Abstract:
A base station apparatus performs radio communications with multiple mobile stations using a multi-input multi-output (MIMO) technique. The base station apparatus includes a scheduler configured to assign a mobile station to a first stream over a first frequency band or a second stream over a second frequency band included in the first frequency band, in accordance with the level of interference of the mobile station with a neighbor cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of International Application PCT/JP2009/070193, filed on Dec. 1, 2009 and designated the U.S., the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present invention relates to a base station apparatus and a communication method in a radio communications network. 
       BACKGROUND 
       [0003]    The increase of mobile stations in radio communications networks has demanded technologies that achieve high throughput in a wide area. 
         [0004]      FIG. 1  is a diagram illustrating a configuration of a radio communications network  10 . While a description is given below, taking an uplink as an example, the same is the case with a downlink. As illustrated in  FIG. 1 , radio base stations BS 1  and BS 2  are located in Cells  1  and  2 , respectively, of the radio communications network  10 . The BS 1  and BS 2  perform communications by receiving radio signals transmitted from mobile stations MS 1  through MS 4  in the cells. 
         [0005]    There is a demand for improvement in the efficiency of use of frequency in order to increase throughput between radio base stations and mobile stations. For example, MIMO (Multi-Input Multi-Output) techniques are used in LTE (Long Term Evolution), which is a standard developed by 3GPP (3rd Generation Partnership Project). 
         [0006]    According to MIMO, a transmitter transmits different information items from multiple transmission antennas in the same frequency band using spatial multiplexing, and a receiver receives transmitted signals with multiple reception antennas. The transmitted signals are received through multiple channels. The throughput may be improved by demodulating the received signals in consideration of the propagation path conditions of the respective channels. 
         [0007]    The MIMO technology includes single-user MIMO and multi-user MIMO.  FIG. 2A  is a diagram for illustrating single-user MIMO. As illustrated in  FIG. 2A , each of the mobile stations MS 1  and MS 2  has multiple antennas, and transmits different data to the base station BS 1  using the same frequency. 
         [0008]      FIG. 2B  is a diagram for illustrating multi-user MIMO. According to multi-user MIMO, the different mobile stations MS 1  through MS 4  transmit signals using the same frequency, and the base station BS 1  receives the transmitted signals using multiple antennas as illustrated in  FIG. 2B , so that MIMO communications may be performed. Multi-user MIMO is applicable and improvement in the efficiency of use of frequency is expected even if the number of antennas of a mobile terminal is one. 
         [0009]      FIG. 3  is a diagram illustrating a frequency band f and transmission power P in multi-user MIMO. This example illustrates the case of 2×2 multi-user MIMO on the uplink. Different mobile stations (for example, MS 1  and MS 2 ) perform transmission in different streams (for example, Streams # 1  and # 2 ) using the same frequency band in the same cell (for example, Cell  1 ), so that the efficiency of use of frequency is improved and the throughput is increased. 
         [0010]    On the other hand, there is a demand for techniques for improving the throughput of a mobile station near a cell boundary in consideration of interference with a neighbor cell. The FFR (Fractional Frequency Reuse) technology is effective for this. This technology assigns a frequency band different from that for a neighbor cell to a mobile station near the cell boundary, thereby preventing interference with the neighbor cell and improving the throughput of the mobile station near the cell boundary. 
         [0011]      FIG. 4  is a diagram illustrating a spatial configuration of a radio communications network  20 . The radio communications network  20  is divided into neighboring Cell  1  and Cell  2 , in which the radio base stations BS 1  and BS 2  are located, respectively. Further, the mobile stations MS 1 , MS 2  . . . are in Cell  1 , MS 1  is in a cell boundary area, and the MS 2  is in an area near BS 1 . Likewise, the mobile stations MS 3 , MS 4  . . . are in Cell  2 , MS 3  is in a cell boundary area, and MS 4  is in an area near BS 2 . 
         [0012]    In the case of directly applying FFR to this radio communications network  20 , the interference between neighboring Cell  1  and Cell  2  may be reduced by assigning frequency bands different from each other to MS 1  and MS 3  in the respective boundary areas. 
       PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
       [0000]    
       
         [Patent Document 1] Japanese Laid-open Patent Publication No. 2007-214993 
         [Patent Document 2] Japanese Laid-open Patent Publication No. 2008-61250 
       
     
       SUMMARY 
       [0015]    According to an aspect of the embodiments, a base station apparatus, which performs radio communications with a plurality of mobile stations using a multiple-input multiple-output (MIMO) technique, includes a scheduler configured to assign a mobile station to a first stream over a first frequency band or a second stream over a second frequency band included in the first frequency band, in accordance with a level of interference of the mobile station with a neighbor cell. 
         [0016]    According to another aspect of the embodiments, a communication method, which performs radio communications between a plurality of mobile stations and a base station apparatus using a multi-input multi-output (MIMO) technique, includes the steps of measuring a level of interference with a neighbor cell in the mobile stations; and assigning, in the radio base station, a mobile station to a first stream over a first frequency band or a second stream over a second frequency band included in the first frequency band, in accordance with the level of interference of the mobile station with the neighbor cell. 
         [0017]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0018]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a diagram illustrating a configuration of a radio communications network; 
           [0020]      FIG. 2A  is a diagram for illustrating single-user MIMO; 
           [0021]      FIG. 2B  is a diagram for illustrating multi-user MIMO; 
           [0022]      FIG. 3  is a diagram illustrating a frequency band and transmission power in multi-user MIMO; 
           [0023]      FIG. 4  is a diagram illustrating a spatial configuration of a radio communications network; 
           [0024]      FIG. 5  is a diagram illustrating a stream configuration in each cell according to an embodiment; 
           [0025]      FIG. 6  is a diagram illustrating a configuration of a radio base station apparatus according to the embodiment; 
           [0026]      FIG. 7  is a diagram illustrating a configuration of a mobile station apparatus according to the embodiment; 
           [0027]      FIG. 8  is a flowchart illustrating a communication method according to the embodiment; and 
           [0028]      FIG. 9  is a flowchart illustrating the details of step  806  of  FIG. 8 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0029]    There is a trade-off between the throughput of all mobile stations and the throughput of a mobile station positioned near a cell boundary. 
         [0030]    If the multi-user MIMO technology is applied in a multi-cell environment, the efficiency of use of frequency increases, while the inter-cell interference due to use of the same frequency band by multiple mobile stations increases. As a result, the signal to interference plus noise ratio (SINR) decreases, thus causing a problem in that the throughput of the mobile stations is significantly reduced compared with an environment where a cell is isolated. 
         [0031]    In order to solve this problem, it is possible to apply FFR, which is a technique that gives consideration to interference with a mobile station near a cell boundary. However, in the case of applying multi-user MIMO, since multiple mobile stations perform communications using the same frequency band in each sector, there is more interference, so that the interference suppression effect due to causing a frequency band assigned to a mobile station near a cell boundary to differ between neighboring cells is reduced. This also prevents the throughput improvement effect from being produced. Therefore, a satisfactory effect is not produced by combining FFR with multi-user MIMO. 
         [0032]    The present invention has an object of providing a base station apparatus and a radio communication method in a radio communications network that make it possible to improve not only the throughput of the mobile stations of the entire cell but also the throughput of a mobile station near a cell boundary by giving consideration to interference with a neighbor cell in the case of applying multi-user MIMO. 
         [0033]    A description is given in detail, with reference to the drawings, of an embodiment of the present invention. 
         [0034]    According to the embodiment, the problem of the interference with a neighbor cell in the case of applying multi-user MIMO is solved by changing the stream configuration of multi-user MIMO. 
         [0035]      FIG. 5  is a diagram illustrating a stream configuration, that is, the relationship between a frequency band f and transmission power P, in each cell according to the embodiment. As illustrated in  FIG. 5 , MS 1  and MS 2  are in Cell  1 , and MS 3  and MS 4  are in Cell  2 , which neighbors Cell  1 . 
         [0036]    In this embodiment, as illustrated in  FIG. 5 , in Cell  1 , MS 2 , which is a mobile station of low interference with other cells (for example, Cell  2 ) (that is, of high SINR) is assigned to Stream # 2 . Further, MS 1 , which is a mobile station of high interference with other cells (that is, of low SINR) is assigned to Stream # 1 . Likewise, in Cell  2 , MS 4 , which is a mobile station of low interference with other cells (for example, Cell  1 ) (that is, of high SINR) is assigned to Stream # 2 . Further, MS 3 , which is a mobile station of high interference with other cells (that is, of low SINR) is assigned to Stream # 1 . 
         [0037]    Thus, by assigning a mobile station of low interference with other cells only to Stream # 2 , it is possible to reduce an increase in interference with other cells due to application of the MIMO technology. This makes it possible to increase the throughput of a mobile station in the peripheral area of a cell. 
         [0038]    Further, in each cell, the entire frequency band is used for Stream # 1 , but only part of the frequency band is used for Stream # 2 . Further, the frequency band used for Stream # 2  is caused to differ between neighboring cells. 
         [0039]    Thus, by reducing an increase in the amount of interference due to application of the MIMO technology by limiting the frequency band used in Stream # 2 , and by using different frequency bands, it is possible to disperse the effect of an increase in interference for a decrease in throughput. 
         [0040]    In the above-described embodiment, the frequency band used in Stream # 2  is caused to differ between neighboring cells. If a cell is constituted of multiple sectors, the frequency band may be caused to differ between neighboring sectors. 
         [0041]      FIG. 6  is a diagram illustrating a configuration of a radio base station apparatus  100  according to the embodiment. In the radio base station apparatus  100 , a signal transmitted from a mobile station (for example, a mobile station apparatus  200  illustrated in  FIG. 7 ) is received by reception antennas  102  to be demultiplexed into a data signal, a control signal, and an other-cell interference signal by a signal demultiplexing part  104 . The data signal is decoded by a data channel (CH) decoding part  106  to be transmitted to a core network  150  of an upper layer. On the other hand, the control signal is decoded by a control channel (CH) decoding part  108  to be transmitted to an uplink scheduler  110  and a downlink scheduler  112  in the radio base station apparatus  100 . Further, the other-cell interference signal, indicating the amount of interference with other cells measured in the mobile station is processed by an other-cell interference receiving part  114  to be transmitted to the uplink scheduler  110  and the downlink scheduler  112 . The uplink scheduler  110  schedules resource allocation, transmission power, etc., on the uplink. 
         [0042]    The downlink scheduler  112  schedules resource allocation, transmission power, etc., on the downlink. A control channel (CH) generating part  116  generates a control signal based on the scheduling results. Further, on the downlink, a data channel (CH) generating part  118  generates a data signal using a signal from the core network  150  based on the scheduling results. The generated control signal and data signal are multiplexed by a signal multiplexing part  120  to be transmitted from a transmission antenna  122 . 
         [0043]    In  FIG. 6 , for an easier understanding of explanations, the reception antennas  102  and the transmission antenna  122  are separately illustrated. However, a reception antenna and a transmission antenna do not always have to be separated. In practice, radio base station apparatuses may use a shared antenna for transmission and reception. 
         [0044]      FIG. 7  is a diagram illustrating a configuration of the mobile station apparatus  200  according to the embodiment. In the mobile station apparatus  200 , a signal transmitted from a base station (for example, the base station apparatus  100  illustrated in  FIG. 6 ) is received by a reception antenna  202  to be demultiplexed into a data signal and a control signal by a signal demultiplexing part  204 . The data signal is application data in the mobile station apparatus  200 , and is decoded by a data channel (CH) decoding part  206  to be used in a data processing part  208 . The control signal is decoded by a control channel (CH) decoding part  210  to be used for data decoding by the data channel (CH) decoding part  206 . Further, an other-cell interference measuring part  212  measures an SIR on the downlink from the control signal decoded by the control channel (CH) decoding part  210 , and transmits the SIR to a CQI generating part  214 . The CQI generating part  214  generates a CQI from the SIR measured by the other-cell interference measuring part  212 . The generated CQI is converted into a control signal by a control channel (CH) generating part  216 . Data from the data processing part  208  are converted into a data signal by a data channel (CH) generating part  218 . The generated control signal and data signal are multiplexed by a signal multiplexing part  220  to be transmitted from a transmission antenna  222 . 
         [0045]    In  FIG. 7 , for an easier understanding of explanations, the reception antennas  202  and the transmission antenna  222  are separately illustrated. However, a reception antenna and a transmission antenna do not always have to be separated. In practice, radio base station apparatuses may use a shared antenna for transmission and reception. 
         [0046]      FIG. 8  is a flowchart illustrating a communication method according to the embodiment. This communication method may be performed by, for example, the uplink scheduler  110  of the radio base station apparatus  100  illustrated in  FIG. 6 . 
         [0047]    First, received data are processed, and it is determined whether to perform retransmission by checking ACK/NACK from a CRC in the signal (step  802 ). Next, it is determined whether there are new data by determining the presence or absence of data to be transmitted and the presence or absence of data to be retransmitted from mobile stations (step  804 ). Then, frequency allocation is performed for mobile stations for which new data exist (step  806 ). A description is given in more detail, with reference to  FIG. 9 , of this step. Next, transmission power and MCS (Modulation and Channel Coding Scheme) are controlled (steps  808  and  810 ), and transmission data are transmitted (step  812 ). 
         [0048]      FIG. 9  is a flowchart illustrating the details of step  806  of  FIG. 8 . 
         [0049]    First, assignment to Stream # 1  is performed. In each frequency band, an instantaneous data rate and an average data rate are measured on a mobile station basis (step  902 ). Then, a mobile station having the largest instantaneous data rate/average data rate value is selected (step  904 ), and is assigned to a corresponding frequency band of Stream # 1  (step  906 ). Steps  902  to  906  are repeated with respect to each sub band. 
         [0050]    Next, assignment to Stream # 2  is performed. First, a frequency band used for Stream # 2  is divided without an overlap of a band to be used with neighbor cells (step  908 ). Next, the amount of interference with other cells of each mobile station is obtained in the base station (step  910 ), and it is determined whether the measured value is less than or equal to a threshold (step  912 ). Mobile stations whose measured values are less than the threshold are determined to be targets of selection (step  914 ). Here, the measured value may be the total amount of interference with mobile stations in other cells. The mobile station may calculate path loss between the mobile station and each of base stations, and inform a base station to communicate with of the value. Therefore, the measurement is performed using the value. The instantaneous data rate of each of target mobile stations is measured in each frequency band (step  916 ). A mobile station having the highest instantaneous data rate is selected (step  918 ), and is assigned to a frequency band of Stream # 2  (step  920 ). Steps  908  through  920  are repeated with respect to each sub band. 
         [0051]    In the above-described embodiment, the measured values are compared with a threshold, but only a mobile station having the smallest measured value may be determined to be a target of selection. 
         [0052]    Next, a description is given of the results of a comparison of the embodiment and the conventional technique based on simulations. The simulations are performed under the conditions illustrated in Tables 1A and 1B. Tables 1A and 1B illustrate simulation data. 
         [0000]    
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
             
           
               
                 TABLE 1A 
               
               
                   
               
               
                 ITEM 
                 CONTENTS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Carrier Frequency 
                 2.0 
                 [GHz] 
               
               
                 Bandwidth 
                 5 
                 [MHz] 
               
             
          
           
               
                 FFR size 
                 512 
               
               
                 Useful subcarrier 
                 300 
               
               
                 Sub Band 
                  3 
               
             
          
           
               
                 Frame Length 
                 1.0 
                 [ms] 
               
             
          
           
               
                 Cell 
                  19 
               
             
          
           
               
                 User 
                 10 
                 [UE/Sector] 
               
             
          
           
               
                 Sector/Cell 
                  3 
               
             
          
           
               
                 Inter-site distance 
                 500 
                 [m] 
               
               
                 Minimum distance between 
                 35 
                 [m] 
               
               
                 UE and cell site 
                   
                   
               
               
                 Max Tx Power 
                 24 
                 [dBm] 
               
             
          
           
               
                 Power Control 
                 Fractional power control 
               
               
                   
                 (α = 0.5, Po = −40) 
               
               
                   
               
               
                 Antenna pattern and gain 
                           A        (   θ   )       =     -     min        [       12          (     θ     θ     3      dB         )     2       ,     A   m       ]             
   θ 3dB  = 70, A m  = 20,14dBi(3 sector) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1B 
               
               
                   
                   
               
               
                   
                 ITEM 
                 CONTENTS 
               
               
                   
                   
               
             
             
               
                   
                 Distance dependent path 
                 128.1 + 37.6*log(R) 
               
               
                   
                 loss 
               
               
                   
                 Penetration loss 
                 20 [dB]  
               
               
                   
                 Shadowing standard 
                 8 [dB] 
               
               
                   
                 deviation 
               
               
                   
                 Shadowing correlation 
                 0.5/1.0 
               
               
                   
                 between cell/sectors 
               
               
                   
                 Antenna configuration 
                 MultiUser MIMO (1, 2) 
               
               
                   
                 (TxAnt, RxAnt) 
               
               
                   
                 UE antenna Gain 
                 0.0 [dB]   
               
               
                   
                 Noise Figure 
                 5 [dB] 
               
               
                   
                 Control Delay 
                 4TTI (4 [ms]) 
               
               
                   
                 Link Mapping 
                 ESM 
               
               
                   
                 HARQ ON or OFF 
                 ON 
               
               
                   
                 HARQ Maximum ReTx Number 
                 6 
               
               
                   
                 HARQ ReTx Interval 
                 5 [frame] 
               
               
                   
                 Fading Channel 
                 TU (fd = 5.5 [Hz]) 
               
               
                   
                 MCS Threshold 
                 BLER = 0.1 
               
               
                   
                 Simulation Length 
                 3000 [frame]*1 [Drop] 
               
               
                   
                   
               
             
          
         
       
     
         [0053]    The results of the above-described simulations are illustrated in Table 2. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Average (Mbps) 
                 Coverage (Mbps) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 MIMO 
                 7.224 
                 0.090 
               
               
                   
                 SIMO 
                 5.381 
                 0.134 
               
               
                   
                 Embodiment 
                 6.392 
                 0.139 
               
               
                   
                   
               
             
          
         
       
     
         [0054]    The simulation results illustrate the following. That is, according to conventional MIMO, while there is an increase in the average (the average throughput of all mobile stations), there is a significant decrease in the coverage (the average throughput of mobile stations near a cell boundary) compared with SIMO. On the other hand, according to the embodiment, the average increase effect due to MIMO is obtained without decreasing the coverage, and it is possible to improve the efficiency of use of frequency while controlling an increase in the amount of interference. 
         [0055]    In the case of applying multi-user MIMO as well, it is possible to improve not only the throughput of the mobile stations of the entire cell but also the throughput of a mobile station near a cell boundary by giving consideration to interference with a neighbor cell. 
         [0056]    All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventors to furthering the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
         [0057]    Further, a description is given above of the case where the maximum number of multiplexed streams is two, but according to an embodiment of the invention, the maximum number of multiplexed streams is not limited to two.