In a code division multiple access mobile communication system, since many channels use the same frequency, the reception power of a signal on a given channel becomes interference wave power that interferes with other channels. On an upstream channel through which a mobile station transmits a signal and a base station receives it, when the ratio of signal power to interference wave power becomes high and excessive reception quality is set, the interference wave power increases. As a consequence, the channel capacity decreases. In order to prevent this, the transmission power on the mobile station must be strictly controlled. Transmission power control on an upstream channel is performed in the following manner. A base station measures reception quality based on a signal-to-interference ratio or the like, and compares it with a control target value. If the reception quality is higher than the control target value, the base station transmits a control instruction to decrease the transmission power to the mobile station. If the reception quality is lower than the control target value, the base station transmits a control instruction to increase the transmission power to the mobile station. The mobile station then increases/decreases the transmission power in accordance with the control instruction. This transmission power control method is disclosed in detail in U.S. Pat. No. 5,056,109 (Gilhousen et al., “Method and apparatus for controlling transmission power in a CDMA cellular mobile telephone system”).
On a downstream channel as well, a high channel capacity is realized by performing transmission power control to set reception quality based on a signal-to-interference ratio or the like to a predetermined control target value. In transmission power control on a downstream channel, a mobile station measures the reception quality of the downstream channel, and compares it with a control target value. If the reception quality is higher than the control target value, the mobile station transmits a control instruction to decrease the transmission power to the base station. If the reception quality is lower than the control target value, the mobile station transmits a control instruction to increase the transmission power to the base station. The base station then increases/decreases the transmission power in accordance with the control instruction.
In an actual propagation environment for a mobile communication system, since mobile stations differ in their multipath effects and moving speeds, if a constant control target value is set for the above transmission power control on upstream and downstream channels, channel quality based on a bit error rate, frame error rate, or the like cannot be kept constant. If a large control target value is uniformly set to satisfy predetermined channel quality in mobile stations in any conditions, the set control target value is unnecessarily large for many mobile stations. As a consequence, the transmission power becomes unnecessarily high accordingly to increase interference wave power that interferes with other channels, resulting in a reduction in channel capacity. Optimal control target values that can minimize an increase in interference wave power and obtain a predetermined channel quality differ from mobile station to mobile station.
As a method of controlling a control target value to an optimal value, a technique called an outer loop is available, which changes a control target value in accordance with channel quality. When a frame error rate is to be used as channel quality, an error detection code is set in each frame, and a frame in which an error is detected by this code is determined as an error. A control target value is changed to set the frame error rate to a predetermined channel quality target value.
A specific method for this operation is described in Higuchi, Ando, Okawa, Sawabashi, and Adachi, “Experimental Study on Adaptive Transmission Power Control Using Outer Loop in W-CDMA” (Technical Report of IEICE, RCS98-18 (1998-04), pp. 51–57). As described in this reference, if the frame error rate in a predetermined time is higher than a target frame error rate, the control target value is increased by a predetermined amount, whereas if the frame error rate in the predetermined time is lower than the target frame error rate, the control target value is decreased by the same predetermined amount. The frame error rate in a predetermined time is obtained by counting the number of frames determined as errors for each predetermined number of frames and dividing the number of frames determined as errors by a predetermined number.
Assume that, in this method, a low frame error rate is set as a channel quality target value, and a control target value is changed by using the frame error rate in a predetermined time which is obtained from a small number of frames. In this case, even with the same channel quality, since the frame error rate in the predetermined time varies, a control target value is often set apart from an optimal value. Assume that in order to prevent this, the control target value is changed by obtaining a frame error rate within the predetermined time from many frames. In this case, when the optimal control target value changes, it takes much time to change the control target value in accordance with this change. Assume that the moving speed abruptly changes, the optimal control target value increases, and many frame errors are caused. Even in this case, the control target value cannot be increased until a predetermined number of frames are received, and the frame error rate in the predetermined time is calculated. As a consequence, the state where many frame errors are caused will continue. In contrast to this, if the optimal control target value decreases, the control target value cannot be decreased until a predetermined number of frames are received. As a consequence, the state where the transmission power is unnecessarily high will continue, resulting in a reduction in channel capacity.
Another problem in this conventional method is that the frame error rate greatly varies over time. In a constant propagation environment, even if the frame error rate is fixed to an ideal control target value as a channel quality target value, the frame error rate within a short time unit varies depending on the number of frame errors caused in the time. According to the conventional method, however, even if a propagation environment such as a moving speed is constant, since a control target value is repeatedly increased and decreased, the frame error rate within a short time unit greatly varies as compared with a case where a control target value is fixed to an ideal control target value. This is because, when the control target value is set to be larger than an optimal value, the frame error rate becomes lower than the channel quality target value, whereas when the control target value is set to lower than the optimal value, the frame error rate becomes higher than the channel quality target value.
When voice and image are to be transmitted in real time, even the frame error rate in a short time unit must be suppressed to be equal to or lower than a predetermined value because the quality of the voice and image deteriorates if frame errors concentrate. Therefore, a large channel quality target value must be set to make the frame error rate fall within a predetermined value while the frame error rate increases. As the control target value is increased, the transmission power increases, resulting in a reduction in channel capacity.