Abstract:
A transmission apparatus which multiplexes control channels for multiple reception apparatuses into an OFDM symbol at the same timing in OFDM downlink radio access includes a pattern generating unit configured to generate a frequency mapping pattern which is specific to the transmission apparatus; and a frequency allocating unit configured to allocate subcarriers to the control channels for the multiple reception apparatuses according to the frequency mapping pattern.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a technical field of radio communications. More specifically, the present invention relates to a transmission apparatus and a communication method used for a communication system in which multicarrier transmission is performed. 
         [0003]    2. Description of the Related Art 
         [0004]    In this technical field, it is becoming more and more important to realize wideband radio access for efficiently performing high speed and large capacity communications. As for downlink channels, a multicarrier scheme, more specifically an Orthogonal Frequency Division Multiplexing (OFDM) scheme, is considered promising from the viewpoint of performing high speed and large capacity communications while effectively suppressing multipath fading. 
         [0005]    As shown in  FIG. 1 , a frequency bandwidth used in the system is divided into multiple resource blocks (divided into three resource blocks in  FIG. 1 ), and each of the resource blocks includes one or more subcarriers. The resource block is also referred to as a frequency chunk or a frequency block. One or more resource blocks are allocated to a mobile station. The technology for dividing a frequency band into multiple resource blocks is described in P. Chow, J. Cioffi, J. Bingham, “A Practical Discrete Multitone Transceiver Loading Algorithm for Data Transmission over Spectrally Shaped Channel”, IEEE Trans. Commun. vol. 43, No. 2/3/4, February/March/April 1995, for example. 
       SUMMARY OF THE INVENTION 
     Problem(s) to be solved by the Invention 
       [0006]    When a frequency bandwidth is divided into multiple resource blocks, multiple control channels (control signals) for multiple scheduled users can be multiplexed into a single subframe.  FIGS. 2A-2C  show examples of multiplexing control channels for multiple users into a single subframe.  FIG. 2A  shows an example of multiplexing control channels for three users (UE 1 , UE 2 , and UE 3 ) into a single OFDM symbol within the subframe. User data are placed (mapped) on shared data channels multiplexed into the subframe.  FIG. 2B  shows an example of multiplexing control channels for three users into two OFDM symbols within the subframe.  FIG. 2C  shows an example of multiplexing control channels for three users into the single subframe. To focus attention on control channels, shared data channels are not illustrated in  FIGS. 2B and 2C . As shown in  FIGS. 2A-2C , the present invention discusses the case where control channels for multiple users are placed within the subframe and these control channels are multiplexed into one or more OFDM symbols at the same timing. 
         [0007]    Since the control channel includes information necessary for modulating the shared data channel, it is desired to improve reception quality on the control channel. However, when transmission power control or transmission beamforming is used, there is a problem in that control channels transmitted from neighboring base stations may cause interference and degrade reception quality on the control channel. Particularly, a mobile station situated at a cell edge may seriously have this problem. 
         [0008]    In view of the aforementioned problem, it is a general object of the invention to improve reception quality on the control channel. 
       Means for solving the Problem  
       [0009]    In one aspect of the present invention, there is provided a transmission apparatus which multiplexes control channels for multiple reception apparatuses into an OFDM symbol at the same timing in OFDM downlink radio access, including: 
         [0010]    a pattern generating unit configured to generate a frequency mapping pattern which is specific to the transmission apparatus; and 
         [0011]    a frequency allocating unit configured to allocate subcarriers to the control channels for the multiple reception apparatuses according to the frequency mapping pattern. 
         [0012]    In another aspect of the present invention, there is provided a communication method in which a transmission apparatus multiplexes control channels for multiple reception apparatuses into an OFDM symbol at the same timing in OFDM downlink radio access, including the steps of: 
         [0013]    generating a frequency mapping pattern which is specific to the transmission apparatus; 
         [0014]    allocating the control channels for the multiple reception apparatuses to subcarriers according to the frequency mapping pattern; and controlling transmission power for the subcarriers. 
       Effect of the Invention  
       [0015]    According to an embodiment of the present invention, reception quality on the control channel can be improved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows an example of dividing a frequency bandwidth into multiple resource blocks. 
           [0017]      FIG. 2A  shows a first example of multiplexing control channels for multiple users into a subframe. 
           [0018]      FIG. 2B  shows a second example of multiplexing control channels for multiple users into a subframe. 
           [0019]      FIG. 2C  shows a third example of multiplexing control channels for multiple users into a subframe. 
           [0020]      FIG. 3  shows interference in the case where base stations perform transmission power control. 
           [0021]      FIG. 4A  shows a first example of FDM-based transmission power control. 
           [0022]      FIG. 4B  shows a second example of FDM-based transmission power control. 
           [0023]      FIG. 4C  shows a third example of FDM-based transmission power control. 
           [0024]      FIG. 5  shows an example of CDM-based transmission power control. 
           [0025]      FIG. 6  shows a combination of FDM-based transmission power control and CDM-based transmission power control. 
           [0026]      FIG. 7  shows interference in the case where base stations perform transmission beamforming. 
           [0027]      FIG. 8  shows a block diagram of a base station in accordance with a first or second embodiment. 
           [0028]      FIG. 9  shows a flowchart of power control in the base station in accordance with the first or second embodiment. 
           [0029]      FIG. 10  shows a block diagram of a mobile station in accordance with a first or second embodiment. 
           [0030]      FIG. 11  shows an approach for achieving orthogonalization of control channels among sectors in the frequency domain. 
           [0031]      FIG. 12  shows an approach for achieving orthogonalization of control channels among sectors in the code domain. 
           [0032]      FIG. 13  shows an approach for using inter-sector FDM-based transmission power control and using CDM-based transmission power control within each sector. 
           [0033]      FIG. 14  shows an approach for using inter-sector FDM-based transmission power control and using FDM-based transmission power control within each sector. 
           [0034]      FIG. 15  shows an approach for using inter-sector CDM-based transmission power control and using CDM-based transmission power control within each sector. 
           [0035]      FIG. 16  shows an approach for using inter-sector CDM-based transmission power control and using FDM-based transmission power control within each sector. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description of Notations  
       [0036]    eNB 1 , eNB 2  base station 
         [0037]    UE 1 , UE 2 , UE 3 , UE 4  mobile station 
         [0038]      10  base station 
         [0039]      101 - 1 ,  101 - 2  pattern generating unit/code multiplying unit 
         [0040]      103 - 1 ,  103 - 2  frequency allocating unit 
         [0041]      105 - 1 ,  105 - 2  power control unit 
         [0042]      107  IFFT unit 
         [0043]      109  CP adding unit 
         [0044]      111  weight multiplying unit 
         [0045]      113  transmission unit 
         [0046]      20  mobile station 
         [0047]      201  reception unit 
         [0048]      203  CP removing unit 
         [0049]      205  FFT unit 
         [0050]      207  demultiplexing unit 
         [0051]      209  pattern/code storing unit 
       BEST MODE OF CARRYING OUT THE INVENTION  
       [0052]    With reference to the accompanying drawings, a description is given below with regard to preferred embodiments of the present invention. 
       First Embodiment  
       [0053]    In a first embodiment, a base station performs transmission power control of signals transmitted to mobile stations, when control channels are arranged as shown in  FIGS. 2A-2C . The transmission power control refers to changing transmission power of signals transmitted to mobile stations in order to improve reception quality at each mobile station. 
         [0054]      FIG. 3  shows transmission power on the frequency axis in the case where base stations perform transmission power control. The base stations are shown as eNB 1  and eNB 2  and mobile stations are shown as UE 1 -UE 4 . When the base station eNB 1  performs transmission power control of signals transmitted to the mobile stations UE 1  and UE 2  which are situated within a cell  1  covered by the base station eNB 1 , the base station eNB 1  decreases transmission power of signals transmitted to the mobile station UE 1  which is situated close to the base station eNB 1 . In addition, the base station eNB 1  increases transmission power of signals transmitted to the mobile station UE 2  which is situated far from the base station eNB 1 . Similarly, when the base station eNB 2  performs transmission power control, the base station eNB 2  decreases transmission power of signals transmitted to the mobile station UE 4  which is situated close to the base station eNB 2 . In addition, the base station eNB 2  increases transmission power of signals transmitted to the mobile station UE 3  which is situated far from the base station eNB 2 . As shown in  FIG. 3 , when subcarriers corresponding to a control channel transmitted from the base station eNB 1  to the mobile station UE 2  coincides with subcarriers corresponding to a control channel transmitted from the base station eNB 2  to the mobile station UE 3 , the control channel for the mobile station UE 2  interferes with the control channel for the mobile station UE 3 , and vice versa. Accordingly, the SIR (signal-to-interference ratio) cannot be improved, even though the base stations eNB 1  and eNB 2  increase transmission power. 
         [0055]    In the first embodiment, each base station uses a frequency mapping pattern which is specific to the base station (cell), in order to solve this problem. This approach is referred to as FDM-based transmission power control. The base station uses the frequency mapping pattern determined in advance for each cell. 
         [0056]    Specifically, each base station uses the frequency mapping pattern which is different from that of other base stations so as to randomize positions (subcarriers) where control channels for the respective mobile stations are placed (mapped), as shown in  FIG. 4A . For example, the base station eNB 1  covering the cell  1  allocates third, fourth, sixth, seventh, tenth, thirteenth, and fourteenth subcarriers to the mobile station UE 1 . Then, the base station eNB 1  allocates the other subcarriers to the mobile station UE 2 . On the other hand, the base station eNB 2  covering the cell  2  allocates first, third, fourth, seventh, ninth, eleventh, and thirteenth subcarriers to the mobile station UE 3 . Then, the base station eNB 2  allocates the other subcarriers to the mobile station UE 4 . This allocation can make portions with a low interference level and portions with a high interference level and reduce interference among subcarriers. 
         [0057]    According to the FDM-based transmission power control shown in  FIG. 4A , transmission power of signals transmitted to a mobile station is at the same level among subcarriers allocated to the mobile station. For example, transmission power of the signals transmitted to the mobile station UE 1  is determined based on average reception quality (for example, SINR (signal-to-interference plus noise ratio)) on the system bandwidth for the mobile station UE 1 . Alternatively, transmission power may be determined for each subcarrier based on reception quality on each subcarrier, as shown in  FIG. 4B . Controlling transmission power for each subcarrier can further reduce interference observed by the mobile station. Alternatively, the base station may group subcarriers into subcarrier groups based on reception quality on each subcarrier and determine transmission power for each subcarrier group based on average reception quality on each subcarrier group, as shown in  FIG. 4C . Alternatively, the base station may group subcarriers into subcarrier groups within close ranges in the frequency domain and determine transmission power for each subcarrier group. In addition, the base station may combine the approach for grouping subcarriers into subcarrier groups based on reception quality with the approach for grouping subcarriers into subcarrier groups within close ranges in the frequency domain. In this manner, the subcarrier groups may be arranged in multiple levels. 
         [0058]    Alternatively, the base station may multiply control channels for the respective mobile stations with orthogonal codes to achive orthogonalization among the mobile stations, instead of using the frequency mapping pattern which is specific to the base station. This approach is referred to as CDM-based transmission power control. 
         [0059]    Specifically, the base station multiplies control channels for the respective mobile stations with orthogonal codes (Walsh codes, Phase shift codes, and the like) to achieve orthogonalization among mobile stations in the code domain, as shown in  FIG. 5 . According to this approach, transmission power of signals transmitted to each mobile station is at the same level among subcarriers. Therefore, this approach can reduce variations in transmission power (interference) among subcarriers. 
         [0060]    As shown in  FIG. 6 , FDM-based transmission power control and CDM-based transmission power control may be combined. It should be noted that  FIGS. 3-5  show multiplexed control channels for two mobile stations and  FIG. 6  shows multiplexed control channels for four mobile stations. 
         [0061]    CDM-based transmission power control has an advantage over FDM-based transmission power control to randomize interference. When control channels to be multiplexed increase in number, however, CDM-based transmission power needs a large spreading factor, and may not maintain orthogonality in the frequency selective fading environment. In other words, CDM-based transmission power has a disadvantage of being vulnerable to interference within the cell. On the other hand, FDM-based transmission power control is tolerant of interference within the cell, because signals among mobile stations do not interfere with each other in the frequency domain. When CDM-based transmission power control and FDM-based transmission power control are combined, interference can be reduced with a small spreading factor. 
       Second Embodiment 
       [0062]    In a second embodiment, a base station performs transmission beamforming of signals transmitted to mobile stations, when control channels are arranged as shown in  FIGS. 2A-2C . The transmission beamforming refers to changing antenna directivity in order to improve reception quality at each mobile station. 
         [0063]      FIG. 7  shows reception power for control channels for respective mobile stations observed by a mobile station UE 2  on the frequency axis in the case where base stations perform transmission beamforming. The base stations are shown as eNB 1  and eNB 2  and the mobile stations are shown as UE 1 -UE 4 . When the base station eNB 1  performs transmission beamforming of signals transmitted to the mobile stations UE 1  and UE 2  which are situated within a cell  1  covered by the base station eNB 1 , the base station eNB 1  changes antenna directivity so as to improve reception quality at the mobile station UE 2  which is situated far from the base station eNB 1 . Similarly, when the base station eNB 2  performs transmission beamforming, the base station eNB 2  changes antenna directivity so as to improve reception quality at the mobile station UE 3  which is situated far from the base station eNB 2 . As shown in  FIG. 7 , when subcarriers corresponding to a control channel transmitted from the base station eNB 1  to the mobile station UE 2  coincides with subcarriers corresponding to a control channel transmitted from the base station eNB 2  to the mobile station UE 3 , the control channel for the mobile station UE 2  interferes with the control channel for the mobile station UE 3 , and vice versa. Accordingly, the effect of transmission beamforming may be reduced. 
         [0064]    In the second embodiment, similar to the first embodiment, each base station uses a frequency mapping pattern which is specific to the base station (cell), in order to solve this problem. This approach is referred to as FDM-based transmission beamforming. The use of the frequency mapping pattern which is specific to the base station can make portions with a low interference level and portions with a high interference level and reduce interference among subcarriers, as is the case with  FIG. 4A . Alternatively, the base station may multiply control channels for the respective mobile stations with orthogonal codes. This approach is referred to as CDM-based transmission beamforming. This approach can reduce variations in interference among subcarriers, as is the case with  FIG. 5 . In addition, FDM-based transmission beamforming and CDM-based transmission beamforming may be combined. 
       Structures of Base Station and Mobile Station in accordance with First or Second Embodiment  
       [0065]    With reference to  FIGS. 8 and 9 , a structure and an operation of a base station  10  are described below. The base station  10  includes pattern generating units/code multiplying units  101 - 1  and  101 - 2 , frequency allocating units  103 - 1  and  103 - 2 , power control units  105 - 1  and  105 - 2 , an IFFT (Inverse Fast Fourier Transform) unit  107 , a CP (Cyclic Prefix) adding unit  109 , a weight multiplying unit  111 , and a transmission unit  113 . Although  FIG. 8  shows the base station  10  including the two pattern generating units/code multiplying units  101 - 1  and  101 - 2 , the two frequency allocating units  103 - 1  and  103 - 2 , and the two power control units  105 - 1  and  105 - 2  for two mobile stations, the base station  10  may include N pattern generating units/code multiplying units  101 , N frequency allocating units  103 , and N power control units  105  for N mobile stations. Alternatively, the base station  10  may use a single pattern generating unit/code multiplying unit  101  and multiple frequency allocating units  103  for multiple mobile stations. 
         [0066]    In the case of FDM-based transmission power control or FDM-based transmission beamforming, the pattern generating unit  101  generates a frequency mapping pattern which is specific to the base station (cell) (S 101 ). Alternatively or in addition, in the case of CDM-based transmission power control or CDM-based transmission beamforming, the pattern generating unit/code multiplying unit  101  multiplies control channels for mobile stations with orthogonal codes to achieve orthogoonalization among the mobile stations (S 103 ). In the case of FDM-based transmission power control or FDM-based transmission beamforming, the frequency allocating unit  103  allocates subcarriers according to the frequency mapping pattern (S 105 ). In the case of CDM-based transmission power control or CDM-based transmission beamforming, the frequency allocating unit  103  may allocate subcarriers (frequencies) sequentially starting from the first mobile station  1 , since the orthogonal codes are multiplied to achieve orthogonalization among the mobile stations (S 107 ). The power control unit  105  controls transmission power based on reception quality at mobile stations (S 109 ). Control channels for the respective mobile stations are multiplexed and transformed into orthogonal multicarrier signals by the IFFT unit  107 . The CP adding unit  109  inserts CPs into the orthogonal muticarrier signals. The weight multiplying unit  111  multiplies the signals with a weight to change antenna directivity based on the positional relationship between the base station and the mobile stations (S 111 ). The transmission unit  113  transmits the signal to the mobile stations. 
         [0067]      FIGS. 8 and 9  show the base station  10  implementing both the first embodiment and the second embodiment. When the base station implements only the first embodiment, the base station  10  may not include the weight multiplying unit  111 . When the base station implements only the second embodiment, the base may not include the power control unit  105 . 
         [0068]    In addition, the base station may notify the mobile stations of the frequency mapping pattern or the orthogonal codes generated by the pattern generating unit/code multiplying unit  101  on a broadcast channel. 
         [0069]      FIG. 10  shows a structure of a mobile station  20  which receives a control channel for the mobile station  20  using the frequency mapping pattern or the orthogonal codes received on the broadcast channel. The mobile station  20  includes a reception unit  201 , a CP removing unit  203 , an FFT unit  205 , a demultiplexing unit  207 , and a pattern/code storing unit  209 . The CP removing unit  203  removes CPs from signals received by the reception unit  201 , and then the FFT unit  205  transforms the signals into the frequency domain. The pattern/code storing unit  209  stores the frequency pattern or the orthogonal codes received on the broadcast channel. The demultiplexing unit  207  retrieves the control channel for the mobile station  20  using the frequency mapping pattern or the orthogonal codes. 
       Third Embodiment  
       [0070]    In a third embodiment, a base station orthogonalizes control channels among sectors, when the base station covers multiple sectors. 
         [0071]      FIG. 11  shows a diagram in which control channels are orthogonalized among sectors in the frequency domain. This approach is referred tows inter-sector FDM-based transmission control. Allocating different subcarriers to control channels in the sectors can orthogonalize the control channels among the sectors. Specifically, when the frequency allocating unit ( 103  in  FIG. 8 ) for a sector  1  allocates subcarriers to control channels, the frequency allocating unit ( 103  in  FIG. 8 ) for a sector  2  does not allocate the same subcarriers to control channels. For example, the base station  10  may include a control unit for controlling the frequency allocating units in this manner among sectors. The control unit controls not to transmit control channels for the sector  2  on the subcarriers to which the control channels for the sector  1  are allocated. 
         [0072]      FIG. 12  shows a diagram in which control channels are orthogonalized among sectors in the code domain. This approach is referred to as inter-sector CDM-based transmission control. Using different orthogonal codes for control channels in the sectors can orthogonalize the control channels among the sectors. Specifically, when the code multiplying unit ( 101  in  FIG. 8 ) for a sector  1  uses orthogonal codes, the code multiplying unit ( 101  in  FIG. 8 ) for a sector  2  does not use the same orthogonal codes to control channels. For example, the base station  10  may include a control unit for controlling the code multiplying units in this manner among sectors. The control unit controls to orthogonalize between the control channels for the sector  1  and the control channels for the sector  2  in the code domain. 
         [0073]    When transmission timings for control channels are synchronized among base stations, control channels can be orthogonalized among base stations, as is the case with  FIGS. 11 and 12  which show control channels orthogonalized among sectors. GPS (Global Positioning System) may be used to synchronize control channels among base stations. 
         [0074]      FIGS. 13-16  show diagrams in which control channels for respective mobile stations are orthogonalized using the combination of the aforementioned approaches.  FIG. 13  corresponds to the combination of inter-sector FDM-based transmission control among sectors and CDM-based transmission power control within each sector.  FIG. 14  corresponds to the combination of inter-sector FDM-based transmission control among sectors and FDM-based transmission power control within each sector.  FIG. 15  corresponds to the combination of inter-sector CDM-based transmission control among sectors and CDM-based transmission power control within each sector.  FIG. 16  corresponds to the combination of inter-sector CDM-based transmission control among sectors and FDM-based transmission power control within each sector. 
         [0075]    According to an embodiment of the present invention, interference among control channels can be reduced and reception quality on the control channel can be improved. 
         [0076]    This international patent application is based on Japanese Priority Application No. 2006-169443 filed on Jun. 19, 2006, the entire contents of which are incorporated herein by reference.