Abstract:
Systems, methods, and apparatuses are provided for linear envelope elimination and restoration transmitters that are based on the polar modulation operating in conjunction with the orthogonal recursive predistortion technique. The polar modulation technique enhances the battery life by dynamically adjusting the bias level. Further, the analog orthogonal recursive predistortion efficiently corrects amplitude and phase errors in radio frequency (RF) power amplifiers (PA) and enhances the PA output capability. Additionally, even-order distortion components are used to predistort the input signal in a multiplicative manner so that the effective correction bandwidth is greatly enhanced. Also, the predistortion scheme, which uses instantaneously feed-backed envelope distortion signals, allows for correction of any distortion that may occur within the correction loop bandwidth, including envelope memory effects.

Description:
RELATED APPLICATION 
   This application claims priority to U.S. Provisional Ser. No. 60/803,871, entitled “Systems, Methods, and Apparatuses for Linear Polar Transmitters,” filed on Jun. 4, 2006, which is incorporated by reference as if fully set forth herein. 

   FIELD OF THE INVENTION 
   The invention relates generally to linear envelope elimination and restoration (EER) transmitters, and more particularly to systems, methods, and apparatuses for the performance enhancement of radio frequency (RF) power amplifiers. 
   BACKGROUND OF THE INVENTION 
   In cost-sensitive mobile transmitters, performance trade-offs must be carefully managed to achieve high efficiency and high output power at the required gain and linearity. With an intrinsically nonlinear power amplifier (PA) itself, the only way to achieve a better linear operation is to restrict the dynamic range of signals to a small fraction of the PA&#39;s overall capability. Unfortunately, such a restriction in the dynamic range to achieve a more linear operation is quite inefficient since it requires the construction of an amplifier that is much larger in size and consumes more power. 
   With the demand to increase data transmission rates and communication capacity, Enhanced Data rate for GSM Evolution (EDGE) has been introduced within the existing GSM (Global System for Mobile communications) specifications and infrastructure. GSM is based on a constant envelope modulation scheme of Gaussian Minimum Shift Keying (GMSK), while EDGE is based on an envelope-varying modulation scheme of 3π/8-shifted 8-phase shift keying (8-PSK) principally to improve spectral efficiency. Because of this envelope-varying modulation scheme, EDGE transmitters are more sensitive to PA nonlinearities, which can significantly and negatively affect the performance of an EDGE handset. Also, Wideband Code Division Multiple Access (WCDMA) is another communication technology that has much higher data rate (˜2 Mbps) than the data rate (384 kbps) of EDGE. It is the leading global wireless broadband standards to deliver 3G multimedia applications like videoconferencing, imaging and video, 3D games and high-end stereo to mobile users. The combination of GSM/EDGE and WCDMA technology on a single, cost-effective system architecture provides mobile users with a more seamless experience as they roam within networks enabled by both technologies while taking advantage of the highest network connection speed available. Mobile device design complexity is also minimized by combining the leading mobile wireless and multimedia technologies onto a single system. However, as for EDGE, WCDMA is based on an envelope-varying modulation scheme of Hybrid Phase Shift Keying (HPSK). As a result, WEGDE (WCDMA and EDGE) transmitters require efficient, accurate amplitude and phase control with additional blocks to compensate for distortion caused by the PA nonlinear characteristics and non-constant envelope variation. 
   To provide for efficiently amplified signal transmissions, many EER transmitter architectures have been proposed in the form of either an open-loop with digital predistortion scheme or a closed-loop with analog feedback scheme. 
   First, in the conventional open-loop with digital predistortion scheme, the PA is characterized by calibration data including powers, temperatures, and frequencies. The calibration data is then stored in look-up tables (LUTs). The correct coefficients for the operating conditions, which come from the LUT, are selected by digital logic and applied for predistortion. The DSP-based linearization can provide an accurate, stable operation as well as easy modification by the power of software programming. However, this technique requires time-consuming calibration on the production line to compensate for part-to-part variations and cannot easily correct any aging effects in the system. When including a feedback path to look at the PA output changes, the circuitry becomes costly and consumes a considerable amount of DC power. 
   Second, a closed-loop envelope feedback control is generally used for analog linearization. In such a feedback control structure, a precise receiver has to be included within the transmitter and the control-loop bandwidth should greatly exceed the signal bandwidth. In addition, the intrinsic gain reduction characteristic in the negative feedback may cause a severe restriction to amplifiers that do not have enough transmission gain. Additionally, conventional closed-loop systems feed back both distortion and signal power, thereby reducing the stability of the closed-loop systems. Likewise, power amplifiers used in these conventional polar modulation systems are operated at highly nonlinear switching modes for efficiency so the cancellation of high-order distortion components becomes more important. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the invention may provide for an analog linear EER transmitter architecture using the orthogonal recursive predistortion technique. This transmitter architecture may operate in a low power mode and achieve greater bandwidth by feeding the low-frequency even-order distortion components (i.e., the deviation from linear gain) back. Moreover, the distortion components may not be added to the input signal as feedback, but rather may be used to predistort the input signal in a multiplicative manner. In particular, the low-frequency even-order distortion components may generate odd-order in-band distortion terms when they are multiplied by the fundamental signal. Thus, such architecture may be inherently more stable than conventional additive polar loop systems. 
   According to an embodiment of the invention, there is a method for providing a linear envelope elimination and restoration transmitter. The method may include generating an input amplitude signal and an input phase signal, where the input amplitude signal and the input phase signal are orthogonal components of an input signal, and where the input amplitude signal and the input phase signal are generated on respective first and second signal paths. The method may also include processing the input amplitude signal along the first signal path using an amplitude error signal to generate a predistorted amplitude signal, processing the input phase signal along the second signal path using an phase error signal to generate a predistorted phase signal, and providing the predistorted amplitude signal along the first signal path and the predistorted phase signal along the second signal path to a power amplifier to generate an output signal. The method may further include applying a first logarithmic amplifier to the output signal of the power amplifier to obtain a log-detected output signal, and applying a second logarithmic amplifier to the predistorted amplitude signal to obtain a log-detected predistorted amplitude signal. The method may also include applying a first amplitude limiter to the output signal of the power amplifier to obtain an amplitude-limited output signal, and applying a second amplitude limiter to the predistorted phase signal to obtain an amplitude-limited predistorted phase signal, where the amplitude error signal is generated from a comparison of at least a log-detected output signal with the log-detected predistorted amplitude signal, and where the phase error signal is generated from a comparison of at least the amplitude-limited output signal with the amplitude-limited predistorted phase signal. 
   According to an embodiment of the invention, there is a system for a linear envelope elimination and restoration transmitter. The system may include an input amplitude signal and an input phase signal, where the input amplitude signal and the input phase signal are orthogonal components of an input signal, and where the input amplitude signal and the input phase signal are provided on respective first and second signal paths. The system may also include a first predistortion module that processes the input amplitude signal along the first signal path using an inverse amplitude error signal to generate a predistorted amplitude signal, a second predistortion module that processes the input phase signal along the second signal path using an inverse phase error signal to generate a predistorted phase signal, and a power amplifier that receives the predistorted amplitude signal along the first signal path and the predistorted phase signal along the second signal path and generates an output signal based upon the predistorted amplitude signal and the predistorted phase signal. The system may further include at least one logarithmic amplifier that retrieves a log-detected output signal from the output signal of the power amplifier and that retrieves a log-detected predistorted amplitude signal from the predistorted amplitude signal, and at least one amplitude limiter that retrieves a limited output signal from the output signal of the power amplifier and that retrieves a limited predistorted phase signal from the predistorted phase signal, where the amplitude error signal is generated from a comparison of at least the log-detected output signal with the log-detected predistorted amplitude signal, and wherein the phase error signal is generated from a comparison of at least the amplitude-limited output signal with the amplitude-limited predistorted phase signal. 
   According to yet another embodiment of the invention, there is a system for providing a linear polar transmitter. The system may include an input amplitude signal and an input phase signal, where the input amplitude signal and the input phase signal are orthogonal components of an input signal, and where the input amplitude signal and the input phase signal are provided on respective first and second signal paths. The system may also include first means for processing the input amplitude signal along the first signal path using an inverse amplitude error signal to generate a predistorted amplitude signal, second means for processing the input phase signal along the second signal path using an inverse phase error signal to generate a predistorted phase signal, and a power amplifier that receives the predistorted amplitude signal along the first signal path and the predistorted phase signal along the second signal path and generates an output signal based upon the predistorted amplitude signal and the predistorted phase signal. The system may further include third means for generating the inverse amplitude error signal from the output signal and the predistorted amplitude signal, and fourth means for generating the inverse phase error signal from the output signal and the predistorted phase signal. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
       FIGS. 1A and 1B  illustrate functional block diagrams of an illustrative EER transmitter system in accordance with an embodiment of the invention. 
       FIG. 2  illustrates an amplitude modulation error correction loop in accordance with an embodiment of the invention. 
       FIG. 3  illustrates a phase modulation error correction loop in accordance with an embodiment of the invention. 
       FIG. 4  illustrates an amplitude modulation scheme in accordance with an embodiment of the invention. 
       FIGS. 5A and 5B  illustrates simulated power amplifier (PA) characteristics without predistortion and with predistortion, respectively, in accordance with an embodiment of the invention. 
       FIGS. 6A and 6B  illustrate the simulated constellation results of a WCDMA signal without predistortion (EVMrms: 14.0%) and with predistortion (EVMrms: 0.07%), in accordance with an embodiment of the invention. 
       FIGS. 7A and 7B  illustrate the simulated spectrum results of a WCDMA signal, in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION  
   The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
   Embodiments of the invention may provide for linear EER transmitters that are based upon a polar modulation technique using two orthogonal paths for amplitude and phase, and an analog orthogonal recursive predistortion linearization technique. The polar modulation technique may enhance the battery life by dynamically adjusting the bias level. Additionally, the analog orthogonal recursive predistortion may provide for a substantially instantaneous correction of amplitude and phase errors in an RF PA, thereby enhancing the linear output power capability and efficiency of the PA. Additionally, embodiments of the invention may utilize even-order distortion components to predistort the input signal in a multiplicative manner, which allows for correction of any distortion that may occur within the correction loop bandwidth, including envelope memory effects. 
     FIG. 1A  illustrates a simplified functional block diagram of an illustrative EER transmitter system  100  in accordance with an embodiment of the invention. As shown in  FIG. 1A , the EER transmitter system  100  may include an amplitude predistortion module  118 , a phase predistortion module  120 , an amplifier power control (APC) module  110 , a diode-based envelope detector  111  at the input of the APC module  110 , a power amplifier module  112 , an amplitude modulation error detection module  114 , and a phase modulation error detection module  116 . During operation of the EER transmitter system  100 , a complex RF signal may be modulated with two orthogonal baseband input signals—one representing the amplitude and one representing the phase of the input signal. The complex RF input signal may be provided to the amplitude predistortion module  118  for amplitude linearization and phase predistortion module  120  for phase linearization. It will be appreciated that while the two orthogonal input signals are associated with amplitude and phase, respectively, other embodiments of the invention may utilize I- and Q-components for a Cartesian system. Furthermore, other orthogonal input signals may be utilized as well without departing from embodiments of the invention. 
   The amplitude predistortion module  118  and the phase predistortion module  120  will now be discussed with respect to  FIG. 1B , which provides a more detailed functional block diagram of the EER transmitter system  100  of  FIG. 1A . As illustrated, the amplitude predistortion module  118  may be a variable gain amplifier (VGA) and the phase predistortion module  120  may be a phase adder such as voltage-controlled variable phase (VVP) shifter. The power amplifier module  112  may include a power amplifier  124  having transfer function G{·}. Also, the power amplifier module  112  may additionally include one or more input matching (IM) circuits  122  and output matching (OM) circuits  126 . The IM circuit  122  may provide for impedance matching at the input of the power amplifier  124  while the OM circuit  126  may provide for impedance matching at the output of the power amplifier  124 . 
   Still referring to  FIG. 1B , the amplitude modulation error detection module  114  may include a subtraction device  132  like a differential error amplifier, an attenuator  128  with an attenuation of 1/a 1 , and logarithmic amplifiers  140 ,  130  to produce the log-detected value z A (t) of the amplitude predistorter  118  output raz(t) and the log-detected output y A (t) of the PA module  112 , respectively. The phase modulation error detection module  116  may include a multiplier  136  and amplitude limiters  134 ,  138  to produce the amplitude-limited output rz P (t) of the phase predistortion module  120  and the amplitude-limited output ry P (t) of the PA module  112 , respectively. 
   As will be also described in further detail below, the amplitude predistortion module  118  and the phase predistortion module  120  may be operative to predistort the amplitude signal and the phase signal of the input RF signal rx(t), respectively. In particular, the amplitude orthogonal portion x A (t) of the RF signal rx(t) may be predistorted by an inverse amplitude error signal e A (t) from the amplitude modulation error detection module  114 , to produce an amplitude-predistorted RF signal raz(t). To produce the inverse amplitude error signal e A (t), the amplitude modulation error detection module  114  generally performs a comparison of the log-detected output z A (t) of the predistortion module  118  output raz(t) with the log-detected output y A (t) of the PA module  112  output ry(t). According to an embodiment of the invention, the inverse amplitude error signal e A (t) may be determined by subtracting the log-detected output y A (t) from the log-detected output z A (t). This process may recursively be performed to achieve the optimum predistortion linearization. 
   Likewise, the phase orthogonal portion x P (t) of the RF signal rx(t) may be predistorted by an inverse phase error signal e P (t) from the phase modulation error detection module  116 , producing a phase-predistorted RF signal rpz(t). To produce the inverse phase error signal e P (t), the phase modulation error detection module  116  generally performs a comparison of the amplitude-limited output rz P (t) of the predistortion module  120  output rpz(t) with the amplitude-limited output ry P (t) of the power amplifier module  112  output ry(t). According to an embodiment of the invention, the inverse phase error signal e P (t) may be determined by multiplying the amplitude-limited output rz P (t) with the amplitude-limited output ry P (t). 
   In  FIG. 1B , the EER transmitter system  100  may provide a linearization scheme to look at any changes of the PA output ry(t) and almost instantaneously predistort the input signal rx(t). More specifically, the predistortion mechanism in accordance with an embodiment of the invention may utilize the predistorted signal toward the PA  124  as the reference for recursive predistortion so that the outputs e A (t) and e P (t) of modulation error detection modules  114 ,  116  may be simply the reciprocals of the PA  124  transfer function G{·}. Accordingly, the calculation of the predistortion function may be performed by analog components. 
   Assuming that the two paths are fully synchronized, the complex baseband input signal z(t) toward the PA  124  may be defined as follows: 
                         z   ⁡     (   t   )       =       ⁢         z   A     ⁡     (   t   )       ⁢       ∠z   P     ⁡     (   t   )                     =       ⁢       {         x   A     ⁡     (   t   )       ·       e   A     ⁡     (   t   )         }     ⁢   ∠   ⁢     {         x   P     ⁡     (   t   )       +       e   P     ⁡     (   t   )         }                   =       ⁢       x   ⁡     (   t   )       ·     e   ⁡     (   t   )                       (   1   )               
where x A (t) and x P (t) are the orthogonal baseband amplitude and phase input signals, respectively. Likewise, e A (t) and e P (t) are the outputs of the amplitude modulation error detection module  114  and the phase modulation error detection module  116 , respectively. When a complex-form analysis are employed for simplicity the baseband output y(t) of the PA  124  may be described as follows:
 
                     e   ⁡     (   t   )       =         a   1     ·     G     -   1         ⁢     {            z   A   ′     ⁡     (   t   )            }         ,           (   2   )                       y   ⁡     (   t   )       =       ⁢         z   ⁡     (   t   )       ·   G     ⁢     {            z   A   ′     ⁡     (   t   )            }                     =       ⁢         [       x   ⁡     (   t   )       ·     e   ⁡     (   t   )         ]     ·   G     ⁢     {            z   A   ′     ⁡     (   t   )            }         ,                 (   3   )               
where G{·} is the PA  124  transfer function, e(t) the complex modulation error signal, x(t) the complex system input signal, z(t) the complex predistorted PA input signal, y(t) the complex PA output signal, z A ′(t) the diode-detected amplitude signal to drive the power controller  110 . As a result obtained from equations (1) to (3) above, a linearly amplified signal a 1 ·x(t) can simply be generated with this architecture.
 
   Amplitude Error Correction. The amplitude error correction loop, which includes the amplitude modulation error detection module  114 , will be described in detail with reference to  FIG. 2 . The amplitude-predistorted signal z A (t) may be extracted from the RF signal output raz(t) of an amplitude predistortion module  118  (e.g., variable gain amplifier (VGA)) by a logarithmic amplifier  140 . In addition, the amplitude signal y A (t) of the RF PA  112  output ry(t) through an attenuator  128  is extracted by a logarithmic amplifier  130 . The amplitude-predistorted signal z A (t) is then compared with the amplitude signal y A (t) using a subtraction device  132  such as a differential error amplifier to obtain the amplitude error signal e A (t). The amplitude error signal e A (t) may then be logarithmically added to the orthogonal amplitude x A (t) of the RF input rx(t) through the amplitude predistortion module  118  (e.g., VGA) to produce a amplitude-predistorted RF signal rpz(t). That is, at the amplitude predistortion module  118 , the amplitude error signal e A (t) may be linearly multiplied with the amplitude x A (t) of the input RF signal rx(t). 
   Phase Error Correction  FIG. 3  illustrates the phase error correction loop, which includes the phase modulation error detection module  116 . As shown in  FIG. 3 , the phase error signal e P (t) may be obtained from the comparison of the amplitude-limited output rz P (t) of a phase predistortion module  120  (e.g., phase shifter) and the amplitude-limited output ry P (t) of the PA output ry(t) through amplitude limiters  138 ,  134 , respectively. The phase error signal e P (t) may then added to the orthogonal phase x P (t) of the RF input rx(t) to produce the phase-predistorted RF signal rpz(f). 
   Amplitude Modulation. In TDMA communication systems such as GSM/EDGE, the power control of a PA output may need to meet the required time mask, while preserving the efficiency of the power supply. It may be done by using a linear regulator, switching regulator, or combined structure. Unlike the GSM system, the EDGE or WCDMA system in accordance with an embodiment of the invention requires the tracking of RF envelope signals as well as the power control. Tracking the envelope signal needs much wider operation bandwidth.  FIG. 4  shows an example of a combined PA controller  110  scheme that may be employed for power efficiency and wideband operation. As shown in  FIG. 4 , the DC-DC converter  404  may provide the DC and low frequency load current, while the Class-AB linear amplifier  402  may provide the high frequency load current, maintaining the tracking loop closed. The DC-DC converter  404  may be controlled by the output current of the Class-AB amplifier  402 . The hysteric current controller of the DC-DC converter  404  may attempt to minimize the output current of the Class-AB amplifier  402 , to maximize the overall efficiency. The output capacitance  428  of the architecture may be low to maintain the high bandwidth of the Class-AB amplifier  402  loop. Moreover, the ripple current of the DC-DC converter  404  may be principally absorbed by the Class-AB linear amplifier  402  operating in conjunction with a feedback loop. Thus, this linear-assisted architecture may be expected to have a high envelope tracking bandwidth, preserving a good linearity and efficiency. 
   Simulation Results. The time-domain signal test shown in  FIGS. 5A and 5B  illustrate the improved performance of a PA  124  in accordance with an embodiment of the invention. In particular,  FIG. 5A  displays the results obtained without the use of the linearizer, while  FIG. 5B  shows the results with the use of the linearizer implemented using the predistortion provided in accordance with an embodiment of the invention. As shown in  FIG. 5B , the PA  124  output with the linearizer turned on tracks the original input signal well, and the nonlinearity in the amplitude and phase is well linearized, even in the situation with memory effects that display scattered PA  124  characteristics over power. 
   Error vector magnitude (EVM) measurement provides a means of characterizing the magnitude and phase variations introduced by the PA nonlinear behavior over a wide dynamic range. In comparison of results shown in  FIGS. 6A and 6B , the EVM simulation results exhibits the improvements of 13.9% in root-mean-square (RMS) by use of the predistortion provided by embodiments of the invention.  FIGS. 7A and 7B  show the spectrum results. Without predistortion, a large amount of intermodulation distortions are produced, as shown in  FIG. 7A . On the other hand, the simulation with predistortion displays that distortions are almost corrected and removed, as shown in  FIG. 7B . 
   Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.