The present invention relates to digital communication systems, in general, and to controlling the transmission power of such systems, In particular.
Batteries are used for powering digital communication transmitters in mobile stations such as cellular handsets. It is desirable to reduce the power consumption of the mobile station as much as possible in order to enable the use of lighter batteries.
When transmitting a signal, the power of the transmitted radio frequency (RF) signal (xe2x80x9ctransmission powerxe2x80x9d) must be within a range defined by the digital communications standard and selected by the base station. Reference is now made to FIGS. 1A and 1B, which are schematic illustrations of prior art transmission power levels. The permissible transmission power varies between the values of MAX_POWER and MIN_POWER, thereby defining a range R. An error range E expresses the possible discrepancy between the desired transmission power and the actual transmission power at the antenna. Many factors affect the size of this error range E. For example, the inherent inaccuracy of the RF amplifier that converts and amplifies the digital transmission signal has an inherent inaccuracy due to temperature fluctuations, instability of its power supply and frequency response flatness. An additional factor is the lack of impedance matching between the antenna and the RF amplifier. Typically, the error range E can be as large as 3 dB.
FIG. 1A shows an attempt to set the desired transmission power too low. Since the error range E is large, the actual transmission power at the antenna may be less than the limit MIN_POWER. This is unacceptable.
As shown in FIG. 1B, in order not to exceed the limits MAX_POWER and MIN_POWER of the range R, conventional prior art transmission systems set the desired transmission power setting to the mean power of the range, MEAN_POWER. Clearly, transmission at the mean power of the range R consumes more power than transmission at powers in the lower half of the range R. It would be beneficial to create a system and method for transmitting at a power which is within the range R and yet is lower than the mean power, MEAN_POWER.
The following patents and patent applications describe a variety of devices and methods for controlling or monitoring the transmission power in a communication system.
U.S. Pat. No. 5,832,373 to Nakanishi et al. discloses a digital power control device in a time division multiple access (TDMA) system. The device measures the actual transmitted power at the antenna and with a digital feedback loop, adjusts the gain of the RF amplifier. The digital feedback loop is synchronized with predetermined transmission bursts. U.S. Pat. No. 5,752,172 to Matero discloses an analog loop with a comparator for maintaining a given level of power in a transmitted signal, based on measurement of the actual transmitted power at the antenna.
U.S. Pat. No. 5,323,239 to Keane disclose a digitally-assisted power-leveling circuit for an RF power generator. U.S. Pat. No. 5,574,991 to Miyama et al. discloses a transmission power control circuit. U.S. Pat. No. 5,564,084 to Hirasawa discloses a transmission power level monitoring apparatus employed in a TDMA (time division multiple access) commutation system.
U.S. Pat. No. 5,287,555 to Wilson et al. disclose power control circuitry for a TDMA radio frequency transmitter. U.S. Pat. No. 5,564,086 to Cygan ct al. discloses a method and apparatus for enhancing an operating characteristic of a radio transmitter EP 0416 613 A2 to Toda, assigned to Fujitsu, discloses a transmission power control circuit. EP 0462 952 A1 to Larsson et al., assigned to Ericsson, discloses a method for regulating power in a digital mobile telephony system.
There a several advanced digital communication system using pseudo-random or random gated output power (xe2x80x9cpseudo random burstsxe2x80x9d), for example, IS-95, wideband code division multiple access (W-CDMA), CDMA2000, both frequency division duplex (FDD) and time division duplex (TDD) modes. None of the above mentioned references is capable of controlling the power of the transmitted RF signal in such systems. It will be appreciated that in such a system, not only is the timing of the transmission random, but the strength of the transmission cannot be anticipated.
There is provided in accordance with a preferred embodiment of the present invention a method for controlling the transmission power of a transmitter in a pseudo-random gated output power digital communication system. The method includes the steps of comparing a measured transmitted power at an antenna to a desired power only when transmission is enabled, and adjusting a power setting of the transmitter according to the difference between the measured transmitted power and the desired power.
Moreover, in accordance with a preferred embodiment of the present invention, the desired power is set lower than the mean power of an allowed range of transmission powers.
Furthermore, in accordance with a preferred embodiment of the present invention, the step of comparing is performed only when the transmitter issues a read request.
Additionally, in accordance with a preferred embodiment of the present invention, the transmitter issues the read request according to previous measurements of transmitted power and factors characterizing the transmitter when the previous measurements were made.
There is also provided in accordance with a preferred embodiment of the present invention, a system for controlling transmission power of a transmitter in a pseudo-random gated output power digital communication system. The system includes a power measurement unit for producing a transmission power measurement only when transmission is enabled, and a processing unit for comparing the transmission power measurement to a desired power and for determining a power setting of the transmitter.
Moreover, in accordance with a preferred embodiment of the present invention, the desired power is set lower than the mean power of an allowed range of transmission powers.
Furthermore, in accordance with a preferred embodiment of the present invention, the transmitter has an antenna, and the power measurement unit includes a power sensor, an analog to digital converter, and a control logic unit. The power sensor measures transmission power at the antenna, thereby producing an analog measurement. The analog to digital converter converts the analog measurement to the transmission power measurement. The control logic unit enables operation of the analog to digital converter only when transmission is enabled.
Additionally, in accordance with a preferred embodiment of the present invention, the processing unit includes a processor for issuing a read request, and the control logic unit enables operation of the analog to digital converter oily when the processor issues the read request.
Finally, in accordance with a preferred embodiment of the present invention, the processor issues the read request according to previous measurements of transmitted power and factors characterizing the transmitter when the previous.