PATENT ABSTRACT
A measurement circuit including: a memory, and a processor coupled to the memory and configured to measure a first signal strength of a received signal within a first frequency band, the received signal including a signal transmitted from a first transmitting apparatus within the first frequency band and a signal transmitted from a second transmitting apparatus within a second frequency band that constitutes a part of the first frequency band, to generate a second signal strength of the signal transmitted from the second transmitting apparatus within the second frequency band, and to generate a third signal strength of the received signal within the second frequency band based on the first signal strength and the second signal strength.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-000703, filed on Jan. 7, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a measurement circuit, a wireless communication device, and a measurement method. 
     BACKGROUND 
     in the related arts, a technology is known by which reception electric field strength that is corrected depending on characteristic change due to variation of temperature and variation of power-supply voltage is detected in a mobile wireless communication device (for example, see Japanese Laid-open Patent Publication No, 7-35809). 
     In recent years, a mobile communication system using orthogonal frequency division multiple access (OFDMA) having high spectrum efficiency has been put to practical use in order to cope with an increase in a data amount of wireless communication. For example, as a mobile phone system, standards of Long Term Evolution (LTE) have been developed in 3rd generation partnership project (3GPP). 
     In LTE, OFDMA is used in a downlink that corresponds to communication from a base station to a wireless terminal, in addition, single carrier-frequency division multiple access (SC-FDMA) is used in an uplink that corresponds to communication from a wireless terminal to a base station. 
     In LTE, when a plurality of base stations exists, handover is used by which a wireless terminal is coupled to an optimal base station. The wireless terminal periodically detects a neighboring cell (cell is the area in which communication with a base station is allowed to be performed) while communicating with a base station to which the wireless terminal is being coupled and reports reception quality of the cell to the base station to which the wireless terminal is being coupled. That procedure is called “Measurement”. The base station that receives the report from the wireless terminal selects an optimal cell from a list of cells and the reception qualities, and executes processing of changing the base station that is a connection destination. That procedure is called “Hand-over”. 
     As the reception quality that is reported to the base station, for example, there is reference signal reception power (RSRP) and reference signal reception quality (RSRQ). The RSRQ is calculated from the RSRP and a received signal strength indicator (RSSI). The RSRP and the RSSI are measured using a received signal of an OFDM symbol that includes a reference signal (RS) that is transmitted in a downlink. 
     For example, in a definition of 3GPP, an RSSI is defined as an average value of the total reception power (serving cell, neighboring cell, thermal noise, and the like) that are observed in OFDM symbols that include RSs on the certain number of resource blocks. As a method of measuring such an RSSI, there is a time domain method of measuring an RSSI in a time domain and a frequency domain method of measuring an RSSI in a frequency domain. 
     SUMMARY 
     According to an aspect of the invention, a measurement circuit includes a memory, and a processor coupled to the memory and configured to measure a first signal strength of a received signal within a first frequency band, the received signal including a signal transmitted from a first transmitting apparatus within the first frequency band and a signal transmitted from a second transmitting apparatus within a second frequency band that constitutes a part of the first frequency band, to generate a second signal strength of the signal transmitted from the second transmitting apparatus within the second frequency band, and to generate a third signal strength of the received signal within the second frequency band based on the first signal strength and the second signal strength. 
     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, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a structure of a measurement circuit according to a first embodiment; 
         FIG. 2  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating an example of a structure of a subframe in a downlink; 
         FIG. 4A  is a diagram illustrating an example of a signal in a time domain; 
         FIG. 4B  is a diagram illustrating an example of a signal in a frequency domain; 
         FIG. 5A  is a diagram illustrating an example of an RSSI that is measured by an RSSI measurement unit; 
         FIG. 5B  is a diagram illustrating an example of an RSSI that is measured in the frequency domain; 
         FIG. 6  is a diagram illustrating an example of a hardware structure of a wireless terminal to which the measurement circuit is applied; 
         FIG. 7  is a diagram illustrating an example of a floe of a signal in the wireless terminal illustrated in  FIG. 6 ; 
         FIG. 8  is a flowchart illustrating an example of an operation of the measurement circuit according to the first embodiment; 
         FIG. 9  is a diagram illustrating a modification of the measurement circuit; 
         FIG. 10  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 9 ; 
         FIG. 11  is a diagram illustrating an example of a structure of a measurement circuit according to a second embodiment; 
         FIG. 12  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 11 ; 
         FIG. 13A  is a diagram illustrating an example of a received signal; 
         FIG. 13B  is a diagram illustrating examples of a frequency response of an analog filter; 
         FIG. 13C  is a diagram illustrating an example of a received signal that is permeated through the analog filter; 
         FIG. 14A  is a diagram illustrating an example of thermal noise that is generated in an analog circuit; 
         FIG. 14B  is a diagram illustrating an example of change in thermal noise due to the analog filter; 
         FIG. 15A  is a diagram illustrating an example of an RSSI that is measured by the RSSI measurement unit; 
         FIG. 15B  is a diagram illustrating an example of an RSSI that is measured in the frequency domain; 
         FIG. 16  is a flowchart illustrating an example of an operation of a wireless terminal according to the second embodiment; and 
         FIG. 17  is a diagram illustrating an example of a measurement result of RSSIs. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the above-described related art, there is a case in which RSRQ may not be measured efficiently. For example, when an RSSI is measured in the frequency domain, a received signal in a time domain is converted into a frequency band at reception timing, so that a computational complexity is increased. On the other hand, when the RSSI is measured in the time domain, power in a band outside of measurement target band is included in the measurement result, so that RSRQ is not calculated accurately. 
     An object of the embodiments discussed herein is to provide a measurement circuit, a wireless communication device, and a measurement method that efficiently measures RSRQ in order to solve the above-described problems in the related art. 
     A measurement circuit, a wireless communication device, and a measurement method according to the embodiments discussed herein are described below in detail with reference to accompanying drawings. 
     First Embodiment 
     Structure of a Measurement Circuit According to a First Embodiment 
       FIG. 1  is a diagram illustrating an example of a structure of a measurement circuit according to a first embodiment.  FIG. 2  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 1 . As illustrated in  FIGS. 1 and 2 , a measurement circuit  100  according to the first embodiment includes an RSSI measurement unit  110 , an RSRP measurement unit  120 , a correction unit  130 , and RSRQ calculation unit  140 . 
     The measurement circuit  100  measures RSRP and RSRQ based on an input received signal. The received signal is, for example, a signal on which multiplexing is performed by orthogonal frequency division multiplexing (OFDM). Various multiplexing schemes may be applied to the received signal in addition to OFDM. 
     &lt;RSSI Measurement Unit  110 &gt; 
     The RSSI measurement unit  110  measures an RSSI based on a received signal that is input to the measurement circuit  100 , in a time domain. For example, the RSSI measurement unit  110  measures average power in OFDM symbols that include RSs out of OFDM symbols in a time domain in a received signal. The RSSI measurement unit  110  outputs the measured RSSI to the correction unit  130 . 
     &lt;RSRP Measurement Unit  120 &gt; 
     The RSRP measurement unit  120  measures RSRP based on a received signal that is input to the measurement circuit  100 . For example, the RSRP measurement unit  120  measures power of an RS that is included in the received signal. Here, the RSRP measurement unit  120  transforms the received signal into a frequency domain by Fast Fourier Transform (FFT), and measures RSRP in the frequency domain on the basis of the transformed signal. Here, the RSRP measurement unit  120  may measure RSRP in a time domain. 
     The RSRP measurement unit  120  outputs the measured RSRP to the correction unit  130  and the RSRQ calculation unit  140 . In addition, the RSRP measurement unit  120  outputs the measured RSRP from the measurement circuit  100 . 
     &lt;Correction Unit  130 &gt; 
     The correction unit  130  corrects the RSSI that is output from the RSSI measurement unit  110  on the basis of the RSRP that is output from the RSRP measurement unit  120 . For example, the correction unit  130  corrects the RSSI in the time domain, which is output from the RSSI measurement unit  110 , by correcting interference power outside an effective bandwidth using the RSRP that is output from the RSRP measurement unit  120 . The correction unit  130  outputs the corrected RSSI to the RSRQ calculation unit  140 . 
     &lt;RSRQ Calculation Unit  140 &gt; 
     The RSRQ calculation unit  140  calculates RSRQ on the basis of the RSSI that is output from the correction unit  130  and the RSRP that is output from the RSRP measurement unit  120 . For example, the RSRQ calculation unit  140  calculates RSRQ by calculating a ratio of the RSSI that is output from the correction unit  130  to the RSRP that is output from the RSRP measurement unit  120 . The RSRQ calculation unit  140  outputs the calculated RSRQ from the measurement circuit  100 . 
     Therefore, output of the RSRP and the RSRQ based on the received signal that is input to the measurement circuit  100  is performed. In addition, the RSRQ is calculated accurately by correcting the RSSI that is measured in the time domain using the RSRP that is measured to be output with the RSRQ, without measuring the RSSI in the frequency domain. Therefore, the RSRQ is calculated accurately while an increase in a computational complexity is suppressed. 
     (Structure of a Subframe in a Downlink) 
       FIG. 3  is a diagram illustrating an example of a structure of a subframe in a downlink. In  FIG. 3 , the horizontal axis indicates a time, and the vertical axis indicates a frequency. A single subframe consists of several OFDM symbols. Nsym is the number of OFDM symbols in a subframe  201 . 
     A single OFDM symbol consists of subcarriers. Nc is the number of subcarriers in an OFDM symbol  202 . A signal that is modulated by quadrature phase shift keying (QPSK), quadrature amplitude modulation (16QAM), 64QAM, or the like is allocated on a subcarrier. In a single radio frame, 10 subframes are included. 
     In addition, in the subframe  201 , RSs that are used to measure a status of a propagation path in the wireless terminal (shaded areas in  FIG. 3 ) are included. For example, in LTE, as illustrated in  FIG. 3 , an RS is allocated for every 6 subcarrier in several OFDM symbols in the subframe. The RS has a certain value known in both the base station and the wireless terminal, so that the wireless terminal may measure the status of the propagation path by comparing the RS that is included in the received signal. 
     The set of 12 subcarriers in a subframe is called “resource block” (RB). RB is the minimum unit of user data allocation. 
     (Signal in the Time Domain) 
       FIG. 4A  is a diagram illustrating an example of a signal in the time domain. In  FIG. 4A , the horizontal direction indicates a time, and the vertical direction indicates strength. A signal  310  illustrated in  FIG. 4A  indicates a signal in the time domain. The base station transforms a signal in the frequency domain into the signal  310  in the time domain, for example, by inverse fast fourier transform (IFFT) and performs wireless transmission on the transformed signal. The wireless terminal receives the signal  310  in the time domain, which is transmitted from the base station. 
     (Signal in the Frequency Domain) 
       FIG. 4B  is a diagram illustrating an example of a signal in the frequency domain. In  FIG. 4B , the horizontal direction indicates a frequency, and the vertical direction indicates strength. A signal  320  illustrated in  FIG. 4B  indicates a signal in the frequency domain. The wireless terminal transforms the signal  310  in the time domain, which is received from the base station, into the signal  320  in the frequency domain, for example, by FFT, and executes reception processing for the transformed signal. 
     The measurement circuit  100  may obtain “dl-Bandwidth” that is a number of RBs in the system bandwidth of a cell (serving cell, which corresponds to a serving base station) in communication and “allowedMeasBandwidth” that is a maximum number of RBs in the bandwidth in which measurement is performed. Hereinafter, “dl-Bandwidth” is referred to as a system bandwidth Ndl, and “allowedMeasBandwidth” is referred to as a measurement bandwidth Nmeas. In this application, a system band having the system bandwidth Ndl may be referred to as a first frequency band. Moreover, a measurement band having the measurement bandwidth Nmeas may be referred to as a second frequency band. 
     Each of the system bandwidth Ndl and the measurement bandwidth Nmeas becomes, for example, any of 6RB, 15RB, 25RB, 50RB, 75RB, and 100RB. For example, in a case in which “system bandwidth Ndl&gt;measurement bandwidth Nmeas” is satisfied, when the measurement is performed by the frequency domain method, an effective bandwidth of a neighboring cell is merely measured accurately. In addition, when the measurement is performed by the time domain method, interference power outside the effective bandwidth that is specified by the measurement bandwidth Nmeas is also included in the measurement. 
     The system bandwidth of the neighboring cell (which corresponds to one of candidates of a target base station for handover performed by the wireless terminal) is obtained, for example, by decoding a physical broadcast channel (PBCH). Here, it takes a long time to execute the decoding processing, so that decoding of the PBCH may not be executed when measurement of the neighboring cell is performed. In this case, the measurement may be performed using the measurement bandwidth Nmeas that is notified from the base station for each center frequency. 
     (RSSI that is Measured by the RSSI Measurement Unit) 
       FIG. 5A  is a diagram illustrating an example of an RSSI that is measured by the RSSI measurement unit. In  FIG. 5A , the horizontal direction indicates a frequency. The system bandwidth Ndl and the measurement bandwidth Nmeas illustrated in  FIG. 5A  respectively correspond to “dl-Bandwidth” and “allowedMeasBandwidth” that are notified from the base station. 
     RSSI  400  illustrated in  FIG. 5A  is an RSSI in the time domain, which is measured by the RSSI measurement unit  110 . As illustrated in  FIG. 5A , in the RSSI  400  that measured in the time domain, signal power  410  of a measurement target cell and interference power  420  from a serving cell are included. In addition, in the RSSI  400  that is measured in the time domain, interference power in the whole system bandwidth Ndl is included. Therefore, in the RSSI  400 , interference power outside the measurement bandwidth Nmeas is also included. 
     The size of the signal power  410  is represented as “S”, and the size of the interference power  420  is represented as “I”. In this case, the size of the RSSI  400  in the time domain, which is measured by the RSSI measurement unit  110  is represented, for example, by the following formula (1). In this application, the “RSSI” may be referred to as a first signal strength. Moreover, the “S” may be referred to as a second signal strength. 
     [Mathematical Expression 1]
 
RSSI= S+I  
 
∴ I =RSSI− S   (1)
 
     (RSSI that is Measured in the Frequency Domain) 
       FIG. 5B  is a diagram illustrating an example of an RSSI that is measured in the frequency domain. In  FIG. 5B , the same reference numerals are given to portions that are similar to those of  FIG. 5A , and the description is omitted herein. When it is assumed that the received signal is transformed into the frequency domain by FFT and the RSSI is measured, as illustrated in  FIG. 5B , an RSSI  401  of the merely measurement bandwidth Nmeas is measured. Interference power  421  illustrated in  FIG. 5B  is interference power that is included in the measurement bandwidth Nmeas. The size of the interference power  421  illustrated in  FIG. 5B  is represented as “Ia”. 
     When the size of the RSSI  401  is represented as “RSSIa”, “RSSIa” is represented, for example, by the following formula (2). That is, the interference power “I” that is measured in the time domain (interference power  420  illustrated in  FIG. 5A ) is converted by a ratio of the system bandwidth Ndl to the measurement bandwidth Nmeas, and “RSSIa” may be calculated by adding the converted interference power to the signal power S of the measurement target cell. In this application, the “RSSIa” may be referred to as a third signal strength. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     RSSIa 
                     = 
                     
                       S 
                       + 
                       
                         
                           Nmeas 
                           Ndl 
                         
                         ⁢ 
                         I 
                       
                     
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     When the above-described formula (1) is substituted into the above-described formula (2), the size “RSSIa” of the RSSI  401  of the merely measurement bandwidth Nmeas may be represented by the following formula (3). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         RSSIa 
                         = 
                           
                         ⁢ 
                         
                           S 
                           + 
                           
                             
                               Nmeas 
                               Ndl 
                             
                             ⁢ 
                             
                               ( 
                               
                                 RSSI 
                                 - 
                                 S 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
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                               Nmeas 
                               · 
                               RSSI 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   Ndl 
                                   - 
                                   Nmeas 
                                 
                                 ) 
                               
                               · 
                               S 
                             
                           
                           Ndl 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
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     In addition, for example, since an RB contains 12 subcarriers, the size S of the signal power  410  is represented by the following formula (4) using the RSRP that is measured by the RSRP measurement unit  120 . That is, the RSRP that is measured by the RSRP measurement unit  120  is multiplied by 12 and the measurement bandwidth Nmeas to perform transformation into an RSSI bandwidth and calculate the size S of the signal power  410 . 
     [Mathematical Expression 4]
 
 S =RSRP×12× N meas  (4)
 
     Therefore, the correction unit  130  corrects the RSSI that is output from the RSSI measurement unit  110 , on the basis of the above-described formulas (3) and (4), the RSRP, the system bandwidth Ndl, and the measurement bandwidth Nmeas, and obtains the size “RSSIa” of the RSSI  401 . The RSRP is RSRP that is output from the RSRP measurement unit  120  to the correction unit  130 . The system bandwidth Ndl and the measurement bandwidth Nmeas have been already notified from the base station or the like. The correction unit  130  outputs the size RSSIa of the RSSI  401  of the merely measurement bandwidth Nmeas to the RSRQ calculation unit  140 . 
     As described above, the correction unit  130  perform correction so that interference power in a band that is different from the measurement bandwidth Nmeas is removed from the RSSI that is measured by the RSSI measurement unit  110 , on the basis of the RSRP, the system bandwidth Ndl, and the measurement bandwidth Nmeas. 
     Therefore, an RSSI that is defined in 3GPP, that is, an average value of the total pieces of reception power (serving cell, neighboring cell, thermal noise, and the like) that are observed in OFDM symbols that include RSs on the certain number of resource blocks is obtained. 
     (Hardware Structure of a Wireless Terminal to which a Measurement Circuit is Applied) 
       FIG. 6  is a diagram illustrating an example of a hardware structure of a wireless terminal to which a measurement circuit is applied.  FIG. 7  is a diagram illustrating an example of a flow of a signal in the wireless terminal illustrated in  FIG. 6 . A wireless terminal  500  illustrated in  FIGS. 6 and 7  includes a reception antenna  511 , a transmission antenna  512 , an analog circuit  520 , and a digital baseband signal processing unit  530 . 
     &lt;Reception Antenna  511  and Transmission Antenna  512 &gt; 
     The reception antenna  511  receives radio waves and outputs a received signal that indicates the reception result to the analog circuit  520 . The transmission antenna  512  transmits a signal that is output from the analog circuit  520 , through radio waves. 
     &lt;Analog Circuit  520 &gt; 
     The analog circuit  520  includes an amplifier  521 , an oscillator  522 , a mixer  523 , an analog filter  524 , an analog/digital converter (ADC)  525 , a digital/analog converter (DAC)  526 , an oscillator  527 , a mixer  528 , and an amplifier  529 . 
     The amplifier  521  amplifies the received signal that is output from the reception antenna  511  by a variable amplification amount and outputs the amplified received signal to the mixer  523 . The oscillator  522  oscillates a dock signal having a certain frequency and outputs the oscillated clock signal to the mixer  523 . The mixer  523  transforms the frequency of the received signal into a baseband by multiplying the received signal that is output from the amplifier  521  by the clock signal that is output from the oscillator  522 . The mixer  523  outputs the received signal the frequency of which is transformed, to the analog filter  524 . 
     The analog filter  524  permeates the received signal that is output from the mixer  523  and outputs the permeated received signal to the ADC  525 . In addition, the analog filter  524  gives a certain permeation characteristic (frequency response) to a received signal to be permeated. For example, the analog filter  524  is a low pass filter (LPF), a band pass filter (BPF), or the like that merely permeates a certain frequency. 
     The ADC  525  converts a received signal that is output from the analog filter  524 , from the analog signal to a digital signal. In addition, the ADC  525  outputs the received signal that is converted into the digital signal, to the digital baseband signal processing unit  530 . 
     The DAC  526  converts a transmitted signal that is output from the digital baseband signal processing unit  530 , from the digital signal to an analog signal. In addition, the DAC  526  outputs the transmitted signal that is converted into the analog signal, to the mixer  528 . The oscillator  527  oscillates a clock signal having a certain frequency and outputs the oscillated clock signal to the mixer  528 . 
     The mixer  528  transforms the frequency of the transmitted signal into a high frequency band by multiplying the transmitted signal that is output from the DAC  526  by the clock signal that is output from the oscillator  527 . The mixer  528  outputs the transmitted signal the frequency of which is transformed, to the amplifier  529 . The amplifier  529  amplifies, by a variable amplification amount, the transmitted signal that is output from the mixer  528 , and outputs the amplified transmitted signal to the transmission antenna  512 . 
     &lt;Digital Baseband Signal Processing Unit  530 &gt; 
     The digital baseband signal processing unit  530  executes reception processing for the received signal that is output from the analog circuit  520 . For example, the digital baseband signal processing unit  530  obtains information that indicates the above-described system bandwidth Ndl and measurement bandwidth Nmeas, which is included in the received signal, from the base station that performs wireless communication with the wireless terminal  500 . 
     In addition, the measurement circuit  100  may be applied to the digital baseband signal processing unit  530 . The measurement circuit  100  measures RSRP and RSRQ on the basis of the received signal that is output from the analog circuit  520 , and the information that indicates the system bandwidth Ndl and the measurement bandwidth Nmeas, which is obtained from the received signal in the digital baseband signal processing unit  530 . 
     In addition, the digital baseband signal processing unit  530  generates a signal to be transmitted from the wireless terminal  500 , and outputs the generated signal to the analog circuit  520 . For example, the digital baseband signal processing unit  530  generates a transmitted signal that is destined for the base station, which includes the RSRP and RSRQ that are measured by the measurement circuit  100 , and outputs the generated transmitted signal to the analog circuit  520 . Therefore, the RSRP and RSRQ that are measured on the basis of the received signal in the wireless terminal  500  are reported to the base station. 
     The digital baseband signal processing unit  530  may realize the above-described processing, for example, by a field programmable gate array (FPGA), a processor such as a digital signal processor (DSP), or the like. 
     (Operation of the Measurement Circuit According to the First Embodiment) 
       FIG. 8  is a flowchart illustrating an example of an operation of the measurement circuit according to the first embodiment. The measurement circuit  100  executes each step illustrated in  FIG. 8 , for example, for each certain report cycle to the base station. First, the measurement circuit  100  measures an RSSI in the time domain on the basis of a received signal (Step S 601 ). After that, the measurement circuit  100  measures RSRP on the basis of the received signal (Step S 602 ). 
     After that, the measurement circuit  100  corrects the RSSI that is measured in Step S 601 , on the basis of the RSRP that is measured in Step S 602  (Step S 603 ). After that, the measurement circuit  100  calculates RSRQ on the basis of the RSRP that is measured in Step S 602  and the RSSI that is corrected in Step S 603  (Step S 604 ). 
     After that, the measurement circuit  100  reports the RSRP that is measured in Step S 602  and the RSRQ that is calculated in Step S 604 , to the base station or the like to which the wireless terminal  500  is being coupled (Step S 605 ), and ends a series of operations. In Step S 605 , for example, the measurement circuit  100  generates a transmitted signal that is destined for the base station and that stores the RSRP and RSRQ, and outputs the generated transmitted signal to the DAC  526 . Therefore, the transmitted signal is wirelessly transmitted from the transmission antenna  512  to the base station, and the RSRP and RSRQ are reported to the base station. 
     (Modification of the Measurement Circuit) 
       FIG. 9  is a diagram illustrating a modification of the measurement circuit.  FIG. 10  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 9 . In  FIGS. 9 and 10 , the same reference numerals are given to portions that are similar to those of  FIGS. 1 and 2 , and the description is omitted herein. As illustrated in  FIGS. 9 and 10 , the RSSI measurement unit  110  may output an RSSI that is measured in the time domain to the RSRQ calculation unit  140 . 
     In this case, the RSRQ calculation unit  140  calculates RSRQ on the basis of the RSSI before correction, which is output from the RSSI measurement unit  110  and the RSRP that is output from the RSRP measurement unit  120 . In addition, the RSRQ calculation unit  140  outputs the calculated RSRQ to the correction unit  130 . The correction unit  130  corrects the RSRQ that is output from the RSRQ calculation unit  140  on the basis of the RSRP that is output from the RSRP measurement unit  120 . The correction unit  130  outputs the corrected RSRQ from the measurement circuit  100 . 
     As illustrated in  FIGS. 9 and 10 , RSRQ is calculated on the basis of the RSRP and the RSSI before correction, and the calculated RSRQ may be corrected using the RSRP. In this case, similar to the measurement circuit  100  illustrated in  FIGS. 1 and 2 , the RSRQ is calculated accurately without measuring the RSSI in the frequency domain. Therefore, the RSRQ is obtained accurately while an increase in a processing amount is suppressed. 
     As described above, in the measurement circuit  100  according to the first embodiment, RSRQ is calculated accurately by correcting the RSSI that is measured in the time domain using the RSRP that is measured to be output with the RSRQ, without measuring the RSSI in the frequency domain. Therefore, the RSRQ is calculated accurately while an increase in a processing amount is suppressed. 
     Alternatively, RSRQ is obtained accurately by calculating the RSRQ on the basis of RSRP and an RSSI before correction, and correcting the calculated RSRQ using the RSRP, without measuring the RSSI in the frequency domain. Therefore, the RSRQ is calculated accurately while an increase in a processing amount is suppressed. 
     Second Embodiment 
     In a second embodiment, a portion that is different from that of the first embodiment is described. For example, when reception electric field strength in the wireless terminal  500  is low, an impact due to thermal noise inside the wireless terminal  500  on an RSSI is increased. 
     Therefore, thermal noise power outside the measurement bandwidth Nmeas is also desired to be corrected similar to that of the interference power, and for example, there is a case in which the thermal noise power varies depending on frequency band due to an effect of the analog filter  524 . In addition, a measurement circuit  100  according to the second embodiment may obtain a highly accurate RSSI by correcting the RSSI using information on thermal noise power and frequency response of the analog filter  524 . 
     (Structure of the Measurement Circuit According to the Second Embodiment) 
       FIG. 11  is a diagram illustrating an example of a structure of the measurement circuit according to the second embodiment.  FIG. 12  is a diagram illustrating an example of a flow of a signal in the measurement circuit illustrated in  FIG. 11 . In  FIGS. 11 and 12 , the same reference numerals are given to portions that are similar to those of  FIGS. 1 and 2 , and the description is omitted herein. As illustrated in  FIGS. 11 and 12 , the measurement circuit  100  according to the second embodiment includes a thermal noise power obtaining unit  810  and a frequency response obtaining unit  820  in addition to the structure illustrated in  FIGS. 1 and 2 . 
     The thermal noise power obtaining unit  810  obtains, for example, thermal noise power information that indicates power of thermal noise that is generated in the analog circuit  520  illustrated in  FIGS. 6 and 7 . For example, the power of the thermal noise that is generated in the analog circuit  520  is allowed to be obtained at the stage of design of the analog circuit  520 , so that thermal noise power information is allowed to be stored in a memory of the measurement circuit  100 . In this case, the thermal noise power obtaining unit  810  obtains the thermal noise power information from the memory of the measurement circuit  100 . The thermal noise power obtaining unit  810  outputs the obtained thermal noise power information to the correction unit  130 . 
     The frequency response obtaining unit  820  obtains, for example, frequency response information that indicates frequency response characteristics in the analog filter  524  illustrated in  FIGS. 6 and 7 . For example, the frequency response characteristics in the analog filter  524  are allowed to be obtained at the stage of design of the analog circuit  520 , so that frequency response information is allowed to be stored in the memory of the measurement circuit  100 . In this case, the frequency response obtaining unit  820  obtains the frequency response information from the memory of the measurement circuit  100 . The frequency response obtaining unit  820  outputs the obtained frequency response information to the correction unit  130 . 
     The correction unit  130  corrects the RSSI that is output from the RSSI measurement unit  110  on the basis of the RSRP that is output from the RSRP measurement unit  120 , the thermal noise power information that is output from the thermal noise power obtaining unit  810 , and the frequency response information that is output from the frequency response obtaining unit  820 . 
     Therefore, even when the thermal noise is large for the reception electric field strength, a highly accurate RSSI is obtained. Therefore, the RSRQ is calculated accurately in the RSRQ calculation unit  140 . 
     (Frequency Response Characteristics of the Analog Filter) 
       FIG. 13A  is a diagram illustrating an example of a received signal. In  FIG. 13A , the horizontal axis indicates a frequency, the vertical axis indicates strength. A received signal  910  indicates a received signal at the former stage of the analog filter  524  of the wireless terminal  500 . An effective bandwidth  901  indicates an effective bandwidth of the received signal  910 . 
     For example, the effective bandwidth  901  corresponds to the above-described system bandwidth Ndl. The received signal  910  includes an interference signal and a noise component, and as illustrated in  FIG. 13A , a noise component and the like are included in a band outside the effective bandwidth  901  as well. In the band outside the effective bandwidth  901 , strength of the received signal  910  is increased by thermal noise power N. 
       FIG. 13B  is a diagram illustrating examples of a permeation characteristic (frequency response) of the analog filter. In  FIG. 13B , the horizontal axis indicates a frequency, and the vertical axis indicates a frequency response. A frequency response  920  is a characteristic of a permeation rate for a frequency in the analog filter  524 . As illustrated in the frequency response  920 , the analog filter  524  has a high permeation rate in the effective bandwidth  901 , and has a low permeation rate in the band outside the effective bandwidth  901 . The frequency response information that is obtained by the frequency response obtaining unit  820  is, for example, information that indicates the frequency response  920  illustrated in  FIG. 13B . 
       FIG. 13C  is a diagram illustrating an example of a received signal that has been permeated through the analog filter. In  FIG. 13C , the horizontal axis indicates a frequency, and the vertical axis indicates strength. A received signal  930  indicates a received signal that has been permeated through the analog filter  524  of the wireless terminal  500 . Due to the frequency response  920  illustrated in  FIG. 13B , the received signal  930  is obtained by extracting a component of the merely effective bandwidth  901  from the received signal  910  illustrated in  FIG. 13A . 
     (Thermal Noise Power of the Analog Circuit) 
       FIG. 14A  is a diagram illustrating an example of thermal noise that is generated in the analog circuit. In  FIG. 14A , the horizontal axis indicates a frequency, and the vertical axis indicates strength. Thermal noise  1010  indicates thermal noise that is generated in the analog circuit  520 . The thermal noise  1010  is, for example, noise that is generated by irregular thermal motion of free electrons in the analog circuit  520 , and the thermal noise power that is viewed from the digital baseband signal processing unit  530  is evaluated as the whole analog circuit  520 . 
     The thermal noise power information that is obtained by the thermal noise power obtaining unit  810  is, for example, information that indicates the thermal noise  1010 . The correction unit  130  may extract thermal noise that is included in the measurement bandwidth Nmeas that is notified from the base station or the like, from the thermal noise  1010  that is indicated by the thermal noise power information that is output from the thermal noise power obtaining unit  810 , and may calculate power of the extracted thermal noise. Therefore, thermal noise power Na that is included in the measurement bandwidth Nmeas is obtained. For example, the thermal noise power Na is calculated by integrating pieces of strength of portions that are included in the measurement bandwidth Nmeas, in the thermal noise  1010 . 
       FIG. 14B  is a diagram illustrating an example of change in thermal noise due to the analog filter. In  FIG. 14B , the horizontal axis indicates a frequency, and the vertical axis indicates strength. The thermal noise  1010  illustrated in  FIG. 14A  becomes noise such as the thermal noise  1020  illustrated in  FIG. 14B  due to the analog filter  524 . That is, due to the frequency response  920  illustrated in  FIG. 13B , the thermal noise  1020  becomes noise that is obtained by extracting the component of the merely effective bandwidth  901  (see  FIG. 13B ). 
     The correction unit  130  obtains the thermal noise  1020  on the basis of the thermal noise  1010  that is indicated by the thermal noise power information that is output from the thermal noise power obtaining unit  810  and the frequency response  920  that is indicated by the frequency response information that is output from the frequency response obtaining unit  820 . For example, the correction unit  130  obtains the thermal noise  1020  by multiplying the strength of the thermal noise  1010  by the permeation rate of the frequency response  920  for each frequency. Therefore, the thermal noise power N that is included in the system bandwidth Ndl (effective bandwidth  901 ) is obtained. For example, the thermal noise power N is calculated by integrating the pieces of strength of the thermal noise  1020 . 
     (RSSI that is Measured by the RSSI Measurement Unit) 
       FIG. 15A  is a diagram illustrating an example of an RSSI that is measured by the RSSI measurement unit. In  FIG. 15A , the same reference numerals are given to portions that are similar to those of  FIG. 5A , and the description is omitted herein. 
     As illustrated in  FIG. 15A , in the RSSI  400  that is measured in the time domain, the signal power  410  of the measurement target cell, the interference power  420  from the serving cell, and thermal noise power  1110  are included. In addition, in the RSSI  400  that is measured in the time domain, thermal noise power in an area broader than the system bandwidth Ndl is also included in addition to the interference power of the whole system bandwidth Ndl. Therefore, in the RSSI  400 , the interference power and thermal noise power outside the measurement bandwidth Nmeas are also included. 
     The size of the signal power  410  is represented as “S”, the size of the interference power  420  is represented as “I”, and the size of the thermal noise power  1110  is represented as “N”. In this case, the size of the RSSI  400  that is measured by the RSSI measurement unit  110  is represented, for example, by the following formula (5). 
     [Mathematical Expression 5]
 
RSSI= S+I+N  
 
∴ I =RSSI− S−N   (5)
 
     (RSSI that is Measured in the Frequency Domain) 
       FIG. 15B  is a diagram illustrating an example of an RSSI that is measured in the frequency domain. In  FIG. 15B , the same reference numerals are given to portions that are similar to those of  FIG. 15A , and the description is omitted herein. When the RSSI is measured by transforming a received signal into a frequency domain by FFT, as illustrated in  FIG. 15B , the RSSI  401  of the measurement bandwidth Nmeas is merely measured. The interference power  421  illustrated in  FIG. 15B  is interference power that is included in the measurement bandwidth Nmeas. The size of the interference power  421  is represented as “Ia”. The thermal noise power  1111  illustrated in  FIG. 15B  is thermal noise power that is included in the measurement bandwidth Nmeas. The size of the thermal noise power  1111  is represented as “Na”. 
     When the size of the RSSI  401  is represented as “RSSIa”, “RSSIa” is represented, for example, by the following formula (6). That is, the interference power I that is measured in the time domain is converted by a ratio of the system bandwidth Ndl to the measurement bandwidth Nmeas, and “RSSIa” is calculated by adding the converted interference power to the thermal noise power Na and the signal power S of the measurement target cell. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   RSSIa 
                   = 
                   
                     S 
                     + 
                     
                       
                         Nmeas 
                         Ndl 
                       
                       ⁢ 
                       I 
                     
                     + 
                     Na 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     When the above-described formula (5) is substituted into the above-described formula (6), the size RSSIa of the RSSI  401  of the measurement bandwidth Nmeas is merely represented by the following formula (7). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         RSSIa 
                         = 
                           
                         ⁢ 
                         
                           S 
                           + 
                           
                             
                               Nmeas 
                               Ndl 
                             
                             ⁢ 
                             
                               ( 
                               
                                 RSSI 
                                 - 
                                 S 
                                 - 
                                 N 
                               
                               ) 
                             
                           
                           + 
                           Na 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 Nmeas 
                                 · 
                                 
                                   ( 
                                   
                                     RSSI 
                                     - 
                                     N 
                                   
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     Ndl 
                                     - 
                                     Nmeas 
                                   
                                   ) 
                                 
                                 · 
                                 S 
                               
                             
                             Ndl 
                           
                           + 
                           Na 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     In addition, as described above, for example, since an RB contains 12 subcarriers, the size S of the signal power  410  is represented by the following formula (8) using the RSRP that is measured by the RSRP measurement unit  120 . 
     [Mathematical Expression 8]
 
 S =RSRP×12× N meas  (8)
 
     The correction unit  130  corrects the RSSI that is output from the RSSI measurement unit  110 , on the basis of the above-described formulas (7) and (8), the RSRP, the system bandwidth Ndl, the measurement bandwidth Nmeas, the size N of the thermal noise power  1110 , and the size Na of the thermal noise power  1111 . Therefore, the size RSSIa of the RSSI  401  is obtained. 
     The correction unit  130  outputs the size RSSIa of the RSSI  401  of the merely measurement bandwidth Nmeas to the RSRQ calculation unit  140 . The RSRP is RSRP that is output from the RSRP measurement unit  120  to the correction unit  130 . For example, the system bandwidth Ndl and the measurement bandwidth Nmeas have been already notified from the base station or the like. 
     The size N of the thermal noise power  1110  is calculated by integrating pieces of strength of the thermal noise  1020  on the basis of the thermal noise power information (for example, see  FIG. 14B ). The size Na of the thermal noise power  1111  is calculated by integrating pieces of strength of portions that are included in the measurement bandwidth Nmeas in the thermal noise  1010 , on the basis of the thermal noise power information and the frequency response information (for example, see  FIG. 14A ). 
     As described above, the correction unit  130  performs correction so that interference power and thermal noise power in a band that is different from the measurement bandwidth Nmeas are removed from the RSSI that is measured by the RSSI measurement unit  110 . At that time, the correction unit  130  uses the RSRP, the system bandwidth Ndl, the measurement bandwidth Nmeas, information that indicates a frequency response characteristic of the analog filter  524 , and information that indicates thermal noise of the analog circuit  520 . 
     Therefore, for example, RSSI that is defined in 3GPP, that is, an average value of total reception power (serving cell, neighboring cell, thermal noise, and the like) that are observed in OFDM symbols that include RSs on the certain number of resource blocks is obtained. 
     (Operation of the Wireless Terminal According to the Second Embodiment) 
       FIG. 16  is a flowchart illustrating an example of an operation of the wireless terminal according to the second embodiment. The measurement circuit  100  executes each step illustrated in  FIG. 16 , for example, for each certain report cycle to the base station. Steps S 1201  to S 1205  illustrated in  FIG. 16  are similar to Step S 601  to S 605  illustrated in  FIG. 8 . 
     In Step S 1203 , the measurement circuit  100  corrects the RSSI that is measured in Step S 1201  on the basis of the RSRP that is measured in Step S 1202 , the thermal noise power information, and the frequency response information (Step S 1203 ). Therefore, a further highly accurate RSSI is obtained. Therefore, in Step S 1205 , further highly accurate RSRQ is reported to the base station or the like. 
     As described above, in the measurement circuit  100  according to the second embodiment, the RSSI that is measured in the time domain is corrected on the basis of the information that indicates the frequency response characteristic of the analog filter  524  and the information that indicates the thermal noise of the analog circuit  520 . Therefore, the RSRQ is calculated further accurately. 
     In addition, in the second embodiment, as illustrated in  FIGS. 9 and 10 , the RSRQ may be calculated on the basis of the RSRP and the RSSI before correction. In this case, the calculated RSRQ is corrected on the basis of the RSRP, the information that indicates the frequency response characteristics of the analog filter  524 , and the information that indicates the thermal noise of the analog circuit  520 . Therefore, similar to the measurement circuit  100  illustrated in  FIGS. 11 and 12 , the RSRQ is calculated further accurately. 
     (Measurement Result of RSSI) 
       FIG. 17  is a diagram illustrating an example of a measurement result of RSSIs. In  FIG. 17 , the horizontal axis indicates an RSSI [dBm], and the vertical axis indicates probability distribution. An ideal value  1301  indicates ideal probability distribution of the RSSIs. A measurement result  1302  indicates probability distribution of the RSSI that is measured in the frequency domain as a reference. In the measurement result  1302 , the RSSIs are distributed using the ideal value  1301  as the center. 
     A measurement result  1303  (time domain before correction) indicates probability distribution of the RSSIs that are measured in the time domain in the RSSI measurement unit  110 . Due to the interference power outside the effective bandwidth, the RSSIs are distributed using a value that is deviated from the ideal value  1301  as the center, in the measurement result  1303 . A measurement result  1304  (time domain after correction) indicates probability distribution of the RSSIs that are obtained by correcting the RSSIs that are measured in the time domain in the RSSI measurement unit  110 , by the correction unit  130 . In the measurement result  1304 , similar to the measurement result  1303  in the frequency domain, the RSSIs are distributed using a value that is deviated from the ideal value  1301  as the center. 
     As described above, by correcting RSSIs that are measured in the time domain by RSRP, an RSSI that has a configuration that is obtained when a RSSI is measured in the frequency domain is obtained. Therefore, RSRQ is accurately obtained while an increase in a processing amount is suppressed. 
     As described above, in the measurement circuit, the wireless communication device, and the measurement method according to the embodiments, RSRQ is measured efficiently. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation 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 the 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.