Abstract:
A radio communication system for conducting the radio communication with a terminal device includes a first radio communication apparatus for processing a first signal; a second radio communication apparatus for processing a second signal, and to combine the first signal and the second signal; a common amplifier for amplifying the first and second signals from the second radio communication apparatus, transmitting the amplified signals to the terminal device, amplifying first and second signals received from the terminal device, and outputting the amplified signals to the second radio communication apparatus, the second radio communication apparatus separating the amplified first and second signals; and a switching apparatus coupled to the first radio communication apparatus, the second radio communication apparatuses and the common amplifier to disconnect the first or second radio communication apparatus when a fault occurs in the first or second radio communication apparatus, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of prior Japanese Patent Application No. 2008-297680, filed on Nov. 21, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Certain aspects of the present invention discussed herein are related to a radio communication apparatus, radio communication system and signal switching method in the radio communication system. 
     BACKGROUND 
       FIG. 18  is a diagram illustrating an example of a configuration of a conventional radio base station apparatus (see CPRI Specification V2.1, for example). A radio base station apparatus  200  includes a radio equipment controller (REC)  210  for processing the baseband signal and a radio equipment (RE)  220  for processing the radio signal. The radio equipment controller  210  and the radio equipment  220  are connected to each other by an optical fiber  230 . For this connection, an interface CPRI is used. 
       FIG. 19  is a diagram illustrating an example of a configuration of a conventional radio communication system using the CPRI (common public radio interface) as an interface (see PCT/JP2007/000565, for example). The radio communication system  100  includes a first radio base station apparatus  2 , a second radio base station apparatus  4  and superordinate apparatuses  1 ,  3  connected to the radio base station apparatuses  2 ,  4 , respectively. The first radio base station apparatus  2  and the second radio base station apparatus  4  are connected to each other through interface conversion units  213 ,  513  of radio equipment controllers  21 ,  51 , respectively. The radio communication system  100  conducts radio communication with terminal devices through an antenna  5 . The radio communication system  100  can provide two radio services A, B to the terminal devices. 
     The conventional radio communication system  100 , however, harbors the problem that in the case where a fault occurs in the interface conversion unit  513 , the signals from the second radio equipment controller  51  adapted for the radio service B and the signal from the first radio equipment controller  21  adapted for the radio service A cannot be output to a radio equipment  52 . In such a case, the radio communication system  100  can not provide radio service A or B to the terminal devices continuously. 
     SUMMARY 
     Accordingly, in an aspect, an object of the invention is to provide a radio communication system, radio communication apparatus and radio communication system to contribute to continuous service providing. 
     According to a certain aspect of the invention, there is provided a radio communication system for conducting radio communication with a terminal device, including: a first radio communication apparatus for processing a first signal; a second radio communication apparatus for processing a second signal, and to combine the first signal and the second signal; a common amplifier for amplifying the first and second signals from the second radio communication apparatus, and transmitting the amplified signals to the terminal device; and a switching apparatus coupled to the first and second radio communication apparatus and the common amplifier to disconnect the first or second radio communication apparatus when a fault occurs in the first or second radio communication apparatus, respectively. This aspect may further include the common amplifier amplifying first and second signals received from the terminal device, outputting the amplified signals from the terminal device to the second radio communication apparatus, and the second radio communication apparatus separating the amplified first and second signals. 
     According to a certain aspect of the invention, there is provided a radio communication apparatus for conducting radio communication with a terminal device, including: a radio controller for combining a first signal with a second signal output from another radio communication apparatus and separating combined first and second signals received from a common amplifier; the common amplifier for amplifying and transmitting to the terminal device the first and second signals output from the radio controller on the one hand, and amplifying and outputting to the radio controller the combined first and second signals received from the terminal device on the other hand; and a switching unit coupled to the other radio communication apparatus and the radio controller and the common amplifier to disconnect the another radio base station apparatus or the radio controller when a fault occurs in the other radio base station apparatus or the radio controller, respectively. 
     According to a certain aspect of the invention, there is provided a signal switching method for a radio communication system including a first radio communication apparatus for processing a first signal, a second radio communication apparatus for processing a second signal, and to combine the first and second signals, and a common amplifier for amplifying and transmitting to a terminal device the first and second signals output from the second radio communication apparatus on the one hand and amplifying and outputting to the second radio communication apparatus the first and second signals received from the terminal device on the other hand, the second radio communication apparatus separating the first and second signals received from the common amplifier, wherein upon occurrence of a fault in the first or second radio communication apparatus, the first or second radio communication apparatus is disconnected by a switching apparatus coupled to the first and second radio communication apparatuses and the common amplifier. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a configuration example of a radio communication system. 
         FIG. 2  is a flowchart illustrating an example of switching control. 
         FIG. 3  illustrates another configuration example of a radio communication system. 
         FIG. 4  illustrates an example of CPRI protocol. 
         FIG. 5  illustrates an operation example when an abnormality is detected. 
         FIG. 6  illustrates another configuration example of a radio communication system. 
         FIG. 7  is a flowchart illustrating a example of switching control. 
         FIG. 8  illustrates still another configuration example of a radio communication system. 
         FIG. 9  is a flowchart illustrating an example of switching control. 
         FIG. 10  is a flowchart illustrating an example of switching control. 
         FIG. 11  illustrates yet another configuration example of a radio communication system. 
         FIG. 12  illustrates a configuration example of the second interface converging unit and the signal switching unit. 
         FIG. 13  illustrates yet another configuration example of a radio communication system. 
         FIG. 14  illustrates yet another configuration example of a radio communication system. 
         FIG. 15  illustrates yet another configuration example of a radio communication system. 
         FIG. 16  illustrates yet another configuration example of a radio communication system. 
         FIG. 17  illustrates yet another configuration example of a radio communication system. 
         FIG. 18  illustrates a configuration example of a radio base station apparatus of related art. 
         FIG. 19  illustrates a configuration example of a radio communication system of related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following embodiments are described with reference to the figures. 
       FIG. 1  is a diagram illustrating an example of the configuration of a radio communication system  300 . The radio communication system  300  includes a first radio base station apparatus (or a radio communication apparatus)  10  adapted for the radio service A, a second radio base station apparatus  20  adapted for the radio service B, a radio equipment (RE)  24  and a switching apparatus  40 . The radio service A and the radio service B are different types of communication methods and the radio communication system  300  can handle the signals of the different types of communication methods. For example, the radio service A is a communication method for an old service, and the radio service B a communication method for a new service. 
     The first radio base station apparatus  10  includes a first radio equipment controller (REC)  11 . The first radio equipment controller  11  includes a first controller  111 , a first baseband signal processor  112  and a first interface conversion unit  113 . 
     The second radio base station apparatus  20 , on the other hand, includes a second radio equipment controller (REC)  22 . The second radio equipment controller  22  includes a second controller  221 , a second baseband signal processor  222  and a second interface conversion unit  223 . 
     The first and second controllers  111 ,  221  control the interior of each radio base station apparatus  10 ,  20  by distributing a sync signal to each radio base station apparatus  10 ,  20  or set individual users. 
     First and second baseband signal processors  112 ,  222  convert the signal from a superordinate apparatus into, for example, a baseband signal having an I signal and a Q signal and also change the format of the baseband signal from the first and second interface conversion units  113 ,  223  into the one capable of transmission to the superordinate apparatus. 
     The first interface conversion unit  113  includes a first CPRI processing unit  1131  and a first conversion unit  1132 . 
     The second interface conversion unit  223  includes a second CPRI processing unit  2231 , a second conversion unit  2232 , a third conversion unit  2233 , a third CPRI processing unit  2234 , a service multiplexer/demultiplexer  2235 , a fourth conversion unit  2236 , a fourth CPRI processing unit  2237  and a fifth conversion unit  2238 . 
     The first and second CPRI processing units  1131 ,  2231  convert the baseband signal into the CPRI signal having the CPRI format and outputs the CPRI signal to the first and second conversion units  1132 ,  2232 , respectively. Also, the first and second CPRI processing units  1131 ,  2231  convert the CPRI signal from the first and second conversion units  1132 ,  2232  into the baseband signal. 
     The first and second conversion units  1132 ,  2232  convert the CPRI signal from the first and second CPRI processing units  1132 ,  2232  into an optical signal, and by converting the optical signal into the CPRI signal, output the CPRI signal to the first and second CPRI processing units  1131 ,  2231 . 
     The third and fourth conversion units  2233 ,  2236  convert the optical signal into the CPRI signal as an electrical signal, and outputs it to the third and fourth CPRI processing units  2234 ,  2237  on the one hand, and convert the CPRI signal from the third and fourth CPRI processing units  2234 ,  2237  into an optical signal on the other hand. 
     The third and fourth CPRI processing units  2234 ,  2237  convert the CPRI signal into the baseband signal, and output it to the service multiplexer/demultiplexer  2235  on the one hand, and convert the baseband signal from the service multiplexer/demultiplexer  2235  into the CPRI signal on the other hand. 
     The service multiplexer/demultiplexer  2235  multiplexes or synthesizes the two CPRI signals corresponding to two radio services A and B, respectively, and outputs by separating the multiplexed CPRI signal from the fifth conversion unit  2238  for each radio service. 
     The fifth conversion unit  2238  converts the multiplexed CPRI signal into an optical signal, and outputs the result to the switching apparatus  40 . The optical signal from the switching apparatus  40  is converted into the CPRI signal and output to the service multiplexer/demultiplexer  2235 . 
     The radio equipment  24  includes a third interface conversion unit  241  and a transmission/reception amplifier  242 . 
     The third interface conversion unit  241  converts the optical signal from the switching apparatus  40  into the CPRI signal, and by separating or extracting the baseband signal corresponding to the radio services A and B from the converted CPRI signal (multiplexed signal), outputs the resulting signal to the transmission/reception amplifier  242 . Also, the third interface conversion unit  241  multiplexes the baseband signal corresponding to the radio services A and B output from the transmission/reception amplifier  24 , and by converting it into an optical signal, outputs the resulting signal to the switching apparatus  40 . 
     The transmission/reception amplifier  242  modulates and amplifies the baseband signal from the third interface conversion unit  241 , and outputs the resulting signal to the antenna  5 . Also, the transmission/reception amplifier  242  amplifies the signal corresponding to the two radio services A and B received through the antenna  5 , and outputs the resulting signal to the third interface conversion unit  241  by down conversion or the like. 
     As described above, the radio equipment  24  or the transmission/reception amplifier  242  is a common amplifier shared by the signals corresponding to the radio services A and B. 
     The antenna  5  transmits and receives, to/from the terminal device, the signals corresponding to the two radio services A and B. The radio communication system  300  can provide the two radio services A and B. 
     The switching apparatus  40  includes a first optical switch  41 , a second optical switch  42 , first and second optical couplers  43 ,  45 , sixth and seventh conversion units  44 ,  46 , a signal monitor/autonomous switching controller (hereinafter referred to as the signal monitor unit)  47  and a switching controller  48 . 
     The first optical switch  41  is connected between the first conversion unit  1132  and the fourth conversion unit  2236 , and further, between the first conversion unit  1132  and the second optical switch  42 . The first optical switch  41 , based on the switching control signal from the switching controller  48 , outputs the signal adapted for the radio service A from the first conversion unit  1132  to one of the second optical switch  42  and the fourth conversion unit  2236  through the terminal “a” or “b”, respectively. Also, the first optical switch  41 , based on the switching control signal, outputs one of the signal from the second optical switch  42  and the signal from the fourth conversion unit  2236  to the first conversion unit  1132 . 
     The first optical coupler  43  diverges (or copies) the signal flowing between the first radio base station apparatus  10  and the switching apparatus  40 , and outputs the resulting signal to the sixth conversion unit  44 . The sixth conversion unit  44  converts the diverged optical signal into an electrical signal and outputs it to the signal monitor unit  47 . 
     The second optical switch  42  is connected between the fifth conversion unit  2238  and the radio equipment  24 , and furthermore, between the first optical switch  41  and the radio equipment  24 . The second optical switch  42 , based on the switching control signal from the switching controller  48 , outputs one of the signals adapted for the radio service A from the first optical switch  41  and the multiplexed signal from the fifth conversion unit  2238  to the radio equipment  24  through the terminal “c” or “d.” Also, the second optical switch  42 , based on the switching control signal, outputs the signal from the radio equipment  24  to the fifth conversion unit  2238  or the first optical switch  41 . 
     The second optical coupler  45  is connected between the second optical switch  42  and the radio equipment  24  and diverges (or copies) the signal flowing therebetween. The seventh conversion unit  46  converts an optical signal into an electrical signal, and outputs the resulting signal to the signal monitor unit  47 . 
     The signal monitor unit  47 , based on the signal from the sixth or seventh conversion unit  44 ,  46 , detects whether a fault has occurred in the first radio base station apparatus  10  or the second radio equipment controller  22 , and notifies the result to the switching controller  48 . 
     The switching controller  48 , based on the notification from the signal monitor unit  47 , outputs a switching control signal to the first and second optical switches  41 ,  42 . 
     Next, the switching operation of the first and second optical switches  41 ,  42  is illustrated. First, the down direction (the direction from each radio base station apparatus  10 ,  20  to the radio equipment  24 ) is illustrated.  FIG. 2  is a flowchart illustrating an example of the switching operation. 
     The switching apparatus  40 , once activated (S 10 ), is initialized (S 11 ). The switching controller  48 , upon detection of the activation of the switching apparatus  40 , outputs a switching control signal to switch the first optical switch  41  to the terminal “b” and the second optical switch  42  to the terminal “d” as an initialize operation. The first and second optical switches  41 ,  42  are operated based on the switching control signal.  FIG. 1  illustrates an example of the switching operation for initialization. 
     In this case, the signal for the radio service A output from the first radio base station apparatus  10  is output to the second radio base station apparatus  20  adapted for the radio service B through the first optical switch  41 . Also, the two multiplexed signals for the radio services A and B are output to the radio equipment  24  through the second optical switch  42  from the second radio base station apparatus  20 . In this case, the two signals of the radio services A, B are transmitted from the antenna  5 , and therefore, the radio communication system  300  can provide the two radio services A, B to the terminal device. 
     Then, the signal monitor unit  47  of the switching apparatus  40  starts to monitor the multiplexed signal output from the second optical switch  42  (S 12 ). 
     The signal monitor unit  47 , upon detection of a fault in the signal output from the second optical switch  42  (YES in S 13 ), notifies the switching controller  48  that a fault has been detected (S 14 ). 
     The switching controller  48 , upon reception of this notice, outputs a switching control signal to the switches  41 ,  42  to switch the first optical switch  41  from terminal “b” to terminal “a” and the second optical switch  42  from terminal “d” to terminal “c”. 
       FIG. 3  is a diagram illustrating an example of the configuration of the radio communication system  300  after the switching operation. After the switching operation, the signal adapted for the radio service A is output from the first optical switch  41  to the second optical switch  42 , and further, from the second optical switch  42  to the radio equipment  24 . The signal adapted for the radio service A is not output to the second radio base station apparatus  20 , and the output signal from the second radio base station apparatus  20  is not output to the radio equipment  24 . By the switching operation, the second radio base station apparatus  20  is separated from the radio communication system  300 , and the signal adapted for the radio service A is output to the radio equipment  24  without being affected by the second radio base station apparatus  20  adapted for the radio service B. 
     Returning to  FIG. 2 , the signal monitor unit  47 , as far as no fault is detected (NO in step S 13 ), repeats the process until a fault is detected (loop of S 13 ). 
     As described above, the radio communication system  300 , upon detection of a fault of the signal output from the second radio base station apparatus  20 , outputs a signal adapted for the radio service A through the switching apparatus  40 . The radio communication system  300 , therefore, can continue to provide the service even after a fault occurs in the second radio equipment controller  22  adapted for the radio service B. 
     Next, an example of fault detection is illustrated.  FIG. 4  is a diagram illustrating an example of the CPRI protocol. In the layer  1  of the CPRI protocol, the disconnection of an optical interface can be detected by removing a connector, for example. By receiving this signal indicating the disconnection, the signal monitor unit  47  can detect a fault. As another method, the signal monitor unit  47  can detect a fault taking advantage of, for example, High-Level Data Link Control (HDLC) of the layer  2 . In the case of “HDLC”, the information indicating a link disconnection or a protocol fault, if developed, is inserted in the “HDLC” field, and therefore, is detected by the signal monitor unit  47 . Further, the signal monitor unit  47  can detect a fault by monitoring the diagnosis data or the data unique to each vendor using “Vender Specific” or “Control &amp; Management Plane” and detecting the data indicating a fault. The information is inserted in each field by, for example, the service multiplexer/demultiplexer  2235  or the fifth conversion unit  2238 . 
     The aforementioned case represents an example in which a fault is detected by the signal monitor unit  47 . In this case, the signal monitor unit  47  can detect a fault up to the stage before the second optical switch  42  from the service multiplexer/demultiplexer  2235  of the second radio equipment controller  22 . 
     A fault which occurs in the range from the second controller  221  and the fourth conversion unit  2236  up to the stage before the service multiplexer/demultiplexer  2235  in the second radio base station apparatus  20 , on the other hand, can be detected by, for example, the service multiplexer/demultiplexer  2235 . The service multiplexer/demultiplexer  2235  handles the CPRI signal, and therefore, can detect a fault in the example illustrated in  FIG. 4 . The operation performed upon detection of a fault in such a case is illustrated.  FIG. 5  is a flowchart illustrating an example of such an operation. 
     The service multiplexer/demultiplexer  2235 , upon detection of a fault in the signal on the radio service A side, i.e. the signal input/output in the range from the fourth conversion unit  2236  up to the fourth CPRI processing unit  2237  (YES in S 20 , S 21 ), outputs a signal for the radio service B side without multiplexing the signal for the radio service A side (S 22 ). 
     In the case where the service multiplexer/demultiplexer  2235  detects a fault in the signal on the radio service B side, i.e. the signal input or output in the range from the second controller  221  up to the third CPRI processing unit  2234  (YES in S 21 ), on the other hand, the signal on the radio service B side is not multiplexed but the signal on the radio service A side is output (S 22 ). 
     In the case where a fault is detected in the service multiplexer/demultiplexer  2235 , the switching apparatus  40  is kept in the initialized state. In the case where no fault is detected or cannot be detected in the service multiplexer/demultiplexer  2235 , however, a fault may be detected by the signal monitor unit  47  of the switching apparatus  40  described above. 
     The switching apparatus  40  can also monitor a fault of the first radio base station apparatus  10 . The signal monitor unit  47  monitors the signal applied through the first optical coupler  43  and the sixth conversion unit  44 , and performs the switching operation as illustrated in  FIG. 1  for initialization. The signal monitor unit  47 , upon detection of a fault, notifies the switching controller  48 . The switching controller  48  outputs a switching control signal in such a manner that the first optical switch  41  is turned to the terminal “a” and the second optical switch turned to the terminal “d.” 
       FIG. 6  is a diagram illustrating an example of the configuration of the radio communication system  300  after switching. The signal adapted for the radio service A is output to the second optical switch  42  through the first optical switch  41 . Since the second optical switch  42  is switched to terminal “d,” however, the signal adapted for the radio service A is not output from the second optical switch  42 . Also, the signal adapted for the radio service A is not transmitted to the second radio base station apparatus  20 . As a result, the signal adapted for the radio service B is output from the second optical switch  42  to the radio equipment  24 . 
     In the case where the switching apparatus  40  detects a fault in the signal from the first radio base station apparatus  10 , the signal from the second radio base station apparatus  20  is output to the radio equipment  24 . Thus, the radio communication system  300  can continuously provide the services. 
     Next, the operation in the up direction is illustrated.  FIG. 7  is a flowchart illustrating an example of the switching operation in the up direction. 
     The switching apparatus  40 , once activated (S 40 ), is initialized (S 41 ). For example, the switching controller  48 , upon detection of the activation of the switching apparatus  40 , outputs a switching control signal in such a manner as to turn the first optical switch  41  to terminal “b” and the second optical switch  42  to terminal “d.” The 
     In this case, the signal from the radio equipment  24  is output to the second radio control apparatus  22  through the second optical switch  42 . Also, the signal adapted for the radio service A output from the second radio base station apparatus  20  is output to the first radio base station apparatus  10  through the first optical switch  41 . 
     Then, the signal monitor unit  47  of the switching apparatus  40  starts to monitor the signal output from the first optical switch  41 , i.e. the signal output from the first optical coupler  43  and the sixth conversion unit  44  (S 42 ). 
     The signal monitor unit  47 , upon detection of a fault in the signal output from the first optical switch  41  (YES in S 43 ), notifies the switching controller  48  that a fault is detected (S 44 ). 
     The switching controller  48 , in response to this notice, outputs a switching control signal to the switches  41 ,  42  in such a manner as to turn the first optical switch  41  to terminal “a” and the second optical switch  42  to terminal “c.” An example of the configuration after the switching operation is illustrated in  FIG. 3 . 
     The signal from the radio equipment  24  is output to the first radio base station apparatus  10  from the second optical switch  42  through the first optical switch  41 . The second radio equipment controller  22  adapted for the radio service B and the radio equipment  24  is disconnected by the two switches  41 ,  42 . Even in the case where a fault occurs in the second radio equipment controller  22 , the signal from the radio equipment  24  can be output to the first radio base station apparatus  10 . As a result, the radio communication system  300  can continuously provide the service to the terminal device also in the up direction. 
     Returning to  FIG. 7 , in the case where no fault is detected in the signal from the sixth conversion unit  44  (NO in S 43 ), the signal monitor unit  47  repeats the process until a fault is detected (loop of S 43 ). 
     Incidentally, in the case where a fault occurs due to trouble between the service multiplexer/demultiplexer  2235  and the fourth conversion unit  2236  of the second radio equipment controller  22 , the signal monitor unit  47  can detect the fault by monitoring the output of the first optical switch  41 . Also, a fault, if developed in the stages before the service multiplexer/demultiplexer  2235  of the second radio equipment controller  22 , can be detected by the service multiplexer/demultiplexer  2235 . In this case, the service multiplexer/demultiplexer  2235  and the switching controller  48  may be connected to each other with a control line, for example, in order that the service multiplexer/demultiplexer  2235  may notify the detected fault occurrence to the switching apparatus  40 . An example of such a configuration is illustrated in  FIG. 8 . The switching controller  48 , upon reception of the notice on the fault occurrence, performs the switching operation in such a manner as to disconnect the second radio equipment controller  22  as illustrated in  FIG. 8 . 
     Next, another embodiment is illustrated. For example, the switching apparatus  40  may be operated based on a command from a superordinate apparatus.  FIGS. 9 to 11  are diagrams illustrating an example of the configuration of the radio communication system  300  in such a case. 
     As illustrated in  FIG. 9 , the radio communication system  300  further includes first and second superordinate apparatuses  60 ,  80 . The first superordinate apparatus  60  is connected to the first radio base station apparatus  10  adapted for the radio service A, and the second superordinate apparatus  80  to the second radio base station apparatus  20  adapted for the radio service B. Further, the first and second controllers  111 ,  221  are each connected to the switching controller  48  of the switching apparatus  40 , respectively, by a control line. 
       FIG. 10  is a flowchart illustrating an example of the switching operation in the first and second controllers  111 ,  221 . The flowchart of  FIG. 10  includes the process both up and down directions. 
     Upon activation of the first and second radio base station apparatuses  10 ,  20  adapted for the radio services A, B (S 50 ), the first or second controller  111 ,  221  judges whether a control signal is received by the switching apparatus  40  from the first or second superordinate apparatus  60 ,  80  (S 51 ). 
     The first or second controller  111 ,  221 , upon reception of the control signal (YES in S 51 ), gives a switch command contained in the control signal to the switching controller  48  through the control line (S 52 ). As a result, the switching apparatus  40  performs the switching operation under the control of the first or second superordinate apparatus  60 ,  80 . As illustrated in  FIG. 3  or  6 , for example, the switching operation is performed in such a manner that the signal from the first or second radio equipment controller  11 ,  22  is output to the radio equipment  24 . As an example, this switching operation is performed for maintenance or to trace the cause of a fault. 
     The first or second controller  111 ,  221 , upon reception of no control signal (NO in S 51 ), on the other hand, judges whether the switching apparatus  40  is controlled due to a fault of the local apparatus (S 53 ). The first and second controllers  111 ,  221 , for example, can detect a signal indicating a fault in the case where a fault occurs in the first or second radio base station apparatus  10 ,  20 . This process can be judged from whether this signal is received or not. 
     In the case where the switching apparatus  40  is controlled due to a fault of the local apparatus (YES in S 53 ), the first or second controller  111 ,  221  instructs the switching controller  48  to perform the switching operation (S 54 ). The switching controller  48  is instructed to perform the switching operation in such a manner that upon detection of a fault in the second radio base station apparatus  20  by the second controller  221 , for example, the first optical switch  41  is turned to terminal “a” and the second optical switch  42  to terminal “c.” As a result, as illustrated in  FIG. 3 , for example, the second radio equipment controller  22  adapted for the radio service B can be separated from the radio communication system  300 . 
       FIG. 11  is a flowchart illustrating an example of the switching operation of the switching apparatus  40 . The flowchart of  FIG. 11  also includes the process in both up and down directions. 
     The switching apparatus  40  is initialized (S 61 ) by the activation thereof (S 60 ). For example, the switching apparatus  40  turns the first optical switch  41  to terminal “b” and the second optical switch to terminal “d.” 
     Then, the signal monitor unit  47  and the switching controller  48  monitors the signal (S 62 ). 
     Next, the switching controller  48 , upon reception of a switching command from the first or second controller  111 ,  221  of the first or second radio base station apparatus  10 ,  20  (YES in S 63 ), outputs a switching control signal to the first and second optical switches  41 ,  42  in accordance with the content of the command (S 64 ). As a result, the switching operation is performed as illustrated in  FIGS. 3 and 6 . 
     On the other hand, in the case where no switch command is received from the first and second controllers  111 ,  221  (NO in S 63 ), the aforementioned process is repeated until such command is received. 
     As another embodiment, the switching apparatus  40  may be arranged not outside but inside of the second radio base station apparatus  20 .  FIGS. 12 and 13  are diagrams illustrating an example of such a configuration of the radio communication system  300 . The second radio base station apparatus  20  includes a signal switching unit  50 . As illustrated in  FIG. 12 , the signal switching unit  50  has the same configuration as the switching apparatus  40  described above. 
     As still another embodiment, the second interface unit  223  of the second radio base station apparatus  20  may be arranged inside the switching apparatus  40 .  FIGS. 14 and 15  illustrate an example of such a configuration of the radio communication system  300 . The switching apparatus  40  further includes a second interface conversion unit  223 . 
     In normal operation, the first and second optical switches  41 ,  42 , as illustrated in  FIG. 14 , are turned to terminals “b” and “d,” respectively. On the other hand, in the case where a fault occurs in the second radio equipment controller  22  or the second interface conversion unit  223 , as illustrated in  FIG. 15 , the first and second optical switches  41 ,  42  are turned to terminals “a” and “c,” respectively. Also in the case where a fault occurs in the first radio base station apparatus  10 , as illustrated in  FIG. 6 , the first and second optical switches  41 ,  42  are turned to terminals “a” and “c,” respectively. Also in this case, the radio services can be provided continuously even if a fault occurs. 
     As a further embodiment, the radio equipment  24  may be arranged inside the second radio base station apparatus  20 .  FIGS. 16 and 17  are diagrams illustrating an example of the configuration of such an example of the radio communication system  300 . As illustrated in  FIG. 16 , the second radio base station apparatus  20  further includes the radio equipment  24 . Also, as illustrated in  FIG. 17 , the second radio base station apparatus  20  may further include a signal switching unit  50  in addition to the radio equipment  24 . 
     According to this embodiment, as described above, there are provided a radio communication system, a radio communication apparatus and a signal switching method for the radio communication system capable of providing the services continuously. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.