Abstract:
Disclosed are a control signal transmitting method and mobile terminal device capable of switching between a plurality of terminal communication modes having different maximum transmissible power values with a high degree of precision, while suppressing increases in signaling overhead. In a mobile station ( 100 ), a PHR transmission evaluating unit ( 115 ) sends, to a base station ( 200 ), power head room (PHR) information for the SC-FDMA mode or the OFDMA mode during a reporting period, and a maximum transmission power information setting unit ( 101 ) provides notification to the base station ( 200 ) of difference information between the transmission modes prior to the beginning of the reporting period. An increase in signaling overhead can be prevented because only a single set of PHR information among the information for the plurality of terminal transmission modes is reported in this way. Providing notification of difference information between the transmission modes enables the base station ( 200 ) to calculate the PHR of each of the terminal transmission modes without receiving PHR information for all of the terminal transmission modes. This therefore enables highly accurate switching among the terminal transmission modes with appropriate timing.

Description:
TECHNICAL FIELD 
     The present invention relates to a power headroom reporting method of a mobile station that switches between a plurality of mobile station transmission modes having different maximum transmissible power values when transmitting, and a mobile station apparatus that switches between that plurality of mobile station transmission modes when transmitting an uplink signal to a base station. 
     BACKGROUND ART 
     In LTE-Advanced, an improved version of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), hybrid transmission, in which switching is performed between SC-FDMA (Single Carrier-Frequency Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) is performed in an uplink, has been investigated (see Non-Patent Literature 1, for example). 
     An advantage of OFDMA is that more flexible frequency resource allocation is possible than in the case of SC-TDMA, and therefore frequency scheduling gain is obtained. Thus, OFDMA enables throughput performance to be improved. On the other hand, an advantage of SC-FDMA is that PAPR (Peak-to-Average Power Ratio) denoting a ratio of peak to average power of a transmission signal, and CM (Cubic Metric), are smaller than in the case of OFDMA. Consequently, if power amplifiers with the same maximum transmission power specification are used for SC-FDMA and OFDMA, power amplifier back-off necessary for transmitting a transmission signal without distortion can be made smaller in the case of SC-FDMA. Thus, SC-FDMA can increase actually transmissible maximum power, enabling coverage performance to be improved. 
     Hybrid transmission enables the respective above advantages to be obtained by switching adaptively between SC-FDMA and OFDMA according to the communication environment of a mobile station. 
     Investigation has been carried out into having control of switching between SC-FDMA and OFDMA performed by a base station based on power headroom (hereinafter referred to as “PHR”) information indicating a margin of power (possible increase in power) of the transmission power of a mobile station. Non-Patent Literature 1 describes applying OFDMA to a mobile station with a PHR margin because transmission power is low, and applying SC-FDMA to a mobile station with no PHR margin because transmission power is high. 
     The PHR definition and transmitting method investigated in LTE will now be described. With LTE, a mobile station transmits PHR by means of a data channel in order for PHR to be used when a base station performs transmission power control, MCS (Modulation and channel Coding Scheme) control, and transmission bandwidth control. Non-Patent Literature 2 includes a PHR definition and PHR transmission conditions according to equation 1.
 
 PHR= 10 log 10 ( P   MAX )−(10 log 10    M+P   0   +αPL+Δ   MCS +ƒ(Δ i ))  (Equation 1)
 
     Here, PHR denotes power headroom [dB], P MAX  denotes maximum transmission power [mW], M denotes an allocated number of frequency resource blocks, P 0  denotes an offset (a parameter signaled from a base station) [dB], PL denotes a path loss level [dB], α denotes a weighting coefficient for path loss, Δ MCS  denotes an MCS-dependent offset, and f(Δ i ) denotes a transmission power control value subject to closed-loop control. 
     When a mobile station moves, path loss fluctuates, and therefore PHR fluctuates temporally. Consequently, it is necessary for a mobile station to report PHR to a base station at a predetermined period or when a predetermined condition is satisfied. Non-Patent Literature 2 discloses reporting of PHR to a base station by a mobile station if PHR is Y [dB] or below or if path loss changes by X [dB], 
     and also describes reporting of PHR at N-frame intervals (where Y, X, and N are parameters). 
     CITATION LIST 
     Non-Patent Literature 
     
         
         [NPL 1] Panasonic, REV-080007, “Technical proposals and considerations for LTE advanced” 3GPP TSG RAN 1MT Advanced Workshop, Shenzhen, China, Apr. 7-8, 2008 
         [NPL 2] Nokia Siemens Networks, Nokia, R1-081464, “Triggers for Power Headroom Reports in EUTRAN Uplink” 3GPP TSG RAN WG1 Meeting #52bis, Shenzhen, China, 31 Mar.-4 Apr. 2008 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Maximum transmissible power in SC-FDMA and OFDMA does not only differ between the two, but also differs for each mobile station. Therefore, in order for the transmission mode of a mobile station to be switched between SC-FDMA and OFDMA with a high degree of precision, it is necessary for a base station to know both SC-FDMA PHR and OFDMA PHR. 
     Knowledge of SC-FDMA and OFDMA PHRs by a base station can be achieved by having a mobile station report both SC-FDMA and OFDMA PHRs to the base station. 
     However, if both these PHRs are reported, the amount of reporting-related information is doubled compared with a 3GPP LTE system in which only SC-FDMA is employed. Control information overhead increases accordingly, resulting in a problem of degradation of data throughput. 
     Also, if only PHR relating to the current transmission mode is reported in order to prevent an increase in overhead, the base station can only obtain either SC-FDMA PHR or OFDMA PHR in one report. Therefore, in this case, the base station cannot switch between SC-MDMA and OFDMA at appropriate timing based on PHR. 
     It is an object of the present invention to provide a control signal transmitting method and mobile station apparatus that enable switching to be performed between a plurality of mobile station transmission modes having different maximum transmissible power values with a high degree of precision, while suppressing an increase in signaling overhead. 
     Solution to Problem 
     A power headroom reporting method of the present invention is a power headroom reporting method of a mobile station that switches between a plurality of mobile station transmission modes having different maximum transmissible power values when transmitting, and has: a step of transmitting PHR information of one of the plurality of mobile station transmission nodes to the base station in a reporting period; and a step of transmitting difference information relating to the maximum values between mobile station transmission modes to the base station before the reporting period starts. 
     A mobile station apparatus of the present invention is a mobile station apparatus that switches between a plurality of mobile station transmission nodes having different maximum transmissible power values when transmitting an uplink signal to a base station, and has: a reporting section that reports PHR information of one of the plurality of mobile station transmission modes having different maximum transmissible power values to the base station in a reporting period; and a notification section that notifies the base station of difference information relating to the maximum values between mobile station transmission modes before the reporting period starts. 
     Advantageous Effects of Invention 
     According to the present invention, a power headroom reporting method and mobile station apparatus can be provided that enable switching to be performed between a plurality of mobile station transmission modes having different maximum transmissible power values with a high degree of precision, while suppressing an increase in signaling overhead. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a mobile station apparatus according to Embodiment 1 of the present invention; 
         FIG. 2  is a drawing for explaining difference information; 
         FIG. 3  is a drawing provided to explain a power headroom information report format; 
         FIG. 4  is a block diagram showing the configuration of a base station apparatus according to Embodiment 1 of the present invention; 
         FIG. 5  is a drawing provided to explain a communication procedure between a mobile station and base station; 
         FIG. 6  is a drawing showing a maximum transmission power information table and difference information table; and 
         FIG. 7  is a drawing showing a table used for reporting maximum transmission power information and difference information according to Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present inventors have found that a difference in power headroom between a plurality of mobile station apparatuses is attributable only to the maximum transmissible power value of each transmission mode, and, while differing for each mobile station apparatus, is a fixed value in each mobile station apparatus irrespective of the transmission state. The present inventors then have found that, if difference information is reported to a base station beforehand, the base station can calculate power headroom information for all mobile station transmission modes by having only power headroom information for one mobile station transmission mode reported by a mobile station apparatus, and so arrived at the present invention. 
     Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the embodiments, identical configuration elements are assigned the same reference codes, and duplicate descriptions thereof are omitted. 
     Embodiment 1 
       FIG. 1  is a block diagram showing the configuration of mobile station apparatus (hereinafter referred to simply as “mobile station”)  100  according to Embodiment 1 of the present invention. Mobile station  100  is configured to allow support for 3GPP LTE-Advanced. 
     In  FIG. 1 , mobile station  100  has maximum transmission power information setting section  101 , data generation section  102 , switch section  103 , DFT section  104 , S/P (serial/parallel) conversion section  105 , mapping section  106 , IDFT section  107 , CP adding section  108 , RF transmitting section  109 , RF receiving section  110 , demodulation section  111 , transmission mode/scheduling information detection section  112 , path loss measurement section  113 , PHR calculation section  114 , and PHR transmission determination section  115 . 
     Maximum transmission power information setting section  101  sets the maximum transmission power used by mobile station  100  in the cell or system to which it currently belongs. This maximum transmission power is the maximum value of power that it is possible for mobile station  100  to transmit in that cell or system. Maximum transmission power is set according to the transmission frequency, number of antennas, or the like, of mobile station  100 . Maximum transmission power information setting section  101  outputs set maximum transmission power information (power class information) to data generation section  102  and PHR calculation section  114 . 
     If mobile station  100  belongs to a cell or system in which a plurality of mobile station transmission modes having different maximum transmission power can be utilized, maximum transmission power information setting section  101  outputs difference information relating to maximum transmission power between transmission modes to data generation section  102  together with maximum transmission power information. Difference information relating to maximum transmission power between transmission modes is equivalent to difference information relating to power headroom (PHR) between transmission modes. Maximum transmission power information setting section  101  outputs difference information to data generation section  102  so that difference information is transmitted before a power headroom information reporting period described later herein starts. In this way, a base station described later herein is notified of difference information. 
     Here, a plurality of mobile station transmission modes having different maximum transmission power are an SC-FDMA mode and an OFDMA mode. In this case, difference information is the difference between SC-FDMA mode maximum power and OFDMA mode maximum power (see  FIG. 2 ). 
     Data generation section  102  generates data that is transmitted by mobile station  100 , and outputs the generated transmission data to switch section  103 . When PHR information is received from PHR transmission determination section  115  (that is, when PHR information is to be reported to a base station), data generation section  102  includes PHR information in the transmission data as MAC information, as shown in  FIG. 3 . This report format is the same for 3GPP LTE and 3GPP LTE-Advanced. In this way, PHR information is reported to base station  200  described later herein. 
     Also, when data generation section  102  receives maximum transmission power information and difference information from maximum transmission power information setting section  101 , data generation section  102  generates transmission data based on both of these pieces of information. 
     Switch section  103  switches the transmission mode in accordance with a command from base station  200  described later herein. Switch section  103  switches between outputting data output from data generation section  102  to DFT section  104  and outputting this data to S/P conversion section  105  based on a transmission mode information detection result from transmission mode/scheduling information detection section  112 . Specifically, switch section  103  outputs data to DFT section  104  if SC-FDMA mode information is detected as transmission mode command information by transmission mode/scheduling information detection section  112 , and outputs data to S/P conversion section  105  if OFDMA mode information is detected. 
     DFT section  104  executes DFT (Discrete Fourier Transform) processing on data output from switch section  103 , and outputs the data to mapping section  106 . 
     S/P conversion section  105  converts data output from switch section  103  from a serial sequence to a parallel sequence, and outputs this to mapping section  106 . 
     Mapping section  106  maps data output from DFT section  104  or data output from S/P conversion section  105  onto a frequency band scheduled by base station  200  described later herein, and outputs the data to IDFT section  107 . That is to say, each data symbol of data output from DFT section  104  is mapped onto an entire transmission frequency band, while each data symbol of data output from S/P conversion section  105  is mapped onto one individual subcarrier. 
     IDFT section  107  executes IDFT (Inverse Discrete Fourier Transform) processing on a frequency-domain signal output from mapping section  106 , performs conversion to a time-domain signal, and outputs this to CP adding section  108 . 
     CP adding section  108  copies part of the end of a frame of a signal output from IDFT section  107  as a CP (Cyclic Prefix), and adds the CP to the head of the frame. A signal to which a CP has been added is output to RF transmitting section  109 . 
     RF transmitting section  109  executes transmission processing such as D/A conversion, amplification, and up-conversion on a signal output from CP adding section  108 , and transmits the signal to base station  200  described later herein from an antenna. 
     RF receiving section  110  receives a signal transmitted from base station  200  described later herein via the antenna, executes reception processing such as down-conversion and A/D conversion on the received signal, and outputs the signal to demodulation section  111 . 
     Demodulation section  111  performs equalization processing and demodulation processing on a signal output from RF receiving section  110 , and outputs the demodulation result to transmission mode/scheduling information detection section  112 . 
     Transmission mode/scheduling information detection section  112  detects scheduling information specified by base station  200  described later herein from the demodulation result. Scheduling information includes an MCS (Modulation and Coding Scheme), transmission bandwidth, and transmission power control information. The detected scheduling information is output to PHR calculation section  114 . 
     Transmission mode/scheduling information detection section  112  also detects transmission mode command information from base station  200  described later herein from the demodulation result. Transmission mode command information includes a switching-destination transmission mode (that is, here, SC-FDMA mode information or OFDMA mode information, which transmission mode information at the switching destination). The detected transmission mode command information is output to switch section  103  and PHR calculation section  114 . 
     Path loss measurement section  113  measures the reception level of a downlink common pilot signal for which transmission power is known included in a signal output from RF receiving section  110 , and measures a downlink channel path loss level. The measured path loss level is output to PHR calculation section  114 . 
     PHR calculation section  114  finds a data channel transmission power level based on the path loss level output from path loss measurement section  113  and scheduling information output from transmission mode/scheduling information detection section  112 , and calculates PHR using equation 1. Here, PHR calculation section  114  calculates PHR of the currently selected transmission mode. The calculated PHR is output to PHR transmission determination section  115 . 
     Periodically in a reporting period, or when a predetermined condition is satisfied, PHR transmission determination section  115  outputs PHR received from PHR calculation section  114  to data generation section  102 . PHR transmission determination section  115  determines whether or not a predetermined condition for transmitting PHR has been satisfied by performing a relative size comparison between the PHR output from PHR calculation section  114  and a PHR threshold value—that is, by performing a threshold value determination. PHR transmission determination section  115  also determines whether or not a timer value counted from the previous PHR periodic transmission has reached a predetermined value. In this way, PHR information is reported to base station  200  described later herein periodically in a reporting period or when a predetermined condition is satisfied. 
       FIG. 4  is a block diagram showing the configuration of base station apparatus (hereinafter referred to simply as “base station”)  200  according to Embodiment 1 of the present invention. Base station  200  is configured to allow support for 3GPP LTE-Advanced. 
     In  FIG. 4 , base station  200  has RF receiving section  201 , CP removal section  202 , demapping section  204 , IDFT section  205 , P/S conversion section  206 , switch section  207 , data decoding section  208 , PHR detection section  209 , PHR correction section  210 , transmission mode decision section  211 , scheduling information decision section  212 , modulation section  213 , and RF transmitting section  214 . 
     RF receiving section  201  receives a signal transmitted from mobile station  100  via an antenna, executes reception processing such as down-conversion and A/D conversion on the received signal, and outputs the signal to CP removal section  202 . 
     CP removal section  202  removes a CP component of a signal output from RF receiving section  201 , and outputs a signal from which the CP component has been removed to DFT section  203 . 
     DFT section  203  executes DFT processing on a signal output from CP removal section  202 , and outputs a signal that has been converted from the time domain to the frequency domain to demapping section  204 . 
     Demapping section  204  extracts received data from a frequency band scheduled by base station  200  in a frequency-domain signal output from DFT section  203 , and outputs the extracted received data to IDFT section  205  and P/S conversion section  206 . 
     IDFT section  205  executes IDFT processing on received data output from demapping section  204 , performs conversion to a time-domain signal, and outputs this to switch section  207 . 
     P/S conversion section  206  converts received data output from demapping section  204  from a parallel sequence to a serial sequence, and outputs this to switch section  207 . 
     Switch section  207  is switched based on a mobile station  100  transmission mode decided by transmission mode decision section  211 . This switching is performed by means of control by a control section (not shown) based on a transmission mode decided by transmission mode decision section  211 . By means of this switching, data output from IDFT section  205  is output to data decoding section  208  if the current transmission mode is SC-FDMA, and data output from P/S conversion section  206  is output to data decoding section  208  if the current transmission mode is OFDMA. 
     Data decoding section  208  decodes data output from switch section  207 , and outputs the decoded data to PHR detection section  209 . 
     PHR detection section  209  detects PHR information and difference information included in data output from data decoding section  208 , and outputs the detected PHR information and difference information to PHR correction section  210 . 
     By performing correction for PHR information reported from mobile station  100  using difference information, PHR correction section  210  calculates PHR for a transmission mode other than a transmission mode corresponding to that PHR information. Here, if SC-TDMA mode PHR is reported from mobile station  100 , PHR correction section  210  calculates OFDMA mode PHR by subtracting the difference information from that reported PHR. On the other hand, if OFDMA mode PHR is reported from mobile station  100 , PHR correction section  210  calculates SC-FDMA mode PHR by adding the difference information to that reported PHR. 
     Transmission mode decision section  211  decides transmission mode (here SC-FDMA mode or OFDMA mode) switching for a data channel for the next transmission by mobile station  100  based on SC-FDMA mode and OFDMA mode PHRs calculated by PHR correction section  210 , and outputs transmission mode command information specifying the switching-destination transmission mode (the transmission mode to be switched to) to modulation section  213 . 
     Scheduling information decision section  212  decides scheduling information such as a transmission signal MCS, allocated resource, transmission power, and so forth, based on SC-FDMA mode and OFDMA mode PHRs calculated by PHR correction section  210 , together with reception quality information obtained separately. This scheduling information is output to modulation section  213 . 
     Modulation section  213  modulates transmission data, transmission mode command information, and scheduling information, and outputs a modulated signal to RF transmitting section  214 . 
     RF transmitting section  214  executes transmission processing such as D/A conversion, amplification, and up-conversion on a modulated signal output from modulation section  213 , and transmits the signal to mobile station  100  from the antenna. 
     The operation of mobile station  100  and base station  200  having the above configurations will now be described. 
       FIG. 5  is a drawing provided to explain a communication procedure between mobile station  100  and base station  200 . It is assumed that base station  200  can use a plurality of mobile station transmission modes having different maximum transmission power. 
     When mobile station  100  power is switched on (step S 1001 ), mobile station  100  performs a cell search (step S 1002 ). 
     Then, when base station  200  to be accessed is decided, mobile station  100  and base station  200  perform RACH processing in step S 1003 , and perform authentication processing in step S 1004 . 
     When mobile station  100  and base station  200  enter a communicable state, in step S 1005  mobile station  100  transmits a mobile station  100  capability (mobile station capability) report to base station  200 . 
     That is to say, mobile station  100  reports mobile station  100  capability, such as transmission power information, number of antennas, maximum data rate, and so forth. As this transmission power information, maximum transmission power information setting section  101  of mobile station  100  reports maximum transmission power information and difference information (information on the difference between SC-FDMA mode maximum power and OFDMA mode maximum power) to base station  200 . This reporting is performed by each mobile station  100 . Then base station  200  holds maximum transmission power difference information for each mobile station  100 . 
     Here, maximum transmission power information and difference information are defined by tables such as shown in  FIG. 6 , for example. That is to say, maximum transmission power information is classified into a plurality of power classes based on maximum power, and difference information is classified into a plurality of back-off classes based on back-off (difference). Similar tables are also held by base station  200 . 
     Mobile station  100  notifies base station  200  of maximum transmission power information and difference information by means of power class information and back-off class information. This is notified before a PHR reporting period starts. Here, in particular, this notification is performed before mobile station  100  and base station  200  enter a communication start state (for example, active mode (also called connected mode)) in step S 1006 . 
     Then, when a PHR reporting period starts, mobile station  100  starts reporting PHR to base station  200  in step S 1007 . This PHR reporting is performed periodically in a reporting period, or when a predetermined condition is satisfied. 
     When PHR is reported, PHR correction section  210  in base station  200  calculates PHR of both modes for each mobile station  100  using previously acquired difference information. 
     In the above description, a case has been described in which difference information is reported during a communication procedure between mobile station  100  and base station  200  performed when mobile station  100  power is switched on. However, difference information may also be reported at a time other than when power is switched on—for example, when mobile station  100  performs handover. Also, difference information may be transferred from handover-source base station  200  to handover-destination base station  200 . 
     As described above, according to this embodiment, periodically in a reporting period, or when a predetermined condition is satisfied, PHR transmission determination section  115  in mobile station  100  transmits either SC-FDMA mode or OFDMA mode power headroom information to base station  200 , and maximum transmission power information setting section  101  notifies base station  200  of difference information between transmission modes before a power headroom information reporting period starts. 
     By this means, since power headroom information transmitted in a reporting period is power headroom information for one of a plurality of mobile station transmission modes, an increase in uplink signaling overhead compared with a case in which there is one mobile station transmission mode can be prevented. 
     Also, by having base station  200  notified of difference information between transmission modes before a reporting period, base station  200  can calculate power headroom relating to each mobile station transmission mode without receiving power headroom information relating to all mobile station transmission modes. Consequently, base station  200  can switch between mobile station transmission modes with a high degree of precision at appropriate timing, and can also give mobile station  100  a transmission power or transmission bandwidth command according to each mobile station transmission mode. By this means, mobile station  100  can transmit using appropriate transmission parameters at all times, enabling uplink throughput to be improved. 
     Furthermore, since one kind of power headroom information is reported in the same way as with 3GPP LTE, power headroom information can be reported using the same format as with 3GPP LTE. This makes simple system implementation possible. 
     A case has been described above in which PHR of the current transmission mode is reported. However, the present invention is not limited to this, and provision may also be made for SC-FDMA mode PHR to be reported irrespective of the transmission mode. 
     By so doing, since the PHR format (for example, the PHR range and so forth) can be shared with LTE, processing for conversion from PHR to an information bit string performed by mobile station  100  can be simplified. Also, since a PHR report trigger condition (such as PHR being Y [dB] or below, for example) can be shared with LTE, a simple system can be implemented. At this time, when the current transmission mode is OFDM mode, mobile station  100  measures OFDM mode PHR, finds SC-FDMA mode PHR by correcting the found PHR using difference information, and reports this SC-TDMA mode PHR to base station  200 . 
     A case has been described above in which a plurality of mobile station transmission modes are an SC-TDMA mode and OFDM mode. However, the present invention is not limited to this, and, for example, a mode in which a plurality of SC-FDMA signals are transmitted at different frequencies, or a mode in which one SC-FDMA signal is transmitted placed on different frequencies (for example, clustered SC-FDMA or clustered DFT-S-OFDM) may be used rather than an OFDM mode, as long as that mode is a multicarrier transmission mode. For example, if there are a plurality of transmission modes supporting transmission in which the numbers of SC-TDMA signals are one, two, and four as a plurality of mobile station transmission modes having different maximum transmission power, mobile station  100  reports to base station  200  maximum transmission power difference information for the case of two and maximum transmission power difference information for the case of four with respect to maximum transmission power for the case of one. 
     Also, a MIMO transmission mode may be used as one case of a plurality of mobile station transmission modes having different maximum transmission power. Since a plurality of data streams multiplied by different weights are multiplex-transmitted from the antennas, the PAPR or CM increases in the same way as in rnulticarrier transmission. Therefore, a MIMO transmission mode can be treated in the same way as an OFDM mode. Furthermore, a PAPR or CM increase differs for each weight matrix—that is, precoding matrix. Therefore, mobile station  100  may report to base station  200  difference information for MIMO transmission mode maximum transmission power with respect to SC-FDMA (single-antenna transmission) mode maximum transmission power for each precoding matrix. 
     Embodiment 2 
     Embodiment 2 relates to a variation of capability (mobile station capability) reporting. The configurations of a mobile station and base station of Embodiment 2 are the same as those in Embodiment 1. 
       FIG. 7  shows a table used for reporting maximum transmission power information and difference information according to Embodiment 2. 
     In  FIG. 7 , mobile station performance is classified according to a combination of maximum power and back-off. That is to say, when reporting is performed using this table, mobile station (UE) performance information reported to a base station from a mobile station is defined by a combination of maximum power and back-off. Specifically, maximum transmission power information setting section  101  of mobile station  100  uses a class number matching the performance of that station in a report. 
     In the table shown in  FIG. 7 , in the combinations that define mobile station (UE) performance information, a higher power class is combined with a correspondingly higher back-off class, because, generally speaking, the greater the maximum power, the greater is the back-off required. 
     In this way, improbable combinations in which the power class is high and hack-off is small can be excluded. Therefore, the number of classes can be reduced, enabling the number of bits required to represent all the class identification information to be reduced. Thus, the amount of report-related signaling can be reduced. Also, only a class number need be reported, making an increase in types of control information unnecessary as compared with LTE. Therefore, a simple system can be constructed. 
     In the above embodiments, cases have been described by way of example in which the present invention is configured as hardware, but it is also possible for the present invention to be implemented by software. 
     The function blocks used in the descriptions of the above embodiments are typically implemented as LSTs, which are integrated circuits. These may be implemented individually as single chips, or a single chip may incorporate some or all of them. Here, the term LSI has been used, but the terms IC, system LSI, super LSI, and ultra LSI may also be used according to differences in the degree of integration. 
     The method of implementing integrated circuitry is not limited to LSI, and implementation by means of dedicated circuitry or a general-purpose processor may also be used. An FPGA (Field Programmable Gate Array) for which programming is possible after LSI fabrication, or a reconfigurable processor allowing reconfiguration of circuit cell connections and settings within an LSI, may also be used. 
     In the event of the introduction of an integrated circuit implementation technology whereby LSI is replaced by a different technology as an advance in, or derivation from, semiconductor technology, integration of the function blocks may of course be performed using that technology. The application of biotechnology or the like is also a possibility. 
     The disclosure of Japanese Patent Application No. 2008-163278, filed on Jun. 23, 2008, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     A power headroom reporting method and mobile station apparatus of the present invention are useful in enabling switching to be performed between a plurality of mobile station transmission modes having different maximum transmissible power values with a high degree of precision, while suppressing an increase in signaling overhead.