Patent Publication Number: US-2017373642-A1

Title: System and method for adaptive power modulation for power amplifier

Description:
TECHNICAL FIELD 
     This disclosure is directed in general to power amplifiers. More specifically, this disclosure relates to a system and method for adaptive power modulation for a power amplifier in various applications, such as communication, electronic warfare, and radar applications. 
     BACKGROUND 
     The power amplifier plays a vital role in many radar, electronic warfare, and communication systems. Performance of these systems is often largely affected by the efficiency of these power amplifiers. Unfortunately most power amplifiers operate in the linear region where their efficiency is pretty low (e.g., approximately 10-30%). At such a poor efficiency, most power is wasted and converted into heat, which requires the corresponding system to be cooled for reliable operation. Power amplifiers are most efficient when they are operated at peak output power just below the compression point. For a typical RF/communication system, it is possible to achieve up to approximately 50% efficiency when a device is operating at or around peak output power. However, much of the efficiency is lost when output power is backed off from the peak power due to the higher peak-to-average power ratio (PAPR) waveform of modern RF/communication systems. 
     SUMMARY 
     This disclosure provides a system and method for adaptive power modulation for a power amplifier. 
     In a first embodiment, a method includes determining one or more characteristics of a system that uses a power amplifier. The method also includes determining, based on the one or more determined characteristics, a switching speed and a supply voltage for the power amplifier. The method further includes modulating a power supply of the power amplifier according to the determined switching speed and supply voltage. 
     In a second embodiment, a system includes a power amplifier having a power supply, and a power supply modulator. The power supply modulator is configured to determine one or more characteristics of the system. The power supply modulator is also configured to determine, based on the one or more determined characteristics, a switching speed and a supply voltage for the power amplifier. The power supply modulator is further configured to modulate the power supply of the power amplifier according to the determined switching speed and supply voltage. 
     In a third embodiment, a non-transitory computer readable medium containing instructions that, when executed by at least one processing device, cause the at least one processing device to determine one or more characteristics of a system that uses a power amplifier; determine, based on the one or more determined characteristics, a switching speed and a supply voltage for the power amplifier; and modulate a power supply of the power amplifier according to the determined switching speed and supply voltage. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a chart showing a power curve for a typical power amplifier; 
         FIG. 2  illustrates a chart showing power aided efficiency (PAE) of a power amplifier as a function of drain supply voltage; 
         FIG. 3  illustrates a chart showing different power tracking techniques for reducing unnecessary power consumption and improving PAE in a power amplifier; 
         FIG. 4  illustrates a digital predistortion (DPD) technique for improving PAE in a power amplifier; 
         FIG. 5  illustrates a system that uses adaptive power modulation according to this disclosure; 
         FIG. 6  illustrates an example look up table for use with the system of  FIG. 5  according to this disclosure; 
         FIG. 7  illustrates an example method for adaptive power modulation according to this disclosure; and 
         FIG. 8  illustrates an example device for performing adaptive power modulation according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 8 , described below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any type of suitably arranged device or system. 
     For simplicity and clarity, some features and components are not explicitly shown in every figure, including those illustrated in connection with other figures. It will be understood that all features illustrated in the figures may be employed in any of the embodiments described. Omission of a feature or component from a particular figure is for purposes of simplicity and clarity, and not meant to imply that the feature or component cannot be employed in the embodiments described in connection with that figure. 
       FIG. 1  illustrates a chart  100  showing a power curve for a typical power amplifier (PA). The point on the chart  100  indicated at  102  is the compression point for this PA. As known in the art, the compression point is the point at which the actual output power of the PA P out  is 1 dB less than a theoretical P out  if the power curve had remained linear as indicated by the dotted line  104 . Power amplifiers are most efficient when they are operated at peak output power just below the compression point  102 . For a typical RF/communication system, it is possible to achieve up to approximately 50% efficiency when a device is operating at or around peak output power. However, much of the efficiency is lost when output power is backed off from the peak power due to the higher peak-to-average power ratio (PAPR) waveform of modern RF/communication systems. For higher PAPR waveforms, a power amplifier operates at an average output power well below its optimum value. 
     In some PAs, the drain supply voltage Vdd may be adjusted to allow for necessary load line swing, instead of operating the PA at the optimum peak-power bias point. A programmable load line provides support to signals without saturating or cutting off. This allows the PA to operate at a smaller Vdd and therefore consume less DC power. 
       FIG. 2  illustrates a chart  200  showing power aided efficiency (PAE; also referred to as power added efficiency) of a power amplifier as a function of drain supply voltage. The small plot in the upper left corner shows the modulated power supply waveform  202  to accommodate the transmitted signal over time. As can be seen in the plot, the power supply waveform  202  varies over time between a low point  204  and a high point  206 . Each point in the waveform  202  corresponds to a drain supply voltage Vdd. Each drain supply voltage Vdd is associated with a corresponding PAE curve, such as the representative PAE curves  211 - 214  where the PAE is plotted for various fixed drain supply voltages Vdd. During linear operation, for a given fixed supply, the PAE rises with a rise of the output power until the output eventually saturates (e.g. close to saturation) and the PAE reaches a maximum practical level. This maximum practical level is a function of the fixed drain supply voltage Vdd. Raising the fixed drain supply voltage Vdd yields a higher maximum value for the PAE as depicted by the rise in the curves  211 - 214 . In general, peak efficiency can also vary based on waveform characteristics. Vdd modulation allows for the efficiency to follow the peaks of the modulated drain voltage. 
       FIG. 3  illustrates a chart  300  showing different power tracking techniques for reducing unnecessary power consumption and improving PAE in a power amplifier. In the chart  300 , the region  302  represents the signal waveform, which varies over time. 
     The plot line  304  shows modulation of the drain supply voltage Vdd using an average power tracking (APT) technique. APT is a widely-implemented approach to reduce unnecessary power consumption in RF PAs. A DC-to-DC converter connected to the PA supply voltage dynamically changes the Vdd based on the PA average output power. When the PA output power is below maximum, the PA supply voltage is reduced and improves PA efficiency. Adjustments in Vdd occur whenever the average output power changes, as indicated by the changes in the plot line  304 . APT modulates Vdd using relatively long term statistics. 
     In contrast, another technique referred to as envelope tracking (ET) uses relatively short term statistics to maximize PAE. The plot line  306  shows modulation of the Vdd using an ET technique. ET uses a dynamic Vdd, which tracks the RF modulation amplitude (the instantaneous output power level) instead of the average output power level. An envelope-tracking power supply (ETPS) is used as a dynamic power supply for the PA, adjusting the Vdd more frequently and optimizing PAE for smaller increments of time. Thus, ET improves efficiency for high-PAPR modulation at high average output power. 
     Both APT and ET techniques have disadvantages. In particular, ET is complex to implement, and is associated with various switcher anomalies, interfrequency modulation (IM) products, and amplitude modulation (AM) to phase modulation (PM) issues. Similarly, APT, while easy to implement, does not fully take advantage of the transmit waveform characteristics and switcher offerings, and thus marginalizes the improvements in PAE. 
       FIG. 4  illustrates a digital predistortion (DPD) technique for improving PAE in a power amplifier. In a system  400  that uses a power amplifier  404 , a digital predistorter  402  compensates for signal distortions inherent to the PA  404 . For example, the plot  406  shows a response power curve for a typical PA, such as the PA  404 . The digital predistorter  402  anticipates the power output for the PA  404  and provides an compensation signal in advance (i.e., digital predistortion), as indicated by the plot  408 . When the predistorted signal is amplified at the PA  404 , the resulting PA response is substantially linearized, as indicated by the plot  410 . In other words, two non-linear components are combined to generate a substantially linear output. While results of DPD can be good, DPD techniques requires specific detailed knowledge of the PA  404  to accurately model and compensate for its behavior. Such knowledge may not be available for some PAs. In addition, the DPD techniques represent a static compensation solution. However, many PA environments are highly dynamic, and the voltage, temperature, and operating conditions of the PA are subject to large variations during use, so it can be very difficult to implement DPD successfully in such a dynamic environment. 
     As discussed above, current techniques for PA power supply modulation, such as described with respect to  FIGS. 3 and 4 , have a number of disadvantages, including poor performance, or high cost and size penalties. Additionally, these techniques do not provide any programmability or adaptability. 
     To address these or other issues, embodiments of this disclosure provide adaptive power modulation techniques that improve the PAE of a PA by allowing the PA to operate at its optimum value based on a number of signal characteristics, which are known a priori. For example, the disclosed techniques take advantage of a priori knowledge of signal statistics, latency, switching, and signal bandwidth to formulate the optimal switching rate. Using the disclosed techniques, the power supply voltage is varied accordingly to obtain optimal efficiency for a given output power. The disclosed techniques allow the PA to have a broad range of operating scenarios based on system requirements and size, weight, and power (SWaP) constraints and offers alternative schemes to improve the power efficiency without adversely affecting other system performance metrics. By optimizing the efficiency of the PA, the disclosed techniques minimize system cooling and overall footprint requirements and improve system reliability. This helps to enhance system performance including mission effectiveness and assurance. 
     It will be understood that embodiments of this disclosure may include any one, more than one, or all of the features described here. In addition, embodiments of this disclosure may additionally or alternatively include other features not listed here. While the disclosed embodiments are described with respect to PAs in communication, radar and electronic warfare applications, these embodiments are also applicable in any other suitable systems or applications. 
       FIG. 5  illustrates a system  500  that uses adaptive power modulation according to this disclosure. The embodiment of the system  500  shown in  FIG. 5  is for illustration only. Other embodiments of the system  500  could be used without departing from the scope of this disclosure. 
     As shown in  FIG. 5 , the system  500  includes a power function block  502 , a power supply modulator  504 , a power supply  506 , a delay element  508 , an RF upconverter  510 , and a power amplifier (PA)  512 . 
     The system  500  receives a baseband signal  520  that is to be amplified by the PA  512  before being transmitted. The baseband signal  520  is divided into in phase (I) and quadrature (Q) components, and the I and Q components are output to the power function block  502 . 
     The power function block  502  receives the I and Q components of the baseband signal  520  and determines a power setting function r(t) for the PA  512  based on the I and Q components. In some embodiments, the function r(t)=f(I,Q) can be defined very generically to support alternative operating conditions. The power setting function r(t) is then output to the power supply modulator  504 . 
     The power supply modulator  504  receives the power setting function r(t) output from the power function block  502 . The power supply modulator  504  generates a power modulation function p(t) based on the power setting function r(t) and various known components of the system  500  using adaptive power modulation as described in greater detail below. The power supply modulator  504  modulates the power supply  506  according to the power modulation function p(t). The modulated power supply  506  then supplies power to the PA  512  according to the function p(t). In some embodiments, the power supply  506  is a DC to DC power supply, although any other suitable power supply could be used. 
     The delay element  508  applies a delay to the baseband signal  520  in order to account for the inherent latency resulting from processing times in the power function block  502  and the power supply modulator  504 . An appropriate delay at the delay element  508  aligns the baseband signal  520  with a corresponding power setting of the PA  512 . 
     The RF upconverter  510  receives the baseband signal  520  from the delay element  508 , upconverts the baseband signal  520 , and outputs the upconverted signal to the PA  512 , where it is amplified before transmission. 
     As discussed above, the power supply modulator  504  modulates the power supply  506  according to the power modulation function p(t). The power modulation function p(t) allows for dynamic frequency planning to avoid in-band harmonics and out-of-band spectral regrowth for wide band applications. In particular, the power supply modulator  504  maps the instantaneous baseband signal amplitude to the supply voltage and switching rate, which are determined using a look up table (LUT)  514 . The determined supply voltage and switching rate, in turn, optimize the performance of the PA  512  by dynamically adjusting the behavior of the PA  512  according to the current inputs. 
     The power supply modulator  504  takes into account a number of platform SWaP requirements and constraints, which are known a priori, to formulate the power modulation function p(t), which determines the switching rate and power supply voltage for the PA  512 . These requirements and constraints can include carrier frequency, PAPR, bandwidth of the signal in space (SiS), short-term and long-term waveform characteristics, power requirements, cooling requirements, frequency planning, in-band spurs and out-of-band ACLR requirements, and switcher characteristics. In general, the power modulation function p(t) is embodied in the LUT  514  and can be programmed to accommodate many design choices based on one, some, or all of the characteristics listed above. 
       FIG. 6  illustrates an example of the LUT  514  according to this disclosure. The embodiment of the LUT  514  shown in  FIG. 6  is for illustration only. Other embodiments of the LUT  514  can be used without departing from the scope of this disclosure. 
     As shown in  FIG. 6 , the LUT  514  is a multidimensional data structure having multiple input parameters  602 . The input parameters  602  correspond to various signal, environmental, and power parameters associated with the system  500  in which the PA  512  operates. Multiple input parameters  602  can be considered together to control the power supply modulation. Each input parameter  602  has a corresponding range of values  604 . While the values  604  are expressed as a range in  FIG. 6 , the LUT  514  can actually include discrete records corresponding to different values within each range of values  604 . For example, instead of the LUT  514  having a single record for a PAPR range of 0-30 dB, the LUT  514  can have different records corresponding to PAPR values of, e.g., 0 dB, 2 dB, 4 dB, and so on, up to 30 dB. 
     In addition to the input parameters  602 , the LUT  514  includes modulation outputs  606 - 608 . These include the voltage switching rate  606  and the supply voltage  608 . The LUT  514  is arranged such that, for a given set of input parameters  602 , the power supply modulator  504  can determine the optimal voltage switching rate  606  and the supply voltage  608  for the PA  512 . The parameters  602  and the corresponding values  604 - 608  in the LUT  514  can be determined empirically and can be configured for a wide range of operating conditions. Thus, use of the LUT  514  provides a wide operating range for the system  500  to achieve power aided efficiency based on SWaP constraints and provide adequate operating headroom or buffer for possible variations in the baseband signal  520  from what is expected. 
     Using the power supply modulator  504  and LUT  514  for adaptive power modulation is superior to current power tracking techniques. For example, many current power tracking techniques are only suitable for low signal bandwidth. Because the switching rate is proportional to signal bandwidth (e.g., in some systems, the switching rate is twice the bandwidth, so 10 MHz bandwidth requires a 20 MHz switching rate), a higher bandwidth signal requires a higher switching rate. Most current switches do not support a switching rate higher than about 10 MHz. This is not adequate to support higher bandwidth signals, such as those with a bandwidth greater than 10 MHz. Additionally, the programmability of such switches is limited and does not adequately handle in-band spurs and out-of-band adjacent channel leakage ratio (ACLR) management. Moreover, many switches support only lower voltage applications (e.g., 3-5 V, which is a common voltage range in mobile devices, for example). In contrast, the adaptive power modulation performed in the system  500  is suitable for implementations with switcher speeds of 100 MHz or higher and voltages of 100 V or higher. 
     Although  FIG. 6  illustrates one example of a LUT  514 , various changes may be made to  FIG. 6 . For example, while shown as having certain rows, columns, and data values, these are provided for illustration purpose only; the values shown can vary significantly in real implementations and can be arranged in different formats according to particular needs. As a particular example, the LUT  514  may additionally or alternatively include values associated with operating and environmental conditions such as humidity, pressure, and the like. 
     Although  FIG. 5  illustrates one example of system  500  that uses adaptive power modulation, various changes may be made to  FIG. 5 . For example, while the power function block  502  and the power supply modulator  504  are shown as separate components, this is merely for clarity of illustration. In some embodiments, these components could be combined into one block. In general, the makeup and arrangement of the system  500  are for illustration only. Components could be added, omitted, combined, or placed in any other configuration according to particular needs. 
       FIG. 7  illustrates an example method  700  for adaptive power modulation according to this disclosure. The method  700  may be performed using the system  500  of  FIG. 5 . However, the method  700  could be used with any other suitable system. 
     At step  701 , one or more characteristics of a system that uses a power amplifier are determined. This may include, for example, the power supply modulator  504  determining a characteristic related to the baseband signal  520  or determining a size, weight, and power (SWaP) characteristic of the system  500 . Specifically, this may include the power supply modulator  504  determining one or more of carrier frequency, PAPR, bandwidth of a signal in space (SiS), short-term and long-term baseband signal waveform characteristics, power requirements, cooling requirements, frequency planning, in-band spurs and out-of-band ACLR requirements, or voltage switcher characteristics. 
     At step  703 , a switching speed and a supply voltage for the power amplifier are determined based on the one or more characteristics determined at step  701 . This may include, for example, the power supply modulator  504  selecting the switching speed and the supply voltage from the look up table  514 . 
     At step  705 , a power supply of the power amplifier is modulated according to the determined switching speed and supply voltage. This may include, for example, the power supply modulator  504  dynamically modulating the power supply according to a changing amplitude of the baseband signal. 
     Although  FIG. 7  illustrates one example of a method  700  for adaptive power modulation, various changes may be made to  FIG. 7 . For example, while shown as a series of steps, various steps shown in  FIG. 7  could overlap, occur in parallel, occur in a different order, or occur multiple times. Moreover, some steps could be combined or removed and additional steps could be added according to particular needs. 
       FIG. 8  illustrates an example device  800  for performing adaptive power modulation according to this disclosure. The device  800  could, for example, represent a computing device or control device in the system  500  of  FIG. 5 , such as the power supply modulator  504 . The device  800  could represent any other suitable device for performing adaptive power modulation. 
     As shown in  FIG. 8 , the device  800  can include a bus system  802 , which supports communication between at least one processing device  804 , at least one storage device  806 , at least one communications unit  808 , and at least one input/output (I/O) unit  810 . The processing device  804  executes instructions that may be loaded into a memory  812 . The processing device  804  may include any suitable number(s) and type(s) of processors or other devices in any suitable arrangement. Example types of processing devices  804  include microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits, and discrete circuitry. 
     The memory  812  and a persistent storage  814  are examples of storage devices  806 , which represent any structure(s) capable of storing and facilitating retrieval of information (such as data, program code, and/or other suitable information on a temporary or permanent basis). The memory  812  may represent a random access memory or any other suitable volatile or non-volatile storage device(s). The persistent storage  814  may contain one or more components or devices supporting longer-term storage of data, such as a ready only memory, hard drive, Flash memory, or optical disc. In accordance with this disclosure, the memory  812  and the persistent storage  814  may be configured to store instructions associated with facilitating dynamic remapping of absolute addresses during a software migration. 
     The communications unit  808  supports communications with other systems, devices, or networks, such as the system  500 . For example, the communications unit  808  could include a network interface that facilitates communications over at least one Ethernet network. The communications unit  808  could also include a wireless transceiver facilitating communications over at least one wireless network. The communications unit  808  may support communications through any suitable physical or wireless communication link(s). 
     The I/O unit  810  allows for input and output of data. For example, the I/O unit  810  may provide a connection for user input through a keyboard, mouse, keypad, touchscreen, or other suitable input device. The I/O unit  810  may also send output to a display, printer, or other suitable output device. 
     Although  FIG. 8  illustrates one example of a device  800  for performing adaptive power modulation, various changes may be made to  FIG. 8 . For example, various components in  FIG. 8  could be combined, further subdivided, or omitted and additional components could be added according to particular needs. Also, computing devices can come in a wide variety of configurations, and  FIG. 8  does not limit this disclosure to any particular configuration of device. 
     The disclosed embodiments provide adaptive power modulation techniques that mitigate many SWaP constraints and issues and provide PAE improvements that allow for better and improved system performance for many applications in communication, radar and electronic warfare domains. The disclosed embodiments leverage commercial off the shelf (COTS) components and matured technologies. These techniques allow for future enhancements as new innovations and technology bring faster switching DC-to-DC converters to support high bandwidth applications (e.g., certain military applications). 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, means to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
     The description in the present application should not be read as implying that any particular element, step, or function is an essential or critical element that must be included in the claim scope. The scope of patented subject matter is defined only by the allowed claims. Moreover, none of the claims is intended to invoke 35 U.S.C. §112(f) with respect to any of the appended claims or claim elements unless the exact words “means for” or “step for” are explicitly used in the particular claim, followed by a participle phrase identifying a function. Use of terms such as (but not limited to) “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” or “system” within a claim is understood and intended to refer to structures known to those skilled in the relevant art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. §112(f). 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.