PATENT DOCUMENT

Publication Number: US-8249531-B2
Application Number: US-26294308-A
Country: US
Kind Code: B2

Title: Transmit power measurement and control methods and apparatus

Abstract:
Embodiments include methods and apparatus for performing transmit power control. A gain application element receives a sequence of digital input samples and a digital gain signal, and combines the digital gain signal with the digital input samples to generate a sequence of gain-compensated digital samples. A power amplifier receives and amplifies an analog version of the gain-compensated digital samples in order to generate an antenna output signal. A feedback path generates an analog feedback signal from the antenna output signal, produces a sequence of digital feedback samples from the analog feedback signal, and generates difference values based on the digital feedback samples. When a specified type of constant modulus symbol is represented in the antenna output symbol, the feedback path accumulates the difference values into an accumulated error value, and produces the digital gain signal from the accumulated error value.

Claims:
1. A wireless device comprising:
 a gain application element adapted to receive a sequence of digital input samples and a digital gain signal, and to combine the digital gain signal with the digital input samples to generate a sequence of gain-compensated digital samples; and 
 a power amplifier adapted to receive and amplify an analog version of the gain-compensated digital samples in order to generate an antenna output signal, wherein the transmitter is adapted to operate in a closed loop power control mode when a specified type of constant modulus symbol is represented in the antenna output signal, and to operate in an open loop power control mode when the specified type of constant modulus symbol is not represented in the antenna output signal. 
 
     
     
       2. The wireless device of  claim 1 , further comprising a feedback path adapted to generate an analog feedback signal from the antenna output signal, to produce a sequence of digital feedback samples from the analog feedback signal, to generate difference values based on the digital feedback samples, to accumulate the difference values into an accumulated error value when the transmitter is operating in the closed loop power control mode, and to produce the digital gain signal from the accumulated error value. 
     
     
       3. The wireless device of  claim 1 , further comprising a processor adapted to produce the digital input samples as a representation of a plurality of symbols that are formatted into a sub-frame structure that includes two slots, wherein each of the two slots includes a plurality of information-bearing symbols and a demodulation reference symbol (DRS) positioned within a first symbol position, and wherein the DRS is a constant modulus symbol of the specified type of constant modulus symbol. 
     
     
       4. The wireless device of  claim 1 , further comprising:
 a digital-to-analog converter adapted to receive the gain-compensated digital samples and to convert the gain-compensated digital samples into a gain-compensated analog signal; 
 a radio frequency (RF) modulator adapted to receive the gain-compensated analog signal and to modulate the gain-compensated analog signal to an RF carrier frequency to generate a pre-adjusted analog signal; and 
 a stepped variable gain amplifier (SVGA) adapted to apply gains indicated in a gain ramp signal to the pre-adjusted analog signal in order to generate the analog version of the gain-compensated digital samples. 
 
     
     
       5. The wireless device of  claim 1 , wherein the gain application element is adapted to receive the digital input samples as a sequence of signal bursts produced using a 3 rd  Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard. 
     
     
       6. The wireless device of  claim 1 , wherein the wireless device forms at least a portion of a device selected from a group of devices that includes a cellular telephone, a radio, a personal data assistant, a computer, and a mobile internet device. 
     
     
       7. A method for performing power control in a wireless device, the method comprising:
 combining a sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples, wherein the sequence of digital input samples represents a plurality of symbols; 
 amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal; 
 generating an analog feedback signal from the antenna output signal; 
 producing a sequence of digital feedback samples from the analog feedback signal; 
 generating difference values based on the digital feedback samples; 
 responsive to the digital feedback samples that correspond to at least one constant modulus symbol, accumulating the difference values into an accumulated error value; 
 generating the compensation gain values represented in the digital gain signal based on the accumulated error value; and 
 producing the digital input samples as a representation of a plurality of symbols that are formatted into a sub-frame structure that includes a plurality of information-bearing symbols and the constant modulus symbol; 
 wherein the sub-frame structure includes a first slot and a second slot, the first slot and the second slot each include a plurality symbol positions, a first symbol position in the first slot is designated for a first demodulation reference symbol (DRS), a first symbol position in the second slot is designated for a second DRS, a seventh symbol position in the second slot is designated for a sounding reference symbol (SRS), and the first DRS, the second DRS, and the SRS all are constant modulus symbols. 
 
     
     
       8. A method for performing power control in a wireless device, the method comprising:
 receiving a sequence of digital input samples that represent a plurality of symbols that are formatted into a sub-frame structure that includes a plurality of information-bearing symbols and at least one constant modulus symbol; 
 combining the sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples; 
 amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal; 
 generating an analog feedback signal from the antenna output signal; 
 producing a sequence of digital feedback samples from the analog feedback signal; and 
 performing closed loop power control using digital feedback samples corresponding to one or more of the at least one constant modulus symbol; 
 wherein the at least one constant modulus symbol includes a first demodulation reference symbol (DRS), and the closed loop power control is performed using digital feedback samples corresponding to the first DRS. 
 
     
     
       9. The method of  claim 8 , wherein the at least one constant modulus symbol further includes a second DRS, and the closed loop power control is performed using digital feedback samples corresponding to the first DRS and the second DRS. 
     
     
       10. The method of  claim 9 , wherein the at least one constant modulus symbol further includes a sounding reference symbol (SRS), and the closed loop power control is performed using digital feedback samples corresponding to the SRS and either or both of the first DRS and the second DRS. 
     
     
       11. A method for performing power control in a wireless device, the method comprising:
 receiving a sequence of digital input samples that represent a plurality of symbols that are formatted into a sub-frame structure that includes a plurality of information-bearing symbols and at least one constant modulus symbol; 
 combining the sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples; 
 amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal; 
 generating an analog feedback signal from the antenna output signal; 
 producing a sequence of digital feedback samples from the analog feedback signal; and 
 performing closed loop power control using digital feedback samples corresponding to one or more of the at least one constant modulus symbol; 
 wherein the at least one constant modulus symbol includes a sounding reference symbol (SRS), and the closed loop power control is performed using digital feedback samples corresponding to the SRS. 
 
     
     
       12. The method of  claim 11 , wherein the at least one constant modulus symbol further includes a first demodulation reference symbol (DRS), and the closed loop power control is performed using digital feedback samples corresponding to the SRS and the first DRS. 
     
     
       13. The method of  claim 12 , wherein the at least one constant modulus symbol further includes a second DRS, and the closed loop power control is performed using digital feedback samples corresponding to the SRS and either or both of the first DRS and the second DRS. 
     
     
       14. A method for performing power control in a wireless device, the method comprising:
 receiving a sequence of digital input samples that represent a plurality of symbols that are formatted into a sub-frame structure that includes a plurality of information-bearing symbols and at least one constant modulus symbol; 
 combining the sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples; 
 amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal; 
 generating an analog feedback signal from the antenna output signal; 
 producing a sequence of digital feedback samples from the analog feedback signal; and 
 performing closed loop power control using digital feedback samples corresponding to one or more of the at least one constant modulus symbol; 
 wherein the at least one constant modulus symbol includes a sounding reference symbol (SRS), and power measurement is performed using digital feedback samples corresponding to the SRS. 
 
     
     
       15. The method of  claim 14 , wherein the at least one constant modulus symbol further includes a first demodulation reference symbol (DRS), and the power measurement is performed using digital feedback samples corresponding to the first DRS. 
     
     
       16. The method of  claim 15 , wherein the at least one constant modulus symbol further includes a second demodulation reference symbol (DRS), and the power measurement is performed using digital feedback samples corresponding to the second DRS. 
     
     
       17. The method of  claim 16 , wherein the at least one constant modulus symbol further includes a second demodulation reference symbol (DRS), and the power measurement is performed using digital feedback samples corresponding to both the first and the second DRS.

Description:
TECHNICAL FIELD 
     Embodiments of the inventive subject matter relate to transmit power measurement and control methods and apparatus, and more particularly to transmit power measurement and control in wireless communication devices. 
     BACKGROUND 
     Current cellular wireless standards include stringent transmit power control requirements. For example, some orthogonal frequency division multiplexing (OFDM) standards (including variants of generic OFDM, such as Single Carrier Frequency Division Multiple Access (SC-FDMA) standards) include transmit power control requirements that deal with peak-to-average ratios (PARs) of 6.5 decibels (dB) or higher for both 16-QAM (Quadrature Amplitude Modulation) and Quadrature Phase Shift Key (QPSK) modulation. In order to address such requirements, a conventional wireless communication device may include a transmit power control system for amplification and transmission of a wireless signal. Such a system may include a power control feed-forward path and a transmit power detector feedback path. A power amplifier and variable gain amplifier (VGA) along the power control feed-forward path amplifies the signal to be transmitted. The transmit power detector feedback path measures the power of the amplified signal, and determines and provides control or bias voltages to apply to the system&#39;s power amplifier and VGA, thus controlling the transmit power ramp up and ramp down curves. 
     The emerging 3 rd  Generation Partnership Project (3GPP) Long Term Evolution (LTE) standards specify that a single 3GPP LTE symbol is to be represented using QPSK, 16-QAM or 64-QAM, where 12 constellation points are selected as a data-dependent subset of a selected QAM constellation. In addition, under emerging 3GPP LTE standards, each transmit sub-frame may include a plurality of information-bearing symbols and two demodulation reference symbols located at fixed symbol positions within the sub-frame. In addition, occasional sub-frames also may include a sounding reference symbol (SRS) that replaces an information-bearing symbol of the sub-frame (e.g., the last symbol). Prior to transmission, the amplitude of the SRS may be transmitted at a different power level from the power level of the information-bearing and demodulation reference symbols. 
     Power measurement and control techniques for 3GPP LTE devices should be adapted to take into account this multi-level, modified QAM modulation. However, because a single 3GPP LTE symbol is represented using only a subset of available constellation points (e.g., 12 of 16 constellation points), not enough samples may be available to achieve sufficiently accurate power measurement and/or control using power measurement and control techniques currently employed in conjunction with conventional OFDM, 16-QAM modulation or other conventional power measurement and control techniques. In addition, conventional power measurement and control techniques are not adapted to take into account the multi-level modulation performed in conjunction with transmitting the SRSs. 
     Accordingly, methods and apparatus are desired for performing transmit power measurement and control for wireless communication devices in which insufficiently accurate power measurement and/or control may result from the use of conventional power measurement and control techniques. In particular, methods and apparatus for performing transmit power measurement and control are desired for wireless communication devices in which multi-level QAM modulation is performed (e.g., devices that employ a 3GPP LTE standard or another standard for which current power measurement and/or control techniques are unsuitable) and/or in which a data-dependent subset of available QAM constellation points are used to represent a symbol. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a simplified block diagram of a wireless device, in accordance with an example embodiment; 
         FIG. 2  is an example of the structure of a sub-frame, according to an example embodiment; 
         FIG. 3  is a first example of a QAM constellation representing selected constellation points for a first symbol, according to an embodiment; 
         FIG. 4  is a second example of a QAM constellation representing selected constellation points for a second symbol, according to an embodiment; 
         FIG. 5  is a plot illustrating relative transmission power for three, consecutive, example sub-frames, according to an embodiment; 
         FIG. 6  is a plot illustrating relative transmission power for three, consecutive, example sub-frames, according to an alternate embodiment; 
         FIG. 7  is a plot illustrating relative transmission power for three, consecutive, example sub-frames, according to another alternate embodiment; 
         FIG. 8  illustrates a simplified block diagram of a portion of a transmitter, in accordance with an example embodiment; 
         FIG. 9  illustrates a flowchart of a method for performing power measurement and control, according to an example embodiment; and 
         FIG. 10  illustrates a flowchart of a method for switching between an open loop power control mode and a closed loop power control mode, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein include methods and apparatus for measuring and controlling the power applied to transmitted radio frequency (RF) signals. Embodiments may be implemented, for example, in wireless systems, networks, and devices adapted to implement 2G (second generation), 2.5G (2.5 generation), 3G (third generation), 4G (fourth generation), and/or other wireless telephone technologies. For example, but not by way of limitation, embodiments may be implemented in wireless systems, networks, and devices that operate in accordance with one or more of various standards within a group that includes the family of 3 rd  Generation Partnership Project (3GPP) standards (e.g., Global System for Mobile Communications (GSM) standards and/or Universal Mobile Telecommunications System (UMTS) standards). Examples of such standards include, but are not limited to, UMTS Rev. 8 (e.g., 3GPP Long Term Evolution (LTE), Enhanced Data Rates for Global System for Mobile Communications (GSM) Evolution (EDGE), Wideband Code Division Multiple Access (W-CDMA), High Speed Orthogonal frequency division multiplexing Packet Access HSOPA)), High Speed Packet Access (HSPA), UMTS Time Division Duplexing (UMTS-TDD) (e.g., TD-CDMA and TD-SCDMA), Freedom of Mobile Multimedia Access (FOMA), General Radio Packet Service (GPRS), Circuit Switched Data (CSD), and High Speed CSD (HSCSD)). Alternate embodiments may be implemented in wireless systems, networks, and devices that operate in accordance with one or more other standards within a group that includes the family of 3 rd  Generation Partnership Project 2 (3GPP2) standards (e.g., CDMAOne, CDMA2000, Evolution-Data Optimized (EV-DO), and Ultra Mobile Broadband (UMB)). Still other alternate embodiments may be implemented in wireless systems, networks, and devices that operate in accordance with one or more other standards within a group that includes the family of Advanced Mobile Phone System (AMPS) standards (e.g., AMPS or digital AMPS (D-AMPS)). Still other alternate embodiments may be implemented in wireless systems, networks, and devices that operate in accordance with one or more other standards within a group that includes Institute of Electrical and Electronics Engineers (IEEE) 802 standards (e.g., 802.16 (e.g., Worldwide Interoperability for Microwave Access (WiMAX)), IEEE 802.20, and IEEE 802.22) and/or standards based on other wireless technologies. Although a particular type of wireless system, network, and/or device may be described herein for example purposes, the example system is not to be interpreted as limiting the scope of the various embodiments or the claims only to the below-described system or device. For example, embodiments below may be described in conjunction with a 3GPP LTE system, network, device, and/or protocol, and the examples are not meant to limit application of the embodiments only to 3GPP LTE systems, networks, devices, and/or protocols. 
       FIG. 1  illustrates a simplified block diagram of a wireless device  100 , in accordance with an example embodiment. Device  100  is adapted to transmit electromagnetic signals over an air interface. In a particular embodiment, wireless device  100  is adapted to transmit 3GPP LTE signals over the air interface according to a 3GPP LTE standard, although wireless device  100  may be adapted to transmit different types of wireless signals over the air interface according to different standards, in other embodiments. Wireless device  100  may form substantially all of or a portion of a variety of different types of apparatus. For example, but not by way of limitation, wireless device  100  may form substantially all of or a portion of a cellular telephone, a radio, a personal data assistant (PDA), a computer (e.g., a laptop, notebook, desktop or other type of computer), a mobile internet device (MID), and/or another device that is adapted to transmit electromagnetic signals over an air interface. 
     Wireless device  100  includes at least one transmit subsystem  102 , receive subsystem  104 , antenna  106 , processing subsystem  108 , memory subsystem  110 , user interface subsystem  112 , and power supply subsystem  114 , in an embodiment. These subsystems are communicatively coupled together as illustrated in  FIG. 1 , where the term “communicatively coupled” means that information signals are transmissible through various interconnections between the subsystems. The interconnections between the subsystems may be direct interconnections that include conductive transmission media, or may be indirect interconnections that include one or more intermediate electrical components. Although certain interconnections are illustrated in  FIG. 1 , it is to be understood that more, fewer or different interconnections may be present in other embodiments. 
     The at least one processing subsystem  108  is adapted to perform various functions. These functions may include, for example, generating outgoing digital signals  134 , processing incoming digital signals  132 , interfacing with the at least one memory subsystem  110  to store and retrieve data, interfacing with the at least one user interface subsystem  112 , and performing various power control functions in conjunction with the at least one power supply system  114 . The at least one power supply system  114  may include, for example, an interface to line power and/or a battery power subsystem. 
     User interface subsystem  112  may include one or more user interface components adapted to enable a user to input commands or other information into device  100  and/or to provide visual, auditory, or mechanical indicia intended to convey information to the user. For example, but not by way of limitation, user interface subsystem  110  may include one or more display screens, touch screens, lights, speakers, vibration devices, keypads, buttons, dials, and/or other components adapted to receive input commands and/or to produce information-conveying indicia. 
     Memory subsystem  110  may include one or more components adapted to store digital information in a retrievable format. For example, but not by way of limitation, memory subsystem  110  may include one or more removable or non-removable, volatile or non-volatile memory components, such as read only memory (ROM)-based memory components, random access memory (RAM)-based memory components, compact disks (CDs), digital video disks (DVDs), and/or magnetic storage media (e.g., hard disks or floppy disks), to name a few. 
     Receive subsystem  104  is adapted to receive incoming RF signals  130  from antenna  106 , and to perform down-conversion, filtering, and analog-to-digital conversion, among other things, to the incoming RF signals  130  in order to generate incoming digital signals  132 . The incoming digital signals  132  may be processed by processing subsystem  108 . In an alternate embodiment, for a transmit-only type of device, receive subsystem  104  may be excluded. 
     According to an embodiment, processing subsystem  108  may include a baseband processor, and outgoing digital signal  134  may be generated as sequence of digital samples representative of symbols to be transmitted. The outgoing digital signal  134  is provided to transmit subsystem  102 . Transmit subsystem  102  (also referred to herein as a “transmitter” or “RF transmitter”) is adapted to receive the outgoing digital signals  134  generated by processing subsystem  108 , and to perform digital-to-analog conversion, up-conversion, gain adjustment, and amplification, among other things, to the outgoing digital signals  134  in order to generate outgoing RF signals  136 . The outgoing RF signals  136  are transmitted over the air interface by antenna  106 . The resulting signal is amplified by an output amplifier of transmit subsystem  102 , and provided to the device&#39;s antenna  106 . 
     RF signals transmitted by the device&#39;s antenna  106  may be received by infrastructure (e.g., a base station or “Node B”, not illustrated) associated with a wireless communication system. According to prevailing regulations, the wireless device  100  may be restricted only to transmit RF signals within a pre-defined “uplink” frequency band. For example, a particular communication system may support communications within a frequency band between about 1920 megahertz (MHz) and about 1980 MHz, which corresponds to about 60 MHz total bandwidth. In alternate embodiments, a communication system may support communications within different frequency bands and/or may have a wider or narrower bandwidth. 
     According to an embodiment, wireless device  100  transmits information in the time domain within the context of a radio frame. A radio frame may be defined as an information-bearing entity within which data and other information (e.g., pilot signals) are represented and transmitted by a wireless device (e.g., wireless device  100 ). In an embodiment, a radio frame is a segment of time having a duration. Each radio frame may be divided into a plurality of sub-frames. In an embodiment, each radio frame has a duration of about 10 milliseconds (ms), and is divided into 20 sub-frames. Accordingly, in an embodiment, a sub-frame may have a duration of about 1 ms, although a sub-frame may have a longer or shorter duration, in other embodiments. In addition, each radio frame may have a longer or shorter duration and/or may be divided into more or fewer sub-frames than indicated above, according to other embodiments. 
     According to an embodiment, frequency division duplexing (FDD) is performed in conjunction with transmitting the sub-frames of a radio frame. For example, in an embodiment in which a radio frame includes 20 sub-frames, 10 sub-frames of the radio frame may be available for uplink transmissions (e.g., wireless device  100  to base station), and the other 10 sub-frames may be available for downlink transmissions (e.g., base station to wireless device  100 ). Uplink and downlink transmissions may be separated in the frequency domain. In an embodiment in which half-duplex FDD is implemented, wireless device  100  may be restricted from transmitting and receiving simultaneously. In an embodiment in which full-duplex FDD is implemented, wireless device  100  may transmit and receive simultaneously. 
       FIG. 2  is an example of the structure of a sub-frame  200 , according to an example embodiment. Sub-frame  200  includes two, equally-sized slots  202 ,  204 , in an embodiment, although a sub-frame may include more or fewer slots and/or the slots may be unequal in size, in other embodiments. The duration of each slot  202 ,  204  is further subdivided into a plurality of symbol positions  211 ,  212 ,  213 ,  214 ,  215 ,  216 ,  217 ,  218 ,  219 ,  220 ,  221 ,  222 ,  223 ,  224  (referred to simply as “symbols”, herein), according to an embodiment. In an embodiment, each slot  202 ,  204  is subdivided into a set of seven symbols (e.g., symbols  211 - 217  and  218 - 224 ), although each slot  202 ,  204  may be divided into sets of more or fewer symbols, in other embodiments. A cyclic prefix (CP) may be appended to each symbol as a guard interval, according to an embodiment. 
     Symbols  211 - 224  are produced using Quadrature Amplitude Modulation (QAM), according to an embodiment, and accordingly may be referred to as QAM symbols. As will be described in more detail in conjunction with  FIGS. 3 and 4 , each symbol  211 - 224  is represented through a subset of available constellation points of a QAM constellation, where different data is represented by different subsets, according to an embodiment. Accordingly, the selection of constellation points for a particular symbol is data dependent, and consecutive symbols may be represented using different subsets of available constellation points. According to an embodiment in which modified 16-QAM modulation is implemented, the number of available constellation points equals 16, and the number of constellation points selected for any given symbol equals 12. In other embodiments, more or fewer constellation points may be available and/or larger of smaller subsets of constellation points may be selected for any given symbol. 
     Each slot  202 ,  204  includes a plurality of information-bearing symbols (e.g., symbols  211 - 213  and  215 - 217  for slot  202 , and symbols  218 - 220  and  222 - 224  for slot  204 ). In addition, in an embodiment, a demodulation reference symbol (DRS) is transmitted at a known symbol position of each slot  202 ,  204 . According to an embodiment, a DRS is transmitted at the fourth (or center) symbol position of each slot  202 ,  204 , or at symbol  214  for slot  202  and symbol  221  for slot  204 . In an alternate embodiment, a DRS is transmitted at the first symbol position of each slot  202 ,  204 , or at symbol  211  for slot  202  and symbol  218  for slot  204 . In still other alternate embodiments, one or more DRS may be transmitted in other symbol positions from those discussed above. 
     According to an embodiment, each DRS includes a constant modulus signal. The constant-modulus signal is constant modulus in the frequency domain (not necessarily in the time domain), and may be defined around the unit circle, according to an embodiment, although it may be modulated in a different (but constant) manner, according to other embodiments. Each DRS may be generated based on a prime-length Zadoff-Chu sequence that is either truncated or cyclically extended to a desired length, according to various embodiments. The DRS may be associated with a Physical Uplink Shared Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH), according to an embodiment. The DRS may be used during the processes of channel estimation and/or coherent demodulation. In addition or alternatively, as will be discussed in more detail later, the power levels of one or more DRS within a sub-frame may be measured by a transmitter during the process of power measurement and control, according to an embodiment. 
     During occasional sub-frames (e.g., during one or more sub-frames within a radio frame, such as the first and sixth sub-frames, for example), a sounding reference symbol (SRS) is transmitted at a known symbol position within a sub-frame  200 . The SRS replaces an information-bearing symbol of the sub-frame  200 . In an embodiment, the SRS is transmitted in the last symbol position within sub-frame  200  (e.g., the seventh symbol position of the second slot  204 , or symbol  224 ). Such an embodiment is described in more detail in conjunction with  FIG. 5 , later. According to other embodiments, which are described in more detail in conjunction with  FIGS. 6 and 7 , later, the SRS may be transmitted at a different symbol position from the last symbol position (e.g., the first symbol position of the first slot  202 , or symbol  211 , the first symbol position of the second slot  204 , or symbol  218 , or some other symbol position). 
     The SRS is not associated with a PUSCH or PUCCH, and the SRS has an independent power scaling compared with the PUSCH and PUCCH, according to an embodiment. Accordingly, prior to transmission, the amplitude of the SRS may be transmitted at a different power level from the power level of the information-bearing symbols (e.g., symbols  211 - 213 ,  215 - 220 , and  222 - 223 ) and the DRS (e.g., symbols  214  and  221 ) within the sub-frame in which the SRS is transmitted. More particularly, according to an embodiment, the power level of the SRS may be significantly higher than the power level of the information-bearing symbols and the DRS within the sub-frame. In addition, as with the DRS, each SRS is a constant-modulus signal. The constant-modulus signal is constant modulus in the frequency domain (not necessarily in the time domain), and may be defined around the unit circle, according to an embodiment, although it may be modulated in a different (but constant) manner, according to other embodiments. Each SRS may be generated based on a prime-length Zadoff-Chu sequence that is either truncated or cyclically extended to a desired length, according to various embodiments. The SRS may be used during the processes of channel estimation and/or coherent demodulation. In addition or alternatively, as will be discussed in more detail later, the power level of an SRS may be measured by a transmitter during the process of power measurement and control, according to an embodiment. 
     A “resource block” (RB) or “physical resource block” may be defined as a time-frequency unit that includes a plurality (e.g., 12 or some other number) of contiguous (in frequency) sub-carriers for one symbol period (e.g., symbol  211 ). According to an embodiment, the plurality of symbols (e.g., symbols  211 - 217 ) associated with a single slot (e.g., slot  202 ) may be transmitted containing a particular number of RBs. Accordingly, the transmitted signal in each slot  202 ,  204  is described by a resource grid of subcarriers and Single-Carrier Frequency-Division Multiple Access (SC-FDMA) symbols. RBs are allocated to a wireless device (e.g., wireless device  100 ,  FIG. 1 ) by the wireless system infrastructure (e.g., by a base station). The transmission bandwidth for the wireless system may be divided into a plurality of consecutive RBs, and the edges of each RB may be defined by a low and a high frequency. According to various embodiments, the number of available RBs (in the frequency domain) may be in a range of about 1 to 100 RBs. For a given allocated channel bandwidth, a maximum number of RBs may be transmitted to provide a sufficient guard band. For example, up to 100 RBs may be transmitted for a channel bandwidth of 20 MHz, and up to 6 RBs may be transmitted for a channel bandwidth of 1.4 MHz. The number of available RBs may be larger or smaller, in other embodiments. 
       FIGS. 3 and 4  are first and second examples of QAM constellations  300  and  400  representing selected constellation points for two different symbols, according to an embodiment. The amplitude and phase of each constellation point may be represented at one of the sub-carrier frequencies within an RB, according to an embodiment. Selected constellation points are indicated with solid black circles, and non-selected constellation points are indicated with white-centered circles. The illustrated example constellations  300 ,  400  represent only two possible constellation point subsets  302 ,  402  selected from 16 available constellation points, where each subset  302 ,  402  includes 12 of the 16 available constellation points. A plurality of other constellation point subsets that include constellation points other than those indicated as being selected in the example subsets  302 ,  402  alternatively may be selected. 
     In both of the illustrated example constellations  300 ,  400 , and according to a QAM modulation technique of a particular embodiment, a subset  302 ,  402  of 12 constellation points is selected from a set of 16 available constellation points for any given symbol. Accordingly, seventy-five percent (75%) of the available constellation points is selected for each symbol. In alternate embodiments, different numbers of constellation points may be available (e.g., 64, 128, or another number of available constellation points) for each symbol, and/or a different percentage of constellation points may be selected from the available constellation points to represent the data. According to various embodiments, the number of selected constellation points equals the number of sub-carrier frequencies allocated to represent any given symbol, and the number of sub-carrier frequencies allocated to represent any given symbol is less than the number of available constellation points. Accordingly, for example, when a single RB is allocated to a transmitter, there may be only 14 symbols in a sub-frame. Given the limited number of samples in the constellation, a measured power reading will be intrinsically data dependent. 
     According to an embodiment, a wireless device (e.g., wireless device  100 ,  FIG. 1 ) receives information from the wireless system infrastructure (e.g., a base station), which indicates the power at which the wireless device should transmit the symbols within a sub-frame. The wireless device may, therefore, measure the power of its transmitted signals, and may adjust the amplification applied by it&#39;s transmit subsystem (e.g., transmit subsystem  102 ,  FIG. 1 ) according to that measurement. The amplification may be adjusted on a sub-frame by sub-frame basis or less frequently, according to various embodiments. In addition, as discussed previously, when a sub-frame includes an SRS, the amplitude of the SRS may be transmitted at a different (e.g., higher) power level from the power level of the information-bearing symbols and the DRS within the sub-frame. 
     To illustrate the power adjustments that may be made between consecutive transmitted sub-frames and power adjustments that may be made in conjunction with transmitting an SRS,  FIG. 5  is a plot  500  illustrating relative transmission power for three, consecutive, example sub-frames  502 ,  503 ,  504 , according to an embodiment. Plot  500  includes a time axis  506  and a transmit signal power axis  508 . A “front” sub-frame boundary refers to a first boundary of a sub-frame that occurs in time at the beginning of transmission of the sub-frame, and a “rear” sub-frame boundary refers to a second boundary of the sub-frame that occurs in time at the end of transmission of the sub-frame. Symbol boundaries within each sub-frame  502 - 504  are indicated with vertical dashed lines. First and third sub-frames  502  and  504  represent sub-frames in which no SRS is transmitted, and second sub-frame  503  represents a sub-frame in which an SRS is transmitted within symbol position  510 . The relative power levels  511 ,  512 ,  513 ,  514  at which the information-bearing symbols, the DRS, and the SRS are transmitted are indicated along the transmit signal power axis  508 . It is to be understood that these relative power levels  511 - 514  are for example purposes only. 
     As plot  500  indicates, the RF signal corresponding to a first sub-frame  502  is transmitted at a first power level  511  for substantially the entire duration of the first sub-frame  502 . The term “substantially the entire duration” means an entire duration minus portions of the duration when a transmit power ramp up or ramp down are being performed (e.g., in proximity to the sub-frame boundaries and/or the SRS boundaries). Prior to the front boundary of the second sub-frame  503 , an upward power ramp  520  is applied to the RF signal, and the information-bearing symbols and DRS within the second sub-frame  503  are transmitted at a second power level  512  for substantially the entire duration of the information-bearing symbols and the DRS. 
     According to an embodiment, and as discussed previously, an SRS may be transmitted at the last symbol position within a sub-frame, or within symbol position  510  of sub-frame  503 , as the example of  FIG. 5  illustrates. A “front” symbol boundary refers to a first boundary of a symbol that occurs in time at the beginning of transmission of the symbol, and a “rear” symbol boundary refers to a second boundary of the symbol that occurs in time at the end of transmission of the symbol. An SRS may be characterized as a symbol, and accordingly an SRS also may have front and rear boundaries. Because the SRS may be transmitted at a relatively high power level, another upward power ramp  521  is applied to the RF signal prior to the front boundary of symbol position  510  (i.e., the front boundary of the SRS), and the SRS is transmitted at a third power level  513  for substantially the entire duration of symbol position  510 . Upon completion of the SRS transmission (e.g., proximate to a rear boundary of symbol position  510  or the SRS), a downward power ramp  522  to a fourth power level  514  is applied to the RF signal, and the third sub-frame  504  is transmitted at the fourth power level  514  for substantially the entire duration of the third sub-frame  504 . 
     In the embodiment illustrated and described in conjunction with  FIG. 5 , the SRS is transmitted in the last symbol position within a sub-frame (e.g., the fourteenth symbol position  510  of sub-frame  503 ). According to other embodiments, the SRS may be transmitted at a different symbol position from the last symbol position of a sub-frame. For example,  FIG. 6  is a plot  600  illustrating relative transmission power for three, consecutive, example sub-frames  602 ,  603 ,  604 , according to an alternate embodiment in which an SRS is transmitted within a first symbol position  610  of a sub-frame  603 . Because sub-frame  603  may be divided into two slots (not indicated in  FIG. 6 ), the first symbol position  610  also corresponds to a first symbol position of a first slot of sub-frame  603 . As an alternate example,  FIG. 7  is a plot  700  illustrating relative transmission power for three, consecutive, example sub-frames  702 ,  703 ,  704 , according to another alternate embodiment in which an SRS is transmitted within an eighth symbol position  710  of a sub-frame  703 . Again, because sub-frame  703  may be divided into two slots (not indicated in  FIG. 7 ), the eighth symbol position  710  also corresponds to a first symbol position of a second slot of sub-frame  703 . Even though  FIGS. 6 and 7  illustrate two specific examples in which an SRS is transmitted within a symbol position other than the last symbol position of a sub-frame, it is to be understood that other embodiments include transmitting an SRS within a symbol position other than the first or eighth symbol positions of a sub-frame. 
     As  FIGS. 5-7  illustrate, power ramping upward and/or downward of the RF signal may occur proximate to sub-frame boundaries and also proximate to front and rear boundaries of a transmitted SRS. According to an embodiment, only a single power transition is performed in conjunction with a sub-frame in which no SRS is transmitted (e.g., sub-frames  502 ,  504 ,  602 ,  604 ,  702 ,  704 ), where that single power transition may be considered to be the power transition to adjust the RF signal power to the power level at which the next, adjacent sub-frame is to be transmitted (e.g., upward power ramps  520 ,  720 ) or, in the case of the embodiment of  FIG. 6 , the power level at which the SRS is to be transmitted (e.g., upward power ramp  620 ). Although  FIGS. 5-7  indicate a power adjustment proximate to each boundary between sub-frames  502 - 504 ,  602 - 604 ,  702 - 704 , power adjustments may occur less frequently. Accordingly, when the power level is substantially the same between adjacent sub-frames, a power transition may not be performed between the adjacent sub-frames. 
     In contrast with sub-frames in which no SRS is transmitted, two or more power transitions may be performed in conjunction with a sub-frame in which an SRS is transmitted (e.g., sub-frames  503 ,  603 ,  703 ). More particularly, for the embodiment illustrated in  FIG. 5 , and as described previously, a first power transition (e.g., upward power ramp  521 ) is performed to increase the RF signal power level  512  to the power level  513  specified for transmitting the SRS, and a second power transition (e.g., downward power ramp  522 ) is performed to decrease the RF signal power from the SRS power level  513  to the power level  514  specified for the next, adjacent sub-frame (e.g., third sub-frame  504 ). For the embodiment illustrated in  FIG. 6 , a first power transition (e.g., downward power ramp  621 ) is performed to decrease the RF signal power from the SRS power level  613  to the power level  612  specified for transmitting the sub-frame in which the SRS is being transmitted (e.g., second sub-frame  603 ), and a second power transition (e.g., upward power ramp  622 ) is performed to increase the RF signal power to the power level specified for the next, adjacent sub-frame (e.g., third sub-frame  604 ). Finally, for the embodiment illustrated in  FIG. 7 , a first power transition (e.g., upward power ramp  721 ) is performed to increase the RF signal power to the power level  713  specified for transmitting the SRS, a second power transition (e.g., downward power ramp  722 ) is performed to adjust the RF signal power from the SRS power level  713  to the power level  712  specified for the sub-frame in which the SRS is being transmitted (e.g., second sub-frame  703 ), and a third power transition (e.g., upward power ramp  723 ) is performed to adjust the RF signal power to the level specified for the next, adjacent sub-frame (e.g., third sub-frame  704 ). Accordingly, more than one (e.g., two or three) power transition may be performed in conjunction with a sub-frame in which an SRS is transmitted. The technique of adjusting the power more than once within a sub-frame (e.g., a sub-frame in which an SRS is transmitted) is referred to herein as “multi-level modulation.” 
     Conventional techniques for power measurement and control do not account for multi-level modulation in conjunction with a sub-frame (e.g., a sub-frame in which an SRS is transmitted). In addition, as discussed previously, embodiments also may implement a modified type of QAM modulation, in which a data-dependent subset of available constellation points is selected for each transmitted, information-bearing symbol. Accordingly, the average transmit signal power between adjacent symbols (e.g., symbols  211  and  212 ,  FIG. 2 ) may vary significantly simply by virtue of the difference in which constellation points are selected between the adjacent symbols (e.g., power differences are inherent in the modulation technique). Because conventional techniques for power measurement and control do not account for the variable selection of subsets of constellation points from symbol-to-symbol, these techniques may be unsuitable for performing accurate power measurement and control in wireless devices in which embodiments of QAM modulation discussed herein are implemented. As described in more detail below, embodiments include power measurement and control methods and apparatus that account for the power characteristics of modified QAM modulation and potential multi-level modulation performed within a sub-frame. Accordingly, such embodiments may result in more accurate power measurement and control for wireless devices in which such modified QAM modulation and/or multi-level modulation is implemented. According to a particular embodiment, closed loop power control is performed using portions of a feedback signal corresponding to one or more constant modulus signals (e.g., constant modulus in the frequency domain) within a sub-frame, where the one or more constant modulus signals may include any combination of a first DRS, a second DRS, and an SRS. As used herein, the terms “first DRS” and “second DRS” are not meant to imply any particular ordering (e.g., in time) of DRSs within a sub-frame or any particular actual or relative symbol position within a sub-frame. 
       FIG. 8  illustrates a simplified block diagram of a portion of a transmitter  800 , in accordance with an example embodiment. Transmitter  800  may correspond, for example, to transmit subsystem  102 ,  FIG. 1 . Transmitter  800  is adapted dynamically to adjust the gain of an RF antenna output signal  832  (e.g., outgoing RF signal  136 ,  FIG. 1 ), according to an embodiment. Essentially, transmitter  800  performs gain adjustments based on power control commands received from other portions of the system (e.g., from processing subsystem  108 ,  FIG. 1 ) and based on power measurements of the RF antenna output signal  832  that are performed in a feedback path  811  of the transmitter  800 , in accordance with an embodiment. 
     In an embodiment, transmitter  800  receives a sequence of digital input samples  812  (e.g., outgoing digital signal  134  from processing subsystem  108 ,  FIG. 1 ). The sequence of digital input samples  812  includes a baseband sequence of multiple input data samples, which may include, for example, a sequence of discrete time samples of a signal to be transmitted (e.g., a transmission burst). In an embodiment, the sequence of digital input samples  812  includes a sequence of complex values represented in Cartesian coordinates, so that each value has a real part (I) and an imaginary part (Q), which are received in parallel. Accordingly, digital input samples  812  may include a sequence of values that may be represented as X(k)=[I(k), Q(k)], where k indicates a sample number and k=1 . . . K, I(k) represents a real part of an input data sample, and Q(k) represents an imaginary part of an input data sample. In alternate embodiments, digital input samples  812  may include sequences of values represented in polar coordinates or some other representation. Digital input samples  812  could represent, for example, baseband, time-domain representations of a sequence of symbols produced using a 3GPP LTE standard. In alternate embodiments, digital input samples  812  could represent a sequence of symbols produced using other standards. According to an embodiment, the sequence of digital input samples  812  is produced in accordance with a sub-frame structure such as that described in conjunction with  FIG. 2 , where the symbols are modulated using a modified QAM modulation technique, as described above. 
     Along a forward path, transmitter  800  includes a pulse shaping filter/up-converter  801 , a gain application element  802 , a digital-to-analog converter (DAC) block  804 , an RF modulator  806 , a stepped variable gain amplifier (SVGA)  808 , and a power amplifier  810 . In addition, according to an embodiment, transmitter  800  includes a feedback path  811 , which will be described in detail later, and which is adapted to measure the power of the transmitted RF signal (e.g., RF antenna output signal  832 ) and to provide information that is used to control the RF gains applied to the RF signal (e.g., RF signal  820 ) prior to transmission. 
     Pulse shaping filter/up-converter  801  receives the sequence of digital input samples  812  and produces filtered digital input samples  813 . According to an embodiment, pulse shaping filter/up-converter  801  is adapted to implement a pulse shaping filter to modify the spectral shape of the digital input samples  812  in order to meet the spectral requirements of an applicable standard (e.g., a 3GPP LTE or other standard). In addition, pulse shaping filter/up-converter  801  is adapted to up-convert the digital input samples  812  to or toward a sampling rate supported by DAC block  804  (e.g., to the DAC sampling rate or to an intermediate frequency). According to an embodiment, pulse shaping filter/up-converter  801  also may include a component adapted to apply a constant gain or attenuation to the digital input samples  812  based on the peak-to-average ratio (PAR) inherent in the modulation technique being implemented. 
     Gain application element  802  receives the filtered digital input samples  813  and a digital gain signal  814 , which is produced by feedback path  811 , as will be described later. Gain application element  802  is adapted to apply compensation gains represented by the digital gain signal  814  to the filtered digital input samples  813 . More particularly, gain application element  802  applies the compensation gains represented by digital gain signal  814  to the filtered digital input samples  813  in order to generate a sequence of gain-compensated digital samples  816 , which may be represented as X c (k)=[(G i (k)×I(k)), (G q (k)×Q(k))], for example, where G i (k) includes a sequence of gain values applied to the real part of the digital input samples  812 , and G q (k) includes a sequence of gain values applied to the imaginary part of the filtered digital input samples  813 . 
     According to an embodiment, the compensation gains represented in digital gain signal  814  may inversely represent differences between a target power of the RF antenna output signal  832  and an actual power of the RF antenna output signal  832 . Digital gain signal  814  includes a sequence of digital values (e.g., compensation gain values) that, when applied to filtered digital input samples  813  by gain application element  802 , should have the effect of pre-distorting the filtered digital input samples  813  in a manner that mitigates gain application inaccuracies and/or non-linear distortion (referred to collectively as “gain inaccuracies”), which may be produced by one or more non-linear devices in the transmit lineup (e.g., SVGA  808  and/or power amplifier  810 ). Essentially, digital gain signal  814  includes a sequence of digital values that are inversely related to the gain inaccuracies that may be applied by SVGA  808  and power amplifier  810  (and possibly other transmitter elements) to the RF signal  820 . 
     During times when transmitter  800  is being operated in an open loop power control mode, the compensation gains represented in digital gain signal  814  may remain constant. More particularly, during the open loop power control mode, differences between the target power and the actual power of the RF antenna output signal  832  may not be measured and/or factored simultaneously into the value for the compensation gain provided in the digital gain signal  814 . However, feedback path  811  still may be used to perform power measurement. The measured results may be used, for example, to compute errors in open loop control, and compensation values may be factored into future transmissions. During times when transmitter  800  is being operated in a closed loop power control mode, the compensation gain represented in the digital gain signal  814  may vary in response to newly measured differences between the actual power of the RF antenna output signal  832 , as will be described in more detail later in conjunction with the discussion of the elements of feedback path  811 . According to an embodiment, transmitter  800  is operated in a closed loop power control mode during times that correspond with transmission of a DRS, an SRS or both (e.g., during times when either a DRS or an SRS is being conveyed in RF antenna output signal  832 ), and transmitter  800  is operated in an open loop power control mode during times that correspond with transmission of information-bearing symbols (e.g., during times when an information-bearing symbol is being conveyed in RF antenna output signal  832 ). As an example, transmitter  800  may operate in an open loop power control mode while ramping up transmit power to a target level, and transmitter  800  may operate in a closed loop power control mode during transmission of a DRS and/or an SRS in order to maintain an output power level while the operation point (e.g., the collector or bias voltage) of the power amplifier  810  is being adjusted to improve current efficiency. 
     The gain-compensated digital samples  816  may be upsampled (not illustrated), and provided to DAC block  804 , according to an embodiment. DAC block  804  performs a digital-to-analog conversion of the gain-compensated digital samples  816  in order to generate a gain-compensated analog signal  818 . The gain-compensated analog signal  818  may be filtered by a baseband filter (not illustrated) in order to attenuate out-of-band components, in an embodiment. RF modulator  806  receives the gain-compensated analog signal  818 , and up-converts the gain-compensated analog signal  818  to an appropriate RF carrier frequency in order to generate an RF signal  820 . 
     SVGA  808  is adapted to receive the RF signal  820  from RF modulator  806  and a gain ramp signal  822  from SVGA ramp generator  826 . SVGA  808  applies a sequence of gains represented by the gain ramp signal  822  to the RF signal  820  in order to generate a gain-adjusted RF signal  824 . In an alternate embodiment, a variable gain amplifier (VGA) may be used instead of SVGA  808  to generate the gain-adjusted RF signal  824 . 
     In order to generate the gain ramp signal  822 , SVGA ramp generator  826  receives and evaluates a power change command  828  from another portion of the system (e.g., from processing subsystem  108 ,  FIG. 1 ). The power change command  828  may specify, for example, a direction of gain change (e.g., indicating a ramp up or a ramp down in gain), and a target gain or gain change for a future sub-frame (e.g., a sub-frame to be transmitted one or two sub-frames later than the sub-frame currently being transmitted), and a direction of change. Alternatively, the power change command  828  may specify a target power level or gain for a future sub-frame. In accordance with the power change command  828 , SVGA ramp generator  826  generates the gain ramp signal  822 . In an embodiment, the gain ramp signal  822  is generated roughly as a raised cosine curve, which will cause the SVGA  808  smoothly to ramp the gains applied to the RF signal  820  from a first value to a second value that corresponds with a target power level. In an embodiment, the gain ramp signal  822  includes a sequence of codes (e.g., 6-bit codes), each of which corresponds to a gain level that may be applied by SVGA  808  to RF signal  820 . An SVGA gain transition, either upward or downward, is implemented by sequentially changing the codes provided within gain ramp signal  822 . In an embodiment, the power change command  828  is provided in adequate time before provision of a corresponding timing strobe  829  (described below) for the SVGA ramp generator  826  to calculate the parameters associated with the desired gain transition, which will be represented in the gain ramp signal  822 . 
     In addition to the power change command  828 , SVGA ramp generator  826  also receives a timing strobe  829  from another portion of the system (e.g., from processing subsystem  108 ,  FIG. 1 ), which indicates when SVGA ramp generator  826  is to begin providing the gain ramp signal  822 . The power change command  828  and the timing strobe  829  may be received at the same or different signaling interfaces, according to various embodiments. A timing strobe  829  may be received in proximity to and/or prior to a sub-frame front boundary or an SRS front or rear boundary. A timing strobe  829  received in proximity to and prior to a sub-frame front boundary will cause the SVGA ramp generator  826  to begin outputting a gain ramp signal  822  that causes the SVGA  808  to ramp the gains applied to the RF signal  820  from a first value to a second value that corresponds with the target power level for the next sub-frame. A timing strobe  829  received in proximity to and prior to a front SRS boundary will cause the SVGA ramp generator  826  to begin outputting a gain ramp signal  822  that causes the SVGA  808  to ramp the gains applied to the RF signal  820  up from a first value to a second value that corresponds with an SRS power level. Another timing strobe  829  received in proximity to a rear SRS boundary will cause the SVGA ramp generator  826  to begin outputting a gain ramp signal  822  that causes the SVGA  808  to ramp the gains applied to the RF signal  820  down from a first value that corresponds with the SRS power level to a second value that corresponds with the power level for the next sub-frame to be transmitted (e.g., in an embodiment in which the SRS is transmitted in a last symbol position of the sub-frame), or to the power level for the sub-frame in which the SRS is transmitted (e.g., in embodiments in which the SRS is transmitted prior to the last symbol position). 
     After SVGA  808  applies the gain to the RF signal  820  in accordance with the gain ramp signal  822 , the resulting gain-adjusted RF signal  824  (also referred to herein as an “analog version of the gain-compensated digital samples”) is de-coupled through transformer  830  (e.g., a balun) and received by power amplifier  810 . Essentially, the de-coupled, gain-adjusted RF signal  824  may be considered to be an analog version of the gain-compensated digital samples  816 . Power amplifier  810  amplifies the de-coupled, gain-adjusted RF signal  824  to generate an RF antenna output signal  832 . The RF antenna output signal  832  is radiated onto the air interface by an antenna (e.g., antenna  106 ,  FIG. 1 ). 
     As mentioned above, gain application element  802  applies compensation gains represented by digital gain signal  814  to the digital input samples  812 . During operations in a closed loop power control mode, elements of feedback path  811  are activated to measure the power of the RF antenna output signal  832  and to adjust the compensation gains represented in the digital gain signal  814 . During operations in an open loop power control mode, adjustments to the compensation gains may not be made simultaneously with measuring signals on feedback path  811 . For description purposes, a switch  833  is illustrated in feedback path  811  to indicate operation in an open loop or closed loop power control mode at various times. When switch  833  is closed, operation in a closed loop power control mode may be assumed, and when switch  833  is open, operation in an open loop power control mode may be assumed. In practice, such a switch  833  may not actually be present, and operation in an open loop or closed loop power control mode may otherwise be implemented. Although a particular embodiment of a feedback path is illustrated in  FIG. 8  and described herein, it is to be understood that any of a number of other types of feedback paths may be incorporated into transmitter  800 , in other embodiments. 
     In an embodiment, feedback path  811  includes an output signal sensing element  834  (e.g., a transformer), an attenuator  836 , a power detector  838 , a reference comparator  840 , an error accumulator  842 , and a transmit (TX) power controller  850 . Output signal sensing element  834  senses the power level of the RF antenna output signal  832 , and converts that power level into an analog feedback signal  835 . In order to prepare the analog feedback signal  835  for further processing, attenuator  836  is adapted to attenuate the analog feedback signal  835  to produce an attenuated analog feedback signal  837 , according to an embodiment. In an alternate embodiment, attenuation of the analog feedback signal  835  is not performed, and accordingly analog feedback signal  835  and attenuated analog feedback signal  837  may generically be referred to as an “analog feedback signal”. 
     The analog feedback signal  835  or  837  typically will have a varying power level. Power detector  838  is adapted to sense the power level of the analog feedback signal  835  or  837 . More particularly, according to an embodiment, power detector  838  is adapted to convert instantaneous power levels of portions of the analog feedback signal  835  or  837  that correspond with transmission of a DRS, an SRS or both into voltage values. In a particular embodiment, when a DRS or SRS includes a CP, power detector  838  may bypass performing power measurements for those portions of the DRS or SRS that correspond with the CP. Alternatively, power detector  838  may perform power measurements for those portions of the DRS or SRS that correspond with a CP, although those measurements may be disregarded at later stages of the power control process. 
     According to an embodiment, power detector  838  includes a linear detector, which measures the power level in dBm (i.e., the ratio in decibels (dB) of the measured power referenced to one milliwatt (mW)). Power detector  838  is adapted to produce a digital feedback signal  839  that includes a sequence of values (e.g., voltage values) that represent the sensed power levels. Accordingly, power detector  838  may include an analog-to-digital converter (ADC, not illustrated) adapted to convert the sensed power level of the analog signal (e.g., as measured in dBm) into a digital feedback signal  839 . According to an embodiment, the ADC performs a sampling and quantizing process to generate a sequence of digital voltage values, and power detector  838  represents the digital voltage values in the digital feedback signal  839  (e.g., power detector  838  may scale and/or quantize the analog feedback signal samples to produce a sequence of voltage values that are represented in the digital feedback signal  839 ). Power detector  838  also may include a power measurement block (not illustrated) that computes the average value of the digital voltage values over a programmable period of time. 
     Reference comparator  840  is adapted to compare the values represented in digital feedback signal  839  with a reference signal  841  in order to generate a difference signal  843 . The reference signal  841  represents the desired power level at which the RF antenna output signal  832  should be transmitted. Accordingly, the difference signal  843  generated by reference comparator  840  includes values that represent differences between the actual power level at which the RF antenna output signal  832  is being transmitted and the desired power level at which the RF antenna output signal  832  should be being transmitted. While the transmitter is being operated in a closed loop power control mode, the values represented in difference signal  843  are accumulated by error accumulator  842  to produce an accumulated error value  845 . 
     TX power controller  850  generates the digital gain signal  814  based on the accumulated error value  845 . According to an embodiment, and as discussed previously, the digital gain signal  814  is generated to include compensation gain values that may inversely represent the differences between a target power of the RF antenna output signal  832  and an actual power of the RF antenna output signal  832 , where the differences are reflected in the accumulated error value  845 . 
     According to an embodiment, TX power controller  850  also may be adapted dynamically to adjust the digital gain signal  814  to maintain a substantially constant transmit power level while the collector or bias voltage of power amplifier  810  is being adjusted based on a target transmit power. In order to optimize the efficiency of power amplifier  810 , power amplifier  810  normally may be operated at a minimum collector or bias voltage that provides sufficient linearity at the transmitted power level. Adjustments to the collector or bias voltage of power amplifier  810  may result in improved efficiency of power amplifier  810  under different loading conditions. In the case of implementing a power increase, the collector or bias voltage may be adjusted prior to the power change. In the case of implementing a power decrease, the collector or bias voltage may be adjusted after the power change. For example, when the power of an upcoming sub-frame is higher than the power of a sub-frame currently being transmitted, the collector or bias voltage of power amplifier  810  may be adjusted at or in proximity to the DRS symbol of the second slot of the current sub-frame. While the collector or bias voltage is being adjusted, transmitter  800  may be operated in a closed loop power control mode in order to compensate for the gain change of power amplifier  810 , and to achieve a substantially constant output power. When the power of an upcoming sub-frame is lower than the power of the current sub-frame, the collector or bias voltage of power amplifier  810  may be adjusted at or in proximity to the DRS symbol of the first slot in the upcoming sub-frame. In both cases, while the collector or bias voltage is being adjusted, transmitter  800  is operated in a closed loop power control mode in order to compensate for gain changes applied by power amplifier  810 , and to achieve a substantially constant output power during the DRS symbol. Operating in a closed loop power control mode during transmission of a DRS may ensure enhanced power accuracy during the closed loop power control mode of operation. For power changes that will occur due to the presence of an SRS, similar methods may be applied. 
       FIG. 9  illustrates a flowchart of a method for performing power measurement and control, according to an example embodiment. Referring also to  FIG. 8 , the method may begin, in block  902 , when a plurality of symbols are formatted (e.g., by processing subsystem  108 ,  FIG. 1 ) into a sub-frame structure (e.g., sub-frame  200 ,  FIG. 2 ), according to an embodiment. Each sub-frame may include one or more constant modulus symbols, which may include, for example, an SRS and/or one or more DRS. As used herein, the term “constant modulus symbol” refers to a symbol in which a constant modulus (e.g., in the frequency domain) signal is represented (e.g., an SRS, DRS or other type of constant modulus signal). In block  904 , a sequence of digital input samples (e.g., digital input samples  812  or filtered digital input samples  813 ) is produced (e.g., by processing subsystem  108 ,  FIG. 1 ), which represents the symbols within the sub-frame. 
     In block  906 , the sequence of digital input samples is received (e.g., by gain application element  802 ) and combined with a digital gain signal (e.g., digital gain signal  814 ) to generate a sequence of gain-compensated digital samples (e.g., gain-compensated digital samples  816 ). Generation of the digital gain signal will be discussed in subsequent steps, and an assumption is made at this point that such steps have been performed based on previously transmitted RF output signals (e.g., RF antenna output signal  832 ). In block  908 , a digital-to-analog (D-to-A) conversion process is performed (e.g., by DAC block  804 ) to convert the gain-compensated digital samples into the analog domain. The resulting gain-compensated analog signal (e.g., gain-compensated analog signal  818 ) is then upconverted (e.g., by RF modulator  806 ) to a carrier frequency, in block  910 , in order to generate an RF signal (e.g., RF signal  820 ). 
     In block  912 , a gain ramp signal (e.g., gain ramp signal  822 ) is generated, and gains are applied to the RF signal (e.g., by SVGA  808 ) based on the gain ramp signal, in order to generate a gain-adjusted RF signal (e.g., gain-adjusted RF signal  824 ). In block  914 , the gain-adjusted RF signal (e.g., gain-adjusted RF signal  824 ) is amplified (e.g., by power amplifier  810 ) and radiated onto the air interface (e.g., by antenna  106 ,  FIG. 1 ). 
     In block  916 , a determination may be made whether the transmitter is operating in a closed loop power control mode or an open loop power control mode. When the transmitter is operating in an open loop power control mode, some or all of blocks  918 - 922  may be bypassed, and the method may iterate as shown. More particularly, block  922  will be bypassed, in accordance with an embodiment, and blocks  918  and  920  may or may not be performed. 
     When transmitter is operating in a closed loop power control mode, then in block  918 , an analog feedback signal (e.g., analog feedback signal  835 ) is generated from the antenna output signal (e.g., RF antenna output signal  832 ). In block  920 , instantaneous power levels of the analog feedback signal are sensed, and the sensed power levels are converted (e.g., by power detector  838 ) into a digital feedback signal (e.g., digital feedback signal  839 ), which may include a sequence of digital voltage values. 
     In block  922 , the voltage values in the digital feedback signal are compared (e.g., by reference comparator  840 ) with a reference signal (e.g., reference signal  841 ) in order to generate a signal that includes a sequence of difference values (e.g., difference signal  843 ). Each difference value represents a difference between a voltage value from the digital feedback signal and a target voltage as represented in the reference signal. The difference values are accumulated (e.g., by error accumulator  842 ) to produce an accumulated error value (e.g., accumulated error value  845 ). In block  924 , the digital gain signal is generated based on the accumulated error value. More particularly, according to an embodiment, the compensated gain values within the digital gain signal are generated based on the accumulated error value. The method may then iterate, as shown in  FIG. 9 , for subsequently received digital input samples. 
     As indicated previously, and according to an embodiment, a transmitter may switch between an open loop power control mode and a closed loop power control mode. During operations in a closed loop power control mode, an RF antenna output signal (e.g., RF antenna output signal  832 ,  FIG. 8 ) is measured, and compensation gains represented in a digital gain signal (e.g., digital gain signal  814 ,  FIG. 8 ) may be adjusted to reflect differences between the actual and target power levels of the RF antenna output signal. Conversely, during operations in an open loop power control mode, adjustments to the compensation gains may be obtained based only on values within a phasing table in order to perform power changes. Power measurement may be performed to compute open loop errors, and the corresponding entries in the phasing table may be updated. According to an embodiment, switching between a closed loop and an open loop power control modes may be performed based on whether the transmitter is transmitting a constant modulus signal (e.g., a DRS and/or SRS), among other reasons. 
       FIG. 10  illustrates a flowchart of a method for switching between an open loop power control mode and a closed loop power control mode, according to an example embodiment. An embodiment of the method depicted in  FIG. 10  may be performed continuously during operation of a transmitter (e.g., during transmission of RF signals by the transmitter), according to an embodiment, although the method may be performed less frequently, in another embodiment. 
     In block  1002 , a determination may be made whether the power of the RF signal being transmitted (e.g., RF antenna output signal  832 ,  FIG. 8 ) is exceeds a power threshold. The power threshold may be, for example, 0 dBm, although higher or lower power thresholds may be implemented, as well. When the RF signal power is less than the power threshold, then the transmitter switches to or remains in an open loop power control mode, as indicated by block  1004 , and the method iterates as shown. 
     When the RF signal power exceeds the power threshold, a further determination may be made, in block  1006 , whether a specified type of constant modulus symbol is being transmitted. For example, a constant modulus symbol may be a DRS, an SRS, or another type of constant modulus symbol. When the specified type of constant modulus symbol is not being transmitted, then the transmitter switches to or remains in an open loop power control mode, as indicated by block  1004 , and the method iterates as shown. As discussed previously, during operations in an open loop power control mode, adjustments to the compensation gains (and thus power changes) may be obtained based on values in a phasing table. More particularly, open loop errors may be determined based on the digital feedback samples, and the compensation gains represented in the digital gain signal may be determined from the phasing table based on the open loop errors. The entries in the phasing table may be updated accordingly. 
     When the specified type of constant modulus symbol is being transmitted, then the transmitter switches to or remains in a closed loop power control mode, as indicated by block  1008 , and the method iterates as shown. According to an embodiment, a DRS may be considered a “specified type of constant modulus symbol”, and the transmitter will switch to or remain in a closed loop power control mode when a DRS is being transmitted. According to another embodiment, an SRS may be considered a “specified type of constant modulus symbol”, and the transmitter will switch to or remain in a closed loop power control mode when an SRS is being transmitted. According to yet another embodiment, both a DRS and an SRS may be considered a “specified type of constant modulus symbol”, and the transmitter will switch to or remain in a closed loop power control mode when either a DRS or an SRS is being transmitted. According to yet another embodiment, the transmitter will switch to or remain in a closed loop power control mode when a different type of constant modulus symbol is being transmitted. As mentioned previously, a transmitter may operate in a closed loop power control mode while the operation point (e.g., the collector voltage) of the power amplifier is being adjusted to improve current efficiency. 
     It is to be understood that certain ones of the process blocks depicted in  FIGS. 9 and 10  may be performed in parallel with each other or with performing other processes. For example, in a closed loop transmitter, an RF signal generated based on previously-received digital input samples may be being fed back and analyzed to determine a digital gain signal in parallel with receiving new digital input samples. In addition, it is to be understood that the particular ordering of the process blocks depicted in  FIGS. 9 and 10  may be modified, while achieving substantially the same result. Accordingly, such modifications are intended to be included within the scope of the inventive subject matter. 
     Thus, various embodiments of wireless devices, transmitters, and methods for performing power measurement and control have been described. A particular embodiment includes a wireless device having a gain application element and a power amplifier. The gain application element is adapted to receive a sequence of digital input samples and a digital gain signal, and to combine the digital gain signal with the digital input samples to generate a sequence of gain-compensated digital samples. The power amplifier is adapted to receive and amplify an analog version of the gain-compensated digital samples in order to generate an antenna output signal. The transmitter is adapted to operate in a closed loop power control mode when a specified type of constant modulus symbol is represented in the antenna output signal, and to operate in an open loop power control mode when the specified type of constant modulus symbol is not represented in the antenna output signal. 
     Another embodiment includes a transmitter having a gain application element, a power amplifier, and a feedback path. The gain application element is adapted to receive a sequence of digital input samples and a digital gain signal, and to combine the digital gain signal with the digital input samples to generate a sequence of gain-compensated digital samples. The power amplifier is adapted to receive and amplify an analog version of the gain-compensated digital samples in order to generate an antenna output signal. The feedback path is adapted to generate an analog feedback signal from the antenna output signal, to produce a sequence of digital feedback samples from the analog feedback signal, to generate difference values based on the digital feedback samples, to accumulate the difference values into an accumulated error value when a specified type of constant modulus symbol is represented in the antenna output signal, and to produce the digital gain signal from the accumulated error value. 
     Another embodiment includes a method for performing power control in a wireless device. The method includes the steps of combining a sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples, where the sequence of digital input samples represents a plurality of symbols, and amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal. The method also includes the steps of generating an analog feedback signal from the antenna output signal, producing a sequence of digital feedback samples from the analog feedback signal, generating difference values based on the digital feedback samples, responsive to the digital feedback samples corresponding to a constant modulus symbol, accumulating the difference values into an accumulated error value, and generating the compensation gain values represented in the digital gain signal based on the accumulated error value. 
     Another embodiment includes a method for performing power control in a wireless device. The method includes the step of receiving a sequence of digital input samples that represent a plurality of symbols that are formatted into a sub-frame structure that includes a plurality of information-bearing symbols and at least one constant modulus symbol. The method also includes the steps of combining the sequence of digital input samples with compensation gain values in a digital gain signal to generate a sequence of gain-compensated digital samples, amplifying an analog version of the gain-compensated digital samples in order to generate an antenna output signal, generating an analog feedback signal from the antenna output signal, and producing a sequence of digital feedback samples from the analog feedback signal. The method also includes the step of performing closed loop power control using digital feedback samples corresponding to one or more of the at least one constant modulus symbol. 
     While the principles of the inventive subject matter have been described above in connection with specific systems, apparatus, and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the inventive subject matter. Further, the phraseology or terminology employed herein is for the purpose of description and not of limitation. 
     The foregoing description of specific embodiments reveals the general nature of the inventive subject matter sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the general concept. Therefore, such adaptations and modifications are within the meaning and range of equivalents of the disclosed embodiments. The inventive subject matter embraces all such alternatives, modifications, equivalents, and variations as fall within the spirit and broad scope of the appended claims.

Metadata:
Filing Date: 20081031
Publication Date: 20120821
Grant Date: 20120821
Priority Date: 20081031
Inventors: XU BING
RAHMAN MAHIBUR
ZHAO CHUNMIG
Assignee: APPLE INC
CPC Classifications: [{"code": "H04L27/0012", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W52/262", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04W52/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W52/08", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04W52/52", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 42132068