Patent Publication Number: US-2009219088-A1

Title: Apparatus and method for correcting non-linear distortion based on characteristic modeling of high power amplifier

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus and a method for correcting a non-linear distortion based on characteristic model of a high power amplifier; and, more particularly, to an apparatus and a method for correcting a non-linear distortion of a high power amplifier by adapting an output signal of a high power amplifying characteristic predictor to a pre-distortion characteristic predictor, and adapting an output signal of the pre-distortion characteristic predictor to the pre-distorter. 
     BACKGROUND ART 
     In a general communication and broadcasting system, a high power amplifier is necessary for transmitting a transmit signal in a wide area. A general high power amplifier has characteristics that a phase and an amplitude of an output signal are non-linear to an amplitude of an input signal. 
     In general, when a communication signal of a communication apparatus including a satellite, a mobile communication, a radio relay link and a radar is transmitted, a high power amplifier amplifies the communication signal. 
     As above mentioned, a high power amplifier is used for amplifying an output signal before transmitting the communication signal. However, the high power amplifier distorts a transmit signal non-linearly because the high power amplifier has an amplitude modulation/amplitude modulation (AM/AM) or amplitude modulation/phase modulation (AM/PM) characteristics. 
     A transmit signal is distorted by a non-linear effect generated from the high power amplifier distort, and an interference between neighboring channels may be generated by enlarging a bandwidth of the communication signal. A pre-distortion technique for linearizing characteristics of the high power amplifier is one of methods to solve the above problem. 
     A conventional method for correcting non-linear characteristics of the high power amplifier is classified into two kinds of schemes. First scheme is an input signal back-off which uses a linear region of the high power amplifier. Secondary scheme is a pre-distortion scheme which pre-distorts an input signal of the high power amplifier. 
     That is, since the high power amplifier has largest non-linear distortion characteristics in a high input signal, a general scheme for decreasing non-linear distortion characteristics is to use not a power of all input signal of the high power amplifier but a power of a linear characteristic region by decreasing the amplitude of the input signal. However, the above scheme is not based on an AM/PM modulation effect or high power amplifier characteristics which are variable according to a time. 
     A pre-distortion which linearizes an input signal of a high power having non-linear amplifying characteristics of a high power amplifier corrects demerit of the first scheme. That is, the pre-distorter performs an inverse distortion for correcting the distortion of the high power amplifier. 
       FIG. 1  is a block diagram showing an apparatus for correcting a pre-distortion using a conventional high power amplifying characteristic predictor. 
     First, a transmit signal is inputted to a pre-distorter  101 . The pre-distorted signal from the pre-distorter  101  is modulated by a modulator  102 . The modulated signal is converted into a radio frequency signal by a frequency up converter  103 . The up-converted radio frequency signal is amplified by a high power amplifier  104 , and the amplified radio frequency signal is transmitted through an antenna. 
     A part of the output signal of the high power amplifier  104  is divided and down-converted by a frequency down converter  105 . The down-converted signal is demodulated by a demodulator  106 . 
     Subsequently, if the pre-distorted signal from the pre-distorter  101  and the demodulated signal from the demodulator  106  are inputted to a high power amplifying characteristic predictor  107  respectively, the high power amplifying characteristic predictor  107  predicts characteristics of the high power amplifier based on the pre-distorted signal and the demodulated signal. 
     An inverse function converter  108  obtains an inverse characteristic function by using the predicted characteristics of the high power amplifier. The inverse characteristic function obtained by the inverse function converter  108  is used as a pre-distorter. 
     Similar schemes to the above scheme described with reference to  FIG. 1  are disclosed in the U.S. Pat. No. 5,049,832 (Amplifier Linearization by adaptive pre-distortion), U.S. Pat. No. 6,072,364 (Adaptive digital pre-distortion for power amplifiers with real time modeling of memoryless complex gains), U.S. Pat. No. 5,929,703 (Method and device for modeling AM-AM and AM-FM characteristics of an amplifier, and corresponding pre-distortion), and Korean patent No. 10-0315425 (Apparatus and method for high speed adaptive pre-distortion using lookup table). 
     However, since theses schemes obtain directly an inverse function after obtaining a forward characteristic function of a high power amplifier, these schemes have problems such that the complexity of a calculation is increased, and are sensitive to a measuring error or a system error. 
       FIG. 2  is a block diagram showing an apparatus for correcting a pre-distortion using another conventional pre-distortion characteristic predictor. 
     First, a transmit signal is inputted to a pre-distorter  201 . The pre-distorted signal from the pre-distorter  201  is modulated by a modulator  202 . The modulated signal is converted into a radio frequency signal by a frequency up converter  203 . The up-converted radio frequency signal is amplified by a high power amplifier  204 , and the amplified radio frequency signal is transmitted through an antenna. 
     A part of the output signal of the high power amplifier  204  is divided and down-converted by a frequency down converter  205 . The down-converted signal is demodulated by a demodulator  206 . 
     Subsequently, if the pre-distorted signal from the pre-distorter  201  and the demodulated signal from the demodulator  206  are inputted to a pre-distortion characteristic predictor  207 , the pre-distortion characteristic predictor  207  is operated as a pre-distorter by obtaining an inverse characteristic function of the high power amplifier based on the pre-distorted signal and the demodulated signal. 
     Similar scheme to the scheme described with reference to  FIG. 2  is described by “Hua Qian” and “G. Tong Zhou”, “Orthogonal Polynomials for Power Amplifier Modeling and Pre-distorter Design”, “IEEE Transaction on Vehicular Technology, Vol. 53, No. 5, pp 1468-1479”. The scheme described in  FIG. 2  reduces a calculation quantity and uses a less amplitude of input/output signal of the high power amplifier than the scheme described in  FIG. 1  by obtaining a characteristic function of a pre-distorter. 
     However, above-mentioned scheme has performance deterioration by an extraction error when an output signal of the high power amplifier is obtained. 
     That is, the above-mentioned scheme needs many signal samples for calculating a characteristic modeling of a high power amplifier, and is sensitive to a measuring error. In a case that a model of pre-distorter is obtained directly, a time variable effect and a noise of the high power amplifier are sensitive. 
     DISCLOSURE 
     Technical Problem 
     It is, therefore, an object of the present invention to provide an apparatus and a method for correcting a non-linear distortion based on a characteristic model of a high power amplifier, for correcting a non-linear distortion of a high power amplifier by adapting an output signal of a high power amplifying characteristic predictor to a pre-distortion characteristic predictor, and adapting an output signal of the pre-distortion characteristic predictor to the pre-distorter. 
     That is, an object of the present invention is to provide an apparatus and a method for correcting a non-linear distortion using a characteristic model of a high power amplifier for correcting a non-linear distortion of a high power amplifier by adapting an output signal of a high power amplifying characteristic predictor to a pre-distortion characteristic predictor, and adapting an output signal of the pre-distortion characteristic predictor to the pre-distorter after a high power amplifying characteristic predictor predicts characteristics of the high power amplifier based on the output signals of the pre-distorter and the high power amplifier. 
     Technical Solution 
     In accordance with one aspect of the present invention, there is provided an apparatus for correcting a non-linear distortion, including: An apparatus for correcting a non-linear distortion includes a pre-distorter for pre-distorting a transmit signal inputted from an external apparatus based on pre-distortion characteristics outputted from a pre-distortion characteristic predictor; a modulator for modulating the pre-distorted signal to thereby generate a modulated signal; a frequency up-converter For up-converting a frequency of the modulated signal to thereby generate an up-converted signal; a high power amplifier for amplifying the up-converted signal to a high power amplified signal; a frequency down-converter for down-converting a frequency of the high power amplified signal, thereby generate a down-converted signal; a demodulator for demodulating the down-converted signal, to hereby generate a demodulated signal; a high power amplifying characteristic predictor for predicting characteristics of the high power amplifier based on the pre-distorted signal and the demodulated signal; and a pre-distortion characteristic predictor for predicting the pre-distortion characteristics based on the pre-distorted signal and the predicted high power amplifying characteristics. 
     In accordance with another aspect of the present invention, there is provided a method for correcting a non-linear distortion, including the steps of: a) at a high power amplifying characteristic predictor, generating a characteristic model of a high power amplifier by predicting characteristics of the high power amplifier based on a pre-distorted signal which is outputted from a pre-distorter and a demodulated signal which is outputted from a demodulator; b) at a pre-distortion characteristic predictor, predicting pre-distortion characteristics based on the pre-distorted signal and the predicted characteristics model of the high power amplifier; and c) adapting the predicted pre-distortion characteristics to the pre-distorter. 
     ADVANTAGEOUS EFFECTS 
     The present invention corrects a non-linear distortion of a high power amplifier by adapting an output signal of a high power amplifying characteristic predictor to a pre-distortion characteristic predictor, and adapting characteristics of a pre-distorter, which is predicted by the pre-distortion characteristic predictor, to the pre-distorter after a high power amplifying characteristic predictor predicts characteristics of the high power amplifier based on the output signals of the pre-distorter and the high power amplifier. 
     That is, the present invention decreases a measuring error based on an output signal of the pre-distorter and an output signal of characteristic model of the high power amplifier by using the high power amplifying characteristic amplifier. And, the present invention decreases a measuring error and a deterioration of performance by adapting the output signal of the characteristic model of the high power amplifier to the pre-distortion characteristic predictor. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The above and other objects and features of the present invention will become better understood with regard to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing an apparatus for correcting a pre-distortion using a conventional high power amplifying characteristic predictor; 
         FIG. 2  is a block diagram showing an apparatus for correcting a pre-distortion using a conventional pre-distortion characteristic predictor; 
         FIG. 3  is a block diagram showing an apparatus for correcting a non-linear distortion using a characteristic modeling or a high power amplifier in accordance with an embodiment of the present invention; and 
         FIG. 4  is a flow chart showing a method for correcting a non-linear distortion using a characteristic modeling of the high power amplifier in accordance with an embodiment of the present invention. 
     
    
    
     BEST MODE OF THE INVENTION 
     Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
       FIG. 3  is a block diagram showing an apparatus for correcting a non-linear distortion using a characteristic modeling of a high power amplifier in accordance with an embodiment of the present invention. 
     As shown in  FIG. 3 , an apparatus for correcting a non-linear distortion using a characteristic modeling of a high power amplifier in accordance with an embodiment of the present invention includes a pre-distorter  301  a modulator  302 , a frequency up-converter  303 , a high power amplifier  304 , a frequency down-converter  305 , a demodulator  306 , a high power amplifying characteristic predictor  307  and a pre-distortion characteristic predictor  308 . 
     The pre-distorter  301  pre-distorts a transmit signal inputted from an external apparatus based on characteristics of the pre-distorter from the pre-distortion characteristic predictor  308 , and outputs the pre-distorted transmit signal to the modulator  302 . 
     The modulator  302  modulates the pre-distorted transmit signal. The frequency up-converter  303  up-converts the signal outputted from the modulator  302 . 
     The high power amplifier  304  amplifies the signal outputted from the frequency up-converter  303  to a high power. The frequency down-converter  305  down-converts the signal outputted from the high power amplifier  304 . 
     The demodulator  306  demodulates the signal outputted from the frequency down-converter  305 . The high power amplifying characteristic predictor  307  predicts characteristics of the high power amplifier  304  based on the output signal of the pre-distorter  301  and the output signal of the demodulator  306 . 
     The pre-distortion characteristic predictor  308  predicts characteristics of the pre-distorter based on the output signal of the pre-distorter and characteristics of the high power amplifier predicted by the high power amplifying characteristic predictor  307 . 
     A detailed configuration and an operation of each component will be described as below. 
     First, a transmit signal is inputted to the pre-distorter  301 . The pre-distorted signal from the pre-distorter  301  is modulated by the modulator  302 . The modulated signal is converted into a radio frequency signal by the frequency up converter  303 . The up-converted radio frequency signal is amplified by the high power amplifier  304 , and the amplified radio frequency signal is transmitted through an antenna. 
     A part of the output signal of the high power amplifier  304  is divided. The divided output signal is down-converted by the frequency down converter  305 . The down-converted signal is demodulated by the demodulator  306 . 
     Subsequently, if the pre-distorted signal from the pre-distorter  301  and the demodulated signal from the demodulator  306  are inputted to the high power amplifying characteristic predictor  307  respectively, the high power amplifying characteristic predictor  307  predicts the characteristics of the high power amplifier based on the output signal of the pre-distorter and the demodulated signal of the demodulator  306 . The high power amplifying characteristic predictor  307  obtains a characteristic model of the high power amplifier  304 . 
     That is, the high power amplifying characteristic predictor  307  receives the pre-distorted signal outputted from the pre-distorter  301 , but does not adapt directly the demodulated signal outputted from the demodulator  306 . 
     After the high power amplifying characteristic predictor  307  obtains the characteristic model of the high power amplifier  304  based on the output signal of the pre-distorter and the demodulated signal of the demodulator, the high power amplifying characteristic predictor  307  outputs a characteristic model signal of the high power amplifier  304 . 
     The pre-distortion characteristic predictor  308  predicts the characteristics of the pre-distorter based on the output signal of the pre-distorter and the characteristics of high power amplifier predicted by the high power amplifying characteristic predictor  307 , and outputs the predicted characteristics to the pre-distorter  301 . 
     The pre-distorter  301  pre-distorts the transmit signal inputted from an external apparatus based on the characteristics of pre-distorter from the pre-distortion characteristic predictor  308 , and outputs the pre-distorted transmit signal to the modulator  302 . 
     As aforementioned, the pre-distortion characteristic predictor  308  accurately obtains an inverse characteristic function of the high power amplifier by using the output signal of the characteristic model of the high power amplifier. 
     The inverse characteristic function of the high power amplifier is adapted to the pre-distorter  301  which receives the transmit signal from an external apparatus. 
     That is, the output signal of the high power amplifier  304  is not directly adapted to the pre-distortion characteristic predictor  308 . 
     After the characteristic model of the high power amplifier  304  is obtained through the high power amplifying characteristic predictor  307 , a measuring error and a deterioration of performance may be decreased by adapting the output signal of the characteristic model of the high power amplifier  304  to the pre-distortion characteristic predictor  308 . 
       FIG. 4  is a flow chart showing a method for correcting a non-linear distortion using a characteristic modeling of the high power amplifier in accordance with an embodiment of the present invention. 
     First, the output signals of the pre-distorter  301  and the demodulator  302  are stored at step S 401 . 
     A characteristic model of the high power amplifier  304  is generated by predicting characteristics of a high power amplifier  304  based on the output signals of a pre-distorter  301  and the demodulator  302  at step S 402 . 
     The output signal of the pre-distorter  301  and the characteristic model of the high power amplifier  304  outputted from the high power amplifying characteristic predictor  307  are re-stored at step S 403 . 
     Subsequently, the pre-distortion characteristic predictor  308  predicts characteristics of the pre-distorter  301  based on the output signals of the pre-distorter  301  and the characteristic model of the high power amplifier from the high power amplifying characteristic predictor  307  at step S 404 . 
     That is, the pre-distortion characteristic predictor  308  obtains coefficients for the pre-distorter  301 . 
     The output signal of the high power amplifier  304  is linearlized by adapting the characteristics of pre-distorter  301 , which is predicted by the pre-distortion characteristic predictor  308 , to the pre-distorter  301  and operating the pre-distorter  301  at step S 405 . 
     Time-variable characteristics of a system are chased by repeatedly adapting the above steps S 401  to S 405 . 
     The method of the present invention as mentioned above may be implemented by a software program and stored in a computer-readable storage medium such as CD-ROM, RAM, ROM, floppy disk, hard disk, optical magnetic disk, etc. This process may be readily carried out by those skilled in the art; and therefore, details of thereof are omitted here. 
     The present application contains subject matter related to Korean patent application No. 2005-0094438, filed in the Korean patent office on Oct. 7, 2005, the entire contents of which being incorporated herein by reference. 
     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scope of the invention as defined in the following claims.