Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-12-30
2001-08-07
Le, Thanh Cong (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S574000, C455S424000, C455S069000, C455S126000
Reexamination Certificate
active
06272336
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to wireless communications systems and, more specifically, to a system for controlling the output power of an RF amplifier using a traffic weighted closed loop power detector.
BACKGROUND OF THE INVENTION
Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered by cellular service providers, by the regional Bell companies, and by the national long distance operators. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
In order to further increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers are eager to implement any innovations that may reduce equipment costs, maintenance/repair costs, and operating costs, or that may increase service quality/reliability, and the number of subscribers that the cellular system can service.
Electrical power is one of the more significant operating costs of a wireless system. Every base transceiver station has a transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. The transmitter uses a comparatively large RF power amplifier to increase the strength of transmitted signals.
Wireless systems cannot tolerate large amounts of signal distortion and therefore require the use of RF amplifiers having good linearity characteristics across a wide range of operating conditions in order not to violate the IS 95 bandwidth requirements due to spectral spreading effects. Unfortunately, the DC-to-RF conversion efficiency for linear RF amplifiers is very low. CDMA amplifiers generally require about 8-10 dB of overhead input power ratio in order to maintain linearity in the RF waveforms.
The transmitter power amplifier consumes a constant and comparatively large amount of power, regardless of the relative strength of the output signal transmitted by the base transceiver station. For example, if the normal traffic load during the daytime requires the RF output power level to be approximately 10 watts, the DC prime power consumed by the transmitter power amplifier is approximately 80-100 watts (i.e., 8-10 dB higher). However, in the middle of the night, when the traffic load is very light, the RF output power level of the transmitter may be reduced in decrements down to, for example, about 1 watt, as power control is exercised over the RF output signal. However, the DC prime power consumed by the transmitter power amplifier will still be approximately 80-100 watts, since the operating bias points of the power amplifiers are fixed. In short, no allowance is made for reduced traffic loads.
There is therefore a need in the art for improved wireless networks that are less expensive to operate. In particular, there is a need for wireless networks that implement power control in the power amplifiers of the base station transmitters. Improved systems are needed that monitor the RF output signal level of a transmitter power amplifier and reduce the DC power level of the transmitter power amplifier according to the traffic load on the base station.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a power control circuit for controlling an RF transmitter having a transmit path capable of receiving an input baseband signal and generating therefrom a modulated RF output signal. The power control circuit controls the combined effective DC bias current of a bank of parallel RF power amplifiers located in the transmit path of the RF transmitter. The power control circuit comprises: 1) a first signal monitor for monitoring the level of the modulated RF output signal; and 2) an amplifier bias current controller for comparing the modulated RF output signal level and the known combined maximum output power of the active RF power amplifiers to determine if the active RF power amplifiers are operating in the linear region.
If the active RF power amplifiers are operating in or near the nonlinear region, the present invention turns on one or more of the inactive RF power amplifiers to increase the power handling capability and linearity of the overall power amplifier. Conversely, if the active RF power amplifiers are operating well within the linear region, the present invention may disable one or more active RF power amplifiers in order to reduce the effective combined DC bias current of the active RF power amplifiers and save power.
In an advantageous embodiment of the present invention, the power control circuit comprises: 1) a first signal monitor capable of monitoring a level of the modulated RF output signal; and 2) an amplifier bias current controller capable of comparing the modulated RF output signal level and a known maximum output power parameter of a first RF power amplifier in the transmit path and, in response to the comparison, enabling a second RF power amplifier in parallel with the first RF power amplifier to thereby increase the power handling capability and linearity of the overall parallel amplifier. The combined DC bias current of the first RF power amplifier and the second RF power amplifier are larger than the single power amplifier and the overall linearity improves.
In one embodiment of the present invention, the amplifier bias current controller is capable of comparing the modulated RF output signal level to a known combined maximum output power parameter of a plurality of parallel RF power amplifiers in the transmit path and, in response to the comparison, disabling at least one of the plurality of parallel RF power amplifiers to thereby reduce a combined DC bias current of the plurality of parallel RF power amplifiers.
In another embodiment of the present invention, the power control circuit further comprises: 1) a first signal monitor capable of monitoring a level of the input baseband signal; and 2) a gain controller capable of comparing the input baseband signal level and the modulated RF output signal level and generating therefrom a gain control signal capable of adjusting a signal gain of the transmit path.
In still another embodiment of the present invention, the gain controller generates the gain control signal as a function of an operating temperature of the RF transmitter.
In yet another embodiment of the present invention, the gain controller generates the gain control signal as a function of an operating frequency of the RF transmitter.
In a further embodiment of the present invention, the power control circuit further comprises first scaling circuitry for scaling the modulated RF output signal level prior to the comparison of the modulated R output signal level and the known maximum output power parameter.
In a still further embodiment of the present invention, the power control circuit further comprises second scaling circuitry for scaling the input baseband
Appel Mark J.
Johnson Mitchell K.
Cong Le Thanh
Han John C.
Samsung Electronics Co,. Ltd.
Woldetatios Yemane
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