Patent Publication Number: US-2007111727-A1

Title: System and method for enhanced mobility tracking of mobile stations

Description:
FIELD OF THE INVENTION  
      The field of the invention relates to tracking mobile stations in networks and, more specifically, to tracking mobile stations in networks so that communications can be correctly routed to these mobile stations.  
     BACKGROUND OF THE INVENTION  
      In telecommunication systems, various Fixed Network Elements (FNEs) can be used to track and store the location of mobile stations. For instance, after the location of a mobile station has been determined to be in its home network, a Home Location Register (HLR) may be used to store the identity of a network in which a mobile station is located and a Visitor Location Register (VLR) may be used to store the location area in which the mobile station is located in the current network. These location areas are also commonly referred to as “paging areas.” 
      As a mobile station moves between various location areas, it normally updates the system as to which location area it has moved. When the mobile station is the target of a call, the system pages all cells within that location area in order to locate the mobile station for that call.  
      However, current systems only update the current location area of a mobile station when the mobile station reports that it has moved. If a mobile station changes location areas while in a call, the system does not update the current location area for that mobile station and the mobile station and the system are typically no longer synchronized.  
      In one specific example of approaches used in current systems, users frequently conduct voice conversations. During these conversations, pauses sometimes occur and the conversations must be extended across multiple calls in order to be completed. For instance, a user might actuate a pause button at a mobile station during a conversation, and if the length of the pause is too long, the call is terminated by the system. In another example, with dispatch communications, if there is no active talker for a predetermined amount of time, the call is terminated by the system. Consequently, a new call or calls must be established to allow the user to continue and complete the conversation.  
      Problems occur if a call is terminated and the mobile station has moved out of its original location area to a new location area or cell. Specifically, if a mobile station has moved to a different location during the call, then when the call has terminated, the mobile station must update the system with its new location before it can receive a page for subsequent calls. However, if the other party in the call attempts to reestablish the call too quickly (and the mobile station has moved to a new, unknown location area outside its original location area), the system pages the wrong location area since a location update has not been performed.  
      If a mobile station changes location areas while in a call, current systems do not update the current location area for that mobile station. The update is made only when the mobile station explicitly reports its current location area to the system. If the system were to automatically update the location area of the mobile station when the mobile station changes location areas during a call, then it is possible that when the call is terminated, the mobile station would return to idle in the old location area. Since the mobile station in this scenario never reports a change in location areas to the system during the call, it does not report that it moved back to the old location area when the call terminates and, therefore, the system no longer has the current location area of the mobile station. Therefore, current systems do not automatically update the location area of the mobile station.  
      As a result of these problems, FNEs in current systems possess stale information concerning the location of the mobile station for some amount of time after a call terminates and, during the call, the mobile station had moved away from its original location area. Because the information is outdated, current systems are unable to correctly locate the mobile station and call attempts to the mobile station often fail. Consequently, the conversation can not be continued or can be continued only after an inconvenient length of time.  
      Furthermore, in some current systems, if the mobile station does not respond to an initial paging cycle, the system pages adjacent location areas. Such approaches delay call setup as the adjacent location areas are paged only after the initial paging attempt fails, are not efficient as many additional cells are paged, and require additional complexity when configuring the system such that the system has knowledge of which location areas are adjacent to other location areas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a system for tracking the movement of a mobile station according to an embodiment of the present invention;  
       FIG. 2  is a flowchart of one example of an approach for tracking the movement of a mobile station according to an embodiment of the present invention;  
       FIG. 3  is a block diagram of a processing device used to track a mobile station according to an embodiment of the present invention;  
       FIG. 4  is a call flow diagram of an approach for tracking a mobile station according to an embodiment of the present invention; and  
       FIG. 5  is a flow chart of an approach for tracking the movement of a mobile station according to an embodiment of the present invention. 
    
    
      Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. In addition, it should be well understood that different elements may have multiple instances, but are not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A system and method is provided that accurately tracks the location of a mobile station so that ongoing conversations between users that have been interrupted may be continued. In particular, the location of a mobile station is tracked even if the mobile station moves from its original location area to a new cell or location area. Consequently, if the conversation is interrupted during or shortly after the movement of the mobile station to the new cell or location area, the mobile station can be located, a new call can be established, and the conversation can be continued.  
      In many of these embodiments, a location area associated with a mobile station is stored at a first memory location. The location area may be a communication cell or a group of cells. A current cell associated with the mobile station is stored at a second memory location. Both the stored location area and the current cell of the mobile station are simultaneously paged when a request is received to contact the mobile station, for example, a request to establish a new call to continue an interrupted or paused conversation. Since the mobile station is at one of these locations, it will receive the page and, consequently, a conversation that has been interrupted may be continued.  
      In some of these embodiments, the current cell may be identified in an in-call mobility message. Examples of in-call mobility messages include a Service Request message, a handover message, and a Reconnection Request message as used in the Integrated Digital Enhanced Network (iDEN) system manufactured by Motorola, Inc. Other examples of in-call mobility messages are possible. In addition, the second memory location may be cleared whenever the mobile station explicitly updates the network with a current location area. In one example, the clearing may be initiated by Location Update messages, Registration Renewal messages or Dispatch Registration messages (also as used in the iDEN system). Other examples of messages can also be used to clear the second memory location.  
      In addition, the approaches described herein may be performed or implemented within any type of communication system. For example, the approaches described herein may be implemented in dispatch communication systems, cellular communication systems, or combinations of these systems.  
      Thus, a system and method are provided that accurately and efficiently tracks the location of a mobile station so that, after a conversation during a call is interrupted, a new call can quickly be established to continue the conversation without the need to go through additional paging cycles in which adjacent location areas are paged. The approaches described herein are easy to implement and result in conversations being automatically continued even after pauses are made during a call, the call is dropped, and a new call needs to be established.  
      It should be appreciated that with knowledge of the current cell of a mobile station, the system could page only the current cell and if that fails optionally page the old location area and/or page the location area to which the current cell belongs. In the event that the current cell belongs to overlapping location areas, the system may page all of the location areas to which the current cell belongs. Other variations also exist, including, for example, paging all cells of a sectored site when the cell belongs to a sectored site.  
      Referring now to  FIG. 1 , one example of a system for tracking a mobile station so that pages may be made to the correct location of a mobile station is described. The system includes a mobile station  104 , which operates within a location area  102 . The location area  102  may comprise one or more communication cells (not shown for simplicity in  FIG. 1 ).  
      The mobile station  104  may be any type of mobile communication device such as a cellular telephone, pager, personal computer, or personal digital assistant (PDA). Other examples of mobile stations are possible. The mobile station  104  may move back-and-forth between the location area  102  and a cell  108  along a path  106 . As shown in  FIG. 1 , the cell  108  is not located within the location area  102 , but instead is located immediately next to the area  102 . Alternatively, the cell  108  may be positioned at another location, which is a significant distance away from the location area  102 . It will be understood that other cells and location areas may also exist in the system illustrated in  FIG. 1  and that the mobile station  104  may move between these cells and location areas as well.  
      The mobile station  104  transmits signals to a Base Transceiver Station (BTS)  110 . The BTS  110  may include base stations, receivers, transmitters, and other devices that allow the mobile station  104  to communicate with other elements in the system. The BTS  110  is connected to a Dispatch Applications Processor (DAP)  112  and a Base Site Controller (BSC)  114 .  
      The DAP  112  is responsible for tracking the movement of the mobile station  104  for dispatch and packet data services and performing the call control for these services. Additionally, the DAP  112  is responsible for routing data and controlling the operation of the BTS  110  for dispatch and packet data services. In this regard, the DAP  112  may access two memory locations  122  and  124  in a memory  120 . The memory  120  may be any type of memory device and the memory  120  may reside inside or outside of the DAP  112 . In addition, the memory locations  122  and  124  may be located at different memory devices. For instance, one of the memory locations may be located at a Visitor Location Register (VLR), the other memory location may be located at another memory register or both may be located in the VLR.  
      The BSC  114  is responsible for routing data, controlling the operation of the BTS  110  for telephony services and is connected to a Mobile Switching Center (MSC)  116 . The MSC  116  is responsible for tracking the movement of the mobile station  104  for telephony services and performing the call control for these services. In this regard, the MSC  116  may access the two memory locations  122  and  124  in a memory  120 . The memory  120  may be any type of memory device and the memory  120  may reside inside or outside of the MSC  116 . In addition, the memory locations  122  and  124  may be located at different memory devices. For instance, one of the memory locations may be located at a Visitor Location Register (VLR) and the other memory location may be located at another memory register or both may be located in the VLR. The memory locations  122  and  124  may either be shared by the DAP and the MSC or the DAP and the MSC may have their own instances of these memory locations.  
      The MSC  116  also provides switching and gateway functions that allow data to be transmitted and received from other networks. In this regard, the MSC  116  is connected to a Public Switched Telephone Network (PSTN)  118  and the Internet  119 . Users operating various types of equipment (e.g., personal computers and telephones) may, in turn, be connected to the networks  118  and  119 .  
      The following discussion is one example of the operation of the system of  FIG. 1  from the perspective of the DAP  112  for dispatch and packet data services, but those skilled in the art should appreciate that this discussion would equally apply to the MSC  116  and BSC  114  for telephony services and any other similar entity which tracks the current location area of a mobile station  104 .  
      In one example of the operation of the system of  FIG. 1 , the location area  102  (associated with the mobile station  104 ) is stored at a first memory location  122 . A current cell  108  associated with the mobile station  104  is stored at a second memory location  124 . Both the stored location area (at memory location  122 ) and the stored current cell of the mobile station  104  (at memory location  124 ) are paged when a request is received to contact the mobile station  104 .  
      The current cell  108  may be identified to the DAP  112  in an in-call mobility message. Receipt of the in-call mobility message indicates that the mobile station  104  has changed cells while actively employed in a cell or a service. One example of a triggering event that creates the in-call mobility message is a handover event. Another example of an in-call mobility message is a Reconnection Request message (or a Reconnection Request-like message) as used in an iDEN system manufactured by Motorola, Inc. Other examples of in-call mobility messages are possible. The current cell  108  may also be identified to the DAP  112  in a Service Request message (or a Service Request-like message).  
      In addition, the second memory location  124  may be cleared whenever the mobile station  104  explicitly updates the network with a current location area. In one example, the clearing may be initiated by receipt of a Registration Renewal or Dispatch Registration message (also as used in the iDEN system). Other types of messages can also be used to initiate the clearing of the second memory location  124 .  
      Referring now to  FIG. 2 , one example of an approach for tracking the location of a mobile station according to the present invention is described. At step  202 , a location area associated with a mobile station is stored at a first memory location. At step  204 , a current cell associated with the mobile station is stored at a second memory location. This may be accomplished, for example, by receiving an in-call mobility message as described above. At step  206 , the system receives a request to communicate with the mobile station. At step  208 , the system pages both the location area and the current cell of the mobile station after the request has been received to contact the mobile station.  
      At step  210 , the system determines if an update of the stored current cell has occurred. The updating may occur periodically (i.e., after the expiration of a predetermined time period) or immediately after the system detects that the mobile station has moved to a new cell either inside or outside of the original location area (i.e., upon receipt of another in-call mobility message). If the answer at step  210  is affirmative, at step  212 , the current cell is updated. At step  214 , the system waits for the next incoming request and then execution continues at step  206  when a request arrives. If the answer at step  210  is negative, then execution continues at step  214  where the system waits for the next request.  
      Referring now to  FIG. 3 , one example of a device  300  for tracking a mobile station is described. The processing device  300  includes a receiver  302 , a transmitter  304 , and a controller  306 . In this case, the controller  304  may be programmed to send a page  308  to the stored location area and a page  310  to the current cell of the mobile station via the transmitter  304  whenever a request is received at the receiver  302  to contact the mobile station. The stored location area and current cell may be stored in a memory inside or outside of the device  300 .  
      The device  300  may be implemented as a single device or across multiple devices. For example, the device  300  may be incorporated into a DAP. In this case, the DAP receives the location area and current cell information regarding the mobile station and may store this information in an appropriate memory location or locations for later use and paging.  
      Referring now to  FIG. 4 , another example of an approach for tracking the location of a mobile station (MS) is described. This example assumes that a mobile station is operating in a system that is the same or similar to the system portrayed in  FIG. 1 . In this example, while involved in a call, the mobile station moves from its original location area to a new cell outside of the original location area. During, or shortly after the mobile station moves, the on-going call becomes interrupted and, in order to reestablish the call to continue the conversation, a call request is received from the other party involved with the call.  
      At step  402 , sometime prior to the establishment of a call, location area information concerning the mobile station is sent to the DAP from the BTS. This location information identifies the original location area of the mobile station. At step  404 , the location information is sent from the DAP to be stored in a first memory location. As previously described, a dispatch registration message, a registration renewal message, or some other type of message which explicitly updates the location area of the mobile station is used to trigger this approach. The DAP also clears the second memory location associated with the current cell of the mobile station.  
      At step  406 , a call is to be established and the (original) location area is retrieved from the first memory location. No current cell information is stored in the second memory location (in this example). At step  408 , the mobile station is paged and at step  410 , the call is established.  
      At step  412 , an in-call mobility message is sent from the BTS to the DAP and at step  414 , a current cell identity is stored in the second memory location. The message includes the current cell of the mobile station. The current cell information included in the message indicates the identity of the cell (either inside or outside of the original location area) where the mobile station is operating. When this message is received, it indicates that the mobile station has changed cells. As indicated above, examples of in-call mobility messages include a handover message (or handover-like message) and a Reconnection Request message (or a Reconnection Request-like message) as used in the iDEN system. If the new cell is within the original location area of the mobile station, the DAP need not store this current cell information.  
      At step  416 , the call ends. At step  418 , a request to establish a new call in order to resume the conversation (or from another mobile station to start a new conversation) is received at the DAP from the BTS before the mobile station has had a chance to complete a location updating procedure as described previously. At step  420 , the location stored at step  404  (“original location”) is retrieved from the first memory location and at step  422 , the current cell is retrieved from the second memory location.  
      At step  424 , both the original location area (including all cells within the original location area) and the current cell are simultaneously paged. Alternatively, rather than paging one additional cell, the system may page several surrounding cells or may page the entire location area which contains the new current cell of the mobile station. At step  426 , a new call is established to continue the conversation since the mobile station will be either within the location area or the current cell.  
      Referring now to  FIG. 5 , an approach for tracking the movement of a mobile station is described. At step  502 , a Dispatch Registration message is received. At step  504 , the new location area of the mobile station is stored. At step  506 , the system becomes idle.  
      At step  508 , a service request is received from the mobile station. At step  510 , it is determined if the cell identified in the request is outside of the stored location area. If the answer is negative, control continues with step  514 . If the answer is affirmative, execution continues at step  512 .  
      At step  512 , the cell identified in the request is stored. At step  514 , the mobile station is engaged in a call. The call may end at step  518  in which case the system becomes idle at step  520 . However, if an in-call mobility message is received at step  516 , at step  522 , it is determined if the current cell is outside of the location area stored at step  504  or step  532 . If the answer is negative, then at step  526  the mobile station continues in the call. If the answer is affirmative at step  524 , the current cell is stored and at step  526 , the mobile station continues in the call.  
      If the system receives a Dispatch Registration message at step  528  or a Registration Renewal message at step  530 , the current location area is stored at step  532 . At step  534 , the current cell is cleared. At step  536 , the system becomes idle.  
      If a Service Request for the mobile station is received at step  538 , at step  540  the current location area of the mobile station is paged. At step  542 , it is determined if the current cell has been stored. If the answer is negative, control continues with step  546 . If the answer is affirmative, at step  544 , the current cell is paged. At step  546 , the mobile station is engaged in a call.  
      Thus, a system and method are provided that accurately track the location of a mobile station so that pages are likely always made to the actual physical location of the mobile station. Consequently, after a conversation has been interrupted, a new call is conveniently established to allow the conversation to continue even though the mobile station has switched location areas and/or cells and has not yet been able to inform the network of its new location.  
      Those skilled in the art will recognize that a wide variety of modifications, alterations, and combinations can be made with respect to the above described embodiments without departing from the spirit and scope of the invention, and that such modifications, alterations, and combinations are to be viewed as being within the scope of the invention.