Abstract:
In a cellular mobile communication network, mobile records are maintained in a first database for indicating a base station area when a mobile station crosses a boundary between adjacent location areas and enters that base station area. A second database is associated with each control group of base station areas for maintaining mobile records for indicating a base station area when the mobile station successively crosses a boundary between adjacent base station areas. In response to a call request, one of the mobile records of the first database is accessed to identify a control group of base station areas and a search request is produced. In response to the search request, one of the mobile records of the second database is accessed and a paging signal is broadcast to at least one base station area indicated in the accessed mobile record to wait for a response from the mobile station. If no response is not returned, the paging signal is broadcast to remaining base station areas of the identified control group.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to cellular mobile communications systems where a location register is provided to maintain mobile records, and more specifically to an efficient method of locating a mobile station from the mobile switching center when there is a need to access the mobile station. 
     2. Description of the Related Art 
     In a cellular mobile communication network, base station areas are assembled into a plurality of location areas and in each location area the base stations are further assembled into a number of control groups. Base stations of each control group are connected to a base station controller. The current mobile locating process of a cellular mobile communication network involves the use of a database called a location register which is usually provided in a mobile switching center to which a number of base station controllers are connected. Mobile stations are constantly receiving base station identification codes and location area identification codes from surrounding base stations. When a mobile station crosses a boundary between adjacent location areas, it sends a location update request containing its mobile identification code and the codes received from a nearby base station. 
     This request is relayed through the nearby base station and through a corresponding base station controller to the mobile switching center, where the location register is updated according to the request from the mobile station. If there is an incoming call destined for the mobile station, the mobile switching center looks up the location register and identifies its location area and all base station of the location area. The mobile switching center proceeds to broadcast a paging signal to the identified base stations. As a result, copies of the paging signal are broadcast simultaneously from such base stations in search of the called mobile station. If the mobile station is located in one of the base station areas, it sends a response message, which is relayed through the base station and the base station controller to the mobile switching center. 
     However, since the paging signal is broadcast from all base stations of a location area, even a single page significantly increases the amount of network&#39;s downlink traffic. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a cellular mobile communication network and method of communication for efficiently locating a mobile station. 
     According to a first aspect of the present invention, there is provided a cellular mobile communication network including a plurality of base stations for respectively covering base station areas, the base station areas being assembled into a plurality of location areas and the base station areas of each location area being assembled into a plurality of control groups. The network comprises a first database for maintaining a plurality of mobile records, each of the mobile records indicating one of the base station areas when a mobile station crosses a boundary between adjacent location areas and enters the base station area. A second database is provided which is associated with one of the control groups of base station areas for maintaining a plurality of mobile records, one of the mobile records indicating a base station area when the mobile station successively crosses a boundary between adjacent base station areas of one of the location areas. A first controller is responsive to a call request for accessing one of the mobile records in the first database to identify one of the control groups of base station areas and producing a search request. A second controller is associated with one of the control groups of base station areas, the second controller being responsive to the search request for accessing one of the mobile records of the second database and transmitting a paging signal to at least one base station area indicated in the accessed mobile record to wait for a response from the mobile station and broadcasting the paging signal to remaining base station areas of the identified control group if the response is not returned from the mobile station. 
     Preferably, the mobile station is arranged to transmit a message indicating that the mobile station is crossing the boundary between adjacent location areas, and the first controller responds to the message for creating the mobile record in the first database and returning an acknowledgment message to the mobile station. The mobile station is further arranged to transmit a message indicating that the mobile station is crossing the boundary between adjacent base station areas, and the second controller responds to the message for creating the mobile record in the second database and returning no acknowledgment message to the mobile station. 
     According to a second aspect, the present invention provides a communication method for a cellular mobile communication network including a plurality of base stations for respectively covering base station areas, the base station areas being assembled into a plurality of location areas and the base station areas of each location area being assembled into a plurality of control groups. The method comprises the steps of (a) maintaining a plurality of mobile records in a first database, each of the mobile records indicating one of the base station areas when a mobile station crosses a boundary between adjacent location areas and enters the one base station area, (b) maintaining a plurality of mobile records in a second database associated with one of the control groups of base station areas, one of the mobile records indicating a base station area when the mobile station successively crosses a boundary between adjacent base station areas of one of the location areas, (c) accessing one of the mobile records in the first database in response to a call request and identifying one of the control groups of base station areas and producing a search request, (d) accessing one of the mobile records of the second database in response to the search request, (e) transmitting a paging signal to at least one base station area indicated in the accessed mobile record to wait for a response from the mobile station, and (f) broadcasting the paging signal to remaining base station areas of the identified control group if the response is not returned from the mobile station. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an exemplary cellular mobile communication network in which a hierarchically layered location memory structure of the present invention is illustrated; 
     FIG. 2 is a block diagram of the cellular communication network for illustrating the contents of the layered memories when a mobile station moves around, hopping from one cell-site area to another; 
     FIG. 3 is a flowchart of the operation of a mobile switching center when a search request is sent to a base station according to the present invention; 
     FIG. 4 is a flowchart of the operation of a base station when locating a mobile station in response to a search request from the mobile switching center according to one embodiment of the present invention; 
     FIG. 5 is a block diagram of a base station controller according to a modified embodiment of the present invention; and 
     FIG. 6 is a flowchart of the operation of the base station when locating a mobile station in response to the search request according to the modified embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown an exemplary cellular mobile communication network in which hierarchically layered memories of the present invention are illustrated. 
     A mobile switching center  110  is connected to a public switched telephone network  100  to accommodate a plurality of base station controllers  121  through  124  for serving cell-site base stations  130  covering respective cell-site areas BA&#39;s. A plurality of location areas are defined in the network. Some of the base stations may be connected to more than one base station controller. Each location area is formed by a group of base stations which are simultaneously paged by broadcast paging signals from the mobile switching center  110  when there is an incoming call from the switched network  100 . In the illustrated example, the base stations served by base station controllers  121  and  122  are grouped to form a location area LA 1  and those served by base station controllers  123  and  124  are grouped to form a location area LA 2 . 
     Mobile switching center  110  has a database, called location register  111  to maintain data indicating the identities (MS) of mobile stations and the identities (BS) of their current base stations. Database  111  is updated when a mobile station travels across a boundary between adjacent location areas and announces its new location. 
     According to the present invention, the base station controllers  121  through  124  are respectively provided with databases, or location memories  125  through  128  to maintain data indicating the identities of mobile stations and the identities of their current base stations. Each of these databases is updated when a mobile station enters or leaves the coverage area (BA) of a base station. 
     Each base station is constantly transmitting its base station (BS) identification code and its LA identification code to allow mobile stations to identify their current location and announce their new location when they travel across a boundary between adjacent cell-site areas. When a mobile station receives BS and LA identification codes from a nearby base station, it returns its mobile (MS) identification code and the received BS and LA identification codes to the nearby base station. 
     In FIG. 2, assume that a mobile station MS 1  is travelling across the boundaries of base stations BS 1  through BS 4  of location area LA 1  and the boundaries of base stations BS 5  through BS 8  of location area LA 2 . When the mobile station MS 1  is in the coverage area of base station BS 1 , it is receiving a set of identification codes BS 1  and LA 1  from the nearby base station BS 1  and returns a message containing its identity MS 1  and the received codes BS 1  and LA 1  to the base station BS 1 . Base station BS 1  receives this message and transmits a copy  201  of the received message to the base station controller BCS 1 . 
     In response to the message  201 , the base station controller BSC 1  assigns an entry of the location memory LM 1  to the mobile station MS 1  and stores the identification code BS 1  in the assigned entry. At the same time, the base station controller BSC 1  sends a location update message  202  to the mobile switching center  110  for mapping the mobile&#39;s identity MS 1  to the base station identity BS 1  in the location register  111  . 
     In order to ensure that the contents of the location register  111  correctly match the actual mobile locations, the mobile switching center  11   0  returns an acknowledgment message  203  to the base station controller BSC 1 , which is retransmitted to the base station BS 1  as a message  204  and relayed to the mobile station MS 1 . After transmitting a return message, the mobile station MS 1  waits a specified time interval for the acknowledgment message. If the signal from the mobile station MS 1  fails to reach the base station BS 1 , the mobile station MS 1  receives no acknowledgment message and retransmits a copy of the message again when the specified time interval expires. 
     In order to reduce the network&#39;s volume of traffic associated with the transmission of location identification codes, the base station controllers do not return their own acknowledgment message when their location memory is updated. 
     Therefore, when the mobile station MS 1  moves out of the coverage area of base station BS 1  enters the coverage area of base station BS 3 , it sends a location update message containing the area identities BS 3  and LA 1  as well as its identity MS 1  to the base station BS 3 . A copy  205  of this update message is sent from the base station BS 3  to the base station controller BSC 1  to store the cell-site identity BS 3  into a storage area of the entry of the location memory LM 1  assigned to the mobile station MS 1  that is next to the storage area of BS 1 . No acknowledgment message is returned from the base station controller to the mobile station in response to the update message  205 . Likewise, when the mobile station MS 1  moves to the coverage area of base station BS 2 , an update message  206  is sent from the base station BS 2  to the base station controller BSC 1 . The location memory LM 1  is thus updated by storing the cell-site identity BS 2  into a storage area of the mobile&#39;s entry next to the storage area of BS 3 . 
     When the mobile station is crossing a boundary between the location areas LA 1  and LA 2  and entering the coverage area of base station BS 5 , leaving the coverage area of base station BS 2 , the location update message from the mobile station MS 1  is received by both base stations BS 2  and BS 5 , causing a location update message  207  to be transmitted from base station BS 2  to the base station controller BSC 1  and a location update message  208  to be transmitted from base station BS 5  to the base station controller BSC 2 . As a result, the base station controller BSC 1  erases the contents of the entry of the location memory LM 1  that is assigned to the mobile station MS 1 . On the other hand, the base station controller BSC 2  responds to the message  208  by assigning an entry of the location memory LM 2  to the mobile station MS 1  and storing the cell-site identity BS 5  into the assigned entry, and reports the location update of MS 1  to the mobile switching center  110  with a message  209 . Mobile switching center  110  knows that the mobile station MS 1  has entered the location area LA 2  and updates its register  111  by overwriting the old cell-site identity BS 1  with the new cell-site identity BS 5 , and then returns an acknowledgment message  210  which is relayed through the base station controller BSC 2  as a message  211  to the base station BS 5  and sent to the mobile station MS 1 . 
     If the mobile station MS 1  continues to travel and enters the coverage area of base station BS 7  and then to the area of base station BS 8 , location update messages  212  and  213  will be successively transmitted from these base stations to the base station controller BSC 2  so that the cell-site identities of these base stations are stored in successive storage areas of the location memory LM 2 , as illustrated. 
     If there is an incoming call from the switched network that is destined for a mobile station, the mobile switching center  110  attempts to locate the called mobile station. Instead of broadcasting a paging signal to all base stations of a location area, the mobile switching center  110  of the present invention identifies one or more base station controllers in which the mobile&#39;s identification code is stored and proceeds to send a search request to such base station controllers. 
     The operation of the mobile switching center and a base station controller according to a first embodiment of the present invention is illustrated in the flowcharts of FIGS. 3 and 4. 
     In FIG. 3, when the mobile switching center  110  receives an incoming call from the PSTN  100 , it makes a search through the location register  111  for a mobile station which is being called (step  301 ). If there is one, flow proceeds from step  301  to step  302  to identify the base station controller that is mapped to the called mobile station in the location register  111 . A search request message is then sent from the mobile switching center  110  to the identified base station controller. If the called mobile station is within the coverage area of a base station which is connected to more than one base station controller, the mobile switching center identifies such base station controllers and sends a search request message to these base station controllers at the same time. 
     In FIG. 4, the base station controller responds to the search request message from the mobile switching center by looking up the most recent storage area of the location memory and identifies a base station which is mapped to the called mobile station (step  401 ). At step  402 , the base station controller transmits a paging signal to the identified base station and waits for a response (step  403 ). 
     If the called mobile station is within the coverage area of the identified base station, the paging signal will be received by the mobile station and a response is returned to the base station controller via that base station. In this case, the decision at step  403  is affirmative and flow proceeds to step  411  to return an answer message to the mobile switching center and terminates the routine. 
     Due to the absence of acknowledgment from the base station controller whenever a mobile station crosses the boundary of adjacent cell-site areas, there is a likelihood of a mismatch between the contents of the memory and the cell-site areas actually travelled by a mobile station. 
     If such a mismatch has occurred, the decision at step  403  is negative and the base station controller recognizes that the paging has failed and proceeds to step  404  to look up the location memory to identify other base stations. At step  405 , the base station controller broadcasts a paging signal to the identified other base stations and waits for a response (step  406 ). If the called mobile station is within one of the coverage areas of these base stations, the paging signal will be received by the mobile station and a response is returned via one of these base stations to the base station controller. If this is the case, the decision at step  406  is affirmative and flow proceeds to step  411  to return an answer message to the mobile switching center, and terminates the routine. 
     If no response is returned, flow proceeds from step  406  to step  407  to check to see if all base stations of the base station controller are paged. If so, flow proceeds from step  407  to step  410  to return a failure message to the mobile switching center. 
     If all base stations have not been paged, flow proceeds from step  407  to step  408  to broadcast a paging signal to the remaining base stations and waits for a response (step  409 ). If a response is returned, flow proceeds from step  409  to step  411 . Otherwise, the base station controller executes step  410  to return a failure message. 
     A modified method of locating a mobile station is illustrated in FIGS. 5 and 6. In this modification, each base station controller  500  includes a geographic memory  502  as well as a location memory  501 , as illustrated in FIG.  5 . In the geographic memory  502 , data indicating the geographic relations of base stations are stored as a tool which can be used as an aid for evaluating the data stored in the location memory  501  to estimate a base station in which the wanted mobile station is likely to be located. 
     The operation of the base station controller  500  proceeds as shown in FIG.  6 . In response to a search request message from the mobile switching center, the base station controller  500  examines the contents of the entry of location memory  501  which is assigned to the called mobile station and select recently stored data, and base stations corresponding to the selected data are identified (step  601 ). A paging signal is then broadcast to the identified base stations (step  602 ) and the base station controller waits for a response (step  603 ). If a response is received, flow proceeds from step  603  to step  611  to return an answer message to the mobile switching center. 
     If no response is returned, flow proceeds from step  603  to step  604  to check to see if data stored in the location memory is sufficient to estimate likely base stations. If not, flow proceeds from step  604  to step  608  to broadcast a paging signal to remaining base stations. 
     If the decision at step  604  is affirmative, flow proceeds to step  605  to read data from geographic memory  502 . Base station controller evaluates the data stored in the location memory  501  and determines base stations which are likely to receive a response message from the called mobile station. At step  606 , a paging signal is broadcast to the likely base stations to wait for a response (step  607 ). 
     If a response is received, flow proceeds from step  607  to step  611 . Otherwise, flow proceeds to step  608  to broadcast the paging signal to remaining base stations to wait for a response (step  609 ). If a response is received, flow proceeds from step  609  to step  611 . Otherwise, step  610  is executed to return a failure message to the mobile switching center. 
     Since no acknowledgment message is returned from the network to a mobile station when it sends a location update message and since the mobile station is not simultaneously paged from all base stations of a location area, the network&#39;s downlink traffic can be reduced significantly in comparison with the conventional broadcast paging method.