Abstract:
An apparatus and method for data transmission in a base station (BS) of a wireless communication system are provided, storing data to be transmitted to a mobile station which performing handover to a target base station; confirming a quantity of stored data; determining whether to transmit the stored data to the target based at least partly on the confirmed quantity of the stored data; transmitting the stored data to the target base station when the base station determines to transmit the stored data to the target base station.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     The present application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Nov. 6, 2007 and assigned Serial No. 10-2007-0112843, the entire disclosure of which is hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to an apparatus and a method for handover of a mobile station (MS) in a wireless communication system. Particularly, the present invention relates to an apparatus and a method for transmitting stored data for an MS to a target base station (BS) in a serving BS of the wireless communication system when the MS performs handover to the target BS. Herein, the serving BS provides a service to the MS before handover, and the target BS provides a service to the MS after handover. 
     BACKGROUND OF THE INVENTION 
     A wireless communication system supports mobility of an MS using handover. For example, the MS selects a target BS from among neighbor BSs of a serving BS when the MS performs handover. After, the MS releases from the serving BS and connects to the target BS. 
     As described above, an MS releases from a serving BS and connects to a target BS for handover. Therefore, there occurs data loss because the MS is disconnected from both the serving BS and the target BS temporarily. 
     In order to solve this problem, a serving BS temporarily stores data to be transmitted to an MS when the serving BS releases from the MS. The serving BS reduces data loss for the MS incurred during handover by transmitting stored data for the MS to the target BS when the MS connects to the target BS. Herein, the serving BS transmits data to the target BS via a backhaul. Hereinafter, it is assumed that stored data to be transmitted to the MS in the serving BS is referred to as handover data. 
     The serving BS transmits the handover data to the target BS as soon as MS is connected to the target BS. 
     However, the handover data transmitted from the serving BS may be lost, and transmission time delay may occur due to a congested situation when a backhaul capacity is limited or an available backhaul capacity is not enough. 
     Therefore, there is a need to provide an improved method and apparatus for solving the above problem. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary aspect of exemplary embodiments of the present invention to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide an apparatus and method for transmitting stored data for an MS to a target BS based at least partly on an available backhaul capacity in a serving BS of a wireless communication system when the MS performs handover to the target BS. 
     Another aspect of exemplary embodiments of the present invention provides an apparatus and method for transmitting stored data for an MS to a target BS based at least partly on a quantity of stored data to a line card buffer in a serving BS of a wireless communication system when the MS performs handover to the target BS. 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method for data transmission in a base station (BS) of a wireless communication system. The method includes storing data to be transmitted to an MS which is performing handover to a target BS; confirming a quantity of stored data; determining whether to transmit the stored data to the target BS based at least partly on the confirmed quantity of stored data; and transmitting the stored data to the target BS when the BS determines to transmit the stored data to the target BS. 
     In accordance with another aspect of exemplary embodiments of the present invention, there is provided an apparatus for data transmission in a base station (BS) of a wireless communication system includes a storage for storing data to be transmitted to an MS which is performing handover to a target BS; a state confirmer for confirming a quantity of stored data in the storage; a transmission controller for determining whether to transmit the stored data to the target BS based at least partly on the confirmed quantity of stored data; and a transmitter for transmitting the stored data to the target BS when the BS determines to transmit the stored data to the target BS. 
     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  illustrates a conventional procedure for operating in a wireless communication system; 
         FIG. 2  is a block diagram illustrating a construction of a base station in a wireless communication system according to an exemplary embodiment of the present invention; and 
         FIG. 3  is a flow diagram illustrating a process of forwarding traffic according to a handover of a mobile station at a base station of a wireless communication system according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIGS. 1 through 3 , 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 communication system. 
     Exemplary embodiments of the present invention are intended to provide a scheme for transmitting data for a mobile station (MS) to a target base station (BS) based at least partly on an available backhaul capacity in a serving BS of a wireless communication system. Herein, the serving BS provides a service to an MS before handover, and the target BS provides a service to an MS after handover. 
     The serving BS stores data to be transmitted to the MS when the MS releases from the serving BS. When the MS connects to the target BS, the serving BS transmits stored data for the MS to the target BS as shown in  FIG. 1  to reduce data loss of the MS. 
       FIG. 1  illustrates a conventional procedure for operating in a wireless communication system. 
     As shown in  FIG. 1 , the wireless communication system includes a server  100 , a BS management unit  110 , a BS  120 ,  130 , and an MS  140 . 
     The MS  140  transmits and receives data with the serving BS  120 . Herein, the MS  140  is located in a service area of the serving BS  120 . 
     The serving BS  120  transmits received data from the server  100  to the MS  140  via the BS management unit  110 . As well, the serving BS  120  transmits received data from the MS  140  to the BS management unit  110 . 
     If the MS  140  performs a handover to the target BS  130  in step  151 , the MS  140  releases from the serving BS  120  and connects to the target BS  130 . 
     Before the MS  140  connects to the target BS  130 , the BS management unit  110  transmits data for the MS  140  to the serving BS  120  in step  153 . Therefore, the serving BS  120  stores received data for the MS  140  from the BS management unit  110  temporarily. 
     The target BS  130  requests transmission of stored data for the MS  140  to the serving BS  120  when the MS  140  connects to the target BS  130 . 
     When the target BS  130  requests transmission of stored data from the serving BS  120 , the serving BS  120  confirms an available capacity of a backhaul from the serving BS  120  to the target BS  130 . If the available capacity of the backhaul is enough, the serving BS  120  transmits stored data for the MS  140  to the target BS  130  via the backhaul in step  155 . 
     However, the serving BS  120  does not transmit stored data for the MS  140  to the target BS  130  when the available capacity of the backhaul is not enough. At this time, the serving BS  120  discards stored data for the MS  140 . 
     As described above, a serving BS stores data to be transmitted to an MS, when the MS releases from the serving BS. After, the serving BS transmits stored data for the MS to a target BS based at least partly on an available backhaul capacity. 
     For another example, a serving BS stores data to be transmitted to an MS when the MS releases from the serving BS. After, the serving BS transmits stored data for the MS to a target BS based at least partly on a quantity of stored data in a buffer as shown in  FIG. 2 . 
       FIG. 2  is a block diagram illustrating a construction of a base station in a wireless communication system according to an exemplary embodiment of the present invention. Herein, the BS is a serving BS. 
     As shown in  FIG. 2 , the serving BS includes a buffer state confirmer  200 , a transmission controller  210 , a channel card buffer  220 , and a line card buffer  230 . 
     The channel card buffer  220  stores copied uplink data in a channel card temporary. Herein, the channel card processes uplink data. 
     The line card buffer  230  stores copied uplink data in a line card temporary. Herein, the line card processes uplink data. 
     The buffer state confirmer  200  confirms a quantity of cumulated data in the line card buffer  230  periodically and reports the confirmed quantity of cumulated data to the transmission controller  210  periodically. For example, the buffer state confirmer  200  compares the confirmed quantity of cumulated data in the line card buffer  230  with a preset reference value and provides the result of the comparison to the transmission controller  210 . At this time, the buffer state confirmer  200  provides the result of the comparison to the transmission controller  210  using a state variable of one bit. When the quantity of cumulated data in the line card buffer  230  is greater than the reference value, the buffer state confirmer  200  provides the state variable which is set at 1 to the transmission controller  210 . By contrast, the buffer state confirmer  200  provides the state variable which is set at 0 to the transmission controller  210  when the quantity of cumulated data in the line card buffer  230  is less than the reference value. 
     When the quantity of cumulated data in the line card buffer  230  is similar to the reference value, the buffer state confirmer  200  sets the state variable at 0 or 1 iteratively. In order to prevent this problem, the buffer state confirmer  200  uses a hysteresis to set the state variable at 0 or 1. 
     For example, the buffer state confirmer  200  sets the state variable at 1 when the quantity of cumulated data plus α is greater than the reference value. By contrast, the buffer state confirmer  200  sets the state variable at 0 when the quantity of cumulated data plus α is less than the reference value. Herein, the α is a hysteresis variable. 
     At this time, the serving BS sets the reference value differently depending on whether the serving BS supports Automatic Repeat reQuest (ARQ). 
     When the serving BS does not support ARQ, the serving BS calculates the reference value as show in Equation 1:
 
 B   th   =T   narq   ×C   b .  [Eqn. 1]
 
     In Equation 1, B th  denotes a reference value which is determined by the serving BS, T narq  denotes a maximum allowance time for transmitting data to an MS, C b  denotes a minimum value in an uplink backhaul capacity of the serving BS and a downlink backhaul capacity of neighbor BSs of the serving BS. For example, T narq  denotes a maximum delay time of an MS. 
     The serving BS in Equation 1 transmits data at a speed of C b  within T narq . Therefore, new data which flows into the line card buffer  230  is transmitted within the time delay which is fewer than T narq  when a quantity of cumulated data in the line card buffer  230  is less than a product of T narq  and C b . By contrast, new data which flows into the line card buffer  230  is not transmitted within the time delay which is fewer than T narq  when a quantity of cumulated data in line card buffer  230  is greater than the product of T narq  and C b . 
     When the serving BS supports ARQ, the serving BS calculates the reference value as show in Equation 2. Herein, the assumption is that a line card transmits cumulated data in the line card buffer  230  at a speed which is fixed (C b ) by performing a rata shaping.
 
 B   th   =T   arq   ×C   b −( W   arq   ×B   arq ).  [Eqn. 2]
 
     In Equation 2, B th  denotes a reference value which is determined by the serving BS, T narq  denotes a maximum allowance time for transmitting data to an MS, C b  denotes a minimum value in an uplink backhaul capacity of the serving BS and a downlink backhaul capacity of neighbor BSs of the serving BS, W arq  denotes ARQ window size, B arq  denotes ARQ block size. Herein, the assumption is that the serving BS uses a Transmission Control Protocol (TCP) as a transport protocol, and it is required that the serving BS transmits data to a target BS within a TCP timeout. Therefore, T narq  is set based at least partly on a TCP timeout. For example, T narq  is set to less than half of a TCP timeout. 
     When the serving BS supports ARQ, the serving BS sets the reference value based at least partly on a product of T narq  and C b  as shown in Equation 2. 
     When the serving BS supports ARQ, the serving BS transmits stored data for an MS and ARQ data via a backhaul. If the serving BS transmits the ARQ data, the serving BS can transmit data at a maximum rate given by the product of W arq  and B arq . Therefore, the serving BS obtains a reference value by subtracting a product of W arq  and B arq  from a product of T narq  and C b  as shown in Equation 2. 
     When the transmission controller  210  receives a request signal for transmitting data from the target BS, the transmission controller  210  confirms a quantity of cumulated data in the line card buffer  230  periodically and determines whether the transmission controller  210  transmits cumulated data to the target BS using the confirmed result. 
     When the quantity of the cumulated data is greater than a reference value, the transmission controller  210  instructs the serving BS to transmit the cumulated data in the line card buffer  230  to the target BS. 
     When the quantity of cumulated data is less than the reference value, the transmission controller  210  instructs the serving BS not to transmit the cumulated data in the line card buffer  230  to the target BS. The transmission controller  210  discards cumulated data in the line card buffer  230 . 
     The transmission controller  210  transmits a response signal with the received state variable when the transmission controller  210  receives the state variable. Therefore, the buffer state confirmer  200  retransmits the state variable when the confirm signal is not received within a preset time. 
     In another exemplary embodiment, the buffer state confirmer  200  compares a confirmed quantity of cumulated data in the line card buffer  230  with a preset reference value periodically and provides the result of the comparison to the transmission controller  210  periodically. 
     On the other hand, the buffer state confirmer  200  compares a confirmed quantity of cumulated data in the line card buffer  230  with a preset reference value when the buffer state confirmer  200  receives a request signal for transmitting stored data in the line card buffer  230  from the transmission controller  210 . After, the buffer state confirmer  200  provides the result of the comparison to the transmission controller  210 . 
       FIG. 3  is a flow diagram illustrating a process of forwarding traffic according to a handover of a mobile station at a base station of a wireless communication system according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 3 , a BS confirms whether a request signal for transmitting stored data in a line card buffer of the BS is received from a target BS in step  301 . 
     When the BS receives the request signal, the BS confirms a quantity of cumulated data in a line card buffer in step  303 . 
     In step  305 , the BS compares the confirmed quantity of cumulated data in the line card buffer with a preset reference value and determines whether the BS transmits cumulated data to the target BS according to the result of the comparison. 
     If the quantity of cumulated data in the line card buffer is less than the reference value, the BS copies stored data to an uplink channel card in step  307 . 
     In step  309 , the BS performs scheduling for copied stored data. For example, the BS performs scheduling based on a Differentiated Service Code Point (DSCP). The BS allocates a DSCP value which has a highest priority level to the copied stored data. After, the BS performs priority scheduling for copied signal according to a priority level of DSCP. Also, the BS allocates a Weighted Round Robin (WRR) to the copied data according to the DSCP value. After, the BS performs scheduling for copied data. 
     In step  311 , the BS transmits selected data through scheduling to the target BS via a backhaul. 
     In step  313 , the BS updates the quantity of cumulated data in the line card buffer. The BS decreases the quantity of cumulated data in the line card buffer according to the size of the data transmitted via the backhaul. 
     By contrast, the BS discards handover data when the quantity of cumulated data in the line card buffer is greater than the reference value in step  305 . 
     Next, the BS finishes this process. 
     In another exemplary embodiment, the BS confirms a quantity of cumulated data in the line card buffer when the BS receives a request signal for transmitting stored data from the target BS. 
     On the other hand, the BS confirms a quantity of cumulated data in the line card buffer periodically. Therefore, the BS determines whether the BS transmits a handover data using recently confirmed quantity of cumulated data in line card buffer. 
     As set forth above, a serving BS of a wireless communication system stores data to be transmitted to an MS when the MS is disconnected. After, the serving BS transmits stored data to a target BS based at least partly on an available backhaul capacity. Therefore, the wireless communication system prevents data loss and a decrease of throughput. 
     While the invention has been shown and described with reference to certain exemplary embodiments of the present invention 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 present invention as defined by the appended claims and their equivalents. 
     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.