Abstract:
A base station is provided that performs time-division duplex radio communication with a terminal, using part of a frequency band shared with another system. The base station includes a receiver that receives an uplink signal from the terminal for each frequency band in use, and a guard-time-length setter that sets a length of a guard time. The base station also includes a reception level measurer, which measures a reception level of the uplink signal and a reception level of the guard time having a predetermined length, and a frequency band selector, which selects a frequency band in which the reception level of the guard time is not greater than a first threshold. The base station further includes a control signal generator that generates a control signal, and a transmitter that transmits a downlink signal, including the control signal, to the terminal for the frequency band in use.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 14/808,294, filed Jul. 24, 2015, which claims priority to Japanese Application Nos. 2015-012250, filed on Jan. 26, 2015 and 2014-153012, filed on Jul. 28, 2014. The disclosures of these documents, including the specifications, drawings, and claims, are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a base station configured to perform radio communication with a terminal. 
       BACKGROUND ART 
       [0003]    Field pickup units (FPUs) are known as apparatuses used in a radio communication system for video transmission such as live television broadcasting or emergency broadcasting. Such an FPU is used for transmitting materials of a broadcasting sector and configured to transmit an uplink (UL) signal of the main content information from of a scene-reporting-side mobile station (terminal) to a broadcast-station-side base station, while transmitting a downlink (DL) signal of feedback information from the broadcast-station-side base station to the scene-reporting-side mobile station (terminal). The video captured by a camera is sent via file transmission in real time and transmitted from the mobile station to the base station as an UL signal and stored in a storage medium and reproduced. In addition, a feedback signal and control signal are transmitted from the mobile station to the base station as a DL signal. 
         [0004]    In such a radio communication system, while the DL signal which is the feedback information from a broadcast-station-side base station to a scene-reporting-side mobile station is necessary, what is desired most is faster transmission of UL signals which are the main content information such as video information used in broadcasting. An increase in the transmission rate of UL signals requires a Time-Division Duplex (TDD) frame configuration in which the UL period is longer than the DL period. 
         [0005]    In the TDD system, a guard time is provided between a UL transmission period and a DL transmission period for the purpose of absorbing delay in radio propagation (see Patent Literature (hereinafter, referred to as “PTL”) 1. The amount of delay decreases as the distance between the mobile station and the base station decreases. Meanwhile, in order to increase the transmission rate, it is favorable to set the guard time to be as short as possible. For this reason, in the related art, the length of guard time is set to be variable so that the guard time becomes short as the distance between the mobile station and the base station decreases. 
         [0006]    In addition, FPUs share a frequency band with another system such as a transceiver, so that the presence or absence of interference in the frequency band in use needs to be always monitored to prevent interference with the other system in FPUs. FPU base stations each measure the reception level (interference amount) of a guard time and determines that the other system has started using the frequency band (interference has occurred) when the reception level exceeds a predetermined threshold, and the base station stops using the frequency band. Note that, the reception level (reception quality) is a received signal strength indicator (RSSI), for example, to be more specific. 
       CITATION LIST 
     Patent Literature 
     PTL 1 
       [0007]    Japanese Patent Application Laid-Open No. HEI 9-51327 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In the related art, however, a short guard time is set when the distance between the terminal and base station is short, so that a sufficient amount of time for measuring the reception level cannot be secured, which leads to a problem in that frequency sensing accuracy deteriorates. 
         [0009]    An object of the present invention is thus to provide a base station capable of performing sensing of a frequency with high accuracy by securing a sufficient amount of time to measure a reception level of a guard time while avoiding a decrease in the transmission rate as much as possible, even when the distance between a terminal and the base station is short. 
       Solution to Problem 
       [0010]    A base station according to an aspect of the present invention is a base station configured to perform time-division duplex radio communication with a terminal, using part of a frequency band shared with another system, the base station including: a receiving section that receives an uplink signal from the terminal for each frequency band in use; a reception level measurement section that measures a reception level of the uplink signal and a reception level of a guard time for the frequency band in use; a guard-time-length setting section that sets a length of the guard time based on the reception level of the uplink signal; a frequency band selection section that selects a frequency band in which the reception level of the guard time is not greater than a first threshold; an Ack/Nack generation section that generates an Ack/Nack for the frequency hand in use, based on an error detection result of the received uplink signal; a control signal generation section that generates, for the frequency band in use, a control signal including information indicating the set length of the guard time, information indicating the selected frequency band, and an Ack/Nack; and a transmission section that transmits a downlink signal including the control signal to the terminal for the frequency band in use, in which, when not generating the control signal in a first frequency band, the control signal generation section adds information including an Ack/Nack for the first frequency band in the control signal in a second frequency band that is different from the first frequency band, and the transmission section does not transmit the down link signal when the control signal generation section does not generate the control signal in the first frequency band. 
       Advantageous Effects of Invention 
       [0011]    According to the present invention, sensing of a frequency can be performed with high accuracy with a transmission rate almost identical to the conventional transmission rate, even when the distance between a terminal and a base station is short. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a block diagram illustrating a configuration example of a terminal according to Embodiment 1 of the present invention; 
           [0013]      FIG. 2  is a block diagram illustrating a configuration example of a base station according to Embodiment 1 of the present invention; 
           [0014]      FIG. 3  is an example of a frame configuration according to Embodiment 1 of the present invention; 
           [0015]      FIG. 4  is a configuration example of a control signal according to Embodiment 1 of the present invention; 
           [0016]      FIG. 5  is a flowchart illustrating an operation example of the base station according to Embodiment 1 of the present invention; 
           [0017]      FIG. 6  is a block diagram illustrating a configuration example of a base station according to Embodiment 2; 
           [0018]      FIG. 7  is a diagram illustrating a configuration example of a control signal according to Embodiment 2; 
           [0019]      FIG. 8  is an example of a frame configuration according to Embodiment 2 of the present invention; 
           [0020]      FIG. 9  is a flowchart illustrating an operation example of the base station according to Embodiment 2 of the present invention; 
           [0021]      FIG. 10  is an example of a frame configuration according to Embodiment 2 of the present invention; 
           [0022]      FIG. 11  is a block diagram illustrating a configuration example of a base station according to Embodiment 3 of the present invention; 
           [0023]      FIGS. 12A and 12B  are each an example of a frame configuration according to Embodiment 3 of the present invention; 
           [0024]      FIG. 13  is a flowchart illustrating an operation example of the base station according to Embodiment 3 of the present invention; 
           [0025]      FIG. 14  is a block diagram illustrating a configuration example of a base station according to Embodiment 4 of the present invention; 
           [0026]      FIG. 15  is a diagram illustrating a configuration example of a control signal according to Embodiment 4 of the present invention; 
           [0027]      FIGS. 16A and 16B  are each a diagram illustrating an example of a frame configuration according to Embodiment 4 of the present invention; 
           [0028]      FIG. 17  is a flowchart illustrating an operation example of the base station according to Embodiment 4 of the present invention; and 
           [0029]      FIGS. 18A and 18B  are each a diagram illustrating an example of a frame configuration according to Embodiment 4 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Hereinafter, a detailed description will be given of embodiments of the present invention with reference to the drawings. 
       Embodiment 1 
       [0031]    A radio communication system according to Embodiment 1 includes terminal  100  illustrated in  FIG. 1  and base station  200  illustrated in  FIG. 2 . Terminal  100  and base station  200  are each an FPU used in transmission of materials in a broadcasting sector, for example. More specifically, terminal  100  transmits video information or the like to base station  200  as a UL signal, and base station  200  transmits feedback information or the like to terminal  100  as a DL signal. 
         [0032]    To begin with, a description will be given of a configuration example of terminal  100  according to Embodiment 1 with reference to  FIG. 1 .  FIG. 1  is a block diagram illustrating a configuration example of terminal  100  according to Embodiment 1. 
         [0033]    In  FIG. 1 , terminal  100  mainly includes radio reception section  101 , baseband reception processing section  102 , Ack/Nack generation section  103 , control signal generation section  104 , control signal processing section  105 , baseband transmission processing section  106 , buffer  107 , transmission timing adjustment section  108 , and radio transmission section  109 . Hereinafter, each section of terminal  100  will be described. 
         [0034]    Radio reception section  101  performs radio reception processing such as amplification, filtering, and/or the like on a radio signal received via an antenna. Radio reception section  101  acquires a baseband signal by down-converting a resultant signal of the radio reception processing using a frequency band selected by base station  200  (frequency-band-selection section  208  to be described, hereinafter) while taking synchronization based on the length of a guard time set by base station  200  (guard-time-length setting section  209  to be described, hereinafter). Radio reception section  101  outputs the baseband signal to baseband reception processing section  102 . 
         [0035]    Baseband reception processing section  102  performs fast Fourier transform (FFT) processing, demodulation and error correction for or error detection based on the transmission rate on the baseband signal received from radio reception section  101 . Baseband reception processing section  102  outputs information indicating the result of error detection to Ack/Nack generation section  103 . 
         [0036]    Moreover, baseband reception processing section  102  performs parallel/serial (P/S) conversion in accordance with the number of frequency bands selected by base station  200  to acquire a control signal and received data. Baseband reception processing section  102  outputs the control signal to control signal processing section  105 . Note that, although details will be given hereinafter, this control signal includes information indicating the length of a guard time (hereinafter, referred to as “guard-time-length information”), information indicating a frequency band (hereinafter, referred to as “frequency information”), Ack/Nack, and link adaptation information and/or the like. 
         [0037]    Ack/Nack generation section  103  generates an Ack/Nack based on the information indicating the result of error detection received from baseband reception processing section  102 . Ack/Nack generation section  103  outputs an Ack/Nack to control signal generation section  104 . 
         [0038]    Control signal generation section  104  generates a control signal including the Ack/Nack received from Ack/Nack generation section  103  and outputs the control signal to baseband transmission processing section  106 . 
         [0039]    Control signal processing section  105  outputs the guard-time-length information included in the control signal received from baseband reception processing section  102  to radio reception section  101 , baseband transmission processing section  106 , and transmission timing adjustment section  108 . 
         [0040]    Moreover, control signal processing section  105  outputs the frequency information included in the control signal received from baseband reception processing section  102  to radio reception section  101 , baseband reception processing section  102 , baseband transmission processing section  106 , transmission timing adjustment section  108 , and radio transmission section  109 . 
         [0041]    Control signal processing section  105  indicates new transmission or retransmission to baseband transmission processing section  106  for each frequency band based on the Ack/Nack included in the control signal received from baseband reception processing section  102 . 
         [0042]    Furthermore, control signal processing section  105  outputs link adaptation information included in the control signal received from baseband reception processing section  102  to baseband reception processing section  102  and baseband transmission processing section  106 . 
         [0043]    Baseband transmission processing section  106  selects new transmission data, or transmission data stored in buffer  107 , based on the indication of control signal processing section  105 , forms a UL frame by inserting the control signal received from control signal generation section  104  into the selected transmission data, and performs S/P conversion on the UL frame in accordance with the number of frequency bands selected by base station  200 . When forming a UL frame, baseband transmission processing section  106  adjusts the length of the frame based on the length of a guard time that is set by base station  200 . Baseband transmission processing section  106  acquires a baseband signal by performing error correction coding and modulation based on the transmission rate and inverse fast Fourier transform (IFFT) processing for each UL frame. Baseband transmission processing section  106  outputs the baseband signal (UL signal) to transmission timing adjustment section  108 . 
         [0044]    Buffer  107  temporarily stores transmission data. 
         [0045]    Transmission timing adjustment section  108  adjusts the transmission timing so as to secure the guard time length set by base station  200  for the signal sequence of the baseband signal received from baseband transmission processing section  106  and outputs the baseband signal to radio transmission section  109 . 
         [0046]    Radio transmission section  109  performs radio transmission processing such as amplification, filtering, and/or the like on the baseband signal received from transmission timing adjustment section  108 . Radio transmission section  109  up-converts a resultant signal of the radio transmission processing into the frequency band selected by base station  200  to acquire a radio signal. Radio transmission section  109  transmits the radio signal (UL signal) via an antenna. 
         [0047]    The configuration example of terminal  100  according to Embodiment 1 has been described thus far. 
         [0048]    Next, a description will be given of a configuration example of base station  200  according to Embodiment 1 with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating a configuration example of base station  200  according to Embodiment 1. 
         [0049]    In  FIG. 2 , base station  200  mainly includes baseband transmission processing section  201 , buffer  202 , transmission timing adjustment section  203 , radio transmission section  204 , radio reception section  205 , timer  206 , reception level measurement section  207 , frequency-band-selection section  208 , guard-time-length setting section  209 , baseband reception processing section  210 , control signal processing section  211 , Ack/Nack generation section  212 , transmission rate setting section  213 , and control signal generation section  214 . Hereinafter, each section of base station  200  will be described. 
         [0050]    Baseband transmission processing section  201  selects new transmission data, or transmission data stored in buffer  202 , based on the indication of control signal processing section  211 , forms a DL frame by inserting the control signal generated by control signal generation section  214  into the selected transmission data, and performs S/P conversion on the DL frame in accordance with the number of frequency bands selected by frequency-band-selection station  208 . When forming a DL frame, baseband transmission processing section  201  adjusts the length of a frame based on the length of a guard time that is set by guard-time-length setting section  209 . Baseband transmission processing section  201  acquires a baseband signal by performing error correction coding and modulation based on the transmission rate and IFFT processing for each DL frame. Baseband transmission processing section  201  outputs the baseband signal (DL signal) to transmission timing adjustment section  203 . 
         [0051]    Buffer  202  temporarily stores transmission data. 
         [0052]    Transmission timing adjustment section  203  adjusts the transmission timing so as to secure the guard time length set by guard-time-length setting section  209  for the signal sequence of the baseband signal received from baseband transmission processing section  201  and outputs the baseband signal to radio transmission section  204 . 
         [0053]    Radio transmission section  204  performs radio transmission processing such as amplification, filtering, and/or the like on the baseband signal received from transmission timing adjustment section  203 . Radio transmission section  204  up-converts a resultant signal of the radio transmission processing into the frequency band selected by frequency-band-selection section  208  to acquire a radio signal. Radio transmission section  204  transmits the radio signal (DL signal) via an antenna. 
         [0054]    Radio reception section  205  performs radio reception processing such as amplification, filtering, and/or the like on the radio signal received via an antenna. Radio reception section  205  acquires a baseband signal by down-converting a resultant signal of the radio reception processing using the frequency band selected by frequency-band-selection section  208  while taking synchronization based on the length of a guard time set by guard-time-length setting section  209 . Radio reception section  205  outputs the baseband signal to reception level measurement section  207  and baseband reception processing section  210 . 
         [0055]    Timer  206  is set for a predetermined first period of time (e.g., 10 ms) and a predetermined second period of time (e.g., 10 s), and upon expiration of the first period of time, timer  206  outputs a first expiration signal indicating expiration of the first period of time to reception level measurement section  207 , and upon expiration of the second period of time, timer  206  outputs a second expiration signal indicating expiration of the second period of time to reception level measurement section  207  and guard-time-length setting section  209 . 
         [0056]    Upon reception of the first expiration signal from timer  206 , reception level measurement section  207  measures the reception level of the UL signal (received signal). Reception level measurement section  207  outputs the information indicating the measured reception level of the UL signal to guard-time-length setting section  209  and transmission rate setting section  213 . 
         [0057]    Upon reception of the second expiration signal from timer  206 , reception level measurement section  207  measures the reception level of the guard time of the subsequent frame. Reception level measurement section  207  outputs the information indicating the measured reception level of the guard time to frequency-band-selection section  208 . 
         [0058]    Frequency-band-selection section  208  selects a frequency band in which the reception level of the guard time measured by reception level measurement section  207  is equal to or less than a first threshold. Frequency-band-selection section  208  outputs the frequency information indicating the selected frequency band to radio transmission section  204 , radio reception section  205 , baseband transmission processing section  201 , baseband reception processing section  210 , and control signal generation section  214 . 
         [0059]    Guard-time-length setting section  209  sets the guard time length of the subsequent frame according to whether or not the second expiration signal has been received from timer  206 . 
         [0060]    In a case where no second expiration signal has been received from timer  206 , for example, guard-time-length setting section  209  sets a first length for the guard time length of the subsequent frame based on the information indicating the reception level of the UL signal received from reception level measurement section  207 . 
         [0061]    Meanwhile, in a case where the second expiration signal has been received from timer  206 , for example, guard-time-length setting section  209  sets a second length (predetermined length required for measurement of reception level) for the guard time length of the subsequent frame. 
         [0062]      FIG. 3  illustrates an example of a frame configuration according to Embodiment 1. Note that, the length of one frame is set to 10 ms in the example illustrated in  FIG. 3 . 
         [0063]    In  FIG. 3 , guard time  10  is a guard time that is set with the first length, while guard time  11  is a guard time that is set with the second length. As illustrated in  FIG. 3 , even when the distance between the terminal and the base station is short and thus a short guard time is supposed to be set for guard time  10 , which is used for the normal period, guard time  11 , which is longer than guard time  10  used for the normal period, is set for the frame subsequent to the frame transmitted when the second period of time passes. Note that, when the distance between the terminal and the base station is long, and guard time  10  longer than guard time  11  is set, guard-time-length setting section  209  sets the first length (guard time  10 ) for the length of the guard time even during measurement of a reception level. 
         [0064]    Guard-time-length setting section  209  outputs the guard-time-length information indicating the set length of the guard time to baseband transmission processing section  201 , transmission timing adjustment section  203 , radio reception section  205 , and control signal generation section  214 . 
         [0065]    Baseband reception processing section  210  performs FFT processing, demodulation and error correction for error detection based on the transmission rate on the baseband signal received from radio reception section  205 . Baseband reception processing section  210  outputs information indicating the result of error detection to Ack/Nack generation section  212 . 
         [0066]    Moreover, baseband reception processing section  210  performs P/S conversion in accordance with the number of frequency bands selected by frequency-band-selection section  208  to acquire a control signal and received data. Baseband reception processing section  210  outputs the control signal to control signal processing section  211 . 
         [0067]    Control signal processing section  211  indicates new transmission or retransmission to baseband transmission processing section  201  for each frequency band based on the Ack/Nack included in the control signal received from baseband reception processing section  210 . 
         [0068]    Ack/Nack generation section  212  generates, for each frequency band, an Ack/Nack based on the information indicating the result of error detection received from baseband reception processing section  210 . Ack/Nack generation section  212  outputs an Ack/Nack to transmission rate setting section  213  and control signal generation section  214 . 
         [0069]    Transmission rate setting section  213  sets a transmission rate for each frequency band based on the Ack/Nack received from Ack/Nack generation section  212  and the information indicating the reception level of the UL signal received from reception level measurement section  207 . Transmission rate setting section  213  outputs link adaptation information indicating the modulation scheme corresponding to the transmission rate and the coding rate for error correction and/or the like to control signal generation section  214 . 
         [0070]    Transmission rate setting section  213  indicates the modulation scheme and coding rate and/or the like for the transmission data for each frequency band to baseband transmission processing section  201 . 
         [0071]    In addition, transmission rate setting section  213  indicates the demodulation scheme and coding rate and/or the like for the baseband signal for each frequency band to baseband reception processing section  210 . 
         [0072]    Control signal generation section  214  generates a control signal and outputs the control signal to baseband transmission processing section  201 . An example of the control signal generated in this processing is illustrated in  FIG. 4 . 
         [0073]    In  FIG. 4 , the guard-time-length information is the information received by control signal generation section  214  from guard-time-length setting section  209  and indicating the first length or the second length. The frequency information is the information received by control signal generation section  214  from frequency-band-selection section  208 . The Ack/Nack is the information received by control signal generation section  214  from Ack/Nack generation section  212 . The link adaptation information is the information received by control signal generation section  214  from transmission rate setting section  213 . Note that, the control signal may include information other than the information illustrated in  FIG. 4 . 
         [0074]    The configuration example of base station  200  according to Embodiment 1 has been described thus far. 
         [0075]    Next, a description will be given of an operation example of base station  200  according to Embodiment 1 with reference to  FIG. 5 .  FIG. 5  is a flowchart illustrating the operation example of base station  200  according to Embodiment 1. 
         [0076]    Timer  206  starts for the second period of time (step S 1 ). 
         [0077]    When the second period of time set for timer  206  has not expired (step S 2 : NO), i.e., when it is not time to measure the reception level of a guard time (hereinafter, may be referred to as “normal period”), guard-time-length setting section  209  sets the first length for the length of the guard time for the subsequent frame based on the distance between the terminal and the base station (step S 3 ). 
         [0078]    Meanwhile, when the second period of time set for timer  206  has expired (step S 2 : YES), i.e., when it is time to measure the reception level of a guard time, guard-time-length setting section  209  sets the second length, which is a predetermined length required for measurement of a reception level, for the length of the guard time for the subsequent frame (step S 4 ). Reception level measurement section  207  measures the reception level of the guard time of the frequency band (step S 5 ). 
         [0079]    Control signal generation section  214  generates a control signal including the guard-time-length information, frequency information, Ack/Nack, and link adaptation information (step S 6 ). 
         [0080]    Radio transmission section  204  transmits a DL signal including the control signal to terminal  100  (step S 7 ). 
         [0081]    The operation example of base station  200  according to Embodiment 1 has been described thus far. 
         [0082]    As described above, the base station according to Embodiment 1 is characterized in that the base station sets a guard time length so as to secure a predetermined length required for measurement of a reception level when it is time to measure the reception level of the guard time. Thus, even when the distance between the terminal and the base station is short, sensing of a frequency can be performed with high accuracy without a decrease in the transmission efficiency. 
         [0083]    Note that, when the length of one frame is 10 ms, and the interval for measuring the reception levels of guard times is 10 s, a long guard time needs to be set only once in every one thousand frames, so that the transmission rate according to Embodiment 1 is almost the same as that of the related art. Embodiment 1 has been described with the case where the length of one frame is 10 ms, and the interval for measuring the reception levels of guard times is 10 s, as an example, but the present invention is not limited to this case, and any value can be set for the length of one frame as well as the interval for measuring the reception levels of guard times. 
       Embodiment 2 
       [0084]    A description will be given of Embodiment 2. The configuration of terminal  100  according to Embodiment 2 is the same as the configuration described in  FIG. 1 , which is described in Embodiment 1, so that the same description will not be repeated, hereinafter. 
         [0085]    A description will be given of a configuration example of base station  200 A according to Embodiment 2 with reference to  FIG. 6 .  FIG. 6  is a block diagram illustrating the configuration example of base station  200 A according to Embodiment 2. Each section of base station  200 A illustrated in  FIG. 6  is assigned the same reference numeral assigned to the corresponding section of base station  200  illustrated in  FIG. 2 . Hereinafter, a description will be given of only a configuration element that performs an operation different from the operations in  FIG. 2 , among the sections illustrated in  FIG. 6 . 
         [0086]    A predetermined first period of time (e.g., 10 ms) and a predetermined second period of time (e.g., 10 s) are set for timer  206 , and upon expiration of the first period of time, timer  206  outputs a first expiration signal indicating expiration of the first period of time to reception level measurement section  207 , and upon expiration of the second period of time, timer  206  outputs a second expiration signal indicating expiration of the second period of time to baseband transmission processing section  201 , reception level measurement section  207 , and control signal generation section  214 . 
         [0087]    Baseband transmission processing section  201  operates as follows, according to whether or not the second expiration signal has been received from timer  206  (i.e., whether or not it is time to measure the reception level of a guard time). 
         [0088]    In a case where no second expiration signal has been received from timer  206  (i.e., it is not time to measure the reception level of a guard time), a DL signal is generated by inserting the control signal generated by control signal generation section  214  into transmission data (new transmission or retransmission) to form a DL in all the frequency bands in use. The operation to be performed after the DL signal is generated is the same as that of Embodiment 1, so that the same description will not be repeated hereinafter. 
         [0089]    Meanwhile, in a case where a second expiration signal has been received from timer  206  (i.e., it is time to measure the reception level of a guard time), baseband transmission processing section  201  does not generate a DL signal for the subsequent frame in a frequency band (first frequency band) that is a measurement target for the reception level of a guard time (hereinafter, referred to as “measurement target”). 
         [0090]    Reception level measurement section  207  measures the reception level of a guard time of a received signal in each frequency band every time receiving the second expiration signal from timer  206 . Reception level measurement section  207  outputs the information indicating the measured reception level of the guard time to frequency-hand-selection section  208 . 
         [0091]    Control signal generation section  214  operates as follows, according to whether or not the second expiration signal has been received from timer  206  (i.e., whether or not it is time to measure the reception level of a guard time). 
         [0092]    In a case where no second expiration signal has been received from timer  206  (i.e., it is not time to measure the reception level of a guard time), control signal generation section  214  generates, in the subsequent frame, a control signal in all the frequency bands in use as in Embodiment 1 and outputs the control signal to baseband transmission processing section  201 . 
         [0093]    Meanwhile, in a case where a second expiration signal has been received from timer  206  (i.e., it is time to measure the reception level of a guard time), control signal generation section  214  generates a control signal in the subsequent frame in all the frequency bands except the frequency band of the measurement target and outputs the control signal to baseband transmission processing section  201 . However, the guard-time-length information, frequency information, Ack/Nack, and link adaptation information of the frequency band of the measurement target (first frequency band) are included in a control signal of another frequency band (second frequency band) (e.g., a frequency band having the highest reception level).  FIG. 7  illustrates an example of this control signal. 
         [0094]    As illustrated in  FIG. 7 , the guard-time-length information, frequency information, Ack/Nack, and link adaptation information of frequency band f 1  of the measurement target are included in the control signal of frequency band f 2 , which is different from first frequency band f 1 . Upon reception of the second expiration signal from timer  206 , baseband transmission processing section  201  forms a DL frame by inserting the control signal illustrated in  FIG. 7  into transmission data in the subsequent frame. 
         [0095]      FIG. 8  illustrates an example of a frame configuration according to Embodiment 2. As illustrated in  FIG. 8 , no DL signal is transmitted in the frame subsequent to the frame transmitted when the second period of time passes in frequency band f 1 , which is the measurement target, and the portion of the frame where no DL signal is transmitted becomes guard time  20 . Thus, a guard time longer than guard time  10 , which is used for the normal period, can be secured. Meanwhile, in frequency band f 2 , which is different from frequency band f 1 , DL signal  21  is transmitted in the portion of the frame corresponding to guard time  20 . 
         [0096]    Note that, when the measurement target frequency band is f 2 , and another frequency band is f 1 , no DL signal is transmitted in the frame subsequent to the frame transmitted when the second period of time passes in frequency band f 2  of the measurement target, and the portion of the frame where no DL signal is transmitted becomes guard time  22 . Meanwhile, in frequency band f 1 , which is another frequency band, DL signal  23  in the portion of the frame corresponding to guard time  22  is transmitted. Note that, the guard-time-length information, frequency information, Ack/Nack, and link adaptation information in frequency band f 2 , which is the measurement target, are included in the control signal of DL signal  23 . 
         [0097]    The configuration example of base station  200 A according to Embodiment 2 has been described thus far. 
         [0098]    Next, a description will be given of an operation example of base station  200 A according to Embodiment 2 with reference to  FIG. 9 .  FIG. 9  is a flowchart illustrating an operation example of base station  200 A according to Embodiment 2. 
         [0099]    Timer  206  starts for a predetermined second period of time (step S 11 ). 
         [0100]    When the second period of time set for timer  206  has not expired (step S 12 : NO), i.e., when it is not time to measure the reception level of a guard time (during the normal period), baseband transmission processing section  201  generates a DL signal including a control signal (step S 13 ), and radio transmission section  204  transmits the DL signal including the control signal to terminal  100  (step S 14 ). 
         [0101]    Meanwhile, when the second period of time set for timer  206  has expired (step S 12 : YES), i.e., when it is time to measure the reception level of a guard time, baseband transmission processing section  201  stops generating a DL signal in the frequency band (step S 15 ). Reception level measurement section  207  measures the reception level of the guard time of the frequency band (step S 16 ). 
         [0102]    The operation example of base station  200 A according to Embodiment 2 has been described thus far. 
         [0103]    As described above, the base station according to Embodiment 2 is characterized in that the base station secures the period normally allocated for transmission of a DL frame, as a guard time when it is time to measure the reception level of a guard time, and includes information related to the measurement target frequency band (first frequency band) in a control signal of another frequency band (second frequency band). Thus, even when the distance between the terminal and base station is short, sensing of a frequency can be performed with high accuracy without a decrease in the transmission efficiency. 
         [0104]    In Embodiment 2, in a frame in which a DL signal is transmitted in a certain frequency band (second frequency band), a UL signal may be transmitted in another frequency band (first frequency band) in the same frame during a period other than a period in which measurement of a reception level is performed. An example of the frame configuration in this case is illustrated in  FIG. 10 . As illustrated in  FIG. 10 , when a DL signal is transmitted in frequency band f 1  during a period when it is not time to measure the reception level of a guard time (during the normal period), a UL signal is transmitted in frequency band f 2  in the portion of the frame corresponding to the portion of the frame where the DL signal is transmitted in frequency band f 1 . Note that, it is possible to alternately use frequency bands f 1  and f 2  for transmitting a DL signal as illustrated in  FIG. 10  or to use any one of frequency bands f 1  and f 2  except for the period when measurement of a reception level is performed. 
       Embodiment 3 
       [0105]    In Embodiment 1, a description has been given of the case where guard-time-length setting section  209  sets the second length (predetermined length required for measurement of a reception level) for the length of a guard time always at the interval of the second periods of time. 
         [0106]    In Embodiment 3, a description will be given of a case where the communication quality is estimated in advance, and only when the communication quality deteriorates, guard-time-length setting section  209  sets a second length for the length of the guard time at the timing when the second period of time passes. Note that, the configuration of terminal  100  according to Embodiment 3 is the same as the configuration illustrated in  FIG. 1 , which is described in Embodiment 1, so that the description of the configuration will not be repeated, hereinafter. 
         [0107]    A description will be given of a configuration example of base station  200 B according to Embodiment 3 with reference to  FIG. 11 .  FIG. 11  is a block diagram illustrating the configuration example of base station  200 B according to Embodiment 3. Note that, in base station  200 B illustrated in  FIG. 11 , the sections which are common to the sections of base station  200  illustrated in  FIG. 2  are assigned the same reference numerals and the detailed descriptions of the sections will not be repeated, hereinafter. Base station  200 B illustrated in  FIG. 11  employs a configuration which is different from base station  200  illustrated in  FIG. 2  in that quality estimation section  215  is added. 
         [0108]    Upon reception of the first expiration signal from timer  206 , reception level measurement section  207  measures the reception level of a UL signal (received signal). Reception level measurement section  207  outputs the information indicating the measured reception level of the UL signal to guard-time-length setting section  209 , transmission rate setting section  213 , and quality estimation section  215 . 
         [0109]    Upon reception of the second expiration signal from timer  206 , reception level measurement section  207  measures the reception level of a guard time in the subsequent frame only when the signal outputted from quality estimation section  215  indicates that the communication quality has deteriorated. Reception level measurement section  207  outputs the information indicating the measured reception level of the guard time to frequency-band-selection section  208 . 
         [0110]    Quality estimation section  215  makes a comparison of a magnitude relationship between the reception level of the UL signal measured by reception level measurement section  207  with a predetermined second threshold. Quality estimation section  215  estimates that the communication quality is favorable when the reception level is greater than the second threshold or estimates that the communication quality has deteriorated when the reception level is not greater than the second threshold. Quality estimation section  215  outputs a signal indicating a result of the estimation to reception level measurement section  207  and guard-time-length setting section  209 . 
         [0111]    Upon reception of the second expiration signal from timer  206 , guard-time-length setting section  209  sets a second length for the length of a guard time in the subsequent frame when the signal outputted from quality estimation section  215  indicates that the communication quality has deteriorated, or guard-time-length setting section  209  sets a first length for the length of a guard time in the subsequent frame when the signal outputted from quality estimation section  215  indicates that the communication quality is favorable based on the information indicating the reception level of the UL signal received from reception level measurement section  207 . 
         [0112]      FIGS. 12A and 12B  each illustrate an example of a frame configuration according to Embodiment 3.  FIG. 12A  illustrates a case where the communication quality has deteriorated, while  FIG. 12B  illustrates a case where the communication quality is favorable. Note that, an assumption is made that the frame length is 10 ms in the examples of  FIGS. 12A and 12B . 
         [0113]    In  FIGS. 12A and 12B , guard time  10  is a guard time that is set with the first length, while guard time  11  is a guard time that is set with the second length. As illustrated in  FIG. 12A , in a case where the communication quality has deteriorated, guard time  11 , which is longer than guard time  10  used for the normal period, is set in the frame subsequent to the frame transmitted when the second period of time passes. Meanwhile, as illustrated in  FIG. 12B , in a case where the communication quality is favorable, guard time  10  used for the normal period is set in the frame subsequent to the frame transmitted when the second period of time passes. Thus, it is made possible to increase the transmission period for the UL signal, thus making it possible to increase the transmission rate. 
         [0114]    Note that, guard-time-length setting section  209  performs the same operation as that in Embodiment 1 in a case where no second expiration signal has been received from timer  206 . 
         [0115]    The configuration example of base station  200 B according to Embodiment 3 has been described thus far. 
         [0116]    Next, a description will be given of an operation example of base station  200 B according to Embodiment 3 with reference to  FIG. 13 .  FIG. 13  is a flowchart illustrating the operation example of base station  200 B according to Embodiment 3. Note that, in the flowchart illustrated in  FIG. 13 , the steps which are common to the flowchart illustrated in 
         [0117]      FIG. 5  are assigned the same reference numerals, and the detailed descriptions of the steps will not be repeated, hereinafter. The flowchart illustrated in  FIG. 13  is different from the flowchart illustrated in  FIG. 5  in that step S 21  is added between steps S 2  and S 4 . 
         [0118]    In a case where the second period of time set for timer  206  has expired (step S 2 : YES) and the communication quality has deteriorated (step S 21 : YES), guard-time-length setting section  209  sets the second length for the length of a guard time of the subsequent frame (step S 4 ). Meanwhile, in a case where the second period of time set for timer  206  has expired (step S 2 : YES) and the communication quality is favorable (step S 21 : NO), guard-time-length setting section  209  sets the first length for the length of a guard time of the subsequent frame (step S 3 ). 
         [0119]    The operation example of base station  200 B according to Embodiment 3 has been described thus far. 
         [0120]    As described above, the base station according to Embodiment 3 is characterized in that the base station sets the length of a guard time so as to secure a predetermined length required for measurement of the reception level in a case where it is time to measure the reception level of a guard time and the communication quality has deteriorated. Thus, in addition to the effects obtained in Embodiment 1, it is possible to achieve a further increase in the transmission rate as compared with Embodiment 1. 
       Embodiment 4 
       [0121]    In Embodiment 2, a description has been given of the case where control signal generation section  214  stops generating a control signal and baseband transmission processing section  201  stops generating a frame and provides a guard time always at the interval of the second periods of time. 
         [0122]    In Embodiment 4, a description will be given of a case where the communication quality is estimated in advance, and only when the communication quality has deteriorated, control signal generation section  214  stops generating a control signal and baseband transmission processing section  201  stops generating a frame and provides a guard time at the time when the second period of time passes. In Embodiment 4, the configuration of terminal  100  is the same as the configuration illustrated in  FIG. 1 , which is described in Embodiment 1, so that the description of the configuration will not be repeated, hereinafter. 
         [0123]    A description will be given of a configuration example of base station  200 C according to Embodiment 4 with reference to  FIG. 14 .  FIG. 14  is a block diagram illustrating the configuration example of base station  200 C according to Embodiment 4. Note that, in base station  200 C illustrated in  FIG. 14 , the sections which are common to the sections of base station  200 A illustrated in  FIG. 6  are assigned the same reference numerals and the detailed descriptions of the sections will not be repeated, hereinafter. Base station  200 C illustrated in  FIG. 14  employs a configuration which is different from base station  200 A illustrated in  FIG. 6  in that quality estimation section  215  is added. 
         [0124]    Upon reception of the first expiration signal from timer  206 , reception level measurement section  207  measures the reception level of a UL signal (received signal). Reception level measurement section  207  outputs the information indicating the measured reception level of the UL signal to guard-time-length setting section  209 , transmission rate setting section  213 , and quality estimation section  215 . 
         [0125]    In addition, upon reception of the second expiration signal from timer  206 , reception level measurement section  207  measures the reception level of a guard time only when the signal outputted from quality estimation section  215  indicates that the communication quality has deteriorated. Reception level measurement section  207  outputs the information indicating the measured reception level of the guard time to frequency-band-selection section  208 . 
         [0126]    Quality estimation section  215  makes a comparison of a magnitude relationship between the reception level of the UL signal measured by reception level measurement section  207  with a predetermined second threshold. Quality estimation section  215  estimates that the communication quality is favorable when the reception level is greater than the second threshold or estimates that the communication quality has deteriorated when the reception level is not greater than the second threshold. Quality estimation section  215  outputs a signal indicating a result of the estimation to reception level measurement section  207  and control signal generation section  214 . 
         [0127]    Upon reception of the second expiration signal from timer  206 , control signal generation section  214  generates a control signal illustrated in  FIG. 15  in the measurement target frequency band (first frequency band) of the current frame and outputs the control signal to baseband transmission processing section  201 . The control signal illustrated in  FIG. 15  is different from the control signal illustrated in  FIG. 4  (guard-time-length information, frequency information, Ack/Nack, and link adaptation information) in that quality information which is a signal indicating an estimation result of quality estimation section  215  is added. 
         [0128]    Control signal generation section  214  performs the same operation as that in Embodiment 2 in a frame other than the current frame transmitted when control signal generation section  214  receives the second expiration signal from timer  206 . 
         [0129]    Baseband transmission processing section  201  forms DL frames (DL 31  and DL 32  in  FIGS. 16A and 16B ) by inserting the control signal illustrated in  FIG. 15  into transmission data in the current frame transmitted when the second period of time passes. 
         [0130]    In a case where the quality information indicates that the communication quality is favorable, no guard time needs to be provided, so that terminal  100  transmits a UL signal in the portion of the frame where a guard time is supposed to be transmitted. 
         [0131]      FIGS. 16A and 16B  each illustrate an example of a frame configuration according to Embodiment 4.  FIG. 16A  illustrates a case where the communication quality has deteriorated, while  FIG. 16B  illustrates a case where the communication quality is favorable. Note that, an assumption is made that the length of one frame in the examples of  FIGS. 16A and 16B  is 10 ms. 
         [0132]    As illustrated in  FIG. 16A , in a case where the communication quality has deteriorated, no DL signal is transmitted in frequency band f 1 (f 2 ) of the measurement target in the frame subsequent to the frame transmitted when the second period of time passes as in the case of Embodiment 2 ( FIG. 8 ), and the portion of the frame where a DL signal would have been transmitted otherwise becomes guard time  20 ( 22 ). Thus, a guard time longer than guard time  10  used for the normal period can be secured. Note that, in frequency band f 2 (f 1 ) which is different from the measurement target frequency band, DL signal  21 ( 23 ) is transmitted using the portion of the frame corresponding to guard time  20 ( 22 ). 
         [0133]    Meanwhile, as illustrated in  FIG. 16B , in a case where the communication quality is favorable, no DL signal is transmitted in frequency band f 1 (f 2 ) of the measurement target in the frame subsequent to the frame transmitted when the second period of time passes, and a UL signal is transmitted using the portion of the frame where a DL signal is supposed to be transmitted. Thus, it is made possible to increase the transmission period for the UL signal, thus making it possible to increase the transmission rate. Note that, DL signal  21 ( 23 ) is transmitted in frequency band f 2 (f 1 ) which is different from the measurement target frequency band. 
         [0134]    The configuration example of base station  200 C according to Embodiment 4 has been described thus far. 
         [0135]    Next, a description will be given of an operation example of base station  200 C according to Embodiment 4 with reference to  FIG. 17 .  FIG. 17  is a flowchart illustrating the operation example of base station  200 C according to Embodiment 4. Note that, in the flowchart illustrated in  FIG. 17 , the steps which are common to the flowchart illustrated in  FIG. 9  are assigned the same reference numerals, and the detailed descriptions of the steps will not be repeated, hereinafter. The flowchart illustrated in  FIG. 17  is different from the flowchart illustrated in  FIG. 9  in that step S 31  is added between steps S 15  and S 16  of the flow illustrated in  FIG. 9  and that step S 32  is added as a destination when a determination result in step S 31  is NO. 
         [0136]    In a case where the second period of time set for timer  206  has expired (step S 12 : YES), baseband transmission processing section  201  stops generating a DL signal in the frequency band (step S 15 ). Meanwhile, in a case where the communication quality has deteriorated (step S 31 : YES), reception level measurement section  207  measures the reception level of the guard time of the frequency band (step S 16 ). 
         [0137]    Meanwhile, in a case where the communication quality is favorable (step S 31 : NO), base station  200 C receives a UL signal (step S 32 ). In this case, reception level measurement section  207  does not measure the reception level of the guard time of the frequency band. 
         [0138]    The operation example of base station  200 C according to Embodiment 4 has been described thus far. 
         [0139]    As described above, the base station according to Embodiment 4 is characterized in that the base station secures, as a guard time, the period normally allocated for transmission of a DL frame, and includes information related to the measurement target frequency hand (first frequency band) in a control signal of another frequency band (second frequency band) in a case where it is time to measure the reception level of a guard time and the communication quality has deteriorated. Thus, in addition to the effects obtained in Embodiment 2, it is possible to achieve a further increase in the transmission rate as compared with Embodiment 2. 
         [0140]    In Embodiment 4, in a frame in which a DL signal is transmitted in a certain frequency band (second frequency band), a UL signal may be transmitted in another frequency band (first frequency band) during a period other than a period in which measurement of the reception level is performed. An example of the frame configuration in this case is illustrated in  FIGS. 18A and 18B .  FIG. 18A  illustrates a case where the communication quality has deteriorated, while  FIG. 18B  illustrates a case where the communication quality is favorable. As illustrated in  FIGS. 18A and 18B , when a DL signal is transmitted in frequency band f 1  during a period when it is not time to measure the reception level of a guard time (during the normal period), a UL signal is transmitted in frequency band f 2  in the portion of the frame corresponding to the portion of the frame where the DL signal is transmitted in frequency band  1 . 
         [0141]    Embodiments of the present invention have been described thus far. The present invention is, however, not limited to these embodiments, and various modifications are possible. 
         [0142]    For example, the same or mutually different modulation scheme to be used and/or error correction coding rate may be set for the frequency bands. 
         [0143]    In the present invention, depending on the communication state or application or the like, it is possible to fixedly use the frame configuration illustrated in  FIG. 16B  or the frame configuration illustrated in  FIG. 18B  (configuration in which a control signal is transmitted from a frequency band other than the measurement target frequency band during a specific period such as a period when frequency sensing is performed). Even when the level of an interference wave is high, if the frequency bandwidth used by the interference wave is narrow, deterioration in characteristics due to the interference occurs only on partial data as long as the communication is multi-carrier communication such as OFDM. Thus, the deterioration in characteristics due to the interference wave can be reduced by frequency interleaving and error correction and/or the like. Accordingly, fixedly using the frame configuration illustrated in  FIG. 16B  or the frame configuration illustrated in  FIG. 18B  makes it possible to further improve the transmission efficiency depending on the communication state or application or the like. 
       INDUSTRIAL APPLICABILITY 
       [0144]    The base station according to the present invention is applicable to a base station configured to perform radio communication with a terminal, for example. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Terminal 
           101  Radio reception section 
           102  Baseband reception processing section 
           103  Ack/Nack generation section 
           104  Control signal generation section 
           105  Control signal processing section 
           106  Baseband transmission processing section 
           107  Buffer 
           108  Transmission timing adjustment section 
           109  Radio transmission section 
           200  Base station 
           201  Baseband transmission processing section 
           202  Buffer 
           203  Transmission timing adjustment section 
           204  Radio transmission section 
           205  Radio reception section 
           206  Timer 
           207  Reception level measurement section 
           208  Frequency-band-selection section 
           209  Guard-time-length setting section 
           210  Baseband reception processing section 
           211  Control signal processing section 
           212  Ack/Nack generation section 
           213  Transmission rate setting section 
           214  Control signal generation section 
           215  Quality estimation section