Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
2000-10-27
2004-04-06
Vo, Nguyen T. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S246100, C455S250100, C455S296000, C455S127500
Reexamination Certificate
active
06718165
ABSTRACT:
PRIORITY
This application claims priority to an application entitled “Apparatus and Method for Reducing Nonlinear Distortion in an Automatic Gain Control System” filed in the Korean Industrial Property Office on Oct. 27, 1999 and assigned Serial No. 99-46847, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for reducing nonlinear distortion in a system having an automatic gain control function.
2. Description of the Related Art
In general, a mobile communication system has an automatic gain control (AGC) function to automatically control a gain of a transceiver depending on a power level of a received signal. During transmission, the mobile communication system with the AGC function (hereinafter, referred to as an AGC system) properly controls a gain of a transmission power amplifier to transmit signals at a proper transmission power level in response to a transmission power control command uniquely transmitted to each mobile station from a base station in order to prevent performance degradation of the system due to interference with other mobile station users. During reception, the AGC system properly controls a gain of a receiving amplifier to compensate for a fading phenomenon which is a time-dependent variation of reception power due to a variation in the distance from the base station and the movement of vehicles or pedestrians.
FIG. 1
illustrates a structure of an AGC system according to the prior art, which is disclosed in U.S. Pat. No. 5,257,283, entitled “Spread Spectrum Transmitter Power Control Method and System”.
Referring to
FIG. 1
, a signal received from an antenna is applied to an intermediate frequency amplifier (IF AMP)
94
through a down-converter
90
and a bandpass filter (BPF)
92
. The output of the IF amplifier
94
is applied to a baseband processor
108
through an analog-to-digital (A/D) converter
106
and is also applied to an AGC detector
96
. The AGC detector
96
has two different uses, as follows:
(1) During reception, the AGC detector
96
generates a control voltage for controlling a gain of the IF amplifier
94
to maintain a level of the received signal.
(2) During transmission, the output of the AGC detector
96
is compared with a reference power level value generated by a control processor
110
in order to adjust transmission power to a value set by a mobile station power control command transmitted from the base station. The reference power level value is applied to a comparator
98
through a digital-to-analog (D/A) converter
112
. The output of the comparator
98
is applied to a nonlinear filter
100
, the output of which is nonlinearly varied according to a variation rate in the output of the comparator
98
. The nonlinear filter
100
has the function of setting an increasing rate of a transmission power control voltage to be less than a decreasing rate thereof in order to prevent performance degradation of the system due to an abrupt increase in the transmission power. In addition, during a transmission mode of the mobile station, an IF amplifier
104
controls a signal received from a transmission modulator (not shown) depending on a control signal generated from the control processor
110
. Here, a D/A converter
114
is required to generate a control voltage for controlling a gain of the IF amplifier
104
. An output signal of the IF amplifier
104
is applied to an IF amplifier
102
which also receives the output of the nonlinear filter
100
. In this manner, the AGC system enables automatic gain control during transmission.
In the AGC system shown in
FIG. 1
, when a high-power input signal is applied to a first stage in a receiving stage, several elements located in an RF (Radio Frequency) stage are saturated causing nonlinear distortion of the received signal. Finally, this distorted signal is applied to an AGC loop, and when the baseband signal is demodulated in a modem, clipping distortion is further added to the distorted signal by the AGC loop, causing degradation of demodulation performance. The same problems occur even during a transmission mode. That is, if transmission power is controlled using only the transmission AGC amplifier when increasing the transmission power, the RF elements in the next stage can be easily saturated due to the high power of the AGC amplifier in the transmission stage. Therefore, for the high-power reception and transmission signals, it is necessary to reduce distortion of the signals input to and output from the AGC loop; otherwise the nonlinearly-distorted signals input to and output from the AGC loop are transmitted to the next stage, causing degradation of the overall system performance.
Of course, when it is necessary to transmit a high-power signal by providing the nonlinear filter
100
in the transmission stage as shown in
FIG. 1
, it is possible to prevent the AGC amplifier in the transmission stage from being saturated. However, it is not so easy to realize such a nonlinear filter
100
having the proper features. Moreover, in order to control a gain of the IF amplifier
102
, an additional circuit is required to convert the output level of the nonlinear filter
100
to a gain varying control voltage for the IF amplifier
102
. Also, it is not easy to construct such a level conversion circuit because of the nonlinear characteristics of the nonlinear filter
100
.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an apparatus and method for reducing nonlinear distortion of transmission/reception signals in an AGC system.
It is another object of the present invention to provide an apparatus and method for preventing a signal input to an AGC loop in a receiving stage from being distorted by an RF stage when a high-power input signal is applied to a first stage of the receiving stage in an AGC system.
It is yet another object of the present invention to provide an apparatus and method for preventing a signal output from an AGC loop in an IF stage from being distorted, when an output signal of a transmission stage has a high power level in an AGC system.
It is still another object of the present invention to provide an apparatus and method for preventing RF elements from being saturated for high-power transmission/reception signals in an AGC system.
It is still another object of the present invention to provide an apparatus and method for preventing degradation of system performance for the high-power transmission/reception signals in an AGC system.
To achieve the above objects, an apparatus for adaptively controlling an output level of a local oscillator is provided to reduce nonlinear distortion which may occur when receiving a high-power RF signal or when it is necessary to transmit a high-power transmission signal. When a high-power RF signal is applied to a first stage of the receiving stage, the apparatus prevents the signal input to a reception AGC loop from being distorted by decreasing an output level of the local oscillator being provided to a down-converter. Further, when the output signal of a transmission stage has a high power level, the apparatus decreases the output level of a transmission AGC amplifier to some extent and instead, increases the output level of the local oscillator being input to an up-converter located in an IF stage so that it is possible to obtain a nonlinear distortion-reduced signal while obtaining the same output level. As a result, the invention expands the linear operation region of the transmission/reception AGC loop, so that it is possible to prevent degradation of the system performance resulting from the limitation on the linear operation region of the AGC loop due to nonlinear distortion.
According to a first aspect of the present invention, an apparatus is provided for reducing nonlinear distortion occurring in a down-converter according to a level of a received RF signal in a receiving stage of a mobile commun
Dilworth & Barrese LLP
Samsung Electronics Co,. Ltd.
Vo Nguyen T.
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