Abstract:
A predistortion system for linearizing the output of a power amplifier includes a first signal representative of an RF modulated signal and a feedback signal representative of nonlinear characteristics of a power amplifier. The system also includes a predistortion controller, comprising at least one lookup table, adapted to receive the first signal and the feedback signal and to generate a correction factor for correcting the nonlinear characteristics of the power amplifier and combining logic which combines the RF modulated signal with a signal corresponding to the correction factor and supplies it to the power amplifier to linearize the output of the power amplifier.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/799,239, filed on Apr. 30, 2007, which is a continuation-in-part of U.S. patent application Ser. No. 11/262,079, filed on Oct. 27, 2005, which is a continuation of U.S. patent application Ser. No. 10/137,556, filed May 1, 2002, now U.S. Pat. No. 6,985,704. The disclosures of each are hereby incorporated by reference in their entirety for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A fundamental component of wireless communications systems is the power amplifier (PA). Such wireless communications systems include a variety of broadband and multimedia services. In such systems, the power amplifier is supported by a wireless RF transmitter. The power efficiency of the PA is an important aspect of the operational efficiency of the system as a whole. For a long time, the linearity and efficiency of power amplifiers have been two of the most difficult issues faced by wireless system designers and users. These problems are closely related to both the power spectrum efficiency and the power efficiency of power amplifiers. 
         [0003]    Prior art predistortion technologies used to linearize the PA in wireless communication systems mainly use analog predistortion approaches. Some have also used a digital adjustment component implemented by means of an analog feed-forward approach and a digital predistortion approach employing a base-band digital signal processing (DSP) technique. 
         [0004]    The traditional analog predistortion technologies are based on the principle of error subtraction and a power match to linearize the PA. These approaches must use an auxiliary power amplifier to match exactly the non-linearity of the main PA and, at the same time, overcome the signal time-delay that is a variable with frequency and environmental temperature. As a result, it is difficult for this approach to meet the requirement of the advanced wireless communication systems. 
         [0005]    Prior art DSP-based predistortion methods generally require that the predistortion sub-system and circuit design rely on the I and Q signal of the base band in the base station, and therefore have to embed the predistortion processing into the base band circuits of the base station. This requires that the existing base band circuit in the base station must be modified, which is an inconvenient high-cost solution for wireless carriers and equipment makers. Also, the typical DSP-based predistortion technology is not suitable to linearization of the PA when multi-modulation schemes and multi-carrier signals go through a PA. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention includes methods and apparatus for improving power efficiency and linearity of a PA by pre-distorting the input signal to the PA. By applying the methods of the present invention to pre-distort the signal, the conversion characteristic of the normally nonlinear PA becomes substantially linear. As a result, the PA uses power more efficiently and provides advanced performance characteristics suitable for wireless signal transmission. The present invention can be applied to a broad array of wireless communication systems, regardless of modulation type, to yield high quality signal transmission and expanded network capacity. It will be appreciated by those skilled in the art that the power amplifiers described herein can also be regarded as high power amplifiers. 
         [0007]    In an embodiment of the invention, a predistortion controller comprises analog peripheral circuits and a predistortion core processor for RF signal predistortion processing in preparation for PA signal transmission from a wireless communication device. 
         [0008]    The analog peripheral circuits in the predistortion controller comprise the analog down-converter circuits prior to the PA input and the output of the PA, respectively, to pick up and compare an idealized signal from the modulator in the wireless system to the distorted feedback signal from PA&#39;s output. The predistortion core processor operates together with a look-up table, where the look-up table stores predistortion information that is obtained by mathematically calculating AM-AM and AM-PM distortion components. The output of the predistortion processor provides a pre-correction signal for the input of the PA. The precorrection signal is combined with the signal from the RF modulator by a multiplication operation to generate a pre-distorted input signal that is provided as the input to the PA. 
         [0009]    The basic architecture of the predistortion sub-system can be better understood from the appended Figures, as set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a block diagram of the PA module with a predistortion controller in accordance with an embodiment of the invention. 
           [0011]      FIG. 2  presents a simplified architecture of an embodiment of the predistortion controller of  FIG. 1 . 
           [0012]      FIG. 3  illustrates in greater detail the embodiment of  FIG. 2 . 
           [0013]      FIG. 4  shows the operation and arrangement of the lookup table in the predistortion controller shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention relates generally to wireless communications systems, and more particularly relates to systems, apparatus and methods for correcting the nonlinearity of a Power Amplifier (PA) such as might be used in a wireless RF transmitter. 
         [0015]    The present invention provides a novel predistortion controller to linearize the power amplifier (PA) used in wireless base stations operating in wireless networks using a wide variety of signal types including CDMA, TDMA, GSM, GPRS, 3G systems (UMTS, W-CDMA, CDMA2000, TDS-CDMA, 3GPP and others), WLAN, WiFi, WiMax and the like, as well as other proposed and upcoming wireless systems (4G/5G) that transmit a complex modulated signal by using a high power amplifier. Because the high power amplifier in wireless RF transmitters typically distorts the RF output signal, the adaptive predistortion controller of the present invention is useful to correct that non-linearity. Embodiments of the present invention are suitable for use with ail wireless base stations, access points and other wireless communication systems such as microwave and satellite communications. The predistortion controller of the present invention offers the additional advantage that it can be added to existing wireless RF transmitters without otherwise changing or modifying the base station structure while at the same time yielding significant performance improvements. Included in these improvements is a substantial increase in the efficiency of the base station, which yields a significant reduction in power usage because, in current systems, significant power is wasted due to the nonlinear characteristics of typical PA&#39;s. 
         [0016]    From the following description, it will be appreciated that the high-efficiency linearization predistortion controller disclosed herein offers the following benefits to the wireless industry: 
         [0017]    1. It significantly enhances the transmitted signal quality and increases wireless network capacity by improving the PA&#39;s transmission characteristics and decreasing the adjacent channel interference. 
         [0018]    2. It can be implemented as an add-on module to the PA, which permits the aforementioned improvements to the performance of the PA without changing or modifying the existing RF and base band circuits in the base station. This differs from the traditional feedback and feed-forward predistortion approach, and therefore is a comparatively low cost, easy to install solution for the wireless network infrastructure whenever the operators desire to upgrade performance and reduce energy costs. 
         [0019]    3. The predistortion control operation is fast and dynamic, which permits it to track and correct for nonlinearities of the PA in a wider, and makes it particularly suitable for use in base stations, repeaters and handsets. 
         [0020]    4. It can be integrated into the design of the PA for convenient installation and replacement of aging PA&#39;s already installed in a wireless system. 
         [0021]    Referring first to  FIG. 1 , the basic arrangement of the present invention can be appreciated. A predistortion controller  5  received as an input a modulated RF signal  6  and also receives a feedback signal  7  which correlates to the output signal of a power amplifier  12 . The predistortion controller modifies the RF signal  6  in accordance with the nonlinear and distortion characteristics of the output signal of the power amplifier  12  as characterized by the feedback signal  7 , such that the output of the predistortion controller  5  is supplied to the power amplifier  12  to yield a substantially linearized and improved out put signal of the power amplifier  12 . 
         [0022]    Referring next to  FIGS. 2 and 3 , an embodiment of the present invention may be appreciated in greater detail. More particularly, the illustrated embodiment includes an Analog Multiplier I 1  which receives a modulated RF signal V RF  from the RF modulator portion  10  [ FIG. 3 ] of the base station, and also receives a predistortion correction signal V p  from a predistortion processor  200 , shown generally in  FIG. 2  and in greater detail in  FIG. 3 , and discussed in greater detail below. The output of the analog multiplier  11  is provided as the input V in  to the power amplifier (PA)  12 , which in turn transmits an output signal V o  to an Antenna  13 . The RF modulator  10  is typically although not necessarily a quadrature modulator or an orthogonal modulator. It will be appreciated that multiplier  11  can be implemented as multiple multipliers, each associated with one or more quadrature signals. 
         [0023]    An input down-converter circuit  20  receives an idealized reference signal V RF  from modulator in base station, and is biased by a local oscillator  40 , such that it provides an output V ct  to an analog-to-digital converter  21 . The ADC  21  converts the signal V ct  to digital form, whereupon it is provided as one input to a Digital Predistortion Processor  200 . 
         [0024]    A feedback down-converter circuit  26 , also biased by a local oscillator  40 , receives a raw feedback signal V o (t) from the output of the PA, and provides a feedback signal V f  to an feedback ADC  25 . The digital output of the ADC  25  then provides a second input, i.e., feedback signal, to the Digital Predistortion Processor  200 . The Digital Predistortion Processor  200 , discussed in greater detail below, provides a digital output signal V r  to a DAC  30 , which converts the digit signal to an analog form, where it is combined with the modulated RF signal in the multiplier  11 . 
         [0025]    As shown in  FIG. 3 , address data formers  32 I- 32 Q receive inputs from the ADC  21 I/Q, and are designed to generate the required signal format for a lookup table  33 I/Q. The data formers  32 I/Q address memory units within the lookup tables  33 I/Q, where the lookup table provides separate I and Q outputs to an adder  31 . It will be appreciated that the lookup table  33  can be implemented as one or more lookup tables. The address provided by the address formers  32 I- 32 Q can be considered a lookup-table key or address. 
         [0026]    The predistortion controller lookup tables  33 I- 33 Q are designed memory units to store the predistortion signal for high power amplifier linearization. The predistortion signals in the tables are based on the error generated by a comparison of the ideal signal V d  and the feedback signal V f  and the presented adaptive algorithm. The data stored in the tables  331 -Q can be updated by adaptive iteration as described hereinafter, and forms digitally indexed data reflecting the nonlinear characteristics of the power amplifier. 
         [0027]    By comparison of AM-AM and AM-PM information between the idealized signal V RF (t) and the feedback signal Vo(t), the Digital Predistortion Processor calculates the error in the amplitude and phase components of the output signal V o (t) caused by the non-linear transmission characteristics of the high power amplifier  12 . 
         [0028]    Based on the error information obtained by the foregoing comparison, the predistortion processor, based on the lookup table algorithm disclosed in U.S. Pat. No. 6,985,704, the disclosure of which is hereby incorporated by reference, calculates and generates adaptively a compensation signal that is of inverse characteristics with the transform function of the PA  12  to pre-distort the AM-AM and AM-PM distortion caused by the PA  12 . 
         [0029]    The outputs V p  of the predistortion lookup table  331 - 33 Q are fed to multiplier  11 , after an adder  31  and a digital-to-analog converter  30 , to modify the modulated RF signal from modulator  10 . The output of the multiplier is the required predistortion signal V in (k) that is of an inverse non-linearity with that of the power amplifier  12  to yield a pre-compensation to the input of high power amplifier. 
         [0030]    It will be appreciated by those skilled in the art that there can be a signal difference between two signals, ideal signal V RF  and feedback signal, V o (t), when they arrive at predistortion controller  5  [ FIG. 1 ], or, in  FIG. 2 , at processor  200 . The time difference results from the time-delay differences between the two signals, which is caused by the different paths each travels in arriving at the processor  200 . This signal time-delay can vary randomly based on the parameters of the circuits and parts, as well as other environmental factors. The result is that it is difficult to estimate, calculate and adjust for such signal differences in the field application environment. To overcome this issue, the present invention adaptively adjusts for this time-delay through the use of an algorithm taught by the previously cited U.S. Pat. No. 6,985,704. 
         [0031]    The use of the look-up tables  33  permits a memory function to be introduced into at least some embodiments of the present invention. The lookup table of the predistortion controller is based on a stored compensation principle that maps a set of input data into a digital output, and updated adaptively. Based on the stored function, each output signal of lookup table is actually related to both the current and the previously transmitted signal, and therefore has a memory function which compensate not only for the non-linearity of the PA, but also avoids the need for a special time-delay compensation circuit such as typically used in the prior art. See particularly U.S. Provisional Patent Application No. 60/898,312, filed Jan. 29, 10 2007, entitled Power Amplifier Time-delay Invariant Predistortion Methods and Apparatus. 
         [0032]    The architecture of an embodiment of a lookup table which can implement the compensation principle discussed above is shown in  FIG. 4 . The data from ADC  21  is supplied to address former  32 , which in turn forms an address and applies it to the look-up tables  33 . At the same time, the feedback signal V feedback  from ADC  25  and the ideal signal V ideal  from ADC  21  are compared in the error generator  23  and the resulting error signal is multiplied by a numerical value I 1  in multiplier  35 . The value of I 1  is typically between 0 and 1, and represents a convergence factor that can be better understood from the teachings in U.S. Pat. No. 6,985,704. The scaled error factor is then added with a feedback factor in adder  34  and supplied back to the lookup table  33 . The result is an output of the lookup table which provides compensation for the time delay as well as nonlinearities of the PA  12 . 
         [0033]    Due to introducing the memorized lookup table processing, there is unnecessary to build another special time-delay circuit for the signal delay processing. Therefore, the memorized lookup table in this patent shows two functions, the nonlinear predistortion of high power amplifier and adaptive signal time-delay adjusting. 
         [0034]    The lookup table of predistortion controller is based on a stored compensation principle that maps a set of input data into a digital output, and are updated adaptively. Based on the stored function, each output signal of lookup table is actually related to both the current and previous transmitted signal, and therefore has a memory function when compensate the non-linearity of PA [ 1 ]. The architecture of lookup table based on the compensation principle is shown in  FIG. 4 . 
         [0035]    It will be appreciated further that the performance of the predistortion controller is, in some respects, related to the number of bits in the analog-to-digital converters in the original and feedback channels, whereby the bigger the number of bits in the ADC, the better the performance or the predistortion controller is. Similarly, the memory function of the predistortion controller is also related to the number of bits in the address-shifting register, such that, within reasonable sizes, the performance of the predistortion controller improves as the number of bits in the register increases. It will further be appreciated that the PA may be operated in any region, such that, for example, its bias or static operation point can be set either in the saturation region or the cut-off region, with appropriate adjustments for operation in each region. It will further be appreciated that the output signal of predistortion controller is a stochastic control signal rather than a modulated high-frequency signal, and the signal is of the inverse characteristic with the AM-AM and AM-PM distortion component of power amplifier. Those skilled in the art will also appreciate that the output signal of the predistortion controller is not a high-frequency signal, and its frequency typically match the bandwidth of the signal to be transmitted by a power amplifier in a wireless transmitter. 
         [0036]    The combination of both memory and store function performed by the lookup table extends the ability of the present system to compensate for the PA&#39;s non-linear characteristics to time-independent aspects. The time-independent feature of the lookup table&#39;s adaptive processing is a key benefit of the addressing arrangement of the lookup table. In an embodiment, the addressing of the lookup table is implemented by a set of N-bit vector data that contains the current input signal and previous N input signal as well. Therefore, the address of the lookup table is a combination of series of input sequences with the length of N. The longer the address of the lookup table, the wider the time-delay information range that the system can accommodate (i.e. the longer duration of time-delay effects that the system can tolerate). 
         [0037]    Having described the invention in detail, including several embodiments and alternatives, those skilled in the art will appreciate that numerous other alternatives and equivalents exist which are within the scope of the present invention. Therefore the invention is intended not to be limited by the above description, but rather only by the appended claims.