Many popular modulation formats used in the field of digital communications assume the availability of a linear amplifier in a transmitter to boost a communication signal to a level at which it may be successfully broadcast from an antenna, then propagate to and be demodulated by a remotely located receiver. Linearity refers to the ability of an amplifier to faithfully reproduce a signal presented to it without causing the output signal from the amplifier to be distorted in some way. To the extent that the amplifier is imperfectly linear, distortion and spectral regrowth result. If this distortion and spectral regrowth are excessive, then the transmitter may fail to successfully operate within a spectral mask imposed by regulations and/or within power specifications.
Non-linearity in an amplifier causes the generation of signal harmonics as a unwanted byproduct of amplification. Even ordered harmonics include near DC, low frequency components, collectively referred to as a video signal. The second harmonic forms a video component occupying double the baseband bandwidth, the fourth harmonic forms a video component occupying quadruple the baseband bandwidth, and so on. This envelope-induced video signal modulates amplifier gain causing further deterioration in amplifier linearity.
In some applications it is desirable to improve the power-added efficiency of the amplifier by the use of one or more variable amplifier bias signals. Such variable bias signals exhibit signal dynamics near DC, with frequency components that fall in the video signal bandwidth. They represent another form of video signal that can further modulate amplifier gain, causing still further deterioration in amplifier linearity.
Through the use of envelope trapping, the video signal is held to a low level, and the distortion it causes in an amplified output signal is likewise reduced. But the video signal is not eliminated, so the distortion it causes remains to some extent. And, as bandwidths increase it becomes increasingly difficult to distribute a sufficient number of decoupling capacitors 30 and the resulting impedance minima 32 throughout the entire video band in a manner that keeps video impedance sufficiently low, yet also achieves a sufficiently high impedance in the fundamental RF band. When impedance in the fundamental RF band is insufficiently high, amplifier efficiency suffers.
Furthermore, several different physical characteristics of an amplifier cause different nonlinearities, with the video-signal-induced nonlinearity being only one. Another form of nonlinearity is a memory effect, where an influence of the communication signal being amplified at one instant in time may be smeared over a considerable period. In essence, an amplifier acts in part like a collection of filters, or a complicated filter, with numerous undiscovered characteristics.
Conventional efforts aimed at expanding amplifier linearization techniques to include memory effects have found only marginal success. The difficulty associated with linearizing memory effects may result from the fact that conventional amplifiers appear to exhibit many different and distinct long term and short term memory effects cross correlated with one another but each having its own unique spectral characteristics and each contributing a different degree of distortion. The difficulty may have been exacerbated by the use of envelope trapping techniques, and exacerbated further by the use extensive envelope trapping techniques to address wider signal bandwidths, because each impedance minima may be responsible for a distinct memory effect.
One of the more successful conventional efforts at addressing memory effects results from the use of a Volterra model which characterizes the actual behavior of an amplifier, with a currently popular form of this approach being called a generalized memory polynomial (GMP) model. Unfortunately, due to numerous unknown terms, a considerable amount of cross correlation between the terms, and a large span of time over which different memory effects play out, a tremendous amount of power must be consumed to derive a system of equations that model the amplifier, then take the inverse of the system of equations, and implement that inverse system of equations in signal processing hardware. Consequently, this approach is generally viewed as being unacceptable for use in battery-powered transmitters. Moreover, the tremendous processing load of this approach usually dictates that compromises be made in loop bandwidths and in precision in modeling and inversing the amplifier transfer function. Consequently, this approach typically has trouble following signal dynamics and in achieving high quality linearization results.
Another conventional effort at addressing amplifier nonlinearities, including both video-signal induced distortion and memory effects, is called envelope injection. Generally, signal processing circuits process the outgoing communication signal along with a feedback signal obtained from the output of the amplifier in an attempt to generate a baseband signal that is added to, or injected with, the amplifier biasing with the aim of canceling the video signal. But the video signal is a wideband signal that results from a complicated assortment of harmonic components acting on a component network of unknown and complicated impedance, in accordance with unknown nonlinear relationships. And, for cancellation techniques to be effective, cancellation signals should be very precisely generated. Only limited success has been achieved without employing an excessive amount of processing power to resolve the unknown parameters.
Accordingly, a need exists for a linearized transmitter and transmitter linearizing method that expand linearization efforts to address video-signal induced distortion and memory effects without employing an excessive amount of processing power.
A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, wherein like reference numbers refer to similar items throughout the Figures, and:
Transmitter 50 includes a communication-signal source 52. Communication-signal source 52 provides a digitally modulated, complex, baseband version of a communication signal 54. A communication signal, such as communication signal 54 and others discussed below, is an electronic signal that may undergo a variety of different processing steps and be represented in a variety of different ways, including as one or more digital streams of data or as one or more analog signals. A communication signal has been modulated with information and/or data. The transmission of this information and/or data is the primary purpose of transmitter 50, and a communication signal could be demodulated or otherwise processed to recover the information and/or data.
Communication-signal source 52 may perform any number of activities well known to those skilled in the art of digital transmitters. For example, raw data to be transmitted from transmitter 50 may be digitally modulated using a suitable form of digital modulation, such as QPSK, CDMA, OFDM, or the like. Multiple data streams 56 may have been digitally modulated and combined together for transmission, as is common in a cellular base station, or a single data stream 56 may have been digitally modulated for transmission, as is common in an end-user's wireless device, such as a cell phone, laptop, netbook, electronic book, wireless network adapter, wireless router, and the like. The digitally modulated signal may have been pulse shaped to limit bandwidth while minimizing intersymbol interference (ISI). Additional processing may have been performed to reduce the peak-to-average power ratio. Any or all of these and other types of signal processing activities may be performed at communication-signal source 52.
As a result of the processing performed at communication-signal source 52, communication signal 54 is a baseband, digitally modulated, complex signal that exhibits a bandwidth roughly equal to the bandwidth allocated to transmitter 50 for the transmission of RF energy. This bandwidth resides at baseband (i.e., near DC). Desirably, communication signal 54 is an analytic signal having a bandwidth centered at or near 0 Hz.
Communication signal 54 drives a nonlinear predistorter 58. Nonlinear predistorter 58 spectrally processes communication signal 54 to intentionally introduce wide bandwidth distortion into the communication signal. This distortion is introduced upstream of an amplifier that will eventually amplify the communication signal, and it is configured to counteract distortion that the amplifier will impart to the version of the communication signal that it amplifies. This distortion extends over a bandwidth that exceeds the bandwidth of communication signal 54. Although not shown in
Predistorted communication signal 60 from nonlinear predistorter 58 drives an automatic gain control (AGC) section 62 which normalizes an amplitude parameter of predistorted communication signal 60. In one embodiment, section 62 is implemented as a one-tap adaptive complex multiplier, where adaptation of the tap through the operation of a control loop controls the gain provided to predistorted communication signal 60 to provide the automatic gain control function. Predistorted communication signal 60 from AGC section 62 then drives a linear predistorter 64.
At section 64 the communication signal is spectrally processed to introduce linear predistortion. The linear predistortion is desirably configured to compensate for linear distortion introduced downstream of linear predistorter 64. Predistorted communication signal 60 is presented at the output of linear predistorter 64 in a form that includes both linear and nonlinear predistortion.
Predistorted communication signal 60 propagates from linear predistorter 64 toward a digital-to-analog converter (DAC) 66. DAC 66 converts predistorted communication signal 60 into an analog signal that drives an upconverter and filter section 68. Section 68 frequency shifts predistorted communication signal 60, now in analog form, to the allocated RF fundamental frequency band for transmitter 50 and filters the frequency-shifted signal to pass only a desired sideband. Section 68 produces an RF form of the communication signal. Predistorted communication signal 60, now in RF form, is then fed to an input 69 of an amplifier 70. In one embodiment, amplifier 70 is a radio-frequency (RF) amplifier, or high-power amplifier (HPA), known to those of skill in the art of wireless communications. But those skilled in the art will appreciate that other applications may employ other types of amplifiers, including cascades of several amplifiers.
In the embodiment depicted in
In order for the upstream predistortions to be of the correct character and composition to cancel distortions introduced downstream of DAC 66 it is desirable that amplified RF signal 76 be monitored and that the upstream predistortions be responsive to amplified RF signal 76. Accordingly, a tap-off port of directional coupler 72 extracts a small portion of amplified RF signal 76 for use as a feedback signal 78. Feedback signal 78 is routed through an analog-to-digital converter (ADC) 82, where it is then presented to a feedback path processing section 84. ADC 82 desirably operates at high speed and is phase coherent with the upconversion of section 68 so as to perform downconversion by digital subharmonic sampling. This form of downconversion is desirable because it lessens the corruption of feedback signal 78 that might occur if downconversion is performed through a more analog-intensive form of downconversion. But other forms of downconversion may also be used provided they introduce sufficiently low distortion into feedback signal 78.
Processing section 84 performs digital processing on feedback signal 78. In particular, processing section 84 desirably includes a Hilbert transformation to place feedback signal 78 in a complex, analytic signal form. And, processing section 84 may include a phase rotation to compensate for phase rotation introduced downstream of DAC 66, primarily in a band-pass filter portion of section 68, and a gain adjustment for the nominal linear HPA gain. Eventually, feedback signal 78, now in digital complex form, is supplied to a negative input of a subtraction circuit 86.
Communication signal 54 from communication-signal source 52 is also fed through a delay element 88 to a positive input of subtraction circuit 86. Although not shown, the sample rate of communication signal 54 may be increased prior to application at subtraction circuit 86 to accommodate the full bandwidth of feedback signal 78, which is wider than the bandwidth of communication signal 54. Delay element 88 is configured to temporally align communication signal 54 with feedback signal 78 at subtraction circuit 86. In other words, delay element 88 is configured so that a sample of communication signal 54 processed through a first path which includes delay element 88 and a second path that includes amplifier 70 and feedback path processing section 84 arrive at subtraction circuit 86 at the same time.
An output of subtraction circuit 86 generates an error signal 90 which describes the manner in which amplified RF signal 76 fails to be a linear amplification of communication signal 54. Error signal 90 and communication signal 54 configured in a delayed form 92 are each presented to control inputs of nonlinear predistorter 58, AGC section 62, and linear predistorter 64.
In one embodiment, linear predistorter 64 is implemented using an adaptive equalizer that adjusts equalizer coefficients in response to a least-means square (LMS) based control loop algorithm. The adaptive equalizer of linear predistorter 64 desirably estimates coefficient values for the taps of a finite impulse response (FIR) filter to influence the amount of linear distortion in amplified RF signal 76, then alters these coefficients over time to adjust the predistortion transformation function (transform) applied by the adaptive equalizer and to achieve decreasing amounts of linear distortion until convergence is reached at a minimum amount of linear distortion. The control loop trains linear predistorter 64 to reduce linear distortion in response to correlation between the conjugated form of error signal 90 and delayed communication signal 92. The control loop may be configured to adapt coefficients only during periods of substantially linear amplifier operation. Those skilled in the art may devise other forms of linear predistorters for use in transmitter 50.
In one embodiment, AGC circuit 62 also adjusts its complex tap coefficient in response to an LMS-based control loop algorithm. AGC circuit 62 implements an update algorithm similar to that of linear predistorter 64, except that only a single tap needs to be adapted to normalize the gain applied by amplifier 70, directional coupler 72, and other components in the control loop. Those skilled in the art will appreciate that gain may be normalized by applying amplification or attenuation as may be needed to maintain a substantially constant gain value, such as one. Normalizing an amplitude parameter of predistorted communication signal 60 at AGC circuit 62 is desirable so that error signal 90 accurately characterizes the difference between communication signal 54 and feedback signal 78 notwithstanding any long-term or average gain than may be applied through the analog components of transmitter 50, including amplifier 70. And, the control loop algorithm for AGC circuit 62 may operate at a much faster loop bandwidth than that of linear predistorter 64 and than that of nonlinear predistorter 58 (discussed below). By operating at a much faster loop bandwidth, e.g., 10 KHz-100 KHz versus 10-200 Hz, some of the slower memory effect nonlinearities, such as thermal memory effects, may be tracked through AGC adaptation. Linear predistorter 64 may desirably operate at a lower loop bandwidth to conserve power consumption and to decouple its control loop from that of AGC section 62. To further isolate the control loop for AGC section 62 from other control loops operating in transmitter 50, AGC section 62 may desirably use a leaky integrator in its LMS-based continuous process adaptation control loop.
In one embodiment (not shown) simple DC signals may be fed to the input and output of amplifier 70 through bias circuits (not shown) for the purpose of biasing amplifier 70. In another embodiment, communication signal 54 from communication-signal source 52 is fed to an input of a variable bias supply 80. Variable bias supply 80 supplies a non-DC input bias signal 81 to input 69 of amplifier 70 and/or a non-DC output bias signal 83 to output 71 of amplifier 70. Bias signals 81 and 83 may be directed to amplifier 70 through suitable bias circuits (not shown).
Variable bias supply 80 may be implemented in a manner consistent with conventional bias control circuits known to those skilled in the art. Thus, variable bias supply 80 may be configured so that one or more of bias signals 81 and 83 roughly track the envelope of communication signal 54. And, when variable bias supply 80 is configured so that one or more of bias signals 81 and 83 roughly track the envelope of communication signal 54, it may be desirable that the variable bias supply 80 implement a process which causes bias signals 81 or 83 to exhibit a bandwidth less than the bandwidth of communication signal 54. Bias signals 81 and 83 represent video bandwidth signals that vary the bias conditions applied to amplifier 70. As discussed below, video bandwidth bias signals 81 and 83 represent a portion of the factors that determine the gain applied by amplifier 70 and the distortion introduced into the communication signal 60 amplified by amplifier 70. In order to account for this portion of the distortion, one or more variable bias parameters 85 are fed from variable bias supply 80 to nonlinear predistorter 58. Variable bias parameters 85 may be precise or imprecise digital representations of varying voltages exhibited by bias signals 81 and/or 83, or other properties of bias signals 81 and/or 83 whose values characterize signals 81 and/or 83.
The block diagram of model 94 presented in
Model 94 depicts amplifier 70 as having its overall transfer function partitioned into a linear component 96 and a nonlinear component, referred to herein as a nonlinear amplifier transfer function and as a nonlinear amplifier transform 98. Linear component 96 describes the constant linear gain value ideally applied by amplifier 70. In other words, if amplifier 70 had a perfectly linear response, then amplified RF signal 76 would be accurately described by the multiplication of input signal 60′ with constant value linear component 96. Input signal 60′ is the communication signal formed from predistorted communication signal 60 provided at input 69 of amplifier 70. But amplifier 70 is not perfectly linear, and nonlinear amplifier transform 98 describes the manner in which it is not.
Model 94 indicates that nonlinear amplifier transform 98 is partitioned into two distinct types of nonlinear components, referred to as nonlinearities herein. Each component is characterized as a distinct distortion in the gain applied by amplifier 70. The two types include memoryless components 100 and memory components 102.
Although not shown, memory components 102 may include any number of individual memory components, or memory effects, coupled in parallel, with each memory component corresponding to a specific memory nonlinearity of amplifier 70. Memory effects 102 include thermal nonlinearities, which may exhibit unknown corner or resonance frequencies in the 10 KHz-100 KHz range, and electrical nonlinearities, which typically exhibit unknown corner or resonance frequencies above 100 KHz. Thermal nonlinearities result from ambient-environment heating and self-heating in the active amplifying device used by amplifier 70. Electrical nonlinearities result from the use of energy storage devices, such as inductances and capacitances, in connection with processing the analog version of predistorted communication signal 60 within amplifier 70 and elsewhere in transmitter 50. Memory components 102 apply a transform labeled “G” in
Memoryless components 100 are discussed in more detail below. RF input signal 60′ drives each of components 100 and 102, as well as a multiplication element 104. The sum of signals output from components 100 and 102, as depicted at an addition element 106, represents a gain factor by which amplifier 70 multiplies communication signal 60. This gain factor is nonlinearly related to communication signal 60. This multiplication operation is depicted at multiplication element 104. Model 94 indicates that the output of multiplication element 104 and the output of linear component 96 drive respective multiplicand inputs of a multiplication element 108, with the output of multiplication element 108 providing the output from amplifier model 94. Although not specifically shown, the output of nonlinear amplifier transform 98 is normalized so that it's output from addition element 106 would always equal one if amplifier 70 were perfectly linear. This may be accomplished by attributing a linear gain of “1” along with nonlinear gain to memoryless component 146.
Those skilled in the art will appreciate that model 94 is configured primarily to characterize the influence of nonlinearities. A more complete model may reflect other considerations. The more complete model is not presented here because it is unnecessary to an understanding of the nonlinearities to which the below-discussed features of the preferred embodiments of the present invention are directed.
RF input signal 60′ is applied to input bias circuit 110 and to an input port of HPA 114. A fixed or variable input bias voltage, Vg, is also applied to input bias circuit 110. Input bias voltage Vg may be provided by variable bias signal 81 from variable bias supply 80 (
A fixed or variable output bias voltage, Vd, is also applied to output bias circuit 112. Output bias voltage Vd may be provided by variable bias signal 83 from variable bias supply 80 (
Amplified RF signal 76 is provided across load 118, which appears across an output port of output matching network 116 and the common potential 126. Load 118 may be primarily resistive, and/or load 118 may also include a significant inductive component. A significantly inductive load may be present if a distributed active transformer (DAT) or similar component is used to couple multiple active devices to load 118. Collectively, input bias circuit 110, output bias circuit 112, output matching network 116, and load 118 provide a network of components 138 which couples to HPA 114.
In one embodiment, input and output bias voltages Vg and Vd are substantially constant DC voltages. A perfectly linear amplifier with constant DC bias voltages would operate at a constant bias condition. And, input signal 60′ and the output signal from HPA 114 would then each consist of an RF fundamental signal combined with a DC component that corresponds to the constant bias condition.
But for real-world amplifier 70, nonlinearity, as described by nonlinear amplifier transform 98 (
Video bandwidth 140 represents the bandwidth of the video signal generated by applying nonlinear amplifier transform 98 to RF input signal 60′ and by using variable bias signals 81 and 83. Video bandwidth 140 typically exceeds a baseband bandwidth for communication signal 54 (
The MOS FET HPA 114 depicted in
In a fundamental RF band 142, the impedance presented to HPA 114 by bias circuits 110 and 112 is desirably as high as practical. Desirably, the impedance is much higher than the highest impedance exhibited in video bandwidth 140. This high impedance substantially blocks fundamental RF energy from flowing into bias circuits 110 and 112. Fundamental RF band 142 represents the RF bandwidth assigned to transmitter 50 and within which RF transmitter 50 transmits. It desirably has a bandwidth approximately equal to the bandwidth of baseband communication signal 54 generated by communication-signal source 52 (
While bias circuits 110 and 112 desirably present a high impedance to HPA 114 within fundamental RF band 142, output matching network 116 and load 118 present a low impedance, causing the bulk of RF fundamental energy to flow through load 118. In an embodiment where load 118 has a significant inductive component, that low impedance may also exhibit a trajectory with constant or increasing impedance for increasing frequency, similar to the video impedance depicted in
In a harmonics band 144, impedance is desirably as low as practical. The impedance values in harmonics band 144 result at least in part from RF trapping implemented using capacitance 137, which is configured to provide a resonance frequency at the second harmonic.
Referring to
Another distinction between memoryless and memory components 100 and 102 is that parameters which accurately characterize memoryless components 100 may be determined using considerably less processing power than is expended determining parameters which accurately characterize a variety of memory components 102. Power is reduced, at least in part, because unknown temporal parameters associated with such resonance frequencies and/or corner frequencies need not be determined to accurately characterize memoryless nonlinearities.
By omitting envelope trapping capacitors from network of components 138, processing power need not be expended resolving memory effects associated with the video bandwidth 140 resonance frequencies such envelope trapping capacitors form. Network of components 138, without envelope trapping capacitors, has an inductive nature throughout video bandwidth 140, which evidences a filtering effect, and in particular a high-pass filtering effect. But the corner frequency of this high-pass filter is desirably located above video bandwidth 140. Consequently, network of components 138 is desirably configured as a high-pass filter operated as a differentiator within video bandwidth 140.
Model 94 in
One nonlinearity, which corresponds to average gain-droop component 146 (
Another nonlinearity, which corresponds to inductive memoryless component 148 (
This video nonlinearity which is expressed in the different trajectories of the different curves shown in
This video memoryless nonlinearity component differs from the memoryless gain-droop nonlinearity component because it arises from different physical characteristics of amplifier 70. While gain-droop results from operating HPA 114 over a range of input signal magnitude at any constant bias condition or at an average bias condition, the video nonlinearity results from operating HPA 114 at a variety of different bias conditions, where the variety results at least in part from interaction between the video signal and the video bandwidth 140 impedance of the network of components 138 coupled to HPA 114.
Accordingly,
In an embodiment of amplifier 70 in which load 118 (
Predistorter 58 includes a magnitude-extracting section 150 which extracts a magnitude parameter from communication signal 54, forming a magnitude signal 152. In the preferred embodiment, the magnitude parameter obtained in section 150 is the pure mathematical magnitude of the complex communication signal 54, but other embodiments may extract other magnitude parameters, such as magnitude squared or square-root of magnitude.
Magnitude signal 152 passes to a processing section 154 and to a processing section 156. Processing section 154 is configured to implement an inverse transform to the FML1 transform applied by memoryless component 146, i.e., FML1−1. Processing section 154 generates a gain-correcting signal 158 that represents the inverse of the gain-modulating signal generated by memoryless component 146 of model 94. Thus, processing section 154 applies an inversing transform FML1−1 that is responsive to gain droop for an average bias condition, average temperature, and an average of all memory effects.
Those skilled in the art will appreciate that precise mathematical averages or means are not explicitly required in identifying the average behavior for bias conditions, temperature deviations, and memory effects. Rather the average refers to, for each possible single value of communication signal 54, a single value that summarizes or represents the general significance of the set of all values that gain droop exhibits over a tracking period for that value of communication signal 54. And, the averages may be determined implicitly rather than explicitly.
The determination of inversing transform FML1−1 is discussed in more detail below in connection with
Processing section 156 is configured to implement a transform corresponding to the FML2 transform applied by inductive memoryless component 148, i.e., ̂FML2. As discussed below, the ̂FML2 transform may correspond to the FML2 transform in more than one way. Processing section 156 generates a gain-correcting signal 160 that represents the gain-modulating signal generated by inductive memoryless component 148 of model 94. Thus, processing section 156 applies a transform ̂FML2 that is desirably substantially unresponsive to average bias conditions, but desirably responsive to the gain modulation exhibited by amplifier 70 as bias conditions deviate from the average bias conditions. As explained above, bias conditions deviate from the average bias conditions at least in part due to amplifier 70 applying nonlinear amplifier transform 98 to RF input signal 60′, causing even-ordered harmonics which, along with the video impedance of network of components 138, are responsible for the video signal which defines the deviations.
Gain-correcting signals 158 and 160 are each configured to address different components of gain distortion in amplifier 70. And, gain-correcting signals 158 and 160 are contemporaneous with one another. In other words, signals 158 and 160 are respectively generated by processing sections 154 and 156 in parallel or at the same time, and each of signals 158 and 160 is desirably capable of exerting an influence on predistorted communication signal 60 during each sample of communication signal 54. Moreover, since gain-correcting signals 158 and 160 are directed to memoryless phenomena, processing sections 154 and 156 may be implemented and updated while consuming only a small amount of power.
Gain-correcting signals 158 and 160 and communication signal 54 pass to a joining and gain adjusting section 162. In general, joining and gain adjusting section 162 joins gain-correcting signals 158 and 160 together into a combined gain-correcting signal 248 that substantially exhibits an inverse behavior with respect to signal magnitude to the behavior of the signal provided by addition element 149 in model 94 for amplifier 70. And, joining and gain adjusting section 162 applies gain to communications signal 54, including amplification and/or attenuation, in a manner defined by the combined gain-correcting signal. Joining and gain-adjusting section 162 is discussed in more detail below in connection with
Nonlinear predistorter 58 also includes an adaptive control section 164. Adaptive control section 164 receives magnitude signal 152, error signal 90, and delayed communication signal 92 as inputs. These input signals are used to generate update signals 166 and 168 respectively provided to processing sections 154 and 156. Update signals 166 and 168 train and maintain processing sections 154 and 156 to define the FML1−1 and ̂FML2 transforms they apply to a magnitude parameter of communication signal 54. Portions of adaptive control section 164 are discussed below in connection with
Processing section 154 desirably implements a form of gain-based predistortion that uses a look-up table (LUT) 170. Desirably, LUT 170 is organized to include a multiplicity of data entries 172, with different data entries 172 corresponding to different magnitude values that may be presented to the address input of LUT 170. Each data entry is desirably configured as a complex value having in-phase and quadrature components. During each look-up operation, the addressed data entry 172 is provided at a data output of LUT 170 and referred to herein as an outgoing data entry 174.
A mode switch 176 signifies that processing section 154 may operate in two different modes. Those skilled in the art will appreciate that no actual switch is required but that switch 176 is depicted to indicate two different operations that take place with respect to LUT 170. For example, if LUT 170 is implemented using a dual-port memory device then both operations may take place simultaneously from the perspective of circuits outside of LUT 170. One mode is a normal mode, during which processing section 154 applies its FML1−1 transform to magnitude signal 152 in order to generate gain-correcting signal 158. The other mode is an update mode, during which processing 154 updates one of its data entries. During a normal mode of operation LUT 170 is addressed by magnitude signal 152. For each sample, the magnitude value is translated into a complex gain value by LUT 170, and the outgoing data entry 174 that defines the complex gain value forms a sample of gain-correcting signal 158. Although not shown, a section may be included at the output of processing section 154 to force gain-correcting signal 158 to generate a stream of a constant, normalized value, such as [0,0] or [0,1], when processing section 156 is being updated to reduce cross-coupling between processing sections 154 and 156.
During the update mode, data entries 172 for LUT 170 are calculated by adaptive control section 164, which implements a control loop that processes amplified RF signal 76 as expressed in error signal 90. In one embodiment (not shown), adaptive control section 164 implements a conventional least-means-squared (LMS) algorithm. In this embodiment, adaptive control section 164 performs conversions between Cartesian and polar coordinate systems in making its calculations. Alternatively, adaptive control section 164 may implement a conventional LMS algorithm using the secant method, which requires the performance of division operations.
Referring to
Communication signal 92 is also provided to a conjugation section 180. Conjugation section 180 implements a conjugation operation, which in the Cartesian coordinate system can be performed by negating the imaginary component of each complex sample. Conjugation section 180 provides a conjugated communication signal 182 that is responsive to communication signal 92. Conjugated communication signal 182 drives a first input of a multiplier 184.
Error signal 90 (also shown in
As shown in
Raw correlation signal 186 is received at a two-quadrant complex multiplier 188 along with a scaling, step size, or loop-control constant 190, which is labeled using the variable “μML1” in
Scaling constant 190 determines how much influence each sample from raw correlation signal 186 will exert on an updated data entry 172 for LUT 170. Greater influence is associated with faster but less stable convergence for LUT 170, more noise represented in data entries 172 of LUT 170, and a faster loop bandwidth for the control loop that updates data entries 172. Scaling constant 190 is desirably chosen to implement a relatively narrow loop bandwidth. This loop bandwidth establishes the tracking period over which gain-droop memoryless component 146 of amplifier model 94 (
However, scaling constant 190 need not be completely time invariant. For example, a faster loop bandwidth may be initially chosen to quickly populate LUT 170 with data entries 172, then the loop bandwidth may be slowed. And, scaling constant 190 may be set to zero for extended periods when desirable to prevent data entries 172 from changing. For example, scaling constant 190 may be set to zero while transmitter 50 is not actively transmitting, and scaling constant 190 may be set to zero while other control loops within transmitter 50 are converging.
Scaled correlation signal 192 drives a positive input of a combiner 194. A negative input of combiner 194 receives outgoing data entries 174 from LUT 170. For each sample of scaled correlation signal 192, the outgoing data entry 174 provided to combiner 194 from LUT 170 corresponds to the sample of communication signal 92 to which the scaled correlation signal 192 sample also corresponds. A magnitude parameter for that sample from communication signal 92 serves as an address to LUT 170 to cause LUT 170 to produce the corresponding data entry 174.
Desirably, combiner 194 performs a Cartesian coordinate system addition operation. An output of combiner 194 couples to a data input port of LUT 170 and provides incoming data entries through update signal 166 for storage in LUT 170. Each incoming data entry is stored at the same memory address from which the corresponding outgoing data entry 174 was previously stored. The incoming data entry carried by update signal 166 is expressed in the Cartesian coordinate system.
Accordingly, adaptive control section 164 applies an update equation to error signal 90, delayed communication signal 92, and outgoing data entries 172 addressed by a delayed magnitude signal. When the control loop converges, processing section 154 implements transform FML−1, which approximates the inverse of the FML1 transform applied by memoryless component 146 of model 94.
Like processing section 154 discussed above, processing section 156 desirably implements a form of gain-based predistortion that uses a look-up table (LUT), labeled LUT 198. But the transform being implemented by processing 154 was dictated by memoryless gain-droop of HPA 114. While the relationship is nonlinear, gain-droop is fairly well characterized considering signal magnitude alone, and without considering other circuit components. Unlike processing section 154, processing section 156 applies a transform ̂FML2 dictated by network of components 138 (
The above-discussed video signal that is responsible for video memoryless component 148 is generated by even harmonics of the input signal. Accordingly, the video current signal generated by these even harmonics at the drain of HPA 114 may be modeled as an instantaneous nonlinear current generator that implements some unspecified nonlinear polynomial function of the magnitude signal, and more specifically a collection of powers of the magnitude signal. This represents a first nonlinear function to be attributed to HPA 114. It is largely accounted for by gain droop, without considering envelope-induced bias modulation.
The video signal then acts upon the video impedance (
LUT 198 is a polynomial generator that produces a polynomial signal 200 which corresponds to the above-discussed nonlinear current generator and voltage-to-gain conversion. A differentiator 202 has an input driven by magnitude signal 152. Differentiator 202 models the application of the video current signal upon the video impedance of network of components 138 in a generic fashion. Differentiator 202 is desirably configured to provide a reasonably accurate derivative over half of video bandwidth 140. Differentiator 202 may be implemented using a FIR, IIR, or other architecture in a manner understood to those of skill in the art. An output of differentiator 202 provides a derivative signal 204 which drives a first input of a multiplier 206. Polynomial signal 200 drives a second input of multiplier 206. An output of multiplier 206 provides gain-correcting signal 160. Together, the polynomial generator of LUT 198 and differentiator 202 provide transform ̂FML2 when LUT 198 has been updated.
Desirably, LUT 198 is organized to include a multiplicity of data entries 208, with different data entries 208 corresponding to different magnitude values that may be presented to the address input of LUT 198. Each data entry 208 is desirably configured as a complex value having in-phase and quadrature components. During each look-up operation, the addressed data entry 208 is provided at a data output of LUT 198 and referred to herein as a sample of polynomial signal 200.
A mode switch 210 signifies that processing section 156 may operate in two different modes. Those skilled in the art will appreciate that no actual switch is required but that switch 210 is depicted to indicate two different operations that take place with respect to LUT 198. LUT 198 may be implemented using a dual-port memory device so that both operations may take place simultaneously from the perspective of circuits outside of LUT 198. One mode is the normal mode, during which processing section 156 applies its ̂FML2 transform to magnitude signal 152 in order to generate gain-correcting signal 160. The other mode is the update mode, during which processing section 156 updates one of its data entries 208. Desirably, processing sections 154 and 156 operate in their normal modes contemporaneously, but sections 154 and 156 need not, and preferably do not, operate in their update modes contemporaneously. During the normal mode of operation, LUT 198 is addressed by magnitude signal 152. For each sample, the magnitude value is translated into a complex gain value by LUT 198 which serves as a sample of polynomial signal 200. The product of a sample from derivative signal 204 and the sample of polynomial signal 200 forms a sample of gain-correcting signal 160. Although not shown, a section may also be included at the output of processing section 156 to force gain-correcting signal 160 to generate a stream of a constant, normalized value, such as [0,0], when one or both of processing sections 154 and 156 are being updated to reduce cross-coupling between processing sections 154 and 156 or to prevent gain-correcting signal 160 from influencing amplified RF signal 76 (
During the update mode, data entries 208 for LUT 198 are calculated by adaptive control section 164, which implements a control loop that processes amplified RF signal 76. For LUT 198 control section 164 applies a different update equation than is used to update LUT 170 (
Referring to
Communication signal 92 is also provided to a conjugation section 214. Conjugation section 214 implements a conjugation operation. Conjugation section 214 provides a conjugated communication signal 216 responsive to communication signal 92. Conjugated communication signal 216 drives a first input of a multiplier 218.
In an alternate embodiment, processing section 156 is updated so that transform ̂FML2 corresponds to transform FML2 of inductive memoryless component 148 by closely approximating transform FML2. In this embodiment, gain-correcting signal 160 and an appropriately delayed version of the communication signal, such as communication signal 92, drive respective multiplicand inputs of a multiplier 224 to generate a communication signal 226 whose gain has been altered to reflect only the estimated component of total gain modulation due to the video signal influence on bias conditions. Communication signal 226 drives a positive input of a combiner 228, and a form of feedback signal 78 (
Each of error signals 90 and 90′ are responsive to amplified RF signal 76 through feedback signal 78. In particular, error signal 90 is responsive to a difference between delayed communication signal 92 and amplified RF signal 76. Alternate error signal 90′ is responsive to a communication signal whose gain has been altered by the current estimate of FML2 and amplified RF signal 76. Error signal 90 generally represents the portion of amplified RF signal 76 (
Derivative signal 204 is delayed in a delay element 230 and then applied to a second input of derivative neutralizer 220 at a sign section 232. Within derivative neutralizer 220, an output of sign section 232 couples to a second input of multiplier 222. An output of multiplier 222 serves as the output for derivative neutralizer 220 and provides a DC-offset enhanced error signal 234.
Derivative signal 204 has no DC component. It exhibits a relatively short-term average value of zero. It roughly describes the slope of magnitude signal 152, which must be positive as much as it is negative since magnitude signal 152 is permanently confined within a fixed magnitude range. The same derivative effect may be attributed to the video voltage signal which is responsible for bias condition deviations experienced by HPA 114 (
Multiplier 218 correlates conjugated communication signal 216 with DC-offset enhanced error signal 234 to produce a raw correlation signal 236. Raw correlation signal 236 is received at a two-quadrant complex multiplier 238 along with a scaling, step size, or loop-control constant 240, which is labeled using the variable “μML2” in
Scaled correlation signal 242 drives a positive input of a combiner 244. A negative input of combiner 244 receives polynomial signal 200 from LUT 198. For each sample of scaled correlation signal 242 and each corresponding sample from polynomial signal 200 provided to combiner 244 from LUT 198 corresponds to the sample of communication signal 92 to which the scaled correlation signal 242 sample also corresponds. A magnitude parameter for that sample from communication signal 92 serves as an address to LUT 198 to cause LUT 198 to produce the corresponding data entry 208.
An output of combiner 244 couples to a data input port of LUT 198 and provides incoming data entries through update signal 168 for storage in LUT 198. Each incoming data entry is stored at the same memory address from which the corresponding sample of polynomial signal 200 was previously stored.
Accordingly, adaptive control section 164 applies an update equation to error signal 90 or alternative error signal 90′, to delayed communication signal 92, and to data entries 208 stored in LUT 198 at addresses accessed by a delayed magnitude signal. When the control loop converges, processing section 164 implements transform ̂F, which corresponds to the FML2 transform applied by memoryless component 148 of model 94. In one embodiment ̂FML2 is configured with FML1−1 to provide a better estimate of the combined inverse of FML1 in parallel with FML2 than is provided by FML1−1 alone. In another embodiment, ̂FML2 is configured to approximate FML2.
Desirably, the parallel transforms of processing sections 154 and 156 (
Transform FML1−1 may be established as discussed above in connection with
Within joining and gain-adjusting section 162, a multiplier 250 receives gain-correcting signals 158 and 160 and produces the term FML2FML1−1. An output of multiplier 250 couples to first and second inputs of a multiplier 252, a first input of a multiplier 254, and a negative input of a combiner 256. An output of multiplier 252 couples to a second input of multiplier 254 and to a positive input of combiner 256, and an output of multiplier 254 couples to a negative input of combiner 256. Multiplier 252 is responsible for the term: (FML2FmL1−1)2 from EQ. 2. Multiplier 254 is responsible for the term: (FML2FML1−1)3 from EQ. 2. A constant value of [1,1] is applied to a positive input of combiner 256. An output from combiner 256 couples to a first input of a multiplier 258, and gain-correcting signal 158 drives a second input of multiplier 258. The output of multiplier 258 generates combined gain-correcting signal 248.
Communication signal 54 drives a first input of a multiplier 260, and gain-correcting signal 248 drives a second input of multiplier 260. An output of multiplier 260 generates predistorted communication signal 60. At multiplier 260, communication signal 54 is predistorted by adjusting the gain of communication signal 54 in accordance with the dictates of combined gain-correcting signal 248.
In particular, a specific amount of gain, that may differ for each magnitude value that communication signal 54 exhibits within the range of magnitude values exhibited by communication signal 54, has been applied to each sample of communication signal 54. The amount of gain applied is responsive to the derivative of the magnitude of communication signal 54 as well as to the magnitude of communication signal 54. For the stream of samples in communication signal 54, the amount of gain applied at multiplier 260 approximates the inverse of the collective gain associated with memoryless components 146 and 148 of nonlinear amplifier transform 98 (
While the
In this
In predistorter 58′, communication signal 54 drives a delay element 264 and a memoryless nonlinear predistorter 58. Memoryless nonlinear predistorter 58 may be configured as discussed above in connection with
The embodiment of predistorter 58 depicted in
Referring to
Look-up table 282 forms a polynomial generator which applies a polynomial function to parameters presented at its address inputs. In this situation, the polynomial generator of LUT 282 applies a polynomial function to magnitude parameter 152 and magnitude derivative parameter 204. The polynomial generator output from LUT 282 provides a gain-correcting signal 284 to a first input of multiplier 260 in gain-adjusting section 162. Communication signal 54 drives a second input of multiplier 260 in gain-adjusting section 162. Gain-adjusting section 162 and multiplier 260 operate as discussed above in connection with
The polynomial function applied by LUT 282 is determined in response to an LMS control loop, in a manner similar to that described above in connection with
Adaptive control section 164 is responsive to error signal 90 and to communication signal 92. Communication signal 92 represents a form of communication signal 54 delayed into time alignment with error signal 90. Adaptive control section 164 generates update signal 166 as described above in connection with
Accordingly, since a single control loop is provided and a single polynomial that is a function of both magnitude parameter 152 and magnitude derivative parameter 204 is provided, improved performance over the embodiment of predistorter 58 shown in
As discussed above, magnitude derivative parameter 204 is responsive to the portion of the video signal that results from even-ordered harmonics of the RF version of predistorted communication signal 60′ input to amplifier 70 (
As shown in
Likewise, variable bias parameters 85 may include a variable gate bias parameter 81′ that characterizes variable bias signal 81 (
And, variable gate signal 81 also indirectly modulates the bias conditions of HPA 114 by being amplified through HPA 114 to generate a video bandwidth component of drain current proportional to variable bias signal 81. This component of drain current acts upon the substantially inductive network of components 138 to generate another video bandwidth drain voltage signal component proportional to the derivative of variable bias signal 81, since network of components 138 is substantively inductive and resonance frequencies in the video bandwidth impedance are avoided by omitting video trapping capacitors. Thus, variable gate bias parameter 81′ is processed through a differentiator 296 and a quantizer 298 to differentiate parameter 81′ with respect to time and to quantize derivative parameter 81′ prior to concatenating the derivative of parameter 81′ with the other address inputs of LUT 282. While delay elements are shown in other signal paths that drive address inputs of LUT 282,
By making the polynomial generator implemented in LUT 282 responsive to variable bias parameters, predistorter 58 achieves an even more accurate definition of the manner in which signal magnitude and bias conditions alter the gain exhibited by HPA 114, and improved performance in the linearization of transmitter 50 results.
With up to five different parameters driving the address inputs of LUT 282, the size of LUT 282 may become undesirably large for some applications, particularly when some of the parameters are presented to LUT 282 using more than just a few bits of resolution. An excessively large LUT may be undesirable in some applications for two reasons. First, a larger LUT drives up costs and consumes more power. And second, a larger LUT requires longer to converge upon a stable and accurate definition of its polynomial.
The
Other than for the use of a smaller memory which implements LUT 282 using fewer address bits than may be required in the
Variable gate bias parameter 81′ is also scaled by a different scaling constant, βg′, in a scaling section 306 and combined at an adder 310 with a version of magnitude parameter 152 that has been delayed in a delay element 308. The output from adder 310 drives differentiator 202. At the output of differentiator 202 a combination parameter represents the combination of the two derivative parameters from the
Each of scaling constants βg, βg′, and βd may be determined empirically during manufacture. Alternately, each of scaling constants βg, βg′, and βd may be independently determined in control loops (not shown) which monitor error signal 90 and dither the respective scaling constants βg, βg′, and βd in a controlled manner until the power of error signal 90 is minimized. Desirably, such control loops are decoupled from one another in any of a variety of ways known to those skilled in the art and exhibit a loop bandwidth sufficiently slow that they do not interfere with the control loops used to update LUT 282, AGC 62, and linear predistorter 64 (
Accordingly, this
In this
In this
In this
The portion of adaptive control section 164 associated with LUT 282′ responds to the same error signal 90 and delayed communication signal 92 that are used in updating the polynomial of LUT 282. So, only the final stages of the LMS algorithm implemented through adaptive control section 164 are repeated for LUT 282′. In particular, raw correlation signal 186 is used for both LUT 282 and LUT 282′. But for LUT 282′, raw correlation signal 186 drives a first input of a multiplier 188′, where a second input receives a scaling constant 190′. An output of multiplier 188′ drives a first input of a combiner 194′, where a second input receives data output from LUT 282′ during the update process. An output from combiner 194′ provides update signal 166′, which drives the “data in” port of LUT 282′. And, a delay element 286′ delays the magnitude parameter that drives the address input of LUT 282′ into temporal alignment with update signal 166′ for presentation at the second port of LUT 282′.
The
Although not shown, since LUT's 282 and 282′ are at least partially converging on a common solution, in some applications their update control loops may conflict with one another to some degree. Any such conflict may be resolved using techniques known to those of skill in the art, including implementing one of the integrators resulting from combiners 194 and 194′ operating in combination with their respective LUT's as a leaky integrator.
The embodiments of predistorter 58 depicted in
In summary, at least one embodiment of the present invention provides a linearized transmitter and a transmitter linearizing method that expand linearization efforts to address inductively induced distortion, including distortion resulting from video signal bias modulation. In accordance with at least one embodiment, effective amounts of linearization are provided at low power. In accordance with at least one embodiment, a memoryless nonlinear predistorter is provided that compensates for video signal effects. In accordance with at least one embodiment, a network of components coupled to an active amplifying device is configured to minimize memory effects. In accordance with at least one embodiment, variable bias signals are used to improve amplifier power added efficiency, and the contribution to non-linearity of such variable bias signals is compensated for. In accordance with at least one embodiment, improved linearity results at little penalty in look-up table size and/or convergence time.
Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications and adaptations may be made without departing from the spirit of the invention or from the scope of the appended claims. For example, those skilled in the art will appreciate that the specific functions depicted herein through the use of block diagrams and circuit diagrams may be partitioned in equivalent but different ways than shown and discussed herein. Such equivalent but different ways and the modifications and adaptations which may be implemented to achieve them are to be included within the scope of the present invention. Likewise, while certain operational conditions have been mentioned herein for the purposes of teaching the invention, the invention may be applied in connection with other operational conditions.
This is a continuation-in-part of “Transmitter Linearized Using Bias Deviation Gain Adjustment And Method Therefor,” Ser. No. 12/917,878, filed 2 Nov. 2010 by the inventors of the present application, which is incorporated herein by reference. The present invention relates generally to the field of communication systems. Specifically, the present invention relates to predistorters which reduce distortion introduced into a communication signal by an imperfectly linear amplifier.
Number | Date | Country | |
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Parent | 12917878 | Nov 2010 | US |
Child | 13019173 | US |