Abstract:
A mobile station which is participating in a Multimedia Broadcast Multicast Service session, and which will continue its session participation in a new cell, need not wirelessly obtain from the new cell an identifier to use for the session in the new cell. Rather, the mobile station wirelessly obtains the identifier from the cell that is to be exited in favor of the new cell. This can reduce the session interruption associated with the cell change.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to wireless communications and, more particularly, to Multimedia Broadcast Multicast Service (MBMS) in wireless communications.  
       BACKGROUND OF THE INVENTION  
       [0002]     Multimedia Broadcast Multicast Service (MBMS), well known in the art of wireless communication, supports broadcast and multicast data communication to multiple users at the same time using shared network resources. This saves resources in the core network and the radio access network. Broadcast services typically transmit data to a certain area so that any users in that area can receive the data. Multicast services typically transmit data that is to be received only by those users who subscribe to the multicast service. Multicast service subscribers typically use an encryption key or other suitable mechanism to decipher the received multicast data.  
         [0003]     The 3rd Generation Partnership Project (3GPP) has specified the provision of MBMS in the GSM/EDGE Radio Access Network (GERAN). The formal specification document, 3GPP Technical Specification 43.246 “Multimedia Broadcast Multicast Service (MBMS) in the GERAN”, is incorporated herein by reference. Related specification documents 3GPP Technical Specification 44.060 “Radio Link Control/Medium Access Control (RLC/MAC) Protocol”, and 3GPP Technical Specification 48.018 “BSS GPRS Protocol (BSSGP)” are also incorporated herein by reference.  
         [0004]     GERAN can support MBMS sessions according to either a feedback mode or a non-feedback mode. In the non-feedback mode, the base station system (BSS) transmits RLC blocks without receiving any feedback from the mobile stations (MSs) regarding reception of the RLC blocks. In order to limit the error rate in the MSs, the BSS typically retransmits each RLC block some predetermined number of times. In feedback mode, the MSs send acknowledgement messages on the RLC layer to the BSS, so the BSS can retransmit only those RLC blocks that have not been received by all MSs. In any given cell that is supporting an MBMS session in feedback mode, each MS that is participating in the MBMS session is allocated an associated identifier (MS_ID). The BSS uses the MS_IDs to request feedback acknowledgement reports from the respective MSs.  
         [0005]     In order to limit the interruption in an MS&#39;s reception of an ongoing MBMS session when the MS moves from one cell to another, the BSS provides, to all MSs that are currently participating in a given MBMS session, information indicative of the location of the bearers that are supporting the MBMS session in neighboring cells. Thus, upon changing cells, an MS can begin listening to the MBMS session in the new cell relatively quickly.  
         [0006]     In the aforementioned feedback mode, the MSs send to the BSS measurement reports for the strongest neighboring cells. If an MS has triggered cell reselection to a neighboring cell, then the MS uses a measurement report to inform the BSS of that cell reselection. The BSS can then give priority to transmitting, in the “old” cell, the location of the corresponding MBMS bearer in the new cell. In such instances, the MS delays transitioning into the new cell up to one second, in order to provide an opportunity to receive from the BSS the MBMS bearer location information for the new cell.  
         [0007]     If the new cell also uses the feedback mode, then the MS must perform an access request in the new cell in order to request an MS_ID (and timing advance parameters) to use during participation in the MBMS session in the new cell. This access request, which is made via the CCCH (Common Control Channel) or PCCCH (Packet Common Control Channel) in the new cell, can result in an interruption of the MS&#39;s reception of the MBMS session. The MS will not be able to provide feedback on the RLC layer until an MS_ID has been allocated to it.  
         [0008]     There have also been proposed MBMS bearers that do not require the MSs to listen to the CCCH/PCCCH in parallel with the MBMS bearer. This implies system configurations and/or operating modes wherein the MS cannot listen to the CCCH/PCCCH and the MBMS bearer at the same time. In such instances, the interruption of the MS&#39;s participation in the MBMS session would be extended, because the MS would need to remain in non-DRX mode for some time.  
         [0009]     It is desirable in view of the foregoing to provide for reducing the amount of MBMS session interruption that occurs when an MS switches from one cell to another where feedback mode is used.  
         [0010]     According to exemplary embodiments of the invention, an MS entering a new cell need not perform an access request procedure in the new cell to obtain an MS_ID for use in that cell. Rather, the MS_ID for the new cell is provided to the MS while the MS is still in communication with the cell that will be exited in favor of the new cell. This can reduce the session interruption associated with the cell change. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  illustrates exemplary operations that can be performed according to the invention when a mobile station changes cells while participating in a feedback mode MBMS session.  
         [0012]      FIG. 2  diagrammatically illustrates a wireless communication network according to exemplary embodiments of the invention.  
         [0013]      FIG. 3  diagrammatically illustrates a mobile wireless communication apparatus according to exemplary embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     According to exemplary embodiments of the invention, when an MS that is operating in feedback mode and participating in an MBMS session announces to the currently serving BSS a transfer form the current cell to a new cell, the currently serving BSS can, while the MS remains in communication with the current cell, allocate an MS_ID which the MS can use to continue participating in the MBMS session in the new cell. Some embodiments transmit the MS_ID parameter from BSS to MS by adding a new information element to a conventionally available message. For example, a new information element, addressed only to the MS that has reported the cell change, can be added to the existing MBMS NEIGHBOURING CELL INFORMATION message. Other embodiments transmit the MS_ID parameter in a suitable message on the PACCH (Packet Associated Control Channel) of the MBMS point-to-multipoint bearer that is supporting the MS&#39;s session in the current cell.  
         [0015]     In situations where the same BSS that handles the current cell also handles the new cell, the BSS can readily allocate an MS_ID for the MS to use in the new cell.  
         [0016]     In situations where the current and new cells are handled by different BSSs, some embodiments use a RIM application to support the allocation of an MS_ID for the MS to use in the new cell. The currently serving BSS (which handles the current cell) can use a suitable Radio Access Network Information Management (RIM) application to request the new BSS (which handles the new cell) to allocate an MS_ID for the MS, and the new BSS can, after allocating the requested MS_ID, use the RIM application to communicate the allocated MS_ID to the currently serving BSS. The RIM procedure, including its different applications, is set forth in the aforementioned specification document 3GPP TS 48.018. Other embodiments signal the MS_ID allocation request from the current BSS and the response from the new BSS on a communication path that includes the interfaces between the BSSs and their associated Serving GPRS Support Nodes (SGSNs), and the interface between the two SGSNs.  
         [0017]     When the new cell is handled by a different BSS than the current cell, the amount of time required to allocate the MS_ID, and then report the allocated MS_ID to the MS, can be longer than the amount of time that the MS conventionally waits for the currently serving BSS to identify the appropriate MBMS bearer in the new cell. In some embodiments, the currently serving BSS notifies the MS to wait longer in the current cell before switching to the new cell. For example, in some embodiments, the currently serving BSS provides in the MBMS NEIGHBOURING CELL INFORMATION message an information element which indicates that, for transitions to certain neighboring cells (i.e., those handled by a different BSS), the MS should wait for the longer period of time.  
         [0018]     As demonstrated above, with the present invention, the MS_ID for use in the new cell can be provided without executing the conventional access request procedure. However, the access request procedure is also used in conventional systems to provide the MS with the timing advance parameters that it will use to send acknowledgement reports to the new BSS. More specifically, the conventional access request procedure is initiated by the MS sending access bursts for the network to use in determining timing advance parameters for use by the MS.  
         [0019]     Exemplary embodiments of the invention provide for the determination of timing advance parameters for the MS, even though the access request procedure is not executed. After the MS transitions to the new cell, the new BSS will begin to poll the MS for acknowledgement reports with respect to the MBMS session. In some embodiments, the MS initially responds to this polling by sending access bursts instead of acknowledgement reports. The BSS uses these access bursts to determine the timing advance parameters for the MS. In some embodiments, rather than having the MS send access bursts instead of acknowledgement reports in response to initial BSS polling, the conventional PACKET POLLING REQUEST procedure is used to elicit from the MS access bursts that can be used by the BSS to determine the timing advance parameters for the MS.  
         [0020]     In some embodiments, the timing advance parameters are reported to the MS in a PACKET POWER CONTROL/TIMING ADVANCE message on the PACCH. Some embodiments address this message specifically to the desired MS by, for example, including in the Global TFI IE field of the message both the MBMS Bearer Identity and the MS_ID of the desired MS. In other embodiments, the BSS uses the PACCH to send to the MS a control message that contains the timing advance parameters.  
         [0021]     After the MS receives the timing advance parameters, the MS can thereafter respond with acknowledgement reports (which include the MS_ID) when the BSS polls for acknowledgement reports.  
         [0022]     The new BSS will know that the MS has moved to the new cell as soon as the BSS begins to receive access bursts in response to its polling of the MS. At that time, the network also knows that the “old” MS_ID that was previously used by the MS in the exited cell can be re-allocated to another MS currently operating in the exited cell. Because the access request procedure has been avoided, the corresponding time delay required to perform the access request procedure is eliminated, so the “old” MS_ID can be re-allocated earlier than would be possible if the access request procedure were performed.  
         [0023]      FIG. 1  illustrates how operations such as described above can be performed according to exemplary embodiments of the invention when a mobile station changes to a cell that is supporting an MBMS session in feedback mode. After a new cell is selected (and reported from MS to BSS) at  100 , a new MS_ID for the MS to use in the new cell is allocated at  101 . After the new MS_ID has been allocated, the current BSS sends it to the MS at  102 . The MS switches to the new cell at  103 . At  104 , the MS receives an acknowledgement request from the new BSS. As illustrated at  104 - 108 , until the MS has received timing advance (TA) parameters from the new BSS (see  105 ,  107  and  108 ), the MS will send access bursts (see  106 ) in response to an acknowledgement request received from the new BSS. As illustrated at  107  and  108 , when the new BSS has determined the TA parameters from the access bursts (see  107 ), the new BSS then sends the TA parameters to the MS (see  108 ). As shown at  105  and  109 , after the MS has received the TA parameters from the new BSS (see  105 ), the MS will send acknowledgement reports (see  109 ) in response to acknowledgement requests received from the new BSS.  
         [0024]      FIG. 2  diagrammatically illustrates a wireless communication network according to exemplary embodiments of the invention. In  FIG. 2 , BSS A serves mobile stations operating in a plurality of cells designated A 1 -An, and BSS B serves mobile stations operating in a plurality of cells designated B 1 -Bk. Network infrastructure supporting the use of RIM applications for communication between BSS A and BSS B is illustrated at  24 . Embodiments that use SGSNs for communication between BSS A and BSS B are illustrated by broken line at  25 . BSS A includes an MBMS transfer controller  22  coupled with a wireless communication interface to the served cells A 1 -An, and BSS B includes an MBMS session controller  23  coupled with a wireless communication interface to the served cells B 1 -Bk. In some embodiments, either of the MBMS transfer controllers  22  and  23  is capable of suitably controlling conventionally available facilities (not all explicitly shown) in the associated BSS in order to effectuate operations described above with respect to either the “currently serving” BSS or the “new” BSS.  
         [0025]      FIG. 3  diagrammatically illustrates a mobile station apparatus  31  according to exemplary embodiments of the invention. The apparatus  31  includes an MBMS transfer controller  32  coupled to a wireless communication interface. In some embodiments, the MBMS session transfer controller  32  is capable of suitably controlling conventionally available facilities (not all explicitly shown) in the apparatus  31  in order to effectuate operations described above with respect to the mobile station MS.  
         [0026]     As demonstrated above, exemplary embodiments of the invention can avoid the use of the access request procedure when the MS enters a new cell. Avoiding the access request procedure provides various advantages, for example: (1) the corresponding interruption of the MS&#39;s participation in the MBMS session is avoided;  
         [0027]     (2) the corresponding load on the common control channels ((P)RACH and (P)CCCH) in the new cell (which would otherwise support the access request procedure) is reduced; and (3) the MS_ID previously used by the MS in the exited cell can be re-allocated to another MS sooner.  
         [0028]     Although exemplary embodiments of the invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.