Abstract:
Provided is a base station for communicating with a mobile station by assigning at least a part of a communication frame including a plurality of wireless communication channels in a frequency direction. The communication frame includes at least one special subframe having a data transmission region available for transmission to the mobile station and a data reception region available for reception from the mobile station, separated from the data transmission region in a time direction. The base station includes an assignment unit  50  for assigning, in the special subframe, the data transmission region for transmission of downlink control information to nearby mobile stations and the data reception region for reception of uplink control information from the nearby mobile stations, and a transmission and reception unit  10  for transmitting the downlink control information in the data transmission region and for receiving the uplink control information in the data reception region.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to base stations and communication methods of base stations. 
       BACKGROUND ART 
       [0002]    A standards body of mobile communication methods, 3GPP, has recently been deliberating about LTE (Long Term Evolution). In the LTE, a frequency band of a system is divided into a plurality of resource blocks (RB), each of which includes one or more (for example, 12) subcarriers. In the LTE, assignment of resource block to a mobile station is performed in every subframe of 1 ms. 
         [0003]    As shown in  FIG. 4 , a communication frame of LTE applicable to TDD consists of 10 subframes. The subframes are categorized into UL subframe available for uplink communication from the mobile station to the base station, DL subframe available for downlink communication from the base station to the mobile station, and special subframe having both a UL region (data reception region) available for the uplink communication and a DL region (data transmission region) available for the downlink communication.  FIG. 5  is a diagram illustrating an exemplary symbol arrangement in the special subframe. As shown in  FIG. 5 , the top 9 symbols in the special subframe are the DL region for the downlink communication and the last 2 symbols across 3 symbols of a guard time are the UL region for the uplink communication. Various configurations of the special subframe, not limited to the symbol arrangement of the  FIG. 5 , are defined as shown in a table in  FIG. 6 . 
         [0004]    Communications between the base station and a mobile station are performed using the subframes of 3 types described above. For example, physical channels such as PDCCH (Physical Downlink Control Channel), PDSCH (Physical Downlink Shared Channel) and the like are mapped on the DL subframes for the downlink communication. Among them, the PDSCH is used for transmission of user data separately transmitted to each mobile station and control information such as paging information and SIB (System Information Block) transmitted simultaneously to nearby mobile stations. In contrast, the UL subframe for the uplink communication is used for transmission of user data from each mobile station to the base station and control information such as random access from the nearby mobile stations to the base station on the physical channels such as PRACH (Physical Random Access Channel) and the like (for example, see 3GPP TS 36.211 (V8.7.0), “Physical Channels and Modulation”, May 2009). 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    The conventional LTE assigns resources without a distinction between communications related to the control information between the base station and nearby mobile stations and communications related to the user data between the base station and each of the mobile stations. That is, the conventional LTE appropriately assigns, for the control information of the downlink and that of the uplink, a DL subframe and a UL subframe arbitrarily selected. 
         [0006]    Such a resource assignment by the LIE causes a problem in introduction of AAS (Adaptive Antenna System). The AAS performs adaptive control on weight of each of a plurality of antenna elements constituent of an array antenna in accordance with a propagation environment, in order to change the directivity of radio waves. An adaptive array base station corresponding to the AAS appropriately controls beam forming, null steering and the like to a desired mobile station, by using antenna weights calculated based on a reference signal transmitted from the mobile station, in downlink transmission. 
         [0007]    It is preferred that the AAS sets a pair of UL subframe and DL subframe and uses the pair for communication between the base station and the mobile station. This is based on that, if receiving the reference signal transmitted in the UL subframe from the mobile station, the base station can calculate an optimum transmission weight from the reference signal and perform the downlink communication in the DL subframe more efficiently. 
         [0008]      FIG. 7  is a diagram illustrating an exemplary resource assignment by the conventional LTE. As shown in  FIG. 7 , a subframe  2  as the UL subframe and a subframe  4  as the DL subframe are paired with each other. Here, although resource blocks  1 - 3  in the subframe  4  are assigned to a mobile station A, only resource blocks  1 - 2  in the subframe  2  are assigned to the mobile station A. In this case, since nearby mobile stations perform random access to the base station in the resource block  3  of the subframe  2 , the base station cannot calculate an optimum transmission weight for the mobile station A in the resource block  3  of the subframe  4 . Therefore, a transmission efficiency of the AAS is deteriorated. 
         [0009]    Similarly,  FIG. 8  is a diagram illustrating an exemplary resource assignment by the conventional LTE. As shown in  FIG. 8 , the subframe  2  as the UL subframe and the subframe  4  as the DL subframe are paired with each other. Here, although resource blocks  4 - 6  in the subframe  2  are assigned to a mobile station B, only resource blocks  4 - 5  in the subframe  4  are assigned to the mobile station B. In this case, since the base station transmits Paging and the SIB to the nearby mobile stations in the resource block  6  of the subframe  4 , a transmission weight based on the reference signal transmitted in the resource block  6  of the subframe  2  from the mobile station is not used. Therefore, the transmission efficiency of the AAS is deteriorated. 
         [0010]    There is therefore a need in the art for a base station and a communication method of a base station capable of assigning resources without deteriorating the transmission efficiency of the AAS. 
       Solution to Problem 
       [0011]    In order to solve the above problems, a base station according to a first aspect of the present invention is a base station communicating with a mobile station by assigning at least a part of a communication frame including a plurality of wireless communication channels in a frequency direction to the mobile station, the communication frame including at least one special subframe having a data transmission region available for transmission to the mobile station and a data reception region available for reception from the mobile station, separated from the data transmission region in a time direction, the base station includes: 
         [0012]    an assignment unit for assigning, in the special subframe, the data transmission region for transmission of downlink control information to a nearby mobile station and assigning the data reception region for reception of uplink control information from the nearby mobile station; and 
         [0013]    a transmission and reception unit for transmitting the downlink control information in the data transmission region and for receiving the uplink control information in the data reception region. 
         [0014]    A second aspect of the present invention is the base station according to the first aspect, wherein, 
         [0015]    if the communication frame includes a plurality of special subframes, 
         [0016]    the assignment unit assigns each data transmission region of the plurality of special subframes for transmission of same downlink control information. 
         [0017]    A third aspect of the present invention is the base station according to the first or second aspect, wherein 
         [0018]    the communication frame further includes an uplink subframe available for uplink communication from the mobile station and a downlink subframe, paired with the uplink subframe, available for downlink communication to the mobile station, 
         [0019]    the base station further includes a calculation unit for calculating a transmission weight for the downlink subframe based on a signal received in the uplink subframe, 
         [0020]    the assignment unit assigns the uplink subframe and the downlink subframe for communication with each mobile station related to user data, and 
         [0021]    the transmission and reception unit transmits the user data to the mobile station by adaptive array control based on the transmission weight. 
         [0022]    Although apparatuses are used as solutions according to the present invention as described above, it is to be understood that the present invention can also be implemented as methods, programs, and storage media storing the programs and hence they are included within a scope of the present invention. 
         [0023]    For example, as a method implementing the present invention, a communication method of a base station, according to a fourth aspect of the present invention, is a method for communicating with a mobile station by assigning at least a part of a communication frame including a plurality of wireless communication channels in a frequency direction to the mobile station, the communication frame including at least one special subframe having a data transmission region available for transmission to the mobile station and a data reception region available for reception from the mobile station, separated from the data transmission region in a time direction, and includes the steps of: 
         [0024]    assigning, in the special subframe, the data transmission region for transmission of downlink control information to a nearby mobile station and assigning the data reception region for reception of uplink control information from the nearby mobile station; and 
         [0025]    transmitting the downlink control information in the data transmission region and receiving the uplink control information in the data reception region. 
         [0026]    A fifth aspect of the present invention is the communication method of the base station according to the fourth aspect, wherein, 
         [0027]    if the communication frame includes a plurality of special subframes, 
         [0028]    at the step of assigning, each data transmission region of the plurality of special subframes is assigned for transmission of the same downlink control information. 
         [0029]    A sixth aspect of the present invention is the communication method of the base station according to the fourth or fifth aspect, wherein 
         [0030]    the communication frame further includes an uplink subframe available for uplink communication from the mobile station and a downlink subframe, paired with the uplink subframe, available for downlink communication to the mobile station, and 
         [0031]    the communication method further includes calculating a transmission weight for the downlink subframe based on a signal received in the uplink subframe, wherein 
         [0032]    at the step of assigning, the uplink subframe and the downlink subframe are assigned for communication with each mobile station related to user data, and 
         [0033]    at the step of transmitting, the user data is transmitted to the mobile station by adaptive array control based on the transmission weight. 
         [0034]    According to the present invention, it is possible to assign resources without deteriorating a transmission efficiency of AAS by assigning a special subframe for communication related to the control information between a base station and nearby mobile stations. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0035]    Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
           [0036]      FIG. 1  is a functional block diagram illustrating a base station according to one embodiment of the present invention; 
           [0037]      FIG. 2  is a diagram illustrating an exemplary resource assignment according to the embodiment of the present invention; 
           [0038]      FIG. 3  is a flowchart illustrating operation of the base station shown in  FIG. 1 ; 
           [0039]      FIG. 4  is a diagram illustrating an exemplary configuration of a communication frame of LTE; 
           [0040]      FIG. 5  is a diagram illustrating an exemplary symbol arrangement of a special subframe; 
           [0041]      FIG. 6  is a diagram illustrating an exemplary configuration of the special subframe; 
           [0042]      FIG. 7  is a diagram illustrating an exemplary resource assignment of a conventional LTE; and 
           [0043]      FIG. 8  is a diagram illustrating another exemplary resource assignment of the conventional LTE. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0044]      FIG. 1  is a diagram illustrating a schematic configuration of an adaptive array base station  1  according to one embodiment of the present invention. The adaptive array base station  1  includes an array antenna ANT, a wireless communication unit (transmission and reception unit)  10 , an AAS processing unit  20  including a weight calculation unit  21  and a weighting unit  22 , a baseband processing unit  30 , a scheduler  40  and a wireless resource assignment unit (assignment unit)  50 . The wireless communication unit  10 , the AAS processing unit  20  and the baseband processing unit  30  may use interface equipment/circuits suitable for LTE, whereas the scheduler  40  and the wireless resource assignment unit  50  may be implemented with suitable processors such as a CPU or the like. The following is a detailed description of each of the units. 
         [0045]    The wireless communication unit  10 , in processing of a reception system, converts a wireless signal with a carrier frequency received by the array antenna ANT into a baseband signal, and outputs the baseband signal to the weight calculation unit  21 . In addition, the wireless communication unit  10 , in processing of a transmission system, converts a baseband signal from the weighting unit  22  into a signal with a carrier frequency and transmits it to a mobile station via the array antenna ANT by adaptive array control. 
         [0046]    In the AAS processing unit  20 , the weight calculation unit  21  for the reception system performs adaptive signal processing on the signal input from the wireless communication unit  10  and outputs the processed signal to the baseband processing unit  30 . In the adaptive signal processing, in particular, the weight calculation unit  21 , by using a reference signal transmitted in a UL subframe from the mobile station and other known information, calculates a transmission weight (phase/amplitude weight of each antenna element) for a DL subframe paired with the UL subframe from phase information obtained for each antenna element of the array antenna ANT and the like, in order to obtain a high transmission gain to the mobile station. On the other hand, the weighting unit  22  for the transmission system weights the signal input from the baseband processing unit  30  with the transmission weight obtained by the weight calculation unit  21 , and outputs the weighted signal to the wireless communication unit  10 . 
         [0047]    The baseband processing unit  30 , in processing of the reception system, demodulates the signal input from the weight calculation unit  21  and outputs results of demodulation to the scheduler  40  separately for each mobile station. In addition, the baseband processing unit  30 , in processing of the transmission system, outputs a symbol stream of data to be transmitted to a mobile station, which is input from the wireless resource assignment unit  50 , to the weighting unit  22 . 
         [0048]    The scheduler  40  determine a mobile station to assign resource blocks, based on received data from each mobile station input from the baseband processing unit  30 . Specifically, the scheduler  40  determines the mobile station to assign the resource blocks, according to a received signal quality of each resource block informed from the mobile station, channel quality information (CQI) or an amount of data to be transmitted. 
         [0049]    The wireless resource assignment unit  50  assigns wireless resources to the mobile station determined by the scheduler  40 . As stated above, the subframes of LTE applicable to TDD are categorized into UL subframe available for uplink communication from a mobile station to the base station, DL subframe available for downlink communication from the base station to the mobile station, and special subframe including both a UL region (data reception region) available for the uplink communication and a DL region (data transmission region) available for the downlink communication. Here, the wireless resource assignment unit  50  assigns the special subframe for the communication related to control information between the base station and nearby mobile stations, such as Paging/SIB of the downlink and PRACH of the uplink. In addition, the wireless resource assignment unit  50  assigns the UL subframe and the DL subframe, paired with each other, for the communication with an individual mobile station related to user data. When assigning a pair of the UL subframe and the DL subframe to a plurality of mobile stations, the wireless resource assignment unit  50  assigns resources such that communication with each mobile station is performed on the same frequency band (resource block) of the UL subframe and the DL subframe. 
         [0050]      FIG. 2  is a diagram illustrating an exemplary resource block assignment by the wireless resource assignment unit  50 . As shown in  FIG. 2 , the DL region of the subframe  1 , which is the special subframe, is allocated to the Paging/SIB, and a part of the UL region is allocated to PRACH. In addition, in the pair of the UL subframe (subframe  2 ) and the DL subframe (subframe  4 ), resource blocks  1 - 3  are assigned for the communication to the mobile station A, and the resource blocks  4 - 6  are assigned for the communication to the mobile station Thereby, upon reception of reference signals in the subframe  2  as the UL subframe transmitted from the mobile station A and the mobile station B, the base station can calculate an optimum transmission weight to the subframe  4  as the DL subframe from the reference signals. Additionally, since the communication related to the control information between the base station and nearby mobile stations can be performed in the special subframe, such communication does not deteriorate AAS communication efficiency. 
         [0051]    The wireless resource assignment unit  50  performs symbol mapping (assignment of amplitude and phase) in accordance with a modulation scheme on the transmission data including the control information to the mobile station and the user data and outputs a generated symbol stream to the baseband processing unit  30 . 
         [0052]      FIG. 3  is a flowchart illustrating operation of the base station  1  shown in  FIG. 1 . Upon reception of wireless signals from a mobile station via the array antenna ANT, the wireless communication unit  10  converts received wireless signals with the carrier frequency into baseband signals, and outputs the baseband signals to the weight calculation unit  21  (step S 101 ). By using the reference signal transmitted from the mobile station and other known information, the weight calculation unit  21  calculates a transmission weight which enables to obtain a high transmission gain to the mobile station, from phase information of each antenna array of the array antenna ANT and the like (step S 102 ). Specifically, the weight calculation unit  21 , based on the reference signal and the like transmitted in the UL subframe from the mobile station, calculates the transmission weight to a DL subframe paired with the UL subframe such that the high transmission gain to the mobile station can be obtained. The baseband processing unit  30  demodulates the signals input from the weight calculation unit  21  and outputs demodulation results to the scheduler  40  separately for each mobile station (step S 103 ). 
         [0053]    The scheduler  40  determines a mobile station to assign resource blocks based on received data from each mobile station input from the baseband processing unit  30  (step S 104 ). The wireless resource assignment unit  50  assigns the wireless resources to the mobile station determined by the scheduler  40  (step S 105 ). Here, the wireless resource assignment unit  50  assigns a special subframe for the communication related to the control information between the base station and the nearby mobile stations. In addition, the wireless resource assignment unit  50  assigns, for the communication with the individual mobile station related to the user data, the UL subframe and the DL subframe paired with each other. 
         [0054]    The wireless resource assignment unit  50  performs symbol mapping, in accordance with the modulation scheme, on the transmission data including the control information to the mobile station and the user data and outputs the generated symbol stream to the baseband processing unit  30  (step S 106 ). The baseband processing unit  30  outputs the symbol stream of the transmission data to the mobile station input from the wireless resource assignment unit  50  to the weighting unit  22  (step S 107 ). The weighting unit  22  weights the signal input from the baseband processing unit  30  with the transmission weight obtained by the weight calculation unit  21 , and outputs the weighted signal to the wireless communication unit  10  (step S 108 ). The wireless communication unit  10  converts the baseband signal from the weighting unit  22  into a signal with a carrier frequency and transmits the signal to the mobile station via the antenna array ANT by the adaptive array control (step S 109 ). 
         [0055]    According to the present embodiment, the wireless resource assignment unit  50  assigns the special subframe for the communication related to the control information between the base station and nearby mobile stations. Accordingly, it prevents the Paging/SIB of the downlink and the PRACH of the uplink from interfering with the user data to the individual mobile station, and thereby enables resource assignment without deteriorating AAS transmission efficiency. In addition, since the special frame is assigned for transmission and reception of the control information, the mobile station near the base station needs only to receive the special subframe at predetermined intervals in Paging, for example. Thereby, it is possible to reduce power consumption of the mobile station. 
         [0056]    In addition, the wireless resource assignment unit  50  assigns a pair of the UL subframe and the DL subframe for the communication with the individual mobile station related to the user data. It thus enables communication with the individual mobile station using an optimum transmission weight, which enhances the AAS transmission efficiency. Moreover, when assigning a pair of the UL subframe and the DL subframe to a plurality of mobile stations, the wireless resource assignment unit  50  assigns the resources such that the communication with each mobile station is performed on the same frequency band (resource block) of the UL subframe and the DL subframe. Thereby, it is possible to enhance the AAS transmission efficiency to the plurality of mobile stations as well. 
         [0057]    Although the present invention is described based on the figures and the embodiment, it is appreciated that those skilled in the art may easily vary or modify in a multiple manner based on disclosure of the present invention. Accordingly, such variation and modification are included in a scope of the present invention. For example, a function or the like of each component or each step can be rearranged avoiding a logical inconsistency, such that a plurality of components or steps are combined or divided. 
         [0058]    For example, if there are a plurality of special subframes in one communication frame, the wireless resource assignment unit  50  may transmit the same downlink control information, such as the Paging/SIB, in each DL region of the plurality of special subframes. Thereby, the base station can transmit the control information to the nearby mobile stations more definitely.