Abstract:
According to a method, a first signal is produced from a digital input signal by pre-emphasis and said signal is supplied, after carrier frequency translation, D/A conversion and modulation, to a power amplifier for producing a carrier-frequency output signal. IN order to linearize the characteristic curve of the power amplifier, pre-emphasis is carried out in a manner controlled by parameters. A test signal is superposed to the first signal, thereby producing an output signal having a carrier-frequency test signal portion in addition to a carrier-frequency input signal portion. A comparison of the carrier-frequency test signal portion of the output signal with the test signal yields the parameter for controlling pre-emphasis. Alternatively, the test signal can be superposed before the pre-emphasis to be carried out.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to Application No. PCT/EP/2005/051564 filed on Apr. 8, 2005 and European Application No. 04015907 filed Jul. 6, 2004, the contents of which are hereby incorporated by reference. 
     
     BACKGROUND 
       [0002]    The invention relates to a method for linearizing the characteristic curve of a power amplifier and also to a system for linearizing the characteristic curve of a power amplifier. 
         [0003]    Power amplifiers, which ideally are designed for highly-linear amplification of broadband carrier-frequency signals, are used for the transmission of telecommunications signals. Pre-emphasis methods are known for optimizing the characteristic curve of a power amplifier in respect of its linearity. In such cases, for forming a linearly-amplified output signal, the signal to be transmitted is pre-emphasized before amplification so as to compensate for non-linearities of the characteristic amplifier curve. 
         [0004]    The pre-emphasis is normally undertaken in what is known as the intermediate frequency range or in the complex baseband of the signal, i.e. before the conversion into the carrier frequency range, with the pre-emphasis being adjusted with the aid of parameters. The parameters in their turn are obtained from a comparison of the power amplifier output signal with the signal before the pre-emphasis and/or after the completed pre-emphasis. This means that the parameters determined depend both on the properties of the signal to be transmitted and on the operating parameters of the power amplifier and are thus influenced both by the electrical characteristic data of the amplifier and by its ambient temperature. 
         [0005]    The parameters for control of the pre-emphasis are usually stored in a multi-dimensional table and can be re-used when appropriate circumstances occur, which also allows account to be taken of changing ambient temperatures. 
         [0006]    Because of the above-mentioned dependencies of the parameters, these types of table are very extensive and the parameters are only able to be adapted using very time-consuming procedures. 
         [0007]    A method for pre-emphasis is known from US 2002/68023 A1, in which a coupled-in test signal is amplified and subsequently analyzed after the pre-emphasis. 
         [0008]    A method for pre-emphasis is known from WO 00/02324 A1, in which a pilot signal is coupled in before the pre-emphasis is executed and analyzed after completed amplification. 
       SUMMARY 
       [0009]    One possible object of the present invention is thus to specify a method and a system for a fast and precise linearization of a power amplifier characteristic curve to be executed in which the linearization is to be performed by a signal pre-emphasis. 
         [0010]    The inventor suggests that a first signal, which is present for example as a multi-carrier signal is superposed by a test signal and thereby a second signal is formed. The second signal is converted into a carrier frequency slot and fed to a power amplifier to form an output signal. Especially by comparing a test signal component contained in the output signal with the test signal, parameters to control a pre-emphasis are formed. 
         [0011]    In a first embodiment an input signal is pre-emphasized in order to form the first signal, with the input signal being present as a multi-carrier signal in the complex digital baseband or translated in an intermediate frequency slot. 
         [0012]    In a second embodiment the first signal is present as a multi-carrier signal in the complex digital baseband or translated in an intermediate frequency slot and is not pre-emphasized until after completed test signal superposition. 
         [0013]    The test signal exhibits particular spectral characteristics. Preferably a pulse signal is used which has a time-variant amplitude value distribution known in advance. The individual amplitudes are selected so that only negligible disturbance components dependent on the test signal are formed in those frequency ranges which are adjacent to a carrier frequency range to be used. 
         [0014]    Advantageously further parameters are obtained especially from the test signal component contained in the output signal by comparison with the test signal, with which the formation of the test signal is controlled. This makes possible the suppression of the above-mentioned disturbance components in the adjacent frequency ranges. 
         [0015]    In an advantageous development of the invention a baseband clipping method is executed in the baseband on the signal which is to be used in subsequent execution for superposition with the test signal. By analyzing the output signal it is possible to form parameters for controlling the baseband clipping method. 
         [0016]    The baseband clipping method is thus applied to the complex baseband signal from which, after the baseband clipping method has been executed, after interpolation and modulation and if necessary after pre-emphasis has been performed, the first signal is produced in the first embodiment. 
         [0017]    The baseband clipping method is used to reduce possible maximum transmit power values of the output signal to a fixed predetermined value. The baseband clipping method is not used for smaller amplitudes of the baseband signal. 
         [0018]    Advantageously in this case a flexibly adjustable clipping threshold is used which can be changed in accordance with the instantaneously available maximum power of the power amplifier. This instantaneously available maximum power is especially dependent on the ambient temperature of the power amplifier. The parameters for controlling the clipping method are determined as a function of the ambient temperature and stored in the table for subsequent variation of the clipping threshold. 
         [0019]    The design of the test signal and the use of the test signal in the assessment of the behavior of the power amplifier make it possible to reduce the number of entries contained in the table. Characteristics or operating states of the amplifier are detected and compensated for more quickly. This reduced-sized table is especially advantageous for rapid changes in the complex baseband input signal. 
         [0020]    It is possible to consider the baseband clipping method and the pre-emphasis method as uniform for the respective overall embodiment, in which case corresponding settings make it possible for them to complement each other. 
         [0021]    With the above, it is easy to perform an adaptive adjustment of the clipping threshold of the baseband clipping method. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0022]    These and other objects and advantages will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0023]      FIG. 1  a block diagram of a system for linearization of a power amplifier characteristic curve according to a first embodiment, 
           [0024]      FIG. 2  a block diagram of a system for linearization of a power amplifier characteristic curve according to a second embodiment, 
           [0025]      FIG. 3  a typical test signal with reference to  FIG. 1  and  FIG. 2 , 
           [0026]      FIG. 4  a test signal frequency response with reference to  FIG. 3 , 
           [0027]      FIG. 5  the signal timing waveform of the output signal with reference to  FIG. 1  and  FIG. 2 , 
           [0028]      FIG. 6  a complex diagram of the output signal with reference to  FIG. 1  and  FIG. 2 , 
           [0029]      FIG. 7  the output signal frequency response with reference to  FIG. 1  and  FIG. 2 , 
           [0030]      FIG. 8  an output signal frequency response measured on a receiver side, and 
           [0031]      FIG. 9  a comparison of an adaptation result obtained with a non-linear amplifier characteristic curve. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0033]      FIG. 1  shows a block diagram of a first arrangement for linearization of a power amplifier characteristic curve of a power amplifier PA 1 . 
         [0034]    One or more complex baseband signals BBS reach an interpolation device IP 1  and are translated using a first modulator MOD 11  into a (multicarrier) input signal IN 11 , with the input signal IN 11  for example being present as an oversample in an intermediate frequency slot. The input signal IN 11  formed reaches a pre-emphasis unit PRE 1  and is pre-emphasized by this unit, producing a pre-emphasized first signal S 11 . The pre-emphasis unit PRE 1  is controlled by a first parameter set PAR 11 . 
         [0035]    The first signal S 11  reaches an additive superposition unit ADD 1 , to which a test signal TS 1  formed by a pulse generator PG 1  is also fed. 
         [0036]    A second signal S 12  is formed for example with the aid of the additive superposition unit ADD 1  by additive superposition to the first signal S 11  of the test signal TS 1 . 
         [0037]    The second signal S 12  arrives via a device for carrier frequency translation UP 1 , via a D/A converter DAW 1  and via a second modulator MOD 12  as a third carrier-frequency signal S 13  at the power amplifier PA 1 , which exhibits a non-linear characteristic amplifier curve. The power amplifier PA 1  forms from the third signal S 13  a power-amplified, carrier-frequency output signal OUT 1 . 
         [0038]    The carrier-frequency output signal OUT 1  thus has both components of the input signal IN 11  and also components of the test signal TS 1 . These are referred to below as test signal component TSA 1  and as input signal component INA 11 . 
         [0039]    Proportions of the output signal OUT 1  arrive for example by uncoupling via a demodulator DEM 1  and via an A/D converter ADW 1  at a control device SE 1 , to which the second signal S 12  is also fed. As described above, the second signal S 12 , because of the superposition, also contains the test signal TS 1  as well as the first signal S 11 . 
         [0040]    The control device SE 1  analyzes the transmission of the test signal TS 1  by comparing a time segment with the test signal component TSA 1  of the output signal OUT 1  with the corresponding time segment of the test signal TS 1 . Based on this comparison, the first parameter set PAR 11 , with which the pre-emphasis unit PRE 1  is controlled, is formed by the control device SE 1 . The linearization of the characteristic curve of the power amplifier PA 1  is achieved by this control. 
         [0041]    For more precise determination of the parameter set PAR 11 , an additional comparison of the input signal component INA 11  contained in the output signal OUT 1  with the input signal IN 11  is possible in an advantageous further development. 
         [0042]    In a further advantageous development a further parameter set PAR 12  with which the pulse generator PG 1  will be controlled is formed by the control device SE 1 . This parameter set PAR 12  is also formed by comparison of the test signal component TSA 1  of the output signal OUT 1  with the test signal TS 1 . 
         [0043]    The control of the formation of the test signal TS 1  makes it possible to minimize disruptive carrier-frequency components of the test signal TSA 1  in those frequency ranges which are adjacent to a carrier frequency range of the output signal OUT 1  to be used. Overloading of the power amplifier is avoided. 
         [0044]    The parameter sets PAR 11  and PAR 12  are determined with the aid of a peak detection method PD, of a power estimation method PE and/or of a so-called “NL system identification” method NLSYSIDENT with target functions. These methods are known for example from the book entitled “Digital Communications”, John G. Proakis, pages 601-635. The associated algorithms of the target functions can be found in this book on pages 636 to 679. 
         [0045]    Methods for pre-emphasis are known for example from the German patent application with the file reference DE 103 20 420 A1, which was submitted to the German patent and trademark office on 07.05 2003. In this application a projection of an undersampled output signal of an AD converter is computed on different basic vectors which are obtained from a pre-emphasized signal. The projection can for example be undertaken in the form of a power series development. 
         [0046]    In an advantageous development, the complex baseband signal BBS is additionally fed to the control device SE 1  and additionally compared with the input signal component INA 11  contained in the output signal OUT and/or with the first signal S 11  contained in the second signal S 12 . This makes a more precise determination of the parameter set PAR 11  possible. 
         [0047]    In addition a baseband clipping method BBC can be applied to the complex baseband signal BBS. In this case a further parameter set PAR 13  which is used for control of the baseband clipping method is formed by the control device SE 1 . When the parameter set PAR 13  is formed the second signal S 12  and/or the input signal IN 11  are taken into account, in addition to the output signal OUT 1 . 
         [0048]    An adaptive setting of a clipping threshold of the baseband clipping process BBC is implemented, with this threshold being adapted to the overall system or to its transmission characteristics. This adaptation can for example be undertaken as described below. A maximum amplitude of the power amplifier PA 1 , which lies far above a maximum value of the third signal S 13 , is known from the computed parameters of the parameter set PAR 11  or from the use of the peak detection method PD. This means that the clipping threshold value can be adapted to characteristics of the power amplifier PA 1 , especially to its ambient temperature, ageing, dispersion, . . . , or to peak values of the output power of the output signal OUT 1  which depend on these characteristics. 
         [0049]    Furthermore, for an impending overload of the power amplifier PA 1 , higher signal levels of the baseband signal BBS are more greatly reduced by the baseband clipping method BBS than would be the case in a normal application. In addition the test signal TS 1  is then superposed to the first signal S 11 , with the correct phase, but with a negative amplitude, in order to reduce the maximum amplitude of the output signal OUT 1 . 
         [0050]      FIG. 2  shows as a block diagram a second arrangement for linearization of the characteristic curve of a power amplifier PA 2 . 
         [0051]    The complex baseband signals BBS reach an interpolation device IP 2  either via a device for executing a baseband clipping method BBC or directly, and are translated using a first modulator MOD 21  into a (multicarrier) input signal IN 21 , with the input signal IN 21  being present oversampled in an intermediate frequency slot. 
         [0052]    The input signal IN 21  reaches an additive superposition device ADD 2  as first signal S 21 , with a test signal formed by a pulse generator PG 2  also being fed to said device. 
         [0053]    A second signal S 22  is formed for example with the aid of the additive superposition device ADD 2  by additive superposition to the first signal S 21  of the test signal TS 2 . 
         [0054]    The second signal S 22  reaches a pre-emphasis unit PRE 2  and is pre-emphasized by this unit, with a pre-emphasized third signal S 23  being formed. The pre-emphasis unit PRE 2  is controlled by a first parameter set PAR 21 . 
         [0055]    The third signal S 23  arrives via a device for carrier frequency translation UP 2 , via a D/A converter DAW 2  and via a second modulator MOD 22  as a fourth carrier-frequency signal S 24  at the power amplifier PA 2  which has a non-linear characteristic amplifier curve. The power amplifier PA 2  forms a power amplifier carrier-frequency output signal OUT 2  from the fourth signal S 24 . 
         [0056]    Thus the carrier-frequency output signal OUT 2  has both components of the first signal S 21  or of the input signal IN 21  and also components of the test signal TS 2 . These will be referred to below as input signal component INA 21  and as test signal component TSA 2 . 
         [0057]    The output signal OUT 2 , for example by uncoupling via a demodulator DEM 2  and via an A/D converter ADW 2 , proportionally reaches a control device SE 2 , to which the second signal S 22  and/or the third signal S 23  are also fed. 
         [0058]    As described above, the second signal S 22 , because of the superposition, also contains the test signal TS 2  in addition to the first signal S 21 . 
         [0059]    The control device SE 2  analyzes the transmission of the test signal TS 2  by comparing the test signal component TSA 2  of the output signal OUT 2  with the test signal TS 2  contained in the second signal S 22 . Based on this comparison, the first parameter set PAR 21  with which the pre-emphasis unit PRE 2  is controlled is formed by the control device SE 2 . A linearization of the characteristic curve of the power amplifier PA 2  is achieved by this control. 
         [0060]    For more precise determination of the parameter set PAR 21  an additional comparison of the input signal component INA 21  contained in the output signal OUT 2  with the first signal S 21  contained in the second signal S 22  and/or with the third signal S 23  is possible in an advantageous further development. 
         [0061]    In an advantageous development a further parameter set PAR 22 , with which the pulse generator PG 2  is controlled, is formed by the control device SE 2 . To form the parameter set PAR 22  the test signal component TSA 2  contained in the output signal OUT 2  is again compared to the test signal TS 2  contained in the second signal S 22  in assigned time segments. 
         [0062]    In an advantageous development signal components of the output signal OUT 2  or of the second signal S 22  which can additionally be assigned to one another are evaluated. 
         [0063]    By controlling the formation of the test signal TS 2  it is possible to minimize disruptive components of the test signal TSA 2  in those frequency ranges which are adjacent to a carrier frequency range of output signal OUT 2  to be used. 
         [0064]    The parameter sets PAR 21  and PAR 22  are determined using the method already described in  FIG. 1 . 
         [0065]    In an advantageous development the control device SE 2  is additionally supplied with the complex baseband signal BBS. It is also possible to determine the parameter set PAR 21  more precisely by an additional comparison of the baseband signal BBS with the input signal component INA 21  contained in the output signal OUT 2  and/or with the input signal IN 21  contained in the second signal S 22  which corresponds to the first signal S 21 , and/or with the corresponding input signal component of the third signal S 23 . 
         [0066]    When the device BBC for executing the baseband clipping method is used, a further parameter set PAR 23  is formed by the control device SE 2  which controls the baseband clipping method. The complex baseband signal BBS and/or the second signal S 22  and/or the third signal S 23  are taken into account in addition to the output signal OUT 2  in the formation of the parameter set PAR 23 . 
         [0067]    This implements an adaptive adjustment of a clipping threshold value which is used for the baseband clipping method BBC. This clipping threshold is adaptively matched to the overall system or to its transmission characteristics. 
         [0068]    This adaptation can for example be undertaken as described below. A maximum amplitude of the power amplifier PA 2  which lies far above a maximum value of the fourth signal S 24  is known from the calculated parameters of the parameter set PAR 21  or from the use of the peak detection method PD. Thus the clipping threshold can be adapted to characteristics of the power amplifier PA 2 , especially to its ambient temperature, ageing, dispersion, etc., or to the peak values of the output power of the output signal OUT 2  which depend on such characteristics. 
         [0069]    In a further application, if there is the threat of overloading of the power amplifier PA 2 , higher signal levels of the baseband signal BBS are further reduced. In addition a test signal TS 2  is additively superimposed with the correct phase but with a negative amplitude onto the input signal IN 21 . A maximum amplitude of the output signal OUT 2  is reduced in this way. 
         [0070]      FIG. 3  shows, with reference to  FIG. 1  and  FIG. 2 , a typical test signal TS 1  or TS 2  which exhibits time-variant amplitude statistics. 
         [0071]    The time is plotted on the horizontal axis and corresponding pulse signals are plotted on the vertical axis. Test signal TS 1  or TS 2  has been selected here as a Chebyshev design with  41  coefficients and a blocking attenuation of 50 dB. In this case the objective of the test signal was to define a signal limited to 31 time values, of which the essential spectral components lie in the carrier frequency band to be used. 
         [0072]      FIG. 4  shows, with reference to  FIG. 3 , the frequency response of the test signal TS 1  or TS 2 , with the frequency plotted on the horizontal axis and associated amplitude values plotted on the vertical axis. 
         [0073]    In this case the test signal has an identical complex phase to the maximum useful signal S 11  or S 21  in the complex baseband. 
         [0074]      FIG. 5  shows, in relation to  FIG. 1  and  FIG. 2 , the timing of output signals OUT 1  or OUT 2 , with the time plotted on the horizontal axis and the amplitudes plotted on the vertical axis. The superposed test signal can be seen clearly in this diagram at t=2*10 5 . 
         [0075]      FIG. 6  shows a more complex diagram of the output signal OUT 1  or OUT 2  in relation to  FIG. 1  and  FIG. 2 . In this case the superposed test signal can clearly be seen in a club-shaped waveform. 
         [0076]      FIG. 7  shows, in relation to  FIG. 6 , the frequency response of the output signal OUT 1  or OUT 2 , with the frequency being plotted on the horizontal axis and amplitudes being plotted on the vertical axis. 
         [0077]      FIG. 8  shows, in relation to  FIG. 7 , an output signal frequency response measured by a receiver, with the frequency being plotted on the horizontal axis and receive amplitudes being plotted on the vertical axis. This assumes the measured signal will be disturbed by additively superposed, white, Gaussian distributed noise. 
         [0078]      FIG. 9  shows a comparison of an adaptation result obtained on a non-linear characteristic amplifier curve, labeled “non-linearity tanh(abs(x))”. This is the middle characteristic curve shown in  FIG. 9 . 
         [0079]    A magnitude of the amplitude of the third signal S 13  or of the fourth signal S 24  is shown on the horizontal axis. A magnitude of the amplitude of the output signal OUT 1  or OUT 2  is shown on the vertical axis—after a demodulation and analog/digital conversion to be performed. 
         [0080]    A variation of a conventional estimate of non-linearity of the characteristic amplifier curve with increasing amplitude curve can be seen with the curve labeled “approximation using signal only”. This is the left-hand characteristic curve shown in  FIG. 9 . 
         [0081]    By contrast, the estimated non-linearity for the application of the test signal labeled as “pulse” is significantly longer in compliance with the non-linear characteristic amplifier curve—curve labeled with “approximation using signal+pulse”. This is the right-hand characteristic curve shown in  FIG. 9 . 
         [0082]    A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).