Abstract:
A method of calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter of a wireless communication unit is described. The method comprising, within at least one signal processing module of the wireless communication unit, applying a training signal comprising an envelope that varies with time to an input of the RF transmitter, receiving at least an indication of instantaneous output signal values for the power amplifier module in response to the training signal, calculating instantaneous gain values based at least partly on the received output power values, and adjusting a mapping function between an instantaneous envelope of a waveform signal to be amplified by the power amplifier module and the power amplifier module supply voltage to achieve a power amplifier module gain, for example that is monotonically increasing as a function of power amplifier output power.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 61/746,685, filed on Dec. 28, 2012 and incorporated herein by reference. 
    
    
     BACKGROUND 
     The field of this invention relates to a method and apparatus for calibrating an envelope tracking system, and in particular to a method and apparatus for calibrating an envelope tracking system of a radio frequency (RF) transmitter of a wireless communication unit using power-aware power amplifier gain mapping. 
     Continuing pressure on the limited spectrum available for radio communication systems is forcing the development of spectrally-efficient linear modulation schemes. Since the envelopes of a number of these linear modulation schemes fluctuate, these result in the average power delivered to an antenna being significantly lower than a maximum available power, leading to poor efficiency use of the power amplifier. Specifically, in this field, there has been a significant amount of research effort in developing high-efficiency topologies capable of providing high performances in the ‘back-off’ (linear) region of the power amplifier. 
     Linear modulation schemes require linear amplification of the modulated signal, in order to minimise undesired out-of-band emissions resulting from spectral re-growth. However, the active devices used within a typical RF amplifying device are inherently non-linear by nature. Only when a small portion of the consumed DC power is transformed into RF power, can the transfer function of the amplifying device be approximated by a straight line, i.e. as in an ideal linear amplifier case. This mode of operation provides a low efficiency of DC-to-RF power conversion, which is unacceptable for portable (subscriber) wireless communication units. Furthermore, low efficiency is also recognised as being problematic for base stations in wireless communication systems. 
     Additionally, an emphasis in portable (subscriber) equipment is to increase battery life. To achieve both linearity and efficiency, so called linearization techniques are used to improve the linearity of the more efficient amplifier classes, for example class ‘AB’, ‘B’ or ‘C’ amplifiers. A number and variety of linearizing techniques exist, which are often used in designing linear transmitters, such as Cartesian Feedback, Feed-forward, and Adaptive Pre-distortion. 
     Voltages at the output of the linear, e.g. Class AB, amplifier are typically set by the requirements of the final RF power amplifier (PA) device. Generally, the minimum voltage of the PA is significantly larger than that required by the output device of the Class AB amplifier. Hence, they are not the most efficient of amplification techniques. The efficiency of the transmitter (primarily the PA) is determined by the voltage across the output components/devices, as well as any excess voltage across any pull-down device components, due to the minimum supply voltage (Vmin) requirement of the PA. 
     Current communication systems and communication units need to support signals with wide bandwidth and high peak-to-average power ratio (PAPR). In order to increase the bit rate used in transmit uplink communication channels, larger constellation modulation schemes, with an amplitude modulation (AM) component, are being investigated and, indeed, becoming required. These modulation schemes, such as sixteen-state quadrature amplitude modulation (16-QAM), require linear PAs and are associated with high ‘crest’ factors (i.e. a degree of fluctuation) of the modulation envelope waveform. This is in contrast to the previously often-used constant envelope modulation schemes and can result in significant reduction in power efficiency and linearity. 
     A conventional power amplifier (PA) with a fixed supply voltage (VPA) should be operated at the so-called ‘back-off’ region. However, the PA supply voltage is also typically set high enough to linearly amplify high PAPR signals. Thus, a large portion of the supplied energy is dissipated as heat when the instantaneous power is lower than the peak power, thereby making the efficiency of the power stages of the transmitter when operating at lower power to be much less than when operating at peak output power. 
     To help overcome such efficiency and linearity issues a number of solutions have been proposed. One of the more popular efficiency improvement techniques is known as envelope tracking (ET) and relates to modulating the PA supply voltage to match (track) the envelope of the radio frequency waveform being transmitted by the RF PA. With envelope tracking (ET), the instantaneous PA supply voltage of the wireless transmitter is caused to approximately track the instantaneous envelope (ENV) (e.g. amplitude) of the transmitted quadrature (I/Q) radio frequency signals. Thus, since the power dissipation in the PA is proportional to the difference between its supply voltage and output voltage, envelope tracking enables an increase in PA efficiency, reduces heat dissipation, improves linearity and increases maximum linear output power, whilst allowing the PA to produce the intended RF output. Since the overall efficiency of an ET systems is proportional to the PA efficiency, optimized design of envelope (ENV) shaping function is very important. 
       FIG. 1  illustrates a graphical representation  100  of the aforementioned two alternative PA supply voltage techniques; a first technique that provides a fixed supply voltage  105  to a PA, and a second technique whereby the PA supply voltage is modulated to track the RF envelope waveform  115 . In the fixed supply voltage case, excess PA supply voltage headroom  110  is used (and thereby wasted), irrespective of the nature of the modulated RF waveform being amplified. 
     However, for example in the PA supply voltage tracking of the RF modulated envelope case  115 , excess PA supply voltage headroom can be reduced  120  by modulating the RF PA supply, thereby enabling the PA supply to accurately track the instant RF envelope. 
     The mapping function between envelope and the variable PA supply voltage (Vpa) is critical for optimum performance (e.g. efficiency, gain, and adjacent channel power (ACP)).  FIG. 1  also shows graphically  150  a plot of PA gain  155  versus output power (Pout)  160  for various fixed supply voltages with a maximum gain  165  of 28 dB for each fixed supply voltage reduced through gain compression  170  to an operating gain  175  of 25 dB. 
     Envelope-tracking can be combined with digital pre-distortion (DPD) on the RF signal to improve ACP robustness. The inventors of the present invention have also recognized and appreciated that envelope tracking with equal-gain mapping has several limitations, for example a lower equal-gain target provides better PA efficiency (but the efficiency improvement is limited due to the limited maximum output power capability of the RF transceiver) and that DPD AM/AM compensation may be insufficient to overcome high PA gain compression. 
     US 2012/0105150 A1, titled ‘joint optimization of supply and bias modulation’, describes an ET LUT that is populated to linearize a PA based solely on determined PA amplitude modulated-to-amplitude modulated (AM/AM) characteristics. The described technique uses an envelope shaping technique to provide maximum linearity whilst using fixed gain, irrespective of the required output power and supply voltage. The inventors have identified that lower equal-gain target provides good PA efficiency, but the efficiency improvement is limited due to the limited maximum output power capability of the RF transceiver. 
     A paper by D. Kim et al., titled ‘optimization for envelope shaped operation of envelope tracking power amplifier’, published in IEEE TMTT, vol. 59, no. 7, July 2011, pp. 1787-1795, describes a sweet-spot tracking technique that populates an ET LUT in order to adjust the supply voltage to follow the sweet spot points at each output power level to maintain good PA linearity. 
     Thus, there is a need for an efficient and cost effective solution to the problem of ET system calibration. In particular, it would be advantageous to provide an increased efficiency-improvement range whilst operating under current RF transceiver output power constraints. 
     SUMMARY 
     Accordingly, the invention seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination. Aspects of the invention provide a method and apparatus for calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency, RF, transmitter of a wireless communication unit. 
     According to a first aspect of the invention, there is provided a method of calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter. The method comprises, within at least one signal processing module of the wireless communication unit, applying a signal comprising an envelope that varies with time to an input of the RF transmitter; determining an indication of at least one instantaneous output signal value for the power amplifier module; mapping a gain of the power amplifier module to an instantaneous power of a waveform signal to be amplified by the power amplifier module; and applying a supply voltage to the power amplifier module based on the mapping. 
     In this manner, gain compensation, for example in a digital predistorter, may be applied to adaptively compress PA gain according to the instantaneous PA output power, hereinafter sometimes referred to as ‘power-aware’ PA gain mapping. For example, when the output power is close to a PA saturation power, the PA gain will be compressed less, thereby relaxing RF transceiver output power requirement at high PA output power. Alternatively, when the output power is several dB smaller than PA saturation power, the PA gain will be compressed more as the output power decreases, thereby improving PA efficiency at low PA output power. In this manner, the ET operation range may be increased as the efficiency at lower output power is improved. 
     According to an optional example, mapping the gain of the power amplifier module to the instantaneous power may comprise monotonically increasing a power amplifier gain as a function of power amplifier output power. 
     According to an optional example, mapping the gain of the power amplifier module to the instantaneous power may comprise mapping a reduction in power amplifier gain as the instantaneous power of the power amplifier output decreases. 
     According to an optional example, mapping the gain of the power amplifier module to the instantaneous power may comprise digital pre-distortion mapping. 
     According to an optional example, mapping the gain of the power amplifier module to the instantaneous power of a waveform signal to be amplified by the power amplifier module may comprise: determining a system linear gain for at least one average output power that may be output from the power amplifier module; determining a transmitter gain setting for at least one average output power that may be output from the power amplifier module; determining a gain setting of the power amplifier module for at least one instantaneous output power from the power amplifier module; and determining therefrom digital predistortion gain compensation to be applied to a digital predistorter to adaptively compress power amplifier module gain according to the at least one instantaneous output power. 
     According to an optional example, the method may further comprise performing envelope tracking (ET) calibration by determining a supply voltage to be applied to the power amplifier module for the at least one instantaneous output power. In one example, the method may further comprise determining therefrom amplitude modulated-to-phase modulated (AM/PM) to be applied to achieve at least one instantaneous output power to be output from the power amplifier module. In one example, the method may further comprise applying the digital pre-distortion amplitude modulated to phase modulated compensation when power amplifier gain compression is high. 
     According to an optional example, the method may further comprise storing the digital pre-distortion amplitude modulated to phase modulated compensation values in the at least one look-up table. 
     According to an optional example, the method may further comprise mapping digital pre-distortion amplitude modulated-to-phase modulated compensation in a first look-up table and mapping digital pre-distortion amplitude modulated-to-amplitude modulated compensation in a second look-up table. 
     According to an optional example, the method may further comprise calibrating determining an indication of at least one instantaneous output signal value for the power amplifier module. 
     According to an optional example, the at least one instantaneous output signal value may comprise at least one of: instantaneous power, instantaneous envelope, instantaneous phase 
     According to a second aspect of the invention, there is provided a non-transitory computer program product comprising executable program code for calibrating an envelope tracking system for a supply voltage for a power amplifier module within a radio frequency (RF) transmitter of a wireless communication unit, the executable program code operable for, when executed at a communication unit, performing the method of the first aspect. 
     According to a third aspect of the invention, there is provided a communication unit comprising: a radio frequency, RF, transmitter comprising an envelope tracking system for a supply voltage for a power amplifier module within the RF transmitter; and at least one signal processing module for calibrating envelope tracking system. The at least one signal processing module is arranged to: apply a signal comprising an envelope that varies with time to an input of the RF transmitter; determine an indication of at least one instantaneous output signal value for the power amplifier module; map a gain of the power amplifier module to an instantaneous power of a waveform signal to be amplified by the power amplifier module; and apply a supply voltage to the power amplifier module based on the mapping. 
     According to a fourth aspect of the invention, there is provided an integrated circuit for a communication unit comprising a radio frequency, RF, transmitter comprising an envelope tracking system for a supply voltage for a power amplifier module within the RF transmitter. The integrated circuit comprises at least one signal processing module for calibrating the envelope tracking system. The at least one signal processing module being arranged to: apply a signal comprising an envelope that varies with time to an input of the RF transmitter; determine an indication of at least one instantaneous output signal value for the power amplifier module; map a gain of the power amplifier module to an instantaneous power of a waveform signal to be amplified by the power amplifier module; and apply a supply voltage to the power amplifier module based on the mapping. 
     These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
         FIG. 1  illustrates a graphical representation of two known alternative power amplifier (PA) supply voltage techniques together with gain versus output power (Pout) for various fixed supply voltages. 
         FIG. 2  illustrates a simplified block diagram of an example of a wireless communication unit. 
         FIG. 3  illustrates a simplified generic block diagram of an example of a part of a radio frequency (RF) transceiver architecture. 
         FIG. 4  illustrates a graph showing power amplifier gain versus power amplifier output power for various power-aware gain mapping techniques employable by the transceiver architecture of  FIG. 3 . 
         FIG. 5  illustrates a simplified flowchart of an example of generating envelope tracking look-up tables within an RF transceiver. 
         FIG. 6  illustrates a simplified flowchart of an example of a use of the envelope tracking look-up tables generated in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     A primary focus and application of the present invention is the field of radio frequency (RF) power amplifiers capable of use in wireless telecommunication applications. Examples of the invention will be described in terms of one or more integrated circuits for use in a wireless communication unit, such as user equipment in third generation partnership project (3GPP™) parlance. However, it will be appreciated by a skilled artisan that the inventive concept herein described may be embodied in any type of integrated circuit, wireless communication unit or wireless/radio frequency transmitter that comprises or forms a part of an envelope tracking system. 
     Examples of the invention are described in terms of a PA gain mapping technique, for example by introducing amplitude-modulation to amplitude-modulation (AM/AM) compensation into a look-up table set of values in a digital pre-distortion (DPD) envelope tracking architecture. In this manner, DPD gain compensation may be applied to adaptively compress PA gain according to the instantaneous PA output power, hereinafter sometimes referred to as ‘power-aware’ PA gain mapping. For example, when the output power is close to a PA saturation power, the PA gain will be compressed less, thereby relaxing RF transceiver output power requirement at high PA output power. Alternatively, when the output power is several dB smaller than PA saturation power, the PA gain will be compressed more as the output power decreases, thereby improving PA efficiency at low PA output power. In this manner, the ET operation range may be increased as the efficiency at lower output power is improved. 
     In some examples, PA AM/PM compensation may be employed for both equal-gain and power-aware gain mapping when PA gain compression is high in order to overcome PA AM/PM non-linearity. 
     In some examples, the envelope tracking operation range may increase as the efficiency at lower output power is improved. In some examples, as the PA supply voltage may be operated using a smaller high frequency component with smaller efficiency improvement. An efficiency improvement using embodiments of the invention may offer a possibility to trade off the efficiency improvement and signal bandwidth on an envelope tracking path. 
     Furthermore, because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated below, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Referring first to  FIG. 2 , a block diagram of a wireless communication unit (sometimes referred to as a mobile subscriber unit (MS) in the context of cellular communications or a user equipment (UE) in terms of a 3 rd  generation partnership project (3GPP™) communication system) is shown, in accordance with one example embodiment of the invention. The wireless communication unit  200  contains an antenna  202  preferably coupled to a duplex filter or antenna switch  204  that provides isolation between receive and transmit chains within the wireless communication unit  200 . 
     The receiver chain  210 , as known in the art, includes receiver front-end circuitry  206  (effectively providing reception, filtering and intermediate or base-band frequency conversion). The front-end circuitry  206  is coupled to a signal processor  208 . An output from the signal processor  208  is provided to a suitable user interface  230 , which may encompass a screen or flat panel display. A controller  214  maintains overall subscriber unit control and is coupled to the receiver front-end circuitry  206  and the signal processor  208  (generally realised by a digital signal processor (DSP)). The controller  214  is also coupled to a memory device  216  that selectively stores various operating regimes, such as decoding/encoding functions, synchronisation patterns, code sequences, and the like. 
     In accordance with examples of the invention, the memory device  216  stores modulation data, and power supply data for use in supply voltage control to track the envelope of the radio frequency waveform to be output by the wireless communication unit  200 . Furthermore, a timer  218  is operably coupled to the controller  214  to control the timing of operations (transmission or reception of time-dependent signals and in a transmit sense the time domain variation of the PA supply voltage within the wireless communication unit  200 ). 
     As regards the transmit chain, this essentially includes the user interface  230 , which may encompass a keypad or touch screen, coupled in series via signal processing function  228  to transmitter/modulation circuitry  222 . The transmitter/modulation circuitry  222  processes input signals for transmission and modulates and up-converts these signals to a radio frequency (RF) signal for amplifying in the power amplifier module or integrated circuit  224 . RF signals amplified by the PA module or PA integrated circuit  224  are passed to the antenna  202 . The transmitter/modulation circuitry  222 , power amplifier  224  and PA supply voltage module  225  are each operationally responsive to the controller  214 , with the PA supply voltage module  225  additionally responding to a reproduction of the envelope modulated waveform from the transmitter/modulation circuitry  222 . 
     The signal processor  228  in the transmit chain may be implemented as distinct from the signal processor  208  in the receive chain  210 . Alternatively, a single processor may be used to implement processing of both transmit and receive signals, as shown in  FIG. 2 . Clearly, the various components within the wireless communication unit  200  can be realised in discrete or integrated component form, with an ultimate structure therefore being merely an application-specific or design selection. 
     Furthermore, in accordance with examples of the invention, the transmitter/modulation circuitry  222 , together with power amplifier  224 , PA supply voltage  225 , memory device  216 , timer function  218  and controller  214  have been adapted to generate a power supply to be applied to the PA  224 . For example, a power supply is generated that is suitable for a wideband linear power amplifier, and configured to track the envelope waveform applied to the PA  224 . In one example, an envelope tracking LUT is adapted by introducing digital pre-distortion (DPD) amplitude modulation-to-amplitude modulation (AM/AM) compensation in contrast to known equal-gain mapping implementations. 
     Referring now to  FIG. 3 , there is illustrated a generic example block diagram of a part of an RF transmitter architecture  300  of a wireless communication unit, such as the wireless communication unit  200  illustrated in  FIG. 2 . The transmitter architecture  300  comprises at least a part of transmitter/modulation circuitry  222 , for example that may be operationally responsive to the signal processor  228  and/or the controller  214  of  FIG. 2 , and a PA module  224 . The transmitter/modulation circuitry  222  in this example comprises a digital input (V in ) applied to both a forward transmit path  320  and an envelope tracking path  310 . The digital input (V in ) on the forward transmit path  320  is applied to a digital pre-distorter (DPD), comprising or operably coupled to at least one (DPD) look-up table (LUT)  302 . 
     In one example, the at least one DPD LUT  302  may be configured as two LUTs, one LUT being designated as an amplitude modulation-to-amplitude modulation (AM/AM) LUT and one LUT being designated as an amplitude modulation to phase modulation (AM/PM) LUT. 
     In use, the at least one DPD LUT  302  provides a DPD output (V out,dpd ) to a transmit (TX) analogue baseband (ABB) and radio frequency (RF) module  304 , which in some examples may include a digital-to-analog converter (DAC), low pass filter (LPF) to filter image signals and mixer to convert from baseband frequency to radio frequency prior to amplification by PA module  224 . The output (V out,tx )  324  from the TX ABB/RF module  304  is input to PA module  224 , which in turn outputs  326  the output RF signal (V out ) via a duplex filter and/or an antenna switch and an antenna (not shown). 
     Thus, in this manner, output RF signal (V out ) may be defined as:
 
 V   out   =G   lin   *V   in   [3]
 
     Where: Glin is defined as the system linear gain. Hence, initially the at least one (DPD) look-up table (LUT)  302  needs to be calibrated/populated with DPD values by defining the system linear gain (G lin ) as:
 
 G   lin   =G   dpd ( A ( t ))+ G   tx ( A ( t ))+ G   pa ( A ( t ))=const  [4]
 
     And a time variance of the input signal defined as:
 
 A ( t )=| V   in ( t )|  [5]
 
     This enables the DPD gain values to be derived and used to populate the at least one (DPD) look-up table (LUT)  302 . 
     The envelope tracking path  310  is input to an envelope tracking module, which comprises or is operably coupled to at least one ET LUT  312 . The envelope tracking module obtains from the at least one ET LUT  312  suitable values to control a supply voltage modulator  314 . Examples of the invention describe a mechanism to acquire improved ET LUT values for an envelope tracking system with regard to system efficiency, RF transceiver capability, and/or baseband bandwidth requirement. The supply voltage modulator  314  is arranged to modulate the supply voltage applied to the PA module  224  in accordance with a power supply level indication obtained from the at least one ET LUT  312 , for example residing within the controller  214  of  FIG. 2 . 
     Hence, following the at least one (DPD) look-up table (LUT)  302  being calibrated/populated with DPD values, the ET LUT  312  requires calibrating/populating in order to obtain a desired PA gain to be achieved, and therefore determine the Vpa to be applied to the PA module  224 . In one example, the ET LUT  312  is calibrated/populated with values to additionally compensate for determined AM-AM values stored within the at least one (DPD) look-up table (LUT)  302 . 
     In this manner, the supply voltage modulator  314 , and corresponding envelope tracking module may be configured to perform envelope tracking modulation of the supply voltage provided to the PA module  224 , such that the supply voltage provided to the PA module  224  substantially tracks an envelope of a RF waveform being amplified by the PA module  224 . In particular, use of DPD gain compensation is applied to adaptively compress PA gain according to instantaneous PA output power, via calibration of the RF transmitter architecture  300  and appropriate population of the LUTs. Advantageously, there is no commensurate increase in RF transmitter output power capability requirement. Hereafter, the term instantaneous PA output power may encompass one or more of: instantaneous power, instantaneous envelope, instantaneous phase. 
     The supply voltage modulator  314 , and corresponding envelope tracking module may form (at least a part of) an envelope tracking system of the RF transmitter architecture  300 . It will be apparent that the present invention is not limited to the specific example RF transmitter architecture  300  illustrated in  FIG. 3 , and may equally be applied to other transceiver architectures. For example, in an alternative architecture the ET LUT  312  may be comprised of two or more LUTs. 
       FIG. 4  illustrates two exemplary graphs  400 ,  450  showing power amplifier gain  410  versus power amplifier output power  420  for various power-aware gain mapping options employable by the transmitter architecture  300  of  FIG. 3 . For example, in a first case  400 , when the output power at  440  is close to a PA saturation power, the PA gain at  440  will be compressed less, thereby relaxing RF transceiver output power requirement at such high output powers. It has been determined that the efficiency at higher output powers (of say 23˜26 dBm) in the first case  400  is close to an equal-gain setting of 24 dB but advantageously the corresponding Vpa has a smaller high-frequency component than a second case  450 . Hence, a trade-off between the efficiency improvement and signal bandwidth on the envelope tracking path  310  can be obtained. 
     Alternatively, a second case  450 , when the output power at  470  is several dB smaller than PA saturation power at  480 , the PA gain will be compressed more, as shown by  470 , as the output power decreases, thereby improving PA efficiency at low PA output power. In this manner, the ET operation range may be increased as the efficiency at lower output power is improved. 
       FIG. 5  illustrates a simplified flowchart  500  of an example of generating at least one envelope tracking look-up table(s) within an RF transceiver. The simplified flowchart  500  commences in  510  with the system linear gain (G lin ) being set/defined for each average output power that may be output from the PA, such as PA  224  in  FIG. 3 . Here, the term average power encompasses the averaged power over a specific period in the communication system, for example one subframe in an LTE implementation. At  520 , the process then moves to the analogue baseband (ABB) (which in one example may encompass a digital to analog converter and low pass filter) &amp; RF transmitter gain (G tx ) being set/defined for each average output power that may be output from the PA, such as PA  224  in  FIG. 3 . 
     At  530 , the process then moves to the PA gain (G pa ) being set/defined for each instantaneous output power that may be output from the PA, such as PA  224  in  FIG. 3 . In one example, the instantaneous output power, and consequently the respective PA gains (G pa ) may follow the relationship:
 
For  P out,1 &lt;=P out,2  . . . Gpa ( P out,1)&lt;= Gpa ( P out,2)  [1]
 
     Where: 
     Pout,1 and Pout,2 are two possible instantaneous PA output powers, and 
     Gpa (Pout,1) and Gpa (Pout,2) are the respective PA gains. 
     At  540 , the process then moves to the DPD gain (G dpd ) being derived for each instantaneous output power and the amplitude modulated-to-amplitude modulated (AM/AM) LUT being completed first. In one example, the LUT values for G dpd  may be determined based on the following equation:
 
 G   dpd   =G   lin   −G   tx   −G   pa   [2]
 
     At  550 , once the AM/AM values have been populated in the LUT, the process then moves to populating the AM/PM values by commencing the envelope tracking (ET) calibration. Here, Vpa is determined for each instantaneous output power in order to obtain a desired system linear gain (G lin ) and determine therefrom the amplitude modulated-to-phase modulated (AM/PM) values to populate an AM/PM LUT. The AM/AM and AM/PM LUTs are then populated and ready for use. 
       FIG. 6  illustrates a simplified flowchart  600  of an example of a use of the at least one envelope tracking look-up table(s) generated in  FIG. 5 . The simplified flowchart  600  commences in  610  with the system linear gain being adjusted to the preset gain (G lin ) for each average output power that is output from the PA, such as PA  224  in  FIG. 3 . Here, the term average power encompasses the averaged power over a specific period in the communication system, for example one subframe in an LTE implementation. At  620 , the process then moves to the analogue baseband (ABB) (which in one example may encompass a digital to analog converter and low pass filter) &amp; RF transmitter gain being adjusted to the preset gain (G tx ) for each average output power that is output from the PA, such as PA  224  in  FIG. 3 . 
     At  630 , the process then moves to using the DPD gain from both the generated AM/AM LUT and AM/PM LUT to obtain a corresponding compensation gain (G dpd ) and phase for setting in the digital baseband, for example DPD AM/AM and AM/PM LUTs  302  in  FIG. 3 . Finally, in  640 , for each instantaneous output power, the values obtained from the ET LUT (say ET LUT  312  from  FIG. 3 ) are used to obtain a corresponding PA supply voltage (V pa ) in order to properly bias the PA, say PA  224  of  FIG. 3 . In this manner, DPD gain compensation may be applied to adaptively compress PA gain according to the instantaneous PA output power. For example, when the output power is close to a PA saturation power, the PA gain will be compressed less, thereby improving PA efficiency at high output power. Alternatively, when the output power is several dB smaller than PA saturation power, the PA gain will be compressed more as the output power decreases. In this manner, the ET operation range may be increased as the efficiency at lower output power is improved. Advantageously, there is no commensurate increase in RF transmitter output power capability requirement. 
     In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. 
     Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. 
     Any arrangement of components to achieve the same functionality is effectively ‘associated’ such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as ‘associated with’ each other such that the desired functionality is achieved, irrespective of architectures or intermediary components. Likewise, any two components so associated can also be viewed as being ‘operably connected’, or ‘operably coupled’, to each other to achieve the desired functionality. 
     Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. 
     Also for example, the various components/modules, or portions thereof, may implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type. 
     Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code, such as mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices, commonly denoted in this application as ‘computer systems’. 
     However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms ‘a’ or ‘an’, as used herein, are defined as one or more than one. Also, the use of introductory phrases such as ‘at least one’ and ‘one or more’ in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles ‘a’ or ‘an’ limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases ‘one or more’ or ‘at least one’ and indefinite articles such as ‘a’ or ‘an’. The same holds true for the use of definite articles. Unless stated otherwise, terms such as ‘first’ and ‘second’ are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.