Abstract:
A protocol processing method and apparatus are disclosed for a multi-SIM terminal holding multiple subscriber identification cards. The method includes: determining whether all the subscriber identification cards are associated with the same base station (e.g., a Public Land Mobile Network); and conducting, when all the subscriber identification cards are associated with the same base station, frequency scanning and measurement for a selected one of the subscriber identification cards, and sharing the results of frequency scanning and measurement with all the subscriber identification cards. Hence, the multi-SIM terminal may reduce battery power consumption without repeated frequency scanning and measurement for multiple subscriber identification cards.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims, pursuant to 35 U.S.C. §119(a), priority to, and the benefit of the earlier filing date of, that Korean patent application filed in the Korean Intellectual Property Office on Sep. 25, 2009 and assigned Serial No. 2009-0091027, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to the field of portable terminals and more particularly, to a method and apparatus for protocol processing in a terminal that is equipped with multiple subscriber identification cards. 
         [0004]    2. Description of the Related Art 
         [0005]    Regular mobile communication terminals employ a single subscriber identification card. However, advances in mobile communication technology have enabled development of various types of mobile communication terminals, such as dual-mode terminals, multi-standby terminals and multi-SIM terminals. A dual-mode terminal may use one of two subscriber identification cards according to a user selection. A multi-standby terminal may make multiple subscriber identification cards active simultaneously for calls. A multi-SIM terminal includes a radio frequency unit and a baseband chip, and holds multiple subscriber identification cards. Particularly, in the multi-SIM terminal, the single radio frequency unit is alternately shared by multiple subscriber identification cards. For example, when a dual-SIM terminal holding first and second subscriber identification cards is turned on, it may perform frequency scanning and measurement for the first subscriber identification card to register the first subscriber identification card in the base station, and perform frequency scanning and measurement for the second subscriber identification card to register the second subscriber identification card in the base station. After registering the first and second subscriber identification cards, the dual-SIM terminal may perform neighbor cell scanning and measurement. However, frequency scanning and measurement, and neighbor cell scanning and measurement cause a large amount of battery power consumption. That is, as a conventional multi-SIM terminal performs frequency scanning and measurement, and neighbor cell scanning and measurement repeatedly for individual subscriber identification cards, the multi-SIM terminal may consume a large amount of battery power. 
         [0006]    Additionally, in a multi-SIM terminal, when the radio frequency unit is used for a call on one of the subscriber identification cards, radio communication channels related to the remaining subscriber identification cards may be released. That is, when an existing multi-SIM terminal handles a call on one of multiple subscriber identification cards, it may release radio communication channels related to the remaining subscriber identification cards. This unexpected channel release may prevent the corresponding base station from keeping track of the remaining subscriber identification cards, causing network confusion. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention has been made in view of the above problems, and the present invention provides a protocol processing method and apparatus for a multi-SIM terminal that enable the multi-SIM terminal to reduce battery power consumption by sharing the results of frequency scanning and measurement conducted for one subscriber identification card with other subscriber identification cards without repeated frequency scanning and measurement. 
         [0008]    The present invention also provides a protocol processing method and apparatus for a multi-SIM terminal that direct the multi-SIM terminal to send, when the radio frequency unit is used to handle a call on one subscriber identification card, a message indicating temporary release of communication channels associated with the remaining subscriber identification cards to the corresponding base station so as to avoid network confusion by preventing the base station from losing track of the subscriber identification cards. 
         [0009]    In accordance with an exemplary embodiment of the present invention, there is provided a protocol processing method for a multi-SIM terminal holding multiple subscriber identification cards, including: determining whether all the subscriber identification cards are associated with the same base station; and conducting, when all the subscriber identification cards are associated with the same base station, frequency scanning and measurement for a selected one of the subscriber identification cards, and sharing the results of frequency scanning and measurement with all the subscriber identification cards. 
         [0010]    In accordance with another exemplary embodiment of the present invention, there is provided a protocol processing apparatus for a multi-SIM terminal holding multiple subscriber identification cards, including: a radio frequency unit conducting frequency scanning and measurement; a network information examiner determining whether all the subscriber identification cards are associated with the same base station; and a control unit controlling, when all the subscriber identification cards are associated with the same base station, a process of conducting frequency scanning and measurement for a selected one of the subscriber identification cards, and sharing the results of frequency scanning and measurement with all the subscriber identification cards. 
         [0011]    In another aspect of the invention, a multi-SIM portable terminal is disclosed. The terminal includes a transmitter/receiving unit for establishing a communication link with at least one base station; a processor in communication with a memory, the memory including code, which when accessed by the processor causes the processor to: determine which of the multi-SIMs are associated with a same one of the at least one base station and which of the multi-SIMs are associated with a different one of the at least one base station; perform a frequency scan and measurement for one of each multi-SIM associated with each of the different base stations; share the results of the frequency scan and measurement for each of the multi-SIMs associated with the same base station; receive a call message, through the transmitter/receiver, from a base station associated with at least one of the multi-SIM cards; and transmit, through the transmitter/receiver, a release message to each base station associated with a multi-SIM not associated with the call message. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a block diagram of a dual-SIM terminal according to an exemplary embodiment of the present invention; and 
           [0014]      FIG. 2  is a flow chart of a protocol processing method for the dual-SIM terminal according to another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The same reference symbols are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
         [0016]    The description of the various embodiments is to be construed as exemplary only and does not describe every possible instance of the invention. Therefore, it should be understood that various changes may be made and equivalents may be substituted for elements of the invention and are considered to be within the scope of the invention as claimed. 
         [0017]    In the description, a dual-SIM terminal holding two subscriber identification cards is used as an example of the multi-SIM terminal. However, the present invention is not limited thereto. It should be apparent to those skilled in the art that the present invention is also applicable to a multi-SIM terminal holding three or more subscriber identification cards. 
         [0018]    In the description, a subscriber identification card refers to a device storing authentication information of a subscriber. A subscriber identification card may also be referred to as a subscriber identity module (SIM), a universal subscriber identity module (USIM) or a user identity module (UIM). For the purpose of description, only the term “subscriber identification card” is used. 
         [0019]      FIG. 1  is a block diagram of a dual-SIM terminal  100  according to an exemplary embodiment of the present invention. 
         [0020]    Referring to  FIG. 1 , the dual-SIM terminal  100  may include a radio frequency unit  150 , a first slot  140  for holding a first subscriber identification card  40 , a second slot  160  for holding a second subscriber identification card  60 , a display unit  130 , a storage unit  120 , and a control unit  110 . 
         [0021]    The first slot  140  and the second slot  160  hold subscriber identification cards. The first slot  140  acts as an interface connecting the first subscriber identification card  40  with the control unit  110 , and the second slot  160  acts as an interface connecting the second subscriber identification card  60  with the control unit  110 . The first and second subscriber identification cards  40  and  60  may be adapted to the same mobile communication scheme. For example, the first and second subscriber identification cards  40  and  60  may be adapted to one of mobile communication schemes such as Global System for Mobile communications (GSM), General Packet Radio Services (GPRS), Enhanced Data rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Code Division Multiple Access (CDMA), and Wideband Code Division Multiple Access (WCDMA). It is assumed that the first subscriber identification card  40  is installed in the first slot  140  and the second subscriber identification card  60  is installed in the second slot  160 . 
         [0022]    The radio frequency unit  150  sends and receives radio signals to and from a wireless communication network. The radio frequency unit  150  may include a duplexer (not shown) for isolating a transmit signal from a receive signal, a radio frequency (RF) transmitter (not shown) for up-converting the frequency of a signal to be transmitted and amplifying the signal, and an RF receiver (not shown) for low-noise amplifying a received signal and down-converting the frequency of the received signal. The radio frequency unit  150  may conduct frequency scanning and measurement when the dual-SIM terminal  100  is booted, located in a non-service coverage area, or moved from a non-service coverage area to a service coverage area. On the basis of the results of frequency scanning and measurement, the radio frequency unit  150  may register the first and second subscriber identification card  40  and  60  in the base station providing the best communication channel environment (for example, highest receive signal level) under the control of the control unit  110 . Particularly, in the case that the first subscriber identification card  40  and the second subscriber identification card  60  are associated with the same mobile network operator (i.e., the same base station), the radio frequency unit  150  may apply the results of frequency scanning and measurement obtained using one of the first and second subscriber identification cards  40  and  60  to the other subscriber identification card under the control of the control unit  110  (that is, the subscriber identification cards share the results of frequency scanning and measurement). When the first subscriber identification card  40  and the second subscriber identification card  60  are associated with different base stations, the radio frequency unit  150  may conduct frequency scanning and measurement separately for the first subscriber identification card  40  and the second subscriber identification card  60 , independently. Frequency scanning may include a search for available frequencies in the vicinity of the dual-SIM terminal  100 , and measurement of the signal level on the found frequency. When a call request is made on one of the first subscriber identification card  40  and the second subscriber identification card  60 , the radio frequency unit  150  may send a message indicating release of a communication channel related to the subscriber identification card not receiving the call to the corresponding base station under the control of the control unit  110 . 
         [0023]    The display unit  130  may display various menus of the dual-SIM terminal  100 , information input by the user, function setting information, and information to be provided to the user. For example, the display unit  130  may display a boot screen, idle screen, menu screen, and video call handling screen. The display unit  130  may be realized using liquid crystal display (LCD) devices or organic light emitting diodes (OLED) or other similar type displays (e.g., LED). When the display unit  130  has a touch screen capability, it may act as an input means. Touch screen technology is well-known in the art and details of the operation and/or implementation of such technology need not be provided herein to understand the operating principles of the subject matter recited in the claims. 
         [0024]    The storage unit  120  may store the operating system (OS) of the dual-SIM terminal  100 , application programs for optional functions (related to, for example, sound reproduction, display of images or playback of moving images, and broadcast reception), user data, and data generated during communication. In particular, the storage unit  120  may store a program that controls frequency scanning and measurement of the radio frequency unit  150  according to whether the first subscriber identification card  40  and the second subscriber identification card  60  are associated with the same base station. The storage unit  120  may also store information regarding base station selection, cell selection, cell reselection, and neighbor cell measurement for the first subscriber identification card  40  and the second subscriber identification card  60 . 
         [0025]    The control unit  110  controls the overall operation of the dual-SIM terminal  100 , signal exchange between internal components thereof, and processing of data sent and received through the radio frequency unit  150 . In particular, during execution of a boot procedure, the control unit  110  may check whether the first subscriber identification card  40  and the second subscriber identification card  60  are associated with the same base station. To achieve this, the control unit  110  may include a network information examiner  111 . The network information examiner  111  may check whether the first and second subscriber identification cards  40  and  60  are associated with the same base station by comparing International Mobile Subscriber Identity (IMSI) information of the first subscriber identification card  40  with that of the second subscriber identification card  60 . To be more specific, when the Mobile Network Code (MNC) in the IMSI information of the first subscriber identification card  40  is equal to that in the IMSI information of the second subscriber identification card  60 , the network information examiner  111  may determine that the first and second subscriber identification cards  40  and  60  are associated with the same base station. Alternatively, when the Mobile Network Code for the first subscriber identification card  40  is different from that for the second subscriber identification card  60 , the network information examiner  111  may determine that the first and second subscriber identification cards  40  and  60  are associated with different base stations. Here, the IMSI information is widely known to those skilled in the art, and a description thereof is omitted. In addition, the network information examiner  111  may also determine whether the first and second subscriber identification cards  40  and  60  are associated with the same base station by comparing network operator information such as the service provider name (SPN) stored in the first subscriber identification card  40  with that stored in the second subscriber identification card  60 . 
         [0026]    When the first and second subscriber identification cards  40  and  60  are determined to be associated with the same base station, the control unit  110  may control an operation to apply the results of frequency scanning and measurement obtained using one of the first and second subscriber identification cards  40  and  60  to the other subscriber identification card (i.e., sharing of frequency scanning and measurement results). When the first and second subscriber identification cards  40  and  60  are determined to be associated with different base stations, the control unit  110  may control an operation to conduct frequency scanning and measurement for the first subscriber identification card  40  and the second subscriber identification card  60  separately. 
         [0027]    When the first and second subscriber identification cards  40  and  60  are registered in the mobile communication network through frequency scanning and measurement and the boot procedure is completed, the control unit  110  may enter the idle state. In the idle state, the control unit  110  may have the first subscriber identification card  40  and the second subscriber identification card  60  active simultaneously for calls. In the idle state, the control unit  110  may periodically conduct location information update (Location Area Update or Router Area Update), base station selection, cell selection, cell reselection, and neighbor cell measurement separately for the first subscriber identification card  40  and the second subscriber identification card  60 . The results obtained by a background process in the idle state may not be shared among the first subscriber identification card  40  and the second subscriber identification card  60 . Alternatively, on the basis of the results of neighbor cell scanning and measurement for the first subscriber identification card  40 , the control unit  110  may conduct cell reselection for the second subscriber identification card  60 . That is, the results of neighbor cell scanning and measurement may be shared among the first subscriber identification card  40  and the second subscriber identification card  60 . 
         [0028]    When a call request is made on one of the first subscriber identification card  40  and the second subscriber identification card  60  (for purposes of describing the invention claimed, operation using the first subscriber identification card  40  is described with regard to the call request) in the idle state, the control unit  110  may control the radio frequency unit  150  to send an IMSI Detach message indicating release of a communication channel assigned to the other subscriber identification card (i.e., the second subscriber identification card  60 ) to the base station before establishing the call channel. When the call ends, in the case that the first and second subscriber identification cards  40  and  60  are associated with the same base station, the control unit  110  may control an operation to register the second subscriber identification card  60  in the mobile communication network utilizing base station and cell information of the first subscriber identification card  40 . In the case that the first and second subscriber identification cards  40  and  60  are not associated with the same base station, the control unit  110  may control the radio frequency unit  150  to conduct frequency scanning and measurement for the second subscriber identification card  60 , and to register the second subscriber identification card  60  in the mobile communication network on the basis of the results of frequency scanning and measurement. 
         [0029]    Hereinabove, a description is given of the configuration of the dual-SIM terminal  100 . Next, a description is given of a protocol processing method for the dual-SIM terminal  100 . 
         [0030]      FIG. 2  is a flow chart of a protocol processing method for the dual-SIM terminal  100  according to another exemplary embodiment of the present invention. 
         [0031]    Referring to  FIGS. 1 and 2 , when the dual-SIM terminal  100  is turned on, the control unit  110  executes a boot procedure ( 201 ), and recognizes the first subscriber identification card  40  and the second subscriber identification card  60  ( 203 ). Recognition of a subscriber identification card is widely known to those skilled in the art, and a description thereof is omitted. 
         [0032]    The control unit  110  checks whether the first and second subscriber identification cards  40  and  60  are associated with the same public land mobile network (PLMN) (referred to as a base station) ( 205 ). For example, when the Mobile Network Code (MNC) in the IMSI information of the first subscriber identification card  40  is equal to that in the IMSI information of the second subscriber identification card  60 , the control unit  110  may determine that the first and second subscriber identification cards  40  and  60  are associated with the same base station. The control unit  110  may also determine whether the first and second subscriber identification cards  40  and  60  are associated with the same base station by comparing network operator information such as the service provider name (SPN) stored in the first subscriber identification card  40  with that stored in the second subscriber identification card  60 . 
         [0033]    When the first and second subscriber identification cards  40  and  60  are associated with the same base station, the control unit  110  controls the radio frequency unit  150  to conduct frequency scanning and measurement for the first subscriber identification card  40  ( 207 ). Here, frequency scanning and measurement may include a search for available frequencies in the vicinity of the dual-SIM terminal  100 , and measurement of the signal level at the found frequency. 
         [0034]    The control unit  110  selects a base station or cell providing the best communication channel environment on the basis of the results of frequency scanning and measurement to register the first subscriber identification card  40  in the mobile communication network ( 209 ), and registers the second subscriber identification card  60  in the mobile communication network utilizing the results of frequency scanning and measurement obtained for the first subscriber identification card  40  ( 211 ). In the above description, frequency scanning and measurement is conducted for the first subscriber identification card  40  and the results are applied to the first and second subscriber identification cards  40  and  60 . However, the present invention is not limited thereto. That is, frequency scanning and measurement may be conducted for the second subscriber identification card  60  and the results may be applied to the first and second subscriber identification cards  40  and  60 . 
         [0035]    When the boot procedure is completed, the control unit  110  enters the idle state ( 212 ). The control unit  110  conducts neighbor cell scanning and measurement for the first subscriber identification card  40  ( 213 ), and applies the results of neighbor cell scanning and measurement to the first subscriber identification card  40  and the second subscriber identification card  60  ( 214 ). For example, the control unit  110  may conduct cell reselection for the second subscriber identification card  60  on the basis of the results of neighbor cell scanning and measurement obtained for the first subscriber identification card  40 . Thereafter, the control unit  110  may alternately assign the radio frequency unit  150  to the first and second subscriber identification cards  40  and  60  to conduct location information update (Location Area Update or Router Area Update), base station selection, and cell selection. 
         [0036]    The control unit  110  checks whether a call request is made on the first subscriber identification card  40  ( 215 ). Here, the call request may correspond to placement or reception of a call to or from another terminal. When a call request is not made, the control unit  110  returns to step  213 , or may perform a requested operation such as music file playback or image capture. When a call request is made on the first subscriber identification card  40 , the control unit  110  performs an IMSI Detach procedure for the second subscriber identification card  60  ( 217 ). That is, the control unit  110  may control the radio frequency unit  150  to send a message indicating release of a channel connection assigned to the second subscriber identification card  60  to the base station. Thereby, the present invention can avoid network confusion that may occur if the base station loses track of the second subscriber identification card  60  while the radio frequency unit  150  is not available to the second subscriber identification card  60 . For example, when a call request is made on the second subscriber identification card  60  while the user of the dual-SIM terminal  100  is still in conversation during a call on the first subscriber identification card  40 , the base station may send a busy signal to that terminal making the call request. 
         [0037]    The control unit  110  establishes a communication channel to the base station and processes the call on the first subscriber identification card  40  ( 219 ). The control unit  110  checks whether the call is ended ( 221 ). When the call is not ended, the control unit  110  returns to step  219  for continued call processing. When the call is ended, the control unit  110  releases the call connection for the first subscriber identification card  40  and registers the second subscriber identification card  60  in the base station ( 223 ). Here, the control unit  110  may register the second subscriber identification card  60  in the mobile communication network on the basis of information on the base station in which the first subscriber identification card  40  is registered. 
         [0038]    The control unit  110  checks whether a termination command is input ( 225 ). When a termination command is input, the control unit  110  performs a termination procedure. When a termination command is not input, the control unit  110  returns to step  212  for continued processing. 
         [0039]    When the first and second subscriber identification cards  40  and  60  are not associated with the same base station at step  205 , the control unit  110  conducts frequency scanning and measurement for the first subscriber identification card  40  and also conducts frequency scanning and measurement for the second subscriber identification card  60  ( 231 ), and subscribes the first subscriber identification card  40  and the second subscriber identification card  60  appropriately in the mobile communication network ( 233 ). 
         [0040]    After completion of the boot procedure by registration of the subscriber identification cards, the control unit  110  enters the idle state ( 235 ). The control unit  110  conducts neighbor cell scanning and measurement separately for the first subscriber identification card  40  and for the second subscriber identification card  60  ( 236 ). Here, the control unit  110  may alternately assign the radio frequency unit  150  to the first and second subscriber identification cards  40  and  60  so as to periodically conduct location information update (Location Area Update or Router Area Update), base station selection, and cell selection separately for the first subscriber identification card  40  and for the second subscriber identification card  60 . 
         [0041]    The control unit  110  checks whether a call request is made on the first subscriber identification card  40  ( 237 ). When a call request is not made, the control unit  110  returns to step  235 , or may perform a requested operation such as music file playback or image capture. When a call request is made on the first subscriber identification card  40 , the control unit  110  controls the radio frequency unit  150  to send an IMSI Detach message indicating release of a channel connection assigned to the second subscriber identification card  60  to the base station ( 239 ). Transmission of the channel connection release message may contribute to avoidance of network confusion that may occur if the base station loses track of the second subscriber identification card  60  while the radio frequency unit  150  is not available to the second subscriber identification card  60 . For example, when a call request is made on the second subscriber identification card  60  while the user of the dual-SIM terminal  100  is still engaged in a call on the first subscriber identification card  40 , the base station may send a busy signal to that terminal making the call request. 
         [0042]    The control unit  110  establishes a communication channel to the base station and processes the call on the first subscriber identification card  40  ( 241 ). The control unit  110  checks whether the call is ended ( 243 ). When the call is not ended, the control unit  110  returns to step  241  for continued call processing. When the call is ended, the control unit  110  releases the call connection for the first subscriber identification card  40  and conducts frequency scanning and measurement for the second subscriber identification card  60  ( 245 ). The control unit  110  registers the second subscriber identification card  60  in the mobile communication network on the basis of the results of frequency scanning and measurement ( 246 ). 
         [0043]    The control unit  110  checks whether a termination command is input ( 247 ). When a termination command is input, the control unit  110  performs a termination procedure. When a termination command is not input, the control unit  110  returns to step  235  for continued processing. 
         [0044]    In the above description, a call request is made on the first subscriber identification card  40 . However, the present invention is not limited thereto. That is, the above procedure may readily handle the case where a call request is made on the second subscriber identification card  60 . 
         [0045]    Hereinabove, a description is given of a dual-SIM terminal holding two subscriber identification cards. However, the present invention is not limited thereto. That is, the present invention is also applicable to a multi-SIM terminal holding three or more subscriber identification cards. Additionally, in the case of a dual-standby terminal employing identical communication schemes (for example, a GSM-GSM dual-standby terminal), when the two subscriber identification cards have the same network operator information, the results of frequency scanning and measurement obtained by the radio frequency unit may be shared by the two subscriber identification cards. 
         [0046]    As apparent from the above description, the protocol processing method and apparatus enable the multi-SIM terminal to reduce battery power consumption due to repeated frequency scanning and measurement for multiple subscriber identification cards. In addition, when the radio frequency unit is used to handle a call on one subscriber identification card, the multi-SIM terminal sends a message (for example, an IMSI Detach message) indicating temporary release of communication channels associated with the remaining subscriber identification cards to the corresponding base station. Hence, network confusion may be avoided by preventing the base station from losing track of the subscriber identification cards. 
         [0047]    The above-described methods according to the present invention can be realized in the controller that is realized in hardware or as software or computer code that can be stored in a recording medium such as a CD ROM, an RAM, a floppy disk, a hard disk, or a magneto-optical disk or downloaded over a network (i.e., The computer program can be provided from an external source which is electronically downloaded over a network, e.g., Internet, POTS, so that the methods described herein can be rendered in such software using a general purpose computer, or a special processor or in programmable or dedicated hardware, such as an ASIC or FPGA. As would be understood in the art, controller, as represented by the computer, the processor or the programmable hardware may include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code that when accessed and executed by the computer, processor or hardware implement the processing methods described herein. The code when loaded into a general purpose computer transformed the general purpose computer into a special purpose computer that may in part be dedicated to the processing shown herein. In addition, controller, as represented by the computer, the processor or the dedicated hardware may be composed of at least one of a single processor, a multi-processor, and a multi-core processor. 
         [0048]    Although the invention has been described with regard to two SIM cards and the two cards may be associated with the same or different base stations. However, it would be recognized that when multiple-SIM cards (i.e., greater than two SIM cards) are included then one or more of the multiple SIM cards may be associated with one base station and each of the remaining SIM cards may be associated with a different base station or with one or more base stations. Hence, when calls are placed with one SIM card, then the other SIM cards may be disconnected, as previously described, and then when the call is terminated, the scanning of the frequencies for re-connection to the other base stations may consider that one or more of the remaining SIM cards may belong to the base station and, thus, frequency results of one SIM card may be shared with the other remaining SIM cards that share the same base station. 
         [0049]    Although exemplary embodiments of the present invention have been described in detail hereinabove, it should be understood that many variations and modifications of the basic inventive concept herein described, which may appear to those skilled in the art, will still fall within the spirit and scope of the exemplary embodiments of the present invention as defined in the appended claims.