Abstract:
A system and a method for forming a cell by using distributed antennas in a World interoperability for Microwave Access (WiMAX) mobile communication system supporting a broadband wireless access communication system. A virtual cell is formed with a specific Mobile Station (MS) as a central part of the virtual cell by using distributed antennas, which improves the transmission capacity and the performance of an overall network system without changing the structure of a backbone network and the interface of the overall network system. The system includes, multiple MSes; distributed antennas for perform simultaneous communications with at least one MS among the multiple MSes, and for forming one virtual cell by communicating with the multiple MSes; and a Base Station (BS), which is typically connected to the distributed antennas through optical fibers, for communicating with the MSes.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) from an application entitled “System and Method for Forming Cell by Using Distributed Antennas in WiMAX Mobile Communication System,” filed in the Korean Intellectual Property Office on Feb. 21, 2007 and assigned Serial No. 2007-17442, the contents of which are hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a broadband wireless communication system. More particularly, the present invention relates to a system and a method that is suitable for forming a virtual cell in a World interoperability for Microwave Access (WiMAX) mobile communication system. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, the WiMAX mobile communication system is based on Institute of Electrical and Electronics Engineers (IEEE) 802.16e Wireless Metropolitan Area Network (WMAN) standards ensuring mobility of the mobile stations (MS), and supports Broadband Wireless Access (BWA). A network structure supported by the IEEE 802.16 standards operates in two schemes, which typically includes a mesh scheme and the PMP scheme. 
         [0006]    In some nations and regions, the WiMAX mobile communication system having the PMP structure is being used for testing purposes, and/or for commercial purposes. The WiMAX mobile communication system is worthy of close attention for future needs due to the favorable aspects of the system to provide high-speed data communications, with a maximum communication range, and relatively cheap costs. However, WiMAX also has problems that are adversely affecting its implementation because of the high costs associated with the installation of hot zones, each of which functions as a wireless Local Area Network (LAN) base station (BS) for relaying radio waves so as to meet multiple MSes of users and their frequent movements. Accordingly, the installation of such hot zones has been limited to concentrating in populous areas that would provide access for a relatively number of users, such as a crowded downtown, or a university library. 
         [0007]    Therefore, with respect to the hot zones, possible changes such as using Multiple Input Multiple Output (MIMO) or cell division technology can accelerate the growth of WiMAX. 
         [0008]    In relation to this technology,  FIG. 1  is a view illustrating a conventional Single Input Single Output (SISO) system including a Base Station (BS) equipped with one antenna and multiple Mobile Stations (MSes), and  FIG. 2  is a view illustrating an embodiment of a conventional MIMO system including a BS equipped with multiple antennas and multiple MSes each of which has one antenna.  FIGS. 1 and 2  respectively show a backbone  101 ,  201 , Access Control Router (ACR)  102 , 202 , base station (BS)  103 , 203  and a plurality of MSs  104 , 204  within each cell/hot zone/transmission/area  105 , 205 . 
         [0009]    As illustrated in  FIGS. 1 and 2 , technology in which one or more antennas are not only established at a BS  103 , but also one or more antennas are set up at a MS  104 , has been previously proposed. The proposed technology improves the capacity of an overall network system, as well as transmission reliability, by using spatial diversity and spatial multiplexing. 
         [0010]    However, while a MIMO scheme, (such as shown in  FIG. 2 ) increases a data transfer rate and reduces errors theoretically in proportion to the number of antennas, drawbacks are raised in that the complexity of data transmission processing and consumption of channel resources also increases enormously, and due to near-far effect, the average capacity in a cell considerably varies according to a geographical position of a mobile communication MS. 
         [0011]      FIG. 3  is view illustrating a conventional cellular system in which a cell is divided into multiple subcells. As can be seen with regard to  FIG. 3 , the technology of reducing the control range of each cell with an increase in the number of cells through the technology of cell division causes the required transmission output of the BSs to be reduced, and the capacity of an overall network system appears to increase because of the decrease of average access distance. 
         [0012]    However, with regard to  FIG. 3 , as the sizes of cells or microcells become smaller due to the cell division process, there can actually be a reduction in capacity of an overall network system caused by a side effect of co-channel interference. Moreover, the increasing number of BSes and frequent handover operations increase system costs and a control load of an upper layer. 
       SUMMARY OF THE INVENTION 
       [0013]    Accordingly, the present invention has been made in part to solve at least some of the above-stated problems occurring in the prior art. The present invention provides a system and a method for forming a cell by using distributed antennas in a WiMAX mobile communication system, which can improve the transmission capacity and the performance of an overall network system without changing the structure of a backbone network and the interface. 
         [0014]    In accordance with an exemplary aspect of the present invention, there is provided a system for forming a cell by using distributed antennas in a World interoperability for Microwave Access (WiMAX) mobile communication system supporting broadband wireless access according to an embodiment of the present invention, which includes: multiple Mobile Stations (MSes); distributed antennas for performing simultaneous communications with at least one among the multiple MSes, and for forming one virtual cell by communicating with the multiple MSes; and a Base Station (BS), connected to the distributed antennas through optical fibers for communicating with the MSes. 
         [0015]    In accordance with another exemplary aspect of the present invention, there is provided a method for forming a cell by using distributed antennas in a World interoperability for Microwave Access (WiMAX) mobile communication system supporting broadband wireless access communication system according to another embodiment of the present invention, including the steps of: making a search, by a Mobile Station (MS), for one or more distributed antennas satisfying the preset service quality in the same cell including one Base Station (BS); selecting one or more distributed antennas satisfying the service quality according to a result of the search; forming a virtual cell with the selected one or more distributed antennas; and accessing a BS located in a cell including the formed virtual cell to one or more distributed antennas located in the virtual cell, and performing communications therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other exemplary features, aspects, and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a view illustrating a conventional SISO system including a BS equipped with one antenna and multiple MSes; 
           [0018]      FIG. 2  is a view illustrating a conventional MIMO system including a BS equipped with multiple antennas and multiple MSes each of which has one antenna; 
           [0019]      FIG. 3  is a view illustrating a conventional cellular system in which a cell is divided into multiple subcells; 
           [0020]      FIG. 4  is a view illustrating a system for forming a virtual cell based on distributed antennas according to an exemplary embodiment of the present invention; 
           [0021]      FIG. 5  is a view illustrating a virtual cell in a Distributed Wireless Communication System (DWCS) according to an exemplary embodiment of the present invention; and 
           [0022]      FIG. 6  is a flowchart illustrating an example of the operation of a system for forming a virtual cell based on distributed antennas according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is to be understood that the present invention is not limited to the examples shown and described herein. The following 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 various modifications may be made to the particulars which does not depart from the spirit of the invention and the scope of the appended claims. 
         [0024]      FIG. 4  illustrates an exemplary configuration of system for forming a virtual cell by using distributed antennas in a WiMAX mobile communication system according to the present invention. Referring to  FIG. 4 , a WiMAX mobile communication system related to an exemplary embodiment of the present invention includes a cell  405  having one or more distributed antennas  407 - 1  to n, a BS  403 , multiple user MSes  404 - 1  to n, an Access Control Router (ACR)  402 , and a backbone  401 . Herein, the one or more distributed antennas  407 - 1  to n may communicate with at least one among the multiple user MSes  404 - 1  to n. The BS  403  is typically connected to the one or more distributed antennas  407  through optical fibers, and communicates with the multiple MSes  404 . Each of the multiple user MSes  404  communicates with the BS via the distributed antennas, and describes the range of one cell. For example, the virtual cell is formed around the MS by communicating the MS ( 404  . . .  n ) with the distributed antennas. That is, since the virtual cell is formed by communicating the MS with the distributed antennas, the MS ( 404  . . .  n ) describes the range of the virtual cell. 
         [0025]    Still referring to  FIG. 4 , the ACR  402  controls the BS  403 . The backbone  401  receives data transmitted from the ACR  402 , and performs a connection with another network. 
         [0026]    More particularly, each of the MSes  404  performs communication with the BS  403  in order to gain access to a network system according to a request for a call connection and a channel condition in its located position. At this time, each of the MSes  404  can select one or more distributed antennas to which each of the MSes  404  can communicate while still satisfying optimal power and quality requirements (i.e., the service quality) of a signal in the range within the same cell where each MS is located, in order to receive communication services offered from the BS  403 , or for a call connection with another MS. Also, the distributed antennas  407  equipped with a transceiver for performing conversion between a Radio Frequency (RF) signal and a digital Intermediate Frequency (IF) signal correspond to one or more distributed antennas located in the range where the distributed antennas  407  can communicate with the base station  403  for performing a transmitted digital IF signal processing. 
         [0027]    Thus, the multiple MSes  404  communicate with one or more distributed antennas  407  belonging to the same BS, and the multiple distributed antennas  407  are connected to the BS  403 , which in turn forms a cell corresponding to the range over which one BS can control. 
         [0028]    Each of the multiple MSes  404  can search for and select one or more distributed antennas  407  so as to communicate with the BS  403  in its located place, and a point of reference on which the multiple MS  404  search for and select one or more distributed antennas  407  distributed over many places signifies one or more distributed antennas through which the multiple MScs  404  can perform communications at more than a preset received signal level. 
         [0029]    Still referring to  FIG. 4 , according to an example of the present invention, at this time, within one cell  405  formed with the multiple MSes  404  and one or more distributed antennas  407  selected by the multiple MSes  404  as the central part of the cell, another virtual cell  406  is formed. Here, the virtual cell  406  is temporarily formed with a specific MS as the central part of the virtual cell  406  during only a time interval for which the specific MS communicates with the one or more distributed antennas  407  satisfying the service quality of the specific MS in a place where the specific MS is located. Then, even if the position of the specific MS changes, the specific MS searches for and selects one or more distributed antennas  407  satisfying the service quality in a place where the specific MS itself is located, and communicates with one or more distributed antennas  407 . Herein, while performing the communications, at least another virtual cell can be formed. 
         [0030]    According to an exemplary aspect of the present invention, the virtual cell  406  is always generated and ceases to exist in a place where each of the multiple MSes is located regardless of the movement of each of the multiple MSes. It causes, as illustrated in  FIG. 5 , a virtual cell to be formed with the specific MS as the central part of the virtual cell, and because the distributed antennas  502  can simultaneously communicate with one MS or multiple MSes adjacent to their area where the distributed antennas  502  can transmit/receive data, the distributed antennas  502  can be included in one or more virtual cells. Namely, each of the multiple MSes existing within the same cell forms a virtual cell for itself, and the virtual cell changes as the MS moves. Also, the BS  403  includes one or more virtual cells according to the number of MSes existing in the range of controllable cells. Furthermore, because the virtual cell exists within the same cell, and because a virtual cell is continuously produced in a current position as long as the MS communicates with the BS via the distributed antennas even if a position of the MA changes, a process of handover with usual meanings due to the movement between cells of the MS is not considered. 
         [0031]    In addition, according to the present invention, the average access distance of a transmission output that the MS outputs for communications with the distributed antennas is reduced as the distributed antennas are set up in many distributed places, thereby more efficiently utilizing the electric power of the MS. 
         [0032]    Moreover, in protocol layers of a WiMAX system according to the present invention, data processing is performed based on two layers, i.e. a physical layer and Media Access Control (MAC), included in a lower layer of an Open Systems Interconnection (OSI) standard reference model widely known in general in a communication system, and because a scheme of Orthogonal Frequency Division Multiple Access (OFDMA), duplexing technique, and the function of packet encapsulation also are not varied by the present invention. Therefore, a more detailed description of same will be omitted. 
         [0033]      FIG. 6  is a flowchart illustrating an example of a method for forming a virtual cell by using one or more distributed antennas in a WiMAX mobile communication system according to the present invention. 
         [0034]    First, in step S 600 , in order to gain access to a network system when a user requests a user MS to connect a call, or according to channel conditions, the user MS makes a search for one or more distributed antennas through which the user MS can communicate in a preset service quality within the same cell including one BS. 
         [0035]    After the search, the user MS selects, in step S 602 , one or more distributed antennas through which a specific user MS can communicate in a preset service quality according to a result of the search. 
         [0036]    Next, in step S 604 , a virtual cell is formed with the selected one or more distributed antennas and the specific user MS as the central part of the virtual cell. Herein, the virtual cell corresponds to one or more cells included in a single cell including the BS and multiple MSes located in the range over which communication control can be implemented. 
         [0037]    In step  606  if the MS has moved, the method reverts back to step S 600  and makes another search, so as to ensure the quality of service does not change as the user MS changes location. 
         [0038]    In step  608 , a BS located in a cell including the virtual cell formed in step  604  gains access to one or more distributed antennas located in the virtual cell, and performs communications via the one or more distributed antennas. 
         [0039]    The merits and effects of exemplary embodiments, as disclosed in the present invention, and as so configured to operate as shown above, will be described as follows. 
         [0040]    As previously described, according to the present invention, by proposing a virtual cell based on the distributed antennas with a specific MS as the central part of the virtual cell, stable communications are performed without handover between the antennas belonging to the same BS, and a space possessed by antennas is not only small in an overall network system but also it can increase communication capacity and a data transfer rate. In addition, since an average access distance between a user MS and an antenna becomes shorter, transmission output power of the user MS is reduced. 
         [0041]    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 of the invention and the scope of the appended claims.