Abstract:
A system and method for adding a burst of a single-input multiple-output mode or collaborative spatial multiplexing mode to a frame in a spatial multiplexing system. A base station is configured to add a burst of a single-input multiple-output mode to a frame in a spatial multiplexing system. The system and method includes backing up a pilot pattern and a number of slots allocated for connections with terminals; releasing slots allocated for a burst; resetting a pilot pattern of a collaborative spatial multiplexing mode burst of the terminals: and recalculating the number of remaining slots; comparing the number of slots that can be additionally allocated to the single-input multiple-output mode with the number of slots released in the releasing step; and determining a transmission format.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
     The present application claims priority to an application entitled “Apparatus and Method for Adding Burst of Single-Input Multiple-Output Mode or Collaborative Spatial Multiplexing Mode to Frame in Spatial Multiplexing System” filed in the Korean Industrial Property Office on Mar. 6, 2008, and assigned Serial No. 10-2008-0021181, the contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a spatial multiplexing system, and more particularly to an apparatus and method for adding a burst of a single-input multiple-output mode or collaborative spatial multiplexing mode to a frame in a spatial multiplexing system. 
     BACKGROUND OF THE INVENTION 
     In general, spatial multiplexing is a scheme for increasing the data rate through use of spatial diversity between divided transmission/reception antennas. In uplink, it is difficult to utilize transmit antenna diversity due to various restrictions. In order to solve such a problem, a Collaborative Spatial Multiplexing (C-SM) mode for obtaining an effect similar to that of the conventional transmit antenna diversity through use of collaboration between terminals is disclosed in IEEE 802.16. In the C-SM mode, when each of the two terminals has one transmission antenna, the two terminals are allocated the same frequency resource and transmit data at the same time. In such a C-SM mode, the two terminals output pilot signals through code multiplexing, wherein the two terminals can output two different data streams. 
       FIG. 1  is a view illustrating a pilot pattern when the general C-SM mode is used. 
     A first terminal using pilot pattern A and a second terminal using pilot pattern B share one tile (that means a resource unit having a predetermined size) with each other. The two terminals use pilot tones constituted by at least one pilot subcarrier on a division basis while sharing and using data tones constituted by at least one data subcarrier. Accordingly, although the data rate that one terminal can obtain does not change, the throughput increases from the viewpoint of a sector. 
     A significant characteristic of a method of allocating a burst to an uplink frame using the C-SM mode, as compared with using only a single-input multiple-output (SIMO) mode, is that an uplink burst is allocated for each layer using two pilot patterns. This characteristic is restricted when a terminal cannot use both pilot patterns of one slot, when a terminal cannot use one slot in the SIMO mode and the C-SM mode at the same time, and when a terminal uses only the SIMO mode from necessity. 
     According to IEEE 802.16, methods for allocating a C-SM burst to a frame are roughly classified into two methods. 
     The first method is to allocate a burst using pilot pattern A and a burst using pilot pattern B to both sides through use of a Multiple-Input-Multiple-Output Uplink Basic Information Element (MIMO UL Basic IE). This method has disadvantages in that a part of the slots is wasted because of the a difference in size between the burst using pilot pattern A and the burst using pilot pattern B, and it is not easy to make both sides effective. 
     The second method is to separately allocate a burst using pilot pattern A and a burst using pilot pattern B one by one through use of a UL H-ARQ Chase Sub-Burst information element. 
     Of these methods, particularly, a method for allocating an uplink burst through use of the C-SM mode requires two functions. 
     First, in order to maintain efficient Rise-over-Thermal (RoT) at a predetermined level, an interference control function of controlling a rate of slots using the C-SM mode to be proper is required. Generally, with respect to the same number of slots, using the slots in the C-SM mode causes greater interference to neighboring cells than using the slots in the SIMO mode. For this reason, one pilot pattern may be used in the same manner as when only the SIMO mode is used, while the other pilot pattern is used only when a specific condition is satisfied. According to the conventional method, an additional pilot pattern is used only for terminals very near to the base station so as to minimize the increase in the efficient RoT as much as possible. In this case, a part of the slots may use one pilot pattern while the other slots use both pilot patterns. The slots using both pilot patterns require greater uplink transmission power than the slots using only one pilot pattern. Meanwhile, with respect to a terminal located in the boundary of a cell, it may be necessary to prevent the terminal from using both pilot patterns in order to ensure coverage. According to the conventional method, since it is not easy to control a specific burst to use one pilot pattern, a separate consideration is required in order to complement the coverage problem. A simple solution to prevent a terminal located in a cell boundary from using both pilot patterns is to make the terminal use the SIMO mode. Since interference is influenced by the conditions for use with the SIMO mode, the conventional interference control scheme needs to be modified. 
     Second, it must be possible to efficiently add a burst with a specific size. Even in the case of using only the C-SM mode, a newly selected current burst may, or may not, be added according to the disposed position of a pre-allocated burst. Such a result is very undesirable when the newly selected burst includes a connection having relay-related Quality-of-Service (QoS) requirements. In order to avoid such a problem, a function of rearranging the disposition of the pre-allocated burst before the newly selected burst is added to a frame is required. In addition, when the SIMO mode is used together with the C-SM mode, the problem becomes more complex. A burst including a connection selected according to the scheduling priorities may use the SIMO mode or the C-SM mode. That is, bursts using the SIMO mode and bursts using the C-SM mode are alternately selected. However, since a scheme of disposing a pre-allocated burst in order to add a burst using the SIMO mode is different from a scheme of disposing a pre-allocated burst in order to add a burst using the C-SM mode, it is necessary to develop supplementary technology for adding the bursts. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object to provide a method for selecting, by a base station, a single-input multiple-output mode or a collaborative spatial multiplexing mode in a spatial multiplexing system. 
     Also, the present invention provides a method for adding, by a base station, a burst of a single-input multiple-output mode to a frame in a spatial multiplexing system. 
     In addition, the present invention provides a method for adding, by a base station, a burst of a collaborative spatial multiplexing mode to a frame in a spatial multiplexing system. 
     In accordance with an aspect of the present invention, there is provided a method for adding, by a base station, a burst of a single-input multiple-output mode in a spatial multiplexing system. The method including the steps of backing up a pilot pattern and the number of slots allocated for connections with terminals and releasing slots allocated for a burst; resetting a pilot pattern of a collaborative spatial multiplexing mode burst of the terminals and recalculating the number of remaining slots; comparing the number of slots that can be additionally allocated to the single-input multiple-output mode with the number of slots released in the releasing step when bursts for which the pilot pattern has not been determined do not remain; and determining a transmission format to be in the single-input multiple-output mode when the number of slots that can be additionally allocated to the single-input multiple-output mode is not less than the number of slots released in the releasing step. 
     In accordance with another aspect of the present invention, there is provided a method for adding, by a base station, a burst of a collaborative spatial multiplexing mode to a frame in a spatial multiplexing system. The method includes the steps of backing up a pilot pattern and the number of slots allocated for connections with terminals and releasing slots allocated for a burst; resetting a pilot pattern of a collaborative spatial multiplexing mode burst of the terminals and recalculating the number of remaining slots; comparing the number of slots that can be additionally allocated to pilot pattern B of the collaborative spatial multiplexing mode with the number of slots released in the releasing step when bursts for which the pilot pattern has not been determined do not remain; and determining a transmission format to be in the collaborative spatial multiplexing mode, when the number of slots that can be additionally allocated to pilot pattern B of the collaborative spatial multiplexing mode is not less than the number of slots released in the releasing step. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
         FIG. 1  is a view illustrating a pilot pattern when the general C-SM mode is used; 
         FIG. 2  is a flowchart illustrating an uplink scheduling and uplink burst allocating algorithm according to an exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating the configuration of a system for determining the SIMO mode or the C-SM mode for a selected burst according to an exemplary embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating a method for determining the SIMO mode or C-SM mode for a selected burst according to an exemplary embodiment of the present invention; 
         FIG. 5  is a flowchart illustrating a method for adding an SIMO mode burst according to an exemplary embodiment of the present invention; 
         FIG. 6  is a view showing an example of a WF heuristic according to an exemplary embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating a method for adding a C-SM mode burst according to an exemplary embodiment of the present invention; 
         FIG. 8  is a view showing an example of an FF heuristic according to an exemplary embodiment of the present invention; 
         FIG. 9  is a view illustrating burst allocations based on pilot patterns according to an exemplary embodiment of the present invention; and 
         FIG. 10  is a view illustrating a result of a simulation of an uplink burst allocation algorithm according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2 through 10 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communications network. 
     The following description of the present invention will be given about a method for allocating an uplink burst to a frame through use of a C-SM mode according to the present invention. The method for allocating an uplink burst to a frame through use of the C-SM mode is divided into a part for determining a SIMO mode or the C-SM mode for a selected burst, a part for adding a burst of the SIMO mode to a frame, and a part for adding a burst of the C-SM mode to a frame. 
       FIG. 2  is a flowchart illustrating an uplink scheduling and uplink burst allocating algorithm according to an exemplary embodiment of the present invention. 
     Hereinafter, the uplink scheduling and uplink burst allocating-algorithm according to an exemplary embodiment of the present invention will be described in detail with reference to  FIG. 2 . More particularly, the following description will be given about an operation for determining a transmission format of a burst according to a selected mode. 
     A base station calculates priorities of connections with a plurality of terminals in step  210 , and proceeds to step  212 . In step  212 , when the maximum value “max(N slot, remain, SIMO , N slot, remain, A , N slot, remain, B )=N slot,remain,max ” among the number “N slot, remain, SIMO ” of slots that can be additionally allocated to the SIMO mode, the number “N slot, remain, A ” of slots that can be additionally allocated to pilot pattern A of the C-SM mode, and the number “N slot, remain, B ” of slots that can be additionally allocated to pilot pattern B of the C-SM mode are greater than zero, and when the number of allocated bursts is less than the maximum number of bursts “N burst, max ,” the base station proceeds to step  216 . 
     In step  216 , the base station selects a connection having the highest priority from among the remaining connections. In step  218 , the base station determines if a burst of the connection selected in step  216  corresponds to a CDMA allocation information element. When the burst of the selected connection corresponds to a CDMA allocation information element, the base station proceeds to step  220  where the base station determines the SIMO-mode-based transmission format for the burst of the selected connection. In contrast, when it is determined that the burst of the selected connection does not correspond to a CDMA allocation information element in step  218 , the base station establishes the selected connection to be used for the transmission of traffic to a terminal and proceeds to step  224 . 
     In step  224 , the base station determines if the selected mode corresponds to the SIMO mode. When the selected mode corresponds to the SIMO mode, the base station proceeds to step  226  where the base station applies a Worst Fit (WF) heuristic for selecting a pilot pattern of the SIMO mode. In contrast, when the selected mode does not correspond to the SIMO mode, the base station proceeds to step  232  where the base station applies an First Fit (FF) heuristic for selecting a pilot pattern of the C-SM mode. When a pilot pattern is successfully selected as a result of the application of the WF heuristic in step  226 , the base station proceeds to step  228  where the base station determines the transmission format to be in the SIMO mode. In contrast, if the base station fails to select a pilot pattern as a result of the application of the WF heuristic in step  226 , the base station proceeds to step  230  where the base station determines the previously backed-up transmission format to be used for the selected connection and skips the selected connection. Meanwhile, when a pilot pattern is successfully selected as a result of the application of the FF heuristic in step  232 , the base station proceeds to step  234  where the base station determines the transmission format to be in the C-SM mode. In contrast, if the base station fails to select a pilot pattern as a result of the application of the FF heuristic in step  232 , the base station proceeds to step  230  where the base station determines the previously backed-up transmission format to be used for the connection selected in step  216  and skips the selected connection. 
     Detailed operations of the WF heuristic and FF heuristic will be described later with reference to  FIGS. 6 and 8 . 
       FIG. 3  is a block diagram illustrating the configuration of a system for selecting the SIMO mode or the C-SM mode for a selected burst according to an exemplary embodiment of the present invention. 
     The system for selecting the SIMO mode or the C-SM mode for a selected burst according to an exemplary embodiment of the present invention includes a base station  320  and a terminal  310 , wherein the base station  320  includes a modem  322  and an uplink scheduler  324 . When the modem  322  of the base station receives uplink transmission power from the terminal, the modem  322  takes the average of the received uplink transmission power and transmits the average to the uplink scheduler  324 . The uplink scheduler  324  determines whether to use the SIMO mode or the C-SM mode according to the average, and determines a transmission format according to an MCS level by performing the procedure described with reference to  FIG. 2 . When the transmission format has been determined, the uplink scheduler  324  transmits the determined transmission format to the modem  322 , and the modem  322  creates MAP information expressing a burst position. In addition, the MAP information includes information representing the transmission format, and SIMO mode or C-SM mode information, as well. Thereafter, the base station  320  transmits the MAP information to the terminal  310 . The terminal  310  generates a burst according to the transmission format information and SIMO mode or C-SM mode information that are included in the MAP information received from the base station  320 . The terminal  310  transmits the generated burst to the base station  320 . 
       FIG. 4  is a flowchart illustrating a method for determining the SIMO mode or C-SM mode for a selected burst according to an exemplary embodiment of the present invention. 
     A base station transmits a burst in either the SIMO mode or the C-SM mode by taking into consideration the type of data to transmit and the channel state. 
     Selecting the SIMO mode or the C-SM mode is performed through use of an average value that is obtained by normalizing uplink transmission power “P UL, TX ,” that is received from a terminal based on uplink transmission power of a non-HARQ burst with QPSK ½. Before allocating an uplink burst to a frame, the base station predetermines which mode of the SIMO and C-SM modes is to be used. Basically, after a specific threshold value has been set, the terminal having a normalized uplink transmission power “P UL, TX ” higher than the specific threshold value is established to use the SIMO mode, while a terminal having a normalized uplink transmission power “P UL, TX ” equal to or less than the specific threshold value is established to use the C-SM mode. Since a terminal located in a boundary area has a high path loss, the terminal has high normalized uplink power so that the terminal comes to use the SIMO mode. Also, the normalized uplink transmission power threshold value for dividing into the SIMO mode and the C-SM mode is determined by taking capacity and coverage into consideration. When the normalized uplink transmission power threshold value is set to a large value, most terminals come to use the SIMO mode, and thus, the frequency of the utilization of the C-SM mode becomes low so that the capacity is reduced. Meanwhile, when the normalized uplink transmission power threshold value is set to a small value, interference to neighboring cells increases so that a coverage problem is created. In order to prevent unnecessary changes to the SIMO mode or the C-SM mode from occurring, a hysteresis margin is used when changing from the SIMO mode to the C-SM mode. That is, a threshold value “T SIMO, C-SM ” used when changing from the SIMO mode to the C-SM mode and a threshold value “T C-SM, SIMO ” used when changing from the C-SM mode to the SIMO mode are set to mutually different values. 
     Hereinafter, a method for determining the SIMO mode or C-SM mode for a selected burst according to an exemplary embodiment of the present invention will be described in detail with reference to  FIG. 4 . 
     In step  410 , a base station checks if a last mode used by a terminal is the SIMO mode. When the last mode used by the terminal is the SIMO mode as a result of the check, the base station proceeds to step  412 . In step  412 , when uplink transmission power “P UL, TX ” transmitted from the terminal is less than a first threshold value “T SIMO, C-SM ” used as a criterion for changing from the SIMO mode to the C-SM mode, the base station proceeds to step  414 , where the base station establishes the C-SM mode for the terminal. The uplink transmission power is a normalized average value. In contrast, when the uplink transmission power transmitted from the terminal is not less than the first threshold value used as a criterion for changing from the SIMO mode to the C-SM mode, the base station proceeds to step  416 , where the base station establishes the SIMO mode. 
     Meanwhile, when the last mode used by the terminal is not the SIMO mode as a result of the check in step  410 , the base station proceeds to step  418 . In step  418 , when the uplink transmission power is less than a second threshold value “T C-SM, SIMO ,” the base station proceeds to step  420  where the base station establishes the C-SM mode for the terminal. In contrast, when the uplink transmission power is not less than the second threshold value in step  418 , the base station proceeds to step  420  where the base station establishes the SIMO mode for the terminal. 
     A part for adding a SIMO burst to a frame determines if a selected burst can be allocated to the SIMO mode by newly determining the disposition of a pre-allocated burst. 
     Minimizing the number of slots allocated to the C-SM mode is maximizing the number of slots that can be allocated to the SIMO mode. Therefore, in order for the number of slots using one-side pilot pattern to be similar to the number of slots using the other-side pilot pattern, pre-allocated C-SM bursts are evenly divided and allocated to a layer using pilot pattern A and a layer using pilot pattern B. Accordingly, in consideration of the complexity in implementation, a WF heuristic is used. 
     The methods of applying the WF heuristic are as follows. 
     A first method is to arrange all pre-allocated C-SM bursts according to the number of slots. That is, since the first method is to add one burst, to decide a sequence, and to determine a pilot pattern whenever the need arises, a separate operation for arrangement is not required. 
     A second method is to determine pilot patterns in the sequence from a burst allocated the most slots to a burst allocated the fewest slots. When a pilot pattern of a certain burst belonging to a terminal is to be determined and a pilot pattern of another burst belonging to the terminal has already been determined, the certain burst is established to use the same pilot pattern as the another pattern. A pilot pattern of a first burst is determined by the WF heuristic. That is, if the number of slots determined for pilot pattern A is relatively greater, the burst is determined to have pilot pattern B, and if not, the burst is determined to have pilot pattern A. When the number of slots determined for pilot pattern A is equal to the number of slots determined for pilot pattern B, the burst is determined to have pilot pattern A. 
     Hereinafter, a method for adding a SIMO burst to a frame according to an exemplary embodiment of the present invention will be described in detail with reference to  FIG. 5 . 
     In step  510 , an uplink scheduler backs up the pilot patterns and the number of slots allocated for every connection relating to a plurality of terminals. The base station releases slots allocated to bursts including selective connections with the terminals in step  512  and proceeds to step  514  where the base station resets pilot patterns of all C-SM bursts and then recalculates the number of remaining slots. 
     In step  516 , if pilot patterns of every burst have been determined, the base station proceeds to step  518 , and if not, the base station proceeds to step  524 . When it is determined, in step  518 , that the number “N slot, remain, SIMO ” of slots that can be additionally allocated to the SIMO mode is not less than the number of released slots, the base station determines that the WF heuristic has been performed successfully in step  520 . In contrast, when it is determined, in step  518 , that the number “N slot, remain, SIMO ” of slots that can be additionally allocated to the SIMO mode is less than the number of slots released in step  512 , the base station proceeds to step  522  where the base station restores the number of slots and the pilot patterns, which have been backed up, for every connection with the plurality of terminals. When one or more bursts for which a pilot pattern has not been determined remain in step  516 , the base station proceeds to step  524  where the base station selects a burst allocated the most slots from among the remaining bursts. Next, in step  526 , when a pilot pattern of another burst belonging to a terminal, to which the selected burst belongs, has already been determined, the base station proceeds to step  528  where the base station determines if the number of allocation slots of the selected burst is less than the number of remaining slots of the determined pilot pattern. When the number of allocation slots of the selected burst is less than the number of remaining slots of the determined pilot pattern, the base station proceeds to step  530  where the determined pilot pattern identified in step  526  is used for the selected burst. In contrast, when it is determined, in step  528 , that the number of allocation slots of the selected burst is not less than the number of remaining slots of the determined pilot pattern identified in step  526 , the base station proceeds to step  522  where the base station restores the pilot patterns and the number of slots, which have been backed up, for every connection with the plurality of terminals. 
     Meanwhile, when it is determined in step  526  that a pilot pattern of another burst (i.e., a burst in step  512 ) belonging to the terminal has not yet been determined, and when it is determined, in step  532 , that the number “N slot, remain, A ” of slots that can be additionally allocated to pilot pattern A of the C-SM mode is not less than the number “N slot, remain, B ” of slots that can be additionally allocated to pilot pattern B of the C-SM mode, the base station proceeds to step  534  where the base station compares the number of allocation slots of the selected burst with the number of slots that can be additionally allocated to pilot pattern A of the C-SM mode. When the number of allocation slots of the selected burst is less than the number of slots that can be additionally allocated to pilot pattern A of the C-SM mode as a result of the comparison in step  534 , the base station proceeds to step  540  where pilot pattern A is determined. 
     When it is determined, in step  536 , that the number of allocation slots of the selected burst in step  512  is less than the number of slots that can be additionally allocated to pilot pattern B of the C-SM mode, the base station proceeds to step  538  where pilot pattern B is determined. In contrast, when it is determined, in step  534 , that the number of allocation slots of the selected burst in step  512  is greater than the number of slots that can be additionally allocated to pilot pattern A of the C-SM mode, or when it is determined, in step  536 , that the number of allocation slots of the selected burst is not less than the number of slots that can be additionally allocated to pilot pattern B of the C-SM mode, the base station proceeds to step  522  where the base station restores the pilot patterns and the number of slots, which have been backed up, for every connection with the plurality of terminals. 
       FIG. 6  is a view explaining the selection of pilot patterns based on a WF heuristic algorithm according to an exemplary embodiment of the present invention. 
     According to the WF heuristic algorithm, all the slots are divided into slots of the SIMO mode, slots of pilot patterns, and the remaining slots that can be allocated. When the number of slots used for the pilot patterns are minimized, the number of remaining slots that can be additionally allocated to the SIMO mode becomes maximized. 
     The part of adding a C-SM burst to a frame newly determines the disposition of a pre-allocated burst, thereby checking if a selected burst can be allocated to the C-SM mode. If the selected burst can be allocated to the C-SM mode as a result of the check, the selected burst is determined to be allocated to the C-SM mode; and if not, the selected burst is not allocated. 
     When as many as possible of the remaining slots, except for slots allocated to the SIMO mode, are used, and simultaneously, the pilot patterns of pre-allocated C-SM bursts are determined to be gathered into one layer as much as possible, the number of slots that can be additionally allocated to the C-SM mode becomes maximized. Therefore, pre-allocated C-SM bursts are divided and allocated to a layer using pilot pattern A and a layer using pilot pattern B so that the number of slots using pilot pattern A is greater than the number of slots using pilot pattern B. Accordingly, in consideration of the complexity in implementation, the FF heuristic is used. 
     The method of applying the FF heuristic is as follows. 
     A first method is to arrange all pre-allocated C-SM bursts according to the number of slots. In this case, since one burst is added whenever the need arises, and a sequence is determined, a separate operation for arrangement is not required. When a slot allocation for a selected connection is added to a pre-allocated burst, the arrangement is performed with the exception of the pre-allocated burst. 
     A second method is to determine pilot patterns in the sequence from a burst allocated the most slots to a burst allocated the fewest slots. A burst belonging to the same terminal as a burst, of which a pilot pattern is to be determined, is established to use pilot pattern B. In addition, when a pilot pattern of a certain burst belonging to a terminal is to be determined, and a pilot pattern of another burst belonging to the terminal has already been determined, the certain burst is established to use the same pilot pattern as the another pattern. In the other cases, if a space where pilot pattern A can be used remains, pilot pattern A is determined, and if not, pilot pattern B is determined. 
     A pilot pattern of a burst to be allocated changes whenever a new connection is selected. The pilot pattern of the burst is finally determined after all connection selections have been completed. That is, during a scheduling process, only a temporary pilot pattern of a burst is determined and changes every time in the direction of increasing the probability of selecting a new connection is as high as possible. This is because it is preferred that pre-allocated bursts are evenly divided into two-side layers when a connection to be next selected uses the SIMO mode and it is performed such that the bursts are gathered into one-side layer so as to empty the other-side layer as much as possible when a connection to be next selected uses the C-SM mode. It should be noted that one terminal must not simultaneously use pilot pattern A and pilot pattern B of one slot. To this end, one terminal is allowed to use only one of pilot pattern A and pilot pattern B in one frame. According to the standard, one terminal may use pilot pattern A and pilot pattern B for differently positioned slots, respectively, but it is allocated to use only one pattern in consideration of the complexity. 
     Hereinafter, a method for adding a C-SM burst to a frame according to an exemplary embodiment of the present invention will be described in detail with reference to  FIG. 7 . 
     In step  710 , an uplink scheduler backs up the pilot patterns and the number of slots allocated for every connection. The base station releases slots allocated to a burst, which is to include a selected connection with a terminal, in step  712 . The base station proceeds to step  714  where the base station resets pilot patterns of all C-SM bursts and recalculates the number of remaining slots. 
     In step  716 , if pilot patterns of all bursts have been determined, the base station proceeds to step  718 , and if not, the base station proceeds to step  722 . When it is determined, in step  718 , that no burst of which a pilot pattern has not been determined remains and the number of slots that can be additionally allocated to pilot pattern B of the C-SM mode is not less than the number of slots released in step  712 , the base station determines that the FF heuristic has been successfully performed in step  720 . In contrast, when it is determined in step  718  that the number “N slot, remain, B ” of slots that can be additionally allocated to pilot pattern B of the C-SM mode is less than the number of slots released in step  712 , the base station proceeds to step  738 , where the base station restores the number of slots and the pilot patterns, which have been backed up, for every connection with a plurality of terminals. Meanwhile, when one or more bursts for which a pilot pattern has not been determined remain in step  716 , the base station proceeds to step  722  where the base station selects a burst allocated the most slots from among the remaining bursts. Next, in step  724 , when a pilot pattern of another burst belonging to a terminal, to which the selected burst belongs, has already been determined, the base station proceeds to step  732  where the base station determines if the number of allocation slots of the selected burst is less than the number of remaining slots of the determined pilot pattern. When the number of allocation slots of the selected burst is less than the number of remaining slots of the determined pilot pattern, the base station proceeds to step  734  where the determined pilot pattern is used for the selected burst. In contrast, when it is determined in step  732  that the number of allocation slots of the selected burst is not less than the number of remaining slots of the determined pilot pattern, the base station proceeds to step  738  where the base station restores the pilot patterns and the number of slots, which have been backed up, for every connection with the plurality of terminals. 
     Meanwhile, when it is determined, in step  724 , that a pilot pattern of another burst belonging to the terminal has not yet been determined and when it is determined, in step  726 , that there is no burst allocated to the terminal, to which a connection selected by the scheduler belongs, the base station proceeds to step  728 . When the number of allocation slots of the selected burst is less than the number “N slot, remain, A ” of slots that can be additionally allocated to pilot pattern A of the C-SM mode, the base station proceeds to step  740  where pilot pattern A is determined. 
     When it is determined in step  730  that the number of allocation slots of the selected burst is less than the number of slots that can be additionally allocated to pilot pattern B of the C-SM mode, the base station proceeds to step  736  where pilot pattern B is determined. In contrast, when it is determined in step  730  that the number of allocation slots of the selected burst is not less than the number of slots that can be additionally allocated to pilot pattern B of the C-SM mode, the base station proceeds to step  738  where the base station restores the pilot patterns and the number of slots, which have been backed up, for every connection with the plurality of terminals. 
       FIG. 8  is a view explaining the selection of pilot patterns based on an FF heuristic algorithm according to an exemplary embodiment of the present invention. 
     The FF heuristic algorithm is to maximize the number of slots that can be additionally allocated to the C-SM mode by maximizing the number of slots for pilot pattern A among all the slots. 
       FIG. 9  is a view illustrating burst allocations based on pilot patterns according to an exemplary embodiment of the present invention. 
     Bursts of the C-SM mode are allocated both pilot patterns A and B. Pilot patterns A and B are allocated together with the SIMO mode and the C-SM mode (wherein, bursts of the SIMO mode using pilot pattern B have not been shown). 
       FIG. 10  is a view illustrating a result of a simulation of an uplink burst allocation algorithm according to an exemplary embodiment of the present invention. 
     The simulation was performed with the following conditions: 
     The number of entire slots: 140 
     The number of SIMO mode slots: 40 
     C-SM mode bursts are arranged in order of size, e.g., in the sequence of 40, 30, 20, and 10. 
     WF heuristic 
     Pilot pattern A: 40, 20 
     Pilot pattern B: 30, 20, 10 
     FF heuristic 
     As shown in  FIG. 10 , when a sufficient number of connections exist, the burst allocation algorithm according to the present invention can obtain a throughput gain of about 10%, as compared with the baseline. 
     According to the present invention, as described above, since a terminal located in a cell boundary is controlled to use the SIMO mode, it is possible to prevent coverage reduction and it is possible to minimize damage in the priority, so that unnecessary increases in delay can be prevented. In addition, according to the present invention, since bursts using the C-SM mode are properly disposed, it is possible to increase the probability that a newly selected burst will be additionally allocated. 
     Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.