Abstract:
Disclosed is a system and a method for performing handover in a Worldwide interoperability for Microwave Access (WiMAX) mobile communication system supporting broadband wireless access. The system includes a plurality of Mobile Stations (MSs); at least one distributed antenna having the ability to perform simultaneous communications with the plurality of MSs; and a base station connected to the at least one distributed antenna through optical fibers for performing communications and handovers with the multiple MSs.

Description:
CLAIM OF PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of an application entitled “System and Method for Performing Handover in WiMAX Mobile Communication System” filed in the Korean Industrial Property Office on Feb. 21, 2007 and assigned Serial No. 2007-17440, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a broadband wireless communication system, and more particularly to a system and a method for performing handover in a Worldwide interoperability for Microwave Access (WiMAX) mobile communication system supporting a Point-to-MultiPoint (PMP) scheme. 
     2. Description of the Related Art 
     The next generation communication system is continuing to develop in such a way that a Mobile Station (MS) is offered various and plentiful high-speed services. 
     An exemplary instance of the next generation communication system is a WiMAX communication system corresponding to a communication system based on Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard. 
     In general, the WiMAX mobile communication system is based on IEEE 802.16e Wireless Metropolitan Area Network (WMAN) standards ensuring mobility of the MS, and supports Broadband Wireless Access (BWA). A network structure supported by the IEEE 802.16 standard is operates in accordance with two schemes, including a mesh scheme and the PMP scheme. 
     Presently, in some nations and regions, the WiMAX mobile communication system having the PMP structure is being used as a test or for commercial purposes. This WiMAX mobile communication system is worthy of close attention with respect to the aspects of high-speed data communications, the maximum communication range, and relatively cheap costs. However, problems have been raised in that the WiMAX mobile communication system having the PMP structure spends high costs in installing hot zones each of which functions as a wireless Local Area Network (LAN) base station (BS) for relaying radio waves so as to service a plurality of user MSs and their frequent movements, and installing the hot zones has no other option than to concentrate in a limited area, such as a crowded downtown, or a university library. 
     Therefore, a reform measure using Multiple Input Multiple Output (MIMO) or cell division technology is essential in respect to these hot zones. 
     In relation to this,  FIG. 1  illustrates a general cellular system in which the prior cell is divided into multiple small cells. Usually, in a mobile communication system having the cellular structure, a base station (BS) controls one cell, and offers services to an MS located in the cell. Referring to  FIG. 1 , as a service coverage area is divided into multiple small zones through the cell division causing the radius of one cell to change from R to R/2, the same frequency is used in two cells that are far away from each other. A frequency reuse factor (i.e., the number of cells representing how many cells are assigned the total frequency band) can increase the total capacity of a cellular system in an environment like above. Therefore, a cell in which traffic congestion occurs is divided into smaller subcells or micro-cells, and there exists a BS in each subcell or micro-cell itself. 
     In this manner, since an increase of the number of cells is matched with the reproductively of cells, i.e., an increase in the reusability of cells, the cell division causes the total capacity of the cellular system to increase whereas a burden is imposed on control over an upper layer due to a frequent handover procedure between cells setting call from a base station (i.e., from a serving base station) of a current cell caused by the cell division to a base station of another cell target (i.e., to a target base station). 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a system and a method for using a base station to perform handover in a WiMAX mobile communication system, which eliminates frequent handover between cells and a corresponding burden on an upper layer caused by the frequent handover performing handover. 
     In accordance with a first exemplary embodiment of the present invention, there is provided a system for performing handover in a Worldwide interoperability for Microwave Access (WiMAX) mobile communication system supporting broadband wireless access, including: a plurality of Mobile Stations (MSs); a distributed antenna having the ability to perform simultaneous communications with at least one of the plurality of MSs; and a base station (BS) connected to the distributed antenna through optical fibers for performing communications and handovers with the plurality of MSs. 
     In accordance with another exemplary embodiment of the present invention, there is provided a method for performing handover in a Worldwide interoperability for Microwave Access (WiMAX) mobile communication system supporting broadband wireless access, including the steps of: receiving information on a distributed antenna to perform handover from a Mobile Station (MS) through the distributed antenna; distinguishing from other processing stacks a relevant processing stack corresponding to the received information; and transmitting a handover message with the relevant processing stack if there exists the relevant processing stack corresponding to the received information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other exemplary features, aspects, and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a general cellular system in which a prior cell is divided into multiple smaller cells; 
         FIG. 2  is a block configuration diagram illustrating a system for performing a handover in a WiMAX mobile communication system, according to an embodiment of the present invention; 
         FIG. 3  is a configuration view illustrating a connection state between cells having a distributed antenna different from each other, according to an embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating signals while performing handover between cells of base stations different from each other in a WiMAX mobile communication system, according to an embodiment of the present invention; and 
         FIG. 5  is a flowchart illustrating signals while performing handover between subcells of the same base station in a WiMAX mobile communication system, according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. The detailed description includes particulars, such as specific configuration elements, which are only presented in support of more comprehensive understanding of the present invention, and it will be obvious to those skilled in the art that prescribed changes in form and modifications may be made to the particulars in the scope of the present invention. 
       FIG. 2  is a block configuration diagram illustrating a system for performing a handover in a WiMAX mobile communication system according to an embodiment of the present invention. With reference to  FIG. 2 , the WiMAX mobile communication system according to an embodiment of the present invention includes a plurality of user MSs  201 , at least one distributed antenna  202 , a BS  205  comprising a plurality of processing stacks  203  and a network access control unit  204  for control thereof, an Access Control Router (ACR)  206 , and a backbone  207 . Herein, the at least one distributed antenna  202  can communicate with at least one of the plurality of user MSs  201 . The plurality of processing stacks  203  are each connected to one of the at least one distributed antenna  202  through optical fibers, and correspond to the one of the at least one distributed antenna  202 , respectively. The BS  205  includes the plurality of processing stacks  203  and the network access control unit  204 , and communicates with the plurality of user MSs  201 . The ACR  206  controls the BS  205 , and performs a connection to another network. The backbone  207  is connected to another network, and delivers data to the ACR  206 . 
     First, each MS of the plurality of user MSs  201  performs communication with the BS  205  in order to gain access to a network system according to a request for a call connection and a channel condition at its location. At this time, each of the MSs  201  selects at least one of the at least one distributed antenna  202  to which each of the MSs  201  can communicate with an optimal signal power and signal quality (i.e., the service quality) within a neighboring distance of its location in order to receive communication services from the BS  205  or for a call connection with another MS, and communicates with the BS  205 . 
     Also, the selected at least one of the at least one distributed antenna  202  equipped with a transceiver for performing conversion between a Radio Frequency (RF) signal and a digital Intermediate Frequency (IF) signal corresponds to at least one distributed antenna located in a range where each of the at least one distributed antenna  202  can communicate with the base station  205  for performing a transmitted digital IF signal processing. 
     Thus, the plurality of MSs  201  communicate with the at least one distributed antenna  202  belonging to the same BS, and each of the at least one distributed antenna  202  is connected to the BS  205  through optical fibers, which in turn forms a cell corresponding to a range over which one BS has control. Also, the at least one distributed antenna  202  located within a certain zone over which a single BS has control forms at least one subcell in an area where radio waves broadcast from each of the at least one distributed antenna  202   i  reach. 
     At this point, the BS  205  connected through optical fibers to the at least one distributed antenna  202 , each of which forms at least one subcell, is equipped with the plurality of processing stacks  203  respectively corresponding to the at least one distributed antenna  202  to which the BS  205  gains access and transmits, to the network access control unit  204 , position information from each of the at least one distributed antenna  202  and information about the communicating MSs. The network access control unit  204  distinguishes between information transmitted by each respective at least one distributed antenna  202  and received by the plurality of processing stacks  203 , and then directs, on the basis of distinguished information, that one of the ACR  206  and a specific processing stack of the plurality of processing stacks  203  perform handover of the MSs  201 . If the relevant processing stack corresponding to received information is prevented from inputting the received information the ACR  206  is directed to perform the handover. 
     The ACR  206 , directed to perform the handover by the network access control unit  204  in the BS  205 , transmits this direction to another network via the backbone  207 , and the specific processing stack transmits the indication to the MSs  201  via the distributed antenna of the at least one distributed antenna  202  that is connected to the specific processing stack. 
       FIG. 3  is a configuration view illustrating a connection state between cells having a distributed antenna different from each other according to the present invention. As illustrated in  FIG. 3 , there exist a base station A and a base station B of each cell including subcells by respective at least one distributed antennas  202 , wherein the BS A or B controls subcells within a zone where the BS A or B can perform communication. Herein, an MS  30  offered services from the BS A via a distributed antenna  300 A moves to a boundary point of the subcell where the distributed antenna  300 A is located, and is about to change its service route from a route through the distributed antenna  300 A to another route through a distributed antenna  301 A within the same cell or to another route through a distributed antenna  350 B within another cell. Accordingly, with reference to  FIGS. 4 and 5 , a description will be made of a process for performing handover between cells and between subcells over which the BS A and the BS B control, respectively. 
       FIG. 4  is a flowchart illustrating signals while performing handover between cells in a WiMAX mobile communication system according to an embodiment of the present invention. As illustrated in  FIG. 4 , first, an MS  40  is obtaining access to an antenna (i.e., a serving antenna  300 A) to which the MS  40  is gaining current access, and communicates with the BS A. 
     When the above MS  40  moves, before performing handover, the MS  40  scans for distributed antennas located within a neighboring distance so as to select an antenna through which the MS  40  can communicate with an optimal quality and power of signal (step  400 ). Then, in order to search for a target antenna, the MS  40  needs to acquire information on adjacent antennas which the MS  40  itself obtained by scanning for adjacent antennas or can be provided from a serving BS that is offering services. Herein, the information on adjacent antennas signifies a position of a distributed antenna, received signal strength indication, and the like. 
     If at least one objective antenna (i.e., a target antenna) is determined through scanning for the antennas in step  400 , the MS  40  transmits a MOB_MSHO_REQ message requesting handover to the BS A within a current service area via the serving antenna  300 A to which the MS  40  is gaining access, and the MS  40  or the serving BS  300 A selects and recommends the target antennas  301 A and  350 B satisfying the service quality by monitoring preset frequency bands (step  410 ). At this time, the MOB_MSHO_REQ message transmitted by the MS  40  corresponds to a message prescribed in the IEEE 802.16e standards. 
     A specific processing stack which is connected to the serving antenna  300 A among the plurality of processing stacks included in a serving BS A, and which has received a handover request from the MS  40  via the serving antenna  300 A, transmits the information on the target antennas  301 A and  350 B recommended by the MS  40  to the network access control unit  204 . The network access control unit  204  distinguishes the selected target antenna and the specific processing stack, and transmits the handover message to Media Access Control (MAC) layers of the relevant processing stack (step  412 ). Herein, the handover message includes IDentification (ID) of the MS, a request for bandwidth, a request for Quality of Service (QoS), and the like. 
     At this moment, if the target antenna  301 A connected to the processing stack exists within the serving BS A to which the MS  40  currently belongs, the handover message is directly transmitted to an antenna located in a service zone of the same BS by the network access control unit  204  (step  412 ). When a processing stack connected to the target antenna  350 B does not exist within the current serving BS A, the target antenna  350 B corresponds to an antenna located in a service zone of another BS. Accordingly, the network access control unit  204  transmits the handover message to the ACR  206  (steps  414  and  415 ). Respective MAC layers related to the target antennas  301 A and  350 B receiving the handover message from the network access control unit of the serving BS A, respond to the network access control unit  204  of the BS A (steps  416  and  418 ). Namely, the target antennas  301 A and  350 B transmit a response (ACK lower QoS level) answering to whether the handover can be performed according to the handover request of the MS  40  (steps  416  and  418 ), including information on frequency bandwidth and a service level which can be provided by each target antenna when the handover of the MS  40  is delivered to each target antenna. 
     More particularly, by analyzing the handover message received from the network access control unit of the serving BS A, when the MS  40  is required for the handover, the MS  40  selects, as the final target antenna through which the MS  40  accomplishes the handover, a target antenna that can optimally provide the frequency bandwidth and the service level required by the MS  40 . Further, an interface for exchanging information with the network in the network access control unit corresponds to an interface used by one of the protocols selected from the group consisting of Internet Protocol (IP) and Asynchronous Transfer Mode (ATM). 
     To give an example, if it is assumed that a service level that the target antenna  301 A can provide is lower than a service level required by the MS  40 , and a service level that the target antenna  350 B can provide equals the service level required by the MS  40 , the serving BS A selects the target antenna  350 B as the final target antenna through which the MS  40  accomplishes the handover. 
     Therefore, when the target antenna  305 B belonging to the BS B easily satisfies a request for QoS of the MS  40  requesting the handover, the serving BS A transmits a response message related to handover acceptance to the BS B through the backbone by way of the ACR, and gives notice that the handover of the MS  40  is about to be delivered to the target antenna  350 B belonging to the BS B (step  420 ). Meanwhile, as the target antenna  350 B is finally selected, a network access control unit in the BS B transmits a message to a MAC layer of a processing stack connected to the target antenna  350 B. 
     Then, the serving BS A transmits a MOBile subscriber station HandOver ReSPonse (MOB_HO_RSP) message to the serving antenna  300 A (step  424 ), and recommends a change over to the target antenna  350 B. Herein, the MOB_HO_RSP includes information on the target antenna to which the handover of the MS  40  is delivered. Also, the above information signifies a message type to be transmitted, time it is expected to begin a handover procedure, and information on a target antenna selected by the serving BS A. 
     In order to select the target antenna  350 B to which the handover is to be delivered, the serving antenna  300 A receiving the MOB_BSHO_RSP message transmits, to the MS  40 , a MOBile subscriber station HandOver INDication (MOB_HO_IND) message corresponding to a message responding to the MOB_BSHO_RSP message (step  426 ). After sensing that the handover is to be delivered to the target antenna  350 B included in the MOB_HO_IND message, the MS  40  receiving the MOB_HO_IND message responds to the MOB_HO_IND, displays time necessary to perform a handover operation, and releases communications with the serving antenna  300 A currently gaining access to the MS  40  (step  426 ). 
     Next, the MS  40  receives a Fast_Ranging Information Element (IE) (UL_MAP) message from the target antenna  350 B (step  428 ). The UL_MAP message includes an MS initial search opportunity which is based on non-contention. The Fast_Ranging is performed whenever there exists a request from a BS so that the BS may acquire synchronization with an MS of a subscriber. The Fast_Ranging matches the MS of the subscriber with the BS for an accurate time offset therebetween, and is performed so as to adjust transmitted power. Namely, after being powered on, the MS of the subscriber acquires the synchronization with the BS on receiving UL_MAP, and performs the Fast_Ranging in order to adjust the transmitted power. By using the initial search opportunity, the MS  40  transmits an RNG_REQ message to the serving antenna  300 A (step  430 ). The MS  40  exchanges the RNG_REQ message and an RNG_RSP message with the serving antenna  300 A (step  432 ), establishes a normal operation state, and then completes a process for performing the handover. 
       FIG. 5  is a flowchart illustrating signals while performing handover between subcells in a WiMAX mobile communication system according to another embodiment of the present invention. As illustrated in  FIG. 5 , an MS  50  is obtaining access to an antenna (i.e., a serving antenna  300 A) to which the MS  50  is gaining current access, and communicates with a network access control unit of the BS A. When the above MS  50  moves, before performing handover, the MS  50  scans for distributed antennas located within a neighboring distance so as to select an antenna through which the MS  50  can communicate with an optimal quality and power of signal (step  500 ). If at least one objective antenna (i.e., a target antenna) is determined through scanning for the antennas in step  500 , the MS  50  transmits a MOB_MSHO_REQ message requesting handover to the BS A within a current service area via the serving antenna  300 A to which the MS  50  is gaining access, and the MS  50  or the serving BS  300 A selects and recommends the target antennas  301 A and  350 B satisfying the requested service quality by monitoring preset frequency bands (step  510 ). 
     The information on the target antennas  301 A and  350 B recommended by the MS  50  is transmitted to the network access control unit in the serving BS A. The network access control unit distinguishes the selected target antenna and the specific processing stack, and transmits the handover message to Media Access Control (MAC) layers of the relevant processing stack (step  512 ). At this moment, if the target antenna  301 A connected to the processing stack exists within the serving BS A to which the MS  50  currently belongs, the handover message is directly transmitted to an antenna located in a service zone of the same BS by the network access control unit (step  514 ), and if a processing stack connected to the target antenna  350 B does not exist within the current serving BS A, the target antenna  350 B corresponds to an antenna located in a service zone of another BS. Accordingly, the network access control unit transmits the handover message to the ACR  206  (step  515 ). Respective MAC layers related to the target antennas  301 A and  350 B receiving the handover message from the network access control unit of the serving BS A, respond to the network access control unit of the BS A. Namely, the target antennas  301 A and  350 B transmit a response (ACK lower QoS level) answering to whether the handover can be performed according to the handover request of the MS  50  (steps  516  and  518 ), including information on frequency bandwidth and a service level which can be provided by each target antenna when the handover of the MS  50  is delivered to each target antenna. Accordingly, when the target antenna  301 A located in the same BS control area satisfies a request for QoS of the MS  50  requesting the handover, the target antenna  301 A is selected (step  520 ). Namely, the handover related to the MS  50  is checked (step  520 ), and the serving BS A directly transmits a response message related to handover acceptance through the network access control unit because a target antenna  301 A connected to the processing stack exists in the serving BS A to which the MS  50  currently belongs. Then, the serving BS A transmits a MOBile subscriber station HandOver ReSPonse (MOB_HO_RSP) message to the serving antenna  300 A (step  522 ). Herein, the MOB_HO_RSP includes information on the target antenna to which the handover of the MS  50  is delivered. 
     In order to select the target antenna  301 A to which the handover is to be delivered, the serving antenna  300 A receiving the MOB_BSHO_RSP message transmits, to the MS  50 , a MOBile subscriber station HandOver INDication (MOB_HO_IND) message corresponding to a message responding to the MOB_BSHO_RSP message (step  524 ). After sensing that the handover is to be delivered to the target antenna  301 A included in the MOB_HO_IND message, the MS  50  receiving the MOB_HO_IND message responds to the MOB_HO_IND, displays time necessary to perform a handover operation, and releases communications with the serving antenna  300 A currently gaining access to the MS  50 . Next, the MS  50  receives a Fast_Ranging Information Element (IE) (UL_MAP) message from the target antenna  301 A (step  526 ). By using the initial search opportunity, the MS  50  transmits an RNG_REQ message to the serving antenna  300 A. The MS  50  exchanges the RNG_REQ message and an RNG_RSP message with the serving antenna  300 A, establishes a normal operation state, and then completes a process for performing the handover. 
     The merits and effects of preferred embodiments, as disclosed in the present invention, and as so configured to operate above, will be described below. 
     As described above, according to the present invention, a frequent handover between cells and a burden of an upper layer caused by the frequent handover can be removed by using a base station to perform handovers. Furthermore, since only an antenna and transceiver are set up instead of providing a BS for each of the prior cells, there is no burden upon control over an upper layer due to cell division, and there is no need to change the structure of a backbone network and the interface, which in turn can reduce costs of an overall system. 
     While the invention has been shown and described with reference to certain exemplary 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. Therefore, the spirit and scope of the present invention must be defined not by the herein described exemplary embodiments thereof but by the appended claims and equivalents of the appended claims.