Abstract:
The present invention relates to a method of performing a handover or reselection procedure for a wireless telecommunication device from a wireless cellular telecommunication source system to a wireless cellular telecommunication target system. The method comprises the steps of determining of a number of potential handover or reselection target cells of the target system by the source system, providing of first data being indicative of the target cells from the source system to the target system, selecting of one of the target cells by the target system, providing of second data being indicative of the selected one of the target cells from the target system to the source system, performing a handover or reselection for the wireless telecommunication device from the source system to the selected target cell of the target system.

Description:
[0001]    This invention is based on a priority application EP 03 290 367.6 which is hereby incorporated by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to the field of wireless cellular telecommunication, and more particularly without limitation to handover and/or reselection procedures.  
         BACKGROUND OF THE INVENTION  
         [0003]    The act of transferring communication with a subscriber station from one base station to another is referred to as handover or reselection in prior art wireless cellular telecommunication systems, such as GSM, UMTS, . . . Generally the term handover is used for voice communication and the term reselection is used for packetized data transmission. The various handover or reselection procedures are specified in the applicable standards for cellular wireless telecommunication systems such as GSM and UMTS.  
           [0004]    For example U.S. Pat. No. 6,466,556 shows a handover method which aims to provide only a small interruption in packet flow during handover. In addition to handover and reselection procedures within a given cellular wireless telecommunication system it is also known that such procedures can be implemented for inter-system handovers. For example, the document TSGR3#(99)544 of the TSG-RAN working group 3 (http://www.3gpp.org/ftp/tsg_ran/WG3_luFTSGR3 — 04/Docs/Pdf/r3-99544.PDF) shows a handover signalling procedure for handover from GSM to UMTS.  
           [0005]    A common disadvantage of known handover and reselection procedures is that handover sometimes fails such that the telecommunication link is interrupted. The present invention therefore aims to provide an improved method of performing a handover or reselection procedure as well as a controller for a wireless cellular telecommunication system.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a method of performing a handover or reselection procedure for a wireless telecommunication device, such as a mobile phone or other user equipment. In order to perform a handover or reselection a number of potential handover target cells are identified from the source system. These potential handover target cells are communicated to the target system.  
           [0007]    The target system itself selects one of the potential handover target cells and informs the source system of its selection. In response the source system initiates the handover or reselection procedure of the wireless telecommunication device.  
           [0008]    The present invention is particularly advantageous in that it enables to increase the reliability of handover and reselection procedures. In contrast to the prior art not the source system but the target system selects the target cell for performing the handover or reselection. This way it can be avoided that a target cell is selected which provides coverage to the wireless telecommunication device but has no more capacity to accommodate the telecommunication link to be handed over. In addition the selection of the target cell can be performed by the target system in order to perform load balancing between potential handover target cells which have the capacity to become serving cells for the telecommunication link to be handed over.  
           [0009]    In accordance with a preferred embodiment of the invention the potential handover target cells are identified by the wireless telecommunication device. For example the wireless telecommunication device periodically scans one or several frequency bands in order to identify neighbouring cells which provide alternative coverage to the wireless telecommunication device. In addition the field strength can be measured in order to provide a quality criterion for any of the neighbouring cells providing the coverage.  
           [0010]    The wireless telecommunication device can communicate these potential handover target cells to a controller of its source system. This can be done by providing the controller of the source system with a list of the cell IDs of the potential handover target cells which have been identified by the wireless telecommunication device. Preferably, some information about the quality of reception from the different cells should also be included in the list. The controller of the source system can then forward the list of cell IDs of the potential handover target cells to the controller of the target system. The controller of the target system can then select one of the cells from the list of cell IDs which is most suitable for accommodating the telecommunication link to be handed over. The controller of the target system signals its selection of this target cell to the controller of the source system. In response the controller of the source system can initiate a handover or reselection procedure to the identified target cell.  
           [0011]    In accordance with a further preferred embodiment of the invention more than one target system is involved. This case becomes practical when the wireless telecommunication device determines potential handover target cells which belong to different target systems.  
           [0012]    In this case, the controller of the source system forwards at least one cell ID of a potential handover target cell to each one of the respective target systems. Each one of the target systems can then check whether the identified potential handover target cell is presently able to become a serving cell for the telecommunication link to be handed over.  
           [0013]    For example one of the target systems refuses the handover or reselection request as the identified potential handover target cell of that target system is already running at full capacity. The other target system accepts the handover request as the potential handover target cell of that target system still has free capacity in the scenario considered here. In this instance the controller of the source system initiates the handover procedure to the target cell of the target system with the free capacity. This way it can be avoided that a handover or reselection to the target cell is initiated which would have failed and which would have likely resulted in an interruption of the telecommunication link to be handed over.  
           [0014]    If more than one target system sends an acceptance of the handover request, the controller of the source system can make a selection. This selection can be random or it can be based on additional quality criteria, such as the respective field strength measured by the wireless telecommunication device, the load situation in the target cells which is communicated from the target systems to the controller of the source system or load balancing criteria.  
           [0015]    In accordance with a further preferred embodiment of the invention the source system and the target system have different air interfaces. In order to perform an inter-system handover or reselection a dual or multiple mode wireless telecommunication device is required which supports the different air interfaces. While operating in the source system the wireless telecommunication device identifies potential handover target cells in the target system by scanning the frequency spectrum by means of the alternative air interface of the target system. Alternatively the source system has a priori knowledge of the potential handover target cells of the target system as the network topologies of the source system and the target system are fixed and covering overlapping regions. Such an inter-system handover can be initiated by the source system when the source system reaches its capacity limit such that some of the telecommunication traffic of the source system is taken over by the target system to free capacity of the source system. Another trigger for inter-system handover is the loss or degradation of coverage in the mobile terminal.  
           [0016]    In general, the selection of the target cell by the target system from the list of potential handover target cells provided by the source system is advantageous as the target system has up-to-date information on the status of its cells and can therefore make an informed decision regarding the selection of one of the potential handover target cells. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    In the following preferred embodiments of the present invention are described in greater detail by making reference to the drawings, in which:  
         [0018]    [0018]FIG. 1 is a block diagram of a source system and a target system,  
         [0019]    [0019]FIG. 2 is a block diagram of a source system and two target systems,  
         [0020]    [0020]FIG. 3 is a block diagram of source and target systems having different air interfaces,  
         [0021]    [0021]FIG. 4 is illustrative of a flow chart of a method of performing a handover procedure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    [0022]FIG. 1 shows a block diagram of a wireless cellular telecommunication system  100 , such as a UMTS-type system. Telecommunication system  100  has a number of radio network controllers (RNCs)  102 ,  104 , . . . which are interconnected by a wired, packet-switched, backbone network. Each one of the RNCs  102 ,  104 , . . . serves to control a number of cells of the telecommunication system  100 . For example, RNC  102  controls the cells C i  where i typically ranges from 1 to 256. Likewise RNC  104  controls 256 cells C j .  
         [0023]    Each one of the cells C i  has a transceiver station which in the case of UMTS-type networks is also referred to as node B. The node B of cell C i  is referred to as N i  in the following. It is to be noted that in a UMTS-type system a single node B can service a plurality of cells.  
         [0024]    All of the N i  are connected to the same RNC  102 . Likewise the node B of cell C j  is referred to as N j  in the following. All the N j  are connected to the same RNC  104 .  
         [0025]    Within the cell C i  there is active user equipment  106 . User equipment  106  can be any wireless telecommunication device, such as a mobile phone or another electronic device having a UMTS-type air interface. A wireless telecommunication link  108  is established between the user equipment  106  and N i .  
         [0026]    Each one of the RNCs  102 ,  104 , . . . together with the respective cells C i , C j  . . . constitutes a sub-system  110 ,  112 , . . . of telecommunication system  100 . When user equipment  106  is moved within sub-system  110  handovers occur from one of the cells C i  to another one of the cells C i .  
         [0027]    For example if user equipment  106  is currently in the cell C i=a  RNC  102  receives control information which enables it to select one of the cells C i=b  as a handover target cell within the same sub-system  110 . This procedure is as such known from the prior art and is specified in the applicable standards as provided by 3GPP GSM/GPRS and UMTS groups.  
         [0028]    When user equipment  106  is moved to a boundry cell C i=c  of sub-system  110  it can become necessary to perform a handover or reselection procedure from the current sub-system  110  to a neighbouring sub-system  112 . In this instance the current sub-system  110  which serves user equipment  106  becomes the “source system” whereas one of the neighbouring sub-system  112  becomes the “target system”.  
         [0029]    In contrast to the prior art the selection of one of the cells C j  of the sub-system  112  as a handover target is performed by RNC  104  of the target system itself rather than by the RNC  102  of the source system.  
         [0030]    In a typical implementation user equipment  106  will not only scan the reception frequency of wireless telecommunication link  108  but the entire frequency band of telecommunication system  100  in order to identify neighbouring cells C i  and/or C j  which also provide coverage for user equipment  106 . For example user equipment  106  measures the respective field strengths in order to determine a quality measure of the respective coverages. In addition user equipment  106  receives signalling messages from those neighbouring cells C i  and/or C j  which indicate the respective cell IDs.  
         [0031]    However, in a typical UMTS implementation not the cell IDs but a fixed pattern which is scrambled with a code which is unique for each cell is received by the user equipment  106 . The user equipment performs measurements on the quality of reception of the pilot channel of the different cells. The cell IDs for neighboring cells have been provided previously by the network by means of signaling messages, together with the information required to carry out the measurements, mainly the frequency and the scrambling code. This way the user equipment is enabled to determine the respective cell IDs on the basis of the scrambled, fixed pattern.  
         [0032]    The information, required to perform measurements on neighboring cells, is sent by the network on the broadcast channel of the cell, but it can also be sent to each particular user by its serving RNC or BSC, using a dedicated control channel.  
         [0033]    The user equipment only measures those neighbouring cells as commanded by the network. But there is an exception. In the case of UMTS, the user equipment may measure other cells using the same frequency. In this case, the scrambling code is not known, and a procedure called “blind detection” must be used, which is more costly than the normal measurement process. In this case, the user equipment has not received any cell ID from the network, but it can use the scrambling code (detected during this procedure) to identify the cell.  
         [0034]    This way a list  114  of potential handover target cells is established by user equipment  106 . For example list  114  contains a list of the potential handover target cell IDs with the corresponding field strengths and/or other signalling information.  
         [0035]    List  114  is transmitted via wireless telecommunication link  108  to N i  of cell C i  of the source system. From there it is forwarded to RNC  102 . When RNC  102  determines that a handover or reselection procedure for user equipment  106  to the target system becomes necessary it forwards a list  116  to RNC  104  of the target system. List  116  contains a list of potential handover target cell IDs of the target system. List  116  can be identical to list  114  or it can be a sub-set of list  114 .  
         [0036]    RNC  104  has program  118  which receives list  116  as input information. In response program  118  selects one of the target cell IDs of list  116  as a handover target in sub-system  112 . Program  118  receives further input information concerning the current state of sub-system  112 , in particular the current load of the cells C j  which is provided by the node Bs of the target system  112 .  
         [0037]    Based on this input information program  118  selects one of the potential handover target cells of list  118  which has sufficient unused capacity in order to accommodate wireless telecommunication link  108 . If more than one potential handover target cell having enough free capacity is available, program  118  can select a target cell out of the available potential handover target cells having the lowest load in order to perform load balancing between the cells.  
         [0038]    The cell ID of the selected target cell of cells C j  of sub-system  112  is sent from RNC  104  to RNC  102 . In response RNC  102  initiates a handover or reselection procedure for user equipment  106  such that wireless telecommunication link  108  is switched over to the target cell with the target cell ID  120  of sub-system  112 . This hand over or reselection procedure is reliable as the selection of the target cell is based on current status information of the target system itself. It can therefore be guaranteed that the selected target cell is actually in a condition to become a serving cell for the wireless telecommunication link  108 . This way an interruption of the wireless telecommunication link  108  during the handover or reselection procedure can be avoided.  
         [0039]    Preferably the above described handover is performed in a single procedure where the source system initiates handover, and the target system can either reject the handover or allocate the required resources and accept the handover. This way extra delays are avoided. In the case where a single target system is contacted, the target system can allocate resources in the target system before sending a response to the source system (i.e. the handover has already been initiated). In the case where there are several potential target systems, there are two different possibilities:  
         [0040]    To allocate resources in all systems accepting handover: in this case, each target system allocates the required resources before sending back a response. As the handover will proceed towards only one of the systems, resources in the other ones must be released, either by the source system sending a message towards each target system canceling the handover or by means of a timer.  
         [0041]    To allocate resources only in one of the systems accepting handover. In this case, the source system must explicitly indicate which target system must allocate resources in case it can accept the handover. If this system accepts the handover, the handover procedure goes on as in the case with a single potential target system. Otherwise, the source system must initiate handover (since there is no previous resource allocation) towards one of the systems which are ready to accept the handover.  
         [0042]    [0042]FIG. 2 illustrates an alternative mode of operation of telecommunication system  100 . In the scenario considered here, user equipment  106  is brought in the vicinity of sub-systems  112  and  122 . The design of sub-system  122  is similar to the design of sub-systems  110  and  112 . Sub-system  122  has RNC  124  which is connected to a number of N k  to establish cells C k . Further RNC  124  has program  118 .  
         [0043]    As in the example of FIG. 1 RNC  102  receives list  114  of neighbouring potential handover target cells from N i . As user equipment  106  is in the proximity of sub-systems  112  and  122  list  114  contains at least one cell ID of a cell C j  of sub-system  112  and one cell ID of a cell C k  of sub-system  122 . When RNC  102  makes a decision that a handover or reselection of wireless telecommunication link  108  to either sub-system  112  or sub-system  122  is necessary those systems become “target system A” and “target system B”, respectively.  
         [0044]    For example, RNC  102  sends the cell ID  126  of the potential handover target cell C j  of target system A to RNC  104 . In response program  118  checks whether this cell C j  is capable of becoming a serving cell for wireless telecommunication link  108 . If this is the case an acceptance  128  is sent from RNC  104  to RNC  102 .  
         [0045]    In the case of several potential target systems, it is also possible to send a list of cell IDs to each of the target systems and not only a cell ID.  
         [0046]    Likewise RNC  102  sends target cell ID  130  of the potential handover target cell C k  of target system B to RNC  124 . In response program  118  of RNC  124  checks whether this cell C k  is capable of becoming a serving cell for the wireless telecommunication link  108 . If the cell C k  is already running at full capacity refusal  132  is sent from RNC  124  to RNC  102 . In response RNC  102  initiates a handover or reselection procedure to cell C j  with target cell ID  126 .  
         [0047]    When more than more acceptance is received by RNC  102  from the target systems A and B, RNC  102  can make a random selection of the accepted target cell IDs. Alternatively the selection can be based on other criteria such as quality of the coverage, i.e. field strength, load balancing, etc.  
         [0048]    [0048]FIG. 3 shows a block diagram of telecommunication system  300 . Elements of telecommunication system  300  which correspond to elements of telecommunication system  100  of FIGS. 1 and 2 are designated by like reference numerals having added  200 .  
         [0049]    In contrast to telecommunication system  100  of FIGS. 1 and 2, telecommunication system  300  encompasses at least two different communication standards and air interfaces. For example sub-system  310  is a GSM-type system whereas sub-system  312  is a UMTS-type system. Typically sub-system  310  and  312  will cover at least overlapping areas. User equipment  306  has dual mode capability, i.e. it is capable of establishing wireless telecommunication link  308  in accordance with the GSM standard as well as in accordance with the UMTS standard. For this purpose user equipment  306  has two corresponding air interfaces.  
         [0050]    It will be understood by a person skilled in the art that the invention is not only applicable for handover between different types of radio communication networks (GSM, UMTS) but also between different modes of one radio communication networks (UMTS-TDD and UMTS FDD) In this case, sub-system  310  could be an UMTS-FDD mode system whereas sub-system  312  is a UMTS-TDD mode system.  
         [0051]    As regards GSM-type sub-system  310  the controller of the sub-system  310  is referred to as “base station controller” (BSC) and the transceiver stations are referred to as “base transceiver stations” (BTS).  
         [0052]    As user equipment  306  periodically scans the frequencies of both air interfaces the list  314  contains a list of cell IDs of both sub-system  310  and sub-system  312 .  
         [0053]    When the cell C i  which serves user equipment  306  becomes overloaded or when the entire sub-system  310  becomes overloaded BSC  302  can make the decision that a handover or reselection procedure for link  308  to the alternative air interface is necessary. Such a situation is also referred to as “inter-system handover”. Another reason for inter-system handover is better reception from one of the target system cells by the mobile terminal.  
         [0054]    In order to initiate the handover or reselection of wireless cellular communication link  308  BSC  302  provides list  316  to RNC  304 . List  316  contains one or more cell IDs of potential handover target cells C j  of sub-system  312 . Again program  318  selects one of the cells indicated in the list  316  which is capable of becoming a serving cell for the wireless telecommunication link  308 . The cell ID  320  of that selected target cell is communicated from RNC  304  to BSC  302 . In response  302  initiates an inter-system handover of wireless telecommunication link  308  to the target cell with cell ID  320 .  
         [0055]    [0055]FIG. 4 is illustrative of a corresponding flow chart. In step  400  a user equipment of the source system detects potential handover target cells within its vicinity. In step  402  the list of the corresponding target cell IDs is provided to the source system controller, such as a RNC in the case of UMTS or a BSC in the case of GSM.  
         [0056]    In step  404  the source system controller sends target cell IDs of potential handover target cells which are outside its scope to one or more target system controllers of the same or a different air interface type. In response a processing routine in the target system is invoked, in order to chose one or more of the possible target cells based on signal strength, cell load, etc.  
         [0057]    In step  406  the source system controller receives the responses of the one or more target system controllers. Based on the responses received in step  406  the source system controller initiates a handover or reselection procedure to a target cell which has been selected by one of the target system controllers.  
       LIST OF REFERENCE NUMERALS  
       [0058]    [0058] 100  telecommunication system  
         [0059]    [0059] 102  radio network controller (RNC)  
         [0060]    [0060] 104  radio network controller (RNC)  
         [0061]    [0061] 106  user equipment  
         [0062]    [0062] 108  wireless telecommunication link  
         [0063]    [0063] 110  sub-system  
         [0064]    [0064] 112  sub-system  
         [0065]    [0065] 114  list  
         [0066]    [0066] 116  list  
         [0067]    [0067] 118  program  
         [0068]    [0068] 120  target cell ID  
         [0069]    [0069] 122  sub-system  
         [0070]    [0070] 124  RNC  
         [0071]    [0071] 126  target cell ID  
         [0072]    [0072] 128  acceptance  
         [0073]    [0073] 130  target cell  
         [0074]    [0074] 132  refusal  
         [0075]    [0075] 300  telecommunication system  
         [0076]    [0076] 302  base station controller (BSC)