Abstract:
A power amplifier of a polar transmitter having separate amplitude and phase paths is configured so that its output power is controlled by power control circuitry disposed in both the amplitude and phase paths of the transmitter. Coarse power control is provided by coarse power control circuitry configured in the phase path. Fine power control is performed by digital power control circuitry configured in the amplitude path. The combined coarse power control circuitry in the phase path and digital power control circuitry in the amplitude path allows the output power of the power amplifier to be controlled at the accuracy and resolution required by wireless communications standards such as, for example, the W-CDMA standard.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to power control in communications transmitters. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wireless communication technologies have undergone tremendous growth over the last decade. The accumulation of large numbers of subscribers and the introduction of high bandwidth applications such as gaming, music downloading and video streaming have placed strains on network capacity. Newer generation wireless communication systems, such as the third generation (3G) Wide-Band Code Division Multiple Access (W-CDMA) wireless interface, strive to improve network capacity by making more efficient use of the radio frequency (RF) spectrum. 
         [0003]    Compared to earlier generation systems, W-CDMA uses more bandwidth-efficient modulation schemes that directly improve network capacity. Network capacity is also indirectly increased by controlling power levels between mobile terminals and associated basestations. Each mobile terminal in a basestation cell of a W-CDMA based system is required to transmit at a power level that results in the basestation receiving the same power level from all mobile terminals. To account for different and varying distances between the various mobile terminals and the basestation, the W-CDMA standard requires that the basestation periodically send a Transmit Power Control (TPC) command (1500 times per second) to each of the mobile terminals. The TPC commands direct the transmitters of the mobile terminals to increase or decrease their output power levels in discrete steps (e.g., +/−1 dB, +/−2 dB, +/−3 dB, etc.), so that the appropriate power levels from all mobile terminals are received at the basestation. Controlling power in this manner reduces interference between mobile terminals and, consequently, allows more mobile terminals to share the same carrier. The result is an increase in network capacity and greater overall power efficiency. 
         [0004]    The W-CDMA specification also requires the RF transmitter of each mobile terminal to be capable of controlling its output power over a wide dynamic range (80 dB in the W-CDMA specification). This ensures that all mobile terminals, irrespective of their distance from the basestation, have the capability of transmitting at the power needed to result in the basestation receiving the same power level from all mobile terminals. 
         [0005]    Wide dynamic range in output power is difficult to achieve in conventional quadrature modulator transmitters. To avoid signal distortion the power amplifier (PA) used in such transmitters must be configured to operate linearly. Unfortunately, linear operation cannot be easily maintained over the wide dynamic range demanded by the W-CDMA standard. 
         [0006]    The polar modulation transmitter is an alternative type of transmitter that is capable of controlling output power over a wide dynamic range. Because of this capability, and because it is more power efficient than the conventional quadrature modulator transmitter, the polar modulation transmitter has gained widespread recognition as a transmitter suitable for W-CDMA and other next generation wireless communication systems. 
         [0007]      FIG. 1  is a diagram of a typical polar modulation transmitter (or “polar transmitter”)  100 . As shown, the polar transmitter  100  comprises a polar signal generation circuit  102 , an amplitude control circuit  104 , a phase-modulated signal generation circuit  106 , a PA  108 , and an antenna  110 . 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 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 phase component signal is coupled to an input of the phase-modulated signal generation circuit  106  along a phase path. The phase-modulated signal generation circuit  106  is configured to receive the phase component signal and generate a constant-amplitude phase-modulated RF drive signal, which is coupled to an RF input of the PA  108  along the 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, which is coupled to a power supply port of the PA  108 . The PA  108  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 which is radiated by the antenna  108  to a remote basestation. 
         [0008]    The polar transmitter  100  achieves wide dynamic range in output power by configuring the PA  108  to operate in compressed mode during times when a high transmission power is required, and configuring the PA  108  to operate in uncompressed mode during times when only a low transmission power is required. When configured in compressed mode the output power of the transmitter  100  is controlled by the amplitude modulated power supply voltage applied to the collector (or drain) node of the PA  108 , 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  108  is controlled by varying the power of the phase-modulated RF drive signal, while the collector (or drain) node of the PA  108  is also modulated with the envelope signal. 
         [0009]    In addition to requiring a wide dynamic range in output power, the W-CDMA standard requires the transmitter of a mobile terminal to comply with certain specified power control tolerances. As shown in the table in  FIG. 2 , compliance with these power control tolerances must be made and maintained after the transmitter changes its output power in response to a Transmit Power Control (TPC) command. So, for example, 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 be capable of adjusting its output power to within a range of +0.5 dBm and 1.5 dBm for a nominal 1 dB step up in power. 
         [0010]    The level of power control accuracy needed for W-CDMA applications is not easily realized using the polar transmitter  100  in  FIG. 1 . The difficulty arises from the fact that analog circuitry is used to perform and control the transmitter&#39;s output power. An analog signal provided by an analog portion of the amplitude control circuit  104  is used to control the output power of the PA  108 , when the PA  108  is configured to operate in compressed mode. Some degree of power control can also be achieved for uncompressed mode operation by inserting a variable gain amplifier  112  in the phase path of the transmitter, as shown in  FIG. 3 . Unfortunately, a variable gain amplifier that is capable of controlling output power at the accuracies and resolution necessary to satisfy the W-CDMA power control tolerance specifications is difficult to design, especially when the design requires operation over a very wide dynamic range. 
         [0011]    In addition to the foregoing problems, analog power control solutions are sensitive to temperature, difficult to consistently manufacture, consume large portions of integrated circuit area, and use significant amounts of power. It would be desirable, therefore, to have methods and apparatus for controlling output power in a polar transmitter, which are capable of controlling output power at the precision necessary to satisfy the power control specifications of the W-CDMA standard, and similar specifications of other standards. 
       SUMMARY OF THE INVENTION 
       [0012]    Methods and apparatus for controlling output power in radio frequency (RF) transmitters are disclosed. An exemplary RF transmitter comprises a polar transmitter having separate amplitude and phase paths. A power amplifier of the transmitter is adapted so that its output power can be controlled by power control circuitry disposed in both the amplitude and phase paths of the transmitter. Coarse power control is provided by coarse power control circuitry (e.g., by a step attenuator or a variable gain amplifier) configured in the phase path. Fine power control is performed by digital power control circuitry configured in the amplitude path. Complementing the coarse power control in the phase path with the fine digital power control in the amplitude path allows the output power of the power amplifier to be controlled at the accuracy and resolution needed to satisfy strict power control standards such as, for example, those specified by the W-CDMA standard. 
         [0013]    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, in which like reference numbers are used to indicate identical or functionally similar elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a diagram of a typical polar modulation transmitter; 
           [0015]      FIG. 2  is a table showing transmitter power control tolerances for various power level step sizes, as specified by the W-CDMA standard; 
           [0016]      FIG. 3  is a diagram of a typical polar modulation transmitter that employs a variable gain amplifier to provide a limited degree of power control; 
           [0017]      FIG. 4  is a diagram of a polar modulation transmitter having digital power control capabilities, according to an embodiment of the present invention; and 
           [0018]      FIG. 5  is a diagram of a polar modulation transmitter having digital power control capabilities and a power measurement feedback loop, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Referring to  FIG. 4 , there is shown diagram of a polar modulation transmitter  400 , according to an embodiment of the present invention. The polar modulation transmitter  400  comprises a polar signal generation circuit  402 ; an amplitude path having an amplitude control circuit  404  including a modulation digital-to-analog converter (DAC)  406 , a multiplying DAC  408  and a power regulator  409 ; a phase path having a phase-modulated signal generation circuit  410  and a variable gain amplifier (or in an alternative embodiment, a step attenuator)  412 ; a power amplifier (PA)  414 ; an antenna  416 ; and a transmit power controller  418 . 
         [0020]    The polar signal generation circuit  402  operates on an input signal to provide an envelope signal containing amplitude information of the input signal and a phase component signal containing phase information of the input signal. The envelope signal is coupled to an input of the modulation DAC  406  of the amplitude control circuit  404 , in an amplitude path of the transmitter  400 . The modulation DAC  406  modulates a power supply voltage, VSUPPLY, according to the shape of the envelope signal and couples the resulting amplitude modulated power supply signal a(t) to a reference voltage input of the multiplying DAC  408 . The output of the multiplying DAC  408  is coupled to the power regulator  409 , the output of which is coupled to the power control input of the power amplifier (PA)  414 . Based on the product of the amplitude modulated power supply signal a(t) and the value of an m-bit (m is a positive integer) digital power control factor k AM  received from the transmit power controller  418 , the multiplying DAC  408  generates an analog power control signal, which is coupled through the power regulator  409  to the power control input of the PA  414 . 
         [0021]    In the phase path of the transmitter  400 , the phase component signal from the polar signal generation circuit  402  is coupled to an input of the phase-modulated signal generation circuit  410 . The phase-modulated signal generation circuit  410  upconverts the phase component signal to radio frequency (RF) to provide a signal cos(ω c t+φ(t)), where ω c  represents the radian frequency of the RF carrier and φ(t) represents the phase modulation of the upconverted signal. The variable gain amplifier (or step attenuator)  412  scales the magnitude of the upconverted phase component signal cos(ω c t+φ(t)), based on the value of an n-bit (n is a positive integer) digital gain control factor k PM  received from the transmit power controller  418 , to provide a scaled upconverted phase component signal k PM ×cos(ω c t+φ(t)). 
         [0022]    The scaled upconverted phase component signal k PM ×cos(ω c t+φ(t)) is coupled to an RF input of the PA  414 , which is operable to amplify the signal according to the analog power control signal k AM ×a(t) applied to the power control input of the PA  414 . The amplified and upconverted signal a(t)×k AM ×k PM ×cos(ω c t+φ(t)) is coupled to the antenna  416 , which radiates the signal to a remote receiver (e.g., a cellular basestation receiver). In accordance with an embodiment of the invention, this is realized in a manner similar to that taught in U.S. Pat. No. 7,010,276, which is incorporated into this disclosure by reference. 
         [0023]    Power control in the polar modulation transmitter  400  is directed by the transmit power controller  418 . Unlike prior art approaches which provide power control in only one of either the amplitude and phase paths, depending on whether the transmitter PA is configured to operate in uncompressed or compressed mode, power control in the polar modulation transmitter  400  of the present invention is provided in both the amplitude and phase paths at the same time. According to an embodiment of the invention, the n-bit digital gain control signal is used to coarsely control (e.g., in 1 dB steps) the output power level of the transmitter  400 , and the m-bit digital power control signal is used to finely control (e.g., at a 0.25 dB resolution) the output power level of the transmitter  400 . More specifically, the value of the n-bit digital gain control factor k PM  is used to set the amplification (or attenuation) of the variable gain amplifier (or step attenuator)  412  in the phase path of the transmitter  400  and, at the same time, the value of the m-bit digital power control factor k AM  is used by the multiplying DAC  408  to adjust the amplitude of analog power control signal applied to the power setting input of the PA  414  in the amplitude path of the transmitter  400 . The fine power control provided by the m-bit digital power control signal in the amplitude path of the transmitter  400  causes the PA  414  to interpolate between the coarse power levels set by the n-bit digital gain control signal in the phase path of the transmitter  400 . The interpolative effect results in greater resolution and more accurate power control than is obtainable by controlling power in the phase path alone. 
         [0024]    According to one aspect of the invention, the values of m and n are selected so that output power can be controlled at the accuracy and resolution needed to satisfy the power control tolerances specified by the W-CDMA standard, as well as other standards that have stringent power control requirements. The transmit power controller  418  determines the actual values needed for the control factors k AM  and k PM  by acting on the value of a Transmit Power Control Signal (TPCS). The TPCS is determined by the baseband as an absolute power control setting, based on the history of TPC and related system commands transmitted to the associated mobile device by the communications system being used (e.g., W-CDMA). 
         [0025]    Providing power control in both the amplitude and phase paths of the polar modulation transmitter  400  is particularly beneficial during times when the PA  414  of the transmitter  400  is configured to operate in uncompressed mode, which is a mode in which power control can be particularly difficult. Providing digital power control in the amplitude path of the transmitter  400  during times when the PA  414  is configured to operate in uncompressed mode avoids limitations that analog devices have in controlling power in the phase path of the transmitter  400 , and simplifies the design requirements of the variable gain amplifier (or step attenuator)  412 , since it must only operate to coarsely control output power. Nevertheless, while the above embodiments have been described in the context of providing power control in both the amplitude and phase paths of the transmitter simultaneously, those of ordinary skill in the art will readily appreciate and understand that if applications dictate or allow, power control in one of the phase and amplitude paths may be applied independently while power control in the other path is either maintained at some constant value or is not provided at all. 
         [0026]      FIG. 5  is a diagram of a polar modulation transmitter  500 , according to another embodiment of the present invention. This embodiment is similar to that shown in  FIG. 4 , except that it also includes a power measurement feedback loop. The power measurement feedback loop includes a power level detector  520  and an analog-to-digital converter (ADC)  522 . The power detector  520  measures the output power level of the transmitter  500  at the output of the PA  412 . The ADC  522  converts the power measurement to a digital signal, which is coupled to a digital input of the transmit power controller  524 . The transmit power controller  524  is then operable to use the digitized version of the measured output power to adjust the m-bit digital power control and/or the n-bit digital gain control factors k AM  and k PM , so that the desired output power level of the transmitter  500  is provided as commanded by the TPCS. 
         [0027]    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.