1. Field of the Invention
The present invention relates to a wireless relay amplification apparatus which relays radio waves between a base station and a mobile station.
2. Description of the Related Art
The wireless relay amplification apparatus relays radio waves between a base station and a user terminal apparatus such as a mobile phone as a mobile station. The wireless relay amplification apparatus amplifies and transmits radio waves from the base station to broaden the area within which the user terminal apparatus can be used. Similarly, the wireless relay amplification apparatus amplifies and transmits radio waves transmitted by the user terminal apparatus to the base station.
FIG. 9 is a block diagram showing a configuration example of a conventional wireless relay amplification apparatus 20 which relays radio waves between a base station 10 and a user terminal apparatus 15 as a mobile station. The base station 10 comprises an antenna 11 which transfers radio waves to/from the wireless relay amplification apparatus 20. From the base station 10, information indicating transmission power, for example, of the base station 10 as well as communication information to the user terminal apparatus 15 is transmitted as notification information 12.
The wireless relay amplification apparatus 20 comprises a base-station-side input/output terminal 21 and a mobile-station-side input/output terminal 22. A base station antenna 23 is connected to the base-station-side input/output terminal 21, and a mobile station antenna 24 is connected to the mobile-station-side input/output terminal 22. Further, a downlink input filter 25a and an uplink output filter 25b are connected to the base-station-side input/output terminal 21, and a downlink output filter 26a and an uplink input filter 26b are connected to the mobile-station-side input/output terminal 22.
A distributor 27, which distributes downlink signals to two, and a downlink amplifier 28 are provided in a downlink signal line between a downlink input filter 25a and a downlink output filter 26a. Further, an uplink amplifier 29 is provided in an uplink signal line between the uplink input filter 26b and the uplink output filter 25b. 
The signals distributed by the distributor 27 are input to an automatic gain control receiver 31. The receiver 31 receives notification information 12 transmitted from the base station 10 and calculates loss of radio waves, i.e., propagation loss of radio waves between the base station 10 and the wireless relay amplification apparatus 20 based on the difference between the transmission power of the base station 10 included in the notification information 12 and the reception power received by the receiver 31, and outputs the calculated result to a controller 32.
The controller 32 controls gain of the uplink amplifier 29 based on the propagation loss calculated by the receiver 31 such that the noise included in the radio waves transmitted from the wireless relay amplification apparatus 20 to the base station 10 does not affect the reception system of the base station 10. Further, the controller 32 controls the gain of the downlink amplifier 28 provided in the downlink signal line, too.
As described above, in the conventional wireless relay amplification apparatus 20, radio waves transmitted from the base station 10 are received by the base station antenna 23, and the received signals are distributed by the distributor 27 and input to the receiver 31. The receiver 31 calculates propagation loss of radio waves based on the difference between the transmission power of the base station 10 included in the notification information 12 from the base station 10 and the reception power received by the receiver 31. Based on the calculated result and the like, the controller 32 controls the gain of the uplink amplifier 29 and the downlink amplifier 28.
As a publicly-known technique relating to the present invention, in a wireless relay amplification apparatus which amplifies and transmits received input waves as output waves, the technique of performing the following operations until the gain reaches an optimum value within the range in which oscillation does not occur due to interference waves is known, i.e., the operations of sufficiently decreasing the gain when the power is turned on to prevent oscillation due to interference waves included in the output waves and received along with the input waves and increasing the gain stepwise, and generating cancellation waves to cancel the interference waves and amplifying the input waves which synthesized the interference waves and the cancellation waves (see Jpn. Pat. Appln. KOKAT Publication No. 2000-286772, for example).
In the conventional wireless relay amplification apparatus 20 as shown in FIG. 9, the receiver 31 is designed to realize an automatic gain control function (intelligent function) of automatically controlling the gain in consideration of noise figure, for example, so as not to affect the reception system of the base station 10, by means of the notification information 12 transmitted from the base station 10 and the level of the received signals. The receiver 31 plays an important role in operation of the apparatus. When the notification information cannot be received, continuation of operation of the apparatus becomes very difficult.
In particular, when the wireless relay amplification apparatus 20 is used in open space, radio wave interference occurs when the isolation degree 33 between the base station antenna 23 and the mobile station antenna 24 deteriorates. Thereby, the input to the apparatus is synthesized with the output from the apparatus, which causes deterioration in waveform quality. When the isolation degree 33 becomes lower than the gain of the apparatus, oscillation occurs, and the relay amplification operation cannot be continued.