Abstract:
Methods and apparatus for aligning output power levels of a transmitter having a power amplifier adapted to operate in first and second operational modes. According to an exemplary embodiment, the transmitter includes a power alignment circuit configured to execute a power alignment algorithm. The power alignment algorithm is operable to align an output power level of the power amplifier when configured in the first operational mode with an output power level of the power amplifier when configured in the second operational mode. When the power amplifier is switched from the first operational mode to the second operational mode, the power alignment circuit references a power table having power entries that ensure that the output power level in the second operational mode is aligned with the output power level in the first operational mode. So that power control tolerances are satisfied for changes in power levels before and after a mode switch, one or more additional power control settings can be inserted in the power table.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/875,084, filed on Dec. 14, 2006, the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless communications transmitters. More specifically, the present invention relates to power control in communications transmitters. 
     BACKGROUND OF THE INVENTION 
     Cellular telecommunications technologies continue to evolve to satisfy consumer demand for fast and reliable mobile communications. First generation (1G) analog communications systems have been superseded by second generation (2G) digital communications systems, such as the Global System for Mobile Communications (GSM) system. In the last several years these 2G systems have been enhanced by the introduction of General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) wireless services (often referred to as 2.5 G and 2.75 G systems), which provide users not only with voice communication capabilities but also data communication capabilities. Currently, a third generation (3G) system known as the Universal Mobile Telecommunications System (UMTS), which employs the Wide-Band Code Division Multiple Access (W-CDMA) wireless service, is being implemented in many parts of the World, to provide even faster and more reliable voice and data communications. 
     While advances in cellular telecommunications systems have benefited consumers, the realization of higher data throughput and the increasing need for efficient use of available radio frequency (RF) spectra has led to more stringent telecommunications standards. These more stringent standards require handset manufacturers to provide solutions that operate according to more complex modulation schemes and enhanced power control conditions. For example, whereas GSM uses a constant envelope modulation scheme, EDGE and W-CDMA technologies employ more sophisticated non-constant envelope signals. EDGE and W-CDMA also require that the RF transmitter of a mobile terminal to control its output power over a wide dynamic range. Specifically, the EDGE standard requires a transmitter to have the ability of controlling output power over a 30 dB range, while the W-CDMA standard requires a transmitter to have the ability of controlling output power over an 80 dB range. 
     The wide dynamic range in output power control specified by the W-CDMA standard results from the fact that the W-CDMA wireless interface utilizes the direct sequence CDMA (Code Division Multiple Access) signaling method. All mobile terminals share the same radio resource in CDMA-based systems. Consequently, it is important that each physical channel between a base station and a mobile terminal not use more power than necessary. To accomplish this level of power control, W-CDMA systems use a transmit power control (TPC) mechanism, in which base stations in the network transmit TPC commands to mobile terminals in a downlink (DL) direction. The TPC commands require the mobile terminals to increase or decrease their transmission power levels in the uplink (UL) direction in increments (e.g., +/−1, 2, 3, etc. decibel (dB) increments), so that system power use is managed and maintained at acceptable levels. 
     Wide dynamic range in output power is difficult to achieve in conventional RF transmitters (e.g., those based upon quadrature modulators). The difficulty derives from the requirement that the power amplifier (PA) used in such transmitters operate with high linearity, so as to prevent, for example, spectral re-growth and unwanted adjacent channel interferers. The linearity requirement becomes especially problematic when non-constant envelope signaling schemes, such as EDGE and W-CDMA are used, since the drive levels to the PA must be reduced to avoid signal distortion caused by clipping of signal peaks. Additional linearization resources must also be provided to ensure signal integrity. Unfortunately, the immediate consequence of these efforts to preserve linearity is an overall reduction in power efficiency. 
     A polar modulation transmitter is an alternative approach that avoids the problems associated with the conventional quadrature-modulator-based transmitter. As explained below, the polar modulation transmitter is energy efficient, since the PA is not required to operate with high linearity, and is capable of controlling output power over a wide dynamic range. 
       FIG. 1  is an architectural diagram of a typical polar modulation transmitter  100 . As shown, the polar modulation transmitter  100  comprises a polar signal generation circuit  102 , an amplitude control circuit  104 , a PA  106 , an antenna  108 , and a phase modulated signal generation circuit  110 . In operation, the polar signal generation circuit  102  operates on an input signal to provide an envelope component signal containing amplitude information of the input signal and a constant-amplitude phase component signal containing phase information of the input signal. The envelope component signal is coupled to an input of the amplitude control circuit  104  along an amplitude path, and the constant-amplitude phase component signal is coupled to an input of the phase modulated signal generation circuit  110 . The phase modulated signal generation circuit  110  is configured to receive the constant-amplitude phase component signal and generate a constant-amplitude phase-modulated RF drive signal, which is coupled to an RF input of the PA  106  along a phase path. The amplitude control circuit  104  is configured to receive the envelope component signal along the amplitude path and provide an amplitude modulated power supply voltage having a power level determined by a transmit power control signal coupled to a power control input of the amplitude control circuit  104 . The amplitude modulated power supply voltage is coupled to a power supply port of the PA  106 , which amplifies the constant-amplitude phase-modulated RF drive signal in the phase path according to the amplitude modulated power supply voltage, thereby providing a modulated RF output signal that is radiated by the antenna  108  to a system base station. 
     Because signal envelope variation and output power control in the polar modulation transmitter  100  are performed by varying the gain of the polar output stage  106 , there is no need for RF circuit linearity, as there is in conventional transmitters. Both in-band and out-of-band output noise are also dramatically lower compared to output noise produced by conventional transmitters. Another benefit of the polar modulation transmitter  100  is that it is capable of controlling output power over a wide dynamic range. This is achieved by configuring the PA  106  to operate in compressed mode during times when a high transmission power is required, and switching to uncompressed mode during times when only a low transmission power is required. When configured in compressed mode the output power of the transmitter is controlled by the amplitude modulated power supply voltage applied to the collector (or drain) node of the PA  106 , while the power of the constant-amplitude phase-modulated RF drive signal is kept constant. When configured in uncompressed mode, the output power of the PA  106  is controlled by varying the power of the phase-modulated RF drive signal, while the collector (or drain) node of the PA  106  is held constant. 
     Although the polar modulation transmitter  100  is fully capable of achieving a wide dynamic range in output power, even for W-CDMA applications where an 80 dB output power control range is required, drift in output power can make power control difficult. The Third Generation Partnership Project (3GPP), which is the standards body responsible for promulgating UMTS and W-CDMA standards, requires that TPC commands from a cellular network base station result in a mobile terminal increasing or decreasing its output power level in discrete steps (e.g., +/−1 dB, +/−2 dB, +/−3 dB, etc.). The UMTS standard also specifies that these power level steps be performed within certain specified tolerances. For example, as shown in the table in  FIG. 2 , a TPC command for a +/−1 dB step in output power level requires that the resulting output power be within +/−0.5 dB of the target output power. So, if a transmitter of a mobile terminal is operating at 0 dBm, and a TPC command of “1” is received, the transmitter of the mobile terminal must adjust its power so that it transmits within a range of +0.5 dBm and 1.5 dBm. Wider tolerances of +/−1 dB and +/−1.5 dB are permitted for larger step sizes of 2 and 3 dB. The 3GPP UMTS standard also imposes cumulative tolerances for groups of power commands, as shown in the table in  FIG. 3 . For example, ten equal TPC command groups of 1 dB step size each, requires that the resulting output power level be within +/−2 dB of the target output power level. 
     Inspection of the table in  FIG. 2  reveals that the most restrictive step size tolerance for a single TPC command is for a TPC command directing a +/−1 dB step size (+/−0.5 dB tolerance is required). Unfortunately, this tolerance is not always easily satisfied by the polar modulation transmitter  100  in  FIG. 1 , during times when the power level step involves a mode switch from uncompressed mode to compressed mode. Ideally, the output power level following a mode switch from uncompressed mode to compressed mode is continuous. When the PA  106  is configured to operate in compressed mode, steps in power levels remain fairly accurate. However, as illustrated in  FIG. 4 , a discontinuity (or “gap”) is observed in the output power curve near the region where a mode switch occurs. This discontinuity is caused by drift in operating characteristics of circuitry within the phase path of the transmitter and in the power amplifier, which can be sensitive to temperature, aging, load conditions, and voltage variations. It has been observed that in some circumstances the discontinuity between the compressed and uncompressed mode power level curves is large enough that the +/−0.5 dB tolerance for 1 dB power step size specified by the UMTS standard is not satisfied. It would be desirable, therefore, to have methods and apparatuses for aligning the power level curves for step sizes involving a mode switch in a polar modulation transmitter, so that the UMTS power control accuracy requirements are satisfied. 
     SUMMARY OF THE INVENTION 
     Methods and apparatus are disclosed for controlling the output power of a communications transmitter. An exemplary communications transmitter includes a power amplifier adapted to be configured in either a first operational mode or a second operational mode and a power alignment circuit coupled to the power amplifier. The power alignment circuit is operable to execute a power alignment algorithm, which aligns an output power level of the power amplifier when configured in the first operational mode with an output power level of the power amplifier when configured in the second operational mode. According to an exemplary embodiment, the power alignment circuit includes a detector that is selectively coupled to an output of the power amplifier. When coupled to the output of the amplifier, the detector measures a first output power level of the power amplifier when the power amplifier is configured to operate according to said first operational mode, and measures a second output power level of the power amplifier when the power amplifier is configured to operate according to the second operational mode. The power alignment algorithm uses the first and second measured output power levels to determine whether the power alignment circuit should configure the power amplifier to operate according to the first operational mode or the second operational mode. When the power amplifier is switched from the first operational mode to the second operational mode, the power alignment circuit references a power table having power entries that ensure that the output power level in the second operational mode is aligned with the output power level in the first operational mode. 
     An exemplary method of switching a transmitter operating in a first mode to operating in a second mode includes creating a power table having a plurality of power control settings used to configure the transmitter to provide different output power levels. An overlap region is defined within which the transmitter can be configured to provide the same output power level when configured in a first operational mode as is provided when configured in a second operational mode. If necessary, one or more additional power control settings are inserted in the power table for the second operational mode. In response to a power level change that results in an output power level of the power amplifier falling within the overlap region, one of the one or more additional power control settings is selected from the power table to switch the transmitter to operate according to the power level change in the second operational mode while satisfying a power control accuracy requirement. 
     Further aspects of the invention are described and claimed below, and a further understanding of the nature and advantages of the invention may be realized by reference to the remaining portions of the specification and the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an architectural diagram of a typical polar modulation transmitter; 
         FIG. 2  is a table showing power control tolerances for various output power step size commands, according to the 3GPP UMTS standard; 
         FIG. 3  is a table showing cumulative power control tolerances for various groups of power commands, according to the 3GPP UMTS standard; 
         FIG. 4  is a graph illustrating how a discontinuity in output power level between compressed and uncompressed modes of the polar modulation transmitter in  FIG. 1  may result due to drift in operating characteristics of circuitry within the phase path of the transmitter, and the power amplifier; 
         FIG. 5  is an architectural drawing of a polar modulation transmitter, according to an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating various regions of operation of the polar modulation transmitter in  FIG. 5 ; 
         FIG. 7  is a graph illustrating amplitude path envelope scaling and phase path magnitude scaling settings as a function of output power when the power amplifier of the polar modulation transmitter in  FIG. 5  is configured in the compressed, uncompressed and overlap regions; 
         FIGS. 8A-C  are mode switch diagrams, illustrating processes by which the power amplifier of the polar modulation transmitter in  FIG. 5  is switched from uncompressed mode to compressed mode for power level step sizes of 1, 2 and 3 dB respectively; and 
         FIGS. 9A  and B show a flowchart that illustrates operation of the polar modulation transmitter in  FIG. 5  beginning with operation in uncompressed mode, including how the power alignment algorithm operates to perform a mode switch from uncompressed mode to compressed mode, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  is an architectural drawing of a polar modulation transmitter  500 , according to an embodiment of the present invention. The polar modulation transmitter  500  comprises a polar signal generation circuit  502 , an amplitude control circuit  504 , a phase modulated signal generation circuit  506 , a variable gain amplifier (VGA) and/or attenuator  508  (referred to herein as “VGA/attenuator”), a power amplifier (PA)  510 , an antenna  512 , and a power alignment loop (PAL)  514 . The PAL  514  comprises a power detector (e.g., a PIN diode or other semiconductor detector)  516  coupled to the RF output of the transmitter  500 , a low-pass filter (LPF)  518 , an analog to digital converter (ADC)  520 , and a transmit power control section  522 . 
     The polar signal generation circuit  502  of the polar modulation transmitter  500  operates on an input signal to provide an envelope component signal containing amplitude information of the input signal, and a constant-amplitude phase component signal containing phase information of the input signal. The envelope component signal is coupled to an input of the amplitude control circuit  504  along an amplitude path, and the constant-amplitude phase component signal is coupled to an input of the phase modulated signal generation circuit  506  along a phase path. The phase modulated signal generation circuit  506  is configured to receive the constant-amplitude phase component signal and generate a constant-amplitude phase-modulated RF signal, which is coupled to an input of the VGA/attenuator  508 . The VGA/attenuator  508  either amplifies or attenuates the constant-amplitude phase-modulated RF signal, depending on a correction coefficient provided by the PAL  514 , and couples the resulting scaled constant-amplitude phase-modulated RF signal to a drive input of the PA  510 . At the same time, the amplitude control circuit adjusts the amplitude of the envelope signal, according to a scaling factor provided by the PAL  514 , thereby providing an amplitude-varying supply voltage which is coupled to a power supply port of the PA  510 . The PA  510  amplifies the scaled constant-amplitude phase-modulated RF signal applied to the drive input of the PA  510 , depending on the scaling factor and correction coefficient provided to the amplitude control circuit  502  and the VGA/attenuator  508  from the transmit power control section  522 . 
     In general, power control is achieved by the VGA/attenuator  508  in the phase path of the polar modulation transmitter  500 , as well as by the amplitude control circuit  502  in the amplitude path. According to an embodiment of the invention, the PA  510  of the transmitter  500  is configured to operate in compressed mode during times when the transmit power is above a first predetermined threshold, and is configured to operate in uncompressed mode during times when the transmit power is below a second predetermined threshold. 
     The compressed and uncompressed regions of operation are more clearly shown in  FIG. 6 . Pu,max corresponds to the aforementioned first predetermined threshold while Pc,min corresponds to the second predetermined threshold. The overlap region is a power level region in which the same output power can be achieved using either compressed mode or uncompressed mode. Pda represents the lowest uncompressed mode power level that, due to tolerances, may actually produce a power level in the overlap region. Pd,min is the lowest possible measured actual output power at power level Pc,min, and represents the lowest registered value corresponding to Pc,min due to imperfections in the detector PAL detector  516 . According to one embodiment of the invention the boundaries of the overlap region are programmable. The programmability allows the polar modulation transmitter  500  to be adapted for operation in various wireless communications systems, and also affords the ability to accommodate PAs having different operating characteristics. 
       FIG. 7  shows amplitude path envelope scaling and phase path magnitude scaling settings as a function of output power when the PA  510  of the polar modulation transmitter  500  is configured in the compressed, uncompressed and overlap regions. When the PA  510  is configured for operation in compressed mode (to the right of the right-most vertical dotted line), power control is achieved by scaling the amplitude path of the transmitter  500  while maintaining a constant amplitude phase path drive signal. When the PA  510  is configured to operate in uncompressed mode (to the left of the left-most vertical dotted line), power control is achieved by scaling the drive signal in the phase path while maintaining a constant scaling factor in the amplitude path. 
     Whether the PA  510  is configured to operate in compressed mode or uncompressed mode is application dependent. According to an exemplary embodiment adapted for W-CDMA operation, the lowest compressed mode power level (Pc,min) is specified as 0 dBm and the overlap region is 6 dB wide (+/−2 dB plus an additional 2 dB of margin). The calibration region refers to an output power range in which a mode switch between the compressed and uncompressed modes is effected. 
     During operation, the polar modulation transmitter  500  is commanded to lower and raise its output power to comply with network operating conditions. For example, as discussed above, in UMTS applications TPC commands require the transmitter to increase or decrease its output power in power increments (e.g., +/−1 dB, +/−2 dB, etc.). If a power level step increase (ΔP) from a current power level within the uncompressed region results in an output power level that is still within the uncompressed region, the transmitter  500  simply retrieves the appropriate amplitude and/or phase path scaling factors from a power table memory, using the new power level as an address into the table. The transmit power control section  522  then applies the scaling factors to the amplitude control circuit and VGA/attenuator  508  to effect the power level change. 
     If the power level increase is so great that it causes a jump over the overlap region (i.e., from within the uncompressed region and into the compressed region), a mode switch is performed by retrieving the appropriate scaling factors for the amplitude and phase paths that will set the PA  510  at the new power level in the new mode. Because the output power level tolerances are wide for these large step sizes, no calibration between the two modes is required. 
     The situation is different when smaller step sizes are commanded, and the power level increase ΔP results in an output power level that falls within the overlap region. When this condition occurs, the PAL  514  of the polar modulation transmitter  500  is activated to determine whether a mode switch from uncompressed mode to compressed mode is required, and what steps must be performed to ensure that the resulting power level change satisfies specified output power level control tolerances. 
     As explained above, a discontinuity or gap may be observed between the output compressed mode and uncompressed mode power level curves, due to drift in operating characteristics of circuitry within the phase path of the transmitter. Unfortunately, this discontinuity can make it difficult to satisfy power control tolerances when a mode switch is performed for power levels in the overlap region. For example, the W-CDMA standard allow a tolerance of no greater than +/−0.5 dB tolerance for 1 dB power step sizes. 
     According to an embodiment of the invention, the PAL  514  is activated when a power level change results in a power level that falls within the overlap region, to ensure that power control tolerances specified by a wireless standard are complied with. More specifically, when a power level change results in a power level within the overlap region, the PAL  514  of the polar modulation transmitter  500  executes an algorithm that determines the scaling factor and/or correction coefficient required by the amplitude control circuit  504  and the VGA/attenuator  508  to provide the target output power level. The algorithm may be implemented as a state machine in one or more integrated circuits, which are either separate from or included with some or all of the other components of the polar modulation transmitter  500 . The actual scaling factors and/or correction coefficients are stored in a table and are indexable by a power control signal received by the transmit power control section  522 . 
     When the power level step ΔP raises the output power level from within the uncompressed mode region into the overlap region, the PAL  514  algorithm operates to determine the closest compressed mode power level that satisfies the applicable wireless standard power control accuracy specification, while maintaining the same VGA/attenuator  508  control settings. Additional power level entries in the power table may be necessary, however, to ensure that on the power control accuracy requirements. 
     Consider, for example, a W-CDMA system in which the transmitter  500  is commanded to change output power level by ΔP=1 dB, and assume that the current output power level, Ptable, is set at a nominal level of −1 dBm and the lowest compressed mode power level is 0 dBm. With an allowable power control tolerance of +/−0.5 dB, this means that the output power level before the power level change may permissibly be anywhere within a range of −1.25 and −0.75 dBm. The ΔP=1 dB increase in power level would result in an output power level of Ptable+ΔP being within the range of −0.25 and +0.25 dBm, as illustrated in the mode transition diagram presented in  FIG. 8A . A mode switch to the closest available compressed mode power level of 0 dBm satisfies the +/−0.5 dB power control tolerance allowed by the W-CDMA specification, and even satisfies a more stringent maximum allowable step size error of +/−0.25 dB. 
     Unfortunately, a maximum allowable step size error of +/−0.25 dB cannot be satisfied for all power level transitions from uncompressed mode to compressed mode. Assume, for example, that the current uncompressed mode power level, Ptable, is at −0.7 dBm. A ΔP=+1 dB power level step would result in an uncompressed mode power level of 0.3 dBm and a tolerance range of 0.3+/−0.5 dBm (i.e., −0.2 dBm to 0.8 dBm). With a maximum allowable step size error of +/−0.25 dB (specification is +/−0.5 dB), a transition to the closest compressed mode power level of 1 dBm would provide a power level range of 1+/−0.25 dBm (i.e., 0.75 dBm to +1.25 dBm). Such a mode transition would result in the upper end of the power level range being greater than the upper tolerance level of the −0.2 dBm to 0.8 dBm tolerance range. Hence, the power control tolerance of the W-CDMA specification would be violated if a mode switch to the compressed mode 1 dBm power level was permitted. A transition to the 0 dBm compressed mode power level (0+/−0.25 dBm or −0.25 dBm to +0.25 dBm) would also result in a violation of the specification since the lower end of the power level range would be less than the lower tolerance level of the −0.2 dBm to 0.8 dBm tolerance range. 
     In accordance with an embodiment of the present invention, extra compressed mode power levels are added to the power table, and made available for mode switches from uncompressed mode to compressed mode, so that the power control accuracies imposed by the wireless standard being used are satisfied. For example, compliance with the +/−0.5 dB power control accuracy requirement for 1 dB step sizes in the W-CDMA standard can be achieved by adding a 0.5 dBm power level to the compressed mode power levels and storing it in the power table. Compliance with the W-CDMA specification can be verified by considering the example above, where a 1 dB increase in power level is received and a mode switch from 0.3 dBm in uncompressed mode to the 0.5 dBm compressed mode power level is performed. As can be seen, the upper and lower power levels of the power level range (0.5 dBm+/−0.25 dB) are both within the −0.2 dBm to 0.8 dBm tolerance range. Compliance at other power levels can be confirmed, as will be readily appreciated by those of ordinary skill in the art. 
     Additional power level values may be added to the compressed mode power table, to ensure that the cumulative tolerances (see  FIG. 3 ) are also satisfied. For example, as shown in  FIG. 8A , a 1.25 dB level is included so that subsequent 1 dB power level commands would move from 0.5 dBm, then to 1.25 dBm, 2 dBm, and so on. 
     For larger power level step sizes, additional power table entries (0.25 dBm and 1.25 dBm in the example above) do not need to be entered in the power table, since the power control tolerances for larger step sizes are more relaxed.  FIGS. 8B and 8C  illustrate, for example, mode transitions from uncompressed mode to compressed mode when the transmitter  500  is commanded to increase its output power level by ΔP=2 dB and ΔP=3 dB. The power control tolerance for a 2 dB step size is +/−1 dB (see the table in  FIG. 2 ), and is +/−2 dB for a 3 dB step size, both of which are wide enough to allow transitions to the existing compressed mode power levels while still satisfying a maximum step size error of +/−0.5 dB. 
     Referring now to  FIGS. 9A and 9B , there is presented a flowchart further illustrating operation of the polar modulation transmitter  500  when the transmitter  500  is initially configured for operation in uncompressed mode. In response to receipt of a new power command ΔP, at decision ST 21  the transmission power controller determines whether ΔP is nonzero. If ΔP=0, the transmitter  500  maintains operation in uncompressed mode at the new power level and the method ends until a new command to change power level is received. If ΔP≠0, the power level is updated to a new power level, Ptable=Ptable+ΔP. Next, at decision ST 23  the transmission power control section  522  determines whether the new power level, Ptable=Ptable+ΔP, falls within the uncompressed region. This operation is performed by comparing Ptable+ΔP to Pda, and determining whether Ptable+ΔP is greater than Pda. (As explained above, Pda corresponds to the lowest uncompressed mode power level that, due to tolerances, may produce a power level in the overlap region.) If Ptable+ΔP is determined not to be greater than Pda, the method ends and the transmitter  500  maintains uncompressed mode operation. On the other hand, if it is determined that Ptable+ΔP is greater than Pda, at decision ST 24  the transmission power control section determines whether ΔP&gt;0. If not, the method ends and the transmitter  500  maintains uncompressed mode operation. 
     If, however, it is determined at decision ST 24  that ΔP&gt;0, at decision ST 25  it is determined whether Ptable+ΔP is greater than the maximum possible uncompressed mode power level, Pu,max. If yes, ΔP is so large that initiation of the PAL  514  is not necessary (i.e., no calibration is required), and at step ST 26  the transmission power controller switches the polar modulation transmitter  500  to compressed mode. 
     On the other hand, if at decision ST 25  it is determined that Ptable+ΔP is less than Pu,max, the result of the power increase ΔP may possibly result in a power level that falls within the overlap region. To determine whether the power increase ΔP results in a power level that falls within the overlap region, steps ST 7 -ST 28  are performed. Specifically, at step ST 27  the detector is enabled, at step ST 28  the actual power level in uncompressed mode Pua is measured, and at decision ST 29  it is determined whether the measured power level in uncompressed mode is greater than or equal to Pd,min, where Pd,min is the lowest registered value corresponding to Pc,min due to detector imperfections. If Pua is not greater than or equal to Pd,min, the detector is disabled at step ST 30  the transmitter  500  maintains operation in uncompressed mode at the new power level setting and the method ends. However, if it is determined that Pua≧Pd,min, it is concluded that the Ptable+ΔP falls within the overlap region. 
     To determine whether a mode switch from uncompressed mode to compressed mode is required, the PAL algorithm determines whether the new power level Ptable+ΔP falls within the shaded regions in  FIGS. 8A ,  8 B or  8 C. To make this determination the transmit power control section  522  of the polar modulation transmitter  500  configures the PA  510  for operation in compressed mode at the lowest available output power level in compressed mode, as indicated in steps ST 31  and ST 32 . Then, at step ST 33  (see  FIG. 9B ), the actual output power in compressed mode (Pca) is measured by the PAL detector  516  and compared to the actual output power that was measured in step ST 28 . Next, at decision ST 34  the PAL algorithm determines whether Pca−Pua is less than or equal to 0.25 dB. If Pca−Pua is less than or equal to 0.25 dB then no power alignment (i.e. calibration) between the uncompressed mode and compressed mode power levels is required. The drift in the uncompressed power mode curve is small enough that no power matching to the compressed mode power levels is required, and the precalibrated power table entries can be used. However, if Pca−Pua is determined to be greater than 0.25 dB, the misalignment is great enough that a power match to the closest compressed mode power levels is required. This is accomplished by making a mode switch to compressed mode at step ST 37  and selecting the appropriate compressed mode power table entry, based on the power difference between Pua and Pca. Should the overlap region change due to device characteristic, the values in the ‘switch’ statement would also change to reflect the new overlap region. Once the calibration is completed, the detector  516  is disabled and the power modulation transmitter  500  continues to operate in the new compressed mode power level until a new power level change command is received. 
     While the above is a complete description of the preferred embodiments of the invention sufficiently detailed to enable those skilled in the art to build and implement the system, it should be understood that various changes, substitutions, and alterations may be made without departing from the spirit and scope of the invention as defined by the appended claims.