Abstract:
A system for controlling a reverse data rate in a mobile communication system including a plurality of mobile stations, a plurality of base transceiver systems (BTSs) in communication with the mobile stations, and a base station controller (BSC) connected to the BTSs. The BSC detects handover states of the mobile stations, and controls a reverse data rate of a mobile station in a handover state. The BTS controls a reverse data rate of a mobile station in a non-handover state.

Description:
PRIORITY  
       [0001]    This application claims priority under 35 U.S.C. § 119 to an application entitled “System and Method for Controlling Traffic Distribution in a Mobile Communication System” filed in the Korean Intellectual Property Office on Jan. 11, 2003 and assigned Serial No. 2003-1874, the contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a traffic control system and method in a mobile communication system, and in particular, to a system and method for controlling reverse traffic.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, in a mobile communication system, data transmission can be divided into forward data transmission and reverse data transmission. “Forward data transmission” refers to data transmission from a base station to a mobile station, while “reverse data transmission” refers to data transmission from a mobile station to a base station. According to the type of transmission data, mobile communication systems can be classified into systems supporting only a voice service, systems supporting a combination of a voice service and a simple data service, and systems supporting only a high-speed data service. The advent of such a mobile communication system providing a data service is the result of rapid development of mobile communication technology in answer to increasing users&#39; demands for transmitting/receiving more information at higher speeds.  
           [0006]    In such a mobile communication system processing data at high speed, reverse data traffic is transmitted over a packet data channel by the physical layer packet (PLP), and a length of the data traffic is fixed. Packets have a variable data rate, and a data rate of each packet is determined depending on power of a mobile station, an amount of transmission data, and a rate control bit (RCB) transmitted from a base station over a rate control channel (RCCH).  
           [0007]    In addition, a data rate of a mobile station is determined by scheduling. A base station performs scheduling using RoT (Rise over Thermal) representing total reception power over thermal noises or a load obtained from a received signal-to-noise ratio of a mobile station belonging to a current base transceiver system (BTS). When RoT is available, scheduling is performed so that RoT is matched to a predetermined reference RoT level, and when RoT is unavailable, scheduling is performed so that a load is matched to a predetermined reference load level. For the convenience of explanation, RoT is used herein.  
           [0008]    Thus, a scheduler in a base station determines whether to increase, decrease or hold a data rate of a corresponding mobile station, considering RoT, and a buffer status and a power status of each mobile station.  
           [0009]    [0009]FIG. 1 is a diagram for explaining the configuration and operation scheme for controlling a mobile station in an existing system. As illustrated in FIG. 1, a base station (BS) is comprised of base transceiver systems (BTSs) and a base station controller (BSC). A BTS manages its cell(s), and a BSC is connected to a plurality of BTSs and controls the BTSs connected thereto. In addition, as illustrated in FIG.  1 , each mobile station undergoes reverse data rate control from a BTS to which it belongs. A mobile station in a non-soft handover (non-SHO) state (hereinafter referred to as “non-SHO mobile station”) undergoes reverse data rate control from only a BTS to which it belongs, while a mobile station in a soft handover (SHO) state (hereinafter referred to as “SHO mobile station”) undergoes reverse data rate control from a plurality of BTSs in an active set. Herein, a non-SHO mobile station refers to a mobile station in a non-SHO state, while an SHO mobile station refers to a mobile station in an SHO state. In FIG. 1, mobile stations  111  and  113  each belonging to one BTS are controlled by their BTSs  101  and  102 , respectively, and another mobile station  112  belonging to both BTSs becomes an SHO mobile station which is simultaneously controlled by the BTSs  101  and  102 .  
           [0010]    In an SHO mobile station controlled by a plurality of BTSs, reverse rate control messages received from the BTSs are different from each other. In this regard, a rate control procedure will now be described with reference to FIG. 1. A BTS# 1   101  and a BTS# 2   102  each transmit a rate control command to a mobile station according to their RoT conditions. For example, BTS# 1   101  can send a rate-down command to the mobile station, while BTS# 2   102  can send a rate-up command to the mobile station. In this case, the mobile station obeys a command from any one of the BTSs, and commonly, according to ‘Or-of-Down’ rule, the mobile station decreases it data rate if any one of the BTSs issues a rate-down command. This is because a scheduler in each BTS determines a data rate of each mobile station so that a received RoT maintains a reference RoT, and if a mobile station receiving a rate-down command increases its data rate, a received RoT of a corresponding BTS will undesirably exceed the reference RoT.  
           [0011]    When a received RoT exceeding the reference RoT in a particular BTS, an increase in an interference level may occur, resulting in a reduction in throughput of a corresponding cell. Therefore, in order to secure stability of the entire system, if a mobile station receives different rate control commands from a plurality of BTSs, the mobile station determines whether there is any rate-down command among the received rate control commands. If there is any rate-down command, the mobile station decreases the current data rate and transmits data at the decreased data rate.  
           [0012]    However, even the use of the ‘Or-of-Down’ rule cannot resolve the inefficiency problem. A scheduler in a BTS determines a data rate of each mobile station so that a received RoT maintains a reference RoT. However, if a mobile station receiving a rate-up command from a particular BTS decreases it data rate, a received RoT of the BTS becomes lower than a reference RoT. This means that available resources are not sufficiently utilized, also leading to a reduction in throughput of a corresponding cell.  
           [0013]    Similar situations occur even in a case where reverse Hybrid Automatic Repeat and Request (HARQ) is used. A description will now be made of an operation performed in such a case. If reverse data is received, a BTS must transmit an ACK or NACK signal to a mobile station over an acknowledgment channel (hereinafter referred to as “ACK channel”). In the case where the mobile station is a non-SHO mobile station, a corresponding BTS determines an ACK or NACK signal and then transmits the determined signal. However, in the case where the mobile station is an SHO mobile station, signals received from respective BTSs connected to the mobile station can be different from each other. This will be described below with reference to FIG. 1. For example, BTS# 1   101  can successfully receive a packet transmitted by the mobile station  112 , while BTS# 2   102  fails to receive the packet transmitted by the mobile station  112 . In this case, BTS# 1   101  transmits an ACK signal to the mobile station  112  and BTS# 2   102  transmits a NACK signal to the mobile station  112 . The mobile station  112  receiving such signals transmits the next packet because it received from BTS# 1   101  an ACK signal indicating successful receipt of a previous packet. However, BTS# 2   102  expects that the previous packet will be retransmitted, since it failed to successfully receive the previous packet, i.e., it transmitted a NACK signal to the mobile station  112 . In this case, a signal for determining whether each cell has successfully received a packet is needed between BTS# 1   101  and BTS# 2   102 . However, such signaling can act as an overhead. That is, when BTS# 1   101  transmits ACK and BTS# 2   102  transmits NACK, the mobile station  112  transmits a new packet since it received ACK, but BTS# 2   102  expects retransmission of the previous packet. Therefore, for more accurate operation, it is necessary to inform BTS# 2  that BTS# 1  transmitted ACK, through signaling via a network.  
         SUMMARY OF THE INVENTION  
         [0014]    It is, therefore, an object of the present invention to provide a system and method for controlling traffic distribution so as to increase BTS efficiency during traffic control in a BTS and a BSC in a mobile communication system.  
           [0015]    It is another object of the present invention to provide a system and method for transmitting a consistent control message to a mobile station in a mobile communication system.  
           [0016]    It is a further object of the present invention to provide a system and method for efficiently controlling an SHO mobile station.  
           [0017]    It is yet another object of the present invention to provide a system and method for increasing throughput of a BTS by transmitting a consistent control message to a mobile station in a mobile communication system using reverse HARQ.  
           [0018]    To achieve the above and other objects, there is provided a system for controlling a reverse data rate in a mobile communication system including a plurality of mobile stations, a plurality of base transceiver systems (BTSs) in communication with the mobile stations, and a base station controller (BSC) connected to the BTSs. The BSC detects handover states of the mobile stations, and controls a reverse data rate of a mobile station in a handover state. The BTS controls a reverse data rate of a mobile station in a non-handover state.  
           [0019]    To achieve the above and other objects, there is provided a method for controlling a reverse data rate of a mobile station by a base station controller (BSC) in a mobile communication system including a plurality of mobile stations, a plurality of base transceiver systems (BTSs) in communication with the mobile stations, and the BSC connected to the BTSs. The method comprises the steps of transmitting a reverse rate control suspend message for a particular mobile station to a BTS controlling a reverse data rate of the mobile station when handover of the mobile station is needed; and controlling a reverse data rate of the mobile station considering remaining capacity of BTSs in communication with the mobile station among BTSs included in an active set of the mobile station.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0021]    [0021]FIG. 1 is a diagram for explaining the configuration and operation scheme for controlling a mobile station in an existing system;  
         [0022]    [0022]FIG. 2 is a block diagram illustrating control function blocks for explaining operations of BTSs and a BSC during rate control on an SHO mobile station and a non-SHO mobile station according to a preferred embodiment of the present invention;  
         [0023]    [0023]FIG. 3 is a flowchart illustrating a procedure for performing handover of a mobile station in a BTS control function block according to a preferred embodiment of the present invention;  
         [0024]    [0024]FIG. 4 is a flowchart illustrating a procedure for performing handover of a mobile station in a BSC control function block according to a preferred embodiment of the present invention; and  
         [0025]    [0025]FIG. 5 is a block diagram illustrating structures of a BTS apparatus and a BSC apparatus according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    A preferred embodiment of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness.  
         [0027]    In an embodiment of the present invention, a BSC performs rate control in an SHO state and rate control in a non-SHO state in a different manner. That is, rate control on an SHO mobile station is performed by a BSC, while rate control on a non-SHO mobile station is performed by BTSs. In the following description, the present invention will be applied to rate control. One of ordinary skill in the art will recognize that the invention can also be applied to response signal control. For this, a reverse control message is used. The reverse control message is classified into a rate control bit (RCB) which is a reverse rate control bit, and a grant message. The reverse rate control bit can be transmitted to instruct increase, decrease or hold of the current rate, or to instruct increase or decrease of the current rate. On the other hand, the grant message can be transmitted to instruct a corresponding mobile station to perform reverse control at a certain rate from the next slot. For example, if a mobile station, currently performing reverse transmission at 9.6 Kbps, receives a grant message granting transmission at 38.4 Kbps, the mobile station can perform reverse transmission at 38.4 Kbps, skipping the next rate of 19.6 Kbps. In the following description, RCB is used for reverse rate control. However, the grant message can also be used for reverse rate control.  
         [0028]    In addition, a response signal (or ACK/NACK signal) can be included in the reverse control message. In some cases, a message received from a SHO mobile station via a particular BTS is good while a message received from the SHO mobile station via another BTS is bad. In this case, a BSC transmits an ACK signal, a response signal indicating ‘Good’ reception, to the SHO mobile station. A description of such an example will be made below.  
         [0029]    [0029]FIG. 2 is a block diagram illustrating control function blocks for explaining operations of BTSs and a BSC during rate control on an SHO mobile station and a non-SHO mobile station according to a preferred embodiment of the present invention. With reference to FIG. 2, a detailed description will now be made of control functions and other functions of BTSs and a BSC during rate control on an SHO mobile station and a non-SHO mobile station according to the present invention.  
         [0030]    Respective reverse control function blocks  201 , . . . ,  20 N of BTSs (hereinafter referred to as “BTS control function blocks”) perform reverse rate control on non-SHO mobile stations. Such rate control is performed in the existing method where a BTS controls a data rate of a mobile station. Therefore, signaling and call assignment on a non-SHO mobile station are performed by the BSC like in the existing method. However, the BTS control function blocks  201 , . . . ,  20 N according to the present invention are designed not to perform reverse rate control on an SHO mobile station. A control function block  210  of a BSC (hereinafter referred to as “BSC control function block”) controls BTSs in the existing method. In addition, the BSC control function block  210  performs reverse rate control on an SHO mobile station according to the present invention. That is, the BSC control function block  210  controls a data rate of an SHO mobile station and controls transmission of a signal transmitted over an ACK channel.  
         [0031]    A detailed description will now be made of operations of the BTS control function blocks  201 , . . . ,  20 N and the BSC control function block  210  when a mobile station transitions from a non-SHO state to an SHO state.  
         [0032]    When a mobile station is in a non-SHO state, i.e., when it belongs to only one BTS, the BTS control function block  201  allows the corresponding mobile station to undergo reverse data rate control only from that BTS. That is, the BTS control function block  201  generates a rate control bit and an ACK bit, and transmits them to the mobile station. However, various functions such as call-in, call-out, signaling, data rate control, and ACK/NACK detection from a received signal, are controlled by a BSC like in the existing method.  
         [0033]    In the meantime, if the mobile station transitions to an SHO state, the BTS control function block  201  generates information on a handover action time and an active set of a mobile station, and transmits the generated information to the corresponding mobile station. In addition, since subsequent control is performed by the BSC, the BSC control function block  210  determines whether assignment of a new rate control channel is needed. At the same time, the BSC control function block  210  determines whether assignment of an ACK channel is necessary. As a result of the determinations, if new channels are needed, the BSC control function block  210  generates new rate control channel information and new ACK channel information, and sends the generated channel information to the mobile station. Since the BTS control function block  201  cannot know the states of other BTSs, the BSC control function block  210  determines whether to assign new channels, and if assignment of new channels is required, the BSC control function block  210  controls the BTS so as to set up new channels to the mobile station transitioning to the SHO state. In other words, the BTS forms a message using handover action time information, active set information and new channel assignment information received from the BSC control function block  210 , and transmits the formed message to the corresponding mobile station. As a result, new channels are assigned between the BTS and the mobile station.  
         [0034]    Next, a description will be made of a control operation when the SHO mobile station enters the area of a particular BTS, ending its SHO state. When the SHO mobile station enters the area of a particular BTS, i.e., when handover is ended, the BSC control function block  210  instructs a corresponding BTS control function block to which the mobile station belongs to individually control a reverse data rate of the mobile station. A description will now be made of an operation performed in such a case.  
         [0035]    A delay time of a control message transmitted to a mobile station that undergoes reverse data rate control from the BSC control function block  210  is different from a delay time of a control message transmitted to a mobile station that undergoes reverse data rate control from a BTS. When a mobile station receives reverse rate control information from a BSC, a delay of about 2 frames occurs. Here, the “frame” becomes a data transmission unit for the reverse rate control information transmitted over a forward rate control channel. Therefore, when the mobile station is subject to reverse data rate control from the BSC, an ACK/NACK signal transmitted over an ACK channel may suffer transmission failure. For example, assuming that a BTS generates and transmits an ACK/NACK signal in answer to a signal received from a mobile station within a time of 2 frames (or 1 frame), if the BTS transmits a NACK signal responsive to data received at a current time (time #1) to the mobile station at a particular time (time #2), the mobile station retransmits a corresponding frame at a time #3 in response to the NACK signal received. The time #1, the time #2 and the time #3 occur in sequence. Therefore, each of the times can be either a transmission time in an air state, or a time required when determining a type of received information after completing error check on a received frame.  
         [0036]    However, since the SHO mobile station is subject to reverse data rate control from the BSC, a longer delay time occurs than when the mobile station undergoes reverse data rate control by the BTS. Actually, a time delay of at least 1 frame occurs when a signal is transmitted from a mobile station to a BSC via a BTS and then a NACK signal responsive to the corresponding signal is generated by the BSC and transmitted to the mobile station via the corresponding BTS. That is, if the BSC generates a NACK signal in response to a reception signal transmitted 1 frame ahead of the current time (or time #1) and transmits the generated NACK signal to a corresponding mobile station, the mobile station retransmits a corresponding frame at the time #3. In this case, if handover has ended, the BSC transmits an ACK/NACK signal to the mobile station in response to a previous frame, and the corresponding BTS controls a data rate of the mobile station from the time when handover ended. Therefore, when the BSC transmits an ACK/NACK signal to the mobile station in response to a frame received at a previous time, the BTS transmits an ACK/NACK signal in response to a frame received after handover ended. In this case, the mobile station receives retransmission requests for different data frames at the same time, so the ACK/NACK signal received from the BSC collides with the ACK/NACK signal received from the BTS.  
         [0037]    Generally, a delay time of a control message transmitted when a mobile station is controlled by a BSC is longer than that of a control message transmitted when the mobile station is controlled by a BTS. Therefore, if a mobile station transitions from a non-SHO state to an SHO state, a transmission time of an ACK/NACK signal becomes longer. In this case, the ACK/NACK signal is normally transmitted regardless of whether the mobile station is assigned a new channel or uses an existing channel. However, if the mobile station transitions from the SHO state to the non-SHO state, a transmission time of an ACK/NACK signal becomes shorter. Therefore, if the existing channel is used, the BTS and the mobile station both should transmit ACK/NACK signals for two packets at the same time, which is undesirable.  
         [0038]    In this case, in the embodiment of the present invention, packet transmission is suspended for a period as much as a difference in a transmission time of the ACK/NACK signal between the BTS and the BSC. In an alternative method, when the mobile station transitions from the non-SHO state to the SHO state, the BTS assigns a new ACK channel to the corresponding mobile station. In this way, a response (ACK/NACK) signal generated while the mobile station undergoes data rate control from the BSC is transmitted over an ACK channel assigned when the mobile station undergoes data rate control by the BTS, and a response signal for a new packet is transmitted over an ACK channel newly assigned by the current BTS. In the latter method, the mobile station has two ACK channels corresponding to a transmission delay time between the BTS and the BSC, and the BTS must hold the two ACK channels assigned to the mobile station.  
         [0039]    A description will now be made of an operation of the BSC control function block  210  according to above-stated two embodiments of the present invention. The BSC control function block  210  performs a general control function. In an SHO state, when an inquiry about whether assignment of a new channel to an SHO mobile station is needed is received from the BTS control function block  201 , the BSC control function block  210  checks a state of target BTSs to which a call is to be handed over, and then sends the result information to the inquiring BTS. In addition, if a reverse rate control request for the SHO mobile station is received from the BTS control function block  201 , the BSC control function block  210  controls a data rate of the SHO mobile station considering a resource state of the corresponding BTSs. Furthermore, the BSC control function block  210  symbol-combines packets received from the mobile station via a source BTS that will hand over a current call and target BTSs to which the call is to be handed over, and checks whether the combined received packet is good or bad. If the received packet is good, the BSC control function block  210  transmits an ACK signal over an ACK channel set up between each BTS in the handover operation and the mobile station, and if the received packet is bad, the BSC control function block  210  transmits a NACK signal over the ACK channel set up between each BTS in the handover operation and the mobile station.  
         [0040]    Thereafter, if handover of the mobile station has ended, the BSC control function block  210  transmits an ACK/NACK signal for the packet received from a corresponding mobile station to the mobile station over a channel that was set up during handover. In a first embodiment of the above-stated two embodiments, the BSC control function block  210  controls a BTS so as to suspend reverse data transmission between the BTS and the mobile station for a predetermined time. Unlike this, in the second embodiment, the BSC control function block  210  transmits an ACK/NACK signal over a separate channel instead of a channel which was in the handover operation, as a channel for ACK/NACK transmission between the mobile station and the BTS. After an ACK signal for a final frame received from the BSC is transmitted, the channel is released. However, the channel transmitting an ACK/NACK signal, set up between the BTS currently controlling the mobile station and the BSC, is continuously held. The BSC control function block  210  instructs the BTS control function block  201  to control a reverse data rate of the handover-ended mobile station.  
         [0041]    When the BSC controls a data rate of an SHO mobile station in this way, a BTS subtracts capacity corresponding to a data rate of a corresponding mobile station from the total capacity and controls a data rate of mobile stations in its area based on RoT or load at the remaining capacity, thus contributing to an increase in throughput of the BTS. In addition, BTSs are prevented from transmitting different ACK/NACK signals to the SHO mobile station, contributing to an increase in BTS efficiency and making it possible to easily control the mobile station.  
         [0042]    As a BSC controls an SHO mobile station, a mobile station is not separately controlled by a plurality of BTSs included in its active set, and receives the same signal from the BTSs by the BSC. Therefore, it is possible to increase reception capability of packet data and an ACK/NACK channel signal through soft-combining on the received signals.  
         [0043]    [0043]FIG. 3 is a flowchart illustrating a procedure for performing handover of a mobile station in a BTS control function block according to a preferred embodiment of the present invention.  
         [0044]    In step  300 , the BTS control function block  201  measures the total RoT or a load of a BTS. The total RoT can be measured at a predetermined time, and the load can be calculated by the BTS depending on a state of the current reverse link. Therefore, when the load is used for control, the BTS control function block  201  continuously measures the load, and when control is performed based on RoT, the BTS control function block  201  measures the RoT every predetermined time.  
         [0045]    When measurement of RoT is completed or calculation of a load is completed, the BTS control function block  201  proceeds to step  302 . In step  302 , the BTS control function block  201  receives a reverse load of a mobile station controlled by a BSC because handover is in progress. The reverse load is a value received from a BSC, and the BSC provides this information to the BTS continuously or at stated periods. Therefore, in step  304 , the BTS receiving the reverse load calculates reverse loads of non-SHO mobile stations, i.e., mobile stations whose reverse data rates are controlled by the BTS control function block  201  according to the present invention.  
         [0046]    Thereafter, the BTS control function block  201  proceeds to step  306 . In step  306 , the BTS control function block  201  calculates capacity of a currently available reverse link from the value determined in steps  300  to  304 . In addition, the BTS control function block  201  controls a reverse data rate of a non-SHO mobile station controlled by a BTS according to the currently available reverse link. Furthermore, in step  306 , the BTS control function block  201  transmits a response (ACK/NACK) signal for a reverse packet received from a mobile station. The BTS control function block  201  determines in step  308  whether a message indicating occurrence of a handover state for a mobile station controlled by the BTS is received from the BSC. If it is determined in step  308  that there is a mobile station in an SHO state, the BTS control function block  201  proceeds to step  310  where it excludes the corresponding mobile station from mobile stations whose reverse rates are being controlled. That is, reverse rate control by the BTS is suspended. Thereafter, the BTS control function block  201  returns to step  300 , and measures RoT or a load. When the BTS control function block  201  operates based on RoT, if the current time is not a predetermined RoT measurement time, the BTS control function block  201  proceeds to step  302  without performing the measurement of RoT or a load. In this manner, rapid reverse rate control can be performed on a mobile station in communication with only a BTS. Also, at step  308 , if it is determined that there are no mobile stations in an SHO state, the process returns to step  300 .  
         [0047]    Meanwhile, though not illustrated in FIG. 3, when a particular mobile station controlled by a BSC enters a particular BTS, there is a case where reverse rate control should be performed on the mobile station. In this case, the BTS measures RoT or load of the corresponding mobile station during RoT measurement or load measurement of step  300 , and performs reverse rate control on the mobile station. In addition, for such a rate control time, a separate channel can be used or a method of suspending reverse transmission for a predetermined time can be used.  
         [0048]    [0048]FIG. 4 is a flowchart illustrating a procedure for performing handover of a mobile station in a BSC control function block according to a preferred embodiment of the present invention.  
         [0049]    In step  400 , the BSC control function block  210  holds a call control state. Here, “call control state” refers to a state in which transmission of call assignment and control messages for a mobile station is controlled through a BTS, and according to the present invention, reverse rate control for a non-SHO mobile station is not included. For such mobile stations, reverse rate control is performed in the BTS as described in conjunction with FIG. 3. Holding such a call control state, the BSC control function block  210  proceeds to step  402  where it determines whether there is a mobile station in an SHO state. If it is determined in step  402  that there is a mobile station in an SHO state, the BSC control function block  210  proceeds to step  404 , and otherwise, the BSC control function block  210  returns to step  400 .  
         [0050]    In step  404 , the BSC control function block  210  transmits a call handover (or call transfer) message to the BTS. That is, the BSC control function block  210  transmits to a corresponding BTS a message for suspending reverse rate control on a mobile station controlled by the BTS. Furthermore, in step  404 , the BSC control function block  210  determines an active set and a handover action time of the mobile station to be handed over (i.e., an SHO mobile station). Thereafter, in step  406 , the BSC control function block  210  determines whether assignment of a reverse rate channel and an ACK channel for transmitting an ACK/NACK signal is needed for the SHO mobile station. If it is determined in step  406  that channel assignment is necessary, the BSC control function block  210  proceeds to step  408 , and otherwise, the BSC control function block  210  proceeds to step  410 .  
         [0051]    In step  408 , the BSC control function block  210  assigns new channels to the SHO mobile station, and transmits a channel assignment message via a BTS currently in communication with the mobile station in order to inform the mobile station of the newly assigned channels.  
         [0052]    Thereafter, in step  410 , the BSC control function block  210  controls a reverse data rate of the SHO mobile station considering states of BTSs in communication with the SHO mobile station among BTSs included in the active set of the SHO mobile station. At this point, an ACK/NACK signal received from the mobile station is also transmitted to the BSC without being processed in the BTS. Therefore, the BSC control function block  210  performs data retransmission or new data transmission depending on information received from the mobile station.  
         [0053]    As the SHO mobile station is controlled by the BSC, the mobile station can receive a consistent ACK signal, and since the mobile station receives the same message, the mobile station can increase reception probability by soft-combining the received message.  
         [0054]    After performing rate control on a reverse link of a particular mobile station in step  410 , the BSC control function block  210  proceeds to step  412  where it determines whether handover of the SHO mobile station is ended. That is, the BSC control function block  210  determines whether the mobile station enters the area of a particular BTS and performs communication only in the BTS. If it is determined in step  412  that handover is ended, the BSC control function block  210  proceeds to step  414  where it generates a control handover (or control transfer) message for instructing a BTS where the mobile station is located to perform reverse rate control, and delivers the generated control handover message. Therefore, if handover of the mobile station is ended, transmission control on a reverse rate and an ACK signal is handed over to another BTS by the particular BTS. After the step  414 , the BSC control function block  210  returns to step  400 . At step  412 , if it is determined that handover has not ended, the process returns to step  410 .  
         [0055]    Next, with reference to FIG. 5, a description will be made of the connection between a BTS apparatus and a BSC apparatus, and their internal structures. FIG. 5 is a block diagram illustrating internal structures of a BTS apparatus and a BSC apparatus according to a preferred embodiment of the present invention.  
         [0056]    In FIG. 5, reference numeral  510  represents an internal structure of a BSC apparatus, and reference numeral  520  represents an internal structure of a BTS apparatus. It should be noted that only the essential elements associated with the present invention are shown in FIG. 5.  
         [0057]    First, a description will be made of internal structure and operation of the BSC  510 . A controller  511  of the BSC  510  includes the BSC control function block  210  described in connection with FIG. 2, and thus performs a control operation according to the present invention. Data needed in the controller  511  is stored in a memory  512 . That is, the memory  512  stores data needed for performing the procedure of FIG. 4. In addition, the memory  512  stores various data necessary for controlling an SHO mobile station. Using such data, the controller  511  generates a message for controlling a corresponding mobile station, or a message for controlling a corresponding BTS. A data processor  514  divides forward data to be transmitted to a particular mobile station in a proper size, or combines data received from the mobile station to transmit it to an upper layer. An interface  513  performs interfacing on data exchanged between the BSC  510  and the BTS  520 .  
         [0058]    Next, a description will be made of internal structure and operation of the BSC  520 . The BTS  520  includes an interface  522  for performing interfacing on data received from the BSC  510 , and a controller  521  for performing a control operation according to the present invention. The controller  521  includes the BTS control function block described in connection with FIG. 2. Thus, the controller  521  performs reverse control on only a non-SHO mobile station. Even when reverse control is performed by the BSC  510 , a message is actually transmitted from the BTS  520  to a mobile station. Therefore, when a request for transmission of a control message for an SHO mobile station is received from the BSC  510 , the controller  521  generates a control message. Alternatively, however, the BSC  510  can directly generate such a message and transmit the generated message.  
         [0059]    A switch  523  performs a switching operation for transmitting forward data to be transmitted to each mobile station or reverse data received from each mobile station to the interface  522 , and transmitting data received from the controller  521  to the BSC  510 . In some cases, the switch  523  can be implemented with dedicated lines. However, it is implemented herein with a general switch. Data to be transmitted to a particular mobile station is processed in a modem section  524  and a radio frequency (RF) section  525 . The processed data is transmitted to a mobile station via an antenna. The modem section  524  and the RF section  525  include N modems  524 - 1  to  524 -N and N RF modules  525 - 1  to  525 -N, respectively, and each modem-RF module pair is associated with its corresponding mobile station. The modem section  524  encodes and modulates data to be transmitted in a forward direction, and demodulates and decodes data received in a reverse direction. The RF section  525  performs up-conversion and power amplification to transmit forward transmission data to a corresponding mobile station, and performs low-noise amplification and down-conversion on reverse reception data to generate a baseband signal. The modem section  524  and the RF section  525  constitute a packet transceiver. During handover of a mobile station, the BSC  510  and the BTS  520  perform the control operation described in connection with FIGS.  2  to  4 , so a detailed described thereof will be omitted for simplicity.  
         [0060]    As described above, mobile stations are divided into a mobile station whose handover is performed by the BSC and a mobile station whose handover is not performed by the BSC, to control reverse traffic on a distributed basis. In this case, the same control information can be transmitted to the mobile station. In addition, rate control on an SHO mobile station and transmission of a signal for HARQ become simple. Moreover, a non-SHO mobile station undergoes reverse rate control from a BTS, thus contributing to rapid rate control.  
         [0061]    While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.