Abstract:
There is disclosed a method for serving radio network subsystem relocation from a first radio network controller to a second radio network controller for an established call to a user equipment, comprising transmitting a transmission power control combination index of a radio link established between the user equipment and the first radio network controller to the second radio network controller.

Description:
FIELD OF THE INVENTION 
   The present invention relates to serving radio network subsystem relocation in a mobile communications system. 
   BACKGROUND OF THE INVENTION 
   In the field of mobile communications, it is well know to provide for the relocation of the network access point of an established call to a user equipment as the user equipment roams in the network. 
   A TPC (transmission power control) CI (combination index) is an integer value in the range 0 to 5. Each time a radio link is established, a TPC CI is allocated for the link. A radio link is a connection from a radio network controller (RNC) to a user equipment (UE) via a base station. All radio links established via a given base station (Node B) must have the same TPC CI. If a new radio link is established using a Node B which has no previous radio links, then a new non-used TPC CI must be allocated for that Node B. 
   The UE uses the TPC CI to control the power for different radio links. Radio links from the same Node B are handled as a group from a power control point of view. 
   Whilst all radio links established via a given Node B have the same TPC CI, the various radio links established through the Node B are uniquely identified by a Primary common pilot channel (CPICH) info. 
   The TPC CI is established between the radio network controller (RNC) and the user equipment (UE), and it is not known by the base stations. The TPC CI is always allocated by the RNC that establishes a radio link. 
   When RNC relocation occurs, the new RNC does not know the TPC CI(s) of the radio links that were established by the original RNC, despite the fact that these radio links are transferred to the new RNC. 
   The TPC CI has a very limited value range, being an integer value in the range 0 to 5. Thus, when a new RNC starts to establish a new radio link to a new base station/user equipment after relocation it may accidentally allocate a TPC CI that is already in use. This creates a situation where the UE has the incorrect power control information. If the new RNC allocates a TPC CI that is already in use by another RNC, then the UE will assume that the new radio link is under the control of the same base station as the old radio link(s). 
   It is an aim of the present invention to provide an improved technique for serving radio network subsystem relocation. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect the invention provides a method for serving radio network subsystem relocation from a first radio network controller to a second radio network controller for an established call to a user equipment, comprising transmitting a transmission power control combination index of a radio link established between the user equipment and the first radio network controller to the second radio network controller. 
   The transmission power control combination index may be transmitted in a serving radio network subsystem relocation container. The transmission power control combination index may be transmitted on the IU interface. 
   The step of transmitting a transmission power control combination index may further include transmitting primary common pilot channel information. 
   The method may further include the step of allocating a transmission power control combination index for a radio link established from the second radio network controller to the user equipment. 
   Each radio link from a radio network controller to a user equipment may be established via a base station. If the base station for the link established from the second radio network controller to the user equipment is the same as that established for the link between the first radio network controller and said user equipment, the transport power control combination index received from said first radio network controller may preferably be used by second radio network controller. If the base station for the link established from the second radio network controller to the user equipment is different to that established for the link between the first radio network controller and said user equipment, the transport power control combination index used by the second radio network controller may preferably be different to that received from said first radio network controller. 
   In a second aspect the invention provides a radio network controller adapted, on serving radio network subsystem relocation, to receive from a further radio network controller from which a radio link is being transferred a transmission power control combination index of the radio link established between a user equipment and said further radio network controller. 
   The radio network controller may further be adapted to receive a transmitting primary common pilot channel information in combination with the transmission power control combination index. 
   The radio network controller may further be adapted to allocate a transmission power control combination index for a radio link established from the radio network controller to the user equipment. 
   Each radio link from a radio network controller to a user equipment may be established via a base station. 
   If the base station for the link established from the radio network controller to the user equipment is the same as that established for the link between the further radio network controller and said user equipment, the transport power control combination index received from said further radio network controller is preferably used by the radio network controller. If the base station for the link established from the radio network controller to the user equipment is different to that established for the link between the further radio network controller and said user equipment, the transport power control combination index used by the radio network controller is preferably different to that received from said further radio network controller. 
   In a third aspect the invention provides a radio network controller adapted, on serving radio network subsystem relocation, to transmit to a further radio network controller to which a radio link is to be transferred a transmission power control combination index of the radio link established between a user equipment and said radio network controller. 
   The radio network controller may be further adapted to transmit a primary common pilot channel information in combination with the transmission power control combination index. 
   In a fourth aspect the invention provides a communication system adapted for serving radio network subsystem relocation from a first radio network controller to a second radio network controller for an established call to a user equipment, comprising means at said first radio network controller for transmitting a transmission power control combination index of a radio link established between the user equipment and the first radio network controller to the second radio network controller; means at said second radio network controller for receiving said transport power control combination index, and means at said second radio network controller for determining a transmission power control combination index for a radio link to the user equipment in dependence on the received transport power control combination index. 
   The second radio network controller may be adapted to further determine the transmission power control combination index in dependence on whether the radio link is established through the same base station as the radio link between the first radio network controller and the user equipment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is now described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates part of the radio network subsystem in a UMTS system; and 
       FIG. 2  illustrates the method steps in a preferred embodiment of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  there is illustrated an example of elements in a UMTS network architecture for describing an example implementation of the invention. 
   A source radio network controller (SRNC)  10  is connected to at least one base station, termed a “Node B”,  14  via an interface  22 , preferably an Iub interface. A target radio network controller (TRNC)  12  is connected to at least one Node B  26  via an interface  24 , again preferably an Iub interface. The SRNC  10  and the TRNC  12  are interconnected via an inter-RNC interface  20 , preferably being the Iur interface. A user equipment (UE)  18  may establish a radio communication with any radio network controller via an associated Node B. In the example of  FIG. 2 , the UE  18  may establish a connection to Node B  14  via a radio link  26 , and/or may establish a connection to Node B  16  via a radio link  28 . The UE  18  is representative only, and in practice a plurality of UEs may establish radio links. 
   For the purpose of discussion of an example implementation, the RNC  10  initially has an established radio link with the UE  18  via Node B  14 . Hence the RNC  10  is identified as the source RNC. The RNC  12  is the RNC to which control of a call is to be transferred to due to roaming of the UE  18 . Hence the RNC  12  is identified as the target RNC. The identification of the source and target RNCs enables the following description of the operation of the invention to be easily understood. However, it will be apparent that once the control of the call is transferred to a new RNC from an original RNC, the new RNC becomes the source RNC. Thus in general the invention relates to the transfer between RNCs, and the terms ‘source’ and ‘target’ should not be considered limiting in any way. 
   The establishment of the original radio link between the SRNC  10  and the UE  18  via the Node B  14  is not described. Establishment techniques for such radio links are well known in the art, and the invention does not propose any modification to such techniques. The invention is independent of any of the various techniques for establishing the original radio link. 
   Following the establishment of the radio link between the SRNC  10  and the UE  18  via the Node B  14 , a TPC CI of  1 , for example, is determined for the established radio link. This is denoted as “TPC CI  1 ”. The established radio link is uniquely identified by a Primary CPICH Info. This is denoted as “CPICH info  1 ” for the established radio link. 
   A serving radio network subsystem (SRNS) relocation may occur due to the roaming of the UE  18 . In an SRNS relocation, the RRC connection established between the SRNC  10  and the UE  18  via the Node B  14  is transferred completely to the TRNC  12  via the Node B  16 . In such case, it is assumed that the UE  18  has roamed into a geographical area supported by the Node B  16 . The techniques by which such relocation is controlled are known in the art, and outside the scope of the present invention. The invention does not propose any modification or adaptation to such techniques. 
   As is known in the art, during SRNS relocation an SRNS relocation container is transmitted from the SRNC to the TRNC via an IU interface. In accordance with the invention, the SRNS relocation container is modified to include an identity of the TPC CI in use for the radio link from the SRNC. This information is provided as a list of Primary CPICH Info/TPC CI pairs. This information is preferably provided on the IU interface to the target RNC  12  from the source RNC  10 . 
   Thus the TRNC is provided with the value TPC CI  1  from the SRNC. The UE itself is already aware of the value TPC CI  1 . The TRNC may then establish a new radio link, and is guaranteed not to establish a link based on a TPC CI value of TPC CI  1 . 
   A radio link must always exist in the TRNC at the time the SRNC relocation occurs. As at the time of the SRNS relocation the TRNC has at least one established radio link, then the TPC CI or TPC CIs for the radio link to the UE  18  is that or are those of the established radio link. On completion of the relocation, if the TRNC establishes new radio links for a user now connected through the TRNC, in accordance with the embodiments of the invention the TPC CI(s) of the original SRNC is known. As such, in establishing any new radio links, the TPC CI(s) of the original SRNC can be taken into account by the TRNC, to ensure there is no conflict. 
   The technique of the present invention is summarised by the flow chart of  FIG. 2 . In a step  200 , a radio link has been established from the SRNC  10  to the UE  18  via the Node B  14 . In a step  202  a SRNS relocation is initiated, to transfer the network access point for the UE  18  from Node B  14  to Node B  16 . In accordance with the invention, in a step  204  a TPC CI and Primary CPICH information pair is sent from the SRNC  10  to the TRNC  12  for each transferred radio link. 
   In SRNC relocation the target radio network controller always has an established radio link. The TPC CI is determined, however, to be different to the TPC CI of the radio link transferred from the SRNC if that radio link is supported by a different base station. 
   If in step  206  it is determined that the radio link to the UE  18  from the TRNC  12  uses the same base station as was previously used, then the same TPC CI is used for that link, as that TPC CI is used for all radio links through that base station, as denoted by step  210 . If in step  206  it is determined that the radio link is established through a different base station, then the TRNC must establish a new TPC CI, i.e. different to that of the other base station, for the radio link, as denoted by step  208 . 
   It should be noted that in the foregoing description, which relates to an exemplary embodiment, reference is made to the primary CPICH information being used. However, the invention and its embodiments is not limited to such use. For example, the secondary CPICH information may alternatively be used. 
   The invention has been described by way of example with reference to a particular embodiment. The invention is not, however, limited to any specifics of the described embodiments. The scope of the invention is defined by the appended claims.