CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0117573 filed in the Korean Intellectual Property Office on Nov. 27, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    (a) Field of the Invention 
         [0003]    The present invention relates to a method and a system for enabling handover in a mobile communication network. More particularly, the present invention relates to a method and system for providing handover of a plurality of subscriber stations that have the same moving path. 
         [0004]    (b) Description of the Related Art 
         [0005]    A wireless portable Internet system such as a wireless broadband (WiBro) system or a high-speed portable internet (HPi) system is a type of mobile communication system. Such a wireless portable Internet system is a next generation communication scheme that provides mobility in local area data communication using a fixed access point (AP) such as a conventional wireless LAN. 
         [0006]    Typically, in a wireless portable Internet system, a handover is a function that provides mobility of a subscriber station. Location and situation where the handover occurs differ for respective subscriber stations, and quality of service (QoS) also differs depending on a service mobile communication network that is used. Accordingly, an entire handover is processed between each subscriber station and a serving base station in a one-to-one correspondence. 
         [0007]    Therefore, the wireless portable Internet system remains in an active state of services longer than a voice-dominated service system, and it performs handover more frequently than the same. For example, when a plurality of users are using the wireless portable Internet service in a transportation means such as a bus or a train, scanning as a pre-process of the handover and signaling as an actual operation of the handover are required to occur at respective terminals of the users at the same time. 
         [0008]      FIG. 1  is a flowchart showing a conventional handover process of a plurality of subscriber stations. 
         [0009]    Referring to  FIG. 1 , a plurality of subscriber stations collect cell information of a neighboring base station by receiving a neighbor advertisement (NBR-ADV) message from a serving base station (S 110 ). 
         [0010]    Subsequently, for a handover operation, each of the plurality of subscriber stations periodically performs a scanning process for checking a wireless state of a cell of the neighboring base station (S 120 ). 
         [0011]    When the subscriber stations move in the same direction and a handover condition for them is thereby satisfied, each of the subscriber stations requests handover by sending an MS-HO-REQ message to the serving base station. Then, the serving base station that has received the handover request separately sends a BS-HO-RSP message to each of the subscriber stations as a response to the handover request (S 130 ). 
         [0012]    Having received the BS-HO-RSP message from a serving base station, each of the subscriber stations separately sends an MS-HO-IND message to the serving base station to notify of handover completion (S 140 ), and then performs a process for ending a connection with the serving base station (S 150 ). 
         [0013]    Subsequently, the subscriber stations start a ranging process with a target base station of a cell to which the subscribe stations has moved, and thereby sets up a connection thereto so as to receive a wireless portable Internet service (S 160 ). 
         [0014]    As describe above, when a plurality of subscriber stations escape from a cell of a serving base station and move to another cell of a target base station, each of the subscriber stations separately requests handover to the serving base station and performs a ranging process with the target base station of the cell to which the subscriber stations are moving. 
         [0015]    When a plurality of subscriber stations simultaneously perform handover while moving together in a transportation means such as a train or a bus, transmission of messages for handover operation between the subscriber station and the serving base station abruptly increases so as to possibly cause insufficiency of a wireless band. 
         [0016]    During a random access of the subscriber stations to the base station in order to request a resource allocation, probability of collisions with subscriber stations that are already handled by the target base station increases. In this case, latency increases. 
         [0017]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention has been made in an effort to provide a method and a system for enabling handover in a mobile communication network having advantages of enhanced efficiency in bandwidth management and reduced latency during a handover process. 
         [0019]    An exemplary handover method according to an embodiment of the present invention includes: receiving neighboring base station information at a wireless relay station providing a wireless communication service moving together with a plurality of subscriber stations that move along a same moving path; performing scanning to check a wireless environment state of a neighboring base station using the received neighboring base station information; performing handover of the plurality of subscriber stations to a target base station as a single handover unit, when a handover condition is met during the scanning; and notifying the subscriber station of completion of handover preparation with the target base station. 
         [0020]    An exemplary system for enabling handover in a mobile communication network according to an embodiment of the present invention includes: a wireless relay station that determines whether a handover condition for a plurality of subscriber stations moving along the same moving path is met, and requests handover of the plurality of subscriber stations as a single handover unit when the handover condition is met; a serving base station that provides neighboring base station information to the wireless relay station and receives a handover request from the wireless relay station; and a target base station that performs the requested handover with the wireless relay station when a handover request of the wireless relay station is received through the serving base station. 
         [0021]    An exemplary handover method of a wireless relay station in a mobile communication network according to an embodiment of the present invention includes: receiving neighboring base station information; performing periodic scanning that checks a wireless environment state of a target base station using the neighboring base station information; determining whether a handover condition is met during the checking of the wireless environment; requesting handover of a plurality of subscriber stations to the target base station as a single handover unit when the handover condition is met, the plurality of subscriber stations moving along the same path; receiving a handover preparation completion message from the target base station; and sending, to the subscriber station, a message for notifying completion of handover preparation to the target base station. 
         [0022]    An exemplary wireless relay station performing handover in a mobile communication network according to an embodiment of the present invention includes: a transmitting/receiving unit that transmits and receives wireless signals with a serving base station and a target base station, the serving base station being currently associated with the wireless relay system; a base station information storage unit that stores neighboring base station information received from the serving base station through the transmitting/receiving unit; a wireless environment checking unit that checks the wireless environment state of the neighboring base station that corresponds to the neighboring base station information of the base station information storage unit; and a control unit that determines whether a handover condition for a plurality of subscriber stations moving along the same moving path is met based on the wireless environment state of the neighboring base station, and requests handover to the target base station when the handover condition is met. 
         [0023]    An exemplary handover method of a subscriber station according to an embodiment of the present invention includes: receiving a message notifying of completion of handover preparation to a target base station, the message being received from a wireless relay station for a plurality of subscriber stations moving along the same moving path; determining whether a dedicated handover region is allocated by the target base station; and performing ranging through the dedicated handover region when the dedicated handover is allocated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a flowchart showing a conventional handover process of a plurality of subscriber stations. 
           [0025]      FIG. 2  is a schematic diagram of a handover system using a wireless relay station according to an exemplary embodiment of the present invention. 
           [0026]      FIG. 3  is a flowchart showing a handover process according to an exemplary embodiment of the present invention. 
           [0027]      FIG. 4  is a drawing that schematically shows a wireless relay station according to an exemplary embodiment of the present invention. 
           [0028]      FIG. 5  is a flowchart showing a handover process taken by the wireless relay station according to an exemplary embodiment of the present invention. 
           [0029]      FIG. 6  is a flowchart showing a handover process taken by a subscriber station according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
         [0031]    In addition, unless explicitly described to the contrary, the word “comprise”, and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
         [0032]      FIG. 2  is a schematic diagram of a handover system using a wireless relay station according to an exemplary embodiment of the present invention. 
         [0033]    For convenience of description, an exemplary embodiment of the present invention is hereinafter described in connection with handover from a serving base station to a target base station according to a movement of a transportation means such as a bus, a train, and an airplane, using a wireless relay station (RS) that is installed in the transportation means and handles a plurality of subscriber stations. 
         [0034]    Referring to  FIG. 2 , a handover system using a wireless relay station according to an exemplary embodiment of the present invention includes a wireless relay station  220 , a serving base station  230 , a serving MSC (mobile switching center)  240 , a target MSC  250 , and target base station  260 . 
         [0035]    Various subscriber stations  211 ,  212 , . . . , and  219  are involved in the exemplary embodiment of the present invention, and they are collectively referred to as subscriber stations  210 . Subscriber stations  210  are wirelessly connected with the wireless relay station  220 , and they provide a voice call function with other mobile communication terminals through a wireless communication scheme. 
         [0036]    Here, the subscriber stations  210  according to an exemplary embodiment of the present invention is wirelessly connected with the serving base station  230  through the wireless relay station  220 , and may use a wireless portable Internet service. 
         [0037]    The wireless relay station  220  manages subscriber stations  210  that are located within a range that it covers. 
         [0038]    The wireless relay station  220  relays a downlink/uplink map that is system information (SI) received from the serving base station  230 , and transfers uplink and downlink traffic data to the subscriber stations  210  or the serving base station  230  in an appropriately modulated state. 
         [0039]    The wireless relay station  220  collects information of a neighboring base station by receiving a neighbor advertisement (NBR-ADV) message from the serving base station  230 . The wireless relay station  220  performs a scanning process for checking a wireless environment state of the neighboring base station based on the collected information of the neighboring base station. During the scanning process, the various subscriber stations  210  covered by the wireless relay station  220  are treated as a single group by the wireless relay station  220 . That is, when a handover condition from the serving base station  230  to the target base station  260  is satisfied, the wireless relay station  220  sends subscriber station information of the subscriber stations  210  and service class information to the target base station  260  through the serving base station  230 , and then receives a handover response therefrom. 
         [0040]    When the subscriber stations  210  are grouped by the wireless relay station  220  depending on service mobile communication networks, uniform quality of service (QoS) is acquired during the handover of the subscriber stations  210 . 
         [0041]    In order to reduce collision between subscriber stations  210  during a random access to the target base station  260  for handover of the subscriber stations  210 , it is preferable that the target base station  260  allocates a dedicated ranging region for the wireless relay station  220 . 
         [0042]    Here, it is preferable that the subscriber station information is unique basic connection identifiers (BCID) of the subscriber stations  210 . 
         [0043]    The information of a dedicated handover ranging region received from the target base station  260  is sent to the subscriber stations  210  by the wireless relay station  220 . While doing so, the wireless relay station  220  performs a process for ending the connection with the serving base station  230 . 
         [0044]    Constitution and operation of the wireless relay station  220  will be described later in further detail with reference to  FIGS. 4 and 5 . 
         [0045]    The serving base station  230  is a wireless base station of a wireless portable Internet network. The serving base station  230  includes a Node-B  231  that enables wireless communication with the wireless relay station  220  according to a predetermined protocol and a radio network control unit (RNC)  233  that controls the Node-B  231  and enables call processing. 
         [0046]    The serving base station  230  according to an exemplary embodiment of the present invention responds to a scanning request of the wireless relay station  220 . In response to the handover request from the wireless relay station  220 , the serving base station  230  sends the subscriber station information and service class information received from the wireless relay station  220  to the target base station  260  using a backbone message. Also, the serving base station  230  sends a handover acknowledgement (Ack) response of the target base station  260  to the wireless relay station  220 . 
         [0047]    The serving MSC  240  and the target MSC  250  are switching centers of the wireless portable Internet network. The serving MSC  240  performs various functions such as basic service processing of wireless portable Internet service, call processing of incoming calls for and originating calls from the subscriber stations, processing of location registration and hand-off, and interworking function with other networks. 
         [0048]    The serving MSC  240  according to an exemplary embodiment of the present invention sends the handover request of the wireless relay station  220  received through the serving base station  230  to the target MSC  250 . Also, the serving MSC  240  sends the handover acknowledgement response received from the target base station  260  to the serving base station  230 . 
         [0049]    The target MSC  250  according to an exemplary embodiment of the present invention sends the handover request of the wireless relay station  220  received through the serving MSC  240  to the target base station  260 . Also, the target MSC  250  sends the handover acknowledgement response received from the target base station  260  to the serving MSC  240 . 
         [0050]    The target base station  260  is a wireless base station of a wireless portable Internet network. The target base station  260  includes a Node-B  261  that enables wireless communication with the wireless relay station  220  according to a predetermined protocol, and a radio network control unit (RNC)  263  that controls the Node-B  261  and enables call processing. 
         [0051]    When receiving a backbone message including the handover request from the serving base station  230 , the target base station  260  according to an exemplary embodiment of the present invention checks its own radio resources and allocates a dedicated ranging region such that the ranging may be enabled for the subscriber stations  210  covered by the wireless relay station  220  after the handover. 
         [0052]    A multiple number of subscriber stations  210  are involved in handover of the wireless relay station  220 , although they are grouped. Therefore, during the ranging operation of the subscriber stations  210  to the target base station  260  after the handover, the multiple subscriber stations  210  simultaneously perform random access. While doing so, probability of collision with other subscriber stations that are already being serviced by the target base station  260  is expected to increase. 
         [0053]    Therefore, in order to reduce the collision probability between the subscriber stations  210  and other subscriber stations that are already being serviced by the target base station  260 , it is preferable that a dedicated ranging region is allocated for the multiple subscriber stations  210  when the radio resources are sufficient. 
         [0054]    In addition, the target base station  260  sends handover preparation completion information to the serving base station, by sending information of the dedicated ranging region to be allocated for the subscriber stations  210  through the backbone message. 
         [0055]      FIG. 3  is a flowchart showing a handover process according to an exemplary embodiment of the present invention, and the handover process according to an exemplary embodiment of the present invention is hereafter described with reference to  FIG. 3 . 
         [0056]    Firstly, the subscriber stations  210  and the wireless relay station  220  collect neighboring base station information received from the serving base station  230  through NBR-ADV message (S 310 ). 
         [0057]    Then, wireless relay station  220  requests scanning to the serving base station  230  in order to obtain the wireless environment state information of the neighboring base station (S 321 ), and then receives a response to the scanning request from the serving base station  230  (S 323 ). The wireless relay station  220  performs the requested scanning operation for a period designated by the serving base station  230  in order to obtain wireless environment state information of the neighboring base station, and it is preferable that a handover beginning time point is determined by a periodic scanning procedure. 
         [0058]    When a handover condition is satisfied according to a movement of the transportation means, the wireless relay station  220  sends a handover request message to the serving base station  230  (S 331 ). The handover request message includes subscriber station information and service class information of the subscriber stations  210  covered by the wireless relay station  220 . Then, the serving base station  230  transmits the handover request of the wireless relay station  220  to the target base station using a backbone message (S 333 ). 
         [0059]    Accordingly, the target base station  260  recognizes subscriber station information and service class information of the subscriber stations  210 , and then allocates a dedicated handover ranging region in order for the subscriber stations  210  to perform handover ranging (S 335 ). Then, the target base station  260  sends handover preparation completion information having allocation information of the dedicated handover ranging region to the serving base station  230  (S 337 ). It is preferable that the target base station  260  sends the handover preparation completion information through the backbone message. According to an exemplary embodiment of the present invention, it is preferable that the backbone message includes information that the handover preparation is completed. 
         [0060]    Subsequently, the serving base station  230  sends the handover preparation completion information received from the target base station  260  to the wireless relay station  220  (S 338 ), and receives handover completion notification from the wireless relay station  220  (S 339 ). 
         [0061]    Upon receiving the handover completion notification from the wireless relay station  220 , the serving base station  430  performs a connection finishing operation by initializing all information related to connection with the wireless relay station  220  (S 350 ). At this time, it is preferable that the serving base station  430  performs the connection finishing operation after waiting a handover retain (HO-retain) period, in order to prevent a ping-pong phenomenon of the wireless relay station  220 . 
         [0062]    After that, the wireless relay station  220  notifies the handover completion to the subscriber stations  210  (S 360 ). In response thereto, each of the subscriber stations  210  recognizes the dedicated handover ranging region allocated by the target base station  260  (S 370 ), and performs a process of access to the target base station  260  (S 380 ). At the step S 360 , it is preferable that the wireless relay station  220  transmits an RS-MS-HO-IND message to the subscriber stations  210  by broadcasting. 
         [0063]    When the dedicated handover ranging region is not allocated by the target base station  260 , it is preferable that the subscriber stations  210  perform the process of access to the target base station  260  by a conventional initial ranging process. 
         [0064]      FIG. 4  is a drawing that schematically shows a wireless relay station according to an exemplary embodiment of the present invention. 
         [0065]    Referring to  FIG. 4 , the wireless relay station  220  according to an exemplary embodiment of the present invention includes a transmitting/receiving unit  221 , a modulation/demodulation unit  223 , a wireless environment checking unit  225 , a base station information storage unit  227 , and a control unit  229 . 
         [0066]    The transmitting/receiving unit  221  transmits and receives a wireless signal through an antenna to and from base stations, which are the serving base station  230 , the target base station  260 , and a neighboring base station in the exemplary embodiment of the present invention. Here, it is preferable that the wireless signal includes an NBR-ADV message, a scanning-related message, a handover-related message, etc. 
         [0067]    The modulation/demodulation unit  223  modulates and demodulates the wireless signal communicated with the base stations, so as to communicate the modulated or demodulated signal with the subscriber stations  210  through the transmitting/receiving unit  221 . 
         [0068]    The wireless environment checking unit  225  performs scanning of the wireless environment state of the neighboring base station through the transmitting/receiving unit  221 . 
         [0069]    The base station information storage unit  227  stores neighboring base station information received from the serving base station  230  through the transmitting/receiving unit  221 . 
         [0070]    The control unit  229  controls respective units  221 ,  223 ,  225 , and  227 , and determines whether the handover condition is satisfied according to the movement of the transportation means during the scanning of the wireless environment checking unit  225 . In addition, the control unit  229  performs the handover to the target base station  260  through the transmitting/receiving unit  221  when the handover occurs. Then, when the handover process is completed, the control unit  229  notifies the dedicated ranging region information and handover completion to the subscriber stations  210 , and performs the process of finishing connection with the serving base station  230 . 
         [0071]      FIG. 5  is a flowchart showing a handover process taken by the wireless relay station according to an exemplary embodiment of the present invention. 
         [0072]    Firstly, the control unit  229  extracts neighboring base station information from the NBR-ADV message received from the serving base station  230  through the transmitting/receiving unit  221 , and then stores the extracted information in the base station information storage unit  227  (S 510 ). 
         [0073]    Subsequently, the control unit  229  determines whether the handover condition is met while the wireless environment checking unit  225  scans the wireless environment state of the target base station  260  (S 530 ). 
         [0074]    When the handover condition is met at the step S 530 , the control unit  229  requests the handover to the target base station  260  through the serving base station  230 , and then receives the handover acknowledgement response from the target base station  260  (S 540 ). At this time, it is preferable that the control unit  229  receives the dedicated ranging region information from the target base station  260 . 
         [0075]    Subsequently, the control unit  229  notifies the subscriber stations  210  of the dedicated ranging region information and the handover completion to the target base station  260  (S 550 ), and performs the process of finishing connection with the serving base station  230  (S 560 ). 
         [0076]      FIG. 6  is a flowchart showing a handover process taken by a subscriber station according to an exemplary embodiment of the present invention. 
         [0077]    The subscriber stations  210  receive the NBR-ADV message from the serving base station  230  and thereby collect the neighboring base station information (S 610 ). 
         [0078]    Subsequently, the subscriber stations  210  receive the handover completion notification message from the serving base station  230  (S 620 ), and then determine whether the dedicated handover ranging region is allocated by the target base station  260  to the subscriber stations  210  (S 630 ). 
         [0079]    When the dedicated handover ranging region is allocated by the target base station, the subscriber stations  210  performs the handover using the dedicated handover region allocated by the target base station  260  (S 640 ). 
         [0080]    When the dedicated handover ranging region is not allocated by the target base station, the subscriber stations  210  performs conventional initial ranging with the target base station  260  (S 650 ). 
         [0081]    Subsequently, when the target base station  260  is accessed by the ranging, the subscriber stations  210  perform the process of ending connection with the serving base station  230  (S 670 ). 
         [0082]    As described above, according an exemplary embodiment of the present invention, a wireless relay station performs a handover process by grouping a plurality of subscriber stations that have the same moving path. Therefore, the level of signaling required for the handover operation of the subscriber stations is reduced in comparison with simultaneous and separate handover operations of the subscriber stations. Consequently, the bandwidth management may become more efficient. 
         [0083]    In addition, a dedicated handover ranging region is allocated to subscriber stations by the target base station, and therefore latency caused by the handover process is reduced since collision probability according to the bandwidth request of the subscriber stations decreases. 
         [0084]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
A handover method according to an embodiment of the present invention includes: receiving neighboring base station information at a wireless relay station providing a wireless communication service moving together with a plurality of subscriber stations that move along the same moving path; performing scanning to check a wireless environment state of a neighboring base station using the received neighboring base station information; performing handover of the plurality of subscriber stations to a target base station as a single handover unit, when a handover condition is met during the scanning; and notifying the subscriber station of completion of handover preparation with the target base station.