Abstract:
The present invention relates to a scheduling apparatus of a base station supporting Automatic Repeat reQuest (ARQ) method. In particular, the present invention discloses scheduling a data unit transmitted to a mobile station from a base station providing ARQ method for correcting an error in wireless data communication and generating a high rate frame. 
     According to the present invention, there is provided a transmission controlling apparatus comprising a receiver to receive mobile station status information from a mobile station connected with a base station, a schedule managing unit to determine transmission priority of a plurality of data units to be transmitted to the mobile station based on the received mobile station status information, and a transmitter to transmit the data units to the mobile station based on the determined transmission priority.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Korean Patent Application No. 10-2007-0125954, filed on Dec. 6, 2007, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a scheduling apparatus of a base station supporting an Automatic Repeat reQuest (ARQ) method. In particular, disclosed is a scheduling apparatus for scheduling a data unit transmitted from an MAC that is a base station providing the ARQ method for correcting an error in wireless data communication and generating a high rate frame. The present invention is derived from work as a part of a new driving force of core IT technology development work performed in at the Ministry of Information and Communication and The Institute for Information Technology Advancement [Ref. No. 2006-S-001-02, title: adaptive wireless access and transmission technique for 4G mobile communication system]. 
         [0004]    2. Description of Related Art 
         [0005]    Research is vigorously performed to provide a variety of Quality of Service (QoS) having more than 100 Mbps transmission rate in the 4 th  generation communication system that is a next-generation communication system. In order to accomplish this, a mobile communication system evolves into a form of Broadband Wireless Access (BWA) such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system, plus guaranteeing mobility and QoS. 
         [0006]    An error may occur in specific data in wireless data communication according to wireless channel status. 
         [0007]    Methods for controlling an error occurring from loss or damage of a frame during data transmission in wireless environment are a stop-and-wait ARQ method, go-back-N-continuous ARQ method, selective-repeat continuous ARQ method, and the like. Further, a hybrid ARQ method is recently used. 
         [0008]    According to the stop-and-wait ARQ method, a transmitter waits for an Acknowledgement (ACK) message after transmitting a single frame, a receiver transmits an ACK message only upon successively receiving a frame and transmits a Negative Acknowledgement (NACK) message when error occurs in the frame. When a receiver fails to receive a frame due to destruction of the frame, the receiver does not transmit any signal and a transmitter retransmits when a signal is not received within a predetermined time. Thus, the stop-and-wait ARQ method has an advantage that it is easily realized but has a disadvantage that transmission efficiency declines as waiting time increases. 
         [0009]    Also, according to the go-back-N-continuous ARQ method, a transmitter transmits a series of frames with a fixed window-size and a receiver does not receive the series of frames when an error is detected from at least one frame in the series of frames and transmit NACK message. The transmitter retransmits all frames after the NACK message. 
         [0010]    Also, according to the selective-repeat continuous ARQ method modified from the go-back-N-continuous ARQ method that retransmits only the frame that receives the NACK, it has good transmission efficiency, but overhead may occur from a memory space for storing a next frame until a frame where an error occurs is retransmitted. Specifically, according to the selective-repeat continuous ARQ, error checking result (e.g., CRC result) for packets received in a receiver should be feedback to a transmitter. First of all, a transmitter performs initial transmission of a packet and a receiver decodes a received packet. In this case, when an error does not occurs, ACK signal is transmitted to the transmitter. Conversely, when an error occurs, NACK signal is transmitted to the transmitter. The transmitter either retransmits a previous packet or transmits a new packet according to ACK/NACK signal from the receiver. 
         [0011]    Generally, scheduling is performed in consideration of channel environment of a mobile station and priority of service. However, when a retransmission packet exists, the retransmission packet must be scheduled to be sent firstly among the packet having the same priority. When retransmission of a retransmission packet is delayed, a timeout may occur and other packets related to the corresponding packet may be discarded. When initial transmission is delayed, it appears as service delay, but when retransmission is delayed, it causes the discard of a packet and appears as service cut off. However, when initial transmission having high-priority competes with retransmission having low-priority under a condition that priority of each service is completely different, a system may fall into a dilemma for determining that one should be firstly transmitted. If scheduling is performed according to priority and if data having high-priority are in a rush in a specific time period, a packet having low-priority may not be retransmitted continuously, so that other packets related to the retransmission packet may be at risk of being discard. 
       SUMMARY OF THE INVENTION 
       [0012]    An aspect of the present invention is to provide an apparatus for controlling transmission supporting an Automatic Repeat reQuest (ARQ) method, that control transmission priority of a data unit in consideration of transmission priority of a data unit and Quality of Service (QoS). 
         [0013]    According to an aspect of the present invention, in order to achieve the above and solve a problem of a conventional art, there is provided an apparatus for controlling transmission including a receiver to receive mobile station status information from a mobile station connected with a base station, a schedule managing unit to determine transmission priority of a plurality of data units to be transmitted to the mobile station based on the received mobile station status information, and a transmitter to transmit the data units to the mobile station based on the determined transmission priority. 
         [0014]    According to another aspect of the present invention, there is provided an apparatus for controlling transmission comprising a transmitter to transmit a data unit to a mobile station connected to a base station, and to update final data transmission time information in relation to the mobile station, a data storing unit to store the transmitted data unit, a receiver to receive an Acknowledgement (ACK) message or a Negative Acknowledgement (NACK) message for the transmitted data unit; and a time managing unit to update the data storing unit based on the initial data transmission time information, the ACK message, or the NACK message, 
         [0015]    According to still another aspect of the present invention, there is provided an apparatus for controlling transmission in a base station supporting a selective-repeat continuous ARQ method, the apparatus comprising a transmitter to transmit a data unit to a mobile station connected to a base station and to update transmission number parameter for the data unit, a data storing unit to store the transmitted data unit, a receiver to receive an ACK message or a NACK message for the transmitted data unit, and a message managing unit to update the data storing unit based on the transmission number parameter, ACK message, or NACK message, wherein the transmitter retransmits a remaining data unit in the data storing unit to the mobile station according to the update of the message managing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other aspects of the present invention will become apparent and more readily appreciated from the following detailed description of certain exemplary embodiments of the invention, taken in conjunction with the accompanying drawings of which: 
           [0017]      FIG. 1  illustrates relationship between a frame period and a scheduling period according to the present invention; 
           [0018]      FIG. 2  illustrates mobile station operation information managed by a mobile station managing unit according to an embodiment of the present invention; 
           [0019]      FIG. 3  illustrates mobile station status information according to an embodiment of the present invention; 
           [0020]      FIG. 4  illustrates ARQ transmission Queue management information according to an embodiment of the present invention; 
           [0021]      FIG. 5  illustrates ARQ block management information according to an embodiment of the present invention; 
           [0022]      FIG. 6  illustrates state transition diagram of an ARQ data unit according to an embodiment of the present invention; 
           [0023]      FIG. 7  illustrates non-ARQ transmission queue management information according to an embodiment of the present invention; 
           [0024]      FIG. 8  is a block diagram illustrating a structure of a transmission controlling apparatus according to an embodiment of the present invention; 
           [0025]      FIG. 9  is a block diagram illustrating a structure of a transmission controlling apparatus according to another embodiment of the present invention; and 
           [0026]      FIG. 10  is a block diagram illustrating a structure of a transmission controlling apparatus according to still another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0027]    Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below in order to explain the present invention by referring to the figures. 
         [0028]      FIG. 1  illustrates relationship between frame period and scheduling period according to the present invention. The relationship between frame period and scheduling period is described in detail below referring to  FIG. 1 . 
         [0029]    In order to provide a variety of Quality of Service (QoS) having more than 100 Mpb transmission rate under a high-speed mobile environment in the 4 th  generation communication system that is a next-generation communication system, frame period is getting smaller, each frame is divided into the unit of a sub-frame, and scheduling is performed in the unit of a sub-frame. For example, in Third Generation Partnership Project Long Term Evolution (3GPP LTE) that is a 4 th  generation communication system, a frame period is 10 ms, each frame includes 20 sub-frames, and the period of each sub-frame is 0.5 ms. A scheduler of a base station performs scheduling in consideration of channel environment of a mobile station, priority of each service, and fairness among users, thus, the scheduler of a base station performs a more complicated calculation than a scheduler of a mobile station. 
         [0030]    A transmission controlling apparatus performs scheduling using a plurality of sub-frames  131 ,  132 , and  133  as the unit. That is, length of time of each sub-frame  131 ,  132 , and  133  is a scheduling period  120 . The transmission controlling apparatus receives either an ACK message or an NACK message  141 ,  142 , and  143  for the transmitted data unit using sub-frame  131 ,  132 , and  133 . The transmission controlling apparatus may recognize success or failure of transmission based on the Acknowledgement (ACK) message or a Negative Acknowledgement (NACK) message. The transmission controlling apparatus may retransmit a data unit in failed transmission to a mobile station. The transmission controlling apparatus may redetermine transmission priority of the data unit in failed transmission. The data unit may be either transmitted or discarded depending on the redetermined transmission priority. 
         [0031]      FIG. 2  illustrates mobile station operation information managed by a mobile station managing unit according to an embodiment of the present invention. The mobile station operation information is described in detail below referring to  FIG. 2 . 
         [0032]    A transmission controlling apparatus manages a plurality of data units according to a mobile station connected to a base station and a plurality of services used by each mobile station. In order to systematically manage the data units, the transmission controlling apparatus manages the data units according to a service that each mobile station uses. 
         [0033]    Mobile station status information  210  is used to manage a mobile station connected to a base station. According to an embodiment of the present invention, a controlling apparatus activates as many mobile station status information arrays as the number of mobile stations connected to a current base station among a plurality of mobile station status information arrays and manages the mobile station status information. 
         [0034]    ARQ transmission queue  220  stores data units transmitted through ARQ transmission method among data units transmitted to a mobile station. According to an embodiment of the present invention, a transmission controlling apparatus receives an ACK message for a transmitted data unit from a receiver, and retransmits a data unit that fails to be transmitted to a mobile station. 
         [0035]    Non-ARQ transmission queue  230  stores data units that are not transmitted through ARQ transmission method among data units transmitted to a mobile station. 
         [0036]      FIG. 3  illustrates mobile station status information  300  according to an embodiment of the present invention. The mobile station status information  300  is described in detail below referring to  FIG. 3 . The mobile station status information  300  according to the present invention includes a mobile station identifier  310 , channel quality information (CQI)  320 , ARQ data unit number  330 , ARQ data queue  340 , non-ARQ data unit number  350 , and non-ARQ data queue  360 . 
         [0037]    The transmission controlling apparatus may determine transmission priority and expected transmission time of data unit to be transmitted to a mobile station based on the mobile station status information  300 . 
         [0038]    The mobile station identifier  310  indicates that the mobile station status information  300  corresponds to a particular mobile station. According to an embodiment of the present invention, telephone number assigned to a mobile station, Electric Serial Number (ESN) used in a CDMA mobile communication system, and the like, may be used as the mobile station identifier  310  The channel quality information (CQI)  320  is CQI of wireless link between a mobile station and a base station. According to an embodiment of the present invention, the CQI may include signal to noise and interference ratio (SINR) of wireless link between a mobile station and a base station. 
         [0039]    The ARQ data unit number  330  indicates the number of transmitted data units through ARQ transmission method among data units transmitted to a mobile station. The ARQ data queue  340  stores data units transmitted through ARQ transmission method among data units transmitted to a mobile station. According to an embodiment of the present invention, the ARQ data queue  340  may store a plurality of ARQ data unit  341 . 
         [0040]    The non-ARQ data unit number  350  indicates the number of data unit that are not transmitted through ARQ transmission method among data units transmitted to a mobile station. 
         [0041]    The non-ARQ data queue  360  stores data units that are not transmitted through ARQ transmission method among data units transmitted to a mobile station. According to an embodiment of the present invention the non-ARQ data queue  360  may store a plurality of non-ARQ data units  361 . 
         [0042]      FIG. 4  illustrates ARQ transmission queue management information  400  according to an embodiment of the present invention. The ARQ transmission queue management information  400  is described in detail below referring to  FIG. 4 . The ARQ transmission queue management information  400  according to the present invention includes a service identifier  410 , transmission priority  420 , initial data transmission time information  430 , transmitted data unit number  440 , transmitted data unit list  450 , non-transmitted data unit number  460 , and non-transmitted data unit list  470 . 
         [0043]    A transmission controlling apparatus may manage data unit transmitted through ARQ transmission method among data units transmitted to a mobile station based on ARQ transmission queue management information. 
         [0044]    The service identifier  410  is to identify service in relation to ARQ transmission queue among a plurality of data services used by a mobile station. 
         [0045]    The transmission priority  420  is transmission priority of a data unit transmitted by a mobile station. In particular, according to an embodiment of the present invention, a transmission controlling apparatus may either redetermine transmission priority of data service related to a data unit or redetermine transmission priority of a specific data unit and schedule the specific data unit. According to an embodiment of the present invention, transmission priority of data service used by a mobile station is determined based on QoS of the data service. 
         [0046]    Initial data transmission time information  430  indicates initial transmission time of data unit that is first transmitted among data units stored in ARQ transmission queue. A transmission controlling apparatus may compare a difference between the initial data transmission time and present time with retransmission time limit, may not transmit data stored in ARQ transmission queue, and may delete the stored data from the ARQ transmission queue. 
         [0047]    Transmitted data unit number  440  indicates the number of data units transmitted to a mobile station through ARQ transmission method among data units stored in the ARQ transmission queue  400 . There is no need to store the number of data units that are not transmitted through ARQ transmission method because the data units not transmitted through ARQ transmission method are not to be retransmitted, 
         [0048]    Transmitted data unit list  450  indicates list of data units transmitted to a mobile station through ARQ transmission method among data units stored in the ARQ transmission queue  400 . According to an embodiment of the present invention, the transmitted data unit list  450  may includes a data unit  451 ,  452 , and  453  transmitted to a mobile station through ARQ transmission method. 
         [0049]    The non-transmitted data unit number  460  indicates the number of data units to be transmitted through ARQ transmission method among data units that are not yet transmitted to a mobile station. 
         [0050]    The non-transmitted data unit list  470  indicates a list of data units  471 ,  472 , and  473  to be transmitted through ARQ transmission method among data units that are not yet transmitted to a mobile station. 
         [0051]      FIG. 5  illustrates ARQ block management information  500  according to an embodiment of the present invention. The ARQ block management information is described in detail below referring to  FIG. 5 . The ARQ block management information  500  according to the present invention includes an ARQ data unit identifier  510 , a service data unit identifier  520 , final transmission time information  530 , initial transmission time information  540 , service data unit combination information  550 , next ARQ data unit address  560 , and previous ARQ data unit address  570 . 
         [0052]    The ARQ data unit identifier  510  is to identify each ARQ data unit. 
         [0053]    A transmission controlling apparatus transmits respectively different kind of service data unit (SDU) according to service used by a mobile station. SDU transmitted by a base station has different size according to a kind of service. In order to easily transmit SDU to a mobile station, the transmission controlling apparatus may divide the SDU and generate an ARQ data unit. In order to enable the transmission controlling apparatus to easily transmit data to a mobile station, an ARQ data unit has predetermined size. The SDU identifier  520  is to identify from which SDU unit an ARQ data unit is generated. A mobile station that receives an ARQ data unit may recombine a service data unit from an ARQ data unit using the SDU identifier. 
         [0054]    The final transmission time information  530  indicates final transmission time of an ARQ data unit to a mobile station. 
         [0055]    The initial transmission time information  540  indicates initial transmission time of an ARQ data unit to a mobile station. Although the initial transmission time information  540  and the final transmission time information  530  is identical, when an ARQ data unit is initially transmitted, the final transmission time information  530  is updated according to retransmission of the ARQ data unit to a mobile station. 
         [0056]    A transmission controlling apparatus may compare the initial transmission time information  540 , the final transmission time information  530 , and retransmission time limit, and may determine whether to retransmit an ARQ data unit to a mobile station. 
         [0057]    The service data unit combination information  550  is needed information for combining an ARQ data unit and generating a service data unit. 
         [0058]    The ARQ data unit according to the present invention may be linked together using doubly-linked list. The ARQ data units linked by a doubly-linked list may be easily managed even when any of the ARQ data units is deleted. 
         [0059]    The next ARQ data unit address  560  indicates ARQ data unit address linked next to a corresponding ARQ data unit in a doubly-linked list. 
         [0060]    The previous ARQ data unit address  570  indicates ARQ data unit address linked previous to a corresponding ARQ data unit in a doubly-linked list. 
         [0061]      FIG. 6  illustrates state transition diagram of an ARQ data unit according to an embodiment of the present invention. The status transition diagram of an ARQ data unit is described in detail as below referring to  FIG. 6 . 
         [0062]    Non-transmitted status  610  indicates status of an ARQ data unit not yet transmitted to a mobile station among ARQ data units to be transmitted. A transmission controlling apparatus determines both transmission priority according to service related to a non-transmitted ARQ data unit and expected transmission time of an ARQ data unit based on the determined transmission priority. Upon transmitting an ARQ data unit  660 , status is transited from non-transmitted status  610  to transmitted status  620 . 
         [0063]    Transmitted status  620  indicates status that a transmission controlling apparatus has already transmitted an ARQ data unit to a mobile station. In particular, the transmission controlling apparatus has not yet received either an ACK message or an NACK message. 
         [0064]    Complete transmission status  640  indicates status that successful transmission of an ARQ data unit to a mobile station is acknowledged. When a transmission controlling apparatus receives an ACK message for an ARQ data unit in operation  661 , status of an ARQ data unit is transited from transmitted status  260  to complete transmission status. 
         [0065]    Standby status  630  indicates status that an ARQ data unit that fails to be transmitted is on standby until expected retransmission time. Upon receiving an ACK message in operation  663 , status of an ARQ unit is transited from standby status  630  to complete transmission status  640 . Also, upon retransmitting an ARQ data unit in operation  662 , status of the ARQ data unit is transited to transmitted status. 
         [0066]    When a transmission controlling apparatus either receives an NACK message for an ARQ data unit in transmitted status  620  or determines to retransmit an ARQ data unit that fails to be transmitted in operation  664 , status of an ARQ data unit may be transited to standby status  630  until retransmission time. 
         [0067]    Deleting status  650  indicates status of discard of retransmission of an ARQ data unit in operation  665  and  666 . A transmission controlling apparatus may discard retransmission of an ARQ data unit that fails to be transmitted due to transmission number limit or transmission time limit. When a transmission controlling apparatus discards retransmission, the transmission controlling apparatus deletes an ARQ data unit that fails to be transmitted. 
         [0068]      FIG. 7  illustrates non-ARQ transmission queue management information  700  according to an embodiment of the present invention. The non-ARQ transmission queue management information  700  is described in detail below referring to  FIG. 7 . The non-ARQ transmission queue management information  700  according to the present invention includes a service identifier  710  and transmission priority  720 . 
         [0069]    The service identifier is to identify service related to a non-ARQ service among a plurality of data service used by a mobile station. 
         [0070]    The transmission priority  720  is transmission priority of data service used by a mobile station. In particular, according to an embodiment of the present invention, a transmission controlling apparatus may redetermine transmission priority in relation to a data unit so as to schedule a data unit. 
         [0071]    In the non ARQ transmission queue, data units that are not transmitted through ARQ transmission method among data units to be transmitted to a mobile station are stored. When the ARQ transmission method is not used, a transmission controlling apparatus neither receives an ACK or an NACK for a transmitted data unit nor redetermines data unit that fails to be transmitted, thus, there is no need of storing information for retransmission. 
         [0072]      FIG. 8  is a block diagram illustrating a structure of a transmission controlling apparatus  800  according to an embodiment of the present invention. The structure of a transmission controlling apparatus  800  is described in detail below referring to  FIG. 8 . The controlling apparatus  800  includes a receiver  810 , a schedule managing unit  820 , and transmitter  830 . 
         [0073]    The receiver  810  receives mobile station status information from a mobile station  840  and  850  connected to a base station. The mobile station status information according to an embodiment of the present invention may include signal to interference and noise ratio (SINR) for wireless link between a mobile station and a base station. 
         [0074]    The schedule managing unit  820  determines transmission priority of a plurality of data units to transmit to the mobile station  840  and  850  based on the mobile station status information received in the receiver. The scheduling managing unit according to an embodiment of the present invention may determine high transmission priority for a data unit to be transmitted to the mobile station  840  having superior mobile station status information and may determine low transmission priority for a data unit to be transmitted to the mobile station  850  having inferior mobile station status information. 
         [0075]    The transmitter  830  transmits data units to a mobile station based on the transmitted determined priority. The transmitter  830  according to an embodiment of the present invention may preferentially transmit a data unit of high transmission priority, and upon finishing transmission of the data unit of high transmission priority, the transmitter  830  may transmit a data unit of low transmission priority. 
         [0076]    According to an embodiment of the present invention, the receiver  810  may receive an NACK message from the mobile station  840  and  850 , the schedule managing unit  820  may identify a data unit corresponding to the NACK message and determine not to transmit the identified data unit. When a data unit that fails to be transmitted is a data unit related to real-time transmission, a data unit that is delayed to be transmitted for more than a predetermined time is no more needed by the mobile station  840  and  850 . Therefore, the schedule managing unit  820  may calculate retransmission time based on a determined transmission priority. When calculated retransmission time is later than retransmission time limit from an initial transmission time of the identified data unit, the schedule managing unit  820  may determine the transmission priority so that the retransmission time is earlier than the retransmission time limit from initial transmission time of the identified data unit. 
         [0077]    According to an embodiment of the present invention, the receiver  810  may receive an NACK message for a transmitted data unit from the mobile station  840  and  850 , and the schedule managing unit  820  may identify data unit corresponding to the NACK message. The schedule managing unit  820  may redetermine transmission priority for the identified data unit, the transmitter  830  may retransmit the identified data unit to the mobile station  840  and  850  based on the redetermined transmission priority. When calculated retransmission time is more than retransmission time limit from initial transmission time of the identified data unit, the schedule managing unit  820  may redetermine transmission priority so that the retransmission time for the identified data unit does not exceed the retransmission time limit from initial transmission time. 
         [0078]    According to an embodiment of the present invention, the transmitter  830  may change a modulation or a coding scheme for a data unit. According to an embodiment of the present invention, the transmitter  830  may determine a modulation or coding scheme to transmit a data unit to the mobile station  840  having superior mobile station status information in a modulation or coding scheme in relation to high transmission rate. 
         [0079]      FIG. 9  is a block diagram illustrating a structure of a transmission controlling apparatus  900  according to another embodiment of the present invention. The structure of a transmission controlling apparatus  900  is described in detail below referring to  FIG. 9 . The transmission controlling apparatus  900  includes a transmitter  910 , a data storing unit  920 , a receiver  930 , and a time managing unit  940 . 
         [0080]    The transmitter  910  transmits a data unit to a mobile station  950  connected to a base station and updates initial transmission time related to the mobile station  950 . 
         [0081]    The data storing unit  920  stores the data unit transmitted to the mobile station  950 . 
         [0082]    The initial data transmission time information updated by the transmitter  910  is initial transmission time information of a data unit among data units stored in the data storing unit  920 . 
         [0083]    The receiver  930  receives an ACK message or an NACK message for a transmitted data unit from the mobile station  950 . 
         [0084]    The time managing unit  940  updates the data storing unit  920  based on initial data transmission time information, an ACK message, or an NACK message. According to an embodiment of the present invention, when the receiver  920  receives an ACK message, the time managing unit  940  may identify a data unit corresponding ACK message. The time managing unit  940  may delete a data unit corresponding to the ACK message from the data storing unit  920 , thereby updating the data storing unit. 
         [0085]    The transmitter  910  transmits remaining data unit in a data storing unit according to the update of the time managing unit. 
         [0086]    According to an embodiment of the present invention, when the receiver  930  receives an NACK message, the time managing unit  940  identifies a data unit corresponding to the NACK message, wherein the identified data unit is a data unit that fails to be transmitted. The time managing unit  940  may update a data storing unit based on initial transmission information for the mobile station  950 , present time information, and a predetermined time limit. 
         [0087]    According to an embodiment of the present invention, when a time from initial transmission time to present time does not exceed a predetermined retransmission time limit, all data units transmitted to the mobile station  950  may be retransmitted to the mobile station  950 . That is, the data units are not deleted from the data storing unit  920 . 
         [0088]    According to an embodiment of the present invention, when a time from initial transmission time information to present time is more than retransmission time limit, partial data units among data units transmitted to the mobile station  950  are not retransmitted to the mobile station  950 . That is, the time managing unit  940  may delete partial units from among data units stored in the data storing unit  920 , thereby updating the data storing unit  920 . 
         [0089]    According to an embodiment of the present invention, the time managing unit  920  may redetermine transmission priority of undeleted data units. According to an embodiment of the present invention, the time managing unit  920  may redetermine transmission priority of the undeleted data units to be high transmission priority. 
         [0090]    According to an embodiment of the present invention, when the receiver  930  receives an NACK message, the time managing unit  940  may calculate an expected time for retransmission of a transmitted data unit and compare the calculated predetermined time for retransmission with initial data transmission. When a time from initial data transmission time to an expected time for retransmission is more than a predetermined time limit, the time managing unit  940  may determine not to retransmit the transmitted data unit. The time managing unit  940  may delete the transmitted data unit from the data storing unit  920 . According to an embodiment of the present invention, the time managing unit  940  may redetermine transmission priority for the data unit that is determined to be transmitted. According to an embodiment of the present invention, the time managing  920  may redetermine transmission priority of undeleted data unit to be high transmission priority. 
         [0091]      FIG. 10  is a block diagram illustrating a structure of a transmission controlling apparatus  1000  according to still another embodiment of the present invention. The structure of a transmission controlling apparatus  1000  is described in detail below referring to  FIG. 10 . The transmission controlling apparatus  1000  according to the present invention includes a transmitter  1010 , a storing unit  1020 , a receiver  1030 , and a message managing unit  1040 . 
         [0092]    The transmitter  1010  transmits a data unit to the mobile station  1050  connected to a base station and updates transmission number parameter for the transmitted data. The transmission controlling apparatus according to the present invention may receive an NACK message for the transmitted data and retransmit data in response to the received NACK message, wherein the transmission number parameter indicates the number of transmission of data units to the mobile station  1050 . 
         [0093]    The data storing unit  1020  stores a transmitted data unit. 
         [0094]    The receiver  1030  receives an ACK message or NACK message for a transmitted data unit. 
         [0095]    The message managing unit  1040  updates a data storing unit based on transmission number parameter, an ACK message, or an NACK message. 
         [0096]    The transmitter  1010  retransmits a remaining data unit in a data storing unit to the mobile station  1050  according to update of the message managing unit  1040 . 
         [0097]    According to an embodiment of the present invention, an ACK message may include an identifier of a data unit corresponding to the ACK message and information for success of transmission of the data unit. 
         [0098]    According to an embodiment of the present invention, an NACK message may include an identifier of a data unit corresponding to the NACK message and information for failure of transmission of the data unit. 
         [0099]    According to an embodiment of the present invention, when the receiver  1030  receives an NACK message, the message managing unit  1040  may compare a transmission number parameter for a data unit corresponding to the NACK with a predetermined retransmission threshold. The message managing unit  1040  may update the data storing unit  1020  according to the comparison result. 
         [0100]    According to an embodiment of the present invention, when transmission number parameter is equal to/more than a predetermined retransmission threshold, the message managing unit  1040  may delete a data unit corresponding to the NACK message, thereby updating the data storing unit  1020 . 
         [0101]    According to an embodiment of the present invention, when transmission number parameter is less than a predetermined retransmission threshold, a schedule managing unit may redetermine transmission priority for a data unit corresponding to a NACK message. According to an embodiment of the present invention, the schedule managing unit may redetermine transmission priority of an undeleted data unit to be high transmission priority. 
         [0102]    According to the present invention, a base station supporting ARQ transmission method may control transmission priority of a data unit in consideration of transmission priority and QoS of the data unit. 
         [0103]    Although a few exemplary embodiments of the present invention have been shown and described, the present invention is not limited to the described exemplary embodiments. Instead, it would be appreciated by those skilled in the art that changes may be made to these exemplary embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.