1.  Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to an optical micro cell transmission system in which the radio communication zone of a radio base station is divided into a plurality of micro cells, with each micro cell having an optical forward base station and the radio base station connected to the optical forward base stations via optical fibers. 
2.  Description of the Related Art
In a mobile communication system, the service area is divided into radio communication zones with each having a radio base station, and subscribers' terminals such as cellular telephones (portable telephones) communicating with the radio base station by radio channels.  The radio base station is provided with several functions for processing a radio signal in a baseband, interfacing with a switching network for call handling, time division and space division processing in signal transmission, and call switching to a suitable radio channel, as well as providing a radio transmitter and receiver.  Although each radio communication zone should be as small as possible for lower power consumption at subscribers' terminals and effective use of frequency resources, the service area divided into smaller radio communication zones results in an increase in the number of radio base stations, thereby increasing costs for constructing the radio base stations as well as required time and labor for maintaining them. 
Thus, new optical micro cell transmission systems in which the radio communication zone of a radio base station is divided into a plurality of micro cells with each micro cell having an optical forward base station, and the radio base station connected to the optical forward base stations via optical fibers have been developed. 
The optical forward base station does not have functions such as a processing a radio signal in a baseband, interfacing with switching networks, and call switching, but simply comprises a radio transmitting and receiving function.  The optical micro cell transmission system is disclosed in, for example, Japanese Patent Laid Open No.  196629/92, No.  283725/91, No.  35234/92, and No.  246929/92.  Since the optical forward base station can be inexpensively constructed in any location and its required maintenance is not difficult or time-consuming, the use of such an optical micro cell transmission system reduces the power consumption of subscribers' terminals and enables frequency resources to be effectively used without increasing costs. 
FIG.  2 is a block diagram showing the configuration of a conventional optical micro cell transmission system. 
A radio base station 51 has an optical interface section 52 connected thereto, and the optical interface section 52 is connected to an optical forward base station 53 via upward and downward optical fibers 56, 55. 
The optical interface section 52 comprises a downward signal amplifier 61 for amplifying a downward transmitted signal 57 from the radio base station 51 to a specified level, an electricity-light (E/O) converter 62 for converting into an optical signal an electric signal amplified by the downward signal amplifier 61 and sending the signal out to the optical forward base station 53 via the optical fiber 55, a light-electricity (O/E) converter 63 for converting into an electric signal an optical signal transmitted from the optical forward base station 53 via the optical fiber 56, and an upward signal amplifier 64 for amplifying to a specified level the electric signal converted by the O/E converter 63 and outputting the signal to the radio base station 51 as an upward received signal 58. 
The optical forward base station 53 has an antenna 54 connected thereto, and comprises a light-electricity (O/E) converter 65 for converting into an electric signal an optical signal input via the optical fiber 55, a downward signal amplifier 66 for amplifying the electric signal from the light-electricity (O/E) converter 65 to a specified output level and transmitting it from the antenna 54, an upward signal amplifier 68 for amplifying the received signal received by the antenna 54 according to a specified amplification degree, an electricity-light (E/O) converter 69 for converting from an electric signal into an optical signal the received signal amplified by the upward signal amplifier 68 and sending the signal out to the optical fiber 56, and a duplexer 67 for enabling the downward signal amplifier 66 and the upward signal amplifier 68 to share the antenna 54 by duplexing both directions of the signals. 
The downward transmitted signal 57 from the radio base station 51 is input to the optical interface section 52, amplified to a specified level by the downward signal amplifier 61, and converted into an optical signal by the E/O converter 62.  The optical signal is then sent out to the optical fiber 55.  The optical signal sent to the optical fiber 55 is then input to the optical forward base station 53, and converted back into an electric signal by the O/E converter 65.  The electric signal is subsequently amplified to a specified output level by the downward signal amplifier 66, and sent to subscribers' terminals (not shown) from the antenna 54 via the duplexer 67. 
On the other hand, the received signal received from a subscriber's terminals by the antenna 54 is input to the upward signal amplifier 68 via the duplexer 67, amplified according to a specified amplification level, converted into an optical signal by the E/O converter 69, and sent out to the optical fiber 56.  The optical signal sent out to the optical fiber 56 is then input to the optical interface section 52, converted back into an electric signal by the O/E converter 63, and amplified by the upward signal amplifier 64 according to a specified amplification degree.  The signal is finally output to the radio base station 51 as an upward received signal 58. 
In the optical micro cell transmission system, not only the level of transmission output from the antenna 54 but also the amplification degree for the received signal received by the antenna 54 and amplified before transmission from the antenna 54 to the radio base station 51 must be maintained at fixed values.  The variation of the amplification degree for each amplifier due to a change in temperature must also be minimized.  A constant amplification degree for the received signal must be maintained because the radio base station 51 traces the location of a subscriber's terminal according to the intensity of the received signal.  Although in some cases, the optical output of the E/O converter has been controlled using, for example, the control method disclosed in Japanese Patent Laid Open No.  257343/88, it is difficult to avoid the effects of the variation of the amplification degree for the amplifier simply by controlling the optical output, and the downward transmission output level and the upward received signal amplification degree may be varied if the length of an optical segment (the optical fiber) is changed when that optical fiber is replaced or the optical forward base station is relocated.  Furthermore, conventional optical micro cell transmission systems are disadvantageous in that they cannot detect faults in optical forward base stations located far apart from the radio base station.