Patent Application
20090011730
The present invention relates to power control in communications transmitters.
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.
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.
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.
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.
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.
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.
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
The level of power control accuracy needed for W-CDMA applications is not easily realized using the polar transmitter 100 in
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.
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.
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.
Referring to
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 kAM 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.
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(ωct+φ(t)), where ωc 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(ωct+φ(t)), based on the value of an n-bit (n is a positive integer) digital gain control factor kPM received from the transmit power controller 418, to provide a scaled upconverted phase component signal kPM×cos(ωct+φ(t)).
The scaled upconverted phase component signal kPM×cos(ωct+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 kAM×a(t) applied to the power control input of the PA 414. The amplified and upconverted signal a(t)×kAM×kPM×cos(ωct+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.
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 kPM 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 kAM 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.
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 kAM and kPM 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).
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.
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.
