Abstract:
An apparatus and method is provided for linearizing the characteristic of a power amplifier in the mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system. The apparatus includes an error detector for detecting an error between input and output signals of the power amplifier by comparing the fed-back output signal with the input signal, a predistortion lookup table for storing predistortion data, a predistortion lookup table controller for updating the data of the predistortion lookup table which corresponds to the position of the present input data by adding an error signal from the error detector to an output of the predistortion lookup table, a feedforward lookup table for storing feedforward control data, a feedforward lookup table controller for outputting corresponding feedforward control data of the feedforward lookup table by detecting a size of the error signal, a linearizer for distorting the input signal in accordance with the predistortion data and then controlling a gain of the input signal in accordance with the feedforward control data, and the power amplifier for power-amplifying an output of the linearizer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a mobile radio communication system, and more particularly to an apparatus and method of linearizing the characteristic of a power amplifier in the mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system. 
     2. Description of the Related Art 
     A high-power amplifier which is used for transmitting analog data or digital data in mobile radio communication systems requires a high spectrum efficiency as well as a high power efficiency in order to construct a low-power consuming system in a limited frequency band. In order to meet such general requirements in the system, baseband data modulation methods such as QPSK and QAM having a high spectrum efficiency have been used. Also, a high-efficiency power amplifier such as a class C amplifier has been used to improve the power efficiency of a transmitter in the system. Such a high-efficiency power amplifier generally has strong non-linear characteristics, consequently producing a distortion phenomenon, such as a sidelobe reproduction, in its output spectrum. This phenomenon is especially prevalent in the case where a modulated signal such as a QPSK or QAM signal, which does not have a constant envelope characteristic, passes through the power amplifier with the non-linear characteristic. 
     Various methods have been proposed for preventing the distortion of the output spectrum of the power amplifier resulting from the non-linear characteristics of the power amplifier. One among them is a method of compensating for the non-linear characteristic of the high-power amplifier by adaptively tracking the non-linear characteristic of the power amplifier, predistorting the baseband data in a manner opposite to the distortion caused by the non-linear characteristic of the power amplifier. 
     FIG. 1 is a block diagram illustrating the construction of a conventional power amplifier employing the above-described adaptive predistortion method. 
     Referring to FIG. 1, K-bit data encoded by an encoder (not illustrated) is inputted to a shift register  10  and a modulation select read only memory (ROM)  52 . The shift register  10  has the length of an L-symbol span, its output has a size of LK bits. At this time, if there is no linear distortion caused by the filtering operation of a filter (not illustrated) existing in the system and the length of one symbol is enough for the length L of the shift register  10 , while if a linear distortion due to the system filtering exists, the length of the shift register  10  should be longer than one symbol. 
     The LK-bit output of the shift register  10  is inputted to a predistort RAM  12 . This predistort RAM  12  is stored with predistortion data mapped for data outputted from the shift register  10 . The predistortion data is updated in accordance with input error data. Upon receiving data from the shift register  10 , the predistort RAM  12  outputs predistortion data corresponding to the received data. That is, the predistort RAM  12  predistorts data outputted from the shift register  10  using error data having a phase opposite to a distortion of the transmission signal so that a radio frequency (RF) transmission section detects the distortion of the transmission signal and compensates for the detected distortion. The output of the predistort RAM  12  is converted to an analog signal for transmission by an I-channel digital-to-analog (D/A) converter  14  and a Q-channel digital-to-analog (D/A) converter  16 , respectively. The analog signals converted by the respective D/A converters  14  and  16  are low-pass-filtered through low-pass filters (LPFs)  18  and  20 , and then inputted to a quadrature modulator  22 . The analog signals inputted to the quadrature modulator  22  are mixed with an output of a first oscillator  32 , and then modulated to an intermediate frequency signal in the quadrature modulator  22 . The intermediate frequency signal modulated by the quadrature modulator  22  is determined by the first intermediate frequency (IF) oscillator  32 , and is mixed with an output of a second oscillator  28  by a mixer  24  to be converted to a final radio frequency (RF) transmission frequency. The RF frequency signal outputted from the mixer  24  is finally amplified by a power amplifier  26 , and then transmitted through an antenna. 
     A portion of the output of the power amplifier  26  is fed back to a mixer  30  by a signal coupler  54 , and the fed-back signal is mixed with the output of the second oscillator  28  by the mixer  30  to be converted to a first IF signal. The converted IF signal is then converted to baseband data by a quadrature demodulator  34  using a local oscillation signal outputted from an IF oscillator  32 . 
     The baseband signal converted by the quadrature demodulator  34  is compared with each output signal of D/A converters  48  and  50 , which is used as a reference signal for generating an error signal, by analog adders  40  and  42 , respectively. Here, output signals of the modulation select ROM  52  are inputted to the D/A converters  48  and  50 , and used as reference signals for comparing with the signals fed back to update the value of the predistort RAM  12 . The reference signals added in the analog adders  40  and  42  and the fed-back signals are respectively converted to digital signals, and then added to the digital signals of the predistort RAM  12  by digital adders  36  and  46  to update the value of the predistort RAM  12 . Error data outputted from the digital adders  36  and  46  are inputted to the predistort RAM  12  via a data bus, and then stored in addresses determined by the shift register  10  to complete the predistortion process with respect to the baseband data. 
     However, there are some disadvantages according to the conventional predistortion method shown in FIG.  1 . First, the shift register  10  for generating addresses and the modulation select ROM  52  for obtaining the reference signal required for generating the error signal must be employed. Also, the high-accuracy adders  40  and  42  for obtaining the error signal must be employed to update the value of the predistort RAM  12 . Constructing such high-accuracy analog adders is difficult, and is highly dependant on accuracy. In addition, it is generally known that the performance obtained by the predistortion method is lower than that obtained by a feedforward method. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention overcomes the disadvantages of the prior art using two objectives. One object of the present invention is to provide an apparatus and method of linearizing the characteristic of a power amplifier in a mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system. 
     Another object of the present invention is to provide an apparatus and method of compensating for the non-linear characteristic of a power amplifier in a mobile radio communication system by a predistortion and feed forward method. 
     In order to achieve the above objects, in accordance with the present invention, an apparatus for linearizing a power amplifier in a mobile radio communication system is provided, comprising: an error detector for detecting an error between input and output signals of the power amplifier by comparing the fed-back output signal with the input signal; a predistortion lookup table for storing predistortion data; a predistortion lookup table controller for updating the data of the predistortion lookup table which corresponds to a position of a present input data by adding an error signal from the error detector to an output of the predistortion lookup table; a feedforward lookup table for storing feedforward control data; a feedforward lookup table controller for outputting corresponding feedforward control data of the feedforward lookup table by detecting a size of the error signal; a linearizer for distorting the input signal in accordance with the predistortion data and then controlling a gain of the input signal in accordance with the feedforward control data; and the power amplifier for power-amplifying an output of the linearizer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating the construction of a prior art power amplifier; and 
     FIG. 2 is a block diagram of the linearizing apparatus of a power amplifier in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention will now be described in greater detail with reference to the drawings, in which the same or similar elements are denoted by the same reference numerals in different drawings. In the following description of the present invention, a detailed description of known functions and configurations will be omitted when it may make the subject matter of the present invention unclear. 
     The apparatus and method of linearizing the characteristics of a power amplifier in a mobile radio communication system according to a preferred embodiment of the present invention utilize a predistortion algorithm for the baseband signal and a feedforward method for an RF-modulated signal to compensate for the non-linear characteristic of active elements, such as the power amplifier included in a transmitting stage of the system, and thus reduce a distortion phenomenon of an output spectrum of the RF signal. 
     The address of a lookup table for generating predistortion data is directly generated in accordance with the size of input digital data. The digital data inputted for the generation of reference signals which is to be compared with fed-back signals is synchronized with fed-back received signals by adjusting a delay time using an algorithmic delay of the digital data. The modulated signal is processed by the feedforward method, and thus an additional performance improvement can be obtained as well as the performance improvement obtained by the predistortion algorithm. In other words, the performance of the system can be further improved by applying the feedforward method to the modulated signal, in comparison to the method utilizing only the baseband data predistortion algorithm. 
     FIG. 2 is a block diagram of the linearizing apparatus of a power amplifier according to a preferred embodiment of the present invention. 
     Referring to FIG. 2, a digital shaping filter  100  receives an input signal and produces I-channel and Q-channel signals of a baseband. Multipliers  102  and  104  receive the I-channel and Q-channel signals outputted from the digital shaping filter  100 , respectively, while receiving predistortion data corresponding to the I-channel and Q-channel from a predistortion lookup table  170 . The multipliers  102  and  104  output products obtained by multiplying the I-channel and Q-channel signals by the predistortion data, respectively. The D/A converters  106  and  108  convert the digital data outputted from the multipliers  102  and  104 , respectively, to analog signals. Low-pass filters  110  and  112  remove unnecessary components from the output signals of the D/A converters  106  and  108 , respectively. A quadrature modulator  114  performs a quadrature-modulation with respect to the output signals of the low-pass filters  110  and  112  using a local oscillation frequency outputted from an oscillator  132 . The oscillator  132  outputs the local oscillation frequency to the quadrature modulator  114  and the quadrature demodulator  144 . 
     A first directional coupler  116  couples a portion of a modulated signal outputted from the quadrature modulator  114  to apply a coupled signal to a phase shifter  126 , and provides a portion of its output to a band-pass filter  118  for passing only the signal of the transmission band. For example, when the power of the modulated signal outputted from the quadrature modulator  114  is 10 dBm, the 9 dBm component of the modulated signal is sent to the band-pass filter while the remaining 1 dBm component is sent to the phase shifter  126 . The band-pass filter  118  passes therethrough only the transmission band signal derived from the output signals of the first directional coupler  116 . The output of the band-pass filter  118  is phase-matched with a feedforward circuit section by a delay  176 , and then inputted to a pre-amplifier  174 . The pre-amplifier  174  power-amplifies the output of the delay  176 . A second directional coupler  120  couples an output of the pre-amplifier  174  to an output of an automatic gain control amplifier  130  as is explained later. A power amplifier  122  finally power- amplifies an output of the second directional coupler  120 . 
     A third directional coupler  124  feeds back a portion of an output of the power amplifier  122 , similarly to the first directional coupler  120 . An attenuator  142  attenuates an output of the third directional coupler  124  to a desired level. A quadrature demodulator  144  performs a quadrature-demodulation with respect to an output of the attenuator  142 , based on the output of the oscillator  132  received thereto, thereby I-channel and Q-channel signals. Low-pass filters  146  and  148  low-pass-filter the I-channel and Q-channel signals from the quadrature demodulator  144 , respectively. A/D converters  150  and  152  convert analog outputs of the low-pass filters  146  and  148 , respectively, to digital signals. 
     A delay  158  delays the I-channel and Q-channel signals from the digital shaping filter  100 . Comparators  154  and  156  receive the I-channel and Q-channel signals of the input signal outputted from the delay  158 , respectively, while receiving I-channel and Q-channel signals of the fed-back output signal from the A/D converters  150  and  152 , respectively. The comparators  154  and  156  compare the received signals, calculating output error signals, respectively. Multipliers  160  and  162  multiply corresponding outputs of the comparators  154  and  156  by adaptation constants μi and μq, respectively. Adders  164  and  166  add outputs of the corresponding multipliers  160  and  162  to I-channel and Q-channel signals of the predistortion lookup table  170 , and output added signals to the predistortion lookup table  170 . An address generator  168  generates addresses of the predistortion lookup table  170  using the respective I-channel and Q-channel signals outputted from the digital shaping filter  100 . The predistortion lookup table  170  is stored with predistortion data for input data therein. The predistortion lookup table  170  also receives an address outputted from the address generator  168  and stores outputs from the adders  164  and  166  in the received address. That is, the predistortion lookup table  170  updates the predistort data corresponding to the address generated from the address generator  168 . When the outputs from the adder  164  and  166  have a value of 0, no predistortion data in the predistortion lookup table  170  is updated. 
     A square block  140  squares and adds the outputs of the comparators  154  and  156 . A normalizer block  172  normalizes an output of the square block  140  to adjust the size of the output signal of the square block  140  to an address range of the feedforward lookup table  138 , as is explained later. The feedforward lookup table  138  is stored with feedforward compensation data for the output from the normalizer block  172 . Upon receiving a signal from the normalizer block  172 , the feedforward lookup table  138  outputs feedforward compensation data corresponding to the received signal. A D/A converter  136  converts the compensation data outputted from the feedforward lookup table  138  to an analog signal. A level shifter  134  receives and shifts an output of the D/A converter  136  to a gain-control voltage range of the automatic gain control amplifier  130 . A band-rejection filter (BRF)  128  removes a signal of the transmission band from the output signal of the phase shifter  126 . The automatic gain control amplifier  130  receives and amplifies an output of the band-rejection filter  128  with a gain determined by a gain control voltage from a level shifter  134 , and outputs an amplified signal to the second directional coupler  120 . 
     The operation of the linearization apparatus of the power amplifier in accordance with the above embodiment of the present invention is explained below in detail with reference to FIG.  2 . The digital shaping filter  100  performs a pulse-shaping of the respective I-channel and Q-channel digital signals of the baseband. The signals pulse-shaped by the digital shaping filter  100  are inputted to the multipliers  102  and  104  to be respectively multiplied by the corresponding outputs of the predistortion lookup table  170 . The output signals of the digital shaping filter  100  are inputted to the address generator  168  to be used for generating the addresses of the predistortion lookup table  170 . 
     The outputs of the digital shaping filter  100  are used for producing the reference signals which are compared with the fed-back signals. At this time, in order to apply the predistortion algorithm to the received data which is the same as the transmitted data, the time point of comparing the transmitted data and the received data must coincide along with a proper delay time for compensating the delay generated at the transmission and the reception stages. The delay  158  delays the output signals of the digital shaping filter  100 . If a digital signal processor is employed in the system, such delay operation can be implemented by an algorithm. 
     In order to update the value of the predistortion lookup table  170 , the I-channel and Q-channel signals outputted from the delay  158  are respectively compared with the outputs of the A/D converters  150  and  152  to calculate the error signals. The error signals are then multiplied by the adaptation constants μi and μq for determining the convergence speed and stability of the algorithm in the respective multipliers  160  and  162 . Thereafter, the signals are added to the values stored in the predistortion lookup table  170  by the adders  164  and  166 , and the added signals are then stored in the address position of the predistortion lookup table  170 , as determined by the address generator  168 . Since the signals multiplied by the multipliers  102  and  104  are the values prior to the update of the predistortion lookup table  170 , the predistortion amount to be applied to following data will be determined by the property of the input data prior to one sample. The output signals of the multipliers  102  and  104  are converted to analog signals by the D/A converters  106  and  108 , and the analog signals are inputted to the quadrature modulator  114  through the low-pass filters  110  and  112  for removing the unnecessary high-frequency components. 
     A portion of the output of the quadrature modulator  114 , after passing through the first directional coupler  116 , is inputted to the band-pass filter  118 , while the other portion thereof is inputted to the phase shifter  126 . At this time, the phase shifter  126  serves to shift the phase of the modulated signal by 180 degrees, and this is necessary to produce the predistortion signal used in the feedforward method. The phase-reversed signal outputted from the phase shifter  126  is filtered by the band-pass filter  128 , so that the signal of the transmission band is removed, but the signal excepting the transmission band passes through the band-pass filter  128  to be inputted to the automatic gain control amplifier  130 . The output of the first directional coupler  116  is filtered by the band-pass filter  118  for passing only the signal of the transmission band, delayed by the delay  176  for compensating for the delay caused by the feedforward circuit, and then amplified by the pre-amplifier  174 . The output of the pre-amplifier  174  is added to the output of the automatic gain control amplifier  130  by the second directional coupler  120 , and the added signal is finally inputted to the power amplifier  122 . 
     A portion of the output of the power amplifier  122  is coupled by the third directional coupler  124 , attenuated by the attenuator  142 , and then demodulated by the quadrature demodulator  144 . The demodulated signal is filtered through the respective low-pass filters  146  and  148 , respectively, and the filtered signals are converted to digital signals by the A/D converters  150  and  152 . The outputs of the A/D converters  150  and  152  are compared with the delayed input signals outputted from the delay  158  by the comparators  154  and  156  to produce the error signals. The error signals are used not only for updating the values of the predistortion lookup table  170 , but also as the input signals of the feedforward circuit according to the present invention. Specifically, the outputs of the comparators  154  and  156  are squared and then added by the square block  140 , and the output of the square block  140  is inputted to the normalizer block  172 , so that the size of the output of the square block  140  is adjusted to be within the address range of the feedforward lookup table  138 . The normalized signal is then applied to the address of the feedforward lookup table  138 . 
     The output of the feedforward lookup table  138  is converted to an analog signal by the D/A converter  136  to be inputted to the level shifter  134 . This is required to match the output voltage of the D/A converter  136  with the gain control voltage range of the automatic gain control amplifier  130 . The gain of the automatic gain control amplifier  130  is adjusted by the control voltage of the automatic gain control amplifier  130 , which is the output of the level shifter  134 , and this causes the size of the transmission signal to be inversely amplified according to the distortion amount of the fed-back signal. Specifically, if the error between the transmission signal and the fed-back signal is large, the distortion signal also becomes large. In this case, the size of the reverse-phased signal which exists outside the transmission band becomes larger by increasing the gain of the automatic gain control amplifier  130 . If the error between the transmission signal and the fed-back signal is small, the distortion signal also becomes small. In this case, the size of the reverse-phased signal which exists outside the transmission band becomes smaller by decreasing the gain of the automatic gain control amplifier  130 . 
     According to the above-described method, the phase-reversed unnecessary signal existing outside the transmission band, which is outputted from the automatic gain control amplifier  130 , is coupled to the original transmission signal by the second directional coupler  120 , and then finally radiated through the power amplifier  122 . The system according to the embodiment of the present invention compensates for the non-linear characteristic of the power amplifier  122  by predistorting the data of the baseband, and then further compensates for the spectrum distortion phenomenon using the feedforward method by utilizing the modulated signal as well. Accordingly, the system performance can be improved in comparison to the system for linearizing the power amplifier only using the data predistortion algorithm. 
     As described above, according to the linearization apparatus of the power amplifier according to the embodiment of the present invention, the address of the lookup table for generating the predistortion data is directly generated in accordance with the size of the input digital data. The digital data inputted for the generation of the reference signals, which is to be compared with fed-back signals, is synchronized with fed-back received signals by adjusting the delay time utilizing the algorithmic delay of the digital data. Also, the modulated signal is processed by the feedforward method, and thus an additional performance improvement can be obtained as well as the performance improvement obtained by the predistortion algorithm. In other words, the performance of the system can be further improved by applying the feedforward method to the modulated signal in comparison to the method utilizing only the baseband data predistortion algorithm. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the invention as described in the appended claims.