Patent Publication Number: US-9907031-B2

Title: Base station, mobile station, wireless communications system, and wireless communications method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/388,687, filed Dec. 22, 2016 which is a continuation of U.S. patent application Ser. No. 15/066,585, filed in the USPTO on Mar. 10, 2016, now U.S. Pat. No. 9,596,661, issued Mar. 14, 2017, which claims the benefit of International Application PCT/JP2013/076154 filed on Sep. 26, 2013, and designating the U.S., the entire contents of each are incorporated herein by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a base station, a mobile station, a wireless communications system, and a wireless communications method. 
     BACKGROUND 
     In a conventional wireless network, control information is transmitted multiple times so as to receive the control information under an environment with a low signal-to-noise ratio (SNR). Additionally, a communications system exists that is called machine-to-machine (M2M) in which machines such as terminals communicate with each other or machine-type communication (MTC) in which a machine and a server on a network communicate with each other (see, for example, Published Japanese-Translation of PCT Application, Publication Nos. 2012-522427, 2013-524563, and 2013-520100). 
     For example, under an environment with a low signal-to-interference and noise power ratio (SINR), transmission power may be increased (power boost) or transmission may be repeated to expand coverage and maintain a communication link between a base station and a mobile station. If transmission is repeated, the base station notifies the mobile station of information necessary for signal reception such as a transmission repetition count. 
     However, a required transmission repetition count varies depending on the transmission power of the base station and also varies depending on physical channels, which have differing quality requirements. Therefore, the base station notifies the mobile station of information concerning a power offset amount for increasing the transmission power and information concerning the transmission repetition count for each of the physical channels, resulting in a problem of increased information that is to be notified. 
     SUMMARY 
     According to an aspect of an embodiment, a base station includes a transmitting circuit that transmits a first wireless signal to a mobile station by using different physical channels; a receiving circuit that receives a second wireless signal transmitted from the mobile station; an acquiring circuit that acquires reception information indicating a reception state of the first wireless signal at the mobile station, from the second wireless signal received by the receiving circuit; a determining circuit that based on the reception information acquired by the acquiring circuit, determines a repetition count of a third wireless signal repeatedly transmitted from the transmitting circuit; and a lookup table that has an index value associated with repetition counts of the different physical channels. The base station uses the receiving circuit to receive the second wireless signal that includes the reception information from the mobile station. The base station uses the acquiring circuit to acquire the reception information from the second wireless signal that includes the reception information. The base station uses the determining circuit to determine based on the reception information, the repetition count for each physical channel among the different physical channels. The base station acquires from the lookup table, the index value that corresponds to the repetition counts of the different physical channels. The base station uses the transmitting circuit to transmit a fourth wireless signal that includes the index value, to the mobile station and notify the mobile station of the index value. 
     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. 
     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 
         FIG. 1  is a diagram of an example of a wireless communications system according to an embodiment; 
         FIG. 2  is a diagram of a functional configuration of a first example of a base station according to the embodiment; 
         FIG. 3  is a diagram of a flow of signals or data in the base station depicted in  FIG. 2 ; 
         FIG. 4  is a diagram of an example of a hardware configuration of the base station according to the embodiment; 
         FIG. 5  is a diagram of an example of a wireless communications method performed by the base station according to the embodiment; 
         FIG. 6  is diagram of a functional configuration of a first example of a mobile station according to the embodiment; 
         FIG. 7  is a diagram of a flow of signals or data in the mobile station depicted in  FIG. 6 ; 
         FIG. 8  is a diagram of an example of a wireless communications method performed by the mobile station according to the embodiment; 
         FIG. 9  is a diagram of an example of maximum coupling loss between uplink and downlink; 
         FIG. 10  is a diagram of an example of a relation between repetition count and power boost for a required coverage expansion level; 
         FIG. 11  is a diagram of a functional configuration of a second example of the base station according to the embodiment; 
         FIG. 12  is a diagram of a flow of signals or data in the base station depicted in  FIG. 11 ; 
         FIG. 13  is a diagram of an example of a lookup table; 
         FIG. 14  is a diagram of a functional configuration of a second example of the mobile station according to the embodiment; 
         FIG. 15  is a diagram of a flow of signals or data in the base station depicted in  FIG. 14 ; 
         FIG. 16  is a diagram of an example of a wireless connection process sequence in a wireless communications system according to the embodiment; 
         FIG. 17  is a diagram of an example of a sequence of notification of an index value common to channels; and 
         FIG. 18  is an example of a sequence of notification of respective repetition counts for the channels. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a base station, a mobile station, a wireless communications system, and a wireless communications method according to the present invention will be described in detail with reference to the accompanying drawings. In the description of the embodiments hereinafter, identical components are given the same reference numeral and redundant description is omitted herein. Furthermore, the present invention is not limited by the following embodiments. 
       FIG. 1  is a diagram of an example of a wireless communications system according to an embodiment. As depicted in  FIG. 1 , the wireless communications system includes a base station  1  and a mobile station  2 . 
     The base station  1  has a transmitting unit  11 , a receiving unit  12 , an acquiring unit  13 , a determining unit  14 , and a lookup table  15 . The transmitting unit  11  is a base-station transmitting unit; the receiving unit  12  is a base-station receiving unit, the acquiring unit  13  is a base-station acquiring unit; and the lookup table  15  is a base-station lookup table. The units  11  to  14  and the lookup table  15  of the base station  1  will be described in detail in a first example of a base station described later. 
     The mobile station  2  has a transmitting unit  21 , a receiving unit  22 , a generating unit  23 , an acquiring unit  24 , and a lookup table  25 . The transmitting unit  21  is a mobile-station transmitting unit; the receiving unit  22  is a mobile-station receiving unit; the acquiring unit  24  is a mobile-station acquiring unit; and the lookup table  25  is a mobile-station lookup table. The units  21  to  24  and the lookup table  25  of the mobile station  2  will be described in detail in a first example of a mobile station described later. 
     The base station  1  transmits a wireless reference signal to the mobile station  2 . The mobile station  2  receives the wireless reference signal, generates reception information indicating a reception state based on the wireless reference signal, and transmits a wireless signal including the reception information to the base station  1 . As a result, the base station  1  is notified of the reception information. 
     The base station  1  receives the wireless signal that includes the reception information, acquires the reception information from the wireless signal, and for each physical channel among multiple different physical channels, determines a repetition count for a repeatedly transmitted wireless signal, based on the reception information. The repetition counts of the different physical channels correlate with each other. Therefore, for each piece of reception information, the repetition counts of the different channels can be associated with one index value. 
     The base station  1  acquires from the lookup table  15  thereof, an index value that corresponds to the respective repetition counts of the physical channels and transmits a wireless signal including the index value to the mobile station  2 . As a result, the mobile station  2  is notified of the index value. 
     The mobile station  2  receives the wireless signal that includes the index value corresponding to the reception information reported by the mobile station  2  and acquires the index value from the wireless signal. The mobile station  2 , from the lookup table  25  thereof, acquires for each physical channel, the repetition count corresponding to the index value. As a result, the mobile station  2  can acquire the repetition counts of the different physical channels. 
       FIG. 2  is a diagram of a functional configuration of a first example of the base station according to the embodiment.  FIG. 3  is a diagram of a flow of signals or data in the base station depicted in  FIG. 2 . As depicted in  FIGS. 2 and 3 , the base station  1  has the transmitting unit  11 , the receiving unit  12 , the acquiring unit  13 , the determining unit  14 , and the lookup table  15 . 
     The transmitting unit  11  is connected to the lookup table  15  and an antenna  16 . The transmitting unit  11  transmits from the antenna  16  to the mobile station  2 , wireless signals by using different physical channels. The transmitting unit  11  transmits the wireless reference signal from the antenna  16  to the mobile station  2 . The transmitting unit  11  transmits from the antenna  16  to the mobile station  2 , a wireless signal that includes an index value acquired from the lookup table  15 . As a result, the base station  1  notifies the mobile station  2  of the index value. 
     The receiving unit  12  is connected to an antenna  17 . The receiving unit  12  receives wireless signals transmitted from the mobile station  2  via the antenna  17 . 
     The acquiring unit  13  is connected to the receiving unit  12 . The acquiring unit  13  acquires from the wireless signal received by the receiving unit  12 , the reception information indicating a reception state of a wireless signal at the mobile station  2 . Based on the reception information acquired by the acquiring unit  13 , the determining unit  14  determines for the respective physical channels, the repetition counts for wireless signals repeatedly transmitted from the transmitting unit  11 . 
     In the lookup table  15 , different index values are respectively associated with the repetition counts of the different physical channels. From the lookup table  15 , index values are obtained that correspond to the repetition counts determined by the determining unit  14  for the different physical channels. 
       FIG. 4  is a diagram of an example of a hardware configuration of the base station according to the embodiment. As depicted in  FIG. 4 , the base station  1  has a processor  101 , memory  102 , and an interface  103 . The processor  101 , the memory  102 , and the interface  103  may be connected to a bus  104 . 
     The processor  101  processes a program implementing a wireless communications method performed by a base station described later. The acquiring unit  13  and the determining unit  14  may be implemented in this way at the base station  1  depicted in  FIG. 2 . Examples of the processor  101  include a central processing unit (CPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), and a programmable logic device such as a field programmable gate array (FPGA). 
     The memory  102  retains the lookup table  15 . The memory  102  stores a boot program and the program implementing the wireless communications method performed by the base station described later. If the processor  101  is a programmable logic device, the memory  102  may store circuit information of the programmable logic device. 
     The lookup table  15 , various programs, or the circuit information may be stored in a non-volatile area of the memory  102 . The non-volatile area of the memory  102  may be implemented by, for example, read only memory (ROM) such as mask ROM, electrically erasable programmable read only memory (EEPROM), and flash memory. 
     In the memory  102 , a volatile area may be used as a work area of the processor  101 . The volatile area of the memory  102  may be implemented by, for example, random access memory (RAM) such as dynamic random access memory (DRAM) and static random access memory (SRAM). 
     The interface  103  manages the input and output of signals and data between the transmitting unit  11  and the receiving unit  12 . The interface  103  may manage the input and output of data from and to an application not depicted, for example. The receiving unit  12  and the transmitting unit  11  of the base station  1  depicted in  FIG. 2  may be implemented by a processor that processes wireless signals. The processor that processes wireless signals may be provided separately from the processor  101 . 
     The wireless communications method performed by a base station may be performed by the base station depicted in  FIG. 2 . In the description of this example, the wireless communications method performed by a base station is assumed to be performed by the base station  1  of  FIG. 2 . 
       FIG. 5  is a diagram of an example of the wireless communications method performed by the base station according to the embodiment. As depicted in  FIG. 5 , when the wireless communications method is started at the base station  1 , the base station  1  uses the receiving unit  12  to receive from the mobile station  2 , a wireless signal that includes reception information (step S 1 ). The base station  1  uses the acquiring unit  13  to acquire the reception information from the wireless signal (step S 2 ). 
     Subsequently, the base station  1  uses the determining unit  14  to determine for the respective physical channels and based on the reception information, repetition counts for repeated transmission of a wireless signal (step S 3 ). The base station  1  acquires from the lookup table  15 , an index value corresponding to the repetition counts for the respective physical channels determined at step S 3  (step S 4 ). 
     Subsequently, the base station  1  uses the transmitting unit  11  to transmit to the mobile station  2 , a wireless signal including the index value acquired at step S 4  (step S 5 ). As a result, the base station  1  notifies the mobile station  2  of the index value. The base station  1  then terminates a sequence of the wireless communications method. The base station  1  subsequently performs communication through the physical channels with the mobile station  2 , at the repetition count for each physical channel corresponding to the index value in the notification to the mobile station  2 . 
     According to the wireless communications system depicted in  FIG. 1 , the base station  1  depicted in  FIG. 2 , or the wireless communications method depicted in  FIG. 5 , the base station  1  notifies the mobile station  2  of the index value corresponding to the repetition counts of the different physical channels. Therefore, as compared to when the base station  1  notifies the mobile station  2  of the repetition counts of the different channels for each physical channel, the amount of information transmitted from the base station  1  to the mobile station  2  is reduced. Thus, the base station  1  can efficiently notify the mobile station  2  of the information necessary for signal reception by the mobile station  2 . 
       FIG. 6  is diagram of a functional configuration of a first example of the mobile station according to the embodiment.  FIG. 7  is a diagram of a flow of signals or data in the mobile station depicted in  FIG. 6 . As depicted in  FIGS. 6 and 7 , the mobile station  2  has the transmitting unit  21 , the receiving unit  22 , the generating unit  23 , the acquiring unit  24 , and the lookup table  25 . 
     The transmitting unit  21  is connected to the generating unit  23  and an antenna  26 . The transmitting unit  21  transmits wireless signals from the antenna  26  to the base station  1 . The transmitting unit  21  transmits from the antenna  26  to the base station  1 , a wireless signal including reception information generated by the generating unit  23 . As a result, the mobile station  2  notifies the base station  1  of the reception information indicating the reception state of the mobile station  2 . 
     The receiving unit  22  is connected to an antenna  27 . The receiving unit  22  receives via the antenna  27 , a wireless signal transmitted from the base station  1  by using the different physical channels. The receiving unit  22  receives a wireless reference signal via the antenna  27  from the base station  1 . The receiving unit  22  receives via the antenna  27  from the base station  1 , a wireless signal that includes the index value corresponding to the reception information in the notification from the receiving unit  22 . 
     The generating unit  23  is connected to the receiving unit  22 . The generating unit  23  generates based on the wireless reference signal received by the receiving unit  22 , the reception information indicating the reception state of the mobile station  2 . The acquiring unit  24  is connected to the receiving unit  22 . The acquiring unit  24  acquires the index value from the wireless signal received by the receiving unit  22 . 
     In the lookup table  25 , the repetition counts of the different physical channels are associated with different index values, respectively. From the lookup table  25 , the repetition counts corresponding to the index value acquired by the acquiring unit  24  are acquired for each physical channel. 
     Information of the repetition counts for each physical channel acquired from the lookup table  25  is sent from an output terminal  28  connected to the lookup table  25 , to a processing unit that executes a data process based on the information of the repetition counts for each physical channel in the mobile station  2 . For example, the information of the repetition counts for each physical channel may be sent from the output terminal  28  to the transmitting unit  21  or from a signal generating unit generating a wireless signal transmitted from the transmitting unit  21 . 
     The hardware configuration of the mobile station  2  is identical to the hardware configuration of the base station  1  depicted in  FIG. 4 . Therefore, the hardware configuration of the mobile station  2  is not depicted and will not be described. It is noted that in the configuration depicted in  FIG. 4 , the processor  101  processes a program implementing a wireless communications method performed by a mobile station described later. The generating unit  23  and the acquiring unit  24  may be implemented in this way in the mobile station  2  depicted in  FIG. 6 . 
     The lookup table  25  is retained in the memory  102 . The interface  103  manages the input and output of signals and data between the transmitting unit  21  and the receiving unit  22 . The transmitting unit  21  and the receiving unit  22  of the mobile station  2  depicted in  FIG. 6  may be implemented by a processor that processes wireless signals. The processor that processes wireless signals may be provided separately from the processor  101 . 
     The wireless communications method performed by a mobile station may be performed by the mobile station depicted in  FIG. 6 . In the description of this example, the wireless communications method performed by a mobile station is assumed to be performed by the mobile station  2  depicted in  FIG. 6 . 
       FIG. 8  is a diagram of an example of the wireless communications method performed by the mobile station according to the embodiment. As depicted in  FIG. 8 , when the wireless communications method is started at the mobile station  2 , the mobile station  2  uses the receiving unit  22  to receive a wireless reference signal from the base station  1  (step S 11 ). The base station  1  transmits the wireless reference signal before step S 1  in the wireless communications method depicted in  FIG. 5 . 
     Subsequently, the mobile station  2  uses the generating unit  23  to generate reception information indicating the reception state of the mobile station  2 , based on the received wireless reference signal (step S 12 ). The mobile station  2  then uses the transmitting unit  21  to transmit the wireless signal including the reception information generated at step S 12  to the base station  1  (step S 13 ). As a result, the mobile station  2  notifies the base station  1  of the reception state of the mobile station  2 . 
     Subsequently, the mobile station  2  uses the receiving unit  22  to receive from the base station  1 , a wireless signal that includes the index value corresponding to the reception information transmitted at step S 13  (step S 14 ). The mobile station  2  uses the acquiring unit  24  to acquire the index value from the wireless signal (step S 15 ). 
     Subsequently, for each physical channel, the mobile station  2  acquires from the lookup table  25 , repetition counts that correspond to the index value acquired at step S 15  (step S 16 ). The mobile station  2  then terminates a sequence of the wireless communications method. The mobile station  2  subsequently performs communication through the channels with base station  1  by using the repetition counts of the respective physical channels, corresponding to the index value in the notification from the base station  1 . 
     According to the mobile station  2  depicted in  FIG. 6  or the wireless communications method depicted in  FIG. 8 , the mobile station  2  notified of an index value by the base station  1  can acquire from the lookup table  25  and for each physical channel, the repetition counts that correspond to the index value. This means that the base station  1  suffices to notify the mobile station  2  of an index value instead of giving notification of the repetition counts for each channel among the different channels. Therefore, as compared to when the base station  1  notifies the mobile station  2  of a repetition count for each channel, the amount of information transmitted from the base station  1  to the mobile station  2  is reduced. Thus, the base station  1  can efficiently notify the mobile station  2  of the information necessary for signal reception by the mobile station  2 . 
     An example of application to an MTC System will be described. Under an environment with a low SINR, for example, when a mobile station is present in the vicinity of a boundary with an adjacent cell, the SINR may become lower than the value required for communication between a base station and the mobile station. In such a case, as described above, for example, transmission power may be increased (power boost) or transmission may be repeated to expand coverage. 
       FIG. 9  is a diagram of an example of maximum coupling loss between uplink and downlink. A table depicted in  FIG. 9  is introduced in 3GPP TR 36.888 V2.0.2, “Study on provision of low-cost MTC UEs based on LTE.” In this table, MCL (Maximum Coupling Loss) stands for a maximum coupling loss. FDD (Frequency Division Duplex) stands for a frequency division duplex mode. TDD (Time Division Duplex) stands for a time division duplex mode. 
     In the table depicted in  FIG. 9 , a UE (User Equipment, mobile station) has one transmission antenna and two reception antennas. Therefore, MCL (FDD, 2×2 eNB) represents a maximum coupling loss when communication is performed in the frequency division duplex mode between an eNB (evolutional Node B, base station) having two transmission antennas and two reception antennas and a UE having one transmission antenna and two reception antennas. MCL (TDD, 8×8 eNB) represents a maximum coupling loss when communication is performed in the time division duplex mode between an eNB having eight transmission antennas and eight reception antennas and a UE having one transmission antenna and two reception antennas. 
     As depicted in  FIG. 9 , the maximum coupling loss is different for each of the physical channels. For example, in the example depicted in  FIG. 9 , the maximum coupling loss in the case of the frequency division duplex mode is 147.2 dB in PUCCH(1a), 141.7 dB in PRACH, and 140.7 dB in PUSCH. The maximum coupling loss in the case of the frequency division duplex mode is 145.4 dB in PDSCH, 149.0 dB in PBCH, 149.3 dB in SCH, and 146.1 dB in PDCCH(1A). 
     As depicted in  FIG. 9 , the maximum coupling loss differs depending on the mode of duplex operation. For example, in the example depicted in  FIG. 9 , the maximum coupling loss in the case of the time division duplex mode is 149.4 dB in PUCCH(1a), 146.7 dB in PRACH, and 147.4 dB in PUSCH. The maximum coupling loss in the case of the time division duplex mode is 148.1 dB in PDSCH, 149.0 dB in PBCH, 149.3 dB in SCH, and 146.9 dB in PDCCH(1A). The maximum coupling loss MCL is calculated by expression (1), for example.
 
MCL=[actual transmission power]−([effective noise power]+[required value of SINR])
 
     The PUCCH is a physical uplink control channel. The PRACH is a physical random access channel. The PUSCH is a physical uplink shared channel. The PDSCH is a physical downlink shared channel. The PBCH is a physical broadcast channel. The SCH is a synchronization channel. The PDCCH is a physical downlink control channel. 
     When the current SINR value is smaller than the minimum SINR value required for communication through a physical channel, a coverage expansion level is required that reduces the difference therebetween to zero. By suitably selecting a level of increase in transmission power and a transmission repetition count, a required coverage expansion level can be satisfied. If a UE is in a coverage hole, the required coverage expansion level is approximately 0 to 20 dB, for example. 
     The minimum SINR required for communication through each of the physical channels differs with consideration of different reception probabilities according to the purposes of the physical channels. 
     The coverage expansion level required for each of the physical channels is defined as a difference between the current SINR value and the minimum SINR value required for communication through the physical channel and is therefore different for each of the physical channels. The difference between the current SINR value and the minimum SINR value required for communication through a physical channel can be eliminated by increasing the transmission power and/or increasing the transmission repetition count for the physical channel. Therefore, the transmission repetition count for a physical channel varies depending on the transmission power for the physical channel. Thus, if a level of increase in transmission power for each physical channel is determined, the transmission repetition count satisfying the coverage expansion level required for the physical channel is determined for each physical channel. It is noted that the values of the maximum coupling loss are not limited to those depicted in  FIG. 9 . 
       FIG. 10  is a diagram of an example of a relation between the repetition count and the power boost for the required coverage expansion level. In the example depicted in  FIG. 10 , for example, a physical channel PHYCH 1  has a required coverage expansion level of 10 dB and a gain of 10 dB may be achieved by repeating transmission 10 times. For example, a physical channel PHYCH 2  has a required coverage expansion level of 10 dB identical to the PHYCH 1  and a gain of 6 dB may be acquired by repeating transmission four times while a gain of 4 dB is achieved by power boost. 
     For example, a physical channel PHYCH 3  has a required coverage expansion level of 12 dB and a gain of 6 dB may be acquired by repeating transmission four times while a gain of 6 dB is achieved by power boost. It is noted that the values of the required coverage expansion level, the transmission repetition count, and the power boost are not limited to those depicted in  FIG. 10 . 
     The PHYCH 1 , the PHYCH 2 , and the PHYCH 3  may be, for example, a PDSCH, a PDCCH, a PHICH, or an EPDCCH. The PHICH is a physical hybrid-ARQ indicator channel. The EPDCCH is an enhanced physical downlink control channel. 
       FIG. 11  is a diagram of a functional configuration of a second example of the base station according to the embodiment.  FIG. 12  is a diagram of a flow of signals or data in the base station depicted in  FIG. 11 . In this example, description will be taking as an example, a case where coverage is expanded for two physical channels, a PDCCH and a PDSCH, in an MTC system. 
     As depicted in  FIGS. 11 and 12 , a base station  31  has a radio frequency (RF) receiving unit  32 , a cyclic prefix (CP) removing unit  33 , and a fast Fourier transform (FFT) unit  34 . The base station  31  has a PUSCH signal demodulating unit  35 , a determining unit  36 , and a lookup table  37 . The base station  31  has a PDSCH signal generating unit  38 , a PDCCH signal generating unit  39 , a PBCH signal generating unit  40 , an inverse fast Fourier transform (IFFT) unit  41 , a CP adding unit  42 , and an RF transmitting unit  43 . 
     The RF receiving unit  32  is connected to an antenna  44 . The RF receiving unit  32  receives via the antenna  44 , a wireless signal transmitted from a mobile station. The RF receiving unit  32  is an example of a receiving unit. 
     The CP removing unit  33  is connected to the RF receiving unit  32 . The CP removing unit  33  removes a cyclic prefix from the wireless signal received by the RF receiving unit  32 . The FFT unit  34  performs fast Fourier transform for the signal from which the cyclic prefix is removed by the CP removing unit  33 . As a result, a time-domain signal is converted into a frequency-domain signal. 
     The PUSCH signal demodulating unit  35  demodulates a PUSCH signal converted into a frequency-domain signal by the FFT unit  34 . The PUSCH signal demodulating unit  35  demodulates the PUSCH signal and thereby, acquires an SINR value in a notification through the PUSCH signal from the mobile station. The SINR value is an example of reception information indicating the reception state of a mobile station. The PUSCH signal demodulating unit  35  is an example of an acquiring unit. 
     The determining unit  36  determines transmission repetition counts and power offset amounts for increasing the transmission power for the PDCCH and the PDSCH, based on the SINR value acquired by the PUSCH signal demodulating unit  35 . The determining unit  36  can determine the repetition counts and the power offset amounts for the physical channels with a known technique based on the SINR value reported from the mobile station. 
     From the lookup table  37 , an index value is acquired based on the respective repetition counts and power offset amounts for the PDCCH and the PDSCH determined by the determining unit  36 . The index value is common to the PDCCH and the PDSCH. The power offset amount is set for the PDCCH and the PDSCH, respectively. 
     The PDSCH signal generating unit  38  generates a PDSCH signal. The PDSCH signal generating unit  38  generates the PDSCH signal including the index value and the power offset amounts for the PDCCH and the PDSCH. The PDCCH signal generating unit  39  generates a PDCCH signal. The PBCH signal generating unit  40  generates a PBCH signal. 
     The IFFT unit  41  performs inverse fast Fourier transform for the PDSCH signal generated by the PDSCH signal generating unit  38 , the PDCCH signal generated by the PDCCH signal generating unit  39 , or the PBCH signal generated by the PBCH signal generating unit  40 . As a result, a frequency-domain signal is converted into a time-domain signal. 
     The CP adding unit  42  adds a cyclic prefix to the time-domain signal converted by the IFFT unit  41 . The RF transmitting unit  43  is connected to the CP adding unit  42  and an antenna  45 . The RF transmitting unit  43  transmits the wireless signal having the cyclic prefix added by the CP adding unit  42 , from the antenna  45  to the mobile station. The RF transmitting unit  43  is an example of a transmitting unit. 
       FIG. 13  is a diagram of an example of a lookup table. As depicted in  FIG. 13 , the lookup table  37  has 16 records corresponding to 16 index values of 0, 1, 2, . . . , 15, for example. For each record, an SINR value corresponding to the SINR value in the notification from the mobile station is set along with respective repetition count reference values and repetition count correction values for the PDSCH and the PDCCH. 
     In the lookup table  37 , the SINR value corresponding to the SINR value in the notification from the mobile station is preset at the installation stage of the base station  31 . The SINR value corresponding to the SINR value in the notification from the mobile station is acquired from simulation, for example. 
     The repetition count reference value is the repetition count when the power offset amount is 0 dB. In the lookup table  37 , the repetition count reference values are preset at the installation stage of the base station  31 . The repetition count reference values are acquired from simulation, for example. From the repetition count reference values, a record to be used in  FIG. 13  is determined and an index value corresponding thereto can be acquired. 
     The repetition count correction value is a value for correcting the repetition count reference value according to the power offset amount. The repetition count correction value is acquired by, for example, multiplying the repetition count reference value by a correction coefficient derived from the power offset amount. 
     For example, in the example depicted in  FIG. 13 , when the index value is denoted by k, a record of an index value k has SINR k  set as the SINR value corresponding to the SINR value reported by the mobile station. It is noted that k is an integer from 0 to 15. The record of the index value k has RL PDSCH,k  as the repetition count reference value for the PDSCH and RL PDCCH,k  as the repetition count reference value for the PDCCH. 
     The record of the index value k has a calculation expression expressed by, for example, expression (2) set as the repetition count correction value for the PDSCH. The record has a calculation expression expressed by, for example, expression (3) as the repetition count correction value for the PDCCH. It is noted that a PDSCH  and a PDCCH  are correction coefficients for the PDSCH and the PDCCH, respectively.
 
RL PDSCH,k ×a PDSCH   (2)
 
RL PDCCH,k ×a PDCCH   (3)
 
     When the power offset amount for the PDSCH is denoted by PO PDSCH , the correction coefficient a PDSCH  for the PDSCH is expressed by expression (4), for example. When the power offset amount for the PDCCH is denoted by PO PDCCH , the correction coefficient a PDCCH  for the PDCCH is expressed by expression (5), for example. 
     
       
         
           
             
               
                 
                   
                     a 
                     PDSCH 
                   
                   = 
                   
                     10 
                     
                       
                         - 
                         
                           PO 
                           PDSCH 
                         
                       
                       10 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     a 
                     PDCCH 
                   
                   = 
                   
                     10 
                     
                       
                         - 
                         
                           PO 
                           PDCCH 
                         
                       
                       10 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     PO PDSCH  may take on values of 0 dB, 2 dB, 4 dB, and 6 dB, for example. PO PDCCH  may take on values of 0 dB, 2 dB, 4 dB, and 6 dB, for example. The values that PO PDSCH  may take and the values that PO PDCCH  may take may differ. 
     For example, once the repetition counts and the power offset amounts for the physical channels are determined by the determining unit  36 , repetition count correction values can be obtained for the physical channels from the lookup table  37  depicted in  FIG. 13  by using the respective power offset amounts and repetition reference values. 
     The hardware configuration of the base station  31  is the same as the hardware configuration of the base station  1  depicted in  FIG. 4 . Therefore, the hardware configuration of the base station  31  is not depicted and redundant description is omitted. It is noted that in the configuration depicted in  FIG. 4 , the processor  101  processes the program that implements the wireless communications method performed by the base station  31 . The units  33  to  36 ,  38  to  42  excluding the RF receiving unit  32  and the RF transmitting unit  43  may be implemented in this way at the base station  31  depicted in  FIG. 11 . 
     The lookup table  37  is retained in the memory  102 . The interface  103  manages the input and output of signals and data between the RF transmitting unit  43  and the RF receiving unit  32 . The RF transmitting unit  43  and the RF receiving unit  32  of the base station  31  depicted in  FIG. 11  may be implemented by a processor that processes wireless signals. The processor that processes wireless signals may be provided separately from the processor  101 . 
     The wireless communications method performed by the base station  31  is identical to the method depicted in  FIG. 5 , for example. Therefore, redundant description of the method is omitted. It is noted that in the method depicted in  FIG. 5 , the reception information is the SINR value. At step S 3 , the base station  31  uses the determining unit  36  to determine the repetition counts and the power offset amounts. At step S 5 , the base station  31  uses the RF transmitting unit  43  to transmit the wireless signal including the index value and the power offset amounts to the mobile station. 
     According to the base station  31  depicted in  FIG. 11 , the base station  31  notifies the mobile station of an index value common to the repetition counts of the different physical channels. Therefore, as compared to when the base station  31  notifies the mobile station of the repetition counts for each physical channel among the different channels, the amount of information sent from the base station  31  to the mobile station is reduced. Thus, the base station  31  can efficiently notify the mobile station of the information necessary for signal reception by the mobile station. 
     According to the base station  31  depicted in  FIG. 11 , the base station  31  notifies the mobile station of the power offset amount for each channel. This enables the mobile station to obtain the repetition counts of the respective physical channels, based on the index value common to the different physical channels and the power offset amount for each physical channel. 
     According to the base station  31  depicted in  FIG. 11 , the repetition count can be corrected according to the power offset amount based on a calculation expression set in the lookup table  37 . Additionally, the index value can be easily acquired by finding the index value for which the repetition count correction values obtained by using the lookup table  37  matches the repetition counts determined based on the SINR value in the notification from the mobile station. Moreover, in the lookup table  37 , since the SINR value and the repetition count reference value are acquired by, for example, simulation, the lookup table  37  can be easily created. 
       FIG. 14  is a diagram of a functional configuration of a second example of the mobile station according to the embodiment.  FIG. 15  is a diagram of a flow of signals or data in the base station depicted in  FIG. 14 . In this example, description will be taken as an example, a case where coverage is expanded for two physical channels, a PDCCH and a PDSCH, in an MTC system. 
     As depicted in  FIGS. 14 and 15 , the mobile station  51  has an RF receiving unit  52 , a CP removing unit  53 , and an FFT unit  54 . The mobile station  51  has a PDSCH signal demodulating unit  55 , a PDCCH signal demodulating unit  56 , a reference signal demodulating unit  57 , a PBCH signal demodulating unit  58 , a lookup table  59 , and a SINR calculating unit  60 . The mobile station  51  has an RF transmitting unit  61 , a CP adding unit  62 , an IFFT unit  63 , a PRACH signal generating unit  64 , a PUSCH signal generating unit  65 , and a user data buffer  66 . 
     The RF receiving unit  52  is connected to an antenna  67 . The RF receiving unit  52  receives via the antenna  67 , a wireless signal transmitted from a base station. The RF receiving unit  52  is an example of a receiving unit. 
     The CP removing unit  53  is connected to the RF receiving unit  52 . The CP removing unit  53  removes a cyclic prefix from the wireless signal received by the RF receiving unit  52 . The FFT unit  54  performs fast Fourier transform for the signal from which the cyclic prefix is removed by the CP removing unit  53 . As a result, a time-domain signal is converted into a frequency-domain signal. 
     The PDSCH signal demodulating unit  55  demodulates a PDSCH signal converted into a frequency-domain signal by the FFT unit  54 . The PDSCH signal demodulating unit  55  demodulates the PUSCH signal and thereby, acquires an index value and a power offset amount in a notification through a PDSCH signal from the base station. The PDSCH signal demodulating unit  55  is an example of an acquiring unit. 
     The PDCCH signal demodulating unit  56  demodulates a PDCCH signal converted into a frequency-domain signal by the FFT unit  54 . The reference signal demodulating unit  57  demodulates a reference signal converted into a frequency-domain signal by the FFT unit  54 . The PBCH signal demodulating unit  58  demodulates a PBCH signal converted into a frequency-domain signal by the FFT unit  54 . 
     From the lookup table  59 , the repetition count for the PDSCH corresponding to the power offset amount for the PDSCH is acquired based on the index value and the power offset amount for the PDSCH acquired by the PDSCH signal demodulating unit  55 . Additionally, from the lookup table  59 , the repetition count for the PDSCH corresponding to the power offset amount for the PDSCH is acquired based on the index value and the power offset amount for the PDSCH acquired by the PDSCH signal demodulating unit  55 . The lookup table  59  may be the same table as the lookup table  37  depicted in  FIG. 13 . 
     The information of the repetition counts of the respective physical channels acquired from the lookup table  59  and the respective power offset amounts of the physical channels is sent from an output terminal  69  connected to the lookup table  59 , to a processing unit that executes a data process based on this information at the mobile station  51 . 
     For example, the information of the repetition count and the power offset amount for the PDSCH is sent to the PDSCH signal demodulating unit  55 . The PDSCH signal demodulating unit  55  demodulates the PDSCH signal based on the information of the repetition count and the power offset amount for the PDSCH. The information of the repetition count and the power offset amount for the PDCCH is sent to the PDCCH signal demodulating unit  56 . The PDCCH signal demodulating unit  56  demodulates the PDCCH signal based on the information of the repetition count and the power offset amount for the PDCCH. 
     As is the case with the PDSCH and the PDCCH, the information of the respective repetition counts and power offset amounts is acquired for the PRACH and the PUSCH. For example, the information of the repetition count and the power offset amount for the PRACH is sent to the PRACH signal generating unit  64 . The PRACH signal generating unit  64  generates the PRACH signal based on the information of the repetition count and the power offset amount for the PRACH. The information of the repetition count and the power offset amount for the PUSCH is sent to the PUSCH signal generating unit  65 . The PUSCH signal generating unit  65  generates the PUSCH signal based on the information of the repetition count and the power offset amount for the PUSCH. 
     The SINR calculating unit  60  calculates an SINR value based on the reception intensity of the reference signal demodulated by the reference signal demodulating unit  57 . The SINR calculating unit  60  can calculate the SINR value by a known technique based on the reception intensity of the reference signal. The SINR value is an example of the reception information indicating the reception state of the mobile station  51 . The SINR calculating unit  60  is an example of a generating unit. 
     The user data buffer  66  temporarily retains the SINR value acquired by the SINR calculating unit  60 . The PUSCH signal generating unit  65  generates a PUSCH signal including the SINR value stored in the user data buffer  66 . The PRACH signal generating unit  64  generates a PRACH signal. 
     The IFFT unit  63  performs inverse fast Fourier transform for the PRACH signal generated by the PRACH signal generating unit  64  or the PUSCH signal generated by the PUSCH signal generating unit  65 . As a result, a frequency-domain signal is converted into a time-domain signal. 
     The CP adding unit  62  adds a cyclic prefix to the time-domain signal converted by the IFFT unit  63 . The RF transmitting unit  61  is connected to the CP adding unit  62  and an antenna  68 . The RF transmitting unit  61  transmits from the antenna  68  to the base station, the wireless signal having the cyclic prefix added by the CP adding unit  62 . The RF transmitting unit  61  is an example of a transmitting unit. 
     The hardware configuration of the mobile station  51  is identical to the hardware configuration of the base station  1  depicted in  FIG. 4 . Therefore, the hardware configuration of the mobile station  51  is not depicted and redundant description is omitted. It is noted that in the configuration depicted in  FIG. 4 , the processor  101  processes the program implementing the wireless communications method performed by the mobile station  51 . The units  53  to  58 ,  60 ,  62  to  65  except the RF receiving unit  52  and the RF transmitting unit  61  may be implemented in this way in the mobile station  51  depicted in  FIG. 14 . 
     The lookup table  59  is retained in the memory  102 . The user data buffer  66  may be implemented by the memory  102 . The interface  103  manages input and output of signals and data between the RF transmitting unit  61  and the RF receiving unit  52 . The RF transmitting unit  61  and the RF receiving unit  52  of the mobile station  51  depicted in  FIG. 14  may be implemented by a processor processing a wireless signal. The processor processing a wireless signal may be provided separately from the processor  101 . 
     The wireless communications method performed by the mobile station  51  is identical to the method depicted in  FIG. 8 , for example. Therefore, redundant description of the method is omitted. It is noted that in the method depicted in  FIG. 8 , the wireless reference signal is the reference signal and that the reception information is the SINR value. At step S 14 , the mobile station  51  receives the wireless signal including the index value and the power offset amounts of the respective channels. 
     According to the mobile station  51  depicted in  FIG. 14 , when notified of the index value from the base station, the mobile station  51  can acquire for the respective physical channels from the lookup table  59 , the repetition counts corresponding to the index value. This means that the base station can notify the mobile station  51  of the index value instead of giving notification of the repetition counts for each physical channel among the different physical channels. Therefore, as compared to when the base station notifies the mobile station  51  of repetition counts for each physical channel, the amount of information sent from the base station to the mobile station  51  is reduced. As a result, the base station can efficiently notify the mobile station  51  of the information necessary for signal reception by the mobile station  51 . 
     According to the mobile station  51  depicted in  FIG. 14 , the base station notifies the mobile station  51  of the respective power offset amounts for the physical channels. This enables the mobile station  51  to obtain the respective repetition counts of the physical channels based on the index value common to the different physical channels and the respective power offset amounts of the physical channels. 
     According to the mobile station  51  depicted in  FIG. 14 , the repetition count can be corrected according to the power offset amount, based on a calculation expression set in the lookup table  59  for each index value. Additionally, since the repetition count reference value in the lookup table  59  is acquired by, for example, simulation, the lookup table  59  can be easily created. 
       FIG. 16  is a diagram of an example of a wireless connection process sequence in a wireless communications system according to the embodiment. As depicted in  FIG. 16 , it is assumed that the wireless communications system includes a base station, a mobile station A, and a mobile station B. The mobile station A and the mobile station B may have required coverage expansion levels different from each other. For example, the coverage expansion level required for the mobile station A may be 15 dB and the coverage expansion level required for the mobile station B may be 20 dB. 
     When the wireless connection process is started, first, the base station transmits synchronization signals to the mobile station A and the mobile station B (step S 21 ). Subsequently, the base station uses the PBCH signals to transmit system information, SIB (system information block), to the mobile station A and the mobile station B. At this point in time, the base station does not know the coverage expansion levels required for the mobile station A or the mobile station B and therefore, transmits the PBCH signals at the maximum repetition count (step S 22 ). 
     Subsequently, the mobile station A and the mobile station B transmit the PRACH signals to the base station at the maximum repetition count (step S 23 ). Subsequently, the base station transmits cell-specific reference signals (CRSs) to the mobile station A and the mobile station B (step S 24 ). The mobile station A and the mobile station B receive the reference signals and then calculate the SINR values to roughly determine the coverage expansion levels (step S 25 ). The base station transmits the PDCCH signals to the mobile station A and the mobile station B at the maximum repetition count (step S 26 ). 
     Subsequently, the mobile station A and the mobile station B transmit the PUSCH signals including the SINR values to the base station at the maximum repetition count to feed back the SINR values to the base station (step S 27 ). The base station receives the PUSCH signals including the SINR values and then determines the repetition counts and the power offset amounts for channels, for example, the PDCCH and the PDSCH, based on the SINR values (step S 28 ). 
     Subsequently, the base station acquires from the lookup table, an index value common to the channels and power offset values for the respective channels. The base station transmits the PDSCH signals including the index value to the mobile station A and the mobile station B to notify the mobile station A and the mobile station B of the index value (step S 29 ). The base station transmits the PDSCH signals including the power offset amounts of the respective channels to the mobile station A and the mobile station B to notify the mobile station A and the mobile station B of the respective power offset amounts of the channels (step S 30 ). 
     The mobile station A and the mobile station B acquire the respective repetition counts of the channels from the lookup tables, based on the index value and the power offset amounts. Subsequently, communication links are maintained between the base station and the mobile stations A, B based on the repetition counts of the respective channels and the power offset amounts of the respective channels. 
     A reduction in the amount of information sent from the base station to the mobile station will be described.  FIG. 17  is a diagram of an example of a sequence of notification of an index value common to channels.  FIG. 18  is an example of a sequence of notification of respective repetition counts for the channels. In the examples depicted in  FIG. 17  and  FIG. 18 , it is assumed that physical channels PHYCH 1 , PHYCH 2 , PHYCH 3 , and PHYCH 4  exist. 
     As depicted in  FIG. 17 , in the case of notification of an index value common to the channels, a base station transmits a PDSCH signal including the index value to a mobile station (step S 41 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 1  to the mobile station (step S 42 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 2  to the mobile station (step S 43 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 3  to the mobile station (step S 44 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 4  to the mobile station (step S 45 ). 
     On the other hand, as depicted in  FIG. 18 , in the case of notification of the respective repetition counts for the channels, the base station transmits a PDSCH signal including the repetition count for the PHYCH 1  to the mobile station (step S 51 ). The base station transmits a PDSCH signal including the repetition count for the PHYCH 2  to the mobile station (step S 52 ). The base station transmits a PDSCH signal including the repetition count for the PHYCH 3  to the mobile station (step S 53 ). The base station transmits a PDSCH signal including the repetition count for the PHYCH 4  to the mobile station (step S 54 ). 
     Additionally, the base station transmits a PDSCH signal including a power offset amount for the PHYCH 1  to the mobile station (step S 55 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 2  to the mobile station (step S 56 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 3  to the mobile station (step S 57 ). The base station transmits a PDSCH signal including a power offset amount for the PHYCH 4  to the mobile station (step S 58 ). 
     In the examples depicted in  FIG. 17  and  FIG. 18 , for example, the power offset amount is assumed to be any of four values of 0 dB, 2 dB, 4 dB, 6 dB and the repetition count is assumed to be any of 1, 2, . . . , 15, and 16. The index value is assumed to be any of 0, 1, . . . , 14, and 15. In this case, the base station requires an information amount of two bits for notifying the mobile station of a power offset amount. The base station requires an information amount of four bits for notifying the mobile station of a repetition count. The base station requires an information amount of four bits for notifying the mobile station of an index value. 
     Therefore, in the case of notification of the index value depicted in  FIG. 17 , since four bits are required for the notification of the index value and eight bits are required for the notification of the power offset amounts of the four channels, a total of 12 bits are required. In contrast, in the case of notification of the repetition counts for the respective channels depicted in  FIG. 18 , since 16 bits are required for notification of the repetition counts of the four channels and eight bits are required for the notification of the power offset amounts of the four channels, a total of 24 bits are required. 
     Therefore, the notification of the index value reduces the number of bits required for notification by half as compared to the case of notification of the repetition counts for each channel. If the number of channels further increases, the number of bits required for notification of the index value does not change while the number of required bits increases in the case of notification of the repetition counts for each channel and therefore, the sufficient number of bits required for notification of the index value is equal to or less than a half of the number of bits required for notification of the repetition counts for each channel. As described above, the notification of the index value can reduce the number of bits required for notification as compared to the case of notification of the repetition counts for each channel. 
     Although a terminal that wirelessly communicates with a base station is a mobile station in the examples described above, the terminal may be fixed at a certain position without moving. 
     The base station, the mobile station, the wireless communications system, and the wireless communications method provide an effect that a base station can efficiently notify a mobile station of information necessary for signal reception. 
     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 inventor to further 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 and 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.