Abstract:
A transmitter includes a Power Amplifier (PA), an antenna, at least one passive component and control circuitry. The PA is controlled by a PA control voltage, is operative to amplify a Radio Frequency (RF) signal and has input and output amplifier terminals. The passive component has an input component terminal coupled to the output amplifier terminal of the PA and an output component terminal coupled to the antenna. The control circuitry is configured to determine an interim power level at the output amplifier terminal that causes the signal at the output component terminal to have a target output power level, to determine, based on the interim power level, a given PA control voltage that makes the interim power level producible by the PA, so that the signal at the output component terminal has the target output power level, and to apply the given PA control voltage to the PA.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application 61/245,235, filed Sep. 23, 2009, and U.S. Provisional Patent Application 61/255,812, filed Oct. 28, 2009, whose disclosures are incorporated herein by reference. This application is related to a U.S. patent application entitled “High-Accuracy Transmit Power Control with High-Efficiency Power Amplifier Operation,” Ser. No. 12/772,212, filed on even date, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to Radio Frequency (RF) transmitters, and particularly to methods and systems for controlling Power Amplifiers (PAs) in RF transmitters. 
     BACKGROUND 
     Various communication transmitters modify the power of transmitted signals, for example in order to adapt to current channel conditions. Such techniques are commonly referred to as transmit power control. For example, Technical Specification Group (TSG) RAN WG4 of the 3 rd  Generation Partnership Project (3GPP) specifies the transmission and reception characteristics of Universal Mobile Telecommunications System (UMTS) User Equipment (UE) in a specification entitled “UE Radio Transmission and Reception (FDD),” TS 25.101, version 8.5.1, January, 2009, which is incorporated herein by reference. In particular, section 6.5 specifies the accuracy and timing requirements of output power setting in UMTS UEs. Conformance test procedures for verifying compliance of UMTS UEs with power control specifications are defined in a 3GPP Technical Specification entitled “Terminal Conformance Specification; Radio Transmission and Reception (FDD) (Release 6),” TS 34.121, version 6.4.0, March, 2006, which is incorporated herein by reference. UMTS compliant transmitters need to be compliant with these specifications. 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     SUMMARY 
     An embodiment that is described herein provides a transmitter, which includes a Power Amplifier (PA), an antenna, at least one passive component and control circuitry. The PA is controlled by a PA control voltage, is operative to amplify a Radio Frequency (RF) signal and has input and output amplifier terminals. The passive component has an input component terminal coupled to the output amplifier terminal of the PA and an output component terminal coupled to the antenna. The control circuitry is configured to determine an interim power level at the output amplifier terminal that causes the signal at the output component terminal to have a target output power level, to determine, based on the interim power level, a given PA control voltage that makes the interim power level producible by the PA, so that the signal at the output component terminal has the target output power level, and to apply the given PA control voltage to the PA. 
     In some embodiments, the transmitter includes a digital transmission chain, which provides the input amplifier terminal of the PA with the signal and has an adjustable gain, and the control circuitry is configured to cause the signal at the output amplifier terminal, while the given PA control voltage is applied to the PA, to have the interim power level. In an embodiment, the control circuitry is configured to measure the signal at the output amplifier terminal and to set the adjustable gain based on the signal measured at the output amplifier terminal. 
     In a disclosed embodiment, the control circuitry is configured to store calibration data that is indicative of insertion losses of the passive component at respective output power levels of the PA, to store characterization data that specifies pre-characterized PA control voltages at the respective output power levels of the PA, and to determine the interim power level and the given PA control voltage by querying the calibration data and the characterization data. In an embodiment, the calibration data and the characterization data apply to reference operating conditions, and the control circuitry is configured to calculate the interim power level and the given PA control voltage for actual operating conditions that are different from the reference operating conditions. In an embodiment, the control circuitry is configured to calculate the interim power level and the given PA control voltage for actual reference temperature, frequency and signal modulation that are different from respective reference temperature, frequency and signal modulation. In another embodiment, the control circuitry is configured to determine the interim power level responsively to a pre-characterized PA control voltage that is mapped to the target output power level in the characterization data. 
     In some embodiments, the control circuitry is configured to further adjust the given PA control voltage based on a modulation scheme used in modulating the signal. In an embodiment, the control circuitry is configured to determine the given PA control voltage based on the interim power level when the target output power level is in a first power range, and to determine the given PA control voltage based on an open-loop characterization when the target output power level is in a second power range, at least partially lower than the first power range. In another embodiment, the control circuitry is configured to set an input power level of the signal at the input amplifier terminal using a closed-loop mechanism when the target output power level is in a first power range, and is configured to set the input power level of the signal at the input amplifier terminal using an open-loop mechanism when the target output power level is in a second power range, at least partially lower than the first power range. 
     An additional embodiment provides a mobile communication terminal that includes the disclosed transmitter. Yet another embodiment provides a chipset for processing signals in a mobile communication terminal, including the disclosed transmitter. 
     There is additionally provided, in accordance with an embodiment that is described herein, a transmission method in a transmitter that includes a Power Amplifier (PA) that amplifies the signal and has input and output amplifier terminals, an antenna, and at least one passive component having an input component terminal coupled to the output amplifier terminal and an output component terminal coupled to the antenna. The method includes determining an interim power level at the output amplifier terminal that causes a Radio Frequency (RF) signal at the output component terminal to have a target output power level. Based on the interim power level, a PA control voltage that makes the interim power level producible by the PA is determined, so that the signal at the output component terminal has the target output power level. The determined PA control voltage is applied to the PA, the RF signal is amplified using the PA, and the amplified RF signal is transmitted. 
     The present disclosure will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that schematically illustrates a transmitter that uses power control, in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a graph showing calibration and characterization data for transmit power control, in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a flow chart that schematically illustrates a method for transmit power control, in accordance with an embodiment of the present disclosure; and 
         FIGS. 4 and 5  are graphs schematically showing processes for determining a Power Amplifier (PA) control voltage during transmit power control, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments that are described herein provide improved methods and systems for controlling Radio Frequency (RF) transmitters. In some embodiments, a transmitter comprises a transmission chain that is coupled to a Power Amplifier (PA). The signal at the PA output passes through one or more passive components, such as for example a duplexer, and then is transmitted by an antenna. The gain of the transmission chain is adjustable by setting a digital gain value, the gain of the PA is adjustable by setting a PA gain step control and the PA efficiency is controlled by adjusting a PA control voltage. In an embodiment, the transmitter comprises control circuitry, which enables controlling the signal power at the PA output in a closed loop. Typically, the closed-loop mechanism measures the signal power at the PA output, and adjusts the digital gain until the signal power at the PA output reaches the desired output power level. 
     In some embodiments, the control circuitry accepts a target output power level, which the signal is requested to have at the transmitter output (i.e., at the antenna, after the passive components). The control circuitry first determines the PA output power level that would produce the target output power level at the transmitter output. (The PA output power differs from the transmitter output power, for example because of losses in the passive components.) In an embodiment, the control circuitry determines the power level at the PA output by querying calibration data, which is indicative of the insertion loss of the passive components. Having determined the desired PA output power level, e.g., the PA output power level that achieves the target output power level at the transmitter output, the above-mentioned closed-loop mechanism adjusts the digital gain of the transmission chain so as to maintain the PA output power at that level. 
     In accordance with an embodiment, the control circuitry then determines a PA control voltage that is (1) sufficiently high to allow the PA to produce the signal at the desired power level and at an acceptable signal fidelity, and (2) sufficiently low to achieve high PA efficiency. In an embodiment, the control circuitry determines the PA control voltage by querying characterization data, which comprises pre-characterized PA control voltages at respective output power levels. The control circuitry applies the determined PA control voltage to the PA. Although changes in PA control voltage may vary the PA gain, this gain variation is automatically corrected, e.g. by the closed-loop mechanism, so that a change to the PA control voltage has no net effect on the PA output power level. 
     Typically, the calibration and characterization data are produced at certain reference operating conditions, e.g., reference frequency, temperature and signal modulation. In some embodiments, the control circuitry corrects the PA output power and PA control voltage to match the actual operating conditions of the transmitter. Example correction methods are described herein. 
     In the embodiments described herein, the PA control voltage is determined based on the actual PA output power level, measured directly at the PA output and before the passive components, rather than based on the transmitter output power. As such, the choice of PA control voltage is unaffected by possible variations in the passive components&#39; insertion loss. The PA is therefore operated at high efficiency, while meeting the output power level and signal fidelity requirements. 
       FIG. 1  is a block diagram that schematically illustrates a transmitter  20  that uses power control, in accordance with an embodiment of the present disclosure. In the example of  FIG. 1 , transmitter  20  is embodied in a mobile communication terminal (also referred to as a User Equipment—UE) that transmits uplink signals to a Base Station (BS) in accordance with the Universal Mobile Telecommunications System (UMTS) specifications. In alternative embodiments, transmitter  20  may operate in accordance with any other suitable communication standard or protocol that involve setting of transmit power level. Although the embodiments described herein refer to uplink transmission, the disclosed techniques can be used in downlink transmission, as well. 
     Transmitter  20  comprises a transmission chain, depicted in the embodiment of  FIG. 1  as a digital TX  24 , which accepts a digital baseband input signal, and processes the signal to produce a modulated, low-power Radio Frequency (RF) signal. Transmission chain  24  typically amplifies, filters and up-converts the input signal. The gain that transmission chain  24  applies to the signal is programmable. In an embodiment, transmission chain  24  accepts a digital value, referred to herein as a digital gain, which sets the gain to be applied to the input signal. 
     The low-power RF signal produced by transmission chain  24  is amplified by a Power Amplifier (PA)  28 . PA  28  has an input terminal for accepting the low-power RF signal from transmission chain  24 , and an output terminal for outputting the amplified RF signal. The power of the RF signal at the output of PA  28  is denoted PA OUT . The gain of PA  28  is controlled by a PA gain step control, which determines the discrete gain step of the PA. The power consumption efficiency of PA  28  is controlled by a PA control voltage denoted V CTRL . In some embodiments, V CTRL  denotes the supply voltage (V CC ) that powers the PA. In alternative embodiments, V CTRL  comprises a bias voltage that biases one or more of the PA devices. 
     In an embodiment, the efficiency of PA  28 , i.e., the power consumption of the PA for a given PA OUT , can be controlled by varying V CTRL . For a given PA OUT , lower V CTRL  values typically correspond to higher efficiency, and vice versa. On the other hand, lowering V CTRL  may also limit the output power that can be achieved by the PA. For a certain desired PA OUT , V CTRL  can be reduced and the PA efficiency can be increased accordingly, up to a limit at which the PA is no longer able to produce the RF signal at the desired output power PA OUT  and at a specified signal fidelity. 
     The RF signal produced by PA  28  is provided to one or more passive components  32 , in the present example comprising a duplexer that filters the signal. The signal is then transmitted toward a base station (BS) (not seen in  FIG. 1 ) using an antenna  36 . In the present embodiment, antenna  36  is also used for receiving downlink signals from the BS. The downlink signals are filtered by the duplexer and provided to a downlink receiver (not seen in  FIG. 1 ). The passive components thus have a transmitter input terminal that is connected to the output terminal of PA  28 , a receiver output terminal that is connected to the receiver, and an antenna terminal that is connected to antenna  36 . Additionally or alternatively to using a duplexer, passive components  32  may comprise any other suitable passive component that is connected between the PA and the antenna, such as filters, matching networks, switches or circulators. 
     Transmitter  20  comprises a controller  40 , which manages the transmitter operation and controls the different transmitter elements. In particular, controller  40  configures TX chain  24  with the appropriate digital gain, PA  28  with the appropriate PA gain step and PA  28  with the appropriate V CTRL , so as to cause the transmitter to transmit uplink signals at the desired output power and to operate at high efficiency. 
     In some embodiments, controller  40  comprises a power control module  44 , which carries out the methods described herein. Module  44  accepts a measurement of the PA output power PA OUT . The measured PA OUT  value reported to module  44  is denoted “Power Detected” (PD). The transmitter may produce PD, for example, using a coupler and power detector that sense the signal at the output of PA  28 . Additionally, module  44  accepts a requested target value of the output power level P OUT . In the embodiments described herein, the term “output power level” (P OUT ) refers to the signal power at the output of passive components  32 , i.e., at the input of antenna  36 . In alternative embodiments, however, P OUT  may denote the power of the signal transmitted by antenna  36 , which can be sensed using any suitable means. 
     In an embodiment, transmitter  20  comprises a memory  48 , which holds calibration and characterization data that is used by module  44  in setting the digital gain, the PA gain step and V CTRL . Module  44  calculates and sets the digital gain, the PA gain step and V CTRL  based on the above-described inputs using methods that are explained in detail below. 
     In some practical cases, changes in V CTRL  affect the gain of PA  28 . In some embodiments, module  44  in controller  40  applies a closed-loop control mechanism that maintains the PA output power (PA OUT ) at a desired level despite changes to V CTRL . Typically, the closed-loop mechanism accepts a certain target value of PD, measures the actual PD (which is indicative of the actual PA OUT ), and adjusts the digital gain of transmission chain  24  so as to cause the actual PD to approach the target PD. When using this closed-loop mechanism, PA OUT  is unaffected by the PA gain. In particular, PA OUT  is unaffected by the choice of V CTRL . 
     The transmitter configuration shown in  FIG. 1  is a simplified example configuration, which is depicted for the sake of conceptual clarity. In alternative embodiments, any other suitable transmitter configuration can also be used. The different components of transmitter  20  may be implemented using dedicated hardware, such as using one or more Application-Specific Integrated Circuits (ASICs) and/or Field-Programmable Gate Arrays (FPGAs). Alternatively, some transmitter components may be implemented using software instructions that run on general-purpose hardware, or using a combination of dedicated hardware and software instructions that run on general purpose hardware. Controller  40 , memory  48  and the power detector in transmitter  20  are referred to herein collectively as control circuitry, which is configured to carry out the methods described herein. 
     Typically, controller  40  comprises a general-purpose processor, which is programmed using software instructions that are stored in a memory, such as memory  48  or other suitable memory device, to carry out the functions described herein, although it too may be implemented on dedicated hardware. The software instructions may be downloaded to the processor in electronic form, over a network, for example, or it may, alternatively or additionally, be provided and/or stored on non-transitory tangible media, such as magnetic, optical, or electronic memory. In some embodiments, some or all of the elements of transmitter  20  may be fabricated in a chip-set. Transmitter elements that are not mandatory for explanation of the disclosed techniques have been omitted from  FIG. 1  for the sake of clarity and to avoid obfuscating the teachings of this disclosure. 
     In some embodiments, transmitter  20  is requested by the BS to transmit at a certain output power level P OUT . For example, in UMTS systems the BS sends to the UE Transmit Power Control (TPC) commands over a downlink channel. The TPC commands request the UE transmitter to increase or decrease its output power by a specified step (e.g., 1 dB or 2 dB). In response to the TPC commands, module  44  in controller  40  adjusts P OUT  by modifying the digital gain and/or PA gain step and/or V CTRL . The UMTS specifications specify the absolute and relative accuracies in setting P OUT , and the power adjustments made by module  44  should typically meet these specifications. At the same time, it is typically desirable to operate PA  28  at the highest possible efficiency that still enables the transmitter to transmit at the target P OUT  and at the specified signal fidelity, so as to reduce the transmitter power consumption. 
       FIG. 2  is a graph showing example calibration and characterization data that are stored in memory  48  for a given PA gain step setting, in accordance with an embodiment of the present disclosure. The top graph in  FIG. 2  shows calibration data  50 , which comprise target PD values at respective P OUT  values. Each [P OUT ,PD] data point in calibration data  50  gives the PD value that would cause transmitter  20  to produce a signal at the respective output power level P OUT . Thus, calibration data is indicative of the insertion loss of passive components  32  as a function of signal power. 
     Typically, calibration data  50  is pre-measured and stored for each individual transmitter  20  at reference operating conditions (e.g., reference frequency and temperature). Calibration data  50  is typically available over a range of power levels at which the power detector that measures PD produces reliable measurements. In an example embodiment, calibration data  50  covers a specified power range at 1 dB increments, although any other suitable resolution can also be used. 
     The bottom graph in  FIG. 2  shows characterization data  54 , which comprise V CTRL  values at respective P OUT  values. Each [P OUT ,V CTRL ] data point in characterization data  54  gives a V CTRL  value that is (1) sufficiently high to enable the PA to produce the signal at the respective P OUT  at an acceptable signal fidelity, and (2) sufficiently low to achieve high PA efficiency. 
     Typically, characterization data  54  is pre-characterized over a group (e.g., type or production batch) of transmitters  20  at reference operating conditions (e.g., reference frequency, temperature and signal modulation). In an example embodiment, characterization data  54  covers a specified power range at 1 dB increments, although any other suitable resolution can also be used. 
     In an example embodiment, module  44  in controller  40  accepts a target P OUT  that should be met by transmitter  20 . Module  44  obtains a target PD that corresponds to the target P OUT  by querying calibration data  50  (see top graph in  FIG. 2 ). Module  44  also obtains a V CTRL  value that corresponds to the target P OUT  by querying characterization data  54  (see bottom graph in  FIG. 2 ). Module  44  applies this V CTRL  to PA  28 , and sets the PA output power PA OUT  to match the target PD using the closed-loop mechanism. At these settings, transmitter  20  transmits the signal at the desired output power level (target P OUT ), while meeting the signal fidelity requirements and while operating the PA at high efficiency. 
     In some practical cases, the optimal values for V CTRL  and the target PD may differ from the values given in calibration data  50  and characterization data  54 . Such differences may occur, for example, when the actual operating conditions of the transmitter differ from the reference operating conditions at which the calibration and characterization data were produced. Methods for correcting V CTRL  and the target PD to account for the actual transmitter operating conditions are described further below. 
       FIG. 3  is a flow chart that schematically illustrates a method for transmit power control, in accordance with an embodiment of the present disclosure. The method begins at an input operation  60 , with module  44  in controller  40  accepting a target P OUT  value that is to be met by transmitter  20 . At a target PD selection operation  64 , module  44  determines the target PD that corresponds to the requested target P OUT  value. Typically, module  44  determines the target PD by querying calibration data  50 . 
     In an embodiment, at a correction operation  66 , module  44  corrects the target PD value, to account for the actual operating conditions (e.g., frequency, temperature and signal modulation) of the transmitter. At a V CTRL  selection operation  68 , module  44  determines the V CTRL  that corresponds to the corrected target PD value. Typically, module  44  determines V CTRL  by querying characterization data  54 . Example correction schemes for V CTRL  are shown in  FIGS. 4 and 5  below. 
     At a loop setting operation  76 , module  44  determines the selected loop mechanism. If the open-loop mechanism is selected then module  44  applies a digital gain value based on the V CTRL  as described in the U.S. patent application entitled “High-Accuracy Transmit Power Control with High-Efficiency Power Amplifier Operation,” Ser. No. 12/772,212, cited above. If the closed-loop mechanism is selected then module  44  uses the target PD value that was determined at operation  64  above, and corrected at operation  72  above. At a V CTRL  setting operation  80 , module  44  applies the V CTRL  value to PA  28 . The V CTRL  value used at this stage is the value that was determined at operation  68  above, and corrected at operation  72  above. 
     At a transmission operation  84 , transmitter  20  transmits the signal at the target output power requested at operation  60  above. Because of the above-described selection of V CTRL , PA  28  operates at high efficiency, while meeting the output power level and signal fidelity requirements. 
       FIG. 4  is a graph showing a process for correcting V CTRL  to account for the actual operating frequency of the transmitter, in accordance with an embodiment of the present disclosure. In the present example, calibration data  50  and characterization data  54  were produced at a reference signal frequency denoted f 1 . At a certain point in time, however, the transmitter transmits signals at a frequency f 2  that is different from f 1 . Assume also that the variation of the target PD as a function of frequency is known. 
     In this scenario, a data point  91  marks the target PD corresponding to the target P OUT  at frequency f 2 . The V CTRL  value that is best suited for the target PD at frequency f 2  is unknown, however, since characterization data  54  was produced at frequency f 1 . In an embodiment, module  44  determines a data point  92  in calibration data  50 , which corresponds to the same target PD as data point  91 . Then, module  54  determines a V CTRL  value  93  in characterization data  54 , which corresponds to data point  92 . Module  44  applies V CTRL  value  93  to PA  28 , and uses it for transmission at frequency f 2 . 
     In some embodiments, the closed-loop mechanism operates over only part of the transmitter output power range, e.g., because the power detector that measures PD has limited sensitivity. At low output power levels, module  44  sets the digital gain using an open-loop mechanism, e.g., based on a pre-calibrated mapping of digital gain to output power that is stored in memory  48 . Typically, some overlap exists between the output power ranges of the open-loop and closed-loop mechanisms. In the overlap region, any of the loops can be used. 
     When transmitter  20  operates using the open-loop mechanism in the overlap region, module  44  may determine the V CTRL  and digital gain values in various ways. In an example embodiment, module  44  queries calibration data  50  (which is valid throughout the overlap region), and obtains the target PD that corresponds to the requested target P OUT . Module  44  then finds the V CTRL  corresponding to this target PD value, as explained above. From the V CTRL  value, module  44  determines the digital gain value to be applied to transmission chain  24 . An example method for finding the digital gain value based on V CTRL  is described in the U.S. patent application entitled “High-Accuracy Transmit Power Control with High-Efficiency Power Amplifier Operation,” Ser. No. 12/772,212, cited above. Alternatively, any other suitable method can be used. 
     When operating at low P OUT  levels where no PD calibration data is available, module  44  sets V CTRL  as a function of P OUT  according to a certain worst-case relation over all operating frequencies. In an embodiment, this worst-case relation is determined by characterization over multiple transmitters  20 . 
       FIG. 5  is a graph showing a process for determining V CTRL  as a function of P OUT , in accordance with an embodiment of the present disclosure. In the present example, the calibrated target PD value for all operational frequencies is available for P OUT ≧0 dBm. 
     In an embodiment, below 0 dBm, module  44  sets V CTRL  as a function of P OUT  according to a worst-case curve  98 . This worst-case relation is typically determined by characterization. For very low output power levels, V CTRL  is set constantly to a certain minimum value V MIN . Above a certain output power level, V CTRL  begins to increase with output power. This section corresponds to a certain worst-case frequency f 0 . 
     Above 0 dBm, module  44  sets V CTRL  as a function of P OUT  according to calibration data  54 , as explained above. Calibration data  54  was obtained at a reference frequency f 1 . For other operating frequencies, the dependence of V CTRL  on of P OUT  is shown by graphs  94 . 
     Consider a region  102  in  FIG. 5 , in which the worst-case characterized V CTRL  value increases as a function of P OUT . In some embodiments, module  44  may reduce the V CTRL  values in this region below the worst-case value, to V CTRL  suited for 0 dBm as seen on graphs  54  or  94  (depending on the operating frequency) or to any other suitable value. This reduction in V CTRL  increases the PA efficiency for P OUT  levels in region  102 . 
     In some embodiments, characterization data  54  is measured for a certain reference modulation scheme. The actual modulation scheme used by transmitter  20  at a given time may differ from the reference modulation scheme for which the characterization data was obtained. In some embodiments, module  44  corrects the V CTRL  value to account for the difference between the actual and reference modulation scheme. In an example embodiment, the actual and reference modulation schemes have respective, different Cubic Metrics (CM). Module  44  corrects V CTRL  by a correction factor that depends on the difference (or ratio) between the CMs of the actual and reference modulation schemes. This sort of correction can be determined, for example, by characterization over multiple transmitters. 
     The correction schemes shown in  FIGS. 4 and 5  above are example schemes, which are depicted purely for the sake of conceptual clarity. In alternative embodiments, module  44  may correct the V CTRL  value in any other suitable way to account for the actual operating conditions of the transmitter (for example the combined affect of the operating frequency and temperature). In some embodiments, module  44  takes into consideration additional factors when correcting V CTRL , such as the matching between the transmitter and the antenna (e.g., the Voltage Standing Wave Ratio—VSWR) or various loop errors in the closed-loop and open-loop mechanisms. 
     It is noted that the embodiments described above are cited by way of example, and that the present disclosure is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present disclosure includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.