Patent Publication Number: US-11394350-B2

Title: Method and system for aligning signals widely spaced in frequency for wideband digital predistortion in wireless communication systems

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application a continuation of U.S. patent application Ser. No. 14/956,040, filed Dec. 12, 2015, which is a divisional of U.S. patent application Ser. No. 13/945,736, filed Jul. 18, 2013, now U.S. Pat. No. 9,225,296, which claims the benefit of U.S. provisional patent application No. 61/674,771, filed Jul. 23, 2012, the disclosures of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Predistortion is a technique used in communications systems to improve the linearity of power amplifiers. Because power amplifiers can have nonlinear input/output characteristics, predistortion is used to linearize the input/output characteristics of the power amplifier. In essence, “inverse distortion” is introduced into the input fed to the power amplifier, thereby cancelling the non-linear characteristics of the power amplifier. 
     Current predistortion technologies used to linearize power amplifiers in mobile communication systems are mainly analog predistorters implemented at IF/RF by means of analog circuitry and a digital predistorter at baseband with digital signal processing (DSP) techniques. 
     The analog predistorter is based on the principle of error subtraction and power matching to produce linearization of the power amplifier. Because nonlinear characteristics of power amplifiers can be complicated and involve many variables, analog predistortion results in less than optimal predistortion accuracy and consumes significant power. 
     Despite the progress made in predistortion technologies, there is a need in the art for improved methods and systems for digital predistortion systems. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a robust method of estimating delay between the transmit and feedback signals for a wideband digital predistortion system. To achieve the above objective, according to an embodiment of the present invention, the technique is based on using a Farrow based fractional delay filter in the feedback path and an algorithm in order to accurately control the feedback path delay. Embodiments of the present invention are able to time align transmit and feedback signals with high accuracy at any time. 
     According to an embodiment of the present invention, a simple and robust delay estimation method for wideband digital predistortion with widely frequency spaced carriers is provided. The present invention provides a method of time aligning a transmit and feedback signal, for a wideband digital predistortion system. To achieve the above objective, according to an embodiment of the present invention, the technique is based on using a programmable fractional delay filter based on a third order Lagrange Farrow structure which is very simple to design and control. Embodiments described herein are able to align signals, in digital predistortion systems, with more than an instantaneous 100 MHz bandwidth. 
     According to an embodiment of the present invention, a system for time aligning widely frequency spaced signals is provided. The system includes a digital predistortion (DPD) processor and a power amplifier coupled to the DPD processor and operable to provide a transmit signal at a power amplifier output. The system also includes a feedback loop coupled to the power amplifier output. The feedback loop includes an adaptive fractional delay filter, a delay estimator coupled to the adaptive fractional delay filter, and a DPD coefficient estimator coupled to the delay estimator. 
     According to another embodiment of the present invention, a method of temporally aligning signals is provided. The method includes a) computing a value of a delay parameter, b) receiving a plurality of transmit signals, and c) receiving a plurality of feedback signals. The method also includes d) determining a function related to a timing error using the plurality of transmit signals and the plurality of feedback signals, e) determining that the function related to the timing error is greater than or equal to a predetermined threshold, and f) incrementing a counter. The method further includes g) repeating one or more of a) through f) one or more times, h) determining that the function related to the timing error is less than the predetermined threshold, and i) fixing the delay parameter. 
     Numerous benefits are achieved by way of the present invention over conventional techniques. For example, embodiments of the present invention provide for enhanced control of delay in the feedback path, improving the performance characteristics of the digital predistortion system. These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 . is a schematic block diagram illustrating a multi-carrier wideband system including digital predistortion with delay estimation according to an embodiment of the present invention; 
         FIG. 2 . is a schematic block diagram showing a system for aligning wideband signals according to an embodiment of the present invention; and 
         FIG. 3  is a simplified flowchart illustrating a method of temporally aligning signals according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The present invention generally relates to wideband communication systems using multiplexing modulation techniques. More specially, the present invention relates to a method of aligning widely frequency spaced signals for wideband digital predistortion linearization in wireless transmitters. 
     The linearity and efficiency of radio frequency (RF) power amplifiers (PAs) have been a critical design issue for non-constant envelope digital modulation schemes which have high peak to average power ratio (PARs), with the increasing importance of spectral efficiency in wireless communication systems. RF PAs have nonlinearities which generate amplitude modulation—amplitude modulation (AM-AM) and amplitude modulation—phase modulation (AM-PM) distortion at the output of the PA. These effects create spectral regrowth in the adjacent channels and in-band distortion which degrades the error vector magnitude (EVM). Therefore, one of the various linearization techniques is typically applied to the RF PAs. Various linearization techniques have been proposed in the literature such as feedback, feedforward and predistortion. 
     The most promising linearization technique is baseband digital predistortion (DPD), which takes advantage of the recent advances in digital signal processors. DPD can achieve good linearity, good power efficiency with a reduced system complexity when compared to the widely used conventional feedforward linearization technique. Moreover, a software implementation provides the digital predistorter with re-configurability suitable for the multi-standard environments. In addition, a PA using efficiency enhancement technique such Doherty power amplifier (DPA) is able to achieve higher efficiencies than traditional PA designs at the expense of linearity. Therefore, combining DPD with a DPA using efficiency enhancement technique has the potential of maximizing system linearity and overall efficiency. 
     The typical wireless communication systems instantaneous bandwidth supports around 20 MHz to 25 MHz. The common delay estimation for digital predistortion algorithm uses magnitude correlation between the transmit signal and feedback signal with two times or more oversampling. 
     However, requirements of the instantaneous bandwidth (&gt;25 MHz) for next generation wireless system continue to increase, which means that wideband multicarrier can be widely frequency spaced, for example, carrier spacing can be up to 60 MHz for systems supporting 65 MHz of instantaneous bandwidth. This can create several correlation peaks with a very small time difference due to the large carrier spacing. This can cause a large delay alignment error, which is undesirable. Thus, embodiments of the present invention provide a wideband digital predistortion system with robust delay estimation. 
       FIG. 1  is a schematic block diagram showing digital predistortion (DPD) circuits, interpolators, digital-to-analog converters, a modulator, a power amplifier, a duplexer, radio frequency down-conversion circuits for the output coupled at the output of the PA, an analog-to-digital converter for digital predistortion feedback path, and a digital down-converter. The digital predistortion system utilizes a delay estimation based on the magnitude of complex signals (I and Q). Normally, the sample rate of the feedback ADC is twice that of the digital pre-distorter. For example, if the digital predistortion sample rate is 125 MHz, then sample rate of feedback ADC is typically at least 250 MHz, which means that minimum resolution of the hardware controllable delay is 4 ns (1/250 MHz). In some implementations, this minimum resolution is not small enough to align the delay between transmit and feedback path with desired accuracy in the case of widely frequency spaced carriers. 
     The delay estimator receives inputs from the feedback path as well as inputs from the output of the DPD circuit. The delay estimator calculates the difference between these inputs and provides inputs to the coefficient estimator in order to time align the signals as part of the error minimization process. In some embodiments of the present invention, the delay estimator provides a value, which is a function of the timing error between the in-phase component at the output of the DPD circuit and the in-phase component of the feedback signal and the quadrature-phase component at the output of the DPD circuit and the quadrature-phase component of the feedback signal. 
     As an example of the computation of the function of the timing error, which can also be referred to as a function related to the timing error, the function can be the mean squared error difference between the complex feedback signal and the complex output of the DPD circuit.
 
Error=( Î−I ) 2 +( {circumflex over (Q)}−Q ) 2  
 
     where: Î is the in-phase feedback signal
         {circumflex over (Q)} is the quadrature-phase feedback signal   I is the in-phase output DPD signal   Q is the quadrature-phase output DPD signal       

       FIG. 2 . is a schematic block diagram showing a system for aligning wideband signals according to an embodiment of the present invention. The system illustrated in  FIG. 2  includes digital predistortion (DPD) circuits, interpolators, digital-to-analog converters, a modulator, a power amplifier, a duplexer, radio frequency down-conversion circuits for the output coupled at the output of the PA, an analog-to-digital converter for feedback path, a digital down-converter, and a fractional delay filter with controllable parameter (mu) which ranges 0 to 1 in some embodiments. 
     According to a present invention of the present invention, the fractional delay filter is implemented based on a third order Lagrange Farrow structure that enables a simple implementation and operates at the digital predistortion sample rate. A higher order Lagrange Farrow filter can be used as appropriate to the particular application. Minimum delay resolution can be 10 times the sample rate or higher, which means that it can be as small as 0.1 ns for a feedback ADC having a sample rate of 1 GHz. Of course, the minimum delay resolution will depend on the number of bits in some implementations. In order to provide a similar minimum delay, conventional systems would use a 10 GHz sample rate interpolator in hardware or complicated and time consuming software filtering algorithm. 
     The fractional delay filter allows for shifting of the signal (i.e., time shifting of a signal) by fractions of the sampling rate. As an example, if the sampling rate were 100 MHz, a conventional system would only sample at a rate resulting in 10 ns (i.e., 1/100 MHz) between each sample. As illustrated in  FIG. 2 , the fractional delay filter includes a parameter illustrated as mu. The parameter enables shifting of the signal to change the delay by a predetermined fraction of the sampling rate, for example, one-tenth of the sampling rate. Thus, the fractional delay filter enables reduction in minimum delays from 10 ns to 1 ns for a mu value ranging from 0 to 1. Embodiments of the present invention thus utilize fractional delay filtering in the context of digital predistortion in wideband communications systems. 
     Referring once again to  FIG. 2 , the fractional delay filter serves as a low pass filter with variable delay as a function of the parameter mu. Additional description related to the variation of the parameter mu during operation is provided below. 
       FIG. 3 . is a flow chart illustrating a method of temporally aligning signals according to an embodiment of the present invention. When delay estimation starts, a counter (n) is set to zero and mu is set to the value of the counter times a step size (mu=n*step), setting mu to zero. The step size can be set to a variety of values, for example, 0.2, 0.1, 0.05, or the like. As an example, if the ADC is operating at a sample rate of 1 MHz (i.e., 1 μs per sample) and the step size is set to 0.1, then mu will be set to multiples of 0.1 as the counter is incremented in order to provide a minimum delay resolution of 0.1 μs. 
     Signals are captured at the outputs of the DPD circuit and the output of the feedback path (i.e., the outputs of the digital down converter). The output of the DPD circuit, and the output of the feedback path are magnitude aligned. Using the aligned captured signals of the two paths, a calculation of the function of the timing error is performed in the delay estimator as illustrated in  FIG. 2 . During the first iteration, n=0 and mu=0, providing the parameter mu=0 to the fractional delay filter. If the resulting function of the timing error is larger than or equal to a predetermined threshold, the counter is incremented and mu is recomputed for the next iteration (i.e., mu=1*step=1*0.1=0.1 for the second iteration in this example). The process iterates until the function of the timing error is less than the predetermined threshold, mu is fixed, and the values from the delay estimator are provided to the coefficient estimator (i.e., the coefficient estimation algorithm). The step size is not limited to the value of 0.1 in this example and can be set at other values as appropriate, for example, 0.2, 0.1, 0.05, or the like. 
     Embodiments of the present invention provide for adaptive processing in real time to reduce signal error to a predetermined level. As will be evident to one of skill in the art, a predetermined number of symbols are captured (e.g., 4000 samples), a calculation of the function of the timing error is performed to determine the delay estimation value and then the coefficient estimator is provided with the delay. 
     Referring to  FIG. 3 , a method of temporally aligning signals is provided. The method includes a) computing a value of a delay parameter, b) receiving a plurality of transmit signals, and c) receiving a plurality of feedback signals. As illustrated in  FIG. 3 , computing the value of the delay parameter can include multiplying the counter times a step size parameter. The step size parameter can range from 0 to 1. The method also includes d) determining a function of the timing error using the plurality of transmit signals and the plurality of feedback signals and e) determining that the function of the timing error is greater than or equal to a predetermined threshold. Determining the function of the timing error using the plurality of transmit signals and the plurality of feedback signals can include filtering the plurality of transmit signals and the plurality of feedback signals and estimating the timing error. 
     The method further includes f) incrementing a counter and g) repeating a) through f) one or more times. In some embodiments, a subset of a) through f) are repeated one or more times. As illustrated in  FIG. 3 , the iteration of a) through f) is performed as long as the function of the timing error is greater than or equal to the predetermined threshold. 
     After a number of iterations and increases in the value of the delay parameter, the method includes h) determining that the function of the timing error is less than the predetermined threshold and i) fixing the delay parameter. In an embodiment, the method also includes estimating predistortion coefficients using the delay parameter. 
     It should be appreciated that the specific steps illustrated in  FIG. 3  provide a particular method of temporally aligning signals according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in  FIG. 3  may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. 
     It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.