description of the mode and operation of the mobile station as follows.

First, when the power of a user terminal (UE: User Equipment, hereinafter " UE ") that is a mobile station is turned on, a state 11 of selecting / reselecting a PLMN (Public Land Mobile Network) is entered. This PLMN selection / reselection has two modes, automatic or manual, where the automatic mode is determined by the value set in advance by the selection of the PLMN (101). On the other hand, in the manual mode, information for selection is displayed to the user (102) and the response (103) is made.

The UE then enters a state 12 that selects / reselects a suitable cell of the PLMN to camp on. Depending on the RAT, the PLMN type can be different as well. And, depending on the PLMN type, the method of recognizing it differs. Information about this PLMN type is conveyed to the PLMN selection 104 information, for which it is selected / reselected via the NAS control signal 106 and radio measurements 107 for the surrounding cells. When the cell is selected / reselected in this way, it conveys information 105 as to whether the selected PLMN is possible.

When the position is changed by the cell selection / reselection 12, the change of the registration position is informed 108, and the position registration state 13 for registering a new position is entered. In the location registration state, the CM request is received (109) and the location is registered and responded to the selected cell and the PLMN (110).

2 is a flowchart illustrating a method for reselecting a cell of a mobile station in a system in which a GSM network and a UMTS network are combined according to the related art.

As shown in FIG. 2, the cell reselection method of a mobile station in a system in which a GSM network and a UMTS network according to the prior art are combined firstly fails from a cell providing a service in an idle state 201. The lower surface 202 measures the reception power of neighboring cells (203). Then, it is determined whether the measured reception power of the neighboring cells is equal to or greater than a predetermined value to find a cell equal to or greater than a predetermined value (204), and reselects a cell equal to or greater than the predetermined value to be camped on (Camping On). 205) If not found, the process proceeds to step 203. Here, the reselection according to the related art does not measure the received power of all of the cells, but immediately camps on the cell if a cell that is equal to or larger than a predetermined value is found during the search.

 The above method is a general conventional technique. The prior art as described above is presented in 3GPP 25.304. The characteristic content thereof will be described with reference to FIG. 2.

The characteristics of the cell reselection proposed in The 3rd Generation Partnership Project (3GPP) 25.304 are as follows. First, tracking and measurement of neighboring UMTS cells (frequency division duplex (FDD) cells in this embodiment) and GSM cells (this corresponds to steps 202 and 203). The second step is to determine whether the measured UMTS cell and the GSM cell are eligible (this is performed by requiring a reception level of a predetermined level or more in step 203).

However, in a dual mode UE, the user will be given the option to select a preferred RAT. By choosing UMTS as the preferred RAT according to these options, the software logic of the UE will be the first to search for all possible UMTS cells. If there is no cell available in the preferred RAT, the search for another cell will be performed. For example, when a user is subscribed to a UMTS service (eg, a video call) and wants to use the service, the GSM cells during the reselection process if the UMTS cell is among the candidate cells in the region where the user is currently using the service. Priority should be given to the UMTS cell even though it is a candidate cell. However, according to the current method, the GSM cell is selected and terminated by reselection and eventually the user desired service cannot be performed. That is, if the UE follows the conventional procedure mentioned in 3GPP TS 25.304 for cell reselection, it is impossible for the dual mode mobile terminal to reselect the cell in consideration of the user's preferred RAT.

In addition, in the case of reselection according to the related art, since the camp is immediately turned on when a reception power exceeding a predetermined value is detected, there is a problem in that the camp on operation is frequently performed.

The present invention has been proposed to solve the above problems, in a mobile terminal operating in dual mode, a cell in a dual mode mobile terminal to give priority to the preferred mode by the user to reselect the cell The purpose is to provide a reselection method.

The present invention for achieving the above object, the first step of determining a Public Land Mobile Network (PLMN) preferred by the mobile terminal; A second step of measuring a power of a received signal from cells adjacent to a registered location of the mobile terminal; A third step of ranking cells whose power of the received received signal is equal to or greater than a predetermined received power value; Classifying the ranked cells according to the preferred PLMN; And a fifth step of camping on one of the ranked cells classified according to the PLMN in the fourth step.

The present invention also provides a cell reselection method in a dual mode mobile communication system, comprising: a first step of searching for a cell corresponding to the selected PLMN around a mobile terminal as a new PLMN is selected; A second step of camping on a suitable cell which is equal to or greater than a predetermined received power value among the cells found in the first step; A third step of searching for cells adjacent to the mobile terminal regardless of the selected PLMN if the suitable cell does not exist in the second step; And a fourth step of camping on an acceptable cell that is equal to or greater than a predetermined received power value among the cells searched in the third step.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the same components in the drawings are represented by the same reference numerals and symbols as much as possible even though they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

1 is a diagram illustrating a mode of a mobile station in an idle state in a system combining a GSM network and a UMTS network.

Mobile stations in mobile communication systems always tend to select any base station that moves or camps in the cell.

As such, the purpose of the UE camping on the cell in idle mode is to first allow the UE to receive system information from the PLMN. Secondly, the wireless connection can be established by initially accessing the network to the control channel of the camped cell when registering or establishing a wireless connection. And, thirdly, if the PLMN receives a call for a registered UE, the UE knows the registration area of the cell where it is camped and sends a “paging” message for the UE to the control channel of all cells of the registration area. will be. Accordingly, the UE can receive the paging message delivered to the control channel of the cell of the registration area and respond to the control channel. And finally, it becomes possible for the UE to receive the cell broadcast service. That is, the UE camps on a base station of an adjacent cell and receives a service from the base station.

However, due to its mobility, the UE changes in a cell or changes between cells according to movement even in an idle state. Accordingly, reselection of the cells is required.

The mode of the mobile station in the idle state normally operates with PLMN selection / reselection 11, cell selection / reselection 12 and location registration 13. Referring to FIG. 1 for a description of the mode and operation of the mobile station as follows.

First, when the power of a user terminal (UE: User Equipment, hereinafter " UE ") that is a mobile station is turned on, a state 11 of selecting / reselecting a PLMN (Public Land Mobile Network) is entered. This PLMN selection / reselection has two modes, automatic or manual, where the automatic mode is determined by the value set in advance by the selection of the PLMN (101). On the other hand, in the manual mode, information for selection is displayed to the user (102) and the response (103) is made.

The UE then enters a state 12 that selects / reselects a suitable cell of the PLMN to camp on. Depending on the RAT, the PLMN type can be different as well. And, depending on the PLMN type, the method of recognizing it differs. Information about this PLMN type is conveyed to the PLMN selection 104 information, for which it is selected / reselected via the NAS control signal 106 and radio measurements 107 for the surrounding cells. When the cell is selected / reselected in this way, it conveys information 105 as to whether the selected PLMN is possible.

When the position is changed by the cell selection / reselection 12, the change of the registration position is informed 108, and the position registration state 13 for registering a new position is entered. In the location registration state, the CM request is received (109) and the location is registered and responded to the selected cell and the PLMN (110).

3 is a flowchart illustrating an embodiment of a method for cell reselection of a mobile station in a system in which a GSM network and a UMTS network are combined according to the present invention.

As shown in FIG. 3, the cell reselection method of a mobile station in a system in which a GSM network and a UMTS network are combined according to the present invention, first, when a service from a cell providing a service in an idle state 301 fails. In operation 302, a reception power of neighboring cells is measured in operation 303. Then, all neighboring cells are ranked 304 according to the received power. Then, the ranked cells are separated according to the system (305). That is, it separates into a UMTS cell and a GSM cell. As described above, it is checked whether the user's preferred system is a system for the cells separated by systems (306). If the user's preferred system is UMTS, it is checked whether there is a cell for reselection among UMTS cells (307). If the preferred system is GSM, it is checked if there is a cell for reselection among the GMS cells (308). In this case, the decision of the system preferred by the user is divided into two types, manual selection by the user and automatic selection to automatically select the preferred system, as shown in 11 of FIG. 1. In the manual case, as shown in FIG. 1, information for selection is displayed to the user (102) and a response (103) is made as the selection is made. On the other hand, in the case of automatic, the selection of the system is determined by a preset value. In the present invention, the present invention proposes that the system is automatically selected as an inexpensive system by comparing service prices between the systems. In addition, it is assumed that a selection for a system preferred by the user has already been made before the operation of this drawing is performed.

The check results 307 and 308 determine whether a cell for reselection is found in the user's preferred system (309). If a cell is found, the cell is selected and camped on (310). In this case, the corresponding cell camping on is the cell having the highest reception power among cells for reselection in a system preferred by the user. Then, if a change occurs in the cell with the highest reception power, it is changed.

On the other hand, if the check result (307, 308) checks whether the user has found a cell for reselection in the preferred system (309), if the cell is not found, whether there is a cell for reselection in the system that the user does not prefer Check (311). If a check finds a cell for reselection in a system that is not preferred by the user (312), the cell is selected and camped on in the cell (313). In this case, the corresponding cell camping on is the cell having the highest reception power among cells for reselection in a system that the user does not prefer. Then, if a change occurs in the cell with the highest reception power, it is changed. On the other hand, if not found (312), the process proceeds to step 303 and repeats the cell reselection process again.

An example of this is as follows.

Assume that there are three GSM cells adjacent to one mobile terminal (GSM 1, GSM 2, GSM 3) and two adjacent UMTS cells (UMTS 1, UMTS 2). Among these, when there is one cell (UMTS 1) which does not reach a predetermined reception power value in the mobile terminal and all the others are greater than or equal to the predetermined reception power value, according to the related art, the cell where the mobile terminal is camped on is GSM 1. , The highest receiving power among the four cells of GSM, GSM 2, GSM 3 and UMTS 2. However, according to the present invention, if the preferred PLMN is GSM, the cell having the highest reception power among GSM 1, GSM 2, and GSM 3 is, and if the preferred PLMN is UMTS, it is UMTS 2.

Ranking according to strength of received power Cell One GSM 2 2 GSM 3 3 UMTS 2 4 GSM 1

That is, when the cells having the received power strengths as shown in Table 1 are adjacent to the mobile terminal, the mobile terminal camps on GSM 2 according to the conventional technology. However, according to the present invention, if the preferred PLMN is GSM, it camps on GSM 2, but if the preferred PLMN is UMTS, it camps on UMTS 2 even though the ranking according to the strength of the received power is only third.

In the above, the cell reselection according to the present invention measures all the received power strengths of all cells adjacent to the corresponding UE, classifies them by each PLMN, and camps on cells corresponding to a preferred PLMN among the classified cells. It is.

4A to 4B are detailed flowcharts of an embodiment of a method for cell reselection of a mobile station in a dual mode system according to the present invention.

First, terms will be defined before describing the contents of the present invention. In the description of this figure, a suitable cell (Suitable Cell) refers to a cell having a reception power of a certain level or more of the cells of the PLMN preferred by the user. In addition, an acceptable cell refers to a cell having a reception power of a predetermined level or higher among cells of a PLMN that the user does not prefer.

According to the embodiment of the present invention shown in Figs. 4A to 4B, it is checked whether the preferred new PLMN has been selected by the user (401). If a new PLMN is selected, it is checked whether there is cell information stored for the selected PLMN (402). Here, the stored cell information means information of a cell according to the selected PLMN. That is, if the selected PLMN is GSM, it means information of GSM cells. If the selected PLMN is UMTS, it means information of UMTS cells.

If there is stored cell information, the cell is searched through the stored cell information (404). That is, only cells of the selected PLMN are searched. If there is a suitable cell (405), then a normal camp operation is performed on that cell (407). The normal camping operation here means that the UE camps on the cell of the PLMN selected by the user.

On the other hand, if there is no cell information stored for the selected PLMN (402) or if there is no suitable cell (405) in step 405, the initial cell search is performed (403). In other words, initial cell search refers to measuring received power from adjacent cells, ranking according to measured values for cells having a certain level or more of received power, and searching them separately according to PLMN. it means. As such, the cells of the selected PLMN are searched through the cell information separated according to the PLMN. As a result, if there is a suitable cell (406), a normal camp operation is performed on that cell (407).

The UE then moves out of the normal camp operation 407 by three cases, leaving the idle mode to enter the connected mode, or the NAS notifying that the registration with the selected PLMN has been denied, or reselecting the cell.

That is, when a change of the environment of the UE occurs during the normal camp operation (407), whether the change is due to the cell reselection operation (408), the switch to the connected mode (410), registration for the selected PLMN is made The NAS recognizes whether or not it is known through the NAS (411) and performs an appropriate operation.

In terms of each operation, if a cell reselection operation is requested (407) during a normal camp operation (408), it is determined whether there is a suitable cell (409), and if there is a suitable cell, a normal camp is applied to the cell. Perform 407. Such cell reselection may occur when a cell having a higher reception power is generated than a cell currently camped on, or a cell reselection due to a movement of a mobile station.

If the connection mode is 410, the UE leaves the idle mode and operates in the connected mode 412. When the operation of the connected mode ends, the mode is switched back to the idle mode (413). At this time, the cell search is performed again (414). As a result, if there is a suitable cell, a normal camp is performed on that cell (407). If there is no suitable cell, the process proceeds to step 404 to perform a cell reselection process.

If it is known through the NAS that registration with respect to the selected PLMN has not been made (411), it searches for (Acceptable) cells among all neighboring cells regardless of the selected PLMN (417). In addition, even when a suitable cell is not found in steps 406 and 409, an acceptable cell of all neighboring cells is searched regardless of the selected PLMN (417). In addition, when a new PLMN is not selected (401), it is checked whether a Universal Subscriber Identity Module (USIM), which stores network information and user information, is inserted into a UMTS smart card (416). In step 417, if it is not inserted, an acceptable cell of all neighboring cells is searched regardless of the selected PLMN (417). In other words, here, whether the USIM is inserted or not is the same as selecting a new PLMN.

 The UE selects a possible cell by searching for an acceptable cell among all neighboring cells regardless of the selected PLMN (417), and the UE camps on the selected possible cell (418).

When the UE's environment changes during the camp operation (418), it is determined whether the change is due to cell reselection operation (419), to switch to the connected mode (421) or whether a suitable cell is found (422). Recognize and perform the appropriate action.

For each operation, when a cell reselection operation is requested (418) during the camp operation (419), it is checked whether there is a possible cell (420), and if there is a possible cell, camping on the corresponding cell (418). ).

In operation 421, the UE leaves the idle mode and operates in the connected mode (423). When the operation of the connected mode ends, the mode is switched back to the idle mode (424). At this time, the cell search is performed again (425). As a result, if there is a possible cell, it camps on that cell (426). If no cell is available, the process proceeds to step 417 and the cell reselection process is performed.

If a suitable cell is found (422), normal camping on that cell is performed (407).

As described above, the method of the present invention may be implemented as a program and stored in a recording medium (CD-ROM, RAM, floppy disk, hard disk, magneto-optical disk, etc.) in a computer-readable form.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the spirit of the present invention for those skilled in the art to which the present invention pertains, and the above-described embodiments and accompanying It is not limited by the drawings.

In the present invention as described above, when reselecting a cell for camping in a mobile communication system operating in a dual mode, the cell selection of the system preferentially is considered by the user, so that the cell reselection occurs. There is an effect that can provide a stable service to the user.

Claims (14)

A cell reselection method in a dual mode mobile communication system, Determining a PLMN (Public Land Mobile Network) preferred by the mobile terminal; A second step of measuring received signal power from cells adjacent to a registered location of the mobile terminal; Ranking cells whose measured received signal power is greater than or equal to a predetermined received power value; Classifying the ranked cells according to the preferred PLMN; And And a fifth step of camping on one of the ranked cells classified according to PLMN in the fourth step. The method of claim 1, The fifth step, A sixth step of searching for a cell of the preferred PLMN among the ranked cells classified according to the PLMN in the fourth step; A seventh step of camping on a cell having the highest reception power among the corresponding cells if a cell of the preferred PLMN exists as a result of the search of the sixth step; And And the eighth step of camping on a cell having the highest reception power among cells of other PLMNs other than the preferred PLMN if there is no cell of the preferred PLMN as a result of the search of the sixth step. Cell Reselection Method in Mode Mobile Communication System. The method according to claim 1 or 2, The first step is a cell reselection method of a dual mode mobile communication system, characterized in that by manually determining the PLMN preferred by the user. The method according to claim 1 or 2, The first step is, the cell reselection method in a dual mode mobile communication system, characterized in that determining the PLMN preferred by the mobile terminal is automatically performed by the dual mode mobile communication system. The method of claim 4, wherein In order to automatically determine the preferred PLMN as described above, a low price PLMN is automatically determined by receiving a difference in service price between PLMNs provided by the dual mode mobile communication system. Cell reselection method in a dual mode mobile communication system. The method according to claim 1 or 2, And measuring the received signal power of the second step is measuring a received signal from all cells adjacent to a registered location of the mobile terminal. A cell reselection method in a dual mode mobile communication system, As a new PLMN is selected, searching for a cell corresponding to the selected PLMN around a mobile terminal; A second step of camping on a suitable cell which is equal to or greater than a predetermined received power value among the cells found in the first step; A third step of searching for cells adjacent to the mobile terminal regardless of the selected PLMN if the suitable cell does not exist in the second step; And And a fourth step of camping on an acceptable cell which is equal to or greater than a predetermined received power value among the cells searched in the third step. The method of claim 7, wherein Searching for the cells of the first step and the third step, And the mobile terminal receives a signal from adjacent cells and measures the power of the received signal. 9. The method according to claim 7 or 8, During the camp on operation of the second step, A fifth step of ending the camp on operation and switching to a call connection state when the mobile terminal is in a call connection state; A sixth step of searching for a cell corresponding to the selected PLMN around the mobile terminal when the call connection state ends; A seventh step of camping on a suitable cell which is equal to or greater than a predetermined received power value among the cells searched in the sixth step; And And the eighth step proceeds to the third step if the suitable cell does not exist in the seventh step. 9. The method according to claim 7 or 8, During the camp on operation of the second step, Detecting a change in the received power of the adjacent cell; A sixth step of searching for a cell corresponding to the selected PLMN around the mobile terminal according to the change in the received power sensed in the fifth step; And And maintaining a camp on state if the found suitable cell is the same as the cell being camped on, and otherwise camping on the found suitable cell. 9. The method according to claim 7 or 8, During the camp on operation of the second step, And if the NAS instructs that registration has not been made to the selected PLMN, proceeding to the third step. 9. The method according to claim 7 or 8, During the camp on operation of the fourth step, A fifth step of ending the camp on operation and switching to a call connection state when the mobile terminal is in a call connection state; A sixth step of searching for all cells around the mobile terminal when the call connection state ends; A seventh step of camping on a possible cell that is equal to or greater than a predetermined received power value among the cells found in the sixth step; And And if the possible cell does not exist in the seventh step, proceeding to the third step, the cell reselection method in a dual mode mobile communication system. 9. The method according to claim 7 or 8, During the camp on operation of the fourth step, Detecting a change in the received power of the adjacent cell; A sixth step of searching for all cells around the mobile terminal according to the change in the received power sensed in the fifth step; And And a seventh step of maintaining a camp on state if the searchable possible cell is the same as the cell being camped on, and camping on the searchable possible cell if it is wrong. 9. The method according to claim 7 or 8, During the camp on operation of the fourth step, And if a suitable cell has been found, further comprising a fifth step of camping on the retrieved suitable cell.

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