Patent Publication Number: US-2007105558-A1

Title: Apparatus and method for performing handover for multi-hop system in a broadband wireless access communication network

Description:
PRIORITY  
      This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Performing Handover For Multi-Hop in a Broadband Wireless Access Communication Network” filed in the Korean Intellectual Property Office on Nov. 10, 2005 and assigned Serial No. 2005-107644, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to handover, and in particular, to an apparatus and method for performing handover for a multi-hop system in a Broadband Wireless Access (BWA) communication network.  
      2. Description of the Related Art  
      Along with the increasing demands for services requiring high data rates, there is a pressing need for communication systems offering higher data rates than 3 rd  Generation (3G) mobile communication systems based on Code Division Multiple Access (CDMA). A multi-hop relay scheme is a requisite for designing a higher-rate system and increasing service coverage.  
      The multi-hop relay scheme increases data rate and expands service coverage by use of a Multi-Hop Base Transceiver Station (MH-BTS) between a BTS or Base Station (BS) and a Mobile Station (MS). The term MH-BTS is herein interchangeable with Relay Station (RS).  
       FIG. 1  is a diagram of a signal flow for a handover procedure in a conventional BWA communication network. Messages exchanged during the handover procedure are compliant with the Institute of Electrical and Electronics Engineers (IEEE) 802.16.  
      Referring to  FIG. 1 , a BTS  160  broadcasts a Mobile Neighbor Advertisement (MOB_NBR-ADV) message including neighboring BTS information to an MS  150  in step  100  and MS  150  sends to BTS  160  a Mobile MS Handover Request (MOB_MSHO-REQ) message based on the indexes of neighbor BTSs indicated by the MOB_NBR-ADV message in step  101 . The MOB_MSHO-REQ message, which requests handover initiation, includes a list of candidate BTSs for an MS handover.  
      MS  150  may use the neighbor BTS information acquired from the MOB_NBR-ADV message or neighboring BTS information acquired by autonomous scanning in sending the MOB_MSHO-REQ message. In the latter case, the Identifiers (IDs) of the neighboring BTSs are used.  
      In step  103 , BTS  160  notifies MS  150  whether the candidate BTSs can accept the handover request by a Mobile BS Handover Response (MOB_BSHO-RSP) message.  
      MS  150  notifies BTS  160  of a target BTS by a Mobile Handover Indication (MOB_HO-IND) message in step  105 . The handover is performed using the ID (CID) of a Connection between MS  150  and BTS  160 .  
      Since the above conventional handover procedure involves no MH-BTSs, a novel handover procedure and signaling method need to be defined to facilitate the implementation of MH-BTSs for relaying.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for performing handover for a multi-hop system in a BWA communication network.  
      The above object is achieved by providing a handover apparatus and method for a multi-hop system in a BWA communication network.  
      According to one aspect of the present invention, in a handover system supporting multi-hop system in a BWA communication network, a BS periodically broadcasts a message including information about neighboring BSs and neighboring RSs. Upon receipt of the message, the BS identifies the sending node of the message by an ID encapsulated in the message and sends a message having the encapsulated ID to the node. The BS also performs a handover operation. An RS receives the broadcast message from the BS. When a new BS or RS needs to be added, the RS includes information about the new BS or RS in the broadcast message and periodically broadcasts it. Upon receipt of the message from the node, the RS sends the message with the ID of the node to the BS. Upon receipt of the message including the ID of the node from the BS, the RS sends the received message to the node. An MS receives the broadcast message from the RS and requests information about BSs or RSs to which the MS can perform a handover by notifying candidate BSs or RSs for the handover. The candidate BSs or RSs may include a BS or RS not included in the broadcast message, if the MS detects the BS or RS by scanning. The MS selects a target BS or RS based on the information about the BSs or RSs to which the node can perform the handover, received from the BS, and performs the handover to the target BS or RS.  
      According to another aspect of the present invention, in a handover method in a BWA communication network supporting a multi-hop system, the RS receives a handover request message from an MS. The RS determines whether candidate BSs or RSs for an MS handover set in the handover request message are BSs or RSs set by a BS, BSs or RSs set by the RS, or BSs or RSs detected by the MS. If the candidate BSs or RSs are the BSs or RSs set by the RS, the RS reconfigures information about the candidate BSs or RSs, including the reconfigured information and an ID of the MS in the handover request message, and sends the handover request message with the reconfigured information and the ID of the MS to the BS. Upon receipt of a response message with the ID of the MS for the handover request message from the BS, the RS eliminates the ID of the MS and sends the response message without the ID of the MS to the MS. Upon receipt of a handover message from the MS, the RS eliminates the ID of the MS in the handover message and sends the handover message with the ID of the MS to the BS and without the ID of the MS to the MS. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a diagram of a signal flow for a handover procedure in a conventional BWA communication network;  
       FIG. 2  is an overview of handover cases that can be considered in a BWA communication network supporting multi-hop system according to the present invention;  
       FIG. 3  is a flowchart of an operation for sending a MOB_NBR-ADV message in an MH-BTS in the BWA communication network supporting the multi-hop system according to the present invention;  
       FIG. 4  is a flowchart of an operation for sending a MOB_NBR-ADV message in a BTS in the BWA communication network supporting the multi-hop system according to the present invention;  
       FIG. 5  is an operation of the MH-BTS when the MH-BTS receives a MOB_MSHO-REQ message according to the present invention;  
       FIG. 6  is a flowchart of an operation for processing a message destined for the BTS in the MH-BTS according to the present invention;  
       FIG. 7  is a flowchart of an operation for processing a message destined for an MS in the BTS according to the present invention; and  
       FIG. 8  is a flowchart of a handover procedure in the BWA communication network according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
      The present invention provides an apparatus and method for performing handover in a multi-hop BWA network communication system.  
       FIG. 2  is an overview of handover cases that can be considered in a BWA communication network supporting a multi-hop system according to the present invention.  
      Referring to  FIG. 2 , an MS is connected to an MH-BTS and thus communicates data with the BTS by the relay function of the MH-BTS.  
      Handover can be triggered depending on the location of the MS, for example where the MS moves from one MH-BTS to another within the same cell (indicated by a)), or the MS moves from a BTS to an MH-BTS within the same cell (indicated by b)), or the MS moves from an MH-BTS to a BTS within the same cell (indicated by c)), or the MS moves from an MH-BTS in one cell to a BTS in another cell (indicated by d)), or the MS moves from an MH-BTS in one cell to an MH-BTS in another cell.  
      When the MS moves to a neighboring cell by handover, it needs information about the MH-BTS through which it will communicate with the neighboring cell, as well as information about the neighboring cell.  
       FIG. 3  is a flowchart of an operation for sending a MOB_NBR-ADV message to an MH-BTS in the BWA communication network supporting the multi-hop system according to the present invention. The MOB_NBR-ADV message is broadcast to an MS periodically from a BTS or an MH-BTS, carrying information about neighboring BSs and/or neighboring MH-BTSs.  
      Referring to  FIG. 3 , the MH-BTS receives information about neighboring BSs and/or neighboring MH-BTSs from the BTS by a MOB_NBR-ADV message in step  300 .  
      In step  301 , the MH-BTS determines whether to add information about other MH-BTSs or BSs neighboring the MH-BTS to information about the neighboring BSs and/or the neighboring MH-BTSs received from the BS. If it is determined that information about new neighboring BTS is to be added in step  302 , the MH-BTS reconfigures the MOB_NBR-ADV message by further including the information of the new neighbor (BTS) in the MOB_NBR-ADV message and sends it to the MS in step  303 .  
      If it is determined that information about new neighbor MH-BTS is to be added in step  302 , the MH-BTS reconfigures the MOB_NBR-ADV message by further including the information of the new neighbor (MH-BTS) in the MOB_NBR-ADV message and sends it to the MS in step  304 .  
      On the other hand, if the information of the new neighbor (BTS or MH-BTS) is required in step  301 , the MH-BTS simply broadcasts the received MOB_NBR-ADV message to the MS in step  305 . Then the MH-BTS ends the process of the present invention.  
      The MOB_NBR-ADV message sent by the MH-BTS has the following configuration.  
                       TABLE 1                       Syntax   Size (bits)   Notes                                            MOB_NBR-               ADV_message_format( )       {       Management message type=53   8       Skip-optional-fields bitmap   8   Bit[0]: if set to 1, omit Operator id field               Bit[1]: if set to 1, omit Neighbor BSID field               Bit[2]: if set to 1, omit HO process               optimization field               Bit[3]: if set to 1, omit QoS related fields               Bit[4]: if set to 1, omit Neighbor RSID field               Bit[5]-[7]: reserved       Operator id   24   Operator ID       Configuration change count   8   Incremented each time the information for               the associated neighbor BS or RS has               changed       N_NEIGHBORS   8   Number of BSs and RSs       For (j=0; j&lt;N_NEIGHBORS;       J++) {       Phy profile id (if set)   8   Aggregated IDs for Phy profile       FA index (if set)   8   Frequency assignment index       Neighbor BSID (if set)   24   BS ID for OFDMA phy       Neighbor RSID (if set)   24   RS ID for OFDMA phy       Preamble index/subchannel index   24       TLV neighbor information       DCD settings, UCD settings       }       }                  
 
      Because the information of the new neighbor (MH-BTS) is added in step  304 , a 24-bit “Neighbor RSID” field is further included in the MOB_NBR-ADV message to provide that information.  
      In addition to the “Neighbor RSID”, a 24-bit “Preamble index/subchannel index” field and a “TLV neighbor information” field indicating DCD settings and UCD settings provide the neighbor MH-BTS information. The fifth bit Bit[ 4 ] of “Skip-optional-fields bitmap” in Table 1 is allocated in case no neighbor MH-BTSs are included.  
      This bit is associated with the “Neighbor RSID” field. When the fifth bit of Skip-optional-fields bitmap is set to 1, this implies the absence of the “Neighbor RSID” field (i.e. the absence of any neighboring MH-BTS). This bit is used to ensure backward compatibility. The “Neighbor RSID” field is included if the BTS sets information about its neighbor BSs and MH-BTSs as shown in  FIG. 4 , or if the MH-BTS sets information about its neighbor BSs or MH-BTSs as shown in  FIG. 3 .  
       FIG. 4  is a flowchart of an operation for sending a MOB_NBR-ADV message in the BTS in the BWA communication network supporting the multi-hop system according to the present invention.  
      Referring to  FIG. 4 , a serving BTS acquires information about neighbor BSs in step  400  and determines whether the serving BTS or the neighbor BSs have MH-BTSs in step  401 . The serving BTS is a BTS currently connected to the MS.  
      In the presence of MH-BTSs, the serving BTS sets information about the neighbor BSs and the neighbor MH-BTSs in the MOB_NBR-ADV message as shown in Table 1 and broadcasts it in step  403 . Conventionally “Operator id” and “Neighbor BSID” are set in the MOB_NBR-ADV message. According to the present invention, in the presence of neighbor MH-BTSs, a 24-bit “Neighbor RSID” field is added to thereby identify MH-BTSs under each BS. That is, “Operator id”, “Neighbor BSID” and “Neighbor RSID” are set in the MOB_NBR-ADV message.  
      In the absence of any MH-BTS, the serving BTS sends the neighbor BTS information by the MOB_NBR-ADV message as formatted conventionally in step  405 . In this case, Bit[ 4 ]=1 in Skip-optional-fields bitmap to indicate the absence of any MH-BTS. Then the serving BTS ends the process of the present invention.  
      Upon receipt of the MOB_NBR-ADV message, the MS sends a MOB_MSHO-REQ message including information about candidate. BSs and/or MH-BTSs for the MS handover. When the MH-BTS receives the MOB_MSHO-REQ message from the MS, it operates as follows.  
      Referring to  FIG. 5 , upon receipt of the MOB_MSHO-REQ message in step  501 , the MH-BTS checks the presence or absence of N_new_BS-index in the received message in step  503 .  
      If the N_new_BS-index field is included in step  503 , the MH-BTS reads Neighbor_BS_index in step  505 .  
      In step  509 , the MH-BTS determines whether information about neighbor BSs and/or MH-BTSs listed in Neighbor_BS_index was created and included in the MOB_NBR-ADV message by the MH-BTS.  
      If the MH-BTS created the neighbor BTS/MH-BTS information, referring to its database in step  511  it reconfigures the MOB_MSHO-REQ message to include the MS-requested neighbor BTS/MH-BTS information as shown in Table 2. Specifically, the MH-BTS encapsulates the Connection ID (CID) for the MS and the MH-BTS to indicate the MS&#39;s request, including a new N_new_BS_RS_index field and its associated fields in the MOB_MSHO-REQ message, and sends the reconfigured MOB_MSHO-REQ message to the BTS. In this case, the neighbor MH-BTS/BS information created by the MH-BTS is moved from N_new_BS_index to N_new_BS_RS index. If N_new_BS_Full is included in the MOB_MSHO-REQ message, this field just carries information received from the MS to the BTS without any change.  
      On the other hand, if the MH-BTS did not create the neighbor BS/MH-BTS information included in Neighbor_BS_index, that is, if the MH_BTS just forwarded the neighbor MH-BTS/BS information received from the BTS to the MS in step  509 , it sends the MOB_MSHO-REQ message to the BTS in step  513  with the CID for the MS and the MH-BTS encapsulated therein. This message does not include the N_new_BS_RS_index field and its associated fields.  
      If N_new_BS_index is not included in step  503 , this implies that the MS recommends MS-scanned BTSs and/or MH-BTSs as candidates for the MS handover. Therefore, the MH-BTS encapsulates the CID for the MS and the MH-BTS in the MOB_MSHO-REQ message with N_new_BS_Full to indicate the MS&#39;s request and sends the MOB_MSHO-REQ message to the BTS in step  507 . Then the MH-BTS ends the process of the present invention.  
      The MOB_MSHO-REQ message configured by the MH-BTS is shown in Table 2 below.  
                       TABLE 2                       Syntax   Size (bits)   Notes                                        MOB_MSHO-           REQ_message_format( )       {       Management message type=57   8                         Report metric   8   Bitmap for BTS CINR mean, BTS RSSI                                 mean, etc.                         N_new_BS_index       Number of new recommended BSs and RSs                                 which are included in MOB_NBR-ADV               message                     {                             Configuration change count for   8   Configuration change count value       MOB_NBR-ADV       For (j=0; j&lt;N_new_BS_index;       J++) {       Neighbor_BS index   8   Neighbor BTS index       Preamble index/subchannel index   8   Phy specific preamble/subchannel index       Service level prediction   3   Expected service level from this BTS or RS       }       }       N_new_BS_Full       Number of neighboring BSs and RSs to be                                 scanned or associated, which are not               included in MOB_NBR-ADV message                     {           For (j=0; j&lt;N_new_BS_Full; j++)       {                         Neighbor_BS_RS ID   48   BS or RS identifier       Preamble index/subchannel index   8   Phy specific preamble/subchannel index       Service level prediction   3   Expected service level from this BTS or RS       }       }       N_new_BS_RS_index   8   Number of new recommended BSs and RSs                                 which are included in MOB_NBR-ADV               message and created by RS                         Configuration change count for   8   Configuration change count value       MOB_NBR-ADV                         For   (j=0;                         j&lt;N_new_BS_RS_index; j++) {                             Neighbor_BS_RS ID   48   BS or RS identifier       Preamble index/subchannel index   8   Phy specific preamble/subchannel index       Service level prediction   3   Expected service level from this BTS or RS       }       Connection ID (CID)   16   Basic CID for MS and MH-BTS       }                  
 
      The N_new_BS_index field indicates candidate BSs and/or MH-BTSs for the MS handover included in the neighbor list of the MOB_NBR-ADV message. The N_new_BS_index field includes an 8-bit Configuration change count for MOB_NBR-ADV field indicating the count of the configuration changes of the MOB_NBR-ADV message, an 8-bit Neighbor_BS_index field indicating the index of a candidate BTS or MH-BTS, a Preamble index/subchannel index field indicating a preamble/subchannel index of the physical layer, and a 3-bit Service level prediction field indicating a service level expected from the candidate BTS or MH-BTS. A plurality of each of Neighbor_BS_index, Preamble index/subchannel index, and Service level prediction fields may exist according to the number of the candidate BSs and/or MH-BTSs.  
      The N_new_BS_RS_index field indicates candidate BSs and/or MH-BTSs that the MS has chosen among from neighbor BSs and/or MH-BTSs which were included in the neighbor list of the MOB_NBR-ADV message and created by the MH-BTS. The N_new_BS_RS_index field is created based on the N_new_BS_index field by the MH-BTS, for transmission to the BS. The N_new_BS_RS_index field includes an 8-bit Configuration change count for MOB_NBR-ADV field indicating the count of configuration changes of the MOB_NBR-ADV message, a 48-bit Neighbor_BS_RS_ID field identifying a candidate BTS or MH-BTS, an 8-bit Preamble index/subchannel index indicating a preamble/subchannel index of the physical layer, and a 3-bit Service level prediction field indicating a service level expected from the candidate BTS or MH-BTS. A plurality of each of Neighbor_BS_RS_index, Preamble index/subchannel index, and Service level prediction fields may exist according to the number of the candidate BTSs and/or MH-BTSs.  
      The N_new_BS_Full field indicates candidate BTSs and/or MH-BTSs that the MS has chosen from among MS-scanned BSs/MH-BTSs not included in the MOB_NBR-ADV message. The N_new_BS_Full field includes a 48-bit Neighbor_BS_RS_ID field identifying a candidate BTS or MH-BTS, an 8-bit Preamble index/subchannel index indicating a predetermined preamble/subchannel index of the physical layer, and a 3-bit Service level prediction field indicating a service level expected from the candidate BTS or MH-BTS. A plurality of each of Neighbor_BS_RS_ID, Preamble index/subchannel index, and Service level prediction fields may exist according to the number of the candidate BTSs and/or MH-BTSs.  
      Table 3 shows the information encapsulated in the MOB_MSHO-REQ message, that is, the CID for the MS and the MH-BTS included to indicate the request was issued from the MS.  
                       TABLE 3                       Syntax   Size (bits)   Notes                  Connection ID (CID)   16   Basic CID for MS and MH-BTS                  
 
       FIG. 6  is a flowchart of an operation for processing a message destined for the BTS in the MH-BTS according to the present invention.  
      Referring to  FIG. 6 , upon receipt of a message from the MS in step  600 , the MH-BTS determines if the message is destined for the BTS in step  601 . If the message is for the BS, the MH-BTS encapsulates the CID of the MS in the message and sends the message to the BTS in step  603 . The message can be MOB_MSHO-REQ or MOB_HO-IND in the present invention.  
      The MON_HO-IND message indicates that the MS will perform a handover to a particular target BTS or MH-BTS, configured as shown in Table 4.  
                       TABLE 4                       Syntax   Size (bits)   Notes                                            MOB_HO-IND_message_format( ){               Management message type=59   8       HO_IND_type   2   0b00: success               0b01: HO cancel               0b10: HO reject               0b11: reserved       Target_BS_ID   48   Target BTS or               RS ID       Preamble index   8   Preamble index       TLV encoded information   Variable   TLV specific       Connection ID (CID)   16   Basic CID for               MS and MH- BTS       }                  
 
      In Table 4, a Target_BS_ID field identifies a BTS or an MH-BTS to which the MS will perform a handover. A CID field indicates the encapsulated CID for the MS and the MH-BTS to notify that the MOB_HO-IND message is a request from the MS. If the received message is not destined for the BTS in step  601 , the MH-BTS processes the message in step  605  and ends the algorithm of the present invention.  
      Referring to  FIG. 7 , the BTS receives from the MH-BTS a message that the MH-BTS has received from the MS in step  700 .  
      The BTS acquires a CID from the message in step  701  and determines whether to send a response message for the received message to the MS via the MH-BTS in step  702 .  
      If the received message requires a response message, the BTS encapsulates the CID of the MS in the response message and sends the response message to the MH-BTS in step  703 .  
      For example, the response message is the MOB_BSHO-RSP message for the MOB_MSHO-REQ message. The MOB_BSHO-RSP message contains BTSs and/or MH-BTSs to which the MS can perform a handover, chosen from among candidate BTSs and/or MH-BTSs set in the MOB_MSHO-REQ message. The MOB_BSHO-RSP message has the following configuration.  
                       TABLE 5                       Syntax   Size (bits)   Notes                                            MOB_BSHO-RSP_message_format( ){               Management Message type=58   8       N_Recommended   8   Number of recommended candidates       For (j=0; j&lt;N_Recommended; j__){       Neighbor BSID   48   Recommended BTS or RS ID       Preamble index   8   Preamble index       Service level prediction   8   Expected service level from               recommended BTS or RS       Connection ID (CID)   16   Basic CID for MS and MH-BTS       }       }                  
 
      Referring to Table 5, a CID field indicates that the MOB_BSHO-RSP message is a response for the MOB_MSHO-REQ message. The CID between the MS and the MH-BTS is encapsulated within the CID field.  
      If the received message does not require a response message in step  702 , the BTS sends a corresponding message without the encapsulated CID in step  705  and ends the algorithm of the present invention.  
      Referring to  FIG. 8 , a first BTS  853  (BTS 1 ) broadcasts a MOB_NBR-ADV message in step  801 . The MOB_NBR-ADV message may include neighbor MH-BTS information by setting a 24-bit optional Neighbor RSID field.  
      Upon receipt of the MOB_NBR-ADV message, a first MH-BTS  852  (MH-BTS 1 ) simply forwards the MOB_NBR-ADV message to a first MS  851  (MS 1 ), or reconfigures the MOB_NBR-ADV message by adding new neighbor BTS or MH-BTS information and sends the reconfigured MOB_NBR-ADV message to MS 1  in step  803 .  
      In step  805 , MS 1  requests initiation of a handover by a MOB_MSHO-REQ message. In the presence of candidate BSs or MH-BTSs among neighboring BSs or MH-BTSs set in the MOB_NBR-ADV message, the MOB_MSHO-REQ message contains the number of new recommended BSs and RSs which are included in MOB_NBR-ADV message (N_new_BS_index)  
      If BSs or MH-BTSs discovered through scanning by MS 1  are candidates for the MS handover, the MOB_MSHO-REQ message contains the number of neighboring BSs and RSs to be scanned or associated, which are not included in MOB_NBR-ADV message (N_new_BS_Full).  
      If information about BSs or MH-BTSs listed in Neighbor_BS_index under N_new_BS_index was created by MH-BTS 1 , MH-BTS 1  adds N_new_BS_RS_index and associated fields to the MOB_MSHO-REQ message based on Neighbor_BS_index, and sends the MOB_MSHO-REQ message to BTS 1  in step  807 . The MOB_MSHO-REQ message has the CID for MS 1  and MH-BTS 1  encapsulated therein.  
      BTS 1  replies with a MOB_BSHO-RSP message for the MOB_MSHO-REQ message in step  809 . The MOB_BSHO-RSP message includes the encapsulated CID for MS 1  and MH-BTS 1 .  
      In step  811 , MH_BTS 1  forwards the MOB_BSHO-RSP message to MS  851 .  
      Upon receipt of the MOB_BSHO-RSP message, MS 1  determines a target BTS or a target MH-BTS and sends a MOB_HO-IND message to MH-BTS 1  in step  813 .  
      In step  815 , MH-BTS 1  encapsulates the CID in the MOB_HO-IND message and sends the resulting MOB_HO-IND message to BTS 1 .  
      As described above, the present invention provides a new handover procedure and new associated messages for an MS handover involving an MH-BTS in a BWA communication network supporting multi-hop system. Therefore, the handover can be performed without errors despite the involvement of the MH-BTS.  
      While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as further defined by the appended claims.