Patent Publication Number: US-2012026960-A1

Title: Radio base station

Description:
TECHNICAL FIELD 
     The present invention relates to a radio base station. 
     BACKGROUND ART 
     In a mobile communication system of the LTE (Long Term Evolution) scheme defined in the 3GPP, a radio base station eNB is configured to be capable of performing link adaptation based on CQI (Channel Quality Indicator) fed back from a mobile station UE. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in the 3GPP, when the MIMO (Multiple Input Multiple Output) scheme is applied in the mobile communication system of the LTE scheme, how the link adaption is performed is not defined, and thus, there is a problem that the appropriate link adaptation cannot be performed. 
     Therefore, the present invention is intended to overcome the above-described problem. An object of the present invention is to provide a radio base station capable of performing an appropriate link adaptation when the MIMO scheme is applied. 
     Means for Solving the Problem 
     A first aspect of the present invention is summarized as a radio base station that can simultaneously transmit a first downlink signal in which a transmission stream number is adaptively changed, and a second downlink signal in which a transmission stream number is fixed to 1, to a mobile station using an MIMO scheme, the radio base station including: a reception quality reception unit configured to receive information indicating reception quality of the first downlink signal transmitted by the mobile station; a reception quality adjustment unit configured to calculate an adjustment value of the information indicating the reception quality based on a stream number of the first downlink signal; and a processing unit configured to perform an adaptive modulation and demodulation process and a transmission power control process on the second downlink signal based on the calculated adjustment value. 
     Effect of the Invention 
     As described above, according to the present invention, it is possible to provide a radio base station capable of performing an appropriate link adaptation when the MIMO scheme is applied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram explaining a mobile communication system according to a first embodiment of the present invention. 
         FIG. 2  is a functional block diagram of a radio base station according to the first embodiment of the present invention. 
         FIG. 3  is a diagram explaining CQI transmitted by a mobile station in the mobile communication system according to the first embodiment of the present invention. 
         FIG. 4  is a table explaining CQI transmitted by a mobile station in the mobile communication system according to the first embodiment of the present invention. 
         FIG. 5  is a diagram explaining the rank adaption control (Rank Adaptation) used in the mobile communication system according to the first embodiment of the present invention. 
         FIG. 6  is a table illustrating one example of a corresponding table managed by an adaptive modulation and demodulation processing unit of the radio base station according to the first embodiment of the present invention. 
         FIG. 7  is a flowchart explaining an operation of the radio base station according to the first embodiment of the present invention. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     (Configuration of Mobile Communication System According to First Embodiment of the Present Invention) 
     With reference to  FIG. 1  to  FIG. 6 , the configuration of a mobile communication system according to a first embodiment of the present invention will be explained. 
     The mobile communication system according to this embodiment is a mobile communication system of the LTE scheme, and in the mobile communication system according to this embodiment, a radio base station eNB is configured to use the MIMO scheme so as to transmit a downlink signal to a mobile station UE, as illustrated in  FIG. 1 . 
     For example, in the mobile communication system, the radio base station eNB can simultaneously transmit a first downlink signal in which a transmission stream number is adaptively changed, and a second downlink signal in which a transmission stream number is fixed to 1, using the MIMO scheme. 
     In this case, the first downlink signal is a downlink data signal which is transmitted via a PDSCH (Physical Downlink Shared Channel) and to which dynamic scheduling is applied. 
     Further, the second downlink signal may be at least one of: a downlink control signal (a PDCCH signal) transmitted via a PDCCH (Physical Downlink Control Channel); a transmission acknowledgement signal (ACK/NACK, a PHICH signal) transmitted via a PHICH (Physical HARQ Indicator Channel); a control format signal (a PCFICH signal) transmitted via a PCFICH (Physical Control Format Indicator Channel); and a downlink data signal (an SPS-PDSCH signal) to which Semi-Persistent Scheduling (SPS) is applied and which is transmitted via a PDSCH (Physical Downlink Shared Channel). 
     Further, a “Downlink Reference Signal” for data demodulation and reception quality measurement is transmitted over the entire system bandwidth. 
     As the MIMO scheme, there are a “closed-loop MIMO scheme (Closed-loop MIMO)” and an “open-loop MIMO scheme (Open-loop MIMO)”. 
     When the closed-loop MIMO scheme is applied, in the mobile communication system according to this embodiment, a common “set (codebook) of precoding vectors” is held between the mobile station UE and the radio base station eNB, and the mobile station UE is configured to select a precoding vector corresponding to the first downlink signal with the highest receiving characteristic and to feedback the precoding vector to the radio base station eNB. 
     Then, the radio base station eNB is configured to determine a transmission antenna weight, i.e., to form an optimal transmission beam, based on the precoding vector fed back by the mobile station UE. 
     Meanwhile, when the open-loop MIMO scheme is applied, in the mobile communication system according to this embodiment, the radio base station eNB is configured to use a fixed precoding vector when the number of transmission antennas is two, or to use a precoding vector periodically changed when the number of transmission antennas is four, and not to use the precoding vector fed back by the mobile station UE. 
     As illustrated in  FIG. 2 , the radio base station eNB according to this embodiment includes a CQI reception unit  11 , a scheduling processing unit  12 , an adaptive modulation and demodulation processing unit  13 , a CQI adjustment unit  14 , an adaptive modulation and demodulation processing unit  15 , a transmission power control processing unit  16 , a rank control unit  17 , and a transmission mode control unit  18 . 
     The CQI reception unit  11  is configured to receive the CQI (information indicating the reception quality of the first downlink signal) transmitted by the mobile station UE. 
     In this case, the mobile station UE is configured to measure the reception quality (e.g., SIR (Signal to Interference Ratio)) of the first downlink signal in a downlink using the “downlink reference signal”, and to transmit the CQI that is information indicating the measured reception quality to the radio base station eNB via a PUCCH (Physical Uplink Control Channel). 
     It is noted that the mobile station UE may be configured to multiplex the above-mentioned CQI to the PUSCH, and to transmit the multiplexed CQI to the radio base station eNB at a sub-frame at which an uplink data signal (PUSCH, UL-SCH as a transport channel) is transmitted. 
     It is noted that, as the CQI, two types of a “wideband CQI” and a “subband CQI” are known. 
     The “wideband CQI” is information indicating a reception quality measured in the entire system bandwidth, and the “subband CQI” is information indicating a reception quality measured in each subband obtained by dividing the system bandwidth as illustrated in  FIG. 3 . 
     As illustrated in  FIG. 3 , when the “subband CQI” is used, the radio base station eNB can transmit the downlink signal using a resource block of a frequency band with a good reception state in the mobile station UE. 
     It is noted that, the SPS-PDSCH signal, which basically is a signal transmitted by using the same resource block in each transmission cycle, is configured not to be subject to scheduling in the frequency band, and to be mapped over the entire system band, and thus, it is preferable to perform link adaptation based on the “wideband CQI”. 
     Further, in the same manner, since the PDCCH signal, the PHICH signal, and the PCFICH signal are configured to be mapped over the entire system bandwidth, in order to obtain a frequency diversity effect, it is preferable to perform the link adaptation based on the “wideband CQI”. 
     Further, basically, the mobile station UE is configured to transmit the “wideband CQI” to the radio base station eNB in all transmission modes. In addition to the “wideband CQI”, the mobile station UE may be configured to transmit the “subband CQI” to the radio base station eNB. 
     Further, the mobile station UE is configured to calculate the CQI in each codeword. 
     In this case, the codeword indicates a codeword (TB: Transport Block) transmitted at one sub-frame (TTI: Transmission Time Interval), and the number of codewords varies according to the transmission mode of the first downlink signal (e.g., the PDSCH signal) as illustrated in  FIG. 4 . 
     As illustrated in  FIG. 4 , when the transmission mode of the first downlink signal is a “single antenna” and “transmission diversity”, the number of codewords is “1” and the number of “wideband CQIs” to be transmitted is “1”. 
     Further, when the transmission mode of the first downlink signal is an “Open-loop MIMO”, the number of codewords is “1” or “2” and the number of “wideband CQIs” to be transmitted is “1”. 
     Moreover, when the transmission mode of the first downlink signal is a “Closed-loop MIMO”, the number of codewords is “1” or “2” and the number of “wideband CQIs” to be transmitted is “1” or “2”. 
     It is noted that when the transmission mode of the first downlink signal is the “Open-loop MIMO” or the “Closed-loop MIMO”, the number of codewords which can be transmitted by the mobile station UE changes based on the reception state (e.g., pathloss or a received SINR) of the first downlink signal in the mobile station UE. 
     Further, when the transmission mode of the first downlink signal is the “Open-loop MIMO”, even if the number of codewords is “2”, the number of “wideband CQIs” to be transmitted is “1” from the standpoint of overhead reduction. 
     Hereinafter, in the present embodiment, since the “wideband CQI” is used as the CQI, the CQI indicates the “wideband CQI” unless otherwise specified. 
     The transmission mode control unit  18  is configured to control the transmission mode of the first downlink signal (e.g., the PDSCH signal) which is to be applied in the radio base station eNB. 
     The rank control unit  17  is configured to determine a rank to be applied to the first downlink signal based on RI (Rank Indicator) notified by the mobile station UE. 
     In this case, the rank is a value indicating the stream number of the first downlink signal to be simultaneously transmitted using the MIMO scheme. 
     The mobile station UE is configured to adaptively control an optimal rank based on a propagation path state (a received SINR, antenna fading correlation and the like) in a downlink. 
     For example, as illustrated in  FIG. 5 , when mobile station UE#2 is positioned near the center of a cell, spatial correlation is low and the reception quality (e.g., pathloss or a received SINR) of the first downlink signal in the mobile station UE#2 is high, a rank is set to “2” (SDM (Spatial Division Multiplexing) is used), that is, the number of codewords is set to “2”, so that it is possible to realize a high transmission speed. 
     Meanwhile, as illustrated in  FIG. 5 , when the mobile station UE#1 is positioned near the edge of the cell, spatial correlation is high and the reception quality (e.g., pathloss or a received SINR) of the first downlink signal in the mobile station UE#1 is low, the rank is set to “1” (transmission diversity is used), that is, the number of codewords is set to “1”, so that it is possible to improve the reception quality of the first downlink signal in the mobile station UE#1 and expand a cover area. 
     In this case, when the number of transmission antennas is two and the rank has been changed in the mobile communication system employing the MIMO scheme, the property and number of CQIs transmitted by the mobile station UE are changed. The CQI transmitted by the mobile station UE will be explained, below. 
     Firstly, when the rank is “1” (i.e., the number of codewords is “1”), the CQI transmitted by the mobile station UE will be explained. 
     In such a case, the CQI is measured for one codeword and corresponds to SIR including a transmission diversity gain. 
     In this case, when the transmission mode of the first downlink signal is the “Open-loop MIMO”, since SFBC (Space-frequency Block Coding) is applied, the above-mentioned SIR ideally corresponds to maximum ratio combining of a propagation path frequency response. 
     Further, when the transmission mode of the first downlink signal is the “Closed-loop MIMO”, since an optimal precoding vector is selected, the above-mentioned SIR is high as compared with the SIR when the transmission mode of the first downlink signal is the “Open-loop MIMO”. 
     Secondly, when the rank is “2” (i.e., the number of codewords is “2”), the CQI transmitted by the mobile station UE will be explained. 
     In such a case, the CQI is measured for each codeword. In this case, since each codeword is affected by interference, it is expected that SIR of this case is reduced beyond the influence caused by a simple reduction to the half of received power, as compared with the SIR when the rank is “1”. 
     It is noted that, when the transmission mode of the first downlink signal is the “Closed-loop MIMO”, since an optimal precoding vector is selected, it is expected that the degree of degradation of the SIR is low as compared with the case in which the transmission mode of the first downlink signal is the “Open-loop MIMO”. 
     As described above, when the MIMO scheme is applied, the CQI significantly depends on the stream number (the rank) of the first downlink signal to be simultaneously transmitted, as well as an actual state of a propagation path. 
     On the other hand, since transmission diversity (SFBC) is always applied to the second downlink signal (e.g., the downlink control signal) regardless of the transmission mode of the first downlink signal, because of the above-described rank, it is probable that the SIR of the first downlink signal (e.g., the PDSCH signal) and the SIR of the second downlink signal are significantly different from each other. 
     That is, when the rank is “1”, since the transmission diversity is applied to both the first downlink signal and the second downlink signal, the difference between the SIR of the first downlink signal and the SIR of the second downlink signal is not noticeable. 
     On the other hand, when the rank is “2”, since spatial division multiplexing with the number “2” of codewords (the stream number) is applied to the first downlink signal, the SIR of the first downlink signal is smaller than the SIR of the second downlink signal. 
     Therefore, when the rank is “2”, if the radio base station eNB performs link adaptation (e.g., a scheduling process, an adaptive modulation and demodulation process, and a transmission power control process) with respect to the second downlink signal (e.g., the downlink control signal) based on the CQI of the first downlink signal, the following problems may occur. 
     A first problem would be that transmission power of the second downlink signal (e.g., the downlink control signal) becomes more than necessary as a result of a transmission power control process based on the CQI, because the CQI reported by the mobile station UE indicates a value smaller than the reception quality of the second downlink signal, which is actually measured by the mobile station UE. 
     Further, a second problem would be that if a coding rate (e.g., an aggregation level) of the second downlink signal (e.g., the PDCCH signal) is determined based on the CQI, then downlink resources may be wasteful, because the CQI reported by the mobile station UE indicates a value smaller than the reception quality of the second downlink signal, which is actually measured by the mobile station UE. 
     In this case, the “aggregation level” indicates the number of CCEs (Control Channel Elements) continuously assigned to the PDCCH signal based on the CQI reported by the mobile station UE. 
     Further, each control channel element includes nine continuous REGs (Resource Element Groups). Further, each resource element group includes four continuous REs (Resource Elements). Moreover, the resource element is configured by one OFDM symbol (time direction) and one sub-carrier (frequency direction). 
     In consideration of the above points, the CQI adjustment unit  14  is configured to calculate a CQI adjustment value based on the rank (the stream number of the first downlink signal simultaneously transmitted) determined by the rank control unit  17 . 
     For example, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to select a larger one of two CQIs received in the CQI reception unit  11  as the CQI adjustment value. 
     Specifically, if the CQI adjustment value is set as “CQI adjust ” and the above-mentioned two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment value “CQI adjust ” is expressed by Equation “CQI adjust =max(CQI wideband.1 ,CQI wideband.2 )”. In this case, max(A,B) is a function returning a larger value of A and B. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to select a smaller one of the two CQIs received in the CQI reception unit 11 as the CQI adjustment value. Specifically, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment value “CQI adjust ” is expressed by Equation “CQI adjust =min(CQI wideband.1 ,CQI wideband.2 )”. In this case, min(A,B) is a function returning a smaller value of A and B. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by summing up the two CQIs received in the CQI reception unit  11 . 
     It is noted that the CQI adjustment unit  14  may sum up the two CQIs as a dB value or a true value. 
     That is, since the CQI is an index obtained by quantizing SIR (a dB value) measured by the mobile station UE, the dimension of the unit of the CQI corresponds to the dB value. 
     Therefore, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment unit  14  can sum up the two CQIs as a true value by (Equation 1) below. 
     
       
         
           
             
               
                 
                   
                     CQI 
                     adjust 
                   
                   = 
                   
                     10 
                      
                     
                         
                     
                      
                     log 
                      
                     
                         
                     
                      
                     10 
                      
                     
                       ( 
                       
                         
                           10 
                           
                             
                               CQIwideband 
                               , 
                               1 
                             
                             10 
                           
                         
                         + 
                         
                           10 
                           
                             
                               CQIwideband 
                               , 
                               2 
                             
                             10 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     It is noted that if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment unit  14  can sum up the two CQIs as the dB value by Equation “CQI adjust =CQI wideband.1 +CQI wideband.2 ”. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to an average value of the two CQIs received in the CQI reception unit  11 . 
     It is noted that, the CQI adjustment unit  14  may calculate the average value of the CQIs as the dB value or the true value. 
     Specifically, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment unit  14  can calculate the average value of the two CQIs as the true value by (Equation 2) below. 
     
       
         
           
             
               
                 
                   
                     CQI 
                     adjust 
                   
                   = 
                   
                     10 
                      
                     
                         
                     
                      
                     log 
                      
                     
                         
                     
                      
                     10 
                      
                     
                       ( 
                       
                         
                           
                             10 
                             
                               
                                 CQIwideband 
                                 , 
                                 1 
                               
                               10 
                             
                           
                           + 
                           
                             10 
                             
                               
                                 CQIwideband 
                                 , 
                                 2 
                               
                               10 
                             
                           
                         
                         2 
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     It is noted that, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment unit  14  can average the two CQIs as the dB value by Equation “CQI adjust =(CQI wideband.1 +CQI wideband.2 )/2”. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to one (either larger or smaller value) of the CQIs received in the CQI reception unit  11 . 
     Specifically, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment value “CQI adjust ” is expressed by Equation “CQI adjust =max(CQI wideband.1 ,CQI wideband.2 )+CQI diff ” or Equation “CQI adjust =min(CQI wideband.1 ,CQI wideband.2 )+CQI diff ”. In this case, the CQI diff  corresponds to the predetermined offset value. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to the sum of the two CQIs received in the CQI reception unit  11 . 
     Specifically, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment value “CQI adjust ” is expressed by the following Equation 3. 
     
       
         
           
             
               
                 
                   
                     CQI 
                     adjust 
                   
                   = 
                   
                     
                       10 
                        
                       
                           
                       
                        
                       log 
                        
                       
                           
                       
                        
                       10 
                        
                       
                         ( 
                         
                           
                             10 
                             
                               
                                 CQIwideband 
                                 , 
                                 1 
                               
                               10 
                             
                           
                           + 
                           
                             10 
                             
                               
                                 CQIwideband 
                                 , 
                                 2 
                               
                               10 
                             
                           
                         
                         ) 
                       
                     
                     + 
                     
                       CQI 
                       diff 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In this case, the CQI diff  corresponds to the predetermined offset value. 
     In this case, the CQI adjustment unit  14  may calculate the sum of the two CQIs as the dB value. In such a case, the CQI adjustment value “CQI adjust ” is expressed by Equation “CQI adjust =(CQI wideband.1 +CQI wideband.2 )+CQI diff ”. 
     Further, when the rank determined by the rank control unit  17  is “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to the average value of the two CQIs received in the CQI reception unit  11 . 
     Specifically, if the CQI adjustment value is set as “CQI adjust ” and the two CQIs are set as “CQI wideband.1 ” and “CQI wideband.2 ”, the CQI adjustment value “CQI adjust ” is expressed by the following Equation 4. 
     
       
         
           
             
               
                 
                   
                     CQI 
                     adjust 
                   
                   = 
                   
                     
                       10 
                        
                       
                           
                       
                        
                       log 
                        
                       
                           
                       
                        
                       10 
                        
                       
                         ( 
                         
                           
                             
                               10 
                               
                                 
                                   CQIwideband 
                                   , 
                                   1 
                                 
                                 10 
                               
                             
                             + 
                             
                               10 
                               
                                 
                                   CQIwideband 
                                   , 
                                   2 
                                 
                                 10 
                               
                             
                           
                           2 
                         
                         ) 
                       
                     
                     + 
                     
                       CQI 
                       diff 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     In this case, the CQI diff  corresponds to the predetermined offset value. 
     In this case, the CQI adjustment unit  14  may calculate the average value of the two CQIs as the dB value. In such a case, the CQI adjustment value “CQI adjust ” is expressed by Equation “CQI adjust =(CQI wideband.1 +CQI wideband.2 )/2+CQI diff ”. 
     It is noted that, when the rank determined by the rank control unit  17  is “1”, the CQI adjustment unit  14  may use the CQI received in the CQI reception unit  11  as the CQI adjustment value. 
     The scheduling processing unit  12  is configured to perform a scheduling process on the first downlink signal (e.g., the PDSCH signal) based on one CQI or a plurality of CQIs received in the CQI reception unit  11 . 
     The adaptive modulation and demodulation processing unit  13  is configured to perform an adaptive modulation and demodulation process on the first downlink signal (e.g., the PDSCH signal), based on one CQI or a plurality of CQIs received in the CQI reception unit  11 . 
     Specifically, the adaptive modulation and demodulation processing unit  13  is configured to determine a coding rate and the like to be applied to the first downlink signal, based on one CQI or a plurality of CQIs received in the CQI reception unit  11 . 
     In this case, the adaptive modulation and demodulation processing unit  13  may be configured to determine a coding rate, which corresponds to one CQI or a plurality of CQIs received in the CQI reception unit  11 , as a coding rate to be applied to the first downlink signal with reference to the corresponding table illustrated in  FIG. 6 . 
     The adaptive modulation and demodulation processing unit  15  is configured to determine a coding rate and the like to be applied to the second downlink signal, based on the CQI adjustment value calculated by the CQI adjustment unit  14 . 
     Specifically, the adaptive modulation and demodulation processing unit  15  is configured to determine a coding rate and the like to be applied to the second downlink signal, based on the CQI adjustment value calculated by the CQI adjustment unit  14 . 
     For example, the adaptive modulation and demodulation processing unit  15  may be configured to determine a coding rate, which corresponds to the CQI adjustment value calculated by the CQI adjustment unit  14 , as a coding rate to be applied to the second downlink signal, with reference to the corresponding table illustrated in  FIG. 6 . 
     In this case, the corresponding table referred to by the adaptive modulation and demodulation processing unit  13  and the corresponding table referred to by the adaptive modulation and demodulation processing unit  15  may be equal to or different from each other. 
     The transmission power control processing unit  16  is configured to determine the transmission power of the second downlink signal, based on the CQI adjustment value calculated by the CQI adjustment unit  14 . 
     For example, the transmission power control processing unit  16  may be configured to determine transmission power, which corresponds to the CQI adjustment value calculated by the CQI adjustment unit  14 , as the transmission power of the second downlink signal, with reference to the corresponding table illustrated in  FIG. 6 . 
     (Operation of the Mobile Communication System According to the First Embodiment of the Present Invention) 
     With reference to  FIG. 7 , the operation of the mobile communication system according to this embodiment, specifically, an operation of the radio base station eNB according to this embodiment, will be explained. 
     As illustrated in  FIG. 7 , in step S 101 , the radio base station eNB determines whether or not a rank to be applied to the first downlink signal is “1”. 
     When the rank is “1”, in step S 102 , the radio base station eNB performs an adaptive modulation and demodulation process and a transmission power control process on the second downlink signal, based on CQI received from the mobile station UE. 
     Meanwhile, when the rank has a value other than “1”, the radio base station eNB adjusts the CQI received from the mobile station UE in step S 103 , and performs an adaptive modulation and demodulation process and a transmission power control process on the second downlink signal based on the adjusted CQI in step S 104 . 
     So far, in the above-mentioned embodiment, the case in which the rank determined by the rank control unit  17  is “2” has been explained. However, the present invention is not limited thereto. For example, the present invention can also be applied to the case in which the rank determined by the rank control unit  17  has a value equal to or larger than “2”. 
     Further, in the above-mentioned embodiment, the case in which the “wideband CQI” is used as CQI has been explained. However, the present invention is not limited thereto. For example, the present invention can also be applied to the case in which the “subband CQI” is used as CQI. 
     (Operation and Effect of the Mobile Communication System According to the First Embodiment of the Present Invention) 
     In accordance with the mobile communication system according to the first embodiment of the present invention, in the case in which the MIMO scheme is applied, even if a rank to be applied to the first downlink signal has a value other than “1”, it is possible to appropriately perform an adaptive modulation and demodulation process and a transmission power control process on the second downlink signal by adjusting CQI reported from the mobile station UE. 
     The above-mentioned aspects of the embodiment may be expressed as follows: 
     A first aspect of this embodiment is summarized as a radio base station eNB capable of simultaneously transmitting a first downlink signal in which a transmission stream number is adaptively changed, and a second downlink signal, in which a transmission stream number is fixed to 1, to a mobile station UE using an MIMO scheme, including: a CQI reception unit  11  configured to receive CQI (information indicating a reception quality of the first downlink signal) transmitted by the mobile station UE, a CQI adjustment unit  14  configured to calculate a CQI adjustment value based on a rank (the stream number of the first downlink signal simultaneously transmitted), an adaptive modulation and demodulation processing unit  15  and a transmission power control unit  16  configured to perform an adaptive modulation and demodulation process and a transmission power control process on the second downlink signal based on the calculated adjustment value. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2” (when the stream number of the first downlink signal simultaneously transmitted is plural), the CQI adjustment unit  14  may be configured to select any one of the largest CQI and the smallest CQI among received CQIs as the CQI adjustment value. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by summing up received CQIs. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by averaging the received CQIs. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to any one of the received CQIs. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to the sum of the received CQIs. 
     In the first aspect of this embodiment, when the rank has a value equal to or larger than “2”, the CQI adjustment unit  14  may be configured to calculate the CQI adjustment value by adding a predetermined offset value to the average value of the received CQIs. 
     In the first aspect of this embodiment, the first downlink signal is a downlink data signal to which dynamic scheduling is applied and which is transmitted via PDSCH (Physical Downlink Shared Channel). 
     In the first aspect of this embodiment, the second downlink signal may be at least one of a downlink control signal transmitted via PDCCH (physical downlink control channel), a transmission acknowledgement signal transmitted via PHICH (physical HARQ indicator channel), a control format signal transmitted via PCFICH (physical control format indicator channel), and a downlink data signal to which Semi-Persistent Scheduling is applied and which is transmitted via PDSCH (physical downlink shared channel). 
     Note that operation of the above described radio base station eNB and the mobile station UE may be implemented by means of hardware, a software module executed by a processor, or a combination of both. 
     The software module may be provided in any type of storage medium such as an RAM (Random Access Memory), a flash memory, a ROM (Read Only Memory), an EPROM (Erasable Programmable ROM), an EEPROM (Electronically Erasable and Programmable ROM), a register, a hard disk, a removable disk, or a CD-ROM. 
     The storage medium is connected to the processor so that the processor can read and write information from and to the storage medium. Also, the storage medium may be integrated into the processor. Also, the storage medium and the processor may be provided in an ASIC. The ASIC may be provided in the radio base station eNB and the mobile station UE. Also, the storage medium and the processor may be provided in the radio base station eNB and the mobile station UE as a discrete component. 
     Hereinabove, the present invention has been described in detail using the above embodiment; however, it is apparent to those skilled in the art that the present invention is not limited to the embodiment described herein. Modifications and variations of the present invention can be made without departing from the spirit and scope of the present invention defined by the description of the scope of claims. Thus, what is described herein is for illustrative purpose, and has no intention whatsoever to limit the present invention.