Abstract:
A method for selecting a base station is applied to a mobile station. The mobile station sets a preset distance corresponding to a datum base station. The method comprises selecting a first base station to be a serving base station; and obtaining a first C 2  parameter of a second base station corresponding to the mobile station, and obtaining a second C 2  parameter as a base for the mobile station in selecting the serving base station when a distance between the second base station and the datum base station is determined to be smaller than the preset distance, wherein the second C 2  parameter is equal to the first C 2  parameter plus a threshold value.

Description:
[0001]    This application claims the benefit of Taiwan application Serial No. 95126823, filed Jul. 21, 2006, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates in general to a method for selecting a base station and a communication system using the same, and more particularly to a method for selecting a base station so as to extend a homezone service and communication system using the same. 
         [0004]    2. Description of the Related Art 
         [0005]    In a communication system, such as a cellular network, the geographical area of a metropolis is normally divided into a number of base station cells. Several base stations are connected to a mobile service switch center (MSC), which is connected to a public switched telephone network (PSTN). Users (mobile clients) of the cellular network communicate with the surrounding base stations and datum base station by using a hand-held device or a device installed on a car (mobile station). The base station communicating with the mobile station is called a serving base station. 
         [0006]    However, cell selection of the mobile station for selecting a suitable base station as a serving base station requires parameters C 1  and C 2  for determination. The parameters C 1  and C 2  are set up according to the global system for mobile communication (GSM) standard. The C 1  parameter is a path loss criterion and the C 2  parameter is a cell reselection criterion. For the mobile station, the best serving base station is considered to have a positive C 1  parameter and the highest C 2  parameter. Therefore, when the mobile station detects a higher C 2  parameter, it changes to use the base station with the higher C 2  parameter as the serving base station. As regulated by the ETSI 05.08, the C 1  parameter is defined as bellows: 
         [0000]        C 1=( A −Max( B, 0)); wherein 
         [0007]    A=RLA_C−RXLEV_ACCESS_MIN; 
         [0008]    RLA_C is an average power of carrier waves of a signal; 
         [0009]    RXLEV_ACCESS_MIN is a minimum signal intensity with which a mobile station can connect to the communication system; 
         [0010]    B=MS_TXPWR MAX_CCH−P; 
         [0011]    MS_TXPWR_MAX_CCH is a maximum of a broadcast control channel without power control; 
         [0012]    P is a maximum output power of the mobile station. 
         [0013]    If a certain base station has the C 1  parameter larger than 0, it represents the base station can be selected by the mobile station to be the serving base station and the base station with the maximum C 1  parameter is selected to be the serving base station the mobile station registers in. If the mobile station is in a mobile state (the detected C 1  parameter of the serving base station may be changed), or the selected serving base station rejects communication, the C 2  parameter should be used for determining whether the selected serving base station is suitable. The C 2  parameter is defined as bellows: 
         [0000]        C 2= C 1+CELL_RESELECTION_OFFSET−TEMPORARY OFFSETH×(PENALTY_TIME− T ); 
         [0014]    Equal to PENALTY_TIME&lt; &gt;11111 
         [0015]    Wherein for other base stations not selected to be the serving base station, H(x)=0 for x&lt;0
       =1 for x≧0;       
 
         [0017]    For the base station area operating in service, 
         [0018]    H(x)=0. 
         [0019]    The homezone service is mainly provided for the users to register with the network proprietor beforehand to obtain a lower payment or better service in a limited range. The setting method is to set a starting point first and the starting point is usually a datum base station in an area. The user also registers with the network proprietor to have a preset distance and the circular area with a center at the datum base station and a radius equal to the preset distance is the user&#39;s homezone provided by the network proprietor. When the mobile station obtains the position coordinates of the serving base station through broadcast of the serving base station, if the serving base station is located in the homezone, the user can have a premium of payment or various services. Inversely, if its position coordinates do not belong to the homezone, the user is charged by a normal rating. In this way of arrangement, the users operating in the same area most of the time can be attracted to order this kind of service, which increases more clients for the system proprietor. 
         [0020]    In the above conventional method, the mobile station uses the position coordinates of the serving base station as its own position coordinates for calculating its distance with the datum base station. However, when the mobile station is located at the edge of the homezone, it may be still located within the preset distance but select to communicate with the base stations not located in the homezone, and consequently, the users cannot use thoroughly the whole homezone provided by the network proprietor, and even have unnecessary dispute. Referring to  FIG. 1 , a schematic diagram of a conventional homezone service is shown. In  FIG. 1 , the homezone is a circular area centered at the datum base station and having a radius of the preset distance R. 
         [0021]    The mobile station may register a first base station  101 , a second base station  102  or a third base station  103  as the serving base station. The first base station  101  is distant from the datum base station  100  by R 1 , the second base station  102  is distant from the datum base station  100  by R 2  and the third base station  103  is distant from the datum base station  100  by R 3 , wherein R 1  and R 3  are smaller than R and R 2  is larger than R. When the mobile station connects to the first base station  101  or the third base station  103 , the user can have the homezone service provided by the network proprietor and have lower payment rating; if the mobile station selects the second base station  102  to be the serving base station, because it is not located in the homezone, the charge rating is higher. As shown in  FIG. 1 , the mobile station is originally located at a point P, and the first base station  101  is the serving base station. When the mobile station moves to a point Q, which is located at the edge of the communication range of the first base station  101 , the C 1  and C 2  parameters of the second base station  102  and third base station  103  are measured for performing base station handover. The second base station  102  may be more suitable than the third base station  103  to be a new serving base station according to a comparison of the C 1  and C 2  parameters of the base stations because the second base station  102  is closer to the mobile station. Therefore, the mobile station will select the second base station  102  to be the new serving base station. However, the mobile station located at the point Q is still distant from the datum base station within the preset distance R and the third base station  103  can still provide signals with suitable intensity for the communication of the mobile station. According to present communication regulations, although the mobile station is located in the homezone, the user cannot have the homezone service provided by the network proprietor and lower charge rating, which results in users&#39; loss. 
       SUMMARY OF THE INVENTION 
       [0022]    The invention is directed to a method for selecting a base station and communication system using the same. By using a weighed C 2  parameter to reselect a serving base station, the homezone service can be provided suitably and reasonably. 
         [0023]    According to a first aspect of the present invention, a method for selecting a base station is provided. The method is applied to a mobile station. The mobile station sets a preset distance corresponding to a datum base station. The method comprises selecting a first base station to be a serving base station; and obtaining a first C 2  parameter of a second base station corresponding to the mobile station, and obtaining a second C 2  parameter as a base for the mobile station in selecting the serving base station when a distance between the second base station and the datum base station is determined to be smaller than the preset distance, wherein the second C 2  parameter is equal to the first C 2  parameter plus a threshold value. 
         [0024]    According to a second aspect of the present invention, a communication system is provided. The communication system comprises a datum base station, a mobile station, a first base station, and a second base station. The mobile station sets a preset distance corresponding to the datum base station. The first base station is served as a serving base station of the mobile station. The second base station has a distance smaller than the preset distance from the datum base station. The mobile station obtains a first C 2  parameter of the second base station corresponding to the mobile station, the mobile station determines whether to select the second base station as the serving base station according to a second C 2  parameter, and the second C 2  parameter is equal to the first C 2  parameter plus a threshold value. 
         [0025]    The invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic diagram of a conventional homezone service. 
           [0027]      FIG. 2A  and  FIG. 2B  are a flow chart of the method for selecting a base station according to a preferred embodiment of the invention. 
           [0028]      FIG. 3  is a comparison table of the first C 2  parameters and second C 2  parameters of several base stations according to the preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The invention is directed to a method for selecting a base station and communication system using the same. By using the weighed C 2  parameter to reselect the serving base station, the homezone service can be extended to provide the users with more rights and interests. 
         [0030]    The method of the invention is applied to a communication system, such as the homezone service of  FIG. 1 . The communication system includes a datum base station  100 , a mobile station and several base stations, such as a first base station  101 , a second base station  102  and a third base station  103 . The mobile station selects one of the base stations to be the serving base station and the other base stations adjacent to the serving base station are neighbor base stations. The mobile station sets a preset distance R corresponding to the datum base station  100  and the location of the mobile station distant from the datum base station  100  within the preset distance R forms the homezone. The broadcasting position coordinates of the serving base station are used for the mobile station to calculate the distance between the serving base station and the datum base station. When the distance between the serving base station and the datum base station is smaller the preset distance R, the serving base station provides the mobile station with a homezone service. 
         [0031]    Referring to  FIG. 2A  and  FIG. 2B , a flow chart of the method for selecting a base station according to a preferred embodiment of the invention is shown. When the user communicates with the serving base station through the mobile station, it may trigger the step  200  in which the mobile station has to reselect one of the neighbor base stations to be the serving base station due to poor signal reception or other factors. 
         [0032]    In order to save power consumption, in step  202 , the mobile station determines whether the present registered serving base station has its position coordinates located in a critical range of the homezone according to a critical value t. A lower limit value of the critical range is a multiplication R×t of the preset distance R and the critical value t. An upper limit value of the critical range is a multiplication (2-t)×R of the difference between 2 and the critical value t and the preset distance R. The critical value t is 0.8 for instance, and the critical range of the homezone is 0.8R˜1.2R. 
         [0033]    If the position coordinates of the serving base station belong to the critical range of the homezone, in step  204 , the mobile station obtains position coordinates of several neighbor base stations from a list of neighbor base stations in the serving base station. That is, the information of neighbor base stations is provided by the serving base station. Or the information can be read directly from the broadcasting information of the neighbor base stations. Following that, in step  206 , the mobile station has to determine whether the position coordinates of all the neighbor base stations are read. If the mobile station has read the position coordinates of all the neighbor base stations, in step  208 , the mobile station calculates the corresponding C 1  parameter and first C 2  parameter of each neighbor base station, wherein the first C 2  parameter is the conventional C 2  parameter. 
         [0034]    Next, in step  210 , the mobile station calculates and finds out weighing base stations located in the homezone, which are distant from the datum base station  100  within the preset distance R. The mobile station adds the corresponding first C 2  parameter of each weighing base station by a threshold value to become a second C 2  parameter. In step  212 , the mobile station selects the neighbor base stations with the maximum C 2  parameter (the first C 2  parameter or the second C 2  parameter) to be the serving base station. That is, the C 2  parameters of the weighing base stations in the neighbor base stations are the second C 2  parameters and the C 2  parameters of the other base stations in the neighbor base stations are the first C 2  parameters. Referring to  FIG. 3 , a comparison table of the first C 2  parameters and second C 2  parameters of several base stations according to the preferred embodiment of the invention is shown. As shown in  FIG. 1  and  FIG. 3 , when the mobile station moves to the point Q, the neighbor base stations are base stations  102 ,  103 ,  104  and  105 . The mobile station first measures the first C 2  parameters of the neighbor base stations to be b, c, d and e, and then calculates and determines the base stations  103  and  104  to be located in the homezone according to the coordinates of the neighbor base stations. Therefore, the base stations  103  and  104  are set to be weighing base stations and the corresponding first C 2  parameter is added by a threshold value f to become the second C 2  parameter. Then, the mobile station determines the following selection of the serving base station according to a maximum of the C 2  parameters b, (c+f), d and (e+f). 
         [0035]    Besides, in step  202 , if the position coordinates of the serving base station do not belong to the critical range of the homezone, in step  214 , the mobile station calculates the corresponding C 1  parameters and first C 2  parameters of all the neighbor base stations and in step  216 , the mobile station selects the neighbor base station with the largest first C 2  parameter to be the serving base station. 
         [0036]    Moreover, in step  206 , if the mobile station cannot read the position coordinates of a certain neighbor base station, the mobile station cannot calculate the corresponding C 1  parameters and first C 2  parameters of all the neighbor base station. Therefore, in step  218 , the mobile station searches the position coordinates of all the neighbor base stations transmitted from the datum base station  100  as communicating with the datum base station  100  before and recorded in the cache of the mobile station in order to read the position coordinates of the neighbor base stations. 
         [0037]    However, if the mobile station still cannot read the address of the neighbor base stations in step  218 , in step  220 , the mobile station reads a broadcast control channel (BCCH) of each neighbor base station. It should be noted that the present regulations of the global mobile communication system standard mention only how the mobile station receives broadcast information of the serving base station but not the mechanism of the mobile station receiving the broadcast information of the neighbor base stations. In the embodiment, a list of neighbor base stations is read from the serving base station to obtain an allocate radio frequency channel number (ARFCN) of each neighbor base station and the broadcast control channels of the neighbor base stations are read from the ARFCNs. 
         [0038]    The broadcast control channels of the neighbor base stations carry system information (SI). Through the system information SI, the mobile station can read information broadcasted by the neighbor base stations. Therefore, in step  222 , the mobile station determines whether the neighbor base stations support the short message service cell broadcast (SMSCB). If the neighbor base stations support the SMSCB, in step  224 , the mobile station decodes the system information Si of the BCCH to obtain the address of each neighbor base station and then the mobile station executes the step  206 . 
         [0039]    If the neighbor base station does not support the SMSCB, in step  226 , the mobile station determines the neighbor base station does not support the homezone service and does not try again to read the addresses of the neighbor base stations and executes the step  208  after the step  206 . 
         [0040]    In the step  202  of the above embodiment, a critical range of the homezone is defined. If the mobile station is not located in the critical range, it represents the mobile station is distant from the boundary of the homezone, and thus, the present flow of reselecting the serving base station, that is, the steps  214 ˜ 216  of  FIG. 2A  can be used. Moreover, the mobile station does not need to decode the BCCHs of the neighbor base stations, and thus can reduce power consumption. If the mobile station is located in the critical range, it represents the mobile station is close to the boundary of the homezone, that is, located in an indefinite region and the method goes from the step  202  to the step  204 . 
         [0041]    Besides, in the present regulations of global mobile communication system standard, the mobile station has to read the BCCH of each neighbor base station every 5 minutes. Owing that the mobile station will not move too much distance in a specific time, the position coordinates of the neighbor base station will not change very often. Therefore, in practical application, the mobile station reads the position coordinates of the neighbor base stations and records them in the cache, and thus in step  218 , the mobile can read the position coordinates of the neighbor base stations from the cache without need to decode again the BCCH of each neighbor base station, which helps to save time. 
         [0042]    In the method for selecting a base station and communication system using the same disclosed by the above embodiment of the invention, the weighed C 2  parameters are used for reselecting the serving base station in order to extend the homezone service and provide the users with more rights and interests. 
         [0043]    While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.