In CDMA system, a signal is transmitted by implementing a spread modulation using a spread signal of a high transmitting speed after normal information modulation. In a receiving side, the received signal is subjected to a spread demodulation using spread signal same as that in the transmitting side to return it to the original information band width and then is subjected to normal information demodulation. A mutually orthogonal spread sign series is allotted to each user, and a plurality of movable stations own same frequency band width. Thus, a signal desired by a movable station may be an interfered signal to the other movable stations.
For example, when a communication between a base station X and a movable station A is implemented with a communication between the base station and another movable station B simultaneously, the station A receives a signal transmitted thereto as a desired wave signal SA and also receives a signal transmitted to the station B as an interfered signal SB. The received signals SA and SB are same in fluctuation characteristic, and a ratio of the received electric power of the desired wave signal to that of the interfered wave signal is constant regardless of position of the movable station, because they are received from the base station X through the same transmitting path to the movable station A.
However, when the movable station A receives the desired wave signal from the base station X and receives the interfered wave signal from other base station Y, the signal component transmitted from the base station X reaches the movable station A through a transmitting path different from the signal component transmitted from the base station Y. As a result, since the fluctuation characteristic of the signal components is different from each other, the movable station A which is located around the boarder of cell is much affected by the interfered wave signal transmitted from the other base station Y.
In the case, deterioration of communication quality in the movable station which is located around the boarder of cell can be prevented by increasing a transmitting electric power of the base station more than a standard electric power. Also, deterioration of communication quality in the movable station which is located in a place receiving a plurality of strong multi-pass signal can be prevented by increasing a transmitting electric power of the base station more than a standard electric power.
On one hand, it is necessary in other movable station of good communication quality that a transmitting electric power of the base station is less than a standard electric power in order to reduce the interfered wave electric power which is received by the other movable station communicating with the base station.
From the above reasons, a down-link (transmission from the base station to the movable station) transmission power control is required to obtain uniform communication quality within a service area.
On the other hand, a dynamic range of transmission power amplifier in the base station is limited, and where overpower is input, an output spectrum may be distorted and an amplifier may be broken. For the reason, the number of communication channels per one carrier is regulated and communication channels more than the number are not allotted.
However, where the down-link transmission power control as mentioned above is carried out, the limitation of communication channel to certain number becomes meaningless because the transmitting power of communication channel depends on its user. If the number of communication channels per one carrier is limited by considering the case where the transmission power in all communication channels becomes maximum as a result of the down-link transmission power control, the system of which a traffic volume is less can be achieved.
An object of the invention is to provide a signal transmission device of base station which is used for a movable communication system using a direct spread CDMA system.
Another object of the invention is to provide a movable communication system using a direct spread CDMA system, which prevents a transmission amplifier from breakage and prevents a transmission spectrum from distortion by input of overpower even during a down-link control of transmission power in a communication channel carried out by a transmission channel of base station, and which makes an area covered by the base station automatic expansion/reduction in accordance with total electric power of the communication channels.