Patent Publication Number: US-2019182025-A1

Title: Base station apparatus and method for controlling base station apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/536,992, filed on Jun. 16, 2017, which is a continuation of International Application No. PCT/JP2015/004613 filed on Sep. 10, 2015, which claims priority from Japanese Patent Application No. 2014-257495, filed Dec. 19, 2014, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a base station apparatus and a method for controlling the base station apparatus, and relates to, for example, a base station apparatus and a method for controlling the base station apparatus which is suitable for reducing cost. 
     BACKGROUND ART 
     Interface standards between a radio equipment control (REC) and a radio equipment (RE) which compose a base station apparatus include CPRI (Common Public Radio Interface) standards (registered trademark). 
     Patent Literature 1 discloses a technique related to an interface between a radio equipment control and a radio equipment. Patent Literature 1 discloses a base station apparatus which connects the radio equipment control and the radio equipment via a cable (transmission link). 
     In addition, Patent Literature 2 discloses a data communication apparatus which switches a dummy clock from a regular synchronization clock when it is determined that serial data communication is not executed between first communication means and second communication means. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Published Japanese Translation of PCT International Publication for Patent Application, No. 2008-516503 
     PTL 2: Japanese Unexamined Patent Application Publication No. H11-074893 
     SUMMARY OF INVENTION 
     Technical Problem 
     The base station apparatus disclosed in Patent Literature 1 delivers data between the radio equipment control and the radio equipment via a cable. Hence, when the base station apparatus is installed, it is necessary to install cables such as optical cables at an installation place of the base station apparatus, and there is a problem that cost increases as a result. Other problems of the prior art and new features of the present invention will become more apparent from the description and the drawings accompanying this description. 
     The present invention has been made to solve the above problem, and an object of the present invention is to provide a base station apparatus and a method for controlling the base station apparatus which can reduce cost necessary to install cables such as optical cables by delivering data between a radio equipment control and a radio equipment by radio. 
     Solution to Problem 
     According to one embodiment, a base station apparatus includes: a radio equipment control that generates a first baseband signal including first data; a first microwave apparatus that modulates the first baseband signal to a first microwave to transmit by radio; a second microwave apparatus that demodulates the received first microwave to the first baseband signal, then extracts a first clock from a cycle of the first data included in the first baseband signal, imports the first baseband signal in synchronization with the first clock, and plays back the first data; and a radio equipment that modulates the first data played back by the second microwave apparatus to a first high-frequency signal, and the second microwave apparatus outputs first dummy data instead of the played back first data when a frequency fluctuation amount of the first clock exceeds a predetermined range. 
     Further, according to one embodiment, a method for controlling a base station apparatus includes: generating a first baseband signal including first data; modulating the first baseband signal to a first microwave to transmit by radio from a first microwave apparatus; demodulating the first microwave received by a second microwave apparatus to the first baseband signal, and then extracting a first clock from a cycle of the first data included in the first baseband signal; importing the first baseband signal in synchronization with the first clock and playing back the first data when a frequency fluctuation amount of the first clock is in a predetermined range; modulating the played back first data to a first high-frequency signal; and generating first dummy data instead of the first data when the frequency fluctuation amount of the first clock exceeds the predetermined range. 
     Advantageous Effects of Invention 
     According to the embodiment, it is possible to provide a radio equipment control and a radio equipment which can reduce cost necessary to install cables such as optical cables by delivering data between the radio equipment control and the radio equipment by radio. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a wireless communication system which includes a base station apparatus according to a first embodiment. 
         FIG. 2  is a block diagram illustrating a specific configuration of a microwave apparatus provided to a side of a radio equipment control of the base station apparatus illustrated in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating a specific configuration of the microwave apparatus provided to a side of a radio equipment of the base station apparatus illustrated in  FIG. 1 . 
         FIG. 4  is a flowchart illustrating a first wireless communication method between a radio equipment control  2  and a radio equipment  3  which compose a base station apparatus  1  illustrated in  FIG. 1 . 
         FIG. 5  is a flowchart illustrating a second wireless communication method between the radio equipment control  2  and the radio equipment  3  which compose the base station apparatus  1  illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments will be described below with reference to the drawings. In this regard, the drawings are simplified, and therefore the technical scope of the embodiments should not be narrowly interpreted on the ground of illustrations of these drawings. Further, the same components will be assigned the same reference numerals and overlapping explanation will be omitted. 
     The embodiments will be divided into a plurality of sections or a plurality of embodiments for ease of explanation when necessary. However, unless explicitly described in particular, a plurality of sections or a plurality of embodiments are not irrelevant to each other, and one of a plurality of sections or a plurality of embodiments and one of a plurality of sections or a plurality of embodiments relates to modified examples, application examples, detailed description and supplementary description of part or all of the other one of a plurality of sections or a plurality of embodiments. Further, in the following embodiments, the numbers (including numbers, numerical values, amounts and ranges) of elements mentioned herein are not limited to specific numbers unless explicitly described in particular and except for a case where the numbers are fundamentally obviously limited to the specific numbers, and may be the specific numbers or more or less. 
     Further, in the following embodiments, components (including operation steps, too) are not necessarily indispensable unless explicitly described in particular or except for a case where the components are fundamentally obviously indispensable. Similarly, in the following embodiments, shapes and positional relationships of the components mentioned herein include shapes substantially approximate to or similar to the shapes unless explicitly described in particular or except for a case where shapes and positional relationships are not fundamentally obviously true. This applies to the above numbers (including the numbers, the numerical values, the amounts and ranges), too. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating a wireless communication system which includes a base station apparatus  1  according to the first embodiment. The base station apparatus  1  according to the present embodiment delivers data between a radio equipment control and a radio equipment by radio. By this means, it is possible to reduce the number of cables such as optical cables installed at an installation place of the base station apparatus  1 , so that it is possible to reduce cost necessary to install the cables. This will be specifically described below. 
     As illustrated in  FIG. 1 , the wireless communication system is a system which performs wireless communication between the base station apparatus  1  and a user terminal  6 . The user terminal  6  is, for example, a mobile terminal such as a mobile telephone and a notebook PC. 
     The base station apparatus  1  includes a radio equipment control (REC)  2 , a radio equipment (RE)  3 , and a microwave apparatus (first microwave apparatus)  4  and a microwave apparatus (second microwave apparatus)  5  which deliver data between the radio equipment control  2  and the radio equipment  3  by radio. 
     A transmission path of the base station apparatus  1  will be described below. 
     The radio equipment control  2  generates a baseband signal (first baseband signal) including data, a clock and a control signal received from a higher level apparatus which is not illustrated. In this regard, a serial data interface between the radio equipment control  2  and the radio equipment  3  employs CPRI standards. Hence, synchronization accuracy between the radio equipment control  2  and the radio equipment  3  needs to be within a range of ±0.002 ppm. 
     The microwave apparatus  4  modulates a baseband signal supplied from the radio equipment control  2  via an optical cable, generates a microwave (first microwave) and transmits the microwave by radio via an antenna A 1 . 
     The microwave apparatus  5  receives via an antenna A 2  the microwave transmitted by radio from the microwave apparatus  4  via the antenna A 1 . Further, the microwave apparatus  5  demodulates the received microwave to a baseband signal, extracts a clock (first clock) from a cycle of data (first data) included in this baseband signal, imports this baseband signal in synchronization with this clock, and plays back the data (first data). 
     The radio equipment  3  modulates playback data supplied from the microwave apparatus  5  via an optical cable to a high-frequency signal (first high-frequency signal). The radio equipment  3  transmits this high-frequency signal by radio to outside via an antenna A 3 , and the user terminal  6  receives the high-frequency signal. 
     Thus, the base station apparatus  1  delivers data between the radio equipment control  2  and the radio equipment  3  by radio. In other words, the base station apparatus  1  includes a radio transmission interval in a serial data interface channel provided between the radio equipment control  2  and the radio equipment  3 . Consequently, it is possible to reduce the number of cables such as optical cables installed at an installation place of the base station apparatus  1  and consequently reduce cost necessary to install the cables. 
     In this regard, as described above, very high synchronization accuracy is demanded between the radio equipment control  2  and the radio equipment  3 . However, there is a concern that as the base station apparatus  1  delivers data between the radio equipment control  2  and the radio equipment  3  by radio, synchronization accuracy will be lowered due to an influence of phase noise unless a countermeasure is taken. Hence, the base station apparatus  1  improves the synchronization accuracy by narrowing a loop bandwidth of a PLL (Phase Locked Loop) circuit (not shown in  FIG. 1 ) which is provided to the microwave apparatus  5  at a reception side thereof, and generates a synchronization clock of a clock extracted from a baseband signal. However, narrowing the loop bandwidth of the PLL circuit is likely to unlock the PLL circuit when a frequency fluctuation amount of the clock becomes large. As a result, an unintentional frequency fluctuation caused by switching a clock and an abrupt frequency fluctuation due to a radio abnormality are likely to make it impossible to satisfy radio standards (CPRI standards) between the radio equipment  3  and the user terminal  6  or maintain a link between the radio equipment control  2  and the user terminal  6 . 
     Hence, the microwave apparatus  5  has a function of controlling and determining whether or not it is possible to execute synchronization processing on data accompanying playback of a clock according to a signal quality of the radio transmission interval. More specifically, the microwave apparatus  5  outputs dummy data (first dummy data) instead of played back data until the frequency fluctuation amount of the clock stabilizes within a predetermined range when the frequency fluctuation amount of the clock exceeds the predetermined range. In this regard, the dummy data is data which satisfies synchronization accuracy to satisfy radio standards which are demanded between the radio equipment  3  and the user terminal  6 . Hence, even when the frequency fluctuation amount of the clock becomes large, the base station apparatus  1  can satisfy the radio standards (CPRI standards) between the radio equipment  3  and the user terminal  6  and maintain the link between the radio equipment control  2  and the user terminal  6 . Consequently, it is possible to quickly resume communication when, for example, the frequency fluctuation amount of the clock becomes small. 
     A case where the interface between the radio equipment control  2  and the radio equipment  3  employs the CPRI standards has been described in the present embodiment. However, the interface is not limited to this and may employ other radio standards which demand high synchronization accuracy. 
     Further, only the transmission path of the base station apparatus  1  has been described in the present embodiment. However, the present invention is also applicable to a reception path of the base station apparatus  1 . This will be briefly described below. 
     In this case, for example, the radio equipment  3  receives a high-frequency signal (second high-frequency signal) transmitted by radio from the user terminal  6  via the antenna A 3 , and demodulates the received high-frequency signal to a baseband signal (second baseband signal). The microwave apparatus  5  modulates the baseband signal demodulated by the radio equipment  3  to a microwave (second microwave), and transmits the microwave by radio via the antenna A 2 . The microwave apparatus  4  receives via the antenna A 1  the microwave transmitted by radio from the microwave apparatus  5  via the antenna A 2 . Further, the microwave apparatus  4  demodulates the received microwave to a baseband signal, then extracts a clock (second clock) from a cycle of data (second data) included in this baseband signal, imports this baseband signal in synchronization with this clock, and plays back the data (second data). The radio equipment control  2  receives the data played back by the microwave apparatus  4 , outputs the data to a higher level apparatus and executes various types of processing based on this data. In this regard, when the frequency fluctuation amount of the extracted clock exceeds the predetermined range, the microwave apparatus  4  outputs dummy data (second dummy data) instead of the played back data. In this regard, the dummy data is data which satisfies synchronization accuracy. Thus, even when the frequency fluctuation amount of the clock becomes large, the base station apparatus  1  can satisfy the radio standards (CPRI standards) between the radio equipment  3  and the user terminal  6 , and maintain the link between the radio equipment control  2  and the user terminal  6 . Consequently, it is possible to quickly resume communication when, for example, the frequency fluctuation amount of the clock becomes small. 
     Second Embodiment 
     Specific configurations and operations of microwave apparatuses  4  and  5  will be described in the present embodiment. 
     (Specific Configuration of Microwave Apparatus  4 ) 
       FIG. 2  is a block diagram illustrating the specific configuration of the microwave apparatus  4  at a side of a radio equipment control  2 . 
     As illustrated in  FIG. 2 , the microwave apparatus  4  includes a signal processing unit  40 , a DA converter (DAC)  42 , a modulator  43 , a RF circuit  44  and a PLL circuit  45 . The signal processing unit  40  includes at least a CDR (Clock Data Recovery) circuit  41 . 
     The signal processing unit  40  is a unit that performs signal processing on a baseband signal (an optical signal according to serial CPRI standards) from the radio equipment control  2 , and outputs a digital parallel signal having a bandwidth for transmitting the baseband signal by radio. In this regard, the CDR circuit  41  provided to the signal processing unit  40  extracts data D 1  and a reference clock RCLK 1  from the baseband signal from the radio equipment control  2 . For example, the CDR circuit  41  extracts the reference clock RCLK 1  from a cycle of the data D 1  included in the baseband signal from the radio equipment control  2 . 
     The PLL circuit  45  is a circuit which outputs a clock CLK 1  having the same phase as that of the reference clock RCLK 1 . 
     The PLL circuit  45  is a circuit which generates the clock CLK 1  synchronized with the reference clock RCLK 1  outputted from the CDR circuit  41 . For example, the PLL circuit  45  includes a phase comparator which compares phases of the reference clock RCLK 1  and a feedback clock FCLK 1 , a lowpass filter which generates a control voltage matching a comparison result of the phase comparator, and a voltage control oscillator which outputs an oscillation signal of a frequency matching the control voltage outputted from the lowpass filter. The oscillation signal outputted from the voltage control oscillator is used as the feedback clock FCLK 1 , and is outputted as the clock CLK 1  to an outside of the PLL circuit  45 . 
     The DA converter  42  imports the digital data D 1  in synchronization with the clock CLK 1  and converts the digital data D 1  into an analog signal. The modulator  43  modulates the analog baseband signal outputted from the DA converter  42 , and outputs an intermediate signal. Further, the RF circuit  44  mixes the intermediate signal outputted from the modulator  43  and a local signal outputted from a local oscillator (not illustrated), and outputs a microwave. 
     Furthermore, the microwave outputted from the RF circuit  44  is transmitted by radio to an outside via an antenna A 1 . In this regard, the microwave outputted from the RF circuit  44  may be amplified by a high output amplifier which is not illustrated, unnecessary components of the microwave may be removed by a bandpass filter which is not illustrated, and then the microwave may be transmitted by radio to the outside via the antenna A 1 . 
     (Specific Configuration of Microwave Apparatus  5 ) 
       FIG. 3  is a block diagram illustrating the specific configuration of the microwave apparatus  5  at a side of a radio equipment  3 . 
     As illustrated in  FIG. 3 , the microwave apparatus  5  includes a RF circuit  51 , a demodulator  52 , an AD converter (ADC)  53 , a signal processing unit  54 , a CPRI signal generating unit (output data generating unit)  55 , a PLL circuit (first PLL circuit)  56 , a PLL circuit (second PLL circuit)  57 , a fixed voltage generating unit  58 , an oscillator  59  and a selection circuit  60 . 
     The RF circuit  51  receives via an antenna A 2  a microwave transmitted by radio from the microwave apparatus  4  via the antenna A 1 . Further, the RF circuit  51  mixes the received microwave and the local signal outputted from the local oscillator (not illustrated) and outputs an intermediate signal. The demodulator  52  demodulates the intermediate signal outputted from the RF circuit  51 , and outputs a baseband signal. The AD converter  53  converts the analog baseband signal in synchronization with a clock CLK 2  outputted from the PLL circuit  56  described below, into a digital signal. 
     The signal processing unit  54  plays back the data D 1  and extracts the clock CLK 1  from the digital baseband signal outputted from the AD converter  53 . For example, the signal processing unit  54  extracts the clock CLK 1  from a cycle of the baseband signal outputted from the AD converter  53 . 
     Further, the signal processing unit  54  also has a function of determining whether or not a frequency fluctuation amount of the clock CLK 1  is in a predetermined range. For example, the signal processing unit  54  outputs a determination result S 1  of an L level when the frequency fluctuation amount of the clock CLK 1  is in the predetermined range. Further, the signal processing unit  54  outputs the determination result S 1  of an H level when the frequency fluctuation amount of the clock CLK 1  exceeds the predetermined range due to a radio abnormality or an abnormality of a transmission side apparatus (the radio equipment control  2  or the microwave apparatus  4 ). 
     In this regard, the signal processing unit  54  may directly detect the frequency fluctuation amount of the clock CLK 1  from the received baseband signal or may indirectly detect the frequency fluctuation amount of the clock CLK 1  by detecting a CN (Carrier-to-Noise) ratio (a signal-to-noise ratio) of the received baseband signal. When the CN ratio deteriorates and becomes a predetermined value or less, it is highly probable that a phenomenon that a jitter deteriorates occurs, and the PLL circuit  56  is unlocked. 
     Further, the predetermined range can be arbitrarily set. For example, the predetermined range is determined based on a demanded specification of the base station apparatus. 
     The PLL circuit  56  is a circuit which generates the clock CLK 2  having the same phase as that of the clock CLK 1  by using the clock CLK 1  extracted by the signal processing unit  54  as a reference clock. The PLL circuit  56  may output a state signal LS 1  indicating a whether or not the PLL circuit  56  is in a locked state. 
     In this regard, the PLL circuit  56  widens a loop bandwidth to make it possible to maintain the locked state even when a clock frequency at the side of the radio equipment control  2  fluctuates. 
     The oscillator  59  outputs an oscillation signal of a fixed frequency as a dummy clock (first dummy clock) DCLK based on a fixed voltage generated by the fixed voltage generating unit  58 . 
     The selection circuit  60  selects and outputs one of the clock CLK 2  (also referred to as a normal clock) and the dummy clock DCLK based on the determination result S 1  generated by the signal processing unit  54 . For example, the selection circuit  60  selects and outputs the clock CLK 2  when the determination result S 1  indicates the L level (when the frequency fluctuation amount of the clock CLK 1  is in the predetermined range), and selects and outputs the dummy clock DCLK when the determination result S 1  indicates the H level (when the frequency fluctuation amount of the clock CLK 1  exceeds the predetermined range). The clock selected and outputted by the selection circuit  60  is inputted as a reference clock RCLK 2  to the PLL circuit  57 . 
     The PLL circuit  57  outputs a clock CLK 3  having the same phase as that of the reference clock RCLK 2  outputted from the selection circuit  60 . More specifically, when the frequency fluctuation amount of the clock CLK 1  is in the predetermined range, the selection circuit  60  selects and outputs the clock CLK 2 , and therefore the PLL circuit  57  outputs the clock CLK 3  having the same phase as that of the clock CLK 2 , and, when the frequency fluctuation amount of the clock CLK 1  exceeds the predetermined range, the selection circuit  60  selects and outputs the dummy clock DCLK, and therefore the PLL circuit  57  outputs the clock CLK 3  having the same phase as that of the dummy clock DCLK. 
     For example, the PLL circuit  57  includes a phase comparator which compares phases of the reference clock RCLK 2  outputted from the selection circuit  60  and a feedback clock FCLK 3 , a lowpass filter which outputs a control voltage matching a comparison result of the phase comparator, and a voltage control oscillator which outputs an oscillation signal of a frequency matching the control voltage outputted from the lowpass filter. The oscillation signal outputted from the voltage control oscillator is used as the feedback clock FCLK 3 , and is outputted as the clock CLK 3  to the outside of the PLL circuit  57 . 
     In this regard, the PLL circuit  57  narrows a loop bandwidth compared to the loop bandwidth of at least the PLL circuit  56  to satisfy synchronization accuracy according to the CPRI standards. 
     The CPRI signal generating unit  55  generates data Dout satisfying the synchronization accuracy according to the CPRI standards by performing synchronization processing (importing and outputting in synchronization with the clock CLK 3 ) on the data D 1  played back by the signal processing unit  54  by using the clock CLK 3 . 
     (First Wireless Communication Method Between Radio Equipment Control  2  and Radio Equipment  3 ) 
     Next, a first wireless communication method between the radio equipment control  2  and the radio equipment  3  will be described.  FIG. 4  is a flowchart illustrating the first wireless communication method between the radio equipment control  2  and the radio equipment  3  which compose the base station apparatus  1 . 
     As illustrated in  FIG. 4 , the microwave apparatus  5  determines whether or not the frequency fluctuation amount of the clock CLK 1  is in a normal range (predetermined range) during a normal operation (step S 101 ). 
     For example, when the frequency fluctuation amount of the clock CLK 1  is in the normal range (YES in step S 101 ), the microwave apparatus  5  maintains selection of the normal clock CLK 2  (step S 104 ) and continues the normal operation. 
     On the other hand, when the frequency fluctuation amount of the clock CLK 1  exceeds the normal range due to a radio abnormality or an abnormality of a transmission side apparatus (the radio equipment control  2  or the microwave apparatus  4 ) (NO in step S 101 ), the microwave apparatus  5  selects the dummy clock DCLK (step S 102 ) and temporarily stops the normal operation. In this case, the microwave apparatus  5  performs synchronization processing on the data D 1  by using the clock CLK 3  generated based on the dummy clock DCLK. Thus, the microwave apparatus  5  outputs the dummy data Dout which satisfies the synchronization accuracy according to the CPRI standards while the normal operation stops, too. 
     Subsequently, the microwave apparatus  5  maintains the selection of the dummy clock DCLK while the frequency fluctuation amount of the clock CLK 1  exceeds the normal range (NO in step S 103 ), and switches the selection from the dummy clock DCLK to the normal clock CLK 2  (step S 104 ) when the frequency fluctuation amount of the clock CLK 1  is in the normal range (YES in step S 103 ), and resumes the normal operation. 
     Thus, while the base station apparatus  1  temporarily stops communication even when the frequency fluctuation amount of the clock increases due to an influence of a radio abnormality or an abnormality of the transmission side apparatus, the base station apparatus  1  can satisfy the radio standards (CPRI standards) between the radio equipment  3  and the user terminal  6  and maintain a link between the radio equipment control  2  and the user terminal  6 . Consequently, when, for example, the frequency fluctuation amount of the clock becomes small, it is possible to quickly resume communication. 
     A case where the signal processing unit  54  determines whether or not the frequency fluctuation amount of the clock CLK 1  is in the predetermined range has been described as an example in the present embodiment. However, the present invention is not limited to this. When receiving a notification for switching the frequency of the clock CLK 1  from the radio equipment control  2  or receiving a notification for switching radio transmission of a microwave from the microwave apparatus  4  to another microwave apparatus (third microwave apparatus) provided in parallel to the microwave apparatus  4 , the signal processing unit  54  may determine that the frequency fluctuation amount of the clock CLK 1  exceeds the predetermined range, and output the determination result S 1  of the H level before the frequency fluctuation amount of the clock CLK 1  exceeds the predetermined range. This will more specifically described with reference to  FIG. 5 . 
     (Wireless Communication Method Between Radio Equipment Control  2  and Radio Equipment  3 ) 
       FIG. 5  is a flowchart illustrating a second wireless communication method between the radio equipment control  2  and the radio equipment  3  which compose the base station apparatus  1 . 
     As illustrated in  FIG. 5 , when receiving no notification from the radio equipment control  2  during the normal operation (NO in step S 201 ), the microwave apparatus  5  maintains selection of the normal clock CLK 2  (step S 204 ) and continues the normal operation. 
     On the other hand, when receiving a notification for switching the frequency of the clock CLK 1  from the radio equipment control  2  or when receiving a notification for switching radio transmission of a microwave from the microwave apparatus  4  to another microwave apparatus (YES in step S 201 ), the microwave apparatus  5  selects the dummy clock DCLK (step S 202 ) and temporarily stops the normal operation. In this case, the microwave apparatus  5  performs synchronization processing on the data D 1  by using the clock CLK 3  generated based on the dummy clock DCLK. By this means, the microwave apparatus  5  outputs the dummy data Dout which satisfies the synchronization accuracy according to the CPRI standards while the normal operations stops, too. 
     Subsequently, the microwave apparatus  5  maintains selection of the dummy clock DCLK when the frequency fluctuation amount of the clock CLK 1  exceeds the normal range (NO in step S 203 ), and switches selection from the dummy clock DCLK to the normal clock CLK 2  (step S 204 ) when the frequency fluctuation amount of the clock CLK 1  is in the normal range (YES in step S 203 ), and resumes the normal operation. 
     Thus, while the base station apparatus  1  temporarily stops communication even when the frequency fluctuation amount of the clock is intentionally increased, the base station apparatus  1  can satisfy the radio standards (CPRI standards) between the radio equipment  3  and the user terminal  6  and maintain the link between the radio equipment control  2  and the user terminal  6 . Consequently, when, for example, the frequency fluctuation amount of the clock becomes small, it is possible to quickly resume communication. 
     (Difference from Related Art) 
     Patent Literature 2 discloses only switching between a regular synchronization clock and a dummy clock, and neither discloses nor suggests switching a regular synchronization clock and a dummy clock based on whether or not the frequency fluctuation amount of the clock used during the normal operation is in the predetermined range. By contrast with this, the base station apparatus according to the present invention switches between a regular synchronization clock and a dummy clock based on whether or not the frequency fluctuation amount of the clock used during the normal operation is in the predetermined range. 
     The present invention has been described above with reference to the embodiments. However, the present invention is not limited to the above. The configurations and the details of the present invention can be variously changed within the scope of the invention as long as one of ordinary skill in the art can understand the changes. 
     This application claims priority to Japanese Patent Application No. 2014-257495 filed on Dec. 19, 2014, the entire contents of which are incorporated by reference herein. 
     REFERENCE SIGNS LIST 
     
         
           1  BASE STATION APPARATUS 
           2  RADIO EQUIPMENT CONTROL 
           3  RADIO EQUIPMENT 
           4  MICROWAVE APPARATUS 
           5  MICROWAVE APPARATUS 
           6  USER TERMINAL 
           40  SIGNAL PROCESSING UNIT 
           41  CDR CIRCUIT 
           42  DA CONVERTER 
           43  MODULATOR 
           44  RF CIRCUIT 
           45  PLL CIRCUIT 
           51  RF CIRCUIT 
           52  DEMODULATOR 
           53  AD CONVERTER 
           54  SIGNAL PROCESSING UNIT 
           55  CPRI SIGNAL GENERATING UNIT 
           56  PLL CIRCUIT 
           57  PLL CIRCUIT 
           58  FIXED VOLTAGE GENERATING UNIT 
           59  OSCILLATOR 
           60  SELECTION CIRCUIT 
         A 1  to A 3  ANTENNA