Patent Publication Number: US-2011053599-A1

Title: System, apparatus, and method for handover procedure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit, pursuant to 35 U.S.C. §119(e), of provisional U.S. Patent Application Ser. No. 61/238,160, filed Aug. 29, 2009, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system, an apparatus, and a method for handover procedure, and more particularly, to the technical field regarding the femto base station. 
     2. Description of the Prior Art 
     The IEEE 802.16 standard working group and LTE standard have included the indoor femto BS (Base Station) for the sakes of throughput enhancement, coverage extension and load balancing. The major difference between indoor femto BS and outdoor BS is the media type of backhaul link. Femto BS uses the wired connection, such as ADSL and Cable link, as the backhaul link. 
     The handover procedure adopted in the IEEE 802.16e, is a technique, which allows an MS (Mobile station) to change the serving BS when the channel condition between MS and serving BS is unsatisfactory. Unlike the outdoor BS, the backhaul link of femto BS may disconnect to the backbone network sometimes. If the backhaul link of femto BS fails, the femto BS can not provide any service to associated MSs and thereby every associated MS should be handed over from the serving femto BS to the other BS (such as outdoor BS or another femto BS) in order to provide continuous services even the channel condition between MS and serving femto BS is still satisfactory. However, the conventional handover procedure is designed to force an individual MS to disassociate with its serving BS and re-associated with target BS (maybe outdoor BS or another femto BS) without any reason and the MS may insist to associate with the original serving BS or immediately perform handover back to the original serving BS because the channel quality between it and the original serving BS is better than the others. It is evident that the design of the conventional handover procedure is not suitable for the femto system. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a method to command at least one mobile station to efficiently leave from an original femto base station when it can not provide service to mobile stations (such as backhaul connection failed, power down, frequency change and so on), and return to the original femto base station when the original femto base station can provide service again (such as backhaul connection recovered and power on). 
     To achieve the aforementioned object, the present invention provides a method for a handover procedure, comprising: detecting a status of a backhaul connection between an original femto base station and a backbone network; and sending a handover request message including a reason code to at least one mobile station served by the original femto base station, then the at least one mobile station re-associates with at least one base station other than the original femto base station. 
     Another object of the present invention is to provide a system to command at least one mobile station to efficiently leave from the original femto base station when it can not provide service to mobile stations (such as backhaul connection failed, power down, frequency change and so on), and return to the original femto base station when the original femto base station can provide service again (such as backhaul connection recovered and power on). 
     To achieve the aforementioned object, the present invention provides a system for a handover procedure, comprising: a backbone network; an original femto station base connected to the backbone network via a backhaul connection; and at least one mobile station connected to the original femto base station; wherein the original femto base station sends a handover request message including a reason code to the at least one mobile station when the original femto base station detects a failure of the backhaul connection; and wherein the at least one mobile station re-associates with at least one base station other than the original femto base station when the at least one mobile station receives the handover request message. 
     Further another object of the present invention is to provide an apparatus to command at least one mobile station to efficiently leave from an original femto base station when it can not provide service to mobile stations (such as backhaul connection failed, power down, frequency change and so on), and return to the original femto base station when the original femto base station can provide service again (such as backhaul connection recovered and power on). 
     To achieve the aforementioned object, the present invention provides a femto base station for serving at least one mobile station, comprising: an antenna; a RF circuit electrically connected to the antenna; a baseband processor electrically connected to the RF circuit; a media access controller including a reliability component electrically connected to the baseband processor; and an ADSL/CATV modem electrically connected to the media access controller and connected to a backhaul connection; wherein the femto base station sends a handover request message including a reason code and/or flag to the at least one mobile station when the reliability component detects a failure of the backhaul connection; and wherein the at least one mobile station re-associates with at least one base station other than the femto base station when the at least one mobile station receives the handover request message. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a system for handover procedure according to one embodiment of the present invention. 
         FIG. 2  is a message flow graph for handover procedure according to one embodiment of the present invention 
         FIG. 3  is a message flow graph for handover procedure according to another embodiment of the present invention. 
         FIG. 4  is a block diagram of a femto BS according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a system, method and apparatus for femto BS to drive associated MSs to another BS when the femto BS loses the backhaul connection and then callback the previously associated MSs after the femto BS recovers its backhaul connection. Advantages of the present invention will become apparent from the following detailed description. 
       FIG. 1  illustrates a schematic diagram of a system for handover procedure according to one embodiment of the present invention. The system comprises an indoor femto BS  101 , backbone network  102 , three MSs  103  (MS 1 , MS 2 , MS 3 ), a network mobility controller  104 , and other BS  105 . The MSs  103  is one type of subscriber station which has mobility, such as mobile phone. The femto BS communicates with the MSs  103  via a wireless connection  107  and connects to the backbone network  102  via a wired backhaul connection  106 , hence the MSs  103  can connect to the backbone network  102  via the femto BS  101 . The femto BS  101  also can communicate with the other BSs  105  and network mobility controller  104  via the backhaul network  102 . The other BS  105  includes macro BS and/or femto BS other than the femto BS  101  that the MSs  103  can connect to. The femto BS  101  commands the MSs  103  to perform handover to the other BS when the backhaul connection  106  between the femto BS  101  and the backbone network  102  failed and commands the MSs  103  to perform handover back to the original femto BS  101  after the backhaul connection  106  is recovered. 
       FIG. 2  illustrates a message flow graph for handover procedure according to one embodiment of the present invention. As the femto BS  101  detects a failure of the backhaul connection  106 , the femto BS  101  sends a handover request message (AAI-HO_CMD) to the MSs  103  in broadcast, multicast, or unicast manner. The handover request message includes a reason code and/or specific flag used to notify the MSs  103  to handover to new BS, record the original femto BS  101  information and wait for a callback handover message. The reason code indicates the cause of the failure of the backhaul connection  106  including power down, power reduction, frequency assignment change, backhaul connection down, and so on. After the MSs  103  receive the handover request message, the MSs  103  re-associate with the other BS  105  and wait for the callback handover message, wherein the other BS  105  includes macro BS and/or femto BS other than the original femto BS  101  that the MSs  103  can connect to. 
     As the backhaul connection  106  is fully recovered, the femto BS  101  sends a callback handover request message (HO_REQ) to the other BS  105  for notifying the MSs  103  about the callback notification. It should be mentioned that the femto BS  101  knows which BS (i.e. the other BS  105 ) connected with MSs  103  because the femto BS  101  sending a BS list with the handover request message at the same time to the MSs  103 . Alternatively, the femto BS  101  can know which BS (i.e. the other BS  105 ) connected with MSs  103  via the network mobility controller  104  or a few of BS in a specific area covering the MSs  103 . After the other BS  105  receives the callback handover request message, the other BS  105  sends a receipt of the callback handover request message (HO_RSP) to the original femto BS  101  in response to the received callback handover request message and then sends a callback handover command message (AAI-HO_CMD) with reason code to the MSs  103 . Upon the MSs  103  receiving the callback handover command message and callback reason from the other BS  105 , the MSs  103  will sends an indication message (AAI-HO_IND) to the other BS  105  to indicate the handover event if the handover policy decides to handover back to the original femto BS  101 . Finally, the MSs  103  can know which BS is the original femto BS  101  and re-associates with the original femto BS  101  because the MSs  103  have recorded the information of the original femto BS  101 . In one embodiment of the present invention, the other BS  105  may sends information of the original femto BS  101  with the callback handover command message to the MSs  103  at the same time so as to notify the MSs  103  that which BS (i.e. the original femto BS) they can connect to if the MSs  103  did not or can not record the information of the original femto BS  101 . 
     Refer to  FIG. 4  illustrating a block diagram of the femto BS  101  according to one embodiment of the present invention, the femto BS  101  includes an antenna  301 , a RF circuit  302 , a baseband processor  303 , a media access controller (MAC)  304 , and an ADSL/CATV modem  306 . The RF circuit  302  electrically connects to the antenna  301  and the baseband processor  303 . The MAC  304  including a reliability component  305  electrically connects to the baseband processor  303  and the ADSL/CATV modem  306 . The reliability component  305  is used to detect a status (failure or recovery) of the backhaul connection  106  and identify cause of failure of the backhaul connection  106 . The reliability component  305  may connect to other components of the femto BS  101  to identify cause of failure of the backhaul connection  106 . For example, the reliability component  305  electrically connects to the ADSL/CATV modem  306  to detect whether the backhaul connection  106  is down or not. For another example, the reliability component  305  electrically connects to a power control circuit (not shown) to detect whether the power is down or not. The femto BS  101  generates the reason code according to cause of failure of the backhaul connection  106  detected by the reliability component  305 . 
       FIG. 3  illustrates a message flow graph for handover procedure according to another embodiment of the present invention. As the femto BS  101  detects a failure of the backhaul connection  106 , the femto BS  101  sends a handover request message (AAI-HO_CMD) to the MSs  103  in broadcast, multicast, or unicast manner. The handover request message includes a reason code and/or specific flag used to notify the MSs  103  to handover to new BS, record the original femto BS  101  information and wait for a callback handover message. The reason code indicates the cause of the failure of the backhaul connection  106  including power down, power reduction, frequency assignment change, backhaul connection down, and so on. After the MSs  103  receive the handover request message, the MSs  103  re-associate with the other BS  105  and wait for the callback handover message, wherein the other BS  105  includes macro BS and/or femto BS other than the original femto BS  101  that the MSs  103  can connect to. The other BS  105  may notify the network mobility controller  104  about the handover events of MSs. 
     As the backhaul connection  106  is fully recovered, the original femto BS  101  sends a re-registration message to the network mobility controller  104  in order to trigger the network mobility controller  104  to send a callback handover command message (HO_REQ) to the other BS  105  for notifying the other BS  105  about the callback notification. After the other BS  105  receives the callback handover request message, the other BS  105  sends a receipt of the callback handover request message (HO_RSP) to the network mobility controller  104  in response to the received callback handover request message and then sends a callback handover command message (AAI-HO_CMD) to the MSs  103 . Upon the MSs  103  receiving the callback handover command message and callback reason from the other BS  105 , the MSs  103  will sends an indication message (AAI-HO_IND) to the other BS  105  to indicate the handover event if the handover policy decides to handover back to the original femto BS  101 . Finally, the MSs  103  re-associates with the original femto BS  101 . In one embodiment of the present invention, the other BS  105  may sends information of the original femto BS  101  with the callback handover command message to the MSs  103  at the same time so as to notify the MSs  103  that which BS (i.e. the original femto BS) they can connect to if the MSs  103  did not or can not record the information of the original femto BS  101 . 
     Several features and aspects of the present invention have been illustrated and described in detail with reference to particular embodiments by way of example only, and not by way of limitation. Those of skill in the art will appreciate that alternative implementations and various modifications to the disclosed embodiments are within the scope and contemplation of the present disclosure. Therefore, it is intended that the invention be considered as limited only by the scope of the appended claims.