Abstract:
Aspects of the present invention relate to improved systems and methods for handing over a UE from a source node to a target node. In some embodiments, the target node receives UL and DL count information directly from the UE being handed over, rather than from the source node or from a core network node.

Description:
TECHNICAL FIELD 
       [0001]    The present invention related to the field of wireless communication systems. More specifically, aspects of the present invention relate to systems and methods for handing over a user equipment between nodes. 
       BACKGROUND 
       [0002]    In the 3rd Generation Partnership Project (“3GPP”) Long Term Evolution (“LTE”) protocol, a user equipment (UE) connects to the core network via nodes (e.g. e-UTRAN NodeB nodes). Typically, the UE connection is maintained over a single node. However, as the UE moves geographically or other network conditions occur, it may be desirable or necessary to transfer the UE connections from the source node (the node to which the UE is currently connected) to an available target node (i.e., perform a handover from the source node to the target node). 
         [0003]    The UE connection with the source node is typically encrypted and the cipher requires uplink (“UL”) and downlink (“DL”) counts for each data unit, which comprise sequence numbers and/or a Hyper Frame Numbers (“HFN”). Thus, during a handover the source node must to transfer the UL and DL count values to the target node in order to enable the encryption features of the target node. In the standard 3GPP LTE protocol, this transfer is done in one of two ways depending upon the protocols enabled within the nodes. 
         [0004]    If the nodes are configured to operate according to the S1 Application Protocol (“S1AP”), then the transfer of the UL and DL counts occurs via a two step process. First, during an eNB Status Transfer, the UL and DL counts are transmitted from the source node to a Mobile Management Entity (“MME”) connected to the core network. Then, during an MME Status Transfer, the UL and DL counts are transmitted from the MME to the target node. 
         [0005]    Alternatively, if the nodes are configured to operate according to the X2 Application Protocol (“X2AP”), then the transfer of the UL and DL counts occurs via a SN Status Transfer. During the SN Status transfer, the UL and DL counts are transmitted directly from the source node to the target node. 
         [0006]    After a source node has sent the UL and DL counts to the target node, it must cease scheduling UE data (e.g. data received from the UE or data intended for the UE) so that the UL and DL counts remain accurate when they are received by the target node. It can also occur that a UE connects to a target node (and thus has disconnected from the source node) before the target node has received the UL and DL counts from the source node. In this situation, the UE cannot send or receive any data units via the target node until the target node acquires the UL and DL counts. Thus, during the handover process there can be an undesirable service interruption while the UE waits for the UL and DL counts to be transferred between the source and target nodes. 
         [0007]    Furthermore, there may be a finite amount of time between when the UE disconnects from the source node (i.e., the source node stops scheduling UE data) and when the core network is notified that the UE has connected to the target node. During this time period, any DL data units intended for the UE may be sent to the wrong node or lost. 
       SUMMARY 
       [0008]    Aspects of the invention provide improved systems and methods for handing over a UE from a source node to a target node. In some embodiments, the target node receives the UL and DL count information from the UE rather than other nodes in the network. In some embodiments, the source node continues to schedule UE data during the handover process and forwards the scheduled UE data to the target node to ensure all UE data is received. 
         [0009]    Thus in one aspect, the invention provides an improved method performed by a source node for handing over a UE from a source node to a target node. In some embodiments, the improved method performed by the source node includes: (a) transmitting from the source node to another node a first handover message that indicates that the UE requires a handover; (b) receiving at the source node a second handover message transmitted from the another node in response to the first handover message; (c) in response to receiving the second handover message, transmitting from the source node a reconfiguration message to the UE; (d) after transmitting the reconfiguration message from the source node to the UE, continuing to schedule the UE in the source node until a particular message is received at the source node; and (e) after performing step (d), receiving at the source node a context release message transmitted from the another node indicating that the UE has been handed over to the target node. 
         [0010]    In some embodiments, the another node is the target node. In such embodiments, the first handover message may be an X2AP Handover Request message, the second handover message may be an X2AP Handover Request Acknowledge message, and the context release message may be an X2AP UE Context Release message. In addition, the particular message may be an end marker transmitted from a gateway. 
         [0011]    In some embodiments, the another node is an MME. In such embodiments, the first handover message may be an S1AP Handover Required message, the second handover message may be an S1AP Handover Command message, and the context release message may be an S1AP UE Context Release Command message. Additionally, the particular message may be the S1AP UE Context Release Command message 
         [0012]    In some embodiments, the improved method further comprises transmitting from the source node to the MME an S1AP UE Context Release Complete message in response to the S1AP UE Context Release Command message. 
         [0013]    In some embodiments, the source node is configured such that the source node does not transmit to the MME a status transfer message in response to receiving the S1AP Handover Command message from the MME. 
         [0014]    In some embodiments, the method further comprises a step of: (f) transmitting UE data from the source node to the target node after transmitting the reconfiguration message to the UE, wherein the UE data comprises data received from the UE or data intended for the UE. 
         [0015]    In some embodiments, the method further comprises a step of: (f) after transmitting the reconfiguration message from the source node to the UE, continuing to (i) assign sequence numbers to DL data units intended for the UE and (ii) deliver data units received from the UE to a core network node until the particular message is received at the source node. 
         [0016]    In another aspect, the invention provides a method performed by a UE. In some embodiments, the method includes the following steps: (a) establishing a connection with a source node; (b) after establishing the connection, transmitting data units to and receiving data units from the source node; (c) after step (b), receiving from the source node a reconfiguration message identifying a target node; and (d) after receiving the reconfiguration message, transmitting to the target node a message (e.g. a reconfiguration complete message) comprising a DL count value and an UL count value. 
         [0017]    In some embodiments, the DL count value comprises (1) a sequence number and/or (2) a HFN that the target node should assign to a DL data unit that does not have a sequence number assigned to it and that is intended for the UE; and the UL count value comprises (1) a sequence number and/or (2) a HFN. 
         [0018]    In some embodiments, the method further comprises the steps of: (e) after receiving the reconfiguration message, receiving a resource allocation from the source node; and (f) using the allocated resources to transmit a data unit to the target node. 
         [0019]    In another aspect, the invention provides a method performed by a target node. In some embodiments, the method includes the following steps: (a) receiving at the target node a handover request message transmitted from another node; (b) transmitting to the another node a handover acknowledge message in response to the handover request message; (c) after transmitting the handover acknowledge message and before scheduling the UE in the target node, receiving at the target node a message transmitted from the UE comprising a DL count value and a UL count value; and (d) after receiving the message from the UE, using the DL count value and/or the UL count value in a communication with the UE. 
         [0020]    In some embodiments, the message transmitted from the UE is a reconfiguration complete message that was transmitted by the UE to the target node in response to a reconfiguration message transmitted to the UE from the source node. 
         [0021]    In some embodiments, the DL count value comprises (1) a sequence number and/or (2) a HFN that the target node should assign to a DL data unit that does not have a sequence number assigned to it and that is intended for the UE, and the UL count value comprises (1) a sequence number and/or (2) a HFN. 
         [0022]    In some embodiments, the method further comprises: receiving from the source node a DL data unit for the UE, and determining whether the DL data unit should be transmitted to the UE, wherein the determination is based on the DL count value received from the UE. 
         [0023]    In another aspect, the invention provides an improved access point (e.g., base station or other access point). In some embodiments, the improved access point includes: (a) a first receiver for receiving data transmitted from a user equipment (UE); (b) a first transmitter for transmitting data to the UE; (c) a second transmitter for transmitting data to a node; (e) a second receiver for receiving data from the node; and (f) a data processing system operable to: (i) transmit to the node a first handover message that indicates that the UE requires a handover; (ii) receive a second handover message transmitted from the node in response to the first handover message; (iii) transmit a reconfiguration message to the UE in response to receiving the second handover message; (iv) continue to schedule the UE after transmitting the reconfiguration message to the UE, until a particular message is received; and (v) receive a context release message transmitted from the another node indicating that the UE has been handed over to a target node. 
         [0024]    In another aspect, the invention provides an improved UE. In some embodiments, the improved UE includes: (a) a transmitter for transmitting data; (b) a receiver for receiving data; and (c) a data processing system operable to (i) establish a connection with a source node; (ii) transmit data units to and receive data units from the source node after establishing the connection; (iii) receive from the source node a reconfiguration message identifying a target node; and (iv) transmit to the target node a message comprising a downlink (DL) count value and an uplink (UL) count value in response to receiving the reconfiguration message. 
         [0025]    The above and other aspects and embodiments are described below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. 
           [0027]      FIG. 1  illustrates a wireless communication network according to an embodiment of the invention. 
           [0028]      FIG. 2  is a flow chart illustrating a process according to some embodiments of the invention. 
           [0029]      FIG. 3  illustrates a message flow according to an embodiment of the invention. 
           [0030]      FIG. 4  is a flow chart illustrating a process according to some embodiments of the invention. 
           [0031]      FIG. 5  illustrates a message flow according to an embodiment of the invention. 
           [0032]      FIG. 6  is a block diagram that illustrates a node according to an embodiment of the invention. 
           [0033]      FIG. 7  is a block diagram that illustrates a UE according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Referring to  FIG. 1 ,  FIG. 1  illustrates a block diagram of a wireless communication network  100 . As shown in  FIG. 1 , the wireless communication network  100  connects a UE  102  with a core network  110  via a plurality of nodes  104   a ,  104   b  (e.g., base stations  104   a ,  104   b  or other access points) connected to an MME  106 . As shown in  FIG. 1 , the UE  102  is in wireless communication with a source node  104   a . In some embodiments, the UE  102  can be a cellular telephone handset, a smartphone, a PDA, or other wireless device configured to interoperate with the wireless network  100 . Each of the nodes  104   a ,  104   b  communicates with the MME  106 , e.g. for sending and receiving UE data and for sending and receiving network management messages. In some embodiments, the nodes  104   a ,  104   b  communicate with the MME  106  according to the S 1  application protocol. Also as shown in  FIG. 1 , in some embodiments the nodes  104   a ,  104   b  may communicate with each other, e.g. for sending and receiving data traffic of the UE  102  and for sending and receiving network management messages. In some embodiments, the nodes  104   a ,  104   b  communicate with each other according to the X2 application protocol. 
         [0035]    Referring to  FIG. 2 ,  FIG. 2  illustrates an improved handover process  200  for transferring the connection for a UE  102  between a source node (e.g., node  104   a ) and a target node (e.g., node  104   b ) according to some embodiments of the invention. 
         [0036]    In the embodiment shown, the handover process  200  begins at step  202  when the source node  104   a  transmits an S1AP Handover Required message  301  (see  FIG. 3 , which shows a message flow according to an embodiment of the invention) to the MME  106 , indicating that the UE  102  requires a handover. 
         [0037]    In response to receiving the S1AP Handover Required message  301  from the source node  104   a , at step  204  the MME  106  transmits an S1AP Handover Request message  302  to the target node  104   b.    
         [0038]    In response to receiving the S1AP Handover Request message  302  from the MME  106 , at step  206  the target node  104   b  returns an S1AP Handover Request Acknowledge message  303  to the MME  106 . 
         [0039]    After the MME  106  receives the S1AP Handover Request Acknowledge message  303  from the target node  104   b , at step  208  the MME  106  transmits an S1AP Handover Command message  304  to source node  104   a.    
         [0040]    Upon receiving the S1AP Handover Command message  304  from the MME  106 , at step  210  the source node  104   a  transmits a Radio Resource Control (“RRC”) Connection Reconfiguration message  305  to the UE  102 . 
         [0041]    In response to receiving the Connection Reconfiguration message  305  from the source node  104   a , the UE  102  performs the reconfigurations necessary for executing the handoff. When the reconfigurations are complete, at step  212  the UE transmits a Connection Reconfiguration Complete message  306  to the target node  104   b . The Connection Reconfiguration Complete message  306  includes a DL count value and an UL count value. In some embodiments, the DL count value includes (1) a sequence number and/or (2) a Hyper Frame Number (HFN) that the target node should assign to a DL data unit that does not have a sequence number assigned to it and that is intended for the UE, and the UL count value comprises (1) a sequence number and/or (2) a Hyper Frame Number (HFN). In some embodiments, the Connection Reconfiguration Complete message  306  comprises at least the information in an RRC Connection Reconfiguration Complete message, the DL count, and the UL count. 
         [0042]    After the target node  104   b  receives the Reconfiguration Complete message  306  from the UE  102 , at step  214  the target node begins scheduling traffic for the UE  102 . For example, in some embodiments the target node  104   b  may allocate resources for the upload and download of data between the UE  102  and the external network. Furthermore, at step  214  the target node  104   b  transmits an S1AP Handover Notify message  307  to the MME  106 . 
         [0043]    In response to receiving the Handover Notify message  307  from the target node  104   b  indicating that the UE  102  is in communication with the target node  104   b , at step  216  the MME  106  transmits an S1AP UE Context Release Command message  308  to the source node  104   a  indicating that the UE  102  has been handed over to the target node  104   b.    
         [0044]    After receiving the Context Release Command message  308 , at step  218  the source node  104   a  ceases scheduling traffic for the UE  102  and transmits an S1AP UE Context Release Complete message  309  to the MME  106 . 
         [0045]    As set forth above, in some embodiments after the source node  104   a  transmits the Connection Reconfiguration message  305 , the source node  104   a  may continue to schedule traffic for the UE  102  until the source node  104   a  receives the Context Release Command message  308 . 
         [0046]    Referring again to  FIG. 3 ,  FIG. 3  illustrates a message flow according to some embodiments of the invention. As shown in  FIG. 3 , after the source node  104   a  transmits the Connection Reconfiguration message  305 , it forwards UE data  310 . In some embodiments, this may comprise continuing to deliver to the MME data units received from the UE until the Context Release Command message  308  is received. Additionally, the source node  104   a  may continue to assign sequence numbers to DL data units intended for the UE and transmit these units to the target node. The target node  104   b  determines whether to transmit forwarded packets to the UE based upon a comparison between the DL count information received in the Connection Reconfiguration Complete message  306  and the sequence numbers of the forwarded packets. 
         [0047]    Referring to  FIG. 4 ,  FIG. 4  illustrates an improved handover process  400  for transferring the connection for a UE  102  between a source node (e.g., node  104   a ) and a target node (e.g., node  104   b ) according to additional embodiments of the invention. 
         [0048]    The handover process  400  begins at step  402  when the source node  104   a  transmits an X2AP Handover Request message  501  (see  FIG. 5 , which shows a message flow according to an embodiment of the invention) to the target node  104   b , indicating that the UE  102  requires a handover. 
         [0049]    In response to receiving the Handover Required message  501  from the source node  104   a , at step  404  the target node  104   b  transmits an X2AP Handover Request Acknowledge message  503  to the source node  104   a.    
         [0050]    Upon receiving the Handover Request Acknowledge message  503  from the target node  104   b , at step  406  the source node  104   a  transmits a Radio Resource Control (“RRC”) Connection Reconfiguration message  505  to the UE  102 . 
         [0051]    After the source node  104   a  transmits the Connection Reconfiguration message  505 , at step  408  the source node  104   a  forwards UE data  510  to the target node  104   b.    
         [0052]    In response to receiving the Connection Reconfiguration message  505  from the source node  104   a , the UE  102  performs the reconfigurations necessary for executing the handoff. When the reconfigurations are complete, at step  410  the UE transmits a Connection Reconfiguration Complete message  506  to the target node  104   b . The Connection Reconfiguration Complete message  506  includes a DL count value and an UL count value. In some embodiments, the Connection Reconfiguration Complete message  306  comprises at least the information in a RRC Connection Reconfiguration Complete message, the DL count, and the UL count. 
         [0053]    After the target node  104   b  receives the Reconfiguration Complete message  506  from the UE  102 , at step  412  the target node begins scheduling traffic for the UE  102 . For example, in some embodiments the target node  104   b  may allocate resources for the upload and download of data between the UE  102  and the external network. Furthermore, at step  412  the target node  104   b  transmits an S1AP Path Switch Request message  507   a  to the MME  106 . 
         [0054]    In response to receiving the Path Switch Request message  507   a  from the target node  104   b  indicating that the UE  102  is in communication with the target node  104   b , at step  414  the MME  106  transmits a User Plane Update Request  507   b  to the serving gateway (“S-GW”). 
         [0055]    At step  416 , in response to receiving the User Plane Update Request  507   b  from the MME  106  the S-GW transmits a User Plane Update Response message  508   a  to the MME  106 . Additionally in step  416 , the S-GW transmits an end marker  511  to the source node  104   a.    
         [0056]    After receiving the end marker  511 , at step  418  the source node  104   a  ceases scheduling traffic for the UE  102 . After the source node  104   a  completes this process, it transmits an end marker message  512  to the target node  104   b.    
         [0057]    In response to receiving the User Plane Update Response message  508   a , at step  420 the MME  106  transmits an S1AP Patch Switch Acknowledge message  508   b  to the target node  104   b.    
         [0058]    After the target node  104   b  receives the Patch Switch Acknowledge message  508   b , at step  422  the target node  104   b  receives additional new UE data from the S-GW and transmits the UE data to the UE  102 . 
         [0059]    After the above steps have completed, at step  424  the target node  104   b  transmits a X2AP UE Context Release message  508   c  to the source node  104   a.    
         [0060]    Referring now to  FIG. 6 ,  FIG. 6  is a functional block diagram of a node  104  according to some embodiments of the invention. As shown, the node  104  may comprise a data processing system  602  (e.g., one or more microprocessors), a data storage system  606  (e.g., one or more non-volatile storage devices) and computer software  608  stored on the storage system  306 . Configuration parameters  610  may also be stored in storage system  606 . The node  104  also includes transmit/receive (Tx/Rx) circuitry  604  and  605  for transmitting data to and receiving data from the UE  102 , and the MME  106 , respectively. 
         [0061]    The software  608  is configured such that when the processing system  602  executes software  608 , node  104  performs steps described herein (e.g., steps described above with reference to the flow chart shown in  FIG. 2  or  4 ). For example, the software  608  may include: (1) computer instructions for transmitting to another node a first handover message that indicates that the UE requires a handover; (2) computer instructions for receiving a second handover message transmitted from the another node in response to the first handover message, (3) computer instructions for transmitting from the source node a reconfiguration message to the UE in response to receiving the second handover message; (4) computer instructions for continuing to schedule the UE after transmitting the reconfiguration message until a particular message is received; and (5) computer instructions for receiving a context release message transmitted from the another node indicating that the UE has been handed over to the target node. 
         [0062]    Additionally, the software  608  may include: (1) computer instructions for receiving a handover request message transmitted from another node; (2) computer instructions for transmitting to the another node a handover acknowledge message in response to the handover request message; (3) computer instructions for receiving at the target node a message transmitted from the UE comprising a DL count value and a UL count value after transmitting the handover acknowledge message and before scheduling the UE in the target node; and (4) using the DL count value and/or the UL count value in a communication with the UE after receiving the message from the UE. 
         [0063]    Referring now to  FIG. 7 ,  FIG. 7  is a functional block diagram of a UE  102  according to some embodiments of the invention. As shown, the UE  102  may comprise a data processing system  702  (e.g., one or more microprocessors), a data storage system  706  (e.g., one or more non-volatile storage devices) and computer software  708  stored on the storage system  706 . Configuration parameters  710  may also be stored in storage system  706 . The UE  102  also includes transmit/receive (Tx/Rx) circuitry  704  for transmitting data to and receiving data from node  104 , respectively. 
         [0064]    The software  708  is configured such that when the processing system  702  executes software  708 , UE  102  performs steps described herein (e.g., steps described above with reference to the flow chart shown in  FIG. 2  or  4 ). For example, the software  708  may include: (1) computer instructions for establishing a connection with a source node; (2) computer instructions for transmitting data units to and receiving data units from the source node after establishing the connection; (3) computer instructions for receiving from the source node a reconfiguration message identifying a target node; and (4) computer instructions for transmitting to the target node a message comprising a DL count value and an UL count value after receiving the reconfiguration message. 
         [0065]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above described exemplary embodiments. 
         [0066]    Additionally, while the process described above and illustrated in the drawings is shown as a sequence of steps, this was done solely for the sake of illustration. Accordingly, it is contemplated that some steps may be added, some steps may be omitted, the order of the steps may be re-arranged, and some steps may be performed in parallel.