CDMA (Code Division Multiple Access), which is one of multiple access of a radio transmission system, is a method for spreading a spectrum of an information signal to a sufficiently wide band as compared with an original information bandwidth to transmit the signal, and it is capable of increasing spectral efficiency highly, and accommodating numerous users.
In CDMA, however, there is a near-far problem, specifically, in the case where a desired transmitting station is located at a far place and an undesired transmitting station (interference station) is located at a near place, reception power of a signal transmitted from the interference station becomes larger than reception power of a signal transmitted from the desired transmitting station, and this makes it impossible to suppress cross-correlation between spreading codes by only processing gain to make it impossible to perform communications.
Hence, a cellular system using CDMA needs transmission power control according to a state of each transmission channel on a reverse link. Moreover, in a terrestrial mobile communication, transmission power control for making compensation for a variation in a momentary value of reception power is needed as measures against fading, which is a cause of deteriorating channel quality.
Herein, a duplex system in multiple access includes TDD (Time Division Duplex) and FDD (Frequency Division Duplex).
TDD is a system that time-divides the same radio frequency into a reverse link and a forward link to perform communication, and the frequency correlation relating to fading variations between transmitting signal and received signal are 1 since the transmission and reception are in the same band. Then, in the case where switching time between both is sufficiently short, since TDD has the high time correlation between transmission and reception propagation path states such as fading variation and the like, it is possible to perform the open-loop transmission power control that controls transmission power based on reception power at a communication terminal.
Also, in FDD that performs communications at different frequencies between the reverse link and the forward link, when the communication terminal originates a call using RACH (Random Access Channel), a transmission power value is determined by the open-loop transmission power control based on a transmission power value of a broadcast channel notified by the broadcast channel, an interference power value at a base station, a target power value at a base station reception end, and reception power of the broadcast channel.
The following will explain the conventional CDMA base station and communication terminal that perform the open-loop transmission power control with reference to the drawings.
FIG. 1 is a block diagram illustrating the configuration of the conventional base station. The base station illustrated in FIG. 1 comprises a modulator 11 for modulating transmitting data, a spreader 12 for multiplying the modulated signal by spreading code A to spread, an antenna 13 for receiving and transmitting the signal, a despreader 14 for multiplying the received signal by spreading code B to despread, and a demodulator 15 for demodulating the despread signal.
Transmitting data is modulated by the modulator 11 and the modulated data is spread by the spreader 12 using spreading code A, and the resultant is transmitted via the antenna 13.
The signal received via the antenna 13 is subjected to despreading processing by the despreader 14 using spreading code B, and the despread signal is demodulated by the demodulator 15 to extract received data.
FIG. 2 is a block diagram of the configuration of the conventional communication terminal. The communication terminal illustrated in FIG. 2 comprises an antenna 21 for receiving and transmitting a signal, a despreader 22 for multiplying the received signal by spreading code A to despread, a demodulator 23 for demodulating the despread signal, a reception power measuring section 24 for measuring a reception power value from the demodulation result, a modulator 25 for modulating transmitting data, a spreader 26 for multiplying the modulated signal by spreading code B to spread, and transmission power controller 27 for performing transmission power control based on the reception power value and the like.
Herein, the reception power measuring section 24 provides average processing to the measured reception power value in order to suppress the momentary variation of the reception power value caused by fading and the like, and outputs the reception power average value to the transmission power controller 27.
The signal received via the antenna 21 is subjected to despreading processing by the despreader 22 using spreading code A, and the despread signal is demodulated by the demodulator 23, so that received data is extracted and the demodulation result is outputted to the reception power measuring section 24. Then, reception power is measured from the demodulation result by the reception power measuring section 24, the measurement result is inputted to the transmission power controller 27, and a transmission power value is determined by the transmission power controller 27 based on the reception power value and the like.
Transmitting data is modulated by the modulator 25, and the modulated data is subjected to spreading processing by the spreader 26 using spreading code B. The power is amplified by the transmission power controller 27 based on the determined transmission power value, and the resultant is transmitted as a radio signal from the antenna 21.
In this way, according to the conventional radio transmission system, the base station transmits a signal from one antenna, and the communication terminal performs the open-loop transmission power control based on the reception power of the received signal.
However, since the communication terminal of the conventional radio transmission system provides average processing to the measured reception power value, it takes much time to suppress the momentary variation to calculate a high accurate reception power average value when the fading variation is slow, and this causes a problem in which the open-loop transmission power control cannot be performed at high speed and with high accuracy.