Abstract:
This disclosure relates to linearization in polar modulators of wireless communication devices and associated methods, to attain linear amplification and high power efficiency during transmission.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional Patent Application of U.S. patent application Ser. No. 12/327,687 filed Dec. 3, 2008 entitled “Polar Feedback Receiver for Modulator” in the name of Nick Shute, et al. and is hereby incorporated by reference. 
    
    
     BACKGROUND 
     A modulator, such as a polar modulator, in a wireless communications device may require spectral quality for modulation schemes. Typically, most wireless communications devices are based on constant-envelope modulation schemes (i.e., phase or frequency modulated). An advantage with constant-envelope modulation scheme provides that a final radio frequency (RF) power amplifier in the polar modulator does not have to be linear, and as a consequence, the final RF power amplifier can be operated in the most power efficient region near saturation. However, a drawback with constant-envelope modulation scheme is the inefficient use of the RF spectrum, where data rate transmission for a given bandwidth is not maximized. 
     To utilize the RF spectrum efficiently, a varying envelope and varying phase modulation scheme may be used. When a varying envelope modulation is applied to a power efficient nonlinear amplifier, distortion may be generated by the nonlinear amplifier which may cause interference with adjacent channels. The distortion may also result in detection error of the information signal at the receiver end of the communication channel. For most applications, the distortion is to be avoided, and may require a linear amplifier; however, linear amplifiers typically have low power efficiency, making a linear amplifier unsuitable for varying envelope and phase modulation scheme. To obtain linear amplification and high power efficiency for transmission in the polar modulator, linearization of a power efficient and nonlinear amplifier may be implemented. 
     Linearization may refer to a method of compensation or correction of non-linearity in a polar modulator component to maintain stability at the output of the polar modulator. Linearization of the polar modulator may require a feedback receiver component to couple the polar modulator output into a signal path of the amplifier&#39;s input. The feedback receiver component may produce linear amplification and power efficiency in the polar modulator. 
     The feedback receiver component may be used as a quadrature demodulator and require additional circuitry (e.g., high frequency local oscillator, mixers, 90 degree shifters, etc.). Such additional circuitry may draw significant amounts of current in the polar modulator. Furthermore, delay sensitivity may further be included due to different delays in signal sources and local oscillator input used in the quadrature demodulator. The delay sensitivity may result in degradation of the circuitry at microwave frequencies due to sub nanosecond delay variation requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components. 
         FIG. 1  is a block diagram of a polar modulator component. 
         FIG. 2  is a block diagram of a Radio Frequency (RF) component for polar modulator. 
         FIG. 3  is a block diagram of a polar feedback receiver. 
         FIG. 4  is a block diagram illustrating a specific implementation of the polar feedback receiver in order to extract and measure the distortion only. 
         FIG. 5  is a flow chart illustrating a process for linearization of a polar modulator using polar feedback receiver. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is directed towards techniques and methods of performing linearization in the polar modulator to attain linear amplification and high power efficiency during transmission. Although a polar modulator is described, it is contemplated that the techniques and methods may be applied to other modulators. The linearization may be implemented through the use of a polar feedback receiver in the polar demodulator to directly extract magnitude and phase baseband signal in modulated radio frequency (RF) output. The polar feedback receiver avoids the use of additional circuitry (e.g., quadrature demodulator) which may include additional current consumption and delay sensitivity in the polar modulator. 
       FIG. 1  illustrates a polar modulator component  100  that includes a baseband component  102  and a RF component  104 . During transmission, baseband component  102  may encode a data signal; identify the data signal&#39;s prior state before modulation; convert the inphase signal (I) and quadrature phase signal (Q) into polar form; and transmit the data signal for modulation. Output from the baseband component  102  is referred to as a baseband signal. During demodulation, baseband component  102  may identify the prior state of the data signal following demodulation. The baseband component  102  decodes the demodulated data signal to re-create the data signal. The prior state of the data signal may include attributes of the data signal, such as amplitudes of I and Q, frequency, and phase amount. 
     During modulation, RF component  104  may combine the baseband signal with RF frequency carrier to produce a modulated RF frequency signal, amplify the modulated RF frequency signal, and further filter the modulated RF frequency signal before transmission. The RF frequency carrier may refer to the frequency of oscillation in the RF component  104  when a modulating baseband signal is not present. During receiving, the RF component  104  may receive the modulated RF frequency signal, amplify the modulated RF frequency signal, filter the modulated RF frequency signal, and demodulate the modulated RF frequency signal. 
     A signal from peripherals, camera, display etc.  106  may be received by Input/Output (I/O) component  108  for initial processing. The I/O component  108  may convert analog data signals into digital data signals, while the digital data signals are maintained in the same state (i.e., remain digital). Furthermore, the I/O component  108  may process the data signals to produce the amplitudes of I and Q. 
     The data signals  110  are received by a digital signal processor (DSP)  112 . The DSP  112  may use a filter to limit the bandwidth forming a spectrum of the equivalent low pass signal or baseband signal. The DSP  112  may include a Coordinate Rotation Digital Computer (CORDIC) component to transform the amplitudes of I and Q of the baseband signal into equivalent polar representations. The equivalent polar representations may contain the phase and magnitude of the baseband signal, where the magnitude of the baseband signal may also refer to amplitude of the baseband signal. 
     After transformation of the baseband signal into the equivalent polar form, the baseband signal from the DSP  112  may pass through digital interface  114 . The digital interface  114  may provide concurrent bi-directional communications between the baseband component  102  and RF component  104 . The digital interface  114  may contain clock signals to provide timing references for transmit and receive communications between baseband component  102  and RF component  104 . 
     During transmission, the baseband signal from digital interface  114  is received by phase modulator/analog signal processing component  116 . The phase modulator/analog signal processing component  116  may include an output that contains the modulated RF signal by varying the phase and magnitude of the RF carrier corresponding to the baseband signal to be transmitted. The phase modulator/analog signal processing component  116  may further support the linearization mechanism for the polar modulator  100 . A modulated RF signal  118 , which is the output of phase modulator/analog signal processing component  116 , is passed to a nonlinear amplifier  120  for further amplification before transmission. The nonlinear amplifier  120  may include a relatively high power efficient amplifier suitable for varying envelope and phase modulation scheme. 
     A modulated RF signal  122  is an output of the nonlinear amplifier  120 . The modulated signal  122  may contain distortions which may cause interference in the adjacent channels. The distortions may be caused by unexpected delay between the phase modulation signal and amplitude modulation signal. The unexpected delay may result in the signals (e.g., phase modulation and amplitude modulation) being applied to different portions of the RF carrier. Furthermore, the amplitude modulation may cause inadvertent phase modulation due to undesired feedback between phase modulation components and amplitude modulation components. The distortions may not only cause interference to the adjacent channels, but may also affect reception at the receiving end. Therefore, the distortions should be avoided in order to produce linear amplification, and for high power efficiency transmission. 
     In an implementation, linearization may be used to compensate for the distortions at the polar modulator  100 . The linearization may include coupling of the polar modulator  100  output (i.e., signal  122 ) into a polar feedback receiver  124 . The polar feedback receiver  124  may be used for polar demodulation of the loop back polar modulator  100  output in order to directly extract the phase and magnitude of the modulated RF signal. A linearization mechanism that includes the polar feedback receiver  124  may be made adaptive by extracting an error signal (e.g., phase and magnitude distortion) coefficient at the output (i.e., signal  122 ) of the polar modulator  100 . The error signal coefficient may include unexpected delay between the phase modulation signal and amplitude modulation signal which may result in distortions in the polar modulator  100 . An input path signal  126  to the nonlinear amplifier  120  may be adjusted in accordance with the extracted error signal coefficient (e.g., phase and magnitude distortion) of the polar modulator  100  during real time operation, to effectively and continuously minimize distortion in the polar modulator  100  output. The adjusted modulated RF signal may be transmitted through antenna  128 . 
       FIG. 2  illustrates a RF component  104  using the polar feedback receiver  124  for linearization of the polar modulator  100 . After transformation of the amplitudes of I and Q into polar form in DSP  112  of the baseband component  102 , a baseband phase signal  200  and baseband magnitude signal  202  is received by RF component  104 . The phase signal component of the baseband signal, may phase modulate the RF frequency carrier through the use of a local oscillator, phase detector, charge pump, filter, voltage controlled oscillator (VCO), and a multi modulus divider (MMD). The phase modulated RF signal may further undergo amplitude modulation by the baseband magnitude signal  202  to produce the varying envelope and phase modulation scheme. 
     The modulated RF signal  122  is coupled from the output of the polar modulator  100 . The modulated RF signal (i.e., signal  122 ) may include the phase and magnitude of the baseband signal, together with the distortions, which may be demodulated by polar feedback receiver  124 . The polar feedback receiver  124  may implement polar demodulation of the coupled modulated RF signal by directly extracting the phase and magnitude baseband signal together with the distortions. The polar feedback receiver  124  may extract the phase and magnitude baseband signal and the distortions to be used for measuring the error signal coefficient at the output of the polar modulator  100 . The error signal coefficient may be used to compensate the phase and magnitude distortions to attain linear amplification and high power efficiency during transmission. 
     The output of polar feedback receiver  124  may include the phase signal information plus distortion signal  204  and the magnitude signal information plus distortion signal  206 . Both signal  204  and signal  206  may enter algorithm component  208  which may be used to calculate error signal coefficient. The error signal coefficient may include the difference in gain between the phase and magnitude input sources, and the extracted phase and magnitude signal (including the distortions). The difference in gain may correspond to the calculated amount of distortion at the output of the polar modulator  100  which may be compensated to attain linear amplification and high power efficiency during transmission. 
     The output  210  of the algorithm component  208  may include the magnitude error signal coefficient, and is received by Look Up Table (LUT)  212  for magnitude compensation. The output or signal  214  from algorithm  208  may include the phase error signal coefficient, and is received by LUT  216  for phase compensation. Both LUT  212  and LUT  216  may include a data structure used to replace a runtime computation with a simpler lookup operation. The LUT  212  may contain magnitude gain factors, corresponding to the magnitude error signal coefficient in signal  210 , which may be multiplied with the baseband magnitude signal  202  to produce the compensated magnitude signal. The LUT  216  may include phase gain factors, corresponding to the phase error signal coefficient in signal  214 , which may be multiplied with the baseband phase signal  200  to produce the compensated phase signal. 
     A compensated magnitude signal  218  may be used for amplitude modulation in the polar modulator  100 . The compensated magnitude signal  218  may further be used as an input signal into the polar feedback receiver  124  in order to produce a limited modulated RF signal. The limited modulated RF signal may include the modulated RF signal whose amplitude modulation content is removed. The amplitude modulation content may be removed through combination of the modulated RF signal with a low frequency inverse baseband magnitude signal to produce the limited modulated RF signal. The amplitude modulation content removal may contain a low frequency local oscillator signal that may be used to split the modulated RF signal into phase modulation and amplitude modulation components. The local oscillator signal is the frequency signal that is normally used to split the modulated RF signal into phase modulation and amplitude modulation components in a quadrature demodulator. However, delay sensitivity may occur due to different processing circuitry of the modulated RF signal and the local oscillator signal which may be included in the quadrature demodulator. 
     The low frequency inverse baseband magnitude may result from reversing the compensated magnitude signal  218  of the polar modulator  100 . The combined output of the modulated RF signal and the low frequency inverse baseband magnitude may include a limited modulated RF signal whose phase signal can be directly extracted through the use of a phase discriminator. The phase discriminator is a component where the phase signal information is extracted for a given limited modulated RF signal. The limited modulated RF signal may further be used to extract the magnitude of the modulated RF signal together with the distortion due to nonlinearities in the polar modulator  100 . 
     The compensated magnitude signal  218  is received by a digital to analog converter (DAC)  220  for amplitude modulation of the polar modulator  100 . The DAC  220  may convert the compensated magnitude signal into analog compensated magnitude signal  222  received by mixer component  224 . The analog compensated magnitude signal  222  may be used to amplitude modulate the phase modulated signal in mixer component  224 . The mixer component  224  may combine the phase modulated RF signal with the analog compensated magnitude signal  222  in order to provide the varying envelope and phase modulation scheme for the polar modulator  100 . 
     The compensated phase signal  226  may be used as a control signal for multi-modulus divider (MMD)  228 . The MMD  228  may be used in the polar modulator  100  for low power, high operating frequencies, and high speed frequency synthesis applications to cover multiple frequency bands. The frequency synthesis applications may refer to a phase locked loop (PLL) based frequency synthesizer where the MMD  228  is placed between the output, and the feedback input includes the ability of the PLL to generate multiple frequencies at high speed applications. The MMD  228  may divide the output frequency of VCO  230 , where the output frequency is received by MMD  228  as signal  232 . The VCO  230  provides the up-converted frequency that is phase modulated by the baseband phase signal. 
     Signal  234  is the output of MMD  228  received by phase detector (PD)  236 . The PD  236  provides a control signal which corresponds to the difference between the output of MMD  228  and signal  238 . The signal  238  may include a reference frequency generated by local oscillator  240 , which produces a constant reference frequency used for phase modulation in the polar modulator  100 . The output or signal  242  of the PD  236  is received by charge pump/filter  244 . The charge pump/filter  244  may include a positive output current and a negative output current activated by the control signal from PD  236 . The positive output current or negative output current may be filtered by a low pass filter to produce a DC voltage or control voltage signal  246  to sustain operation of the VCO  230  at a desired frequency. Signal  248  may include the phase modulated output of VCO  230  entering into mixer component  224  for amplitude modulation. The output of mixer component  228  which may contain the varying envelope and phase modulated RF signal may pass through signal  118  for amplification in the nonlinear amplifier  120 . 
       FIG. 3  illustrates a polar feedback receiver  124  used for linearization of the polar modulator  100 . A compensated magnitude signal  218  is received by an inverse baseband magnitude component  300 . The inverse baseband magnitude component  300  may include a gain that reverses the compensated magnitude signal in signal  218 . The reversed baseband magnitude signal  302  may include a low frequency inverse baseband magnitude signal used to directly extract the phase and magnitude signals in the modulated RF signal. Signal  302  may include low frequency inverse baseband magnitude received by DAC  304 . The DAC  304  may convert the digital low frequency inverse baseband magnitude into analog low frequency inverse baseband magnitude to match the analog modulated RF output in signal  122 . 
     The analog signal low frequency inverse baseband magnitude signal  306  is received by mixer component  308  for amplitude modulation content removal. The mixer component  308  may operate as a variable gain stage where one input signal may include the modulated RF signal  122 , and the other signal may include the analog low frequency inverse baseband magnitude signal  306 . The mixer component  308 , operating as a variable gain stage, may strip away the amplitude modulation content without using additional circuitry like a local oscillator in quadrature demodulator. Removal of the amplitude modulation content in mixer component  308  may result in a limited modulated RF signal, and the phase information may be restored efficiently as compared to using a typical quadrature demodulator. 
     Limited modulated RF signal  310  may include a constant envelope phase modulated signal. The limited modulated RF signal  310  may be used to directly extract the phase component plus distortion of the modulated RF signal  122  through the use of a phase discriminator  312 . The phase discriminator  312  may include amplitude variations in the output, which is a function of phase variation in the limited modulated RF signal  310 . The limited modulated RF signal  310  may also be used to extract the magnitude of the modulated RF signal  122  through mixer component  312 . The mixer component  312  may combine the magnitude of the modulated RF signal  122 , and the limited modulated RF signal  310 . The mixer component  312  may include an output that includes the magnitude of the baseband signal plus the distortion. 
       FIG. 4  illustrates an implementation of a polar feedback receiver  124 , which includes direct extraction of distortions without the phase or magnitude component. To extract the phase distortion only in the polar modulator  100 , the phase discriminator  312  may be implemented using a sigma delta converter, MMD component, and time to digital converter (TDC) component. An MMD  400  component divides the limited modulated RF signal  310 , and removes the phase modulation, without the phase distortion, through the use of a sigma to delta converter  402 . The sigma to delta converter  402  may include high precision conversion of the low frequency baseband phase signal  226  into an analog baseband phase signal  404 . The output signal  406  of MMD  400  may include the analog phase distortion output and is received by TDC  408 . The TDC  408  compares the signal edges of signal  406  to generate a delta time signal which can be converted to a delta time phase signal. The output signal  410  of TDC  408  may include only phase distortion. 
     In an implementation, the mixer component  314  may operate in linear mode (i.e., acts as a multiplier), and the signal  412  may include the magnitude of the modulated RF signal  122  and the square of the distortion. An analog to digital converter (ADC)  414  converts the extracted magnitude signal together with the square of the distortion, into a digital signal  416 . The digital signal  416  is received by 1/M Square Root component  418 . The 1/M Square Root component  418  divides the signal  416  by the baseband magnitude signal and calculates the square root of the distortion signal to produce signal  420  which may only include the amount of magnitude distortion. 
       FIG. 5  illustrates an exemplary method  500  for feedback receiver used for linearization in a modulator. In an implementation, the exemplary method  500  can be implemented in the polar modulator  100 . The exemplary method  500  is described with reference to  FIGS. 1-4 . The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or alternate method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the method can be implemented in any suitable hardware, software, firmware, or a combination thereof, without departing from the scope of the invention. 
     At block  502 , inverting a magnitude of a baseband signal is performed. For example, inverse baseband magnitude component  300  may include a gain that reverses the magnitude of the baseband signal to produce an inverse magnitude of the baseband signal in the polar modulator (e.g., polar modulator  100 ). 
     At block  504 , converting the inverse magnitude of the baseband signal into analog form. In an implementation, a digital to analog converter component (e.g., DAC  304 ) may convert the inverse magnitude of the baseband signal into analog inverse magnitude of the baseband signal. 
     At block  506 , removing an amplitude modulation content of a modulated RF signal is performed. A mixer (e.g., mixer component  308 ), operating as a variable gain stage, may combine the analog inverse magnitude of the baseband signal with the modulated RF signal to remove the amplitude modulation content of the modulated RF signal. In an implementation, the mixer (e.g., mixer component  308 ) contains an output that is referred to as a limited modulated RF signal. 
     At block  506 , extracting a phase signal and distortion from the limited modulated RF signal is performed. A phase discriminator (e.g., phase discriminator  312 ) may directly extract the phase signal and distortion from the limited modulated RF signal. 
     At block  508 , extracting a magnitude signal and distortion is performed. The limited modulated RF signal may be used as an input to a mixer (e.g., mixer component  314 ) which directly extracts the magnitude and distortion of the modulated RF signal by combining the modulated RF signal with the limited modulated RF signal. 
     At block  510 , calculating an error signal coefficient for linearization of the polar modulator is performed. The algorithm (e.g., algorithm  208 ) may provide the error signal coefficient by calculating a difference between an input baseband signal sources (phase and magnitude) and the extracted signals (phase and magnitude signal with distortions) from the modulated RF signal. 
     CONCLUSION 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as preferred forms of implementing the claims. For example, the different circuits and components may be configured to perform linearization in a polar modulator.