Patent Publication Number: US-2009221238-A1

Title: Transmitting apparatus and transmitting method of base station, and receiving apparatus and communication method of ue in mobile communication system

Description:
TECHNICAL FIELD 
     The present invention relates to a transmitting apparatus in a base station and a transmission method thereof, and a receiving apparatus in a user equipment (UE) and a communication method thereof in a mobile communication system. More particularly, the present invention relates to an adaptive transmission method in a downlink of an orthogonal frequency division multiplexing access (OFDMA)-based mobile communication system. 
     BACKGROUND ART 
     In general, each user equipment (UE) in an OFDMA-based mobile communication system has a different wireless channel environment. Therefore, the UE estimates channel state information transmitted from a base station and feeds back the estimated channel state information to the base station so as to increase performance and capacity of the OFDMA-based mobile communication system. 
     In more detail, when a transmitting end of the base station transmits a pilot or a preamble to a receiving end of the UE through a radio channel, the receiving end of the UE estimates a radio channel state by using the pilot or the preamble, estimates a signal to noise ratio (SNR) of the estimated radio channel state, and feeds back the estimated radio channel state and the SNR of the radio channel to the transmitting end of the base station in the OFDMA-based mobile communication system. 
     Then, the transmitting end of the base station adaptively transmits traffic data by applying a modulation method, an encoding method, and power allocation based on the SNR that is fed back to the transmitting end of the base station. 
     A downlink frame structure of the OFDMA-based mobile communication system is formed of a series of slots and each slot is formed by at least one symbol. Each slot includes a plurality of pilot symbols for channel estimation that are arranged dispersively, and a plurality of data channels. In case of multi-carrier system such as OFDMA, the pilot symbols are spread along both time and frequency domains. The receiving end of the UE estimates a channel state by using the pilot symbol, and generates channel state information by using the estimated channel state. 
     However, when estimating a channel estimate by using a pilot symbol of an n-th slot, the receiving end of the UE may estimate a channel state after a predetermined delay due to a delay in a channel estimation filter. In addition, the receiving end of the UE generates channel state information using the outdated channel state and transmits the generated channel state information to the transmitting end of the base station through an uplink channel. 
     Accordingly, a time difference between channel state estimation in the receiving end of the UE and actual data transmission in the transmitting end of the base station corresponds to at least more than two slot times, and one or two slots may be added to the time difference depending on a frame structure, a delay in a channel estimation filter in a receiving apparatus, and a time for a transmission/receiving operation between the base station and the UE. As described, a channel state may be changed due to the time difference between the SNR estimation in the receiving end of the UE and the data transmission in the transmitting end of the base station. 
     As a related technology, a channel prediction method may be used for predicting a channel state to be used for data transmission in the transmitting end of the base station. That is, a receiving end of the UE predicts future channel state information by using information from previous channel state information through to current channel state information that the UE has received, and transmits feedback to a transmitting end of the base station. Then, the transmitting end of the base station performs data transmission by using the predicted channel state information. 
     Such a method has been proposed by A. Duel-Hallen, S. Hu, and H. Hallen, in “Long-range prediction of fading signals” (IEEE Signal Processing Magazine, vol. 17, pp. 62-75, May 2000). However, the predicted channel state information includes prediction errors, and therefore, system performance may experience severe degradation when transmit power is determined by compensating a required signal to noise ratio (SNR) with only a predicted SNR. 
     As another prior art, a method for deriving an error bit rate (BER) using statistic characteristics of predicted channel state information and calculating transmit power by using the derived BER has been disclosed by S. Falahati, A. Svensson, T. EKman, and M. Sternad (entitled, “Adaptive Modulation Systems for Predicted Wireless Channels,” IEEE Trans., Vol. 52, pp. 307-316, February 2004). However, since this method is aimed at utilization of adaptive modulation by a single user of a single subcarrier system in a flat fading environment, this method cannot be applied to a mobile communication system including various channel environments, user equipment using various channel state information generation algorithms, and variation in mobile speed. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     DISCLOSURE 
     Technical Problem 
     The present invention has been made in an effort to provide a transmitting apparatus of a base station and a transmission method thereof, and a receiving apparatus of a UE and a communication method thereof having the advantage of performing efficient adaptive transmission in a mobile communication system under a multi-path radio channel environment. 
     Technical Solution 
     An exemplary receiving apparatus according to an embodiment of the present invention is provided to a user equipment (UE) that communicates with a base station in a mobile communication system in a multi-path radio channel environment. 
     The receiving apparatus includes a path estimator, a channel estimator, an average predictor, a variance predictor, long-term channel information generator, and a transmitter. The path estimator estimates a receiving path from a received signal. The channel estimator estimates a channel for each of the estimated receiving paths. The average predictor generates predicted channel average information after a minimum transmission delay by using the estimated channel. The variance predictor generates predicted channel variance information by using the predicted channel average information. The long-term channel information generator generates an average signal to noise ration (SNR) by using the estimated channel and generates statistical information on an error of the predicted channel average information by using the predicted channel average information. The transmitter transmits at least one of the predicted channel average information, the predicted channel variance information, the number of receiving paths, and the statistical information on the predicted channel average information error, as well as the average SNR. 
     An exemplary transmitting apparatus according to another embodiment of the present invention is provided to a base station for transmitting a signal to a UE of a mobile communication system in a multi-path radio channel environment. 
     The transmitting apparatus includes a receiver, a transmit power controller, a scheduler, and a transmission controller. The receiver receives at least one of an average signal to noise ratio (SNR), predicted channel average information, predicted channel variance information, the number of receiving paths, and statistical information on an error in the predicted channel average information. The transmit power controller acquires transmit power for each UE and each encoding/modulation method by using information received at the receiver. The scheduler determines an encoding/modulation method for each UE and selects a UE to be served. The transmission controller encodes and modulates the signal according to the determined encoding/modulation method and transmits the encoded and modulated signal according to a predetermined adaptive transmission method. 
     An exemplary communication method according to another exemplary embodiment of the present invention is provided to a receiving apparatus of a UE for communication with a base station of a mobile communication system in a multi-path radio channel environment. The communication method includes: a) estimating a channel for each receiving path from a received signal; b) generating a plurality of channel state information by using the received signal and the channels respectively corresponding to the receiving paths; and c) transmitting at least a portion of the plurality of channel state information to the base station. 
     An exemplary transmission method according to another embodiment of the present invention transmits a signal from a transmitting apparatus of a base station to a UE in a mobile communication system under a multi-path radio channel environment. The transmission method includes: a) partially or fully receiving a plurality of channel information from the UE; b) acquiring transmit power for an encoding/modulation method of each UE by using the partially received channel information; c) determining an encoding/modulation method for each UE, and selecting a UE to be served; and d) encoding/modulating the signal according to the determined encoding/modulation method and transmitting the encoded/modulated signal to the UE. 
     ADVANTAGEOUS EFFECTS 
     Accordingly to the present invention, a UE in a good channel state can acquire a gain by feeding back channel information, and a UE in a bad channel state can reduce the amount of feedback information without causing performance degradation by feeding back only effective channel information for the bad channel state. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a transmitting apparatus of a base station and a receiving apparatus of a mobile station in an OFMDA-based mobile communication system according to a first exemplary embodiment of the present invention. 
         FIG. 2  shows the receiving apparatus of the mobile station according to the first exemplary embodiment of the present invention in more detail. 
         FIG. 3  is a flowchart of a process of transmitting channel state information in the receiving apparatus of the mobile station according to the first exemplary embodiment of the present invention. 
         FIG. 4  shows the transmitting apparatus of the base station according to the first exemplary embodiment of the present invention in more detail. 
         FIG. 5  is a flowchart of a process for transmitting traffic data in the transmitting apparatus of the base station according to the first exemplary embodiment of the present invention. 
         FIG. 6  schematically shows a structure of a transmitting apparatus of a base station and a receiving apparatus of a mobile station in a mobile communication system according to a second exemplary embodiment of the present invention. 
         FIG. 7  is a flowchart of an operation of the transmitting apparatus of the base station and the receiving apparatus of the mobile station in the wireless communication system according to the second exemplary embodiment of the present invention. 
     
    
    
     BEST MODE 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. 
     Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     In addition, unless explicitly described to the contrary, the word “comprise” or variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     A base station transmitting apparatus and a transmission method thereof, and a UE receiving apparatus and a communication method thereof in a mobile communication system according to an exemplary embodiment of the present invention will now be described in more detail with reference to the accompanying drawings. According to a first exemplary embodiment of the present invention, a mobile communication system is an OFDMA-based mobile communication system. 
       FIG. 1  shows a transmitting apparatus of a base station and a receiving apparatus of a user equipment (UE) in an OFDMA-based mobile communication system according to the first exemplary embodiment of the present invention. 
     As shown in  FIG. 1 , a transmitting apparatus  100  of a base station includes a receiver  110 , a scheduler  130 , a transmit power controller  120 , and a transmission controller  140 , and a receiving apparatus  200  of the UE includes a channel estimator  210 , a channel information generator  220 , a transmitter  230 , and a demodulating/decoding unit  240 . 
     The channel estimator  210  estimates a channel by using a pilot or a preamble transmitted from the transmitting apparatus  100 , and the channel information generator  220  generates predicted channel average information and predicted channel variance information after a minimum transmission delay D by using the estimated channel, and transmits the information as short-term channel information to the transmitter  230 . 
     The channel information generator  220  separates a signal by estimating a receiving path from a received signal, acquires statistical characteristic information of an error in the predicted channel average information by using predicted channel average information and substantial channel state information that can be obtained after the delay D passes, and obtains an average signal to noise ratio (SNR) by using the channel state estimated by the channel estimator  210 . 
     The channel information generator  220  transmits a total number of receiving paths, statistical characteristic information on a channel prediction error, and the average SNR as long-term channel information to the transmitter  230 . Herein, the minimum transmission delay D indicates a difference between a slot time of channel estimation and a slot time of traffic data transmission when the transmitting apparatus  100  of the base station transmits traffic data with the highest priority, and the difference may vary depending on a system. 
     The transmitter  230  transmits channel state information generated by the channel state information generator  220  to the transmitting apparatus  100  of the base station. 
     The demodulating/decoding unit  240  demodulates and decodes the traffic data transmitted from the transmitting apparatus  100  of the base station. 
     The receiver  110  receives at least a portion of the short-term channel information or a portion of the long-term channel information transmitted from the receiving apparatus  200 , and delivers the received information to the transmit power controller  120 . The transmit power controller  120  determines transmit power for a user-desired encoding/modulating algorithm by using the short-term channel information and the long-term channel information, and the scheduler  130  determines an appropriate encoding/modulation algorithm and selects users to be served. 
     The transmission controller  140  encodes/modulates traffic data by using the determined encoding/modulation algorithm and transmits the encoded/modulated traffic data to the receiving apparatus  200 . 
       FIG. 2  shows a detailed configuration of the UE receiving apparatus according to the first exemplary embodiment of the present invention, and  FIG. 3  is a flowchart of channel information transmission in the UE receiving apparatus according to the first exemplary embodiment of the present invention. 
     As shown in  FIG. 2  and  FIG. 3 , the channel information generator  220  includes a path estimator  221 , a long-term channel information generator  222 , an average predictor  223 , a variance predictor  224 , a bias eliminator  225 , and delay units  226  and  227 . 
     The path estimator  221  estimates a receiving path from a received signal and separates a signal from each of the receiving paths, in step S 310 . The channel estimator  210  estimates a channel state for each receiving path from the received signal, in step S 320 . 
     The long-term channel information generator  222  receives a total number of estimated receiving paths L from the path estimator  221  and forwards L as long-term channel information to the transmitting apparatus  100 , in step S 350 . 
     The average predictor  223  uses the channel estimated by the channel estimator  210  to predict a channel state, including the minimum transmission delay D, through a prediction filter (not shown) as given in Math Figure 1. That is, the average predictor  223  predicts the channel state, including the minimum transmission delay D, based on information on a current channel state and P outdated channel states, in step S 330 . 
         ĥ   l   [n+D]=f ( h   l   [n],h   l   [n− 1], . . . ,  h   l   [n−P+ 1])  [Math Figure 1] 
     Where h 1 [n] denotes a conjugate channel at the time n in the l-th path, P denotes a degree of the average predictor  223 , and f denotes a prediction filter. Hereinafter, in order to simplify notations, a time parameter is set to n+D when the parameter is omitted. That is, ĥ l =ĥ l [n+D]. 
     In addition, the transmitting apparatus  100  can determine transmit power not by a complex value of required transmit power but by a square of an absolute value of hi, and therefore, the average predictor  223  obtains a combined power value as given in Math Figure 2. 
     
       
         
           
             
               
                 
                   
                     
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     However, since the power value calculated through Math Figure 2 is biased, the bias eliminator  225  should eliminate the bias by using an average value of the predicted channel average information and the substantial channel state information as given in Math Figure 3. 
         {circumflex over (p)}={circumflex over (p)}   biased   +E{p−{circumflex over (p)}   biased }  [Math Figure 3] 
     Where E{p−{circumflex over (p)} biased } denotes a moving average obtained by accumulating a difference between the substantial channel state estimated at the time n+D and the channel state estimated at the time n for a longer period of time. 
     As a result, the bias eliminator  225  may use a moving average scheme to obtain the average moving. However, when a power of a pilot is P pilot , the receiving apparatus  200  does not know the power of the pilot, and therefore the average predictor  223  obtains an average SNR that is proportional to {circumflex over (p)} as given in Math Figure 4. The average SNR, including the minimum transmission delay D, is transmitted as the short-term channel information to the transmitting apparatus  100  of the base station, in step S 340 . Such an SNR, including the minimum transmission delay D, obtained through Math Figure 4 will be referred to as “predicted channel average information” in the following description. 
     
       
         
           
               
             
               
                 
                   
                     
                       
                         
                           
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     Where σ n   2  denotes noise variance. 
     The variance predictor  224  performs fast Fourier transform (FFT) on the channel state information ĥ l , including the minimum transmission delay D, to obtain a channel on the frequency axis and channel power variance   on the frequency axis. However, the substantial channel power variance that the variance predictor  224  obtains is   Since the channel power variance   of the frequency axis is also biased, the bias eliminator  225  eliminates the bias as given in Math Figure 5 and reports the bias-eliminated channel power variance as short-term channel information. The bias-eliminated channel power variance will be referred to as “predicted channel variance information”. 
     
       
         
           
             
               
                 
                   
                     
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     In addition, the long-term channel information generator  222  receives a difference (SNR− ) between the substantial channel state information and the predicted channel average information from the bias eliminator  225  and obtains statistical information on a channel prediction error, and transmits the statistical information as one of the long-term channel state information to the transmitting apparatus  100 . 
     In this case, the long-term channel information generator  222  transmits the total number of paths L estimated by the path estimator  221  and an average SNR obtained by using a channel state estimated by the channel estimator  210  as the long-term channel information to the transmitting apparatus  100 , in steps S 350  and S 360 . 
     In this case, the long-term channel information generator  222  transmits the average SNR to the transmitting apparatus  100  with an interval that is longer than an interval of the transmission of the statistical information on the channel prediction error and the total number of paths L. Herein, the statistical information on the channel prediction error can be used to calculate an average value of |SNR− | 2  for a long period of time by using the moving average method as given in Math Figure 6. 
     
       
         
           
             
               
                 
                   
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     That is, a channel state in the time (n+D) is predicted at the time n, and the channel estimator  210  estimates a substantial channel state at the time (n+D). In addition, the average predictor  223  transmits substantial channel power P to the bias eliminator  225 . In order to calculate E{(p−{circumflex over (p)})} of Math Figure 6, the delay units  226  and  227  are required to transmit channel power that was predicted at the time n to the bias eliminator  225  at the time (n+D). 
     The delay units  226  and  227  respectively delay outputting of channel state information that have been predicted by the average predictor  223  and the variance predictor  224  for a predetermined time period, and output the delayed channel state information to the bias controller  225 . 
       FIG. 4  shows a detailed diagram of the transmitting apparatus of the base station according to the first exemplary embodiment of the present invention, and  FIG. 5  is a flowchart of a traffic data transmission process of the transmitting apparatus of the base station according to the first exemplary embodiment of the present invention. 
     As shown in  FIG. 4  and  FIG. 5 , the receiver  110  receives the short-term channel information and the long-term channel information transmitted from the receiving apparatus  200  of the UE, and transmits at least a portion of the long-term channel information and at least a portion of the short-term channel information transmitted from the receiving apparatus  200  of the UE to the transmit power controller  120  according to an assigned adaptive transmission method, in step S 510 . 
     In addition, the transmit power controller  120  includes a transmit power table  122  and a transmit power determiner  124 . The transmit power table  122  stores a transmit power value according to an average deviation region for each encoding/modulation method, and the values stored in the transmit power table  122  are obtained by a simulation. 
     In the simulation, the predicted channel variance information   which is one of the short-term channel information, is quantized for each included in the short-term channel information of each encoding/modulation method is quantized with a constant interval for each encoding/modulation method, and the predicted channel average information   the statistical information σ ε,SNR   2  on the error in the predicted channel average information   and the number of paths L are quantized with a constant interval such that the transmit power table  122  becomes a table of required transmit power in three-dimensions. 
     The transmit power determiner  124  determines transmit power for an encoding/modulation method of each user by using the channel state information transmitted from the receiver  110 , in step S 520 . That is, the transmit power determiner  124  searches for transmit power that corresponds to the long-term channel information and the short-term channel information from the transmit power table  122  for each encoding/modulation method. Herein, the long-term channel information includes the statistic information σ ε,SNR   2  of the error in the predicted channel average information   and the number of receiving paths L, and the short-term channel information includes the predicted channel variance information   and the predicted channel average information    
     The scheduler  130  selects a proper encoding/modulation method for each user, and selects users to be served, in steps S 530  and S 540 . In this case, the scheduler  130  selects an encoding/modulation method having the highest transmission rate among encoding/modulation methods requiring transmission power less than the maximum available power. 
     The transmission controller  140  includes an encoding/modulation unit  142  and a transmitter  144 , and the encoding/modulation unit  142  encodes/modulates traffic data by using an encoding/modulation method that is appropriate for traffic data of the user selected by the scheduler  130 , in step S 540 . 
     The transmitter  144  transmits the encoded/modulated traffic data to the receiving apparatus  200 , in step S 540 . 
     As described, the transmitting apparatus  100  according to the first exemplary embodiment of the present invention receives the short-term channel information and the long-term channel information from the receiving apparatus  200  and performs adaptive modulation so that system performance can be improved. 
     However, when the error in the predicted channel average information   increases, the system performance may be the same as in the adaptive transmission without using the predicted channel average information   Therefore, receiving apparatuses  200  of all UEs must report both long-term channel information and short-term channel information to the transmitting apparatus  100  of the base station. An exemplary embodiment aiming to increase system capacity while minimizing the amount of fed-back channel state information will be described in more detail with reference to  FIG. 6  and  FIG. 7 . 
       FIG. 6  schematically shows structures of a transmitting apparatus of a base station and a receiving apparatus of a UE in an OFDMA-based mobile communication system according to a second exemplary embodiment of the present invention, and  FIG. 7  is a flowchart of the transmitting apparatus and the receiving apparatus in the OFDMA-based communication system according to the second exemplary embodiment of the present invention. 
     As shown in  FIG. 6  and  FIG. 7 , a receiving apparatus  200 ′ of the UE is the same as that of the first exemplary embodiment of the present invention, except that the receiving apparatus  200 ′ further includes a channel state information generator  250  in addition to the constituent elements of the receiving apparatus  200  of the first exemplary embodiment. The channel state information generator  250  compares the statistical information σ ε,SNR   2  on the error of the predicted channel average information   with predetermined threshold values, and reports channel state information corresponding to a range between the predetermined threshold values with an interval that is longer than an interval of the transmission of the average SNR to the transmitting apparatus  100  of the base station, in step S 710 . 
     Since the channel state information is fed back to the transmitting apparatus  100  of the base station with a relatively longer interval, an overhead in the feedback information can be ignored. That is, as given in Math Figure 7, the channel state information generator  250  compares a value ν with predetermined threshold values ν th,1 , ν th,2 , wherein the value is obtained by dividing the statistical information σ ε,SNR   2  on the error of the predicted channel average information   with the average SNR E{SNR}, which is substantial channel state information. In this case, the threshold value ν th,1  the is set to be smaller than the threshold value ν th,2   
     
       
         
           
             
               
                 
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     Where E{SNR} may represent the average SNR generated by using the channel state estimated by the channel estimator  210 . In addition, the channel state information generator  250  compares the value ν with the predetermined threshold values ν th,1 , ν th,2 , and reports channel state information corresponding to ν≦ν th,1 , ν th,1 &lt;ν≦ν th,2 , and ν th,2 &lt;ν to the transmitting apparatus  100  of the base station with an interval that is longer than the interval of transmission of the statistical information on the error of the predicted channel average information. Herein, the threshold values ν th,1 , ν th,2  are determined by a simulation. 
     According to the second exemplary embodiment of the present invention, a transmitting apparatus  100 ′ of the base station may adopt three adaptive transmission methods, respectively called a “first adaptive transmission method”, a “second adaptive transmission method”, and a “third transmission method”. The first adaptive transmission method may be used when ν≦ν th,1 , the second adaptive transmission method may be used when ν th,1 &lt;ν≦ν th,2 , and the third adaptive transmission method may be used when ν th,2 &lt;ν. 
     That is, the first adaptive transmission method uses predicted channel average information   predicted channel variance information   statistical information σ ε,SNR   2  on the error of the predicted channel average information   and the number of receiving paths L (see  FIG. 4 ). The second adaptive transmission method uses the short-term channel information including the predicted channel average information   statistical information σ ε,SNR   2  on the error of the predicted channel average information   and the number of receiving paths L, excluding the predicted channel variance information   since the system performance is not improved even though the predicted channel variance information   is used compared to the system performance when using the predicted channel variance information    
     Since the predicted channel variance information   is not used, the transmit power table  122  has a target transmit power table for each encoding/modulation method, wherein the target transmit power table is a three-dimensional table generated by quantizing the predicted channel average information   the statistical information σ ε,SNR   2  on the error of the predicted channel average information   and the number of receiving paths I, with respect to a constant interval. 
     The third adaptive transmission method uses only the average SNR E{SNR} since there is no difference in the system performance when the predicted channel average information is used and when predicted channel average information is not used. 
     As shown in  FIG. 6  and  FIG. 7 , the transmitting apparatus  100 ′ according to the second exemplary embodiment of the present invention includes an adaptive transmission method determiner  110 ′ and an adaptive transmission controller  120 ′. 
     The adaptive transmission method determiner  110 ′ is reported with channel state information by the receiving apparatus  200  of each UE, determines an appropriate adaptive transmission method for the respective receiving apparatuses  200  in consideration of the amount of feedback channel and a quality of service (QoS) among various adaptive transmission methods, and provides the determined adaptive transmission method to the adaptive transmission controller  120 ′, in step S 720 . 
     The adaptive transmission controller  120 ′ corresponds to the receiver  100 , the transmit power controller  120 , the scheduler  130 , and the transmission controller  140  of  FIG. 1  and  FIG. 4 . Herein, the adaptive transmission method determiner  110 ′ may be placed between the receiver  110  and the transmit power controller  120 . The adaptive transmission controller  120 ′ selects an encoding/modulation method by using channel state information allocated in accordance with an adaptive transmission method that has been determined for each user among the various adaptive transmission methods, and transmits user-specific traffic data to the receiving apparatus  200 . 
     In this case, the adaptive transmission controller  120 ′ uses the same transmission method described in the first exemplary embodiment when the first adaptive transmission method is used (see  FIG. 4 ). When the second adaptive transmission method is used, the predicted channel variance information   is not used, and therefore, the adaptive transmission controller  120 ′ uses only the predicted channel average information   among the short-term channel information and acquires a target transmit power by using the transmit power table  122  that has been formed by quantizing predicted channel average information   and statistical information σ ε,SNR   2  on errors on the predicted channel average information   for each encoding/modulation, and transmits traffic data with the acquired target transmit power. 
     In addition, the adaptive transmission controller  120 ′ determines transmit power that satisfies a targeted packet error rate by using the reported average SNR E{SNR} and transmit traffic data, when the third adaptive transmission method is used. 
     In addition, the adaptive transmission controller  120 ′ may individually use the three adaptive transmission methods for each user. For example, when a plurality UEs exist with a good channel state, the first adaptive transmission method is allocated to a portion of the UEs and the second adaptive transmission method is allocated to the rest of the UEs since the amount of feedback channel state information is limited. 
     The above-described exemplary embodiments of the present invention can be realized not only through a method and an apparatus, but also through a program that can perform functions corresponding to configurations of the exemplary embodiments of the present invention or a recording medium storing the program, and this can be easily realized by a person skilled in the art. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.