Abstract:
In a digital broadcast transmission system ( 60 ), a power amplifier ( 66 ) amplifies an information signal to a power level suitable to excite an antenna to emit a broadcast signal. Prior to, an subsequent to, the amplification processing circuitry ( 68, 70 ) processes the signal. In particular, the information signal is bandpass filtered. The power amplifier ( 66 ) causes non-linear distortion to the information signal. The processing circuitry ( 68, 70 ) causes linear distortions to the information signal. A single, memoryful filter ( 76 ) pre-distorts the information signal to compensate for all of the linear and non-linear distortions. Preferably, the memoryful filter ( 76 ) is a volterra filter. In another embodiment, a memoryful filter ( 112 ) pre-distorts an information signal to compensate for linear distortion from a power amplifier ( 102 ) and non-linear distortion from pre-amplification processing circuitry ( 104 ). Linear distortion from post-amplification processing circuitry is compensated by a linear equalizer ( 114 ). For each embodiment, the number of compensation steps is reduced compared to a sequence in which separate compensation occurs.

Description:
TECHNICAL FIELD 
     The present invention relates to broadcast transmission systems and is particularly directed to compensation of distortion within a digital transmission system, such as a digital TV (“DTV”) transmission system. 
     BACKGROUND OF THE INVENTION 
     A high-speed broadcast transmission system such as a DTV broadcast system includes components that distort an information signal away from intended values. Specifically, the system includes a power amplifier that imposes non-linear distortion upon the signal, as a signal is amplified. Also, the broadcast transmission system includes filters, such as band-limiting filters, that impose linear distortion upon the information signal as the signal is filtered. 
     As a result of such distortions within the transmission system, instantaneous amplitude variations (AM/AM) and instantaneous phase variations (AM/PM) occur. In addition, frequency dependent amplitude and phase variations occur. It is to be appreciated that within a phase-amplitude modulated system, integrity of amplitude and phase must be preserved for optimum system performance. 
     There is a need for a high-speed technique for adaptive correction of linear an non-linear distortion within a digital broadcast transmission system. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect, the present invention provides a digital broadcast transmission system. The system includes a signal provision means for providing an electrical information-conveying signal. Antenna means emits a broadcast signal that contains the contents of the information signal. Amplifier means amplifies the information signal to a power level suitable to excite the antenna means to emit the broadcast signal. The amplifier means causes non-linear distortion to the information signal. Signal processing means processes the information signal. The signal processing means causes linear distortion to the information signal. A single filter pre-distorts the information signal to compensate for the linear and non-linear distortions. 
     In accordance with another aspect, the present invention provides a digital broadcast system that includes signal provision means or providing an electrical information-conveying signal along a signal stream. Antenna means, located the end of the signal stream, emits a broadcast signal that conveys the contents of the information signal. A plurality of components is located along the signal stream between the signal provision means and the antenna means. The plurality of components causes distortion to the information signal. At least one of the plurality of components causes linear distortion to the information signal and at least one of the plurality of components causes non-linear distortion to the information signal. A single filter pre-distorts the information signal to compensate for all of the distortion caused by the plurality of components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, wherein: 
     FIG. 1 is a function block diagram of components of a portion of a transmission system configured to provide one approach to distortion correction; 
     FIG. 2 is a function block diagram of components of a portion of a transmission system configured to provide distortion correction in accordance with the present invention; 
     FIG. 3 is the transmission system that includes the structure of FIG. 2; 
     FIG. 4 is a diagram illustrating a manner in which the structure of FIG. 2 determines and adapts correction; 
     FIG. 5 is a diagram similar to FIG. 2, but shows another example of system components configured to provide distortion correction in accordance with the present invention; and 
     FIG. 6 is an expanded block diagram of components in the system for FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a function block diagram of components of a portion of a digital broadcast transmission system  10  that employs one technique of Correction for linear and non-linear distortion. The present invention is an improvement over the correction technique of FIG. 1. A discussion of the technique employed within the system  10  of FIG. 1 will be helpful to understand the present invention. 
     In FIG. 1, a plurality of components of the system  10  is located sequentially along a data stream path  12 . The data stream is for an information data signal that is transmitted at a relatively high rate. Further, the data signal typically has a relatively wide band (i.e., 18 MHz). 
     The high data rate and bandwidth are related to the type/environment of the system  10 . Specifically, the system  10  is a high definition (“HD”) digital television (“DTV”) system. The system  10  includes a power amplifier  14 , pre-amp signal processing circuitry (i.e., filters and/or the like)  1  , and post-amp signal processing circuitry (i.e., filters and/or the like)  18 . The power amplifier  14  and the processing circuitry  16 ,  18  operate in analog format and at a frequency suitable for radio broadcast transmission from an antenna  20 . For example, the broadcast signal output from the antenna  20  is at amplification power level of 50 kilowatts and is in the ultrahigh frequency range (300-3000 MHz), and is preferably in the range of 470-860 MHz. 
     The power amplifier  14  may De comprised of an array of amplifying devices. If a plurality of amplifying devices is present, the power amplifier  14  includes a suitable combiner device. The processing circuitry  16 ,  18  include such components as bandpass filters. In particular, the post-amp processing circuitry  18  includes a high-power bandpass filter. 
     Non-linear distortion is imposed upon the information signal by the power amplifier  14  during amplification of the information signal. Specifically, the non-linear distortion is directed to changes in instantaneous amplitude and phase variations. The is processing circuitry  16 ,  18 , and in particular the bandpass filters of the processing circuitry, impose linear deformation upon the information signal. Thus, in the sequence of the pre-amp processing circuitry  16 , the power amplifier  14 , and the post-amp processing circuitry  18 , a sequence of linear distortion, nonlinear distortion, and linear distortion is imposed upon the information signal as the signal proceeds toward the antenna  20 . 
     In the system  10  of FIG. 1, each of the three distortions (i.e., linear, non-linear, and linear) is corrected or compensated for separately. Specifically, the system  10  includes a first linear equalizer  22  that compensates (i.e., pre-distorts) for the linear distortion caused by the post-amp processing circuitry  18 . A non-linear corrector  24  compensates for the non-linear distortion caused by the power amplifier  14 . A second linear equalizer  26  compensates for the linear distortion caused by the pre-amp processing circuitry  16 . 
     The three compensating components  22 - 26  are located sequentially along the data stream  12  of the system  10 , and are located upstream of the distortion causing components (i.e., the power amplifier  14  and the processing circuitry  16 ,  18 ). The compensating components  22 - 26  operate in digital format. A digital-to-analog converter (DAC)  28  and an upconverter  30  are located along the signal stream between the group of compensating components  22 - 26  and the group of distortion causing components  14 - 18 . 
     Compensation provided by each of the compensating component  22 - 26  is changeable (i.e., adaptable). Each compensating component  22 - 26  requires a separate determination in order to accomplish the change of compensation (adaptation of the pre-distortion). Accordingly, three determination function blocks  32 - 36  are present within the system. The first determination function block  32  determines the pre-distortion that is to be provided by the first linear equalizer  22 . The first determination function block  32  is operatively coupled to receive the input  38  or the first linear equalizer  22  and to receive the output  40  of the post-amp processing circuitry  18 . 
     The second determination function block  34  determines the pre-distortion that is to be provided by the non-linear corrector  24 . The second determination function block  34  is operatively coupled to receive the input  42  of the non-linear corrector  24  and to receive the output  44  of the power amplifier  14 . The third determination function block  36  determines the pre-distortion that is to be provide by the second linear equalizer  26 . The third determination function block  36  is operatively coupled to receive the input  46  of the second linear equalizer  26  and to receive the output  48  of the pre-amp processing circuitry  16 . 
     It is to be appreciated that three separate compensation “steps” are present. Further, it is to be appreciated that each step of compensation requires sampling signal values, determining needed adaptation, and applying the determined adaptation. Associated with each compensation “step” is a need for appropriate processing capability, memory storage capability, and the like. 
     The function block diagram of FIG. 2 shows details of a portion of a system  60  that incorporates the present invention. An overview of the complete system  60  is shown in FIG.  3 . The components shown in FIG. 2 are located within an  8 VSB exciter  62  and transmitter  64  of the system  60  shown within FIG.  3 . 
     The components shown in FIG. 2 include a power amplifier  66 , pre-amp signal processing circuitry  68 , post-amp signal processing circuitry  70 , a DAC  72 , and an upconverter  74 . In one example, the components  66 - 74  of the system  60  of FIG. 2 are identical to the corresponding components  14 - 18 ,  28  and  30  of the system  10  of FIG.  1 . 
     In distinction from the system  10  of FIG. 1, the system  60  of FIG. 2 includes a non-linear memoryful filter  76  instead of the sequence of the first linear equalizer  22  (FIG.  1 ), the non-linear corrector  24 , and the linear equalizer  26 . Preferably, the non-linear memoryful filter  76  is a volterra filter. The single volterra filter  76  is an adaptive filter that corrects for both linear and non-linear distortion of the downstream components (i.e., the power amplifier  66  and the processing circuitry  68 ,  70 ). The volterra filter  76  is modeled by a polynomial. 
     In order to determine adaptation, a determination function block  78  is operatively connected to receive the input  80  of the volterra filter  76  and the output  82  of the post-amp processing circuitry  70 . Accordingly, it is to be appreciated that only a single compensation “step” is present. Further, it is to be appreciated that within the single “step” of compensation, only one group of sampling signal values is needed, only one determination regarding adaptation is needed, and only one application of the determined adaptation is needed. 
     It is to be appreciated that the system  60  may have various types of components, such as solid-state, tube, etc. The volterra filter  7  can be configured to operate within any of such variation of the system  60 . Accordingly, the present invention is not limited to any one volterra filter configuration. 
     FIG. 4 is a block diagram that shows the determination of compensation and adaptation of the volterra filter  76 . Specifically, within FIG. 4, the distortion causing components (i e., the power amplifier and the processing circuitry)  66 - 72  are condensed into a single block located downstream of the volterra filter  76 . The output  2  from the distortion causing components  66 - 72  and the input  80  are provided to the determination function block  78 . Within the determination function block  78 , the input  80  that is provided to the volterra filter  76  is provided as a first input to a comparison circuit  88  (i.e., a SUM circuit having an addition input and a subtraction input). 
     A non-linear memoryful filter  90  receives the output signal  82  from the distortion causing components and outputs a signal  92  to the comparison circuit  88 . An output  94  of the comparison circuit  88  is termed an error signal and is indicative of the amount of distortion (both linear and non-linear) that occurs along the data stream. Thus, the error signal  94  is indicative of the amount of distortion caused by the distortion causing components  66 - 74 . The error signal  94  is provided as an input to the non-linear memoryful filter  90 . The non-linear memoryful filter  90  determines an amount of correction necessary to reduce the error signal to zero or effectively zero (i.e., a null condition). Once the prescribed error value is reached, filter coefficients are downloaded to the volterra filter  76 . 
     To initiate the process of Determining the distortion caused by the distort on causing components  66 - 74 , the volterra filter  76  is initially disabled or turned OFF. The “OFF” state of the volterra filter  76  is represented by the switch  96  and bypass line  98  that extends around the volterra filter  76 . Accordingly, the output from the distortion causing components  66 - 72  does not include any pre-distortion imposed by the volterra filter  76 , and only includes distortion caused by the distortion causing components. Once the non-linear memoryful filter  90  of the determination function makes an initial determination regarding distortion values, the non-linear memoryful filter  90  is downloaded to the volterra filter  76  and the switch  96  is set such that the volterra filter  76  is no longer by-passed (the volterra filter is turned ON). Additional adaptation requires this sequence to be repeated. 
     It is to be noted that the use of the switch  96  to initially by-pass the volterra filter  76  may be omitted such that adaptation is initially based upon the use of the volterra filter  76 . The advantage of the use of the non-linear memoryful filter  90  (i.e., an off-line approach) is that coefficients can be adapted in groups, and the entire memory space can be used. An in-line compensator approach requires the memory to be reset or loaded again after each volterra kernel update. 
     Another example of a system  100  in accordance with the present invention is shown in FIG. 5 (only the pertinent portion of the system  100  is shown in FIG.  5 ). The system  100  of FIG. 5 is similar to the system  60  of FIG.  2 . Specifically, the components of the system  100  of FIG. 5 may be part of the overall system shown in FIG.  3 . Further the system  100  of FIG. 5 includes the components of a power amplifier  102 , pre-amp processing circuitry  104 , post-amp processing circuitry  106 , a DAC  108 , and upconverter  110 . 
     The system  100  of FIG. 5 differs from the system  60  of FIG. 2, in that only the linear distortion of the pre-amp processing circuitry  104  and the non-linear distortion of the power amplifier  102  are corrected within a volterra filter  112  located upstream of the DAC  108 . The linear distortion caused by the post-amp processing circuitry  10  is corrected via pre-distortion within a linear equalizer  114  located upstream of the volterra filter  12 . 
     A determination function block  116  for the volterra filter  112  is connected to an input  118  of the volterra filter  112  and is connected to an output  120  of the power amplifier  102 . A determination function block  122  for the linear equalizer  114  is operatively connected to the input  124  of the linear equalizer and the output  126  of the post-amp processing circuitry  106 . Compared to the system  10  shown in FIG. 1, the system  100  of FIG. 5 includes one less “step” of compensation (i.e., sampling, adaptation determination, and pre-distortion. Specifically, the functions of compensation performed by the non-linear corrector  24  (FIG. 1) and the second linear equalizer  26  of the system  10  of FIG. 1 are performed by the volterra filter  112  (FIG. 5) of the system  100 . 
     The use of the linear equalizer  114  upstream of the volterra filter  112  has certain benefits, especially if the post-amp processing circuitry  106  includes a high-power filter. The high-power filter may be a relatively high order filter. As such, it requires a significant order of volterra filter to compensate for it. In general, the order of a volterra-type filter will increase as the square of the system order. Whereas, a linear equalizer is typically about the same order as the system for which it compensates. Therefore, if the high-power filter is compensated by linear filter means external to the volterra filter, it may reduce system cost by allowing a much lower order volterra filter. 
     Also, the high-power filter attenuates (eliminates) the high order non-linear information needed for non-linear equalizer compensation. Although the information may still be there, it will be sufficiently attenuated so that very high dynamic ranges would be required of the system to provide equalization. Rather than burden the non-linear equalizer with the difficulties of high dynamic range issues, it is prudent to pick-off the return signal before the high-power filter. 
     A more detailed representation of the system  100  of FIG. 5 is shown in the function block diagram of FIG.  6 . Specifically, a Nyquist filter  130  of the  8 VSB exciter provides the information signal in a digital and complex format to a complex-to-real converter  132 . The output of the complex-to-real converter  132  is a digital signal in real format that is provided as the input to the linear equalizer  11 . The linear equalizer  114  is a digital filter that has suitable structure for pre-compensating or pre-equalizing the information signal to compensate for the linear distortion caused by the high-powered filter  134 . The linear equalizer  114  may be comprised, or include, a microprocessor that performs a process and/or may be comprised of, or include, discrete “hard-wired” circuitry. 
     The information signal passe from the linear equalizer  114  to the volterra filter  112 . The volterra filter  112  may have any suitable structure for pre-distorting (e.g., pre-equalizing) the signal to compensate for both the non-linear distortion caused by a power amplifier  102 , and pre-amp linear distortion. The volterra filter  112  may be comprised of, or include, a microprocessor that performs a program process and/or may be comprised or include, discrete “hard-wired” circuitry. 
     In the example of FIG. 6, the pre-amp processing circuitry  104  and upconverter  11  includes a first upconverter  136  driven by a first local oscillator  138 , a first bandpass filter  140 , a second upconverter  142  driven by a second local oscillator  144 , a second bandpass filter  146 , and a third bandpass filter  148 . In addition a fourth bandpass filter  150  is located just downstream of the power amplifier  102 . The volterra filter  112  also corrects the distortion from the fourth bandpass filter  150 . 
     The output of the volterra filter  112  is provided to an interpolation component  152 . Preferably, the interpolation that occurs is a factor of two. The DAC  108  receives the output of the interpolation component  152  and provides the signal to the first upconverter  136 . 
     For the adaptation of the volterra filter  112 , the sample  120  is taken after the power amplifier  102  and after the fourth bandpass filter  150 , but prior to the high-power filter  134 . Along the path of the sample is located a first down-converter  154 , a first low-pass filter  156 , a second down-converter  158 , and a second low-pass filter  160 . Additionally, an analog-to-digital converter (A/D)  162  and a decimation (i.e., by two) function component  164  are located to provide the input to the correction determination function block  116  for the volterra filter  112 . 
     The sample  126  for the linear equalizer correction determination  122  for the linear equalizer  114  is operatively connected to the output of the high-power filter  134 . Similar to the sample path for the volterra filter correction determination  116 , the path for the linear equalizer correction determination  122  includes suitable components  168  for down-converting, filtering, decimation, and the like. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. For example, other types of components could be used in place of the volterra filter. Examples of such components with non-linear components are perceptrons and neural networks. The modeling of such other non-linear components may be logarithmic, curve fitting, or the like. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.