EP3155721A1

EP3155721A1 - Method and apparatus for envelope shaping in envelope tracking power amplification

Info

Publication number: EP3155721A1
Application number: EP14894315.2A
Authority: EP
Inventors: Zhancang WANG
Original assignee: Nokia Technologies Oy
Current assignee: Provenance Asset Group LLC
Priority date: 2014-06-13
Filing date: 2014-06-13
Publication date: 2017-04-19
Also published as: WO2015188363A1; EP3155721A4

Abstract

A method, corresponding apparatuses, and a non-transitory computer readable medium for envelope shaping in envelope tracking power amplification are provided. The method comprises detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The method also comprises clipping the one or more detected troughs in the envelope signal. The method further comprises shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs. With the claimed inventions, the efficiency and linearity of an envelope tracking power amplifier could be improved.

Description

METHOD AND APPARATUS FOR ENVELOPE SHAPING IN ENVELOPE TRACKING POWER AMPLIFICATION

FIELD OF THE INVENTION

[0001] Example embodiments of the present disclosure generally relate to power amplification techniques. More particularly, example embodiments of the present disclosure relate to a method and an apparatus for envelope shaping in envelope tracking power amplification.

BACKGROUND OF THE INVENTION

[0002] The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present disclosure but provided by the present disclosure. Some such contributions of the present disclosure may be specifically pointed out below, while other such contributions of the present disclosure will be apparent from their context.

[0003] The efficiency of a Radio Frequency ("RF") Power Amplifier ("PA") is generally defined as a ratio between a desired transmitted radio power and a total power from a power supply and this ratio appears to be rather low in the future wideband applications if traditional architectures are still applied. For the purposes of enhancing the efficiency, an Envelope Tracking ("ET") technique has been proposed and utilized in the wireless communication infrastructure industry and has been considered as a most promising efficiency enhancement solution for the fourth Generation ("4G") and beyond wireless communications.

[0004] It is known that in the ET PA, the power supply voltage applied to the PA is constantly adjusted according to the envelope version of an original input signal to ensure that the PA is operating at the peak efficiency over the output power range. However, when the PA power supply is changed from low to high instantaneously and dynamically or vice-versa, the PA operating condition on the drain-side would change dramatically accordingly. This significant change of the PA operating point would give rise to undesirable distortions and memory effects, which may cause gain collapse and unpredictable and non-correctable distortions and adversely affect both efficiency and linearity of an ET PA system.

SUMMARY OF THE INVENTION [0005] The following presents a simplified summary of the present disclosure in order to provide a basic understanding of some aspects of the present disclosure. It should be noted that this summary is not an extensive overview of the present disclosure and that it is not intended to identify key/critical elements of the present disclosure or to delineate the scope of the present disclosure. Its sole purpose is to present some concepts of the present disclosure in a simplified form as a prelude to the more detailed description that is presented later.

[0006] To diminish or eliminate at least one of the above-mentioned problems, various aspects of the present disclosure provide a method, an apparatus and non-transitory computer readable medium for performing envelope tracking power amplification such that it is possible to provide a less memory effect and linearizable envelope tracking power amplifier.

[0007] According to an aspect of the present disclosure, there is provided a method. The method comprises detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The method also comprises clipping the one or more detected troughs in the envelope signal. The method further comprises shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0008] In one embodiment, the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

[0009] In another embodiment, the detecting the one or more troughs in the envelope signal comprises inverting the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks; comparing the one or more peaks with the shaping threshold; and identifying one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing.

[0010] In yet another embodiment, the identifying comprises identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal.

[0011] In a further embodiment, the clipping the one or more detected troughs in the envelope signal comprises obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal; generating one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function, wherein the shaping the envelope signal comprises subtracting a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal.

[0012] In one embodiment, the clipping and the subtracting are performed reiteratively a number of times such that the one or more inverted troughs are sequentially processed according to processing capability.

[0013] In another embodiment, the identifying comprises identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold.

[0014] In one embodiment, the clipping the one or more detected troughs in the envelope signal comprises obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal; clipping each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function; and wherein the shaping the envelope signal comprises multiplying the one or more clipped troughs with the delayed version of the envelope signal.

[0015] In an additional embodiment, the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed.

[0016] According to an aspect of the present disclosure, there is provided an apparatus. The apparatus comprises at least one processor and at least one memory including computer program instructions. The memory and the computer program instructions are configured to, with the at least one processor, cause the apparatus at least to detect, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The memory and the computer program instructions are also configured to, with the at least one processor, cause the apparatus at least to clip the one or more detected troughs in the envelope signal. The memory and the computer program instructions are also configured to, with the at least one processor, cause the apparatus at least to shape the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0017] According to an aspect of the present disclosure, there is provided an apparatus. The apparatus comprises means for detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The apparatus also comprises means for clipping the one or more detected troughs in the envelope signal. The apparatus further comprises means for shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0018] According to an aspect of the present disclosure, there is provided a non-transitory computer readable medium having program code stored thereon, the program code configured to direct an apparatus, when executed, to detect, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The program code is also configured to direct the apparatus, when executed, to clip the one or more detected troughs in the envelope signal. The program code is further configured to direct the apparatus, when executed, to shape the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0019] The aspects and embodiments of the present disclosure as described above may be utilized separately or in combination and different combining forms may be established to aim at addressing at least one of the problems as mentioned in the above and achieve some of the notable technical effects as set forth below.

[0020] By virtue of the method, apparatuses and the non-transitory computer readable medium according to multiple aspects of the present disclosure, envelope shaping and biasing are introduced to improve ET PA efficiency and linearity. For instance, the shaping of the envelope signal may reduce the non-linearity at a low supply voltage range so that the ET PA works more linearly as compared to the absence of envelope shaping. Further, the envelope shaping at issue may engender negligible degradation of efficiency having regards to the gain increase in the low voltage region and may obtain higher efficiency of the ET supply modulator attributable to the reduced Peak To Average Power Ratio ("PAPR") of the shaped envelope. Further, in dependence on different shaped waveforms of the envelope signal, it may be very flexible to compromise the ET supply modulator and RF PA design for efficiency, linearity and bandwidth. Additionally, by removing the unwanted one or more troughs, dynamically adaptive changing gate bias via RF transistor sinking current sensing may be accomplished to reduce wasted sunk power. Furthermore, since the PA power supply being changed from low to high instantaneously and dynamically or vice-versa is effectively controlled, the operating point of the ET PA may become more stable and distortions of ET PA may be reduced or eliminated to a certain extent. In addition, since the solutions of the present disclosure may reuse RF transistor quiescent current sinking, the ET modulator design could be greatly simplified and cost could be significantly lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The embodiments of the present disclosure that are presented in the sense of examples and their advantages are explained in greater detail below with reference to the accompanying drawings, in which:

[0022] Fig. 1 is a block diagram exemplarily illustrating a number of functional blocks for envelope tracking power amplification according to an embodiment of the present disclosure;

[0023] Fig. 2 is a block diagram exemplarily illustrating a method for envelope shaping in envelope tracking power amplification according to an embodiment of the present disclosure;

[0024] Fig. 3 is a diagram exemplarily illustrating inversion operations of the envelope signal according to an embodiment of the present disclosure;

[0025] Fig. 4 is a block diagram exemplarily illustrating a method for envelope shaping in envelope tracking power amplification according to an embodiment of the present disclosure; [0026] Fig. 5 is a block diagram exemplarily illustrating details regarding generation of one or more trough cancellation pulses according to an embodiment of the present disclosure;

[0027] Fig. 6 is a block diagram exemplarily illustrating a method for envelope shaping in envelope tracking power amplification according to another embodiment of the present disclosure;

[0028] Fig. 7 is a block diagram exemplarily illustrating a method for envelope shaping in envelope tracking power amplification according to yet another embodiment of the present disclosure;

[0029] Fig. 8 is a diagram exemplarily illustrating probability distribution function of the shaped envelope signal versus the output supply voltage of the ET supply modulator with typical high-PAPR modulated signals as stimulus;

[0030] Fig. 9 is a simplified schematic block diagram illustrating a representative apparatus according to an embodiment of the present disclosure; and

[0031] Fig. 10 is a simplified schematic block diagram illustrating another representative apparatus according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMB ODEVIENT S [0032] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the present disclosure are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Like numbers refer to like elements throughout the specification.

[0033] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, or step" are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, or step unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The discussion above and below in respect of any of the aspects of the present disclosure is also in applicable parts relevant to any other aspect of the present disclosure.

[0034] The following will discuss the details of the embodiments of the present disclosure with reference to the accompanying drawings.

[0035] Fig. 1 is a block diagram exemplarily illustrating a number of functional blocks for envelope tracking power amplification according to an embodiment of the present disclosure. As illustrated in Fig. 1, an envelope shaping block 101 may receive an envelope signal resulted from a baseband signal or a digital Intermediate Frequency ("IF") signal, and shape the envelope signal according to the example embodiments of the present disclosure, as will be discussed in detail later. Then, the resulted shaped envelope signal may be fed into an ET supply modulator 102, which may then modulate the power supply voltage based on the shaped envelope signal and feed the modulated supply voltage to an RF power amplifier 103 as a power supply. Then, the RF power amplifier 103 may amplify the RF signal using the modulated power supply. During the amplification processing, thanks to the example embodiments, the PAPR of the envelope signal may be reduced before entering into the ET supply modulator 102 and the efficiency of the ET supply modulator 102 could be improved since it is more likely to operate in a high efficiency region.

[0036] Fig. 2 is a block diagram exemplarily illustrating a method 200 for envelope shaping in envelope tracking power amplification according to an embodiment of the present disclosure. As illustrated in Fig. 2, at block 201, the method 200 detects, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The envelope tracking power amplifier and the envelope tracking supply modulator may be the ones as depicted in Fig. 1.

[0037] In an example embodiment, the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier. In another example embodiment, the shaping threshold may be a trough threshold V_t for detecting the presence of the troughs, as illustrated in the left drawing of Fig. 3 by a number of thick points at the bottom of the envelope signal. In yet another example embodiment, the shaping threshold may be a peak threshold V'_t for detecting the presence of the peaks as illustrated in the right drawing of Fig. 3, which is an inverted version of the envelope signal in the left drawing via waveform inverting processing. In addition, the shaping threshold herein may be in accordance with the precision of the input data, which is a function of both the target PAPR reduction and the amplitude variance of the envelope signal. Furthermore, the shaping threshold may be a runtime configurable parameter whose actual floating value may be scaled and truncated for various implementations, such as the Digital Signal Processing ("DSP") implementation.

[0038] In an example embodiment, the detecting at block 201 may comprise inverting the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks, comparing the one or more peaks with the shaping threshold and then identifying one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing. Due to the waveform inversion, it becomes easy to detect the troughs in the envelope signal by detecting the peaks.

[0039] In an example embodiment, the identifying as above may comprise identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal. The advantage brought about by this identifying approach is that it is easy to be implemented and a delay from the detecting of the trough to the locating of the trough is relatively fixed. In another example embodiment, the identifying may identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold. The advantages brought about by this identifying approach is that only one trough could be detected as a candidate from a cluster of close troughs to alleviate effects of trough re-growth and ease the allocation of shaped trough noise, as will be discussed in detail later with reference to Figs. 4 and 5.

[0040] At block 202, the method 200 clips the one or more detected troughs in the envelope signal. Then, at block 203, the method 200 shapes the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0041] In an example embodiment, the clipping at block 202 may comprise obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling, and phase determination to the one or more identified peaks of the inverted envelope signal, generating one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function. Based on this example embodiment, the shaping at block 203 may comprise subtracting a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal. In addition, the clipping and the subtracting herein may be performed reiteratively a number of times such that the one or more inverted troughs are sequentially processed according to processing capability. For example, if nine troughs have been detected and need to be shaped, then these troughs could be ranked according to the differences between their respective amplitudes and the shaping threshold. After that, in dependence on the processing capability of a processor, such as a DSP, the first three troughs which have the relatively highest amplitudes than the rest of the six troughs may be shaped first and then the next second three troughs may be shaped. Finally, the last three troughs which have the lowest amplitudes than the previous six troughs may be shaped. It can be seen that the shaping of the detected troughs herein could be done in an iterative and batch-wise manner.

[0042] In an example embodiment, the clipping the one or more detected troughs in the envelope signal at block 202 may comprise obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal, clipping each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function. Based on this example embodiment, the shaping the envelope signal at block 203 may comprise multiplying the one or more clipped troughs with the delayed version of the envelope signal.

[0043] In an example embodiment, the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed. In this way, a precise synchronization could be realized and no mismatch could arise since the time taken to clip the one or more detected troughs is taken into account when cancelling the one or more troughs from the envelope signal.

[0044] With the method 200 and its multiple extensions and variations as discussed in the example embodiments above, the PAPR of the envelope signal prior to entering into the ET supply modulator may be reduced and less knee- voltage-region gain collapse may be obtained at the output of the ET power amplifier since it is more likely to operate close to the compression region for more efficient amplification with less back-off

[0045] It should be noted that the shaping operations or functions as discussed above intend to maintain either constant gain for linearity or constant gain compression for maximum efficiency according to application scenarios. For example, if an ET PA operates in a mobile station, an energy-consuming Digital Pre-Distortion ("DPD") is generally unsuitable for providing the required linearity. Thus, a shape function for linearity may be employed to simultaneously satisfy the linearity and efficiency requirements without additional digital processing. Further, various shaping functions may be designed and selected to reduce the nonlinearity at the expense of average efficiency to a midway. For instance, one of key shaping functions is to remove near cross-zero region tracking voltage at which the RF PA tends to shut down while drain or collector voltage reaches to zero.

[0046] Fig. 3 is a diagram exemplarily illustrating inversion operations of the envelope signal according to an embodiment of the present disclosure. As discussed before with respect to the method 200, in order for easy detection of one or more troughs in the envelope signal, the example embodiments of the present disclosure propose inverting the envelope signal through the waveform inverting block as illustrated in Fig. 3. In this manner, the detection of the one or more troughs is equivalent to the detection of the one or more peaks, which is relatively easier than the former detection.

[0047] As seen from Fig. 3, the processing area of the envelope signal in the left drawing is from 0 to 2V, which is the trough threshold and may be equal to or greater than a knee voltage of the RF PA. Upon the waveform inverting, the processing area of the inverted envelope signal is changed from IV to 3V where the IV is selected as the peak threshold corresponding to the through threshold and may be used as the shaping threshold as discussed before. It should be noted that the peak threshold could be selected according to the trough threshold such that the delta values of both are the same. For example, the delta values as shown in Fig. 3 remain the same as 2 before and after the waveform inverting. Additionally, the shaping threshold may be adjusted according to the knee voltage of the RF PA such that the shaped envelope signal is more adaptive to the operations of the RF PA.

[0048] Fig. 4 is a block diagram exemplarily illustrating a method 400 for envelope shaping in envelope tracking power amplification according to one embodiment of the present disclosure. As shown in Fig. 4, the original envelope signal is split into two branches for processing. The first branch is simply a delay line which is used to synchronize the envelope waveform to the second processing branch. The second branch performs trough noise generation for cancelling troughs from the waveform in the first branch via a subtracting node 410.

[0049] At the outset, the original envelope signal, whose waveforms in the frequency and time domains are respectively shown, is received and at block 401, the waveform of the original envelope signal in the time domain is inverted. At block 403, the trough detection is performed for the inverted envelope signal. For example, a shaping threshold V'th such as the peak threshold in Fig. 3 may be used to identify one or more peaks in the inverted envelope signal whose amplitudes are higher than those of neighboring samples. The detection of the one or more peaks may be more convenient for the utilization of spectra cancellation pulse in the frequency domain to extract the trough noise, i.e., error signals representing the differences between each of the peaks and the target shaped peaks based on the shaping threshold. As depicted in the drawing at the lower left of Fig. 4, troughs (or more precisely, peaks) "a"-"k" have been detected at respective locations as indicated by lines.

[0050] Through the trough detection at block 403, two kinds of information could be obtained, wherein the first kind of information is amplitude and phase information of the original envelope signal and the second kind of information is indexes/positions of detected troughs (i.e., peaks) for clipping. Then, at block 404, the amplitude and phase information of the original envelope signal is converted into a polar format such that a polar clipping method could be used to clip the detected troughs while keeping the phase of the original envelope signal unchanged.

[0051] At block 405, amplitude scaling and phase determination could be performed for the detected troughs so that shaping window filter coefficients of a shaping window function may be obtained. For instance, the differences between the trough amplitudes and the shaping threshold are created by the scaling and rotation block 405 and then the shaping window filter coefficients of the shaping window function may be obtained after the difference being combined with the phase information.

[0052] At block 406, for the one or more indexed troughs, each of them can be allocated to respective Trough Noise Generators ("TNGs") 407, such as TNG-1, TNG-2, . . ., TNG-N as shown, to generate respective trough cancellation pulses for subsequent clipping. The amplitude and phase information of the trough cancellation pulses may be provided by the polar parameters. Then, one or more trough cancellation pulses respectively associated with the one or more inverted troughs (i.e., peaks) may be generated by multiplying each of the inverted troughs and the shaping window function. In other words, the amplitude of each of one or more trough cancellation pulses is difference between the respective each of clipped troughs with the desired amplitude and the original envelope signal and the phase is rotated by 180 degrees relative to the detected troughs. In this manner, the peaks in the inverted envelope signal may be cancelled with shaping cancellation pulses to produce the shaped envelope. The details regarding generation of the trough cancellation pluses will be discussed in detail with reference to Fig. 5.

[0053] Then, a summing node 408 may sum one or more trough cancellation pulses and then at block 409, the resulting waveform of the summed pulses, collectively referred to as "trough noise," is inverted, whose spectrum is shown in the drawing "Trough Noise Spectrum." After that, a subtracting node 410 subtracts the trough noise from the delayed version of the original envelope signal to generate a shaped envelope signal.

[0054] It is to be understood by those skilled in the art that the number of the trough noise generators 407 is dependent on the processing capability of hardware resources, such as the DSP. For example, if only three trough noise generators are supported by the DSP, then the troughs "a"-"k" may be processed in an order in which the three troughs "a"-"c" should be processed first, then the troughs "d"-"f," after that, the troughs "g"-"i," and so on until all the troughs have been processed sequentially and in an iterative and batch-wise manner.

[0055] Further, according to the above description made with reference to Fig. 4, a person skilled in the art would appreciate that the method 400 employs trough cancellation noise shaping with iterations to find troughs below the shaping threshold and creates the waveform of band-limited trough cancellation noise and subtracts it from the waveform of the original envelope signal. Further, it can be understood that the utilization of nonlinear shaping around the troughs by the method 400 may reduce the PAPR of the envelope signal and enable the ET supply modulator to work more efficiently. In additional, since the scaling and summation of a limited number of trough noise pulses may replace the computationally intensive convolution, it is possible to save a notable amount of computation resources.

[0056] Fig. 5 is a block diagram 500 exemplarily illustrating details regarding generation of one or more trough cancellation pulses according to an embodiment of the present disclosure.

[0057] Subsequent to the trough detection processing, e.g., as depicted at block 403 in the method 400, the amplitude information of the detected peaks is provided into a TNG allocator 501 by a trough indicator signal which indicates the existence of a identified trough using a Boolean expression. The number of filter taps is also provided into the TNG allocator 501 and is indicated by a variable filter numtaps to define the order of the shaping filter (i.e., a specific from of the shaping window function). Further provided into the TNG allocator 501 is the allocation spacing information, which is denoted by a variable alloc spacing to allow the TNG allocator 501 to allocate the identified trough information.

[0058] The TNG multiplexer module 502 may perform time-division multiplexing of the TNG addresses and the peak scale values, which are the amplitudes of identified peaks. Further, the TNG multiplexer module 502 may combine the allocated identified peak information altogether as one data stream. The TNG address array is used to access the cancellation pulse coefficients stored in the trough filter RAM module 503, which may consist of a dual port RAM that is used to store the cancellation pulse coefficients. The peak scale value is used to scale the coefficient based on the cancellation pulse amplitude and phase for a given peak.

[0059] The Complex MAC module 504 may perform the complex scaling of the cancellation pulse coefficients with the peak scale values so that the trough cancellation pulses could be produced. One side of the complex multiply is driven from the output of the trough filter RAM 503, and the other side is driven from the time division multiplexed peak scale values after some matching pipe delays from the output of the TNG multiplexer module 502. Then, it may accumulate the results of the time division multiplexed values to produce a single composite cancellation pulse signal. As seen from the Fig. 5, since the data has been divided into in-phase components and quadrature components, one or more trough noise pulses could be summed for the in-phase components and quadrature components, respectively.

[0060] It should be noted that the foregoing descriptions are only illustrative of one possible implementation to generate the trough noise and the example embodiments of the present disclosure should not be limited herein. [0061] Fig. 6 is a block diagram exemplarily illustrating a method 600 for envelope shaping in envelope tracking power amplification according to one embodiment of the present disclosure. First, it should be noted that the dashed box enclosing a number of blocks 601-606 may represent a "trough windowing envelope shaping" block, which may be embodied as or serve as a kind of simple Cartesian shaping in which the in-phase and quadrature components of the baseband signal (i.e., "I component data" and "Q component data") are shaped independently.

[0062] As shown in Fig. 6, similar to the method 400, the I component data and Q component data should be delayed at block 601 in a processing branch by an amount of time for processing the troughs to be detected. Then, in another processing branch, at block 602, the amplitude of the original envelope signal is inverted, such as illustrated in Fig. 3. Then, at block 603, the amplitude of the input envelope signal is compared with a shaping threshold so as to calculate and determine a shaping window function. The shaping window filter coefficients of the shaping window function may be generated by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal. The shaping window function may include but not be limited to Cosine, Hamming, Hanning, Blackman, Blackman-Harris, Kaiser, Gaussian and etc. Then, at block 604, the shaping window function may clip the inverted troughs (i.e., peaks) with a smooth windowing scaling factor by a multiplication operation. After this multiplication operation, the amplitudes of the inverted troughs are above the shaping threshold.

[0063] At block 605, the resulted waveform of clipped inverted troughs would be inverted back and restored. After that, the one or more clipped troughs would be multiplied with the delayed version of the envelope signal, i.e., I component data and Q component data, respectively, to shape the envelope signal. Then, the shaped envelope signal may be transmitted to the ET supply modulator for modulation. In an example embodiment, there may be post-shaping filtering at block 606, via which the spectral leakage caused by shaping the envelope signal could be alleviated.

[0064] According to the method 600, the shaping window coefficients may remain the same for both single and multi-carrier cases, but it may require large filter. In a practical system, smooth clipping function may be additionally applied so that the shaped envelope signal could be smoothed by the feed forward and feedback signals. The feedback structure may scale the values of the incoming signal, if necessary, and prevent over-shaping effect. This implementation may be one of the most cost effective solutions to implement the envelope shaping and reduce the PAPR of the envelope signal, which consumes very little hardware resources. Based on the notion of smooth attenuation of troughs to avoid sharp corners caused by direct hard shaping in prior arts, trough windowing envelope shaping as presented by the method 600 may produce much smaller out-of-band emissions than hard shaping in the prior art.

[0065] Further, since the multiplication in the time domain is the dual of convolution in the frequency domain, the spectral leakage of this envelope shaping approach may be controlled by the length and the type of the shaping window function used. In general, a long window in the time domain may result in better spectral performance at the expense of the increased EVM in that many signal samples are affected. Conversely, a short window may result in a lower EVM but cause more spectral leakage. Additionally, the method 600 may treat each inverted trough through a common symmetric window weighted with the height of trough below the shaping threshold, and the total shaping window function could be the superposition of the sequence of all shaping window functions.

[0066] Fig. 7 is a block diagram exemplarily illustrating a method 700 for envelope shaping in envelope tracking power amplification according to yet another embodiment of the present disclosure. It is to be understood that the method 700 is a relatively general method and as appropriate, may be embodied as the method 400 or 600 through some necessary changes. As shown in Fig. 7, upon obtaining the envelope signal at block 701 and inverting the waveform of the envelope signal at block 702, the detection of the inverted troughs could be done at block 703, which is similar to the operations of the block 403 in the method 400 and the block 603 in the method 600.

[0067] Also at block 703, by performing amplitude scaling and potential phase determination (not shown) to the detected troughs, a shaping window function may be determined for use at block 704. In particular, the shaping window function may reduce the amplitude of the envelope signal and meanwhile maintain the phase of the envelope signal for the trough noise.

[0068] At block 704, trough noise could be generated through the shaping window function as mentioned before. The trough noise herein represents the difference between the shaped envelope signal and the original one. The shaping window function may be implemented as, e.g., the Since Cartesian clipping or polar clipping, which is capable of clipping the amplitude of the envelope while preserving the phase. Take the polar clipping as an example, it may limit the amplitudes of the inverted troughs to the shaping threshold while maintaining their phases. In particular, the polar clipping may compare the amplitude squared of the inverted envelope signal to the square of the shaping threshold. When the amplitude squared of the inverted envelope signal is smaller than the threshold squared, the inverted envelope signal may pass through to the output. Otherwise, the output of polar clipping is a scaled version of the inverted envelope signal whose corresponding value may be corrected to the shaping threshold value. By selecting an appropriate shaping threshold, the minimum trough level of the shaped envelope may be maintained at a desired level.

[0069] At block 705, in order to comply with the spectral emission requirements of the ET PA, the spectrum of the trough noise may be limited by a post-processing windowing filtering in the same band. In this manner, the sharp troughs of the trough noise may be smoothed.

[0070] After that, at block 707, the waveform of the inverted envelope signal including the resulted trough noise should be inverted again and then may be subtracted from the waveform of the original envelope signal, which has been delayed through block 706. That is, at block 708, the original envelope signal may be shaped by trough cancellation. Although the subtraction operation is shown herein similar to the operations as performed by the subtracting node 410 in the method 400, the envelope shaping could also be completed by the multiplication operation such as discussed with respect to the method 600.

[0071] At block 709, further or optional post processing may be performed, if necessary, for further smoothing the shaped envelope signal and meeting the envelope shaping target, e.g., the trough level and PAPR of the envelope signal at input of an ET supply modulator, which may be determined at block 710. If the target is satisfied, then the method 700 may end at block 711. Otherwise, the flow may be iterated and therefore go back to blocks 702 and 706 for next round of processing until the target is met. The iteration herein may be similar to the iteration discussed with regards to the method 400. A person skilled in the art may understand that when the method 700 is specifically embodied as the method 600, no iteration is needed since all of the detected troughs would be processed at a time.

[0072] From the foregoing description, a person skilled in the art may understand that the example embodiments of the present disclosure may reduce the troughs and PAPR of the envelope waveform by subtracting spectrally shaped trough noise from envelope troughs below the shaping threshold. Since the clipped trough noise is designed to provide a spectrum that matches that of the input envelope signal, there would result in negligible out-of-band distortions and better control of the spectral properties of the output envelope signal. Further, by virtue of the example embodiments of the present disclosure, the EVM versus shaping ratio performance becomes better and advantageous for broadband high-PAPR signals of 4G and beyond wireless communication.

[0073] Regarding the envelope signal to be shaped by the example embodiments of the present disclosure, there may be typically two types of envelope signals, i.e., baseband envelope signals or digital intermediate frequency (IF) envelope signals which are generated by processing the baseband envelope signals through a Digital Up Converter ("DUC"), which may be placed before the apparatus of the present disclosure as discussed at block 101 in Fig. 1. Baseband envelope shaping may be achieved with better out-of-band spectrum emission performance compared to IF counterpart, but may engender worse trough level control and PAPR reduction effect because the trough re-growth phenomena after DUC and other DSP blocks would weaken the final processing results. On the other hand, the digital IF envelope shaping may prevent from trough re-growth problem by arranging envelope shaping at last so that all the troughs re-growth can be reduced by envelope shaping functions. However, digital IF envelope shaping often have worse out-of-band spectrum emissions and more complex digital filters design, especially when considering pre-distortion techniques to reserve extra bandwidth.

[0074] With the methods and its variants as discussed before in the various embodiments, the operating point of the ET PA on the drain-side may be stabilized since the PA supply would not be changed from low to high instantaneously and dynamically or vice-versa due to the shaped envelope signal and therefore distortions and memory effects could also be diminished or eliminated. Further, in view of the fact that the efficiency of an ET supply modulator is a function of the transmitted envelope and high PAPR envelope signal imposes significant operating restrictions on the ET supply modulator, based on the shaping operations or functions as discussed above, the PAPR of the shaped envelope signal could be reduced to a target value for ET supply modulator efficiency performance without heavily backing off from its most efficient operating point.

[0075] In other words, in order not to degrade Error Vector Magnitude ("EVM") and Adjacent Channel Power Ratio ("ACPR") due to serious distortions and increase the efficiency of the ET supply modulator, the envelope shaping function or algorithm as set forth before could be used in a Digital Front End ("DFE") to reduce the PAPR of the envelope signal before entering the ET supply modulator, thereby allowing additional gains and more efficient amplification at the output of the ET supply modulator since it may operate closely on the compression region with less back-off for more efficient amplification.

[0076] Fig. 8 is a diagram exemplarily illustrating probability distribution function of the shaped envelope signal versus the output supply voltage of the ET supply modulator with typical high-PAPR envelopes as stimulus.

[0077] As illustrated in Fig. 8, the horizontal axis is the output voltage of the ET supply modulator and the vertical axis is Probability Distribution Function ("PDF") of the envelope signal. The envelope signal without shaping operations according to the example embodiments is distributed into low voltage region centered with -13 V. The PDF corresponding to this non-shaping envelope signal is much lower and narrower than the shaped ones, each of which is associated with a respective PAPR. However, the shaped versions of envelope waveforms are approaching to average output voltage of the ET supply modulator as shown by a histogram chart. From the ET PA perspective, much flatter response of the PDF, as those shaped, over the operation voltages could provide much better linearity due to the PldB point moving into high power level dynamically. In other words, the ET PA could provide more efficient power conversion with the same linearity metrics. Take transistor cost into consideration, the ET supply modulator operation with the shaped envelopes may cover higher voltages with higher probabilities, which may draw out more power from the RF power transistor than the lower voltage operations of the ET PA. Therefore, based on the depiction of the Fig. 8, a person skilled in the art may appreciate that the example embodiments of the present disclosure may be capable of further optimizing ET PA performance.

[0078] Fig. 9 is a simplified schematic block diagram illustrating a representative apparatus 900 according to an embodiment of the present disclosure. As illustrated in Fig. 9, the apparatus 900 includes at least one processor 901, such as a data processor, at least one memory (MEM) 902 coupled to the processor 901, and a suitable RF transmitter TX and receiver RX 903 coupled to the processor 901. The MEM 902 stores a program (PROG) 904. The TX/RX 903 is for bidirectional wireless communications.

[0079] The PROG 904 is assumed to include instructions that, when executed by the processor 901, enable the apparatus 900 to operate in accordance with the exemplary embodiments of the present disclosure, as discussed herein with the methods 200, 400, 600, and 700. For example, the apparatus 900 may be embodied as a terminal device or a part thereof when the example embodiments of the present disclosure are carried out in the terminal device, such as a mobile station.

[0080] In general, embodiments of the present disclosure may be implemented by computer software executable by at least one processor 901 of the apparatus 900, or by hardware, or by a combination of software and hardware.

[0081] The MEM 902 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory, as non-limiting examples. While only one MEM is shown in the apparatus 900, there may be several physically distinct memory units in the apparatus 900. The processor 901 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multicore processor architecture, as non limiting examples. The apparatus 900 may have multiple processors, such as for example an application specific integrated circuit chip that is slaved in time to a clock which synchronizes the main processor.

[0082] In one aspect of the present disclosure, the apparatus 900 may comprise at least one processor and at least one memory including compute program instructions, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 at least to detect, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The at least one memory and computer program instructions are configured to, with the at least one processor, also cause the apparatus 900 at least to clip the one or more detected troughs in the envelope signal. The at least one memory and computer program instructions are configured to, with the at least one processor, further cause the apparatus 900 at least to shape the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0083] In an example embodiment, the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

[0084] In another example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to invert the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks, compare the one or more peaks with the shaping threshold, and identify one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing.

[0085] In yet another example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to identify, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal.

[0086] In an example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to obtain shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal, generate one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function, and subtract a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal.

[0087] In another example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to reiteratively perform the clipping and the subtracting a number of times such that the one or more inverted troughs are sequentially processed according to processing capability. [0088] In yet another example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to identify, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold.

[0089] In a further example embodiment, the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus 900 further to obtain shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal, clip each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function, and multiply the one or more clipped troughs with the delayed version of the envelope signal.

[0090] In an additional example embodiment, the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed.

[0091] Fig. 10 is a block diagram exemplarily illustrating RF signal amplification processing including the apparatus 1000 according to various embodiments of the present disclosure. As illustrated in Fig. 10, the apparatus 1000 comprises means 1001 for detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator. The apparatus 1000 also comprises means 1002 for clipping the one or more detected troughs in the envelope signal. The apparatus 1000 further comprises means 1003 for shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

[0092] In an example embodiment, the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

[0093] In another example embodiment, the means 1001 for detecting comprises means for inverting the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks, means for comparing the one or more peaks with the shaping threshold, and means for identifying one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing.

[0094] In yet another example embodiment, the means for identifying comprises means for identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal.

[0095] In an additional example embodiment, the means 1002 for clipping comprises means for obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of inverted envelope signal, means for generating one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function, and means 1003 for the shaping the envelope signal further comprises subtracting a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal.

[0096] In a further example embodiment, the means for clipping and the subtracting are performed reiteratively a number of times such that the one or more inverted troughs are sequentially processed according to processing capability.

[0097] In an example embodiment, the means for identifying comprises means for identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold.

[0098] In another example embodiment, the means 1003 for clipping comprises means for obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal, means for clipping each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function, and the means 1003 for shaping further comprises means for multiplying the one or more clipped troughs with the delayed version of the envelope signal.

[0099] In yet another example embodiment, the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed.

[00100] It is to be understood that the apparatus 1000 is able to carry out the methods as discussed with respect to the accompanying drawings according to the embodiments of the present disclosure and may be embodied as another form of a terminal device, an ET PA or a part thereof.

[00101] The techniques described herein may be implemented by various means so that an apparatus implementing one or more functions of a corresponding mobile entity described with an embodiment comprises not only prior art means, but also means for implementing the one or more functions of a corresponding apparatus described with an embodiment and it may comprise separate means for each separate function, or means may be configured to perform two or more functions. For example, these techniques may be implemented in hardware (one or more apparatuses), firmware (one or more apparatuses), software (one or more modules), or combinations thereof. For a firmware or software, implementation can be through modules (for example, procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in any suitable, processor/computer-readable data storage medium(s) or memory unit(s) or article(s) of manufacture and executed by one or more processors/computers. The data storage medium or the memory unit may be implemented within the processor/computer or external to the processor/computer, in which case it can be communicatively coupled to the processor/computer via various means as is known in the art.

[00102] Many modifications and other embodiments of the disclosures set forth herein will come to mind to one skilled in the art to which these embodiments of the disclosure pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the embodiments of the disclosure are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

WAHT IS CLAIMED IS:

1. A method, comprising:

detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator;

clipping the one or more detected troughs in the envelope signal; and

shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

2. The method according to claim 1, wherein the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

3. The method according to claim 1, wherein the detecting the one or more troughs in the envelope signal comprises:

inverting the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks;

comparing the one or more peaks with the shaping threshold; and

identifying one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing.

4. The method according to claim 3, wherein the identifying comprises:

identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal.

5. The method according to claim 4, wherein the clipping the one or more detected troughs in the envelope signal comprises:

obtaining shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal;

generating one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function; and

the shaping the envelope signal comprises:

subtracting a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal.

6. The method according to claim 5, wherein the clipping and the subtracting are performed reiteratively a number of times such that the one or more inverted troughs are sequentially processed according to processing capability.

7. The method according to claim 3, wherein the identifying comprises:

identifying, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold.

8. The method according to claim 7, wherein the clipping the one or more detected troughs in the envelope signal comprises:

clipping each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function; and

the shaping the envelope signal comprises:

multiplying the one or more clipped troughs with the delayed version of the envelope signal.

9. The method according to any of claims 1-8, wherein the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed.

10. An apparatus, comprising:

at least one processor; and

at least one memory including compute program instructions,

wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus at least to:

detect, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator;

clip the one or more detected troughs in the envelope signal; and

shape the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

11. The apparatus according to claim 10, wherein the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

12. The apparatus according to claim 10, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to:

invert the envelope signal in a time domain such that one or more troughs to be detected are inverted into one or more peaks;

compare the one or more peaks with the shaping threshold; and

identify one or more peaks of the inverted envelope signal as the one or more troughs based on the comparing.

13. The apparatus according to claim 12, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to:

identify, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes have the highest amplitudes greater than the shaping threshold in one or more specified scope of the envelope signal.

14. The apparatus according to claim 13, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to:

obtain shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of the inverted envelope signal;

generate one or more trough cancellation pulses respectively associated with the one or more inverted troughs by multiplying each of the inverted troughs and the shaping window function; and

subtract a sum of the one or more trough cancellation pulses from the delayed version of the envelope signal.

15. The apparatus according to claim 14, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to reiteratively perform the clipping and the subtracting a number of times such that the one or more inverted troughs are sequentially processed according to processing capability.

16. The apparatus according to claim 14, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to:

identify, as the one or more troughs, one or more peaks of the inverted envelope signal whose amplitudes are greater than the shaping threshold.

17. The apparatus according to claim 16, wherein the at least one memory and computer program instructions are configured to, with the at least one processor, cause the apparatus further to:

clip each of the one or more inverted troughs by multiplying each of the one or more inverted troughs with the windowing filtering function; and

multiply the one or more clipped troughs with the delayed version of the envelope signal.

18. The apparatus according to any of claims 10-17, wherein the delayed version of the envelope signal is obtained by delaying the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed.

19. An apparatus, comprising:

means for detecting, based on a shaping threshold, one or more troughs in an envelope signal representing an envelope of a signal to be amplified by an envelope tracking power amplifier connected with an envelope tracking supply modulator; means for clipping the one or more detected troughs in the envelope signal; and means for shaping the envelope signal for the envelope tracking supply modulator by processing a delayed version of the envelope signal with the one or more clipped troughs.

20. A non-transitory computer readable medium having program code stored thereon, the program code configured to direct an apparatus, when executed, to:

clip the one or more detected troughs in the envelope signal; and

21. The non-transitory computer readable medium according to claim 20, wherein the shaping threshold is adjusted based on a knee voltage level of the envelope tracking power amplifier.

22. The non-transitory computer readable medium according to claim 20, wherein the program code is configured to further direct the apparatus, when executed, to:

compare the one or more peaks with the shaping threshold; and

23. The non-transitory computer readable medium according to claim 22, wherein the program code is configured to further direct the apparatus, when executed, to:

24. The non-transitory computer readable medium according to claim 23, wherein the program code is configured to further direct the apparatus, when executed, to:

obtain shaping window filter coefficients of a shaping window function by performing polar conversion, amplitude scaling and phase determination to the one or more identified peaks of inverted envelope signal;

25. The non-transitory computer readable medium according to claim 24, wherein the program code is configured to further direct the apparatus, when executed, to:

perform the clipping and subtracting reiteratively a number of times such that the one or more inverted troughs are sequentially processed according to processing capability.

26. The non-transitory computer readable medium according to claim 22, wherein the program code is configured to further direct the apparatus, when executed, to:

27. The non-transitory computer readable medium according to claim 26, wherein the program code is configured to further direct the apparatus, when executed, to:

28. The non-transitory computer readable medium according to any of claims 20-27, wherein the program code is configured to further direct the apparatus, when executed, to:

delay the envelope signal by a period of time during which the clipping of the one or more detected troughs in the envelope signal is performed to obtain the delayed version of the envelope signal.