Abstract:
Handover of a mobile station (MS) from a serving base station (BS) to a target base station in a mobile communication system supporting both MS-initiated handover and BS-initiated handover, is coordinated by ensuring that, both in the case of BS-initiated handover and in the case of MS-initiated handover the mobile station (MS) and the base station (BS) share information, either via a MO_BSHO-REQ message sent from the BS to the MS in the case of BS-initiated HO or via a MO_MSHO-RETQ message sent from the MS to the BS in the case of MS-initiated HO, indicating a target base station for the mobile station (MS) to attempt handover; and the mobile station (MS) sends an information message (MOB_HO-IND) indicating the base station to which handover was attempted.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates to techniques for supporting handover (HO) in mobile communication networks. 
         [0002]    This disclosure was devised with specific attention paid to its possible use in Mobile Wireless Broadband Access network conforming to the IEEE 802.16Rev2 specification. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    At present, the IEEE802.16Rev2 specification (IEEE Computer Society and IEEE Microwave Theory and Techniques Society, DRAFT Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Broadband Wireless Access Systems, P802.16Rev2/D1 (October 2007) Revision of IEEE Std 802.16-2004 as amended by IEEE Std 802.16f-2005 and IEEE Std 802.16e-2005) supports both MS (i.e. Mobile Station)-initiated handover and BS (i.e. Base Station)-initiated handover. 
         [0004]    However the current specification does not allow using the BS-initiated handover mechanism alone: MS-initiated handover cannot be disabled in the system when BS-initiated handover is used. Also, the MS-initiated handover mechanism always takes precedence in case a MS-initiated handover decision and a BS-initiated handover decision occur at the same time. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0005]    The behaviour as described above leads to high risk of system instability when a BS-initiated handover feature is enabled in the system. 
         [0006]    Different handover algorithms and criteria are typically used at the Base Station (BS) side and at the Mobile Station (MS) side. MS-initiated handover aims at individual link optimisation by taking into account channel quality state at the MS receiver. BS-initiated handover has a wider scope, since the base station can take into account the overall network resources status when taking the handover decision. The base station may typically command handover toward a target BS which is not the best serving cell in order to set free valuable radio resources on the best serving cell (e.g. in case of Underlay/Overlay cell scenario). The mobile station may immediately trigger handover back to the best serving cell and so on. 
         [0007]    The Inventors have noted that aforementioned problem has not been solved up to now in the IEEE802.16Rev2 specification i.e. in a mobile communication system supporting both MS-initiated handover and BS-initiated handover. In order to avoid system instability, the need exists of ensuring that MS-initiated handover and BS-initiated handover processes are coordinated. 
         [0008]    The object of the invention is thus to provide a satisfactory response to that need. 
         [0009]    According to the present invention, that object is achieved by means of a method having the features set forth in the claims that follow. 
         [0010]    The invention also relates to a corresponding system, as well as corresponding apparatus (i.e. mobile station/base station) and a computer program product, loadable in the memory of at least one computer and including software code portions for performing the steps of the method of the invention when the product is run on a computer. As used herein, reference to such a computer program product is intended to be equivalent to reference to a computer-readable medium containing instructions for controlling a computer system to coordinate the performance of the method of the invention. Reference to “at least one computer” highlights the possibility for the present invention to be implemented in a distributed/modular fashion. 
         [0011]    The claims are an integral part of the disclosure of the invention provided herein. 
         [0012]    In an embodiment, a method is proposed to coordinate MS-initiated handover process and BS-initiated handover process, in such a way that the two procedures can be supported together in the system, in a manner which can be defined a “BS-controlled MS-initiated handover”. 
         [0013]    An embodiment of the solution described herein lends itself to be incorporated in the IEEE802.16Rev2 specification by introducing some modifications in section 6.3.22.2.2 and other related sections thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE ANNEXED REPRESENTATIONS 
         [0014]    The invention will now be described, by way of example only, with reference to the enclosed representations, wherein: 
           [0015]      FIG. 1  is a schematic representation of a BS-initiated handover in a mobile communication network with no trigger events detected by a mobile station (MS); 
           [0016]      FIG. 2  is a schematic representation of a BS-initiated handover in a mobile communication network in the presence of a trigger event detected by a mobile station; 
           [0017]      FIG. 3  is a schematic representation of a MS-initiated handover in a mobile communication network with no new trigger events detected by the mobile station; and 
           [0018]      FIG. 4  is a schematic representation of a MS-initiated handover in a mobile communication network in the pence of a new trigger event detected by the mobile station. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments. 
         [0020]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0021]    The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
         [0022]    The mechanism underlying the arrangement disclosed herein will be described in the following in connection with  FIGS. 1 to 4  by referring to the interplay between a Mobile Station (MS) and a Base Station (BS) in a mobile communication network. The general principles of operation of such a network are well known in the art and do not require to be described in detail herein. 
         [0023]    By way of background, the BS may set an MS-initiated handover trigger event via Trigger TLV in DCD message (as described in section 11.4.1 of the IEEE802.16Rev2 specification) or via Neighbor BS Trigger TLV in MOB-NBR-ADV message (as described in section 11.18.2 of the same specification). Trigger TLV and Neighbor BS Trigger TLV allow configuring the conditions under which a HandOver (HO) can be initiated by MS. 
         [0024]    In the exemplary embodiment described herein, the HO shall always proceed with a notification through either MOB_MSHO-REQ or MOB_BSHO-REQ messages, so that the MS and BS will in any case share (either via a MOB_BSHO-REQ message sent from the BS to the MS in the case of BS-initiated HO or via a MOB_MSHO-REQ message sent from the MS to the BS in the case of MS-initiated HO) information indicating a target base station for the mobile station to attempt handover. 
         [0025]    This is in contrast with the prior art, where notification in case of MS initiated HO (i.e. MOB_MSHO-REQ message) was not mandatory. 
         [0026]    MS shall send MOB_MSHO-REQ in case a trigger conditions specified in Trigger TLV or Neighbor BS Trigger TLV has occurred. 
         [0027]    MS actual pursuit of HO to one of BSs specified in MOB_BSHO-RSP is required, apart from those cases stated in next paragraph. 
         [0028]    MS may decide to attempt HO to a BS not included in MOB_BSHO-RSP, sending a MOB_HO-IND indicating the selected BS, only in case a trigger condition specified in Trigger TLV or Neighbor BS Trigger TLV has occurred for that target BS. 
         [0029]    In an embodiment of the arrangement disclosed herein, system behavior in case of handover is as described in the following. 
         [0030]    As illustrated in  FIG. 1 , when the BS detects the need for HO (step  100 ) HO is initiated by the BS (i.e. in a step  102  the serving BS sends a MOB_BSHO-REQ message indicating a new target BS, e.g. BS “A”). If the MS does not detect trigger conditions for a target BS not included in MOB_BSHO-REQ (i.e. trigger events specified in Trigger TLV and Neighbor BS Trigger TLV are not detected for BSs different from “A”—step  104 ), then in a step  106  the MS shall conduct HO toward the target BS indicated by the serving BS (i.e. in a step  108  the MS sends an MOB_HO-IND message indicating a BS included in MOB 13  BSHO-REQ, thus attempting HO to BS “A”). 
         [0031]    Conversely, as illustrated in  FIG. 2 , when the BS detects the need for HO (step  200 ) and HO is initiated by the BS (i.e. in a step  202  the serving BS sends a MOB_BSHO-REQ message indicating a new target BS, e.g. BS “A”) and the MS does detect trigger conditions for a target BS not included in MOB_BSHO-REQ (i.e. trigger events specified in Trigger TLV and Neighbor BS Trigger TLV are detected for, say, BS “B”, different from “A”—step  204 ), in a step  206  the MS may conduct HO toward a target BS indicated by the serving BS. Specifically, in a step  208  the MS will send an MOB_HO-IND message either towards a BS included in the MOB_BSHO-REQ message or toward the target BS for which a trigger condition is met (i. e. the MS sends an MOB_HO-IND message indicating the BS for which the trigger condition is met). 
         [0032]      FIG. 3  is representative of the case where HO is initiated by the MS, (i.e. in a step  300  the MS detects trigger events specified in Trigger TLV and Neighbor BS Trigger TLV for HO towards BS “A”.). When the trigger condition is met, the MS sends an MOB_MSHO-REQ message (step  302 ) indicating a new target BS (e.g. “A”) and receives from the BS (in a step  304 ) a MOB_BSHO-RSP message indicating to the MS to attempt HO to the BS “A”. After receiving the MOB_BSHO-RSP message, if the MS channel conditions are not changed (which is verified in a step  306 ) and the MS does not detect new trigger conditions (step  308 ), in a step  310  the MS shall conduct HO toward a target BS for which the trigger condition is met. Specifically, in a step  312  the MS sends an MOB_HO-IND message indicating the BS for which the trigger condition is met. 
         [0033]      FIG. 4  refers to the situation where HO is initiated by the MS (i.e. in a step  400  the MS detects trigger events specified in Trigger TLV and Neighbor BS Trigger TLV for HO towards BS “A”.) and the trigger condition being met leads to the MS sending (in a step  402 ) a MOB_MSHO-REQ message indicating a new target BS, say, “A”. In a step  404 , the BS issues a MOB_BSHO-RSP message indicating to the MS to attempt HO to BS “A”. The MS detects (in a step  406 ) new trigger conditions i.e. the channel conditions are changed and BS “B” is now the best target. 
         [0034]    In fact, detection of the new trigger condition may (and typically does) take place even before receiving the MOB_BSHO-RSP message. New trigger conditions can be detected at any time during the time interval between transmission of the MOB_MSHO-REQ message and transmission of the MOB_HO-IND message. 
         [0035]    In a step  408  trigger events specified in Trigger TLV and Neighbor BS trigger TLV are detected for BS “B”, and the MS may conduct, in a step  410 , HO towards either of target BSs “B” or “A”. Specifically, step  412  is representative of the MS attempting HO to the BS for which the first trigger condition is met (i.e. the MS sends a MOB_HO-IND message indicating the BS for which the first trigger condition is met) or to the target BS for which the second trigger condition is met (i.e. MS sends a MOB_HO-IND message indicating the BS for which the second trigger condition is met). 
         [0036]    It will be appreciated that operation as described in connection with  FIGS. 1 to 4  requires—in contrast with the prior art—that transmission of the MOB_HO-IND message is—always—mandatory. 
         [0037]    An advantage of the arrangement disclosed herein lies is that it allows Mobile WiMAX systems to support a BS-initiated handover feature without risks of system instability. BS-initiated handover is a basic functionality supporting value added features as Multicarrier networks, Overlay/Underlay cells and Layered networks (micro/macro cells). 
         [0038]    Large capacity networks can be obtained by deploying layered network structures. A certain geographical location can be served by signals from few cells presenting different radio channel conditions (e.g. small/large cells, micro/macro cells). In a similar scenario, a MS may regularly tend to select larger cells as a target cell for handover since they provide best radio channel conditions, leaving the smaller cells unloaded. BS-initiated handover can be used to the purpose of distributing traffic load efficiently from network and spectrum resource perspective. The arrangement disclosed herein ensures that the BS-initiated decision is enforced at the MS side and that it is not wiped off by a subsequent decision from the MS to handover back to the old serving cell. 
         [0039]    The following is a description of certain proposed modifications which might be introduced in IEEE802.16Rev2 specification to implement an embodiment. 
         [0000]    Enhancement of Subsection 6.3.2.3.5, “Ranging request (RNG-REQ) message” 
         [0040]    A new parameter BS Controlled_MSHO may be introduced in Table 622 “RNG-REQ” message encodings. The parameter defines if BS Controlled MS-initiated handover is supported by the MS. 
         [0000]    Enhancement of Subsection Table 615 “DCD channel encoding” 
         [0041]    A new value may be introduced for HO type support TLV. Bit  3  may be named BS_Controlled_MSHO (or BS_Controlled_HO). This flag may be used to indicate support of BS Controlled MSHO mechanism in the cell. 
         [0000]    Amendment of Subsection 6.3.22.2.2, “HO decision and initiation” 
         [0042]    Certain possible proposed modifications are reported in the following sentences. 
         [0043]    In case of MS-initiated HO, HO initiation and notification mode depends on the settings of the BS_Controlled_MSHO (or BS_Controlled_HO) flag in HO type support field of DCD message and on the BS_Controlled_MSHO field in the RNG-REQ message. 
         [0044]    In case BS_Controlled_MSHO field is not included in RNG-REQ message, the default value “0” shall be applied. 
         [0045]    If BS_Controlled_MSHO flag is set to “0” in HO type support field of DCD message or Value field of BS_Controlled_MSHO in RNG-REQ message is set to “0”, the HO may proceed with a notification through either MOB_MSHO-REQ or MOB_BSHO-REQ messages. The HO notification is recommended, but not required. Acknowledgement of MOB_MSHO-REQ with MOB_BSHO-RSP is required. 
         [0046]    If BS_Controlled_MSHO flag is set to “1” in HO type support field of DCD message and Value field of BS_Controlled_MSHO in RNG-REQ message is set to “1”, HO notification is required. The HO shall proceed with a notification through either MOB_MSHO-REQ or MOB_BSHO-REQ messages. Acknowledgement of MOB_MSHO-REQ with MOB_BSHO-RSP is required. MS shall send MOB_MSHO-REQ only in case a triggering conditions specified in Trigger TLV or Neighbor BS Trigger TLV has occurred. 
         [0047]    If BS_Controlled_MSHO flag is set to “0” in HO type support field of DCD message or Value field of BS_Controlled_MSHO in RNG-REQ message is set to “0”, MS actual pursuit of HO to one of BSs specified in MOB_BSHO-RSP is recommended, but not required. MS may decide to attempt HO to a different BS that may or may not have been included in MOB_BSHO-RSP. 
         [0048]    If BS_Controlled_MSHO flag is set to “1” in HO type support field of DCD message and Value field of BS_Controlled_MSHO in RNG-REQ message is set to “1”, MS actual pursuit of HO to one of BSs specified in MOB_BSHO-RSP is required. MS may decide to attempt HO to a BS not included in MOB_BSHO-RSP, sending a MOB_HO-IND indicating the selected BS, only in case a triggering condition specified in Trigger TLV or Neighbor BS Trigger TLV has occurred for that target BS. 
         [0049]    If BS_Controlled_MSHO flag is set to “0” in HO type support field of DCD message or Value field of BS_Controlled_MSHO in RNG-REQ message is set to “0”, a MS required to conduct HO is not restricted to conducting HO to those BS included in the notifying message. In other words, the MS may attempt HO to a different BS that may or may not have been included in either the MOB_BSHO-REQ or MOB_BSHO-RSP. 
         [0050]    If BS_Controlled_MSHO flag is set to “1” in HO type support field of DCD message and Value field of BS_Controlled_MSHO in RNG-REQ message is set to “1”, a MS required to conduct HO is restricted to conducting HO to those BS included in the notifying message. In other words, the MS may not attempt HO to a BS not included in the MOB_BSHO-REQ or in the MOB_BSHO-RSP unless a trigger event specified in Trigger TLV or Neighbor BS Trigger TLV is detected for that target BS. 
         [0051]    Without prejudice to the underlying principles of the invention, the details and embodiments may vary, even appreciably, with respect to what has been described by way of example only, without departing from the scope of the invention as defined by the annexed claims.