Abstract:
The present invention relates to providing a method and a network node that prevents radio bearers from being dropped if a mismatch in RLC Mode exists between source Node and target Node.

Description:
This application is a continuation of U.S. patent application Ser. No. 13/318560, filed 2 Nov. 2011, which is a national stage application of PCT/SE2009/050480, filed 4 May 2009, the disclosures of each of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to the field of telecommunications. More specifically, the present invention relates to improved handover in a radio communications network. 
     BACKGROUND 
     The present invention finds application particularly in next generation networks as defined by 3GPP and which is commonly known as Long Term Evolution (LTE). However, the techniques may be applicable also in connection with other types of radio networks, such as WCDMA, GSM, CDMA etc. Thus, may for instance Radio Base Station (RBS) as used hereinafter refer to eNodeB (eNB) in LTE as well as NodeB (NB) in WCDMA or any other node handling a radio interface in a radio network. 
     The radio network equipment vendors choose in the product implementation, in cooperation with the operator, which Radio Link Control (RLC) Mode to use for a certain radio bearer with a certain Quality of Service Class Identifier (QCI). With the low round trip time in LTE, different RLC Modes may be suitable for real-time services. Therefore, it can be foreseen that some vendors select unacknowledged RLC for real-time services like Voice over Internet Protocol (VoIP), while others select acknowledged RLC. 
     According to the 3GPP standard reconfiguration of RLC Mode of data radio bearers (DRBs) is not supported in E-UTRAN, making the radio bearer being dropped during handover if there is a mismatch in RLC Mode between Source eNB and Target eNB for a certain radio bearer with a certain QCI. 
     SUMMARY 
     One object according to one aspect of the present invention is to at least alleviate problems mentioned above. 
     One object of one aspect according to the present invention is to provide a method and a network node that prevents radio bearers from being dropped if a mismatch in RLC Mode exists between source Node and target Node. 
     If a mismatch in RLC Mode is identified by Target eNB in an intra-LTE handover from a Source eNb, the preferred pre-configured RLC Configuration in Target eNB is overridden and an alternative RLC configuration with the same RLC Mode as in Source eNB is assigned. 
     These objects, amongst others, are achieved, according to one aspect of the present invention, by a method in a target radio network node supporting a handover from a source radio network node. The target network node comprises a table having preferred relations between a specific QCI and a specific RLC configuration. 
     The method comprises the steps of receiving, from the source radio network node, at least a Quality of Service information element including at least a QCI parameter, and a RLC Configuration information element including at least a RLC Mode parameter, checking if the received RLC Mode corresponds to the RLC Mode for the preferred RLC configuration in said first table for the received QCI, and if said check fails, mapping said received RLC configuration to an alternative RLC configuration having same RLC Mode as the received RLC Mode and being associated with a QCI corresponding to said received QCI. 
     One advantage according to one embodiment of the invention is that the RBS will not be sensitive to future decisions on relation between QCI and RLC Mode with respect to intra-LTE handover in multi-vendor RAN. 
     Another advantage according to one embodiment of the invention is that radio bearer re-establishment, e.g. causing unacceptable breaks in voice calls, will be avoided if different vendors realize radio bearers differently with respect to RLC Mode and QCI. 
     Another advantage according to one embodiment of the invention is that the RBS solution will be general, without need for specific solutions required by specific operators when other vendors make design choices. 
     Another advantage according to one embodiment of the invention is that the invention provides advantages in flexibility for proprietary, operator-defined, QCIs. 
     Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description of embodiments of the present invention given herein below and the accompanying  FIGS. 1 to 3 , which are given by way of illustration only, and thus are not limitative of the present invention. 
         FIG. 1  is a signalling diagram showing signalling between a source and a target node for handover. 
         FIG. 2  is a flowchart showing the steps according to an embodiment of the present invention. 
         FIG. 3  is a schematic block diagram illustrating one embodiment according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details. 
       FIG. 1  is a signalling diagram showing the handover preparation between a source eNB  101  and a target eNB  102 . The handover decision is taken by the Source eNB  101  based on measurement reports from the UE (not shown). The Source eNB  101  starts the handover process by sending a Handover Request message  103  to the Target eNB  102 , which after making an admission control  104  acknowledges the handover request  105 . 
     The Handover Request message  103  conveys the information needed by the Target eNB  102  to correctly set up the required radio bearers in the target cell. Such information include:
         QCI, included in the System Architecture Evolution (SAE) E-RAB Level QoS Parameters information element (IE);   RLC Mode, included in the RLC-Configuration IE, which is a part of the AS-Configuration IE.       

     The bearer level Quality of Service (QoS) includes at least the following QoS parameters:
         Quality of service class identifier, (QCI)   Guaranteed bit rate, (GBR)   Maximum bit rate, (MBR)   Allocation retention priority, (ARP)       

     When setting up a session, each service data flow is mapped to a QCI, which is a pointer, represented by a single integer number, pointing at an access node-specific, or eNB specific, configuration that controls the bearer level packet forwarding treatment, and that have been pre-configured by the operator of the eNodeB  101  and  102 . 
     Each QCI, representing a service or service aggregate, is associated with a set of 
     QCI Characteristics such as:
         Resource type (i.e. GBR or non-GBR)   Priority   Packet Delay Budget, (PDB)   Packet Error Loss Rate, (PELR)       

     The QCI Characteristics are used to characterize the configurations of the eNB  101  and  102 . 
     In 3GPP Release 8, nine different Standardized QCI Characteristics are being defined, used to ensure interoperability between operators. Besides these Standardized QCI Characteristics, the operator is free to define its own QCI Characteristics, mainly for operation within the operator&#39;s own network, since no interoperability is secured through the standard for these. 
     The most important parameter in the RLC protocol configuration is the RLC Mode. Three RLC modes are possible:
         Transparent Mode, TM   Unacknowledged Mode, UM   Acknowledged Mode, AM       

     In RLC TM no overhead information is added, and therefore functions like segmentation and reordering are not possible. 
     In RLC UM functions like segmentation, concatenation and reordering are possible. Since this is an unacknowledged mode no retransmissions are possible, this mode is typically used for delay critical real-time services like voice and video. 
     In RLC AM retransmissions by an ARQ protocol is possible. This mode is therefore suitable to provide low packet error rates. 
     In an SAE/LTE network with eNBs from multiple vendors it is possible that certain QCIs are realized by different RLC Modes in different eNBs. 
     Thus, in the present embodiment, given only as an example, the source eNB  101  communicates, in the handover request message  103 , a combination of QCI and RLC Mode not supported by target eNB  102 . 
     According to the 3GPP standard reconfiguration of RLC Mode of DRBs is not supported in E-UTRAN. Therefore, at handover between eNBs  101  and  102 , which are configured with different RLC Mode for the same QCI, the radio bearer should have been dropped. A re-establishment is possible, but will take time and give insufficient handover performance for real-time services. 
     However, according to the invention and as is shown in  FIG. 2 , the target eNB  102  provides innovative means for handling the situation. 
       FIG. 2  is a flowchart showing the different steps according to one embodiment of the invention. In step  201  a handover request is received from the source eNB  101 . A check is made in step  202  to verify if the RLC Mode as received in the handover request message  103  is equal to the RLC Mode for the preferred RLC configuration as defined in the target eNB  102  for the CQI received with the handover request message  103 . If the RLC Modes are identical, and usual admission control (not shown) is positive, the handover request acknowledge message is sent  105  the source eNB  101  in step  203 . 
     However, if there is a mismatch between the received RLC Mode and the RLC Mode for the preferred RLC configuration associated with the received QCI, the preferred RLC configuration is mapped to an alternative RLC configuration having the same RLC Mode as the received RLC Mode and associated with the received QCI, step  204 . The handover request acknowledgement message  105  is sent to the source eNB  101  in step  203 . 
       FIG. 3  is a schematic block diagram showing the target eNB  102  comprising a transceiver unit  301  for receiving and sending messages, such as the handover request message  101  and the handover request acknowledgement message  105 . A control and checking unit  302  performs the check if the received RLC Mode is identical to the RLC Mode for the preferred RLC configuration and, if not, maps to an alternative RLC configuration as disclosed above in connection with  FIG. 2 . 
     The target eNB  101  also comprises a table  303  mapping QCI to RLC configuration. As stated before, each RLC configuration comprises an RLC Mode. Thus, the same QCI may be mapped to several RLC configurations of which one is the preferred RLC configuration. 
     It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.