Abstract:
The invention relates to a method for performing a handover procedure in a wireless coordinated multipoint (CoMP) transmission network ( 200; 700 ) of a mobile station ( 100 ) from a first base station ( 1 ) to a second base station ( 2 ), wherein the first base station coordinates a first group ( 202 ) of base stations, the first base station being a member of the first group, and the second base station coordinates a second group ( 204 ) of base stations, the second base station being a member of the second group, and wherein a network component ( 102 ) is adapted to transmit data to the first base station and the first group of base stations and/or to the second base station and the second group of base stations.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for performing a handover procedure in a wireless coordinated multipoint (CoMP) transmission network. 
       BACKGROUND AND RELATED ART 
       [0002]    In common radio communication systems handover procedures are performed, when a mobile station moves from one cell of the network to another cell of the network. The connection of a mobile station is always controlled by only one base station. The mobile station receives connection data from this base station. 
         [0003]    In coordinated multipoint networks multiple base stations send information in a coordinated manner to the mobile station. Common handover mechanisms are not applicable for CoMP networks. 
         [0004]    3GPP TS 36.300 version 8.9.0 Release 8 discloses an overview and overall description of the E-UTRAN radio interface protocol architecture. The Intra-E-UTRAN-Access Mobility Support for mobile stations handles all necessary steps for relocation/handover procedures, like processes that precede the final handover decision on the source network side (control and evaluation of mobile station and base station measurements taking into account certain mobile station specific are restrictions), preparation of resources on the target network side, commanding the mobile station to the new radio resource and finally releasing resources on the (old) source network side. It contains mechanisms to transfer context data between evolved nodes, and to update node relations on C-plane and U-plane. 
         [0005]    WO 00/67512A1 discloses a method for performing a handover procedure for a mobile station, said method comprising the steps of processing location information related to the mobile stations and position information related to base transceiver station, and deciding on the basis of the result of said processing, whether a first handover condition is fulfilled. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides a method for performing a handover procedure in a wireless coordinated multipoint (CoMP) transmission network of a mobile station from a first base station to a second base station. The first base station coordinates a first group of base stations. The first base station is a member of the first group. The second base station coordinates a second group of base stations. The second base station is a member of the second group. A network component is adapted to transmit data to the first base station and the first group of base stations and/or to the second base station and the second group of base stations. 
         [0007]    In other words, a data stream meant to be transmitted to the mobile station is transmitted from the network component to the first group of base stations and then forwarded to the mobile station. When a handover of the mobile station from the first base station to the second base station shall be performed the data which shall be transmitted to the mobile station must be directed to the second base station and the second group of base stations instead of the first group in the first group of base stations. The same applies for data being sent from the mobile station to a base station of the first group, the data being meant to be sent to another component in the network, for example another mobile station or a gateway to another network. The data being transmitted from the mobile station to the first group of base stations is first transmitted from the first group of base stations to the network component which then forwards the data to other network means. 
         [0008]    Both data streams, the first one from the network component to the mobile station and the second one from the mobile station to the network component, must be redirected in case of a handover procedure. Therefore, the location of the mobile station is estimated. Preferably, the location estimation is based on channel quality information. Most preferably, the first base station has requested channel quality information from the second base station and the first base station has received channel quality information from the second base station. Channel quality information of the channels between the mobile station and each base station of the first group of base stations are transmitted periodically from the mobile station to the first base station or on request of the first base station. 
         [0009]    After location estimation a first signal is sent to the second base station, the first signal being indicative of requesting the handover procedure. Preferably, the first signal is transmitted from the first base station to the second base station. 
         [0010]    A second signal is sent to the network component, preferably from the second base station. The second signal is indicative of initiating the handover procedure. Then, the data stream from the network component is directed to the second base station and the second group. Then, the connection of the mobile station to the network is controlled by the second base station and every base station of the second group is used for transmitting and receiving data to and from the mobile station according to a coordinated multipoint transmission scheme. 
         [0011]    According to embodiments of the invention the location estimation is performed based on channel quality information, the channel quality information being information of connections between the mobile station and the first group and/or information of connections between the mobile station and the second group. 
         [0012]    According to embodiments of the invention a fourth signal is sent to the second base station before location estimation. The fourth signal is indicative of requesting a channel quality report. Preferably the fourth signal is transmitted from the first base station to the second base station. A channel quality report always comprises the quality of the channel of the connection between a mobile station and at least one base station. In a coordinated multipoint transmission network one channel quality report always comprises the channel quality of the connection between the mobile station and each base station of one of the first and the second group. Thus, a channel quality report being transmitted from the mobile station to the first base station comprises the channel quality of the connections between the mobile station and each base station of the first group. A channel quality report be ing sent from the mobile station to the second base station comprises the channel quality of the connection of the mobile station to each base station of the second group. 
         [0013]    By knowing the channel quality of the connections of the mobile station to each base station of the first and of the second group the location of the mobile station can be estimated. Location estimation is also feasible based only on the channel quality of the connections of one group. Preferably this is the first group. Alternatively it can also be the channel quality of only the second group. 
         [0014]    According to embodiments of the invention first channel quality information is transmitted from the mobile station to the first base station and second channel quality information is transmitted from the second base station to the first base station. Preferably, the second channel quality information comprises information about the channel quality of the connection between the mobile station and the second group of base stations. 
         [0015]    According to embodiments of the invention the first channel quality information comprises information about a first connection quality between the mobile station and the first base station. The second channel quality information comprises information about a second connection quality between the base station and the second base station. 
         [0016]    According to embodiments of the invention the handover procedure is initiated when the second connection quality of the connection between the mobile station and the second base station is above a first threshold. 
         [0017]    According to embodiments of the invention the handover procedure is initiated when the first connection quality of the connection between the mobile station and the first base station is below a second threshold. 
         [0018]    According to embodiments of the invention the first base station is also a member of the second group and the second base station is also a member of the first group. 
         [0019]    According to embodiments of the invention the method comprises a further step of sending a third signal from the network component to the first base station and a fourth signal from the network component to the second base station. The third and the fourth signal are indicative of initiating the handover procedure. 
         [0020]    In another aspect the invention relates to a computer readable storage medium containing instructions that when executed by a base station cause the base station to perform a method according to embodiments of the invention. 
         [0021]    In yet another aspect the invention relates to a base station apparatus, which comprises means for estimating a location of the mobile station, means for sending a first signal to the second base station, means for sending a second signal to the network component and means for transmitting data to the second base station. 
         [0022]    In yet another aspect the invention relates to a telecommunication system comprising a plurality of base stations as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which: 
           [0024]      FIG. 1  is a schematic view showing the data stream between a network component and a mobile station; 
           [0025]      FIG. 2  is a schematic view of a wireless telecommunication network; 
           [0026]      FIG. 3  is a diagram of set of channel quality information; 
           [0027]      FIG. 4  is a diagram of set of channel quality information; 
           [0028]      FIG. 5  is a diagram of set of channel quality information; 
           [0029]      FIG. 6  is messaging diagram; and 
           [0030]      FIG. 7  is a block diagram of a wireless telecommunication network. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Like numbered elements in these Figs. are either identical elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later Figs. if the function is identical. 
         [0032]      FIG. 1  is a schematic view showing a plurality of base stations  104 , a mobile station  100 , a network component  102  and the core network  106 . 
         [0033]    Data can be transmitted from the mobile station to the plurality of base stations  104  or from the base stations  104  to the mobile station  100  according to a coordinated multipoint transmission scheme. The data being transmitted from the mobile station  100  to the base stations  104  is then forwarded to the network component  102 , which then transmits the data to the core network  106 . In the other direction, the data being sent to the mobile station  100  from the base stations  104  originates from the network component  102 . The network component  102  has received this data from the core network  106 . 
         [0034]      FIG. 2  is a schematic view of a wireless telecommunication network  200  comprising a plurality of cells  1 - 37  and a mobile station  100 . Each cell  1 - 37  is served by a corresponding base station  1 - 37  (not depicted) 
         [0035]    The mobile station  100  is first located at position A in cluster  202 . Cluster  202  is a CoMP cluster, which is coordinated by the base station of cell  1 . At a second time point the mobile station  100  is located at position B in cell  2 . Cell  2  being also in the CoMP cluster  202  and in the CoMP cluster  204 , which comprises the cells  1 ,  2 ,  3 ,  7 ,  8 ,  9  and  10 , the base station of cell  2  coordinating cluster  204 . Cluster  202  consists of cells  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 , the base station of cell  1  coordinating duster  202 . 
         [0036]    A handover is initiated when the mobile station  100  is located at position B because the connection quality of the connection to the base stations of cluster  204  is much better than the connection quality to the base stations of cluster  202 , when the mobile station  100  is located at position B. 
         [0037]      FIG. 3  is a diagram of the set of channel quality information (CQIS) of the mobile station  100  at location A in  FIG. 2 . The CQIS is an indicator for the channel quality. CQIS reports are sent periodically from the mobile station to the serving base station. In CoMP networks CQIS reports are transmitted from the mobile base station to the base station coordinating the cluster, to which the mobile station is currently connected. 
         [0038]    The CQIS report in  FIG. 3  shows the channel state, which corresponds to the channel quality, between the mobile station  100  and the base stations serving cells  1 ,  2 ,  3 ,  4 ,  5 ,  6  and  7  in  FIG. 2 . At location A the channel quality of the connection between the mobile station  100  and the base station of cell  1  is the best channel quality in comparison to the other depicted cells because the mobile station  100  is located in cell  1  with a similar distance to each of the other cells of cluster  202 . 
         [0039]      FIG. 4  shows the CQIS of mobile station  100  at location B in  FIG. 2 . This CQIS report is reported periodically from the mobile station to the first base station serving cell  1 . The channel quality between the mobile station  100  and the base station serving cell  2  is better than the channel quality to any other base station. Especially the channel quality between the mobile station  100  and the base station of cells  4 ,  5  and  6  is much worse than the channel quality of the connection between the mobile station  100  and base station serving cell number  2 . The channel quality of the connection between the mobile station  100  and the base stations of cells  1 ,  3  and  7  is in between the other channel qualities. The channel quality between the mobile station  100  and base station number  1  is a bit worse than the channel quality between the mobile station  100  and base stations number  3  and  7 . 
         [0040]      FIG. 5  shows a CQIS report transmitted from the mobile station to the second base station number  2 . The channel state, which corresponds to the channel quality, between the mobile station  100  and the base station serving cell number  2  is best in comparison to the other base stations belonging to cluster  204 . Preferably, this CQIS reported is requested by the base station serving cell number  1  after having determined that the channel quality between base station number  2  and mobile station  100  is better than the channel quality between base station number  1  and mobile station  100 . 
         [0041]    By knowing the diagrams of  FIG. 4  and  FIG. 5 , base station number  1  initiates a handover to base station number  2 . This means, the data flow from mobile station  100  to the core network  106  and vice versa has to be directed to the cluster  204  coordinated by base station number  2 . The handover procedure can be performed according to common handover procedures. 
         [0042]      FIG. 6  shows a detailed view of the messaging between mobile station  100 , base station number  1 , base station number  2  and the network component  102 . First, base station number  1  receives a CQIS report  600  from mobile station  100 . CQIS report  600  is reported periodically to base station number  1  from mobile station  100 . Then, base station number  1  requests a CQIS report from base station number  2  with a first signal  602 . Then, with a second signal  604 , base station number  2  requests a CQIS report from mobile station  100 . Mobile station  100  sends a second CQIS report  606  to base station number  2 . The CQIS report  606  comprises the channel states between the mobile station  100  and each base station of cluster  204 , cluster  204  being coordinated by base station number  2 . Then, CQIS report  606  is transmitted from base station number  2  to base station number  1  with the CQIS report acknowledgement signal  608 . 
         [0043]    The two received CQIS reports  600  and  606  are compared in base station number  1  during CQIS comparison  610 . This is the location estimation of the mobile station  100 . After CQIS comparison  610  the handover procedure is initiated with a handover request message  612  transmitted from base station number  1  to base station number  2 . The handover procedure is indicated to network component  106  from base station number  2  by a handover indication signal  614 . The handover indication is acknowledged with a signal  616  and  618  transmitted to the base station number  1  and base station number  2 . Afterwards the data flow is switched  620 . The data flow is switched such that the data is not any longer transmitted from the core network  106  to the first group of base stations but to the second group of base stations. In the other direction data transmitted from the mobile station  100  to the core network  106  is received from the second group of base stations instead of the first group of base stations. The first group of base stations belongs to cluster  202 , while the second group of base stations belongs to cluster  204 . 
         [0044]    Periodically the master basestation  1  of CoMP cluster  202  inspects the CQIS of every mobile station  100  which is administered by this master basestation. The time period is preconfigured. Based on the CQIS of the different basestations  2 ,  3 ,  4 ,  5 ,  6  and  7 , the master basestation  1  computes a performance index from on the CQIS and a coarse location of the mobile station. 
         [0045]    Basestation  1 , which acts as the master basestation of the blue CoMP cluster  202 , inspects the CQIS as shown in  FIG. 3 . Based in this CQIS the basestation  1  computes a performance index and a location estimation of the mobile station  100 . The result of this location estimation ( FIG. 3 ) indicates that the mobile station is probably located in the centre of cell  1 , because CQI  1  is significantly better than all other and the CQIs of the surrounding cells and the CQIs  2  to  7  are nearly similar. The performance index and the location estimation point out, that a hand over is currently not necessary. 
         [0046]    When the mobile station changes its location from A to B, CQIS values change as shown in  FIG. 4 . By inspecting these CQIS basestation  1  assumes that the mobile station has changed its location and is now located in the coverage area of basestation  2 , because CQIs of basestation  2  is the best channel condition, while Cats of the opposite located basestation  4 ,  5  and  6  are very low. Basestation  1  initiates now a report request from basestation  2  which is the master of a neighbour cluster  204 . Basestation  2  collects CQIS from its cluster, as shown in  FIG. 5 , and reports this information back to basestation  1  by sending a report acknowledge message ( FIG. 6 ). Basestation  1  generates a performance index and location estimation based on the received CQIS of cluster  204  ( FIG. 5 ). The CQIS of cluster  204  ( FIG. 5 ) gives a significantly better performance index and the location estimation points out that the mobile station is now located in the coverage area of basestation  2 , Basestation  1  decides that the mobile station should be serviced from basestation  2  and sends a handover request message to basestation  2 . 
         [0047]    Basestation  2  sends a handover indication message to the network component  102  which servers both basestations  1  and  2 . The backhaul node figures out the data packet number at which a flow switch has to be performed and sends a handover indication message to both basestations  1  and  2  including the packet number. The packet number defines the last packet which has to send out to CoMP cluster  202  which is controlled by basestation  1 . Packet number n+1 defines the first packet which has to be sent to CoMP cluster  204  which is controlled by basestation  2 . 
         [0048]      FIG. 7  is a block diagram of a wireless telecommunication network  700  comprising a first base station  702 , a plurality of base stations  104  and a plurality of mobile stations  100 . The base stations  104  form a cluster coordinated by the first base station  702 . The mobile stations  100  are connected to this coordinated multipoint cluster. The first base station  702  comprises means  704  for sending and receiving signals to the mobile stations  100 . Also data can be transmitted and received via means  704 . Means  704  serve as communication means for communications between the first base station  702  and mobile stations  100 . 
         [0049]    Further, base station  702  comprises means  716  for location estimation of a mobile station  100 . With means  718  base station  702  can transmit and receive signals to and from the network component  102 . 
         [0050]    Preferably a processor  710  executes a program  714  in storage  712 , the program causing the processor to perform a method as described above. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
         
           
               100  mobile station 
               102  network component 
               104  base station 
               106  core network 
               200  wireless telecommunication network 
               202  GoMP cluster 
               204  GoMP cluster 
               600  GQIS report 
               602  signal 
               604  signal 
               606  CQIS report 
               608  CQIS report ack 
               610  CQIS comparison 
               612  handover request 
               614  handover indication 
               616  handover indication ack 
               618  handover indication ack 
               620  flow switch 
               700  wireless telecommunication network 
               702  base station 
               704  communication means 
               710  processor 
               712  data storage 
               714  program 
               716  location estimation means 
               718  communication means