Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1997-11-25
2001-06-26
Chang, Vivian (Department: 2682)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S126000
Reexamination Certificate
active
06253092
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to mobile communications systems. More specifically, the present invention relates to systems and techniques for controlling the power radiated by cellular transmitters.
2. Description of the Related Art
In a cellular network a large number of mobile telephone system users, each having a cellular phone, communicate through repeaters or terrestrial stations organized in a number of cells which extend over a limited geographical area. The terrestrial stations are referred to as ‘base stations’.
Each mobile station (MS), e.g. cellular telephone, broadcasts a signal that is received by a base station. The signal is then relayed to a mobile switching center (MSC) which in turn routes the signal to the public switched telephone network (PTSN) and to telephone lines or other mobile stations. Similarly, a signal may be transmitted from the public switched telephone network to a mobile station via a base station and a mobile switching center.
In certain cellular systems such as GSM (Global Systems for Mobile Communication) there is a requirement to control, inter alia, the radio frequency (RF) power level of each cellular telephone transmitter in order to maximize the capacity of the system. The output power must be controlled over a wide range i.e., 30-40 dB.
One conventional technique for effecting control of the power level of each cellular phone is a closed loop approach wherein a signal is transmitted from a base station which sets the required output power level. The conventional, closed loop approach involves a feedback of the transmitter output to the input thereof via a peak detector, comparator, and variable gain amplifier.
However, this technique has been somewhat problematic inasmuch as the peak detector is generally implemented with a diode and a low pass filter. The response curve of the diode, with regard to its output voltage as a function of input power, is nonlinear over a wide range of input power levels. The nonlinearity is most extreme at low power levels. As a result, the feedback loop is nonlinear and extremely sensitive at low power levels. Inasmuch as the control voltage is typically applied using a digital-to-analog converter or ‘DAC’, a high resolution DAC is required to provide adequate resolution in the control of the applied voltage at the low end where the detector is most sensitive. Moreover, to ensure loop accuracy is maintained, steps must be taken to ensure the closed-loop remains stable. Methods such as changing the loop gain of the transmitter have been employed with limited success.
Hence a need exists in the art for an inexpensive system or technique for controlling the output power of a cellular telephone transmitter.
SUMMARY OF THE INVENTION
The need in the art is addressed by the present invention which in a most general sense is a power control circuit for use with a closed loop transmitter such as a cellular telephone. The inventive circuit includes a first element for providing a transmitter output signal. A second element serves to receive a power level signal. A third element is included for adjusting the transmitter output signal in response to the power level signal in accordance with a first scale factor over a first range if the power level signal is below a first threshold and for adjusting the transmitter output signal in response to the power level signal in accordance with a second scale factor over a second range if the power level signal is not below the first threshold.
In the illustrative embodiment, the system is adapted for use in a mobile station of a cellular telephone system to which a power level control signal is supplied by a base station thereof. The first element is a variable gain amplifier, the second element is a microprocessor. The third element includes a digital-to-analog converter, a detector for detecting a peak amplitude of the output, and a subtractor for subtracting the output of the detector from a reference voltage. The reference voltage is supplied by the digital-to-analog converter. The output of the subtractor is input to a variable gain control input of the variable gain amplifier.
The invention affords an improved design for a cellular telephone transmitter including a modulator for upconverting in input signal; a variable gain amplifier, a feedback loop coupled to the output of the amplifier and a gain varying input thereof and a circuit for adjusting the response of the feedback loop in response to changes in an applied control voltage.
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Brown Stephen J.
Nguyen Hoi
Benman William J.
Chang Vivian
Ramos-Feliciano Eliseo
Uniden Financial, Inc.
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