Abstract:
An apparatus and method are provided for determining a data rate by means of control information in a mobile communication system which includes a User Equipment (UE) and a Node B. The UE transmits data to the Node B, and the Node B transmits the control information to the UE. The data rate is determined by means of the control information and is used for transmission of the data by the UE. The apparatus and method comprise determining a preliminary data rate in consideration of a quantity of data waiting for transmission; comparing the preliminary data rate with a previous data rate used for previous data transmission; and determining the data rate according to a result of comparison so that the UE can transmit the data at the data rate.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. 119(a) of an application entitled “apparatus and method for data rate scheduling of user equipment in mobile communication system” filed in the Korean Industrial Property Office on Nov. 7, 2003 and assigned Serial No. 2003-78756, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to data transmission of a mobile communication system. More particularly, the present invention relates to an apparatus and a method for scheduling a data rate at which a user equipment (UE) transmits data to a Node B. 
     2. Description of the Related Art 
     A mobile communication system is a general term for systems providing voice, data, or other types of information through a wireless network. The mobile communication systems can be classified according to multiplexing schemes used, an example of which is a Code Division Multiple Access (CDMA) mobile communication system which provides wireless mobile communication service using a CDMA scheme. For the CDMA mobile communication system, an IS-95 standard mainly for transmission/reception of voice signals was initially developed and an IMT-2000 standard for transmission of not only voice signals but also high speed data is now being discussed. Specifically, the IMT-2000 standard provides services such as high quality voice reproduction, dynamic image reproduction, Internet access service, etc. 
     In the CDMA mobile communication system, the same frequency band is used by multiple users, and multiplexing is implemented by spreading data by means of a specific code for each of the multiple users. An increase in the data transmission speed of a UE of each of the multiple users implies an increase in transmission power, and the transmission power increase may serve as an interference factor to other UEs. Therefore, it is necessary to discuss a method for reducing the interference factor by scheduling the data transmission speed of the UE. 
     In a network of the existing 2G or 3G mobile communication system, when a new UE accesses the network, a maximum data transmission speed allowable to the UE is determined in consideration of a reception signal level and noise rise of each UE and is reported to the newly accessed UE. Then, the UE sets the transmission speed in consideration of the reported maximum data transmission speed, the quantity of data to be transmitted, and priorities of the data. The UE transmits the data at the set transmission speed. 
       FIG. 1  illustrates a Transport Format Combination Set (TFCS) generated using interference levels of multiple Node Bs measured by a Radio Network Controller (RNC) and transmitted to each UE. The TFCS contains data transmission speed which is allowed for each UE receiving the TFCS. The TFCS includes Transport Format Combination (TFC)  0  through TFC 10 . TFC 0  represents the highest data transmission speed and TFC 10  represents the lowest data transmission speed. The UE selects a TFC in consideration of the received TFCS, buffer occupancy, and maximum transmit power Max_Tx_Pwr.  FIG. 1  shows the UE&#39;s selection of TFC 6  as indicated by arrow  102 . As described above, the data transmission speed of the UE is determined through scheduling by the UE itself based on the received TFCS. 
     When the UE spontaneously determines the TFC, the RNC inevitably takes a long time to reflect a rise in noise of the Node B. The longer the time taken by the RNC in reflecting the rise in noise of the Node B which changes instantaneously, the more difficult it is to precisely reflect the rise in noise in the TFCS to be transmitted to a UE newly accessing a Node B controlled by the RNC. 
     Further, packet data having a burst data transmission characteristic has a larger noise rise dispersion than that of voice data, so the noise rise of the Node B shows an increased variance (i.e., dispersion). 
       FIG. 2  illustrates noise rise variance of the Node B according to time. 
     As shown in  FIG. 2 , the interference elements of the Node B are classified into thermal noise, interference of other Node Bs, interference by a voice channel, and interference on a packet channel. Variance in the thermal noise, the interference of other Node Bs, and the interference by a voice channel according to the passage of time are either predictable or very little. However, it should be noted that the interference on a packet channel has a large variance according to time. That is, it should be noted that most of the noise rise variance of the Node B is determined by the variance of the interference on a packet channel. In  FIG. 2 , ‘max’ signifies a maximum allowable interference level and ‘target_ 1 ’ signifies a target interference level reflecting variance of the interference level according to time. Also, ‘margin’ signifies the difference between the maximum allowable interference level and the target interference level. The ‘margin’ is determined according to the variance of the interference level. In other words, because the sum of the noise rise is not allowed to exceed the value ‘max’ in performing the scheduling, the ‘margin’ must be increased in proportion to the noise rise variance when the noise rise has a large variance. Therefore, an increase in the variance of the interference level causes an increase of the margin and a decrease in the variance of the interference level causes a decrease of the margin. However, a mobile communication system creates a large amount of interference with a packet channel, which causes a large variance in the interference level, thereby increasing the ‘margin’. 
     As described above, each UE determines by itself the data transmission speed, thereby increasing the noise rise variance and accordingly increasing the margin. This implies that the power which the Node B can assign for use of each UE for data transmission is reduced according to an increase of the ‘margin’. That is, as the ‘margin’ increases, inefficient use of radio resources increase.  FIG. 2  also shows inefficient use of radio resources. 
     Hereinafter, the noise rise will be discussed. The noise rise can be expressed as shown in the following equation (1). 
     
       
         
           
             
               
                 
                   Noise_rise 
                   = 
                   
                     
                       
                         I 
                         or 
                       
                       + 
                       
                         I 
                         oc 
                       
                       + 
                       
                         N 
                         t 
                       
                     
                     
                       N 
                       t 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In equation (1), I or  represents the power of reception signals transmitted from UEs located in a particular cell, I oc  represents the power of reception signals transmitted from UEs located in another cell, and N t  represents the power of noise. 
       FIG. 3  is a block diagram illustrating a general form of uplink data transmission from a UE to a Node B. The system shown in  FIG. 3  includes an RNC  300 , a Node B  302 , and a UE  304 . 
     The UE  304  requests a data rate and control information to the Node B  302  through an Enhanced Dedicated Physical Control Channel (E-DPCCH) as shown by arrow  306 . The Node B  302  transmits the control information at a determined data rate in response to the request of the UE  304  as shown by arrow  310 . Specifically, the control information and the data rate are determined by the RNC  300  and are then transmitted to the Node B  302  and then to the UE  304 . The UE  304  transmits data by means of the received data rate and the control information as shown by arrow  308 . Herein, the data is transmitted through an Enhanced Dedicated Physical Data Channel (E-DPDCH). 
       FIG. 4  illustrates a process in which a UE selects a TFC. 
     In step  402 , the UE determines whether a UE pointer j has been received from the Node B or not. When the UE pointer j has been received, step  424  is executed. When the UE pointer j has not been received, step  404  is executed. Hereinafter, a Node B pointer and the UE pointer j will be described with reference to  FIG. 5 . The Node B pointer  502  refers to a TFCS assigned and transmitted to a particular Node B belonging to a cell controlled by an RNC. The UE pointer j  504  refers to a TFC which the Node B assigns to the UE in consideration of the TFCS transferred from the RNC and a received interference level, etc. Usually, the UE transmits data by means of the UE pointer j at an initial stage of transmission. Therefore, at the initial stage of transmission, the UE proceeds to step  404  from step  402 . 
     In step  404 , the UE checks the buffer. If the buffer contains data to transmit, the UE proceeds to step  406 . If the buffer contains no data to transmit, the UE proceeds to step  426  and ends the process. In step  406 , the UE sets the buffer occupancy, the maximum transmission power, the Node B pointer  502 , and TFCS. Although not shown in  FIG. 4 , the UE transmits at the initial stage of transmission the data stored in the buffer by means of the data rate corresponding to the TFC indicated by the received UE pointer j. 
     In step  408 , the UE checks the buffer occupancy at a particular time interval in the course of the data transmission. According to the quantity of the data stored in the buffer, the UE may report the quantity to the Node B. The UE determines an optimum data rate in consideration of the maximum transmission power and the quantity of the data stored in the buffer. Further, the UE selects a TFC corresponding to the determined data rate from the TFCS. The selected TFC is set as TFCi. Referring to  FIG. 5 , i has one value from among 0 through 10. 
     In step  410 , the UE compares the TFC corresponding to the data rate of current transmission with the TFC selected in step  408 . In step  410 , p refers to a level of the TFC corresponding to the data rate of the current transmission. Referring to  FIG. 5 , p has one value from among 2 through 10. The reason why p cannot have a value of 0 or 1 is that data cannot be transmitted at a higher data rate than the data rate assigned to the Node B. As a result of the comparison, the UE proceeds to step  412  if i is larger than p and proceeds to step  414  if i is not larger than p. 
     In step  412 , the UE requests the Node B to assign a data rate that is one step higher and receives a response to the request. In consideration of the reception interference level, the Node B determines whether or not to perform assignment of the data rate requested by the UE. In step  416 , the UE determines the response from the Node B. According to the result of the determination, the UE performs step  422  when information “DOWN” has been received and performs step  420  when information “KEEP” has been received. The UE performs step  418  when information “UP” has been received. 
     In step  414 , the UE compares i and p. According to the result of the comparison, the UE performs step  420  when i and p are equal and performs step  422  when i and p are not equal. 
     In step  418 , the UE replaces the current TFC by a new TFC one step higher than the current TFC. In step  418 , the UE maintains the current TFC. In step  422 , the UE replaces the current TFC by a new TFC one step lower than the current TFC. Table 1 shows an example of the result by the process from step  418  to step  422 . 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Current TFC 
                 Step 418 
                 Step 420 
                 Step 422 
               
               
                   
                   
               
             
             
               
                   
                 TFC 6 
                 TFC 5 
                 TFC 6 
                 TFC 7 
               
               
                   
                   
               
             
          
         
       
     
     In step  424 , the UE transmits the data stored in the buffer at the data rate corresponding to the TFC set in one of steps  418 ,  420 , and  422 . As shown in  FIG. 4 , the UE requests data rate assignment to the Node B only when the UE wants a higher data rate than a current data rate and does not request data rate assignment to the Node B when the UE wants a lower data rate than the current data rate. 
       FIG. 6  illustrates variance in reception interference of the Node B according to the process shown in  FIG. 4 . As shown in  FIG. 4 , the data rate variance is adjusted with reference to one level, so that the variance becomes less abrupt and fluctuating. Therefore, as opposed to  FIG. 1 ,  FIG. 6  shows ‘margin’ with a reduced width. The reduction of the ‘margin’ increases the target interference level target_ 2 , thereby enabling efficient use of radio resources. ‘G’ in  FIG. 6  represents the difference between target_ 1  and target_ 2 . 
     In the conventional system as described above, a radio resource is assigned for signaling between the UE and the Node B, thereby causing capacity reduction of the system and causing time delay due to the signaling transmission. Further, because scheduling is necessary in order to assign radio resources to a plurality of UEs, the Node B has an increased complexity. Therefore, a solution for solving the above-described problems is highly required. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an apparatus and a method for reducing interference signals in Node B. 
     It is another object of the present invention to provide an apparatus and a method for efficiently assigning a radio resource used for signaling between a User Equipment (UE) and a Node B, thereby preventing capacity reduction of a system. 
     It is another object of the present invention to provide an apparatus and a method for preventing time delay due to the signaling between a UE and a Node B. 
     It is another object of the present invention to provide an apparatus and a method for reducing the complexity of a Node B performing scheduling for distribution of radio resources within a particular duration. 
     In order to accomplish this object, there is provided a method for determining a data rate by means of control information in a mobile communication system which includes a UE and a Node B. The UE transmits data to the Node B, and the Node B transmits the control information to the UE. The data rate is determined by means of the control information and used for transmission of the data by the UE. The method comprising the steps of determining a preliminary data rate in consideration of a quantity of data waiting for transmission; comparing the preliminary data rate with a previous data rate used for previous data transmission; and determining the data rate according to a result of comparison so that the UE can transmit the data at the data rate. 
     In accordance with another aspect of the present invention, there is provided an apparatus for determining a data rate by means of control information in a mobile communication system. The data rate is determined by means of the control information and used for transmission of data by the UE. The apparatus comprising a UE for transmitting the data to the Node B, the UE determining a preliminary data rate in consideration of a quantity of data waiting for transmission, comparing the preliminary data rate with a previous data rate used for previous data transmission, and determining the data rate according to a result of comparison so that the UE can transmit the data at the data rate; and a Node B for transmitting the control information to the UE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a structure of a conventional Transport Format Combination Set (TFCS); 
         FIG. 2  illustrates various interference signals generated in a conventional mobile communication system; 
         FIG. 3  is a block diagram for illustrating a conventional signaling between a User Equipment (UE) and a Node B in order to reduce interference signals; 
         FIG. 4  is a flowchart of a conventional process for determining a data rate in a UE; 
         FIG. 5  illustrates a structure of a TFCS transmitted to the UE shown in  FIG. 4 ; 
         FIG. 6  is a graph for illustrating reduction of the interference signals through the conventional signaling between the UE and the Node B; 
         FIG. 7  is a block diagram for illustrating a signaling between a UE and a Node B according to an embodiment of the present invention; 
         FIG. 8  illustrates a structure of a TFCS according to the first embodiment of the present invention; 
         FIG. 9  is a block diagram for illustrating a structure of a UE determining a data rate according to the first embodiment of the present invention; 
         FIG. 10  is a flowchart of an operation of a UE according to the first embodiment of the present invention; 
         FIG. 11  illustrates a structure of a TFCS according to a second embodiment of the present invention; 
         FIG. 12  is a block diagram for illustrating a structure of a UE determining a data rate according to the second embodiment of the present invention; 
         FIG. 13  is a flowchart of an operation of a UE according to the second embodiment of the present invention; and 
         FIG. 14  is a flowchart of an operation of a UE according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted for conciseness. 
       FIG. 7  is a block diagram for illustrating uplink data transmission from a User Equipment (UE) to a Node B according to an embodiment of the present invention. The system shown in  FIG. 7  includes an Radio Network Controller (RNC)  700 , a Node B  702 , and a UE  704 . Although a mobile communication system may include further elements to the above-mentioned ones, only the necessary elements for practicing an embodiment of the present invention as shown in  FIG. 7  are discussed here. In the following description, the signaling between the Node B  701  and the UE  704  is mainly discussed. 
     The UE  704  requests control information to the Node B  702  through an Enhanced Dedicated Physical Control Channel (E-DPCCH) as shown by arrow  706 . Here, as opposed to the case shown in  FIG. 3 , the UE  704  does not request the data rate but requests only the control information. 
     The Node B  702  transmits the requested control information to the UE  704  through a broadcast channel as shown by arrow  710 . The control information transmitted to the UE  704  through the broadcast channel includes information for controlling uplink transmission speed (i.e., data rate) of the UE  704 . In other words, the UE  704  transmits uplink data through Enhanced Dedicated Physical Data Channel (E-DPDCH) as shown by arrow  708  by using the control information transmitted from the Node B  702  for the uplink data transmission. Hereinafter, the information transmitted through the broadcast channel will be referred to as a rate grant probability RG_up_prob or P_i. The rate grant probability is not transmitted at a regular time interval but is transmitted only when the Node B  702  has an increased traffic load. Therefore, the Node B  702  can reduce both the load on the Node B  702  and the interference between the multiple UEs within the Node B  702  by transmitting the rate grant probability. Specifically, in transmitting the rate grant probability, the Node B  702  may set the rate grant probability to have a small value when the traffic load increases and a large value when the traffic load decreases. 
       FIG. 8  illustrates a structure of a Transport Format Combination Set (TFCS) according to a first embodiment of the present invention. Hereinafter, the structure of a TFCS according to a first embodiment of the present invention will be described in detail with reference to  FIG. 8 . The TFCS  800  includes TFC 0  through TFC 10 . TFC 0  represents the highest data rate and TFC 10  represents the lowest rate. Although the TFCS shown in  FIG. 8  employs TFCs in 11 steps, more steps of TFCs may be employed. 
     The Node B pointer  806 , which is information transmitted to the Node B by the RNC refers to a maximum data rate for the Node B, determined from a measurement of noise rise of the multiple Node Bs by the RNC. The Node B determines a UE pointer  2  ( 802 ) with reference to the Node B pointer  806  and transmits the determined UE pointer  2  to the UE. The UE pointer  2  implies a maximum data rate at which the UE can transmit data. The UE pointer  2  is assigned to the UE by the Node B in consideration of the noise rise of the Node B, etc.  FIG. 8  shows that the Node B pointer  806  and the UE pointer  2  ( 802 ) are set as TFC 2 . 
     A UE pointer  1  ( 804 ) is determined by the UE using the information transmitted from the Node B and used in actual transmission of data. Referring to  FIG. 8 , the UE transmits data by means of one TFC from among TFC 2  through TFC 10 , i.e., TFC 6 , which has been determined based on the Node B pointer  806  and the UE pointer  2  ( 802 ) transmitted from the Node B. 
     In other words, in the first embodiment of the present invention, the UE determines the TFC based on the information transmitted from the Node B, as opposed to what is done in the prior art. 
       FIG. 9  is a block diagram showing a construction of a UE pointer  1  selection unit of the UE according to the first embodiment of the present invention. 
     Referring to  FIG. 9 , the UE pointer  1  selection unit  900  includes a rate request unit  902  and a rate grant unit  904 . The rate request unit  902  determines the data rate necessary for the UE and the rate grant unit  904  detects whether the data rate determined by the rate request unit  902  is available or not. 
     The rate request unit  902  receives a Buffer Occupancy (BO)  906 , a maximum allowed transmission power  908 , and the UE pointer  2  ( 910 ) from the Node B, and calculates an optimum data rate based on the received information. In calculating the optimum data rate, an optimum TFC is selected using the TFCS, i.e., the UE pointer  2 . In other words, the rate request unit  902  selects a higher data rate or a data rate of a higher level when the buffer contains large data. However, when the requested data rate exceeds the maximum allowed transmission power  908  for the UE, the rate request unit  902  determines the data rate within a range satisfying the condition of maximum allowed power  908 . Further, the TFC determined in this way is compared with the UE pointer  2  transmitted from the Node B, so that the finally selected TFC cannot exceed the range limited by the UE pointer  2 . The TFC selection proposed above is only one example of various possible methods and can be obviously modified or changed in various ways in its actual application. 
     Then, the rate request unit  902  compares the determined TFC with the UE pointer  1  [n- 1 ]  912 . From the result of the comparison, the rate request unit  902  outputs “UP” when the determined TFC is higher than the TFC of the previous transmission and outputs “DOWN” when the determined TFC is lower than the TFC of the previous transmission. Further, the rate request unit  902  outputs “KEEP” when the determined TFC is equal to the TFC transmitted at the previous time. Hereinafter, each of the output “UP”, “KEEP”, and “DOWN” will be referred to as a rate request message  914 . 
     The data rate request message  914  is transmitted to the rate request unit  904 . The rate request unit  904  receives a rate grant probability message  916  from the Node B as well as the data rate request message  914  and the UE pointer  1 [n- 1 ]  912  used at the previous time from the rate request unit  902 . The rate grant probability message  916  has a value between 0 and 1. The rate grant unit  904  outputs the UE point  1  ( 918 ) at the current time by using the received information. Here, the rate grant unit  904  may generate a random variable between 0 and 1, output the received UE pointer  1 [n]  918  only when the generated random variable is smaller than the received rate grant probability message  916 , and output the previously used UE pointer  1 [n- 1 ] intact when the generated random variable is not smaller than the received rate grant probability message  916 . 
     Hereinafter, a process of outputting the UE pointer  1  ( 918 ) at the current time will be described in detail with reference to  FIG. 10 . 
     First, in step  1002 , the UE checks the buffer occupancy. If the buffer contains data to transmit, the UE proceeds to step  1004 . If the buffer contains no data to transmit, the UE proceeds to step  1024  and ends the process. In step  1004 , the UE sets the buffer occupancy, the maximum allowable transmission power, the UE pointer  2 , the UE pointer  1 , TFCS, and the rate grant probability message. In step  1006 , the UE calculates an optimum data rate by means of the buffer occupancy, the maximum allowable transmission power, and the UE pointer  2 . The calculated data rate is used in selecting a TFC for transmission of data using the UE pointer  2 . In  FIG. 10 , the selected TFC is expressed as TFCi. 
     In step  1008 , the UE compares TFCi with TFCp corresponding to the data rate of previous transmission. “UP” is output when TFCi is higher than TFCp and “KEEP” is output when TFCi is equal to TFCp. “DOWN” is output when TFCi is lower than TFCp. As described above, “UP”, “KEEP”, and “DOWN” are referred to as Rate Request (RR) messages. In step  1010 , the RR message is checked. When the RR message is “UP”, the UE performs step  1012 . When the RR message is “KEEP”, the UE proceeds to step  1020  and transmits data at TFCp corresponding to the data rate of the previous data transmission. In contrast, when the RR message is “DOWN”, the UE proceeds to step  1018  in which the UE pointer  1  is determined as the TFCi selected in step  1006 . 
     When an RR message of “UP” is received in step  1012 , the UE generates a random variable x between 0 and 1 with a uniform generation probability. In step  1014 , the UE compares the rate grant probability message with the variable x generated in step  1012 . When the variable x is smaller than the rate grant probability message, step  1016  is performed. When the variable x is larger than or equal to the rate grant probability message, step  1020  is performed. In step  1016 , the UE determines the UE pointer  1  as TFC(p- 1 ). That is, the UE selects a TFC one step higher than the previously used TFC. In step  1020 , the UE determines the UE pointer  1  as the same TFC as that used in the previous time. In step  1022 , the UE transmits data by means of TFC corresponding to the determined UE pointer  1 . 
     Hereinafter, a structure of a TFCS according to a second embodiment of the present invention will be described in detail with reference to  FIG. 11 . 
     The TFCS  1100  shown in  FIG. 11  includes TFC 0  through TFC 10 . TFC 0  represents the highest data rate and TFC  10  represents the lowest rate. Although the TFCS shown in  FIG. 11  employs TFCs in 11 steps, more steps of TFCs may be employed. 
     The Node B pointer  1108  is transmitted to the Node B by the RNC and is a value determined based on the measurement of the noise rise in multiple Node Bs by the RNC. The UE pointer  2  transmitted from the Node B to the UE implies a maximum data rate at which the UE can transmit data. That is, the UE pointer  2  ( 1102 ) is assigned to the UE by the Node B in consideration of the noise rise of the Node B, etc.  FIG. 11  shows that the Node B pointer  1108  and the UE pointer  2  ( 1102 ) are set as TFC 2 . 
     A UE pointer  1  ( 1104 ) is determined by the UE based on the information transmitted from the Node B and is used in actual transmission of data. Referring to  FIG. 11 , the UE transmits data by means of one TFC selected from among TFC 2  through TFC 10  based on the Node B pointer  1108  and the UE pointer  2  ( 802 ) transmitted from the Node B. Specifically, the UE transmits data by means of TFC 6  in  FIG. 11 . In other words, in the second embodiment of the present invention, the UE determines the TFC based on the information transmitted from the Node B, as opposed to how it is performed in the prior art. 
     Further, the TFC corresponds to an identifier having a particular probability. In  FIG. 11 , the identifier having a particular probability is expressed as P_i( 1106 ). The identifier P_i( 106 ) includes P_ 0  corresponding to TFC 0  and P_ 10  corresponding to TFC 10 . The probability implies a grant probability of the data rate requested by the UE. Therefore, the UE determines whether to transmit data or not by means of a new data rate calculated using the requested data rate and the grant probability. 
     The probability for the TFC is determined in consideration of the data rate allowable for the UE and the interference quantity of the Node B. Usually, P_i of a higher TFC having a lower data rate is set to be higher and P_i of a lower TFC having a higher data rate is set to be lower. For example, P_ 0  through P_ 5  may be set to have a probability of 0.2 and P_ 6  through P_ 10  may be set to have a probability of 1. As another example, P_ 0  may be set to have a probability of 0.1, P_ 1  may be set to have a probability of 0.2, P_ 2  may be set to have a probability of 0.3, and P_ 3  through P_ 10  may be set to have a probability of 1. Correlation between TFCi and P_i may be contained in a table stored in advance between the Node B and the UE in order to reduce signaling between the UE and an upper layer or between the Node B and an upper layer. 
       FIG. 12  is a block diagram showing a construction of a UE pointer  1  selection unit of the UE according to the second embodiment of the present invention. 
     The UE pointer  1  selection unit  1200  includes a rate request unit  1202  and a rate grant unit  1204 . The rate request unit  1202  determines the data rate necessary for the UE and the rate grant unit  1204  detects whether the data rate determined by the rate request unit  1202  is available or not. Hereinafter, the rate request unit  1202  and the rate grant unit  1204  will be discussed. 
     The rate request unit  1202  receives a Buffer Occupancy (BO)  1206 , a maximum allowed transmission power  1208 , and the UE pointer  2  ( 1210 ) from the Node B, and determines an optimum TFC by means of the received information. In this case, the rate request unit  1202  selects a higher data rate or a TFC of a higher level when the buffer contains large data. However, when the requested data rate exceeds the maximum allowed transmission power  1208  for the UE, the rate request unit  1202  determines the data rate within a range satisfying the condition of the maximum allowed power  1208 . Further, the TFC determined in this way is compared with the UE pointer  2  transmitted from the Node B so that the finally selected TFC cannot exceed the range limited by the UE pointer  2 . In other words, the rate request unit  1202  determines and outputs the TFCi  1214  in consideration of the buffer occupancy  1206  and the maximum allowed power  1208 . The TFC selection proposed above is only one example of various possible methods and can be obviously modified or changed in various ways in its actual application. 
     The rate grant unit  1204  determines a data rate for the current transmission in consideration of the received TFCi  1214 , the UE pointer  1 [n- 1 ]  1212  for the previous transmission, and the probability factors P_i  1216  for the TFCs from the Node B. That is, the rate grant unit  1204  outputs the UE pointer  1  ( 1218 ). 
     Here, the rate grant unit  1204  may generate a random variable between 0 and 1 for the determined data rate, i.e., the UE pointer  1  ( 1218 ), output the received UE pointer  1 [n]  1218  only when the generated random variable is smaller than the received rate grant probability message  1216 , and output the previously used UE pointer  1 [n- 1 ] intact when the generated random variable is not smaller than the received rate grant probability message  1216 . 
     Hereinafter, a process of outputting the UE pointer  1  ( 1218 ) at the current time will be described in detail with reference to  FIG. 13 . 
     First, in step  1302 , the UE checks the buffer occupancy. When the buffer contains data to transmit, the UE proceeds to step  1304 . When the buffer contains no data to transmit, the UE proceeds to step  1320  and ends the process. In step  1304 , the UE sets the buffer occupancy, the maximum allowed transmission power, the UE pointer  2 , the UE pointer  1 , TFCS, and P_i. In step  1306 , the UE calculates an optimum data rate by means of the buffer occupancy and the maximum allowed transmission power. The calculated data rate is used in selecting a TFC within the range of the UE pointer  2 . In  FIG. 13 , the selected TFC is expressed as TFCi. 
     In step  1308 , the UE compares TFCi with TFCp. TFCp corresponds to the data rate of previous transmission. Step  1310  is performed when TFCp is larger than TFCi and step  1316  is performed when TFCp is smaller than or equal to TFCi. In step  1310 , the UE generates a random variable x between 0 and 1 with a uniform generation probability. In step  1312 , the UE compares P_i (corresponding to TFCi) with the variable x generated in step  1310 . If x is smaller than P_i, step  1316  is performed. If x is larger than or equal to P_i, step  1314  is performed. In step  1314 , the UE increases one for the value of i. That is, the UE selects a new TFC of one-step reduced data rate and proceeds to step  1310 . In step  1316 , the UE determines the UE pointer  1  as TFCi. In step  1318 , the UE transmits data at the data rate corresponding to the determined UE pointer  1 . 
       FIG. 14  is a flowchart for illustrating an operation of the UE in response to a request of retransmission according to the third embodiment of the present invention. In step  1402 , the UE determines whether to perform the retransmission or not. The UE proceeds to step  1406  if retransmission is necessary and to step  1404  if retransmission is unnecessary. In step  1404 , the UE determines the UE pointer  1  through performing the process according to the first or second embodiment of the present invention and then proceeds to step  1408 . In step  1406 , the UE determines the UE pointer  1  using the TFC corresponding to the data rate of the previous transmission. In step  1408 , the UE transmits data stored in the buffer using the determined pointer  1  and then returns to step  1402 . 
     According to the embodiments of the present invention as described above, a UE can determine a data rate for the UE itself, thereby reducing the complexity of the Node B. Further, the determined data rate for the UE is selected within a particular range with reference to a previously used data rate, so that the influence of the interference signal can be reduced. 
     While the invention has been shown and described with reference to certain embodiments thereof, it should 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.