This invention generally relates to spread spectrum communication systems. More particularly, the present invention relates to controlling transmission power levels in such systems.
In many spread spectrum communication systems, multiple user communications are sent over a shared frequency spectrum. In code division multiple access (CDMA) communication systems, the multiple communications are send using different codes to distinguish them. In time division duplex/code division multiple access (TDD/CDMA) and time division synchronous CDMA (TD-SCDMA) communication systems, the spectrum is also time divided to better utilize the spectrum.
Since multiple communications are sent in the same spectrum, the communications may interfere with each other. To reduce the interference that the communications induce on one another, adaptive power control is used. The purpose of adaptive power control is to transmit each communication at a minimum power level to achieve a specified reception quality, such as a target signal to interference ratio (SIR).
One scheme to implement adaptive power control is closed loop power control. In closed loop power control, a transmitter transmits a communication at a certain power level. A target receiver receives the communication and measures the SIR of the received signal. The measured SIR is compared to a target SIR. If the measured SIR is larger than the target SIR, a power control command indicating that the transmitter should decrease its transmission power is sent. If the measured SIR is less than or equal to the target SIR, a power command indicating that the transmitter should increase its power level is sent. The power control command is, typically, a single bit indicating either an increase or decrease in power level. The commands are typically sent at a frequency of 100 to 200 commands per second in TDD/CDMA and TD-SCDMA.
The transmitter receives the power command and changes its transmission power level in response to the power command by a fixed amount, such as by increasing or decreasing the transmission power level by 1 dB. When channel conditions are highly dynamic, closed loop power control may not be able to compensate fast enough to compensate for the channel conditions. As a result, received signal quality may drop below acceptable levels.
Accordingly, it is desirable to have other approaches to power control.