US 20020190787 A1 Abstract Disclosed is an apparatus and method for iteratively calculating the predistortion function from a power amplifier when the inverse to the power amplifier characterization does not exist analytically. A predistortion module generates a predistorted signal in response to an input signal and a predistortion function received from a predistortion function calculating module. The predistortion function calculating module generates the predistortion function in response to given amplifier characteristics. In particular, the predistortion function calculating module uses a magnitude of the input signal at a given time to estimate the predistorted signal at that given time in order to calculate the predistortion function. Optionally, subsequent iterations based on the magnitude of the estimate occur until a desired accuracy of the predistorted signal approximation and of the predistortion function is achieved.
Claims(18) 1. An apparatus for calculating a predistortion function from characteristics of a power amplifier, comprising:
a predistortion module responsive to a predistortion function and an input signal by generating a predistorted signal; the amplifier responsive to said predistorted signal by generating an output signal; and a predistortion function calculating module responsive to said amplifier characteristics by generating said predistortion function, said predistortion function calculating module using a magnitude of said input signal at a given time to estimate said predistorted signal at said given time to calculate said predistortion function. 2. The apparatus of 3. The apparatus of 4. The apparatus of 5. The apparatus of wherein “n” is a time index, wherein x
_{n }is a current value of said predistorted signal, wherein u_{n }is a current value of said input signal, wherein β is a complex polynomial representation of said amplifier, wherein coefficients are generated for said complex polynomial representation for each of a plurality of delay taps, wherein M is an order of amplifier memory equal to the number of said plurality of delay taps and wherein |x_{n}| is estimated by |u_{n}|. 6. A wireless radio frequency communications system including an apparatus for calculating a predistortion function from characteristics of a power amplifier with memory, comprising:
a predistortion module responsive to a predistortion function and an input signal by generating a predistorted signal; the amplifier responsive to said predistorted signal by generating an output signal; and a predistortion function calculating module responsive to said amplifier characteristics by generating said predistortion function, said predistortion function calculating module using a magnitude of said input signal at a given time to estimate said predistorted signal at said given time to calculate said predistortion function. 7. The system of 8. The system of 9. The system of 10. The system of _{n }is a current value of said predistorted signal, wherein u_{n }is a current value of said input signal, wherein β is a complex polynomial representation of said amplifier, wherein coefficients are generated for said complex polynomial representation for each of a plurality of delay taps, wherein M is an order of amplifier memory equal to the number of said plurality of delay taps and wherein |x_{n}| is estimated by |u_{n}|. 11. A method for calculating a predistortion function from characteristics of a power amplifier with memory, comprising the steps of:
generating a predistorted signal in response to a predistortion function and an input signal; generating an output signal in response to said predistorted signal; and generating a predistortion function in response to said amplifier characteristics, said step of generating a predistortion function including the step of substituting a magnitude of said input signal at a given time to estimate said predistorted signal at said given time to generate said predistortion function. 12. The method of 13. The method of 14. The method of 15. The method of _{n }is a current value of said predistorted signal, wherein u_{n }is a current value of said input signal, wherein β is a complex polynomial representation of said amplifier, wherein coefficients are generated for said complex polynomial representation for each of a plurality of delay taps, wherein M is an order of amplifier memory equal to the number of said plurality of delay taps and wherein |x_{n}| is estimated by |u_{n}|. 16. An apparatus for calculating a predistortion function from a characteristics curve of a power amplifier, comprising:
a predistortion module responsive to a predistortion function representative of an inverse to said characteristics curve and an input signal by generating a predistorted signal; the amplifier responsive to said predistorted signal by generating an output signal; and a predistortion function calculating module responsive to said amplifier characteristics curve by generating said predistortion function, said predistortion function calculating module using a magnitude of said input signal at a given time as an estimate of said predistorted signal at said given time to calculate said predistortion function. 17. The apparatus of 18. The apparatus of Description [0023] The following description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and the scope of the invention. Thus, the present invention is not intended to be limited to the embodiments disclosed, but is to be accorded the widest scope consistent with the principles and features disclosed herein. [0024] The specification initially discusses the general concept and principles of adaptive digital predistortion in view of the novel device for employing predistortion techniques. Exemplary embodiments of the present invention are then described. [0025] Overview of Adaptive Power Amplifier Predistortion [0026] The principal benefit of the present invention is the ability to more efficiently model the power amplifier characteristics in order to improve the ability to employ adaptive digital predistortion (ADPD). The structure of an exemplary ADPD system is seen in FIG. 1. An initial baseband digital signal [0027] As implemented in FIG. 1, the baseband digital input signal [0028] By neglecting the effect of up-conversion and down-conversion process, the whole predistortion process can be considered in the baseband domain. In FIG. 2, an exemplary baseband model for predistortion processing is illustrated. The power amplifier [0029] Calculation of Predistortion Function [0030]FIG. 3 illustrates a block diagram [0031] In FIG. 3, the power amplifier [0032] In general, predistortion module [0033] The operation of the predistortion function calculating module [0034] First, a quick discussion of polynomial modeling of power amplifiers without memory is necessary. If it is assumed that power amplifier is operating in the radio frequency (RF) domain, then the baseband model of the power amplifier as shown in FIG. 3, must be considered in complex numbers. Let x and y be the complex input and output of a power amplifier [0035] where {b [0036] The power amplifier model described above has the same characteristics over different frequency ranges. However, as the bandwidth of the signal increases, actual power amplifiers have memory and its characteristics depend on signal frequencies. Memory, as it relates to a power amplifier, means that the output of the power amplifier is not only a function of the current input but also a function of past inputs. Thus, greater accuracy in modeling a power amplifier can be achieved if memory impacts as well as non-linear responses are accommodated. Let x [0037] where p [0038] Given the above power amplifier model, the predistortion function can be calculated by obtaining the inverse function of the polynomial in equation (2). This is denoted as A(•) in FIG. 1. By using equation (3), equation (2) now becomes: [0039] Then x [0040] Let u [0041] The problem of equation (6) is that it is necessary to know the value of |x [0042] Step 1. Calculate the estimate of x [0043] Step 2. Calculate a better estimate, {tilde over (x)} [0044] Step 3. Repeat Step 2 a given number of iterations. Use the final value of the estimate of x [0045] In Step 1, a magnitude of the input signal at a given time is used as an estimate of the predistorted signal, and particularly of the magnitude of the predistorted signal, at that given time. In Step 2, the predistortion function calculation, responsive to the amplifier characteristics, uses the estimate. That use generates an approximation {tilde over (x)} [0046]FIG. 4 illustrates a base station [0047] As shown in FIG. 4, base station [0048] Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. For example, although an amplifier with memory is described, the iteration technique of the present invention is applicable to amplifiers without memory. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications that come within the scope of the appended claim is reserved. [0018] For a better understanding of the present invention, reference may be had to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which: [0019]FIG. 1 is a block diagram providing an overview of an exemplary system employing adaptive power amplifier predistortion; [0020]FIG. 2 is a block diagram of the simplified baseband model for power amplifier predistortion; [0021]FIG. 3 is a block diagram of a power amplifier employing a predistortion linearization technique in accordance with the principles of the present invention; and [0022]FIG. 4 is a block diagram illustrating the power amplifier of FIG. 3 as used in a RF transmission system in accordance with the principles of the present invention. [0002] The present invention relates to a method and apparatus for providing a predistortion function in a power amplifier. [0003] In the field of radio communication systems, it is a well-known problem that the power amplifiers present in transmission equipment operate in a non-linear fashion when the power amplifiers are operated near their peak output. As a result, the power amplifier introduces significant signal distortion that can appear in various forms. For example, if more than one signal is input into the power amplifier or power amplification stage, its non-linear characteristics can cause an undesirable multiplicative interaction of the signals being amplified, and the power amplifier's output can contain intermodulation products. These intermodulation products cause interference and crosstalk over the power amplifier's operational frequency range. [0004] In power amplifier design, there is a trade off between distortion performance and efficiency. Linear amplifiers that operate under “Class A” conditions create little distortion but are inefficient, whereas nonlinear amplifiers operated under “Class C” conditions are reasonably efficient but introduce significant distortions. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, nonlinear amplifiers are largely preferred, leaving the user with the problem of distortion. [0005] In order to employ nonlinear power amplifiers, techniques have been used to improve linearity and thereby reduce the effects of interference and crosstalk. Linearity can be achieved by application of various linearization techniques that reduce the distortion caused by nonlinear amplification. Conventional amplifier linearization techniques can be broadly categorized as feedback, feedforward, or predistortion. [0006] The last mentioned technique, predistortion, intentionally distorts the signal before the power amplifier so that the non-linearity of the power amplifier can be compensated. According to this technique, linearization is achieved by distorting an input signal according to a predistortion function in a manner that is inverse to the amplifier characteristic function. The predistortion technique can be applied at radio frequency (RF), intermediate frequency (IF), or at baseband. [0007] In the baseband domain, the input signal information is at a much lower frequency, allowing digital methods to be employed. The predistortion function is applied to the input signal with the resulting predistorted signal being upconverted to IF and then finally to the RF carrier frequency. It is also possible to apply adaptive predistortion techniques where feedback from the output of the amplifier is used to update and correct the predistortion function. [0008] The form of the predistortion function is dependent upon the model used to characterize the output of the amplifier. Predistortion functions in the baseband domain are typically implemented as a table of gain and phase weighting values within a digital signal processor. A Cartesian feedback method employs a quadrature representation of the signal being amplified. The incoming quadrature signals I and Q are compared to the feedback quadrature signals. Thus, there are two sets of coefficients, one for each quadrature channel, that are being updated to model the predistortion characteristics. In this manner, gain and phase non-linearities within the amplifier can be compensated. Performance is dependent on the size of the look up table and the number of bits used to represent the signal. Better performance and more adaptivity is achieved with larger look up tables and more bits albeit at the expense of longer processing times. [0009] Predistortion functions are also modeled as polynomials. Here, the complex polynomial must be able to characterize the inverse of the amplifier, which may not analytically exist and which must then be approximated. In order to accurately estimate the inverse, the polynomial requires high order terms, with associated quantization errors and a less accurate polynomial fit. [0010] Adaptive methods generally process and model current amplifier characteristics. The current output signal of the amplifier is contrasted against the current input signal to the amplifier. Past inputs are not considered. However, amplifier characteristics are dependent upon frequency due to the speed in which input signals change amplitude as a function of frequency. Exclusion of past inputs precludes modeling those changes and limits the accuracy with which the amplifier can be modeled, thereby limiting the bandwidth. [0011] Accordingly, there is a need for a power amplifier predistortion system that can quickly and efficiently obtain an optimum predistortion function for frequency dependent amplifiers. [0012] The present invention teaches an apparatus and method for calculating the predistortion function from a power amplifier. A predistortion module generates a predistorted signal in response to an input signal and a predistortion function. The predistortion function calculating module generates a predistortion function in response to given amplifier characteristics. To more readily calculate the predistortion function, the predistortion function calculating module uses a magnitude of the input signal at a given time as an estimate of the predistorted signal at that given time to calculate the predistortion function. For example, in calculating the inverse of an amplifier characteristic curve, the magnitude of the current signal sample can be a function of the magnitude of the current predistorted signal. By replacing the magnitude of the current predistorted signal sample with the magnitude of the current input signal sample, the predistorted signal can determined. [0013] In an exemplary embodiment of the present device, the predistortion function calculating module, in calculating the predistortion function, estimates the magnitude of the predistorted signal on an iterative basis. [0014] In another exemplary embodiment of the present device, time spaced samples of the predistorted signal are processed by an amplifier with memory, and corresponding time indexed coefficients are used in characterizing the amplifier and in the calculation of the predistortion function. Each time spaced sample of the predistorted signal is generated from a corresponding time spaced instance of the input signal. [0015] Importantly, power amplifiers have memory and their characteristics depend upon signal frequencies. By including prior instances of the input signal, the present invention can model the impact of both current and past instances of the input signal and therefore model non-linear and frequency dependent aspects of the power amplifier. [0016] By incorporating the frequency dependency of the power amplifier, the present invention allows efficient power amplifier operation at wider bandwidths where sensitivity to frequency is more problematic. The invention further enables power amplifiers to be operated in the nonlinear region near saturation, yet suppresses undesirable intermodulation products. Resort to a larger amplifier, to keep operation within the linear region, is avoided. Power amplifier sizes are kept small with associated cost savings, particularly important in the field of wireless communications. [0017] The above factors make the present invention essential for effective power amplifier predistortion. [0001] The present patent application is related to U.S. patent application Ser. No. ______KIM 6______ entitled, “METHOD AND APPARATUS FOR MODELING AND ESTIMATING THE CHARACTERISTICS OF A POWER AMPLIFIER BY RETAINING EVEN-ORDER TERMS IN ESTIMATING CHARACTERISTICS”, being concurrently filed herewith and having a filing date of ______,; to U.S. patent application Ser. No. ______KIM 7______, entitled, “METHOD AND APPARATUS FOR MODELING AND ESTIMATING THE CHARACTERISTICS OF A POWER AMPLIFIER WITH MEMORY”, being concurrently filed herewith and having a filing date of ______,; and to U.S. patent application Ser. No. ______KIM 8-4______, entitled, “METHOD AND APPARATUS FOR CALCULATING THE PREDISTORTION FUNCTION FROM A POWER AMPLIFIER”, being concurrently filed herewith and having a filing date of ______,; all of which have a common inventor and assignee and being incorporated herein by reference. Referenced by
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