Abstract:
A feedforward amplifier which is one of the leading element of linearization systems for mobile communications base stations, and a method of improving the performance thereof, are provided. More particularly, provided are a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a feedforward amplifier, which is one of the leading devices of linearization systems for mobile communications base stations, and a method of improving the performance thereof, and more particularly, to a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.  
           [0003]    2. Description of the Related Art  
           [0004]    In long distance telecommunications such as satellite communications and mobile communications, a signal cannot be delivered in one direct transmission. Hence, relay stations or base stations are located at predetermined intervals so that a signal can be amplified and transmitted again. A power amplifier used in the relay stations or base stations for this operates in a saturation region having a non-linear characteristic in order to obtain a maximum output. However, in this case, when multiple carriers (multi-carriers) are input to the power amplifier, these multiple carriers cause intermodulation distortion (IMD) such that the performance of the amplifier is greatly degraded. The intermodulation distortion component is generated by interactions between two carrier waves when two or more carriers are provided and pass a non-linear device.  
           [0005]    It has been known that removal of these intermodulation distortion components with existing filters is very difficult and these components also work as a kind of noise greatly affecting the quality of calls. In order to reduce the non-linear characteristic of the power amplifier, a variety of methods have been suggested and used. One of the leading methods is a linearization method using feedforward. Feedforward is a method by which the characteristics of a system improve by providing a part of input to an output end to modify the output result, and linearization is to convert a device or a signal having a nonlinear characteristic to one having a linear characteristic.  
           [0006]    Meanwhile, a linear amplifier is a kind of amplifier for providing power to a load. Linear amplifiers are classified as low frequency power amplifiers and high frequency power amplifiers. In a power amplifier, it is important to provide power efficiently with less distortion.  
           [0007]    In ordinary feedforward linearization, only the intermodulation distortion component is extracted from a signal intermodulation-distorted with a carrier wave obtained from the output of a power amplifier, and the extracted intermodulation distortion component is again coupled, with the phase of the component inversed, to the output of the amplifier. By doing so, the intermodulation distortion signal is canceled and so that a carrier to intermodulation ratio, that is, the ratio of the power of the carrier to the power of the intermodulation distortion signal, improves.  
           [0008]    [0008]FIG. 1 is a schematic diagram of the structure of an ordinary feedforward amplifier. Referring to FIG. 1, the feedforward amplifier basically includes  2  cancellation loops  10  and  20 . The cancellation loop  10  is a main signal cancellation loop and the cancellation loop  20  is an error signal cancellation loop. Each loop  10  and  20  comprises a power amplifier  12  and  22 , a power coupler  14  and  24 , a vector modulator  16  and  26 , and a delay line  18  and  28 .  
           [0009]    The main signal cancellation loop  10  of FIG. 1 extracts only an error component of the main amplifier  12 , using the difference of the input signal of the entire system and a non-linear signal amplified by the main amplifier  12 . The error signal cancellation loop  20  amplifies the error component, which is provided from the main signal cancellation loop  10 , through an error amplifier  22 , inverses the phase of the error component, and couples the result with the output signal of the main amplifier  12  so that only the error component of the output signal of the main amplifier  12  is removed.  
           [0010]    As described above, after being amplified by the main amplifier  12  through the two loops  10  and  20 , the non-linear signal becomes linearized. It has been known that this feedforward amplifier  1  has good performance in linearity improvement, covers a wide band at the same time, and has good stability.  
           [0011]    The prior art feedforward amplifier  1  as shown in FIG. 1 usually uses an error signal with a perfectly canceled main signal, which is schematically shown at the top of FIG. 3. As shown at the top of FIG. 3, a control method for generating an error signal with a perfectly canceled main signal is realized by the main signal cancellation loop  10  and the error signal cancellation loop  20  detecting and controlling different signals, respectively, as shown in FIG. 2. That is, in the adaptive control method of the prior art feedforward amplifier  1 , the detection unit  1  detects an error signal, the control unit  1  controls the main signal cancellation loop  10 , the detection unit  2  detects the error component of the final output signal, and the control unit  2  controls the error signal cancellation loop  20 , as shown in FIG. 2. Here, the main signal cancellation loop  10  is adaptively controlled so that the component of the main signal in the detected error signal is minimized. In this way, the main signal in the error signal is perfectly canceled.  
           [0012]    However, since most systems use a modulated signal as an input signal, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.  
         SUMMARY OF THE INVENTION  
         [0013]    To solve the above problems, it is an object of the present invention to provide a feedforward amplifier and a method of improving the performance thereof, in which a predetermined amount of a main signal is made to be included in an error signal input to the error signal cancellation loop of a feedforward amplifier, considering that in the feedforward amplifier, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.  
           [0014]    That is, it is an object of the present invention to provide a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.  
           [0015]    According to an aspect of the present invention, there is provided a feedforward amplifier comprising a main signal cancellation loop which comprises a main amplifier and a vector modulator and extracts an error component of the main amplifier; an error signal cancellation loop which comprises an error amplifier and a vector modulator and removes the error component in the output signal of the main amplifier; a final error signal detection unit which detects an error component of a final output signal of the feedforward amplifier; a main signal cancellation loop control unit which controls the main signal cancellation loop based on the error component detected by the final error signal detection unit; and an error signal cancellation loop control unit which controls the error signal cancellation loop based on the error component detected by the final error signal detection unit, wherein the error signal generated by the main signal cancellation loop is optimized in the direction linearizing the final output signal and the main signal cancellation loop is adaptively controlled by the main signal cancellation loop control unit.  
           [0016]    It is preferable that the error signal output from the main signal cancellation loop has an imperfectly canceled main signal.  
           [0017]    It is preferable that the output signal of the main signal cancellation loop control unit is input to the vector modulator of the main signal cancellation loop, and the output signal of the error signal cancellation loop control unit is input to the vector modulator of the error signal cancellation loop.  
           [0018]    According to another aspect of the present invention, there is provided a method for improving the performance of a feedforward amplifier comprising a main signal cancellation loop and an error signal cancellation loop, the method comprising detecting the error component of a final output signal of the feedforward amplifier; and controlling the main signal cancellation loop and the error signal cancellation loop at the same time using the detected error component.  
           [0019]    It is preferable that the method further comprises, before controlling the loops, detecting the error signal of the main signal cancellation loop and using the error signal, controlling the main signal cancellation loop; and detecting the error component of a final output signal of the feedforward amplifier and controlling the error signal cancellation loop. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
         [0021]    [0021]FIGS. 1 and 2 are schematic diagrams of the structures of feedforward amplifiers according to the prior art;  
         [0022]    [0022]FIG. 3 is a conceptual diagram of a method of improving the performance of a feedforward amplifier according to the present invention;  
         [0023]    [0023]FIG. 4 is a diagram of the structure of a feedforward amplifier according to the present invention;  
         [0024]    [0024]FIG. 5 is a flowchart of the steps performed in the method of improving performance of a feedforward amplifier according to the present invention; and  
         [0025]    [0025]FIGS. 6 a  and  6   b  are spectrum graphs comparing the feedforward amplifier according to the present invention with the prior art feedforward amplifier, and more specifically, FIG. 6 a  is a graph of an error signal spectrum and FIG. 6 b  is a graph of a final output signal spectrum. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    [0026]FIG. 3 is a conceptual diagram of a method for improving the performance of a feedforward amplifier according to the present invention, and FIG. 4 is a diagram of the structure of a feedforward amplifier according to the present invention. FIG. 5 is a flowchart of the steps performed in the method of improving the performance of a feedforward amplifier according to the present invention, and FIGS. 6 a  and  6   b  are spectrum graphs comparing the feedforward amplifier according to the present invention with the prior art feedforward amplifier.  
         [0027]    Referring to FIG. 4, the feedforward amplifier  100  according to the present invention comprises a main signal cancellation loop  110  which comprises a main amplifier  112  and a vector modulator  116  and extracts an error component of the main amplifier  112 ; an error signal cancellation loop  120  which comprises an error amplifier  122  and a vector modulator  126  and removes an error component in the output signal of the main amplifier  112 ; a final error signal detection unit  152  which detects the error component of a final output signal of the feedforward amplifier  100 ; a main signal cancellation loop control unit  161  which controls the main signal cancellation loop  110  based on the error component detected by the final error signal detection unit  152 ; and an error signal cancellation loop control unit  162  which controls the error signal cancellation loop  120  based on the error component detected by the final error signal detection unit  152 . Here, the main signal cancellation loop  110  is adaptively controlled by the main signal cancellation loop control unit  161  so that the error signal generated by the main signal cancellation loop  110  can be optimized in the direction linearizing the final output signal. Also, in the feedforward amplifier  100  of the present invention, the error signal output from the main signal cancellation loop  110  has an imperfectly canceled main signal, as shown at the bottom of FIG. 3.  
         [0028]    Meanwhile, in the feedforward amplifier of the present invention, the output signal of the main signal cancellation loop control unit  161  is input to the vector modulator  116  of the main signal cancellation loop  110  and the output signal of the error signal cancellation loop control unit  162  is input to the vector modulator  126  of the error signal cancellation loop  120 .  
         [0029]    Reference numbers  118  and  128  of FIG. 4 indicate delay compensation lines that perform the same functions as the prior art delay compensation lines  18  and  28 .  
         [0030]    Referring to FIG. 5, the method of improving the performance of a feedforward amplifier according to the present invention comprises step S 200  in which the error component of the final output signal of the feedforward amplifier  100  is detected and, using the detected error component, the main signal cancellation loop  110  and the error signal cancellation loop  120  are controlled at the same time. Here, the simultaneous control step S 200  comprises step S 22  for detecting the error component of the final output signal and controlling the error signal cancellation loop  120 , step S 24  for determining whether or not convergence of the step S 22  is accomplished, step S 32  for controlling the main signal cancellation loop using the error component of the final output signal, and step S 34  for determining whether or not convergence of the step S 32  is accomplished.  
         [0031]    Meanwhile, the method of improving the performance of a feedforward amplifier of the present invention comprises a preceding step S 100  which is performed preceding the simultaneous control step S 200  that is the major characteristic of the method of the present invention. The preceding step S 100  comprises step S 12  for detecting the error signal of the main signal cancellation loop  110  and controlling the main signal cancellation loop  110 , step S 14  for determining whether or not the convergence of step S 12  is accomplished, step S 22  for detecting the error component of the final output signal and controlling the error signal cancellation loop  120 , and step S 24  for determining whether or not convergence of the step S 22  is accomplished.  
         [0032]    Referring to FIG. 5, it is shown that because of the characteristic of the method of improving the performance of a feedforward amplifier of the present invention, steps S 22  and S 24  are included both in the preceding step S 100  and the simultaneous control step S 200 .  
         [0033]    The operation of the thus constructed feedforward amplifier and the method of improving the performance thereof according to the present invention will now be explained referring to FIGS. 2 through 6 b.    
         [0034]    [0034]FIG. 3 briefly shows the effect of imperfect cancellation of a main signal in the error signal of the feedforward amplifier according to the present invention. In the prior art feedforward amplifier, an error signal with a perfectly cancelled main signal is used. In this case, as shown at the top of FIG. 3, due to the non-linearity of the error amplifier  22 , distortion components of the error amplifier  22  itself occur greatly. This causes degradation in linearity of the feedforward amplifier, and the less the capacity of the error amplifier  22 , the greater this degradation becomes.  
         [0035]    Shown at the bottom of FIG. 3 is a conceptual diagram of a method used in the present invention. This shows the output of the error amplifier  122  when the main signal is imperfectly canceled. This imperfect cancellation signal comprises the remaining main signal and the pure error signal, and these two signals cause a signal intermodulated by the error amplifier  122 . The generated intermodulation signal can be made to have a phase opposite to the phase of the distortion component of the error amplifier  122  itself through phase change by the vector modulator  116  of the main signal cancellation loop  110  shown in FIG. 4. If an intermodulation signal having the same size as, but a phase opposite to, the intermodulation signal is generated by appropriate control, the distortion of the error amplifier  122  itself can be perfectly removed. This has the same effect as increasing the capacity of the error amplifier  122 . Accordingly, the final output signal does not contain the distortion component of the error amplifier  122  itself such that the linearity improves more than in the prior art method.  
         [0036]    Referring to FIG. 4, the error signal of the final output signal of the feedforward amplifier  100  is detected by the final error signal detection unit  152 . If the error component of the final output signal is detected by the final error signal detection unit  152 , the main signal cancellation loop control unit  161  and the error signal cancellation loop control unit  162  control, at the same time, the main signal cancellation loop  110  and the error signal cancellation loop  120 , respectively. In this case, since the main signal cancellation loop  110  is adaptively controlled so that the error component of the final output signal is minimized, the error signal generated by the main signal cancellation loop  100  is optimized in the direction linearizing the final output signal. That is, the error signal is not a signal with a perfectly canceled main signal, but an optimum imperfect cancellation signal. This imperfect cancellation signal generates a signal intermodulated with the two signals by the error amplifier  122 , as described above, and works as an important factor for improving linearity. Because of this factor, the entire feedforward performance improves greatly over the prior art.  
         [0037]    Referring to FIG. 5, the method of improving the performance of a feedforward amplifier according to the present invention comprises two processes S 100  and S 200 . In the preceding step S 100 , an error signal is detected and the main signal cancellation loop  110  is controlled in steps S 12  and S 14 , and the error component of the final output signal is detected and the error signal cancellation loop  120  is controlled in steps S 22  and S 24 , as in the prior art method.  
         [0038]    The preceding step S 100  is needed in order to obtain initial values of the two vector modulators  116  and  126 . Through this preceding step S 100 , the convergence speed of the simultaneous control step S 200 , which will now be explained, increases, and a danger of falling into a local minimum point decreases. If the convergence of the preceding step S 100  is accomplished, the simultaneous control step S 200  is performed. In the simultaneous control step S 200 , the error component of the final output signal is detected, and control of the main signal cancellation loop  100  and control of the error signal cancellation loop  120  are performed at the same time in steps S 32  and S 34 . When there is no external interrupt, the adaptive control according to the present invention stays in the simultaneous control step S 200  and the gain of imperfect cancellation is obtained such that optimum performance of the feedforward  100  is obtained.  
         [0039]    In order to inspect the performance of the feedforward amplifier according to the present invention, a 50W-level adaptive control feedforward amplifier for WCDMA base stations which operate at a 2.14 GHz band was designed and manufactured, and the characteristics were measured as shown in FIG. 6. Here, a down link WCDMA 2-carrier signal was used as the input signal.  
         [0040]    [0040]FIGS. 6 a  and  6   b  are spectrum graphs comparing the results of measuring the error signal, that is, the output signal of the main amplifier, of the prior art adaptive control method, and the error signal of the adaptive control method of the present invention, when an average output power is 50 W. FIG.  6   a  is a graph of the error signal spectrum and FIG. 6 b  is the graph of the final output signal spectrum.  
         [0041]    Referring to FIGS. 6 a  and  6   b,  the main signal component in the error signal in the prior art method is almost removed, but removal of the main signal component in the method of the present invention is by about 8 dB less than the prior art method. The remaining main signal that is not removed works for linearity improvement as described above. When the down link WCDMA 2-carrier signal was input and the output power was about 50W, an adjacent channel leakage ratio (ACLR) with about 7 dB improvement was confirmed at a 5MHz offset location.  
         [0042]    As described above, in the feedforward amplifier and the method of improving the performance thereof according to the present invention, by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.  
         [0043]    Also, according to the present invention, only by changing an adaptive control method, without additional expense, the linearity of a feedforward amplifier and price competitiveness can be improved greatly.  
         [0044]    An optimum embodiment of the present invention has been described in above and are shown in the attached drawings. However, the present invention is not limited to the preferred embodiment described above, and it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention defined in the appended claims. Therefore, the scope of the present invention is not determined by the above description but by the accompanying claims.