Method and apparatus for multiple-output partial envelope tracking in handheld wireless computing devices

A partial envelope tracking (PET) circuitry for improving the dynamic range of a plurality of power amplifiers amplifying radio frequency signals in a MIMO based handheld wireless computing device. The circuitry includes a plurality of sub-PET circuits respectively connected to the plurality of power amplifiers; and a common charging circuit connected to each of the sub-PET circuits and a power source, wherein the common charging circuit comprises a storage capacitor and a logic configured to control the charging of the storage capacitor respective of an operation mode of each of the sub-PET circuits, wherein the operation mode is any one of: a tracking mode and normal mode, wherein during the normal mode of all of the sub-PET circuits the storage capacitor is charged at the voltage level provided by the power source and during the tracking mode of at least one of the sub-PET circuits the storage capacitor is discharged.

TECHNICAL FIELD

This disclosure relates to the field of power amplifiers, and more particularly to techniques for efficient partial envelope tracking in handheld wireless computing devices.

BACKGROUND

Envelope tracking (ET) is a known technique for improving the efficiency of power amplifiers. In a conventional implementation of an envelope tracking technique, a voltage signal at the drain input of a radio frequency (RF) power amplifier (PA) is varied to be proportional to the envelope of a RF signal. Tracking is performed in order to match the dynamic range of the supply voltage of the power amplifier to the instantaneous requirements of the RF signal envelope.

A subclass of the ET technique is partial envelope tracking (PET). A conventional PET circuit replaces the tracking below a certain envelope voltage level by providing a constant voltage of the power supply to the drain input of the RF PA. A conventional PET circuit also tracks the envelope peaks above that voltage level. One of the advantages of a PET technique is low power consumption and, specifically, low power consumption of a power supply when tracking signals with strongly varying envelopes. Examples for such signals include long-term evolution (LTE) signals, wireless local area network (WLAN) signals, and the like.

Advanced wireless and handled devices designed for LTE and WLAN communications are based on multi-input-multi-output (MIMO) architectures. In such architectures, the information (input) signal is distributed and transmitted via several transmit channels.

As illustrated inFIG. 1, each transmit channel includes a power amplifier110connected to an antenna120. The antennas120-1through120-M form the MIMO architecture. In order to allow the partial envelope tracking of transmitted RF signals, each power amplifier110is connected to a PET circuity that includes a voltage enhancement circuitry (VEC)140and an excess envelope calculator (EEC) circuit130. For example, a power amplifier110-1is connected to a VEC140-1and VEC140-1is connected to EEC130-1.

Generally, each EEC130, e.g. EEC130-1, determines the mode of operation of the respective VEC140, e.g. VEC140-1, based on an access of the input signal (el). The operation may be either a tracking mode or a normal mode. The tracking mode is active when the envelope of the transmitted signal e1is above a predefined threshold. In this mode, the voltage signal VDprovided to the power amplifier is continuously adapted according to the changes of the envelope of the transmitted signal in order to match the required dynamic range of the power amplifier. In the normal mode, the voltage signal VSis provided to the power amplifier from the power source150.

Referring toFIG. 2where the operation of the PET circuity including the VEC140is shown, it should be noted that the description provided with reference toFIG. 2is applicable for each VEC140shown inFIG. 1. In a conventional implementation, the VEC140includes a main valve (MV)142connected to the power amplifier (RF AMP)110and the power source150that outputs a voltage signal (VS) filtered by a capacitor (CS). The VEC140further includes an envelope tracking function realized by a tracking unit145that includes at least a tracking valve (TV)145-1connected to a feedback resistor (RFB) and a linear feedback amplifier (FB AMP)145-2.

The VEC140also includes a diversion valve (DV)147which is connected in series to a grounding valve (GV)148. In a conventional implementation, a Voltage Control Unit (VCU)141activates normal mode when the envelope ‘e’ of a transmitted RF signal is below a predefined voltage threshold. The tracking mode is active when the envelope ‘e’ of the transmitted signal is above the predefined voltage threshold. In this mode, the voltage signal provided to the power amplifier is continuously adapted according to the changes of the envelope of the transmitted signal in order to match the required dynamic range of the power amplifier.

Also connected in the VEC140is a storage capacitor CT144, which, together with DV147, allows a smooth transition between the normal and tracking mode. Specifically, during the normal mode, the storage capacitor144is charged at the voltage level provided by the power source and during the tracking mode the storage capacitor144is discharged. When the VEC140switches from the normal mode to the tracking mode, a source of a drain current to the power amplifier110switches from the power source150to a current path through the DV147and the storage capacitor144. To minimize voltage fluctuations of the drain voltage VD provided to the power amplifier, the capacitance of the storage capacitor CT144is large, typically several microfarads. Therefore, the size of the capacitor in terms of area is also large.

Consequently, whether the storage capacitor is implemented as a discrete component or as part of an integrated circuit (IC), the size of the storage capacitor144is relatively big. This problem is magnified when more than one storage capacitor is required in a system that includes more than one transmit channel (e.g., the system shown inFIG. 1). For example, for a MIMO-based system with 4 transmit channels, 4 storage capacitors are required.

As conventional PET circuits are large in size and primarily designed to support power amplifiers that are stand-alone modules, such circuits cannot provide efficient solutions for MIMO-based systems. In particular, such conventional PET circuits cannot be efficiently utilized in handheld wireless devices, for example, smartphones and tablet computers in which the size is a critical constraint.

It would therefore be advantageous to provide a PET solution that would overcome the deficiencies noted above and be efficiently implemented in MIMO-based wireless handheld devices.

SUMMARY

Certain embodiments disclosed herein include a partial envelope tracking (PET) circuitry for improving the dynamic range of a plurality of power amplifiers amplifying radio frequency (RF) signals in a multi-input-multi-output (MIMO) based handheld wireless computing device. The circuitry comprises a plurality of sub-PET circuits respectively connected to the plurality of power amplifiers; and a common charging circuit connected to each of the plurality of sub-PET circuits and a power source, wherein the common charging circuit comprises a storage capacitor and a logic configured to control the charging of the storage capacitor respective of an operation mode of each of the plurality of sub-PET circuits, wherein the operation mode is any one of: a tracking mode and a normal mode, wherein during the normal mode of all of the plurality of sub-PET circuits the storage capacitor is charged at the voltage level provided by the power source and during the tracking mode of at least one of the plurality of sub-PET circuits the storage capacitor is discharged.

Certain embodiments disclosed herein also include a wireless computing device. The device comprises a radio frequency transmitter including a multi-input-multi-output (MIMO) module including a plurality of power amplifiers connected to a plurality of antennas, wherein the radio frequency transmitter is configured to transmit radio frequency (RF) signals; a partial envelope tracking (PET) circuity for improving the dynamic range of the plurality of power amplifiers amplifying RF signals, including: a plurality of sub-PET circuits respectively connected to the plurality of power amplifiers; and a common charging circuit connected to each of the plurality of sub-PET circuits and a power source, wherein the common charging circuit comprises a storage capacitor and a logic configured to control the charging of the storage capacitor respective of an operation mode of each of the plurality of sub-PET circuits, wherein the operation mode is any one of: a tracking mode and a normal mode, wherein during the normal mode of all of the plurality of sub-PET circuits the storage capacitor is charged at the voltage level provided by the power source and during the tracking mode of at least one of the plurality of sub-PET circuits the storage capacitor is discharged.

Certain embodiments disclosed herein also include a method for improving the dynamic range of a plurality of power amplifiers amplifying radio frequency (RF) signals in a multi-input-multi-output (MIMO) based wireless computing device. The method comprises providing a plurality of sub-PET circuits respectively connected to the plurality of power amplifiers; and providing a common charging circuit connected to each of the plurality of sub-PET circuits and a power source, wherein the common charging circuit comprises a single storage capacitor and a logic; monitoring a plurality of binary envelope tracking parameters to determine an operation mode of the common charging circuit, wherein the operation mode is any one of: a tracking mode and a normal mode; and controlling the charging of the single storage capacitor respective of the determined operation mode, wherein during the normal mode the storage capacitor is charged at the voltage level provided by the power source and during the tracking mode the storage capacitor is discharged.

DETAILED DESCRIPTION

FIG. 3is an exemplary and non-limiting block diagram of a partial envelope tracking (PET) circuity300utilized in MIMO-based systems according to one embodiment. The PET circuity300can be integrated in any handheld wireless computing device including, but not limited to, a smartphone, a tablet computer, a laptop computer, a notebook computer, a wearable computing device, and the like. Such a handheld wireless computing device receives and transmits radio frequency (RF) modulated signals, for example, by means of a RF receiver/transmitter. Such signals can be, but are not limited to, single carrier modulated signals, multi-carrier modulated signals, signals from the Orthogonal Frequency-Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple-Access (OFDMA) families, and the like. The PET circuit300can process signals of wireless communication protocols including, but not limited to, 3G, LTE, LTE Advanced, IEEE 802.11 ac, IEEE 802.11 ax, and so on.

According to the disclosed embodiment, the PET circuity300provides an efficient PET solution for MIMO-based systems as only one storage capacitor is utilized to provide supplemental drain current to each of the M transmit channels when required. That is, instead of implementing a PET circuit with M storage capacitors (M>1), the PET circuity300includes only a single storage capacitor.

Additionally, as will be discussed in detail below, the design and the topology of the various components in the PET circuity300enables smooth transitions between normal and tracking modes in each of the M transmission paths. Smooth transitions are of high importance for ensuring and maintaining spectral purity in wireless communication systems that transmit and receive signals with very high bandwidth for information transmission. In an exemplary embodiment, a “very high bandwidth” is higher than 50 Mbps.

To this end, the PET circuity300includes a common charging circuit310and a number of M of sub-PET circuits320-1through320-M (M is an integer greater than 1). In an embodiment, each sub-PET circuit320-i (i=1, . . . , M) is connected to a different and independent transmit channel, that is, to a different power amplifier301and a corresponding antenna302. As will be discussed below, none of the sub-PET circuits320-1through320-M includes a storage capacitor as this function/element is implemented by the common storage capacitor316in the common charging circuit310. A power source305feeds the power amplifiers301-1through301-M in the normal mode of operation of each of the sub-PET circuits.

Specifically, according to an embodiment, the common charging circuit310includes logic OR and NOT gates311and312, respectively, three valves313,314,315, and a storage capacitor316. The power source305is coupled to the valves313and315. The storage capacitor316is also connected to each sub-PET circuit320-i (i=1, . . . , M). In practice, the storage capacitor316is connected to a Diversion valve (DV) of each VEC (seeFIG. 4). In one implementation, the valves are field-effect transistors (FETs).

An EEC330-i outputs envelope tracking parameters viand Vi(i=1, . . . , M). In an embodiment, the parameter viis an analog voltage equal to the excess of the signal envelope (ei) over the CD-threshold. That is, the parameter viis determined as follows:vi=ei−CD-threshold; when ei≧CD-thresholdvi=0; when ei<CD-threshold

The tracking control parameter Viis a binary value determined as follows:Vi=“0” when vi=0; andVi=“1” when vi>0

The CD-threshold is a voltage level at which the current diversion operation is activated, when the envelope of the signal is crossing this threshold level. The CD-threshold can be either fixed during a transmission session, or may vary during a transmission session according to the average power level of the transmitted signal during the session. Also, the CD-threshold for each of the sub-PET circuits320-i (i=1, . . . , M) can be set to a different value.

The tracking mode of the common charging circuit310is active when at least one Viparameter equals to “1”. That is, when an envelope of one or more of the transmitted signals 1, 2, . . . , M is above its CD-threshold. The normal mode is active when all of the Viparameters equal to “0”. The Vi(i=1, . . . , M) are used to generate the control signals N andNof the common charging circuit310. Specifically, the control signals N andNare utilized to control the operation of the valves313,314, and315.

In a preferred embodiment, the control signals N andNare set to binary values respective of the mode of operation of each VEC340-i. The control signal N equals to ‘0’ if and only if each VEC340-i (i=1, 2, . . . , M) operates in the normal mode. The control signal N equals to ‘1’ if at least one VEC340-i (i=1, 2 . . . , M) operates in the tracking mode. The tracking and normal mode of each VEC340-i are determined respective of the Viparameters as discussed above. The control signalNis the opposite of the N signal. Thus, the control signalNequals to 0 if at least one of the VEC340-i is in the tracking mode and N=1 if all of the VECs340-i are in the normal mode.

Each VEC340-i (i=1, 2, . . . , M) includes a main supply path having a main valve (MV) connecting the respective power amplifier301-i to the power source305. The VEC340-i further includes a supplementary supply path including a diversion valve (DV) and a tracking valve (TV) connected, in parallel, between the power source and a power amplifier301-i. The structure and operation of a VEC340-i are discussed in greater detail with reference toFIG. 4.

At one end, the storage capacitor316is connected to the DV (not shown inFIG. 3) of each of the VEC340-i while at the other end is connected to the valve314. The storage capacitor CT316discharges during the tracking mode (i.e., when N=1) of one or more of the VEC340-i and recharges during the normal mode periods.

Specifically, during a normal mode of the common charging circuit310, the control signal N equals 0 (N=0). As a result, the valves314and315are fully conducting and valve313is blocked. This would cause charging the storage capacitor316to a voltage level of VSprovided by the power source305. During the tracking mode, the values of N andNare switched, that is, N=1 andN=0. As such the valves314and315are blocked and the valve313fully conducts. This would cause discharging the storage capacitor316through the DV of each VEC340-i (i=1, . . . , M) operating in a tracking mode at that time.

Specifically, in such a case of a VEC340-i operating in the tracking mode, a series connection of the power supply305with the storage capacitor CT316and the valve DV of a VEC340-i is formed. As a result, the effective voltage supplied to a respective power amplifier301-i during tracking mode is up to 2 times VSless the voltage level drops on the valve DV-i of a VEC340-i. When the valve DV-i conducts (i.e., VEC340-I is in tracking mode), the valve DV supplies current IDthat is approximately equal to the nominal current IS. The current ISis supplied by the power source305through the respective MV-i valve of a VEC340-i in normal mode. At the end of the tracking mode, i.e., when N switches to 0 again, the storage capacitor316is recharged to the voltage level VSvia the valve313.

Therefore, it should be appreciated that when switching to tracking mode, a valve DV-i of a VEC340-i (i=1, . . . , M) immediately supplies current IDthat is approximately equal to the nominal current ISsupplied by the power source305through the respective MV-i valve in normal mode, thus a smooth switching between the two modes is achieved. This is crucial in communication systems comprising high bandwidth signals, where the frequency of the switching between modes is high, in order to maintain spectral purity of the transmitted signal.

In an embodiment, the capacitance of the storage capacitor316is determined as a function of the number (M) of transmit channels. In one embodiment, the capacitance of the storage capacitor316is M times the capacitance of a storage capacitor required in a PET circuit of a system with only one transmit channel (i.e. M=1). In this case, the storage capacitor316is capable of handling M simultaneous tracking events (one in each transmission path), as to enable supply of enough current to each of the M sub-PET circuits.

In another embodiment, the capacitance of the storage capacitor316is a function of both M, as well as the average number of simultaneous tracking events. For example, if on the average, there are L (L<M) simultaneous tracking events per unit time (i.e., L out of the M transmission paths are in tracking mode simultaneously), the capacitance of the storage capacitor316is L times the capacitance of a storage capacitor required in a PET circuit of a system with only one transmission path (i.e. M=1).

In an embodiment, the functionality of the PET circuity300can be implemented in an integrated circuit (IC) of a RF module of a wireless and/or handheld computing device. The other components are passive electrical components that are relatively small in size, easy to implement, and inexpensive. Thus, the disclosed PET circuity300can be advantageously implemented in handheld wireless devices.

FIG. 4shows an exemplary and non-limiting block diagram of a VEC400implemented according to one embodiment. The VEC400may be any of the VEC's340-i discussed with reference toFIG. 3. The VEC400is dynamically designed to match the dynamic range of the supply voltage of a power amplifier301to the instantaneous requirements of the RF signal envelope.

The VEC400includes a valves control unit (VCU)410, a main valve (MV)420connected to a power amplifier (RF AMP)301, and a power source305that outputs a voltage signal (VS) filtered by a capacitor (CS). It should be noted that the power source305is typically the power source of the device in which the PET circuity300operates. The envelope tracking function is realized by a tracking unit that includes at least a tracking valve (TV)430connected to a feedback resistor (RFB)441and a linear feedback amplifier (FB AMP)442. In an embodiment, the power amplifier301is implemented in an RF module of the wireless handheld device.

The VEC400also includes a diversion valve (DV)450which is connected to the common charging circuit310. The operation of the MV420and the DV450is controlled by the VCU410. In one implementation, the MV420and the DV450are field-effect transistors (FETs).

In an embodiment, the VCU410operates in two different modes: normal and tracking. As described in detail above, the normal mode is active when the envelope of a transmitted RF signal is below the CD-threshold and the tracking mode is active when the envelope of a transmitted RF signal is above the CD-threshold. Specifically, the VCU410outputs two control signals Q andQrespective of the tracking parameter Vi. As illustrated inFIG. 4, the tracking parameter Viis input to the VCU410, which activates one of the modes respective thereof. The control signals Q andQdetermine the states of the MV420and the DV450during the normal and tracking modes.

Specifically, in the normal mode of operation, the Viparameter equals “0” and the Q signal is ‘on’ (e.g., Q is set to a logic value “1”, a high voltage level, and so on) andQis ‘off’ (e.g.,Qis set to a logic value of “0”, a low voltage level, and so on). As a result, the MV420fully conducts and the DV450is blocked during the normal mode of operation. In addition, as the voltage level of the analog envelope tracking parameter vi(input to FB AMP422) is0during a normal mode, the TV430is also blocked. Therefore, in the normal mode of operation, the power amplifier301is driven by the current iMand the drain voltage vDis equal to the supply voltage VSprovided by the power source305.

In the tracking mode of operation, the binary envelope tracking parameter Viequals “1”, and the control voltages for Q andQare ‘off’ and ‘on,’ respectively. As a result, the MV420is blocked and the DV450conducts during the tracking mode of operation. As a result, the current ICTprovided by the storage capacitor (316,FIG. 3) flows through the DV450.

In an embodiment, the DV450is biased such that, when the DV450conducts, the DV450supplies current iDthat is approximately equal to a nominal current IS. The nominal current ISis supplied by the power source305through the MV420to the drain input of power amplifier301. The nominal current ISis required by the power amplifier301immediately before the transition to the tracking mode occurs. In the normal mode of operation, a drain voltage (VD) of the power amplifier301equals the voltage signal (VS) supplied by the power source305.

The design of the VEC400ensures that when the control signal Q is “off”, the MV420is blocked and its output iMis 0 (iM=0). When switching to the tracking mode, the DV440replaces the MV420in supplying the nominal current ISrequired for the optimal operation of the RF amplifier301. Specifically, during the tracking mode the current Idrainsupplied to the RF amplifier301is the sum of the current (iD) provided by DV450and the tracking current (iT) provided by the TV430. That is, Idrain=iD+iT.

As noted above, the drain current idrainis the sum of iD+iT. The tracking current iTis proportional to the analog envelope tracking parameter vi. Thus, the current idrainincreases vDto be proportional to any excess of the analog envelope tracking parameter Vi. When turning back to the normal mode, vi=0 and Vi=1, the tracking capacitor CT316is recharged back to VSand is ready for the next tracking session when required.

It should be appreciated that during the normal mode and at the moment of transition from one mode to another, the drain voltage vDequals to VSsupplied by the power source305.

FIG. 5illustrates the various signals illustrating the operation of the PET circuit300with only two transmit channels (M=2). Graphs501and502are an example of an instantaneous envelopes ‘e’ and ‘e2’ of two transmitted signals with respect to the CD-threshold. The graphs503and504illustrate the drain voltage signals vD1and vD2, respectively. The voltages vD1and vD2are the drain voltages of the power amplifiers connected in each transmit channel (e.g., PA301-1and301-2,FIG. 3). The graphs505and506show transition between the normal mode and the tracking mode of two different VEC's (e.g., VEC340-1and340-2) while the graph507shows the control signal N of the common charging circuit310.

In this example, during a time period T1, when the envelope e1(501) is above the CD-threshold, then the envelope e2(502) is below the CD-threshold. As such, one VEC, tracking the signal e1, is in a tracking mode (Q1=1) and the second VEC, tracking the signal e2, is in a normal mode (Q2=0). Therefore, the control signal N is also in a tracking node (N=1). As illustrated in graph503, the drain voltage VD1is propositional to the excess of the envelope signal ‘e1’. When the envelope signal e1of the transmitted signal falls again below the CD-threshold, VD1is set to its respective value as of immediately prior to switching to tracking mode, i.e., VD1equals to VS.

During a time period T2, both VECs are in their tracking mode as the respective envelope signals e1and e2are over the CD-threshold. In this case, Q1=1 and Q2=1. As shown inFIG. 5, each drain voltage VD1and VD2is proportional to its respective envelope signals e1and e2. During a time period T3, only the second VEC is in its tracking mode (Q1=0 and Q2=1). Therefore, only VD2is proportional to its respective e2while VD1is set to VS.