Abstract:
Apparatus and methods are provided to calibrate a closed-loop envelop tracking system for a power amplifier of a wireless transmitter using standardized modulation signals. In one novel aspect, a closed-loop adaptive method is used to track changes in PA nonlinearity due to environmental or circuitry changes during operating condition using standardized modulation signals. In one embodiment, the PA supply voltage compensation lookup table (LUT) and/or the phase compensation LUT is updated during normal operation to sustain good linearity and efficiency performance. In another novel aspect, the PA target response is adjusted in response to changes in PA-related system configuration, such as changes in PA bias settings, and/or indicators measured from the transmitter such as changes in impedance or temperature changes for the RF module or the PA module. The PA supply voltage compensation LUT and/or the phase LUT is updated accordingly upon adjustment of the PA target response.

Description:
TECHNICAL FIELD 
       [0001]    The disclosed embodiments relate generally to envelope tracking for wireless devices, and, more particularly, to envelope tracking closed-loop on-the-fly calibration. 
       BACKGROUND 
       [0002]    The wireless communications industry has grown exponentially in recent years. In any wireless communications system, the radio frequency (RF) transmitter plays an important role. The power amplifier (PA) for an RF transmitter is the last stage of amplification in the transmitter chain. The performance of the PA for a wireless transmitter is critical and challenging in the transmission path for a wireless system. Since the PA is designed to drive large power into a load, such as an antenna, by a supply circuit, it is power-hungry and dominates the power dissipation of the transceiver unit. Further, since the PA is a critical element of the transmission path, it is expected to provide the desired output power with designed gains, high efficiency and linearity. New technologies have been developed to improve the efficiency of the PA. One of the key techniques is to use envelope tracking (ET) to provide supply voltage for the PA. An envelope tracking system constantly adjusts the supply voltage to the PA according to the input signal such that the power amplifier is operating more efficiently. Moreover, predistortion is often employed to mitigate AM/PM effect and further improve PA&#39;s linearity performance. For example, the digital predistortion, including an AM/AM predistortion and/or an AM/PM distortion, is used to improve system linearity. The problem occurs when the PA characteristics changes over time. PA characteristics are known to vary with circuitry or environmental factors such as load mismatch, temperature shift, voltage standing wave ratio (VSWR) changes, device aging and so on. Changes in PA characteristics result in loss of linearity and efficiency. One way to combat the problem is using open-loop calibration. However, open-loop calibration requires prior information about PA characteristics, which requires long factory calibration time due to the complexity of characterizing all combinations of different factors described above. Closed-loop adaptive calibration is preferred to track changes in PA characteristics. The challenges for closed-loop calibration are how to track the changes of PA characteristics adaptively and efficiently. Further, with the PA-related changes in the system, such as circuitry changes or environmental factors, setting a new PA target response is required or preferred to improve the overall system performance. 
       SUMMARY 
       [0003]    Apparatus and methods are provided to calibrate a closed-loop envelop tracking system for a power amplifier of a wireless transmitter using standardized modulation signals. In one novel aspect, a closed-loop adaptive method is used to track changes in PA nonlinearity due to environmental or circuitry changes during operating conditions using standardized modulation signals. In one embodiment, the PA supply voltage compensation lookup table (LUT) is updated during normal operation to sustain good linearity and efficiency performance. In another embodiment, an AM/PM phase LUT is updated during normal operation. In yet another embodiment, an AM/AM LUT for a digital predistortion process is updated during normal operation. The wireless transmitter transmits a wireless signal utilizing ET adapting a PA supply voltage with a PA target response. The wireless transmitter obtains a reference signal, which is a baseband digital signal of the transmitted wireless signal using a standardized modulation signal specified by the wireless system and a detection signal, which is partial feedback from down-sampled PA outputs. The wireless transmitter computes a difference between a measured response and the PA target response based on the reference signal and the detection signal and adjusts a PA supply voltage compensation LUT based on the computed difference to maintain the linearity of the ET. 
         [0004]    In another embodiment, an AM/PM compensation LUT is also updated when digital pre-distortion (DPD) is employed. The wireless transmitter computes a difference between a measured phase response and a predefined target phase response based on the reference signal and the detection signal. The wireless transmitter adjusts an AM/PM phase compensation LUT for the DPD module based on the computed phase difference to maintain the linearity of the envelope tracking system. 
         [0005]    In another novel aspect, the PA target response is adjusted in response to changes in PA-related system configuration and/or indicators measured from the transmitter. The PA supply voltage compensation LUT and/or the phase LUT and/or the AM/AM predistortion LUT is updated accordingly upon adjustment of the PA target response. In one embodiment, the wireless transmitter adjusts the PA target response based on one or more changes of PA-related system configuration, for example, the PA gate bias setting. In another embodiment, the wireless transmitter detects changes in PA impedance. The wireless transmitter adjusts the PA target response based on the changes in PA impedance. In another embodiment, the wireless transmitter detects one or more temperature changes, such as temperature changes in the RF module or in the PA. The wireless transmitter adjusts the PA target response based on the detected temperature changes. 
         [0006]    Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
           [0008]      FIG. 1  illustrates an exemplary wireless communication system using closed-loop adaptive calibration for an envelope tracking system of a wireless transmitter. 
           [0009]      FIG. 2  shows exemplary diagrams of output power to supply voltage LUTs mapped in different target responses. 
           [0010]      FIG. 3  shows exemplary diagrams of output power to phase LUTs mapped in different target responses. 
           [0011]      FIG. 4  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its LUT on-the-fly using system modulation signals in accordance with embodiments of the current invention. 
           [0012]      FIG. 5  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its PA setting based on changes in system configuration information. 
           [0013]      FIG. 6  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its PA target response based on detected changes in PA impedance. 
           [0014]      FIG. 7  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its PA target response based on detected changes in RF and/or PA temperatures. 
           [0015]      FIG. 8  is an exemplary flow chart for on-the-fly calibration for a closed-loop envelope tracking system of a power amplifier for a wireless transmitter in accordance with embodiments of the current invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0017]      FIG. 1  illustrates an exemplary wireless communication system using closed-loop adaptive calibration for an envelope tracking system of a wireless transmitter. A wireless user equipment (UE)  101  connects with a wireless base station  102  in a wireless system with modulation signals  103  specified by a wireless standard, such as 802.11, WLAN, WCMDA, LTE, LTE-A and other wireless standards. Wireless UE  101  and base station  102  both have radio frequency (RF) modules that transmit and receive wireless signals. UE  101  includes a baseband processor  111 , an RF transmitter  112 , a power amplifier  113 , a radio front end  114 , and an antenna  115 . Similarly, base station  102  includes a baseband processor  121 , an RF transmitter  122 , a power amplifier  123 , a radio front end  124 , and an antenna  125 . Baseband processors  111  and  121  process digital representations of selected channels and extract the raw data through demodulation and other signal processing. RF transmitters  112  and  122  up-convert the baseband data from the processor to RF frequency, typically through I/Q modulator. Power amplifiers  113  and  123  provide gain to the generated RF signal. Radio front-end modules  114  and  124  take the amplified RF signal and send it to the antenna. Antennae  115  and  125  transmit the amplified RF signals. 
         [0018]    A power amplifier, such as PA  113  or PA  123 , takes input power with a voltage of V in , and applies a supply power V supply  (or V pa ) to the PA such that the output signal voltage V out  is amplified. Traditionally, the supply power is a constant DC voltage. The constant supply power results in large power waste. Many modern day wireless communication systems, such as 3G and 4G mobile networks, have a high peak to average power ratio (PAPR), suggesting that the dynamic range of signals to be amplified is very wide. Orthogonal frequency-division multiplexing (OFDM) modulation used in LTE is particularly susceptible to high crest factors, producing multiple peaks and valleys of power. As a result, the amplifier spends a lot of time amplifying small signals where the power efficiency is low. Further, due to high data rates required by demanding wireless applications, linear amplifiers must be used to preserve the modulation content. Envelope tracking (ET) solves the energy efficiency problem. 
         [0019]    As shown in  FIG. 1 , both UE  101  and base station  102  can use envelope tracking. Envelope tracking system  116  of UE  101  takes input signal  104  from baseband processor  111 , the baseband envelope signal  105  is sent to an ET power supply modulator that varies the supply voltage following baseband envelope signal  105 . As a result, a supply voltage signal  106  varies based on the input signal. While input signal  104  at its peak signal levels, the supply voltage from ET  116  is high. While input signal  104  decreases, the supply voltage from ET  116  decreases to low signal levels. The result is that PA  113  operates near its optimum point for best efficiency by minimizing the dissipated heat energy. Similarly, base station  102  uses an envelope tracking system  126 . ET  126  of base station  102  takes the input signal from RF transmitter  122 , the baseband envelope signal is sent to an ET power supply modulator that varies the supply voltage following the baseband envelope signal. ET  126  supplies PA  123  with a supply voltage that varies based on the input signal. 
         [0020]      FIG. 1  further shows simplified block diagrams of a wireless device in accordance with embodiments of the current invention. A wireless device, such as UE  101  or base station  102 , has an antenna  135 , which transmits and receives radio signals. An RF transceiver module  133 , coupled with the antenna, receives RF signals from antenna  135 , converts them to baseband signals and sends them to processor  132 . RF transceiver  133  also receives baseband signals from processor  132 , converts them to RF signals, and sends out to antenna  135 . Processor  132  processes the received baseband signals and invokes different functional modules to perform features in the wireless device. Memory  131  stores program instructions and data  134  to control the operations of the wireless device. 
         [0021]    The wireless device also includes multiple function modules that carry out different tasks in accordance with embodiments of the current invention. A closed-loop ET module  141  performs closed-loop envelope tracking for the power amplifier. An adaptive process engine  142  adaptively update the AM/AM LUT and/or AM/PM LUT based on predefined or preconfigured conditions. A PA target response module  143  adjusts the PA target response based on detected and/or preconfigured conditions. An impedance calculator  144  detects and calculates impedance changes. One or more temperature sensors  145  detects temperature changes in modules such the RF module and/or the PA module. In addition, Adaptive process engine  142  includes multiple modules to carry out different tasks in accordance with embodiments of the current invention. A reference signal detector  151  obtains a reference signal, wherein the reference signal is a baseband digital signal of the transmitted wireless signal. A detection signal detector  152  obtains a detection signal through a coupler, wherein the detection signal is a partial feedback from down-sampled PA outputs. A power/gain difference calculator  153  computes a difference between a measured amplitude response and the PA target response based on the reference signal and the detection signal. A PA supply voltage LUT module  154  adjusts the PA supply voltage compensation LUT based on the computed power/gain difference. A DPD LUT module  155  adjusts an AM/PM phase compensation LUT and/or an AM/AM compensation LUT for DPD based on the computed response difference to maintain the linearity of the ET. 
         [0022]    Envelope tracking enables the optimum performance point to be selected for any given level of RF power output by varying the voltage on the RF power amplifier. With the growing demand for higher data rates, linear amplifiers must be used to preserve the modulation content. 
         [0023]    In one novel aspect of the current invention, the PA target response can be adjusted based on PA related system setting or detected environment settings. The adjusted PA gain is then used to update dynamically the LUT such that the system achieves the newly adjusted target response. The LUT is normally calibrated such that the resulted PA gain meets predefined PA performance/response. However, if PA characteristics change over the time, the actual gains for different input amplitudes may no longer meet the predefined gain criterion resulting in degradation in the system performance. For many modern wireless transmitters, maintaining the linearity is crucial for the overall system performance. 
         [0024]      FIG. 2  shows exemplary diagrams of output power to supply voltage LUT mapped in different target responses. A graph  201  shows an exemplary amplitude LUT curve calibrated in a default setting. The calibrated LUT is designed to result in PA gain stays at PA target response for any inputs. However, PA characteristics are known to be prone to change due to various factors, such as temperature changes.  FIG. 2  also shows a graph  202 , which is an exemplary amplitude LUT curve calibrated at a PA target response due to environmental changes. As shown, graph  202  shifts away from graph  201 . If the ET system continues using the LUT calibrated default setting, the system cannot obtain the required linearity when operating in a changed environment. The LUT needs to be updated adaptively in response to changes that result in PA characteristics changes, such as the temperature changes. 
         [0025]      FIG. 3  shows exemplary diagrams of output power to phase LUT mapped in different temperatures. A graph  301  shows an exemplary phase LUT curve calibrated in a default/original setting. The calibrated LUT is designed to result in PA gain stays at PA target response for any inputs. However, PA characteristics are known to be prone to change due to various factors, such as temperature changes.  FIG. 3  also shows a graph  302 , which is an exemplary phase LUT curve calibrated in a changed environment. As shown, graph  302  shifts away from graph  301  under different environment. If the ET system continues using the phase LUT calibrated in the default setting, the system cannot obtain the required linearity when operating at the changed environment. The phase LUT needs to be updated adaptively in response to changes that results in PA characteristics changes, such as the temperature changes. 
         [0026]      FIG. 4  shows an exemplary block diagram of a closed-loop ET system that adaptively adjust its LUT on-the-fly using system modulation signals in accordance with embodiments of the current invention. The closed-loop ET system includes a transmission path, an ET path, and a feedback path. 
         [0027]    The transmission path transmits the amplified signal using a modulation signal defined by a wireless standard. A digital predistortion (DPD) module  401  receives baseband signals and passes the processed signal to transmitter digital frontend processor  402 . DPD  401  receives baseband signals and performs AM/PM phase compensation and AM/AM compensation. AM/PM phase compensation controls AM/PM distortion. AM/AM compensation controls AM/AM distortion. The output of DPD  401  is sent to transmitter digital frontend processor (TxDFE)  402 . TxDFE  402  sends the signal to digital-analog converters (DAC)  403  and  404 , which convert the baseband digital signals to analog signals. The converted analog signal is sent to an RF module  405 . The output of RF module  405  is the input signal for a PA  406 . PA  406  amplifies the received input signals and sends the amplified output signal to a radio frontend module  407 . Radio frontend module  407  drives the amplified output signal from PA  406  to an antenna  408 , which transmits the RF signal according a wireless standard. 
         [0028]    The ET path tracks modulation envelope and adjusts the supply voltage to PA  406  based on the detected envelope. An envelope detector  411  receives the same signal to DPD  401 . Envelope detector  411  detects the envelope of the received signal and sends it to a power scaler  412 . The scaler is not limited to power scaling; for example, a gain scaler could be utilized as well. Power scaler  412  sends the scaled signal to a Vpa LUT mapper  413 . Vpa LUT mapper  413  maintains a Vpa mapping LUT to achieve the maximum linearity for the system. Upon receiving the input signal, the ET path applies an LUT to generate a supply power signal. Vpa LUT mapper  413  sends the generated supply power signal to a DAC  414 . The output of DAC  414  is passed to an ET modulation module  415 , which generates the supply voltage for PA  406  based on the detected envelope and the amplitude. 
         [0029]    The feedback path takes output signals into an adaptive feedback process engine and compares with the input signal. In one novel aspect, the feedback path also sends other PA-related system information to the adaptive process engine. An IQ feedback receiver  424  takes the output RF signal and sends it to an analog to digital converters (ADC)  422  and  423 . In one embodiment, IQ feedback receiver  424  takes the output RF signal before the radio FE. In another embodiment, IQ feedback receiver  424  takes the output RF signal after the radio FE. The digital signals from the outputs of ADC  422  and  423  are sent to an adaptive processing engine  421 . In one novel aspect, adaptive process engine  421  takes inputs from two paths. In one path, adaptive process engine  421  receives the same input baseband signal as DPD  401 , also referred as the reference signal. The output signal received from ADC  422  and  423 , also referred as the detection signal, is fed to adaptive process engine  421  as well. 
         [0030]    Ideally, the detection signal is the multiple of the reference signal with the target loop gain. Practically, variety of signal noises added to the path and the detection signal is not purely linear to the reference signal. Thereby, with the actual gain and phase error measured for the n-th LUT entry, adaptive processing engine  421  would update the corresponding supply voltage and phase LUT contents according to the measured gain difference or phase error, either stepwise or linearly. 
         [0031]    Upon detecting and calculating new entries for the AM/AM LUT and AM/PM LUT, adaptive processing engine  421  updates the AM/AM LUT for Vpa LUT mapper  413  and the AM/PM LUT and the AM/AM LUT for DPD  401 . The feedback path takes the output signal from the transmission path and compares the output signal with the input signal. Based on the comparison of the live traffic, adaptive processing engine  421  adaptively maintains the linearity of the ET system under operating condition of the PA using standardized modulation signal specified by the wireless standard by updating the PA supply voltage compensation LUT. By using the standardized modulation signal specified by the wireless standard that the wireless device is operating under, there is no training signal required. The calibration is done on-the-fly without any operation interruption. The on-the-fly calibration refers to performing the detection and compensation to the communication signal without hindering, interrupting or stopping the normal operation of the transceiver of the UE. The normal operation includes circuit initialization, data transmission, data reception, and every operation after power-on for the UE. In some implementations, the on-the-fly calibration involves determining the mismatch and updating the LUTs at a regular time interval when the transceiver is in operation. In other implementation, the on-the-fly calibration is carried out whenever a specific environment condition is met, e.g., the ambient temperature has changed, while the transceiver is performing any other normal operation. In any case, the on-the-fly process is carried out by a number of times after power-on, without any interruption to the normal process. 
         [0032]    In another novel aspect, the PA target response is adjusted in response to PA-related changes, such as system configuration changes and changes in indicators measured from the transmitter. In one embodiment, the PA target response/performance is adjusted by adjusting the PA supply voltage compensation LUT.  FIGS. 5 to 8  show different embodiments in adjusting the PA target response/performance and updating the PA supply voltage LUT and/or phase LUT (also referred as AM/PM LUT) and/or gain LUT (also referred as AM/AM LUT) based on detected changes from the feedback channels. 
         [0033]      FIG. 5  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its PA target response based on changes in system configuration information. Adaptive process engine  421 , as also shown in  FIG. 4 , compares input signals and detection/output signals and adjusts the PA supply voltage LUT and/or phases LUT and/or gain LUT based on the comparison. The purpose of the adjustment is to retain the linearity for the PA by adjusting distortions. In adjusting the PA supply voltage LUT and/or phase LUT and/or gain LUT, adaptive process engine  421  would first assume target response for the PA. In one novel aspect, the target response/performance of the PA can be dynamically adjusted based on changes in transmission parameters. In one embodiment, a system central controller  501  communicates with adaptive process engine  421 . System central controller  501  obtains system information, such as transmission parameter changes, such as output power target or resource allocation information. For example, when the output power target is low or when the allocated resource is small, the target PA amplitude may be adjusted to a lower level. In another example, upon configuring PA  406  to a new PA gate bias setting, system central controller  501  informs adaptive process engine  421  such that a new target response may be selected based on the new configuration. System central controller  501  communicates the obtained information to adaptive processing engine  421 . Adaptive processing engine  421  determines if the PA target response needs to be adjusted based on information received from system central controller  501 . In one embodiment, system central controller  501  determines that the PA target response needs to be adjusted. Subsequently, adaptive process engine  421  updates the PA supply voltage LUT and/or phase LUT and/or gain LUT based on the new PA target response. In another embodiment, system central controller  501  directly communicates with PA  406 . In one embodiment, system central controller  501  determines whether to adjust the PA target response to a new value based on the transmission parameters scheduled by the base station. If so, system central controller  501  updates PA  406  with the new gain target. 
         [0034]      FIG. 6  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its PA target response based on detected changes in PA impedance. In one embodiment, the assumed PA target response is modified based on changes in impedance. An impedance calculator  602  detects and calculates impedance from the output of ADC  422  and ADC  423 . Impedance calculator  602  communicates with system central controller  601 . System central controller  601  determines whether the change of impedance is bigger than a predefined threshold. If so, system central controller  601  determines a new PA target response for the PA and updates the PA supply voltage LUT and/or phase LUT and/or gain LUT accordingly. In another embodiment, system central controller  601  determines whether to use a new PA target response based on the calculated impedance received. If so, system central controller  790  updates the PA supply voltage LUT and/or phase LUT using the new PA target response accordingly. 
         [0035]      FIG. 7  shows an exemplary block diagram of a closed-loop ET system that adaptively adjusts its assumed PA target response based on detected changes in RF and/or PA temperatures. A temperature sensor  702  detects temperature changes for RF  405  and/or PA  406 . System central controller  701  reads measured temperature from the temperature sensor and determines whether to modify the assumed PA target response based on the temperature information of PA  406  and/or RF  405 . If so, system central controller  701  updates the PA supply voltage LUT and/or phase LUT and/or gain LUT based on the modified assumed PA target response accordingly. 
         [0036]      FIG. 8  is an exemplary flow chart for an on-the-fly calibration for a closed-loop envelope tracking system of a power amplifier for a wireless transmitter in accordance with embodiments of the current invention. At step  800 , the wireless transmitter adjusts the PA target response based on the one or more changes of PA-related system configuration or the one or more transmitter indicators. At step  801 , the wireless transmitter transmits a wireless signal in a wireless system by a wireless transmitter with a radio frequency (RF) module utilizing an envelope tracking (ET) adapting a power amplifier (PA) supply voltage with a PA target response. At step  802 , the wireless transmitter obtains a reference signal on-the-fly, wherein the reference signal is a baseband digital signal of the transmitted wireless signal using a standardized modulation signal specified by the wireless system. At step  803 , the wireless transmitter obtains a detection signal on-the-fly through a coupler, wherein the detection signal is a partial feedback from down-sampled PA outputs. At step  804 , the wireless transmitter computes a difference between a measured response and the PA target response based on the reference signal and the detection signal. At step  805 , the wireless transmitter adjusts a PA supply voltage compensation look up table (LUT) based on the computed difference to maintain the linearity of the ET. At step  806 , the wireless transmitter computes a difference between a measured phase and a predefined target phase based on the phase difference between the reference signal and the detection signal. At step  807 , the wireless transmitter adjusts an AM/PM phase compensation LUT based on the computed phase difference to maintain the linearity of the ET. 
         [0037]    Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.