Battery-life, or efficiency, is a primary consideration when designing a mobile transmitter. One method of increasing the battery-life is to decrease current consumption of the transmitter during low power operation by decreasing the bias current. Changing the bias current of the transmitter also changes the gain, and thus the power step, of the transmitter. Thus, in order to achieve a target output power, it is desirable to have a power control system that corrects the gain of the transmitter after a change in bias current.
Traditional power control systems use a closed loop architecture where the output of the transmitter is coupled to a power detector. In order to measure the gain, the power detector uses an averaging filter that requires approximately 200 microseconds to remove the amplitude modulation from the detected signal. However, unlike the Global System for Mobile Communications (GSM), continuous time transmit schemes such as the Wideband Code Division Multiple Access (W-CDMA) modulation scheme of the Universal Mobile Telecommunications System (UMTS) require that mobile terminals continuously transmit in each transmit slot. Further, for W-CDMA in the UMTS, any gain adjustments must be completed within approximately the first 50 microseconds of each time slot. Accordingly, the traditional power control systems are not suitable for measuring and adjusting the gain of the W-CDMA transmitter during the first 50 microseconds of a time slot.
One solution would be to use traditional power control systems to detect the gain in one time slot and adjust the gain in the next time slot. However, the bias current may change from one time slot to the next. As such, correcting the gain for one time slot based on the gain during the previous time slot is not desirable.
Commonly owned and assigned U.S. patent application Ser. No. 11/113,873, entitled POWER CONTROL SYSTEM FOR A CONTINUOUS TIME MOBILE TRANSMITTER, which was filed Apr. 25, 2005 and is hereby incorporated herein by reference in its entirety, discloses a system for controlling the output power of a continuous time mobile transmitter by controlling the gain of the transmitter that addresses the issues discussed above. In this system and other systems controlling output power by controlling the gain of the transmitter, the output power of the transmitter is a function of both the RMS value of the input signal as well as the gain of the transmitter (Pout=Gain×RMSIN).
W-CDMA in the UMTS provides multi-code operation which may result in variations in the Root Mean Square (RMS) value of the input signals provided to the transmitter. More specifically, by using orthogonal spreading codes, multiple channels may be combined to provide a digital input signal to the transmitter. However, due to changes in the number of combined channels, which channels are combined, and the gains applied to the channels, the RMS value of the input signal provided to the transmitter may vary. Thus, in order to provide accurate power control by controlling gain, there is a need for a system that corrects or compensates for variations in the RMS value of the input signal provided to the transmitter.
In order to compensate for variations in the RMS value of the input signal, some baseband processor manufacturers scale the input signal and provide the scaled input signal to the transmitter. However, due to errors inherent in the scaling process such as digital multiplication and bit resolution, the RMS value of the scaled input signal may still vary significantly. Thus, even in these systems, there is a need for a system and method for correcting compensating for variations in the RMS value of the input signal provided to the transmitter.