US20170047952A1 - Multi-band wide band power amplifier digital predistortion system - Google Patents
Multi-band wide band power amplifier digital predistortion system Download PDFInfo
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- US20170047952A1 US20170047952A1 US15/181,033 US201615181033A US2017047952A1 US 20170047952 A1 US20170047952 A1 US 20170047952A1 US 201615181033 A US201615181033 A US 201615181033A US 2017047952 A1 US2017047952 A1 US 2017047952A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/02—Transmitters
- H04B1/04—Circuits
- H04B1/0475—Circuits with means for limiting noise, interference or distortion
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/32—Modifications of amplifiers to reduce non-linear distortion
- H03F1/3241—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits
- H03F1/3247—Modifications of amplifiers to reduce non-linear distortion using predistortion circuits using feedback acting on predistortion circuits
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/62—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission for providing a predistortion of the signal in the transmitter and corresponding correction in the receiver, e.g. for improving the signal/noise ratio
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2626—Arrangements specific to the transmitter only
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
- H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
- H04L27/36—Modulator circuits; Transmitter circuits
- H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator
- H04L27/367—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion
- H04L27/368—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion adaptive predistortion
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F2200/00—Indexing scheme relating to amplifiers
- H03F2200/111—Indexing scheme relating to amplifiers the amplifier being a dual or triple band amplifier, e.g. 900 and 1800 MHz, e.g. switched or not switched, simultaneously or not
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F2200/00—Indexing scheme relating to amplifiers
- H03F2200/451—Indexing scheme relating to amplifiers the amplifier being a radio frequency amplifier
Definitions
- the present invention generally relates to wireless communication systems using complex modulation techniques. More specifically, the present invention relates to power amplifier systems for wireless communications.
- a wideband mobile communication system using complex•modulation techniques such as wideband code division access (WCDMA) and orthogonal frequency division multiplexing
- DSP-based DPD schemes utilize FPGAs, DSPs or microprocessors to compute, calculate and correct the PA's nonlinearities: they perform fast tracking and adjustments of signals in the PA system.
- conventional DSP-based DPD schemes are challenged by variations of the linearity performance of the power amplifier over wide bandwidths due to the environment changing such as temperature and the asymmetric distortions of the output signal of the PA resulting from memory effects.
- Conventional DPD algorithms are based on a wideband feedback signal, they require a high speed analog-to-digital converter (ADC) in order to capture the necessary information.
- ADC analog-to-digital converter
- Multi-frequency band, or simply multi-band, applications can have their operating frequencies spaced significantly apart.
- Conventional DPD architectures use an ADC sampling rate that is greater than twice the nonlinear distortion bandwidth of the input signal. This sampling rate is typically more than double a factor of five times the operating bandwidth of the complex modulated signal. The factor of five accounts for the spectral regrowth attributed to the nonlinear distortion created by the power amplifier. This restriction on sampling rate, limits the feasibility of the conventional predistortion architectures to single band applications. Higher sampling rate ADCs have lower resolution, consume more power and are more expensive.
- the present invention has been made in view of the above problems, and it is an object of the present invention to provide a high performance and cost effective method of power amplifier systems with high linearity and high efficiency for multi-frequency band wideband communication system applications.
- the present disclosure enables a power amplifier system to be field reconfigurable and support multiple operating frequency bands on the same PA system over a very wide bandwidth.
- the present invention supports multi-modulation schemes (modulation agnostic), multi-carriers, and multi-channels.
- the technique is based on the method of adaptive digital predistortion to linearize a RF power amplifier.
- the present invention is based on using distinct signals from different frequencies (Multi-Band Signals). These Multi-Band Signals will experience distortion by the power amplifier and create nonlinear distortion centered on each carrier that is approximately five times their individual bandwidths.
- the feedback signal from the power amplifier's are down converted to an intermediate frequency (IF) that insures that the fundamental carrier bandwidths will not be aliased onto each other after sampling in the ADC.
- IF intermediate frequency
- the present invention can accommodate aliasing of the nonlinear distortion of the individual carriers.
- crest factor reduction CFR
- DPD DPD
- QAM analog quadrature modulator
- Some embodiments of the present invention are able to monitor the fluctuation of the power amplifier characteristics and to self-adjust by means of a self-adaptation algorithm.
- One such self-adaptation algorithm presently disclosed is called an adaptive DPD algorithm, which is implemented in the digital domain and taught in the applications incorporated herein by reference and attached as an Appendix.
- Applications of the present invention are suitable for use with all wireless base-stations, access points, mobile equipment and wireless terminals, portable wireless devices, and other wireless communication systems such as microwave and satellite communications.
- FIG. 1 is a block diagram showing the basic form of a digital predistortion power amplifier system.
- FIG. 2 is a block diagram showing a simple digital predistortion block diagram for a power amplifier system according to one embodiment of the present invention.
- FIG. 3 is a block diagram showing polynomial based predistorter of the present invention.
- FIG. 4 is a block diagram of the digital predistortion Direct Learning algorithm applied for self-adaptation in a digital predistortion power amplifier system of the present invention.
- FIG. 5 is a block diagram of the digital predistortion In-direct Learning algorithm applied for self-adaptation in a digital predistortion power amplifier system of the present invention.
- FIG. 6 is a depiction of the frequency domain signals present in the aliased digital predistortion system.
- FIG. 7 is an embodiment of the Quadrature Modulator compensation block architecture.
- the present invention is a novel multi-band predistortion system that utilizes an adaptive digital predistortion algorithm.
- the present invention is a hybrid system of digital and analog modules.
- the interplay of the digital and analog modules of the hybrid system both linearize the spectral regrowth and enhance the power efficiency of the PA while maintaining or increasing the wide bandwidth.
- the present invention therefore, achieves higher efficiency and higher linearity for wideband complex modulation carriers that operate simultaneously over multiple, distinct frequency bands.
- FIG. 1 is a high level block diagram showing the basic system architecture which can be thought of, at least for some embodiments, as comprising digital and analog modules and a feedback path.
- the digital module is the digital predistortion controller 101 which comprises the DPD algorithm, other auxiliary DSP algorithms, and related digital circuitries.
- the analog module is the main power amplifier 102 , other auxiliary analog circuitries such as DPA, and related peripheral analog circuitries of the overall system, as further discussed in the applications incorporated by reference.
- the present invention is a “black box”, plug-and-play type system because it accepts RF modulated signal 100 as its input, and provides a substantially identical but amplified RF signal 103 as its output, therefore, it is RF-in/RF-out.
- Baseband input signals can be applied directly to the Digital Predistorter Controller according to one embodiment of the present invention.
- An Optical input signal can be applied directly to the Digital Predistorter Controller according to one embodiment of the present invention.
- the feedback path essentially provides a representation of the output signal to the predistortion controller 101 .
- the memory effects due to self-heating, bias networks, and frequency dependencies of the active device are compensated by the adaptation algorithm 204 in the DPD 201 , FIG. 2 .
- the multi-band RF input x[n] is provided to a DPD 201 and to DPD Algorithm logic 204 .
- the output of the DPD 201 , z[n] is provided to a DAC 202 and the logic 204 .
- the output of the DAC 202 provides the input to the power amplifier 203 .
- the distortion characteristic of the PA is sensed by the feedback samples y(t), which are converted in the ADC 206 into data ya[n] and provided to alignment logic 205 . After alignment the data ya[n] provides feedback data to the algorithm logic 204 .
- the coefficients of the DPD are adapted by synchronizing the wideband captured, aliased output Multi-Band Signal ya[n] from the feedback path (Sampled Feedback Aliased Signal) with the reference Multi-Band Signal x[n] (Input Signal).
- the DPD algorithm performs the synchronization and compensation.
- the synchronization aligns the reference signal with the aliased feedback signal in the alignment block.
- the reference signal and the aliased Sampled Feedback Aliased Signal ya[n] are used in the Direct Learning adaptive algorithm.
- the aliased predistorted signal za[n] (Predistorted Output Aliased Signal) and the Sampled Feedback Aliased Signal ya[n] are used in an Indirect Learning adaptive algorithm.
- Some embodiments apply crest factor reduction (CFR) prior to the DPD with an adaptation algorithm in one digital processor, so as to reduce the PAPR, EVM and ACPR and compensate the memory effects and variation of the linearity due to the temperature changing of the PA.
- the digital processor can take nearly any form for convenience, an FPGA implementation is typically used, but a general purpose processor is also acceptable in many embodiments.
- the CFR implemented in the digital module of the embodiments is based on the scaled iterative pulse cancellation presented in patent application U.S. 61/041,164, filed Mar. 31, 2008, entitled An Efficient Peak Cancellation Method For Reducing The Peak-To-Average Power Ratio In Wideband Communication Systems, incorporated herein by reference.
- the CFR is included to enhance performance and hence optional.
- the CFR can be removed from the embodiments without affecting the overall functionality.
- the memory effects due to self-heating, bias networks, and frequency dependencies of the active device are compensated by the adaptation algorithm in the DPD.
- the coefficients of the DPD are adapted by synchronizing the wideband captured output signal from the feedback path with the reference signal.
- the digital predistortion algorithm performs the synchronization and compensation.
- the predistorted signal is passed through a DQM in order to generate the real signal and then converted to an IF analog signal via a DAC.
- the DQM is not required to be implemented in the FPGA, or at all, in all embodiments. If the DQM is not used in the FPGA, then the AQM Implementation can be implemented with two DACs to generate real and imaginary signals, respectively.
- FIG. 3 is a block diagram showing a predistortion (PD) part in the DPD system of the present invention.
- the PD in the present invention generally utilizes an adaptive polynomial-based digital predistortion system.
- Another embodiment of the PD utilizes a LUT-based digital predistortion system.
- the PD illustrated in FIG. 3 are processed in the digital processor by an adaptive algorithm, presented in U.S. patent application Ser. No. 11/961,969, entitled A Method for Baseband•Predistortion Linearization in Multi-Channel Wideband Communication Systems.
- the PD 3 has multiple finite impulse response (FIR) filters, that is, FIR 1 301 , FIR 2 303 , FIR 3 305 , and FIR 4 307 .
- the PD also contains the third order product generation block 302 , the fifth order product generation block 304 , and the seventh order product generation block 306 .
- the output signals from FIR filters are combined in the summation block 308 . Coefficients for multiple FIR filters are updated by the digital predistortion algorithm.
- DPD Digital Predistortion
- PA power amplifier
- FIG. 2 shows the block diagram of a digitally predistorted PA system.
- a memory polynomial model is used as the predistortion function ( FIG. 3 ).
- a least square algorithm is utilized to find the DPD coefficients a ij , and then transfer them to DPD block.
- the detailed DPD algorithm is shown in FIG. 4 and FIG. 5 .
- the coefficients are obtained using the QR RLS adaptive algorithm in the DPD estimator block.
- FIG. 4 shows one embodiment of the multi-band digital predistorter.
- the Direct Learning adaptive algorithm has two inputs into the DPD estimator.
- the DPD estimator uses the aliased sampled Multi-Band Signal xa[n] (Sampled Input Aliased Signal) as a reference and the Sampled Feedback Aliased Signal ya[n] as an input.
- the x(n) signal is provided to DPD 400 and also frequency translation/aliasing logic 420 .
- the DPD outputs signal z(n), while the aliasing logic 420 outputs•xa(n) and provides it to integer delay logic 402 which then supplies a signal to fractional delay logic 402 and mux 414 , which also receives the output of the logic 402 .
- the mux also receives a control signal from delay estimator 406 , and provides an output to block xa′ 404 , which determines xa′ (n ⁇ m).
- the mux output is also supplied to data buffer 405 , which provides its output to DPD estimator 412 , and control signals to phase shift block 41 and gain correction block 411 .
- the feedback signal y(t) is provided to ADC 421 , also with sampling frequency F s , which is selected to create appropriate Nyquist zones for the aliased signals.
- the ADC output is provided to feedback data buffer 408 , as is ya(n) data from block 407 .
- the data buffer 408 provides data to delay estimator 406 as well as providing the data to phase shifter 410 and gain correction block 411 .
- the gain correction is supplied to the DPD estimator, which, together with the predistortion coefficients already in memory, shown at 401 , are muxed back to DPD 400 .
- FIG. 5 shows another embodiment of the multi-band digital predistorter.
- the in-direct Learning adaptive algorithm of FIG. 5 has two inputs into the DPD estimator.
- the DPD estimator uses the Predistorted Output Aliased Signal za[n] as a reference and the Sampled Feedback Aliased Signal ya[n] as an input, but is otherwise very similar to FIG. 4 and therefore is not explained further.
- FIG. 6 A depiction of the spectrum domain plots are shown in FIG. 6 .
- the reference Input Signal x[n] shows two distinct bands centered at frequencies F a and F b .
- the operating bandwidth of the individual carriers is small in comparison to the frequency spacing between the carriers.
- the nonlinearities in the power amplifier will cause spectral re-growth as shown in the analog feedback Multi-Band Signal y(t) (Analog Feedback Signal).
- the Analog Feedback Signal is down converted to an intermediate frequency F i which is selected to be in the 1st Nyquist zone as shown in FIG. 6 .
- the choice of frequency F i is dependent on the ADC sampling rate used, or F s , the operating bandwidth of the carriers and the frequency separation (F a ⁇ F b ) between the carriers.
- the ADC sampling rate F s is typically the limiting factor in the choice of F i .
- F a is positioned in the 1st Nyquist zone and F b is positioned in the 2nd Nyquist zone in at least some embodiments, although either signal F a or F b could be positioned in the third, fourth or nth Nyquist zones, depending upon the particular implementation.
- the primary requirement is simply that the two signal are located in different Nyquist zones.
- the choice of F i limits how close F a and F b can be separated together.
- Sampling on the Analog Feedback Signal y(t) generates images as shown in the spectrum for the Sampled Feedback Aliased Signal ya[n].
- the nonlinear distortion from the individual carriers are allowed to alias onto each other as long as the aliased part of the signal does not adversely impact the original Multi-Band signal.
- a Direct Learning algorithm uses the difference between xa[n] and ya[n] to minimize the resultant error signal.
- the QR RLS algorithm uses this error to adapt the predistorter coefficients in the DPD estimator.
- the In-Direct learning algorithm first models the power amplifier using the Predistorted Output Aliased Signal za[n] and the Sampled Feedback Aliased Signal ya[n]. The modelled power amplifier coefficients are then used to calculate the predistorter coefficients.
- FIG. 7 is a block diagram showing the analog quadrature modulator compensation structure.
- the analog quadrature modulator will translate the baseband signal output from the DACs to an RF frequency.
- the input signal is separated into an in-phase component X I and a quadrature component X Q .
- the analog quadrature modulator compensation structure comprises four real filters ⁇ g 11 , g 12 , g 21 , g 22 ⁇ and two DC offset compensation parameters c 1 , c 2 .
- the DC offsets in the AQM will be compensated by the parameters c 1 , c 2 .
- the frequency dependence of the AQM will be compensated by the filters ⁇ g 11 , g 12 , g 21 , g 22 ⁇ .
- the order of the real filters is dependent on the level of compensation required.
- the output signals Y 1 and•Y a will be presented to the AQM's in-phase and quadrature ports.
- the multi-band wideband power amplifier predistortion system of the present invention can significantly reduce the feedback ADC sampling rate requirements. This will enable multi-band wideband applications and reduce the power consumption and cost.
- the system is also reconfigurable and field-programmable since the algorithms and power efficiency enhancing features can be adjusted like software in the digital processor at anytime, as discussed in greater detail in the applications incorporated by reference and attached as an Appendix.
- the multi-band wideband DPD system is agnostic to modulation schemes such as QPSK, QAM, OFDM, etc. in CDMA, GSM, WCDMA, CDMA2000, and wireless LAN systems.
- modulation schemes such as QPSK, QAM, OFDM, etc.
- CDMA Code Division Multiple Access
- GSM Global System for Mobile communications
- WCDMA Wideband Code Division Multiple Access
- CDMA2000 Code Division Multiple Access 2000
- wireless LAN systems wireless LAN systems.
- the DPD system is capable of supporting multi-modulation schemes, multi-carriers and multi-channels.
- Other benefits of the DPD system includes correction of PA non-linearities in repeater or indoor coverage systems that do not have the necessary baseband signals information readily available.
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Abstract
A high performance and cost effective method of RF-digital hybrid mode power amplifier systems with high linearity and high efficiency for multi-frequency band wideband communication system applications is disclosed. The present disclosure enables a power amplifier system to be field reconfigurable and support multiple operating frequency bands on the same PA system over a very wide bandwidth. In addition, the present invention supports multi-modulation schemes (modulation agnostic), multi-carriers and multi-channels.
Description
- This application is a continuation of U.S. application Ser. No. 14/553,828, filed Nov. 25, 2014; which is a continuation of Ser. No. 13/894,987, filed May 15, 2013 now U.S. Pat. No. 8,903,337; which is a continuation of U.S. application Ser. No. 13/705,022, filed Dec. 4, 2012, now U.S. Pat. No. 8,467,747; which is a divisional of U.S. application Ser. No. 12/928,934, filed Dec. 21, 2010, now U.S. Pat. No. 8,351,877; which is a non-provisional of and claims the benefit of U.S. Provisional Application No. 61/288,838, filed Dec. 21, 2009. Each of these applications is hereby incorporated by reference for all purposes.
- The present invention generally relates to wireless communication systems using complex modulation techniques. More specifically, the present invention relates to power amplifier systems for wireless communications.
- A wideband mobile communication system using complex•modulation techniques, such as wideband code division access (WCDMA) and orthogonal frequency division multiplexing
- (OFDM), has large peak-to-average power ratio (PAPR) specifications and hence requires highly linear power amplifiers for its RF transmissions. Conventional digital predistortion (DPD) techniques have an operational bandwidth limitation.
- Conventional DSP-based DPD schemes utilize FPGAs, DSPs or microprocessors to compute, calculate and correct the PA's nonlinearities: they perform fast tracking and adjustments of signals in the PA system. However, conventional DSP-based DPD schemes are challenged by variations of the linearity performance of the power amplifier over wide bandwidths due to the environment changing such as temperature and the asymmetric distortions of the output signal of the PA resulting from memory effects. Conventional DPD algorithms are based on a wideband feedback signal, they require a high speed analog-to-digital converter (ADC) in order to capture the necessary information. Multi-frequency band, or simply multi-band, applications can have their operating frequencies spaced significantly apart. Conventional DPD architectures use an ADC sampling rate that is greater than twice the nonlinear distortion bandwidth of the input signal. This sampling rate is typically more than double a factor of five times the operating bandwidth of the complex modulated signal. The factor of five accounts for the spectral regrowth attributed to the nonlinear distortion created by the power amplifier. This restriction on sampling rate, limits the feasibility of the conventional predistortion architectures to single band applications. Higher sampling rate ADCs have lower resolution, consume more power and are more expensive.
- Accordingly, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a high performance and cost effective method of power amplifier systems with high linearity and high efficiency for multi-frequency band wideband communication system applications. The present disclosure enables a power amplifier system to be field reconfigurable and support multiple operating frequency bands on the same PA system over a very wide bandwidth. In addition, the present invention supports multi-modulation schemes (modulation agnostic), multi-carriers, and multi-channels.
- To achieve the above objects, according to the present invention, the technique is based on the method of adaptive digital predistortion to linearize a RF power amplifier. The present invention is based on using distinct signals from different frequencies (Multi-Band Signals). These Multi-Band Signals will experience distortion by the power amplifier and create nonlinear distortion centered on each carrier that is approximately five times their individual bandwidths. The feedback signal from the power amplifier's are down converted to an intermediate frequency (IF) that insures that the fundamental carrier bandwidths will not be aliased onto each other after sampling in the ADC. The present invention can accommodate aliasing of the nonlinear distortion of the individual carriers.
- Various embodiments of the invention are disclosed. In an embodiment, the combination of crest factor reduction (CFR), DPD, power efficiency boosting techniques as well as coefficient adaptive algorithms are utilized within a PA system. In another embodiment, analog quadrature modulator (AQM) compensation structure is also utilized to enhance performance.
- Some embodiments of the present invention are able to monitor the fluctuation of the power amplifier characteristics and to self-adjust by means of a self-adaptation algorithm. One such self-adaptation algorithm, presently disclosed is called an adaptive DPD algorithm, which is implemented in the digital domain and taught in the applications incorporated herein by reference and attached as an Appendix.
- Applications of the present invention are suitable for use with all wireless base-stations, access points, mobile equipment and wireless terminals, portable wireless devices, and other wireless communication systems such as microwave and satellite communications.
- Further objects and advantages of the present invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:
-
FIG. 1 is a block diagram showing the basic form of a digital predistortion power amplifier system. -
FIG. 2 is a block diagram showing a simple digital predistortion block diagram for a power amplifier system according to one embodiment of the present invention. -
FIG. 3 is a block diagram showing polynomial based predistorter of the present invention. -
FIG. 4 is a block diagram of the digital predistortion Direct Learning algorithm applied for self-adaptation in a digital predistortion power amplifier system of the present invention. -
FIG. 5 is a block diagram of the digital predistortion In-direct Learning algorithm applied for self-adaptation in a digital predistortion power amplifier system of the present invention. -
FIG. 6 is a depiction of the frequency domain signals present in the aliased digital predistortion system. -
FIG. 7 is an embodiment of the Quadrature Modulator compensation block architecture. -
Glossary of Terms ACLR Adjacent Channel Leakage Ratio ACPR Adjacent Channel Power Ratio !1′ ADC Analog to Digital Converter AQDM Analog Quadrature Demodulator AQM Analog Quadrature Modulator AQDMC Analog Quadrature Demodulator Corrector AQMC Analog Quadrature Modulator Corrector BPF Bandpass Filter CDMA Code Division Multiple Access CFR Crest Factor Reduction DAC Digital to Analog Converter DET Detector DHMPA Digital Hybrid Mode Power Amplifier DOC Digital Down Converter DNC Down Converter DPA Doherty Power Amplifier DPD Digital Predistortion DQDM Digital Quadrature Demodulator DQM Digital Quadrature Modulator DSP Digital Signal Processing DUC Digital Up Converter EER Envelope Elimination and Restoration EF Envelope Following ET Envelope Tracking EVM Error Vector Magnitude FFLPA Feedforward Linear Power Amplifier FIR Finite Impulse Response FPGA Field-Programmable Gate Array GSM Global System for Mobile communications 1-Q In-phase I Quadrature IF Intermediate Frequency LINC Linear Amplification using Nonlinear Components LO Local Oscillator LPF Low Pass Filter MCPA Multi-Carrier Power Amplifier MDS Multi-Directional Search OFDM Orthogonal Frequency Division Multiplexing PA Power Amplifier PAPR Peak-to-Average Power Ratio PD Predistortion PLL Phase Locked Loop QAM Quadrature Amplitude Modulation QPSK Quadrature Phase Shift Keying RF Radio Frequency SAW Surface Acoustic Wave Filter UMTS Universal Mobile Telecommunications System UPC Up Converter WCDMA Wideband Code Division Multiple Access WLAN Wireless Local Area Network - The present invention is a novel multi-band predistortion system that utilizes an adaptive digital predistortion algorithm. The present invention is a hybrid system of digital and analog modules. The interplay of the digital and analog modules of the hybrid system both linearize the spectral regrowth and enhance the power efficiency of the PA while maintaining or increasing the wide bandwidth. The present invention, therefore, achieves higher efficiency and higher linearity for wideband complex modulation carriers that operate simultaneously over multiple, distinct frequency bands.
-
FIG. 1 is a high level block diagram showing the basic system architecture which can be thought of, at least for some embodiments, as comprising digital and analog modules and a feedback path. The digital module is thedigital predistortion controller 101 which comprises the DPD algorithm, other auxiliary DSP algorithms, and related digital circuitries. The analog module is themain power amplifier 102, other auxiliary analog circuitries such as DPA, and related peripheral analog circuitries of the overall system, as further discussed in the applications incorporated by reference. The present invention is a “black box”, plug-and-play type system because it accepts RF modulatedsignal 100 as its input, and provides a substantially identical but amplifiedRF signal 103 as its output, therefore, it is RF-in/RF-out. Baseband input signals can be applied directly to the Digital Predistorter Controller according to one embodiment of the present invention. An Optical input signal can be applied directly to the Digital Predistorter Controller according to one embodiment of the present invention. The feedback path essentially provides a representation of the output signal to thepredistortion controller 101. - In either input mode, the memory effects due to self-heating, bias networks, and frequency dependencies of the active device are compensated by the
adaptation algorithm 204 in theDPD 201,FIG. 2 . InFIG. 2 , the multi-band RF input x[n] is provided to aDPD 201 and toDPD Algorithm logic 204. The output of theDPD 201, z[n] is provided to aDAC 202 and thelogic 204. The output of theDAC 202 provides the input to thepower amplifier 203. The distortion characteristic of the PA is sensed by the feedback samples y(t), which are converted in theADC 206 into data ya[n] and provided toalignment logic 205. After alignment the data ya[n] provides feedback data to thealgorithm logic 204. - The coefficients of the DPD are adapted by synchronizing the wideband captured, aliased output Multi-Band Signal ya[n] from the feedback path (Sampled Feedback Aliased Signal) with the reference Multi-Band Signal x[n] (Input Signal). The DPD algorithm performs the synchronization and compensation. The synchronization aligns the reference signal with the aliased feedback signal in the alignment block. In one embodiment of the DPD algorithm, the reference signal and the aliased Sampled Feedback Aliased Signal ya[n] are used in the Direct Learning adaptive algorithm. In another embodiment of the DPD algorithm, the aliased predistorted signal za[n] (Predistorted Output Aliased Signal) and the Sampled Feedback Aliased Signal ya[n] are used in an Indirect Learning adaptive algorithm.
- Some embodiments apply crest factor reduction (CFR) prior to the DPD with an adaptation algorithm in one digital processor, so as to reduce the PAPR, EVM and ACPR and compensate the memory effects and variation of the linearity due to the temperature changing of the PA. The digital processor can take nearly any form for convenience, an FPGA implementation is typically used, but a general purpose processor is also acceptable in many embodiments. The CFR implemented in the digital module of the embodiments is based on the scaled iterative pulse cancellation presented in patent application U.S. 61/041,164, filed Mar. 31, 2008, entitled An Efficient Peak Cancellation Method For Reducing The Peak-To-Average Power Ratio In Wideband Communication Systems, incorporated herein by reference. The CFR is included to enhance performance and hence optional. The CFR can be removed from the embodiments without affecting the overall functionality.
- In all embodiments, the memory effects due to self-heating, bias networks, and frequency dependencies of the active device are compensated by the adaptation algorithm in the DPD. The coefficients of the DPD are adapted by synchronizing the wideband captured output signal from the feedback path with the reference signal. The digital predistortion algorithm performs the synchronization and compensation. The predistorted signal is passed through a DQM in order to generate the real signal and then converted to an IF analog signal via a DAC. The DQM is not required to be implemented in the FPGA, or at all, in all embodiments. If the DQM is not used in the FPGA, then the AQM Implementation can be implemented with two DACs to generate real and imaginary signals, respectively.
-
FIG. 3 is a block diagram showing a predistortion (PD) part in the DPD system of the present invention. The PD in the present invention generally utilizes an adaptive polynomial-based digital predistortion system. Another embodiment of the PD utilizes a LUT-based digital predistortion system. More specifically, the PD illustrated inFIG. 3 are processed in the digital processor by an adaptive algorithm, presented in U.S. patent application Ser. No. 11/961,969, entitled A Method for Baseband•Predistortion Linearization in Multi-Channel Wideband Communication Systems. The PD for the PD system inFIG. 3 has multiple finite impulse response (FIR) filters, that is,FIR1 301,FIR2 303,FIR3 305, andFIR4 307. The PD also contains the third orderproduct generation block 302, the fifth orderproduct generation block 304, and the seventh orderproduct generation block 306. The output signals from FIR filters are combined in thesummation block 308. Coefficients for multiple FIR filters are updated by the digital predistortion algorithm. - Digital Predistortion (DPD) is a technique to linearize a power amplifier (PA).
FIG. 2 shows the block diagram of a digitally predistorted PA system. In the DPD block, a memory polynomial model is used as the predistortion function (FIG. 3 ). -
- where aij; are the DPD coefficients.
- In the DPD estimator block, a least square algorithm is utilized to find the DPD coefficients aij, and then transfer them to DPD block. The detailed DPD algorithm is shown in
FIG. 4 andFIG. 5 . The coefficients are obtained using the QR RLS adaptive algorithm in the DPD estimator block. -
FIG. 4 shows one embodiment of the multi-band digital predistorter. The Direct Learning adaptive algorithm has two inputs into the DPD estimator. The DPD estimator uses the aliased sampled Multi-Band Signal xa[n] (Sampled Input Aliased Signal) as a reference and the Sampled Feedback Aliased Signal ya[n] as an input. Thus the x(n) signal is provided toDPD 400 and also frequency translation/aliasing logic 420. The DPD outputs signal z(n), while thealiasing logic 420 outputs•xa(n) and provides it to integerdelay logic 402 which then supplies a signal tofractional delay logic 402 andmux 414, which also receives the output of thelogic 402. The mux also receives a control signal fromdelay estimator 406, and provides an output to block xa′ 404, which determines xa′ (n−m). The mux output is also supplied todata buffer 405, which provides its output toDPD estimator 412, and control signals to phase shift block 41 andgain correction block 411. The feedback signal y(t) is provided toADC 421, also with sampling frequency Fs, which is selected to create appropriate Nyquist zones for the aliased signals. The ADC output is provided tofeedback data buffer 408, as is ya(n) data fromblock 407. Thedata buffer 408 provides data to delayestimator 406 as well as providing the data to phaseshifter 410 and gaincorrection block 411. The gain correction is supplied to the DPD estimator, which, together with the predistortion coefficients already in memory, shown at 401, are muxed back toDPD 400. -
FIG. 5 shows another embodiment of the multi-band digital predistorter. The in-direct Learning adaptive algorithm ofFIG. 5 has two inputs into the DPD estimator. The DPD estimator uses the Predistorted Output Aliased Signal za[n] as a reference and the Sampled Feedback Aliased Signal ya[n] as an input, but is otherwise very similar toFIG. 4 and therefore is not explained further. - A depiction of the spectrum domain plots are shown in
FIG. 6 . The reference Input Signal x[n] shows two distinct bands centered at frequencies Fa and Fb. The operating bandwidth of the individual carriers is small in comparison to the frequency spacing between the carriers. The nonlinearities in the power amplifier will cause spectral re-growth as shown in the analog feedback Multi-Band Signal y(t) (Analog Feedback Signal). The Analog Feedback Signal is down converted to an intermediate frequency Fi which is selected to be in the 1st Nyquist zone as shown inFIG. 6 . The choice of frequency Fi is dependent on the ADC sampling rate used, or Fs, the operating bandwidth of the carriers and the frequency separation (Fa−Fb) between the carriers. The ADC sampling rate Fs is typically the limiting factor in the choice of Fi. For a two carrier system Fa is positioned in the 1st Nyquist zone and Fb is positioned in the 2nd Nyquist zone in at least some embodiments, although either signal Fa or Fb could be positioned in the third, fourth or nth Nyquist zones, depending upon the particular implementation. The primary requirement is simply that the two signal are located in different Nyquist zones. The choice of Fi limits how close Fa and Fb can be separated together. - Sampling on the Analog Feedback Signal y(t) generates images as shown in the spectrum for the Sampled Feedback Aliased Signal ya[n]. The nonlinear distortion from the individual carriers are allowed to alias onto each other as long as the aliased part of the signal does not adversely impact the original Multi-Band signal. A Direct Learning algorithm uses the difference between xa[n] and ya[n] to minimize the resultant error signal. The QR RLS algorithm uses this error to adapt the predistorter coefficients in the DPD estimator. The In-Direct learning algorithm first models the power amplifier using the Predistorted Output Aliased Signal za[n] and the Sampled Feedback Aliased Signal ya[n]. The modelled power amplifier coefficients are then used to calculate the predistorter coefficients.
-
FIG. 7 . is a block diagram showing the analog quadrature modulator compensation structure. The analog quadrature modulator will translate the baseband signal output from the DACs to an RF frequency. The input signal is separated into an in-phase component XI and a quadrature component XQ. The analog quadrature modulator compensation structure comprises four real filters {g11, g12 , g21, g22} and two DC offset compensation parameters c1, c2. The DC offsets in the AQM will be compensated by the parameters c1, c2. The frequency dependence of the AQM will be compensated by the filters {g11, g12, g21, g22}. The order of the real filters is dependent on the level of compensation required. The output signals Y1 and•Ya will be presented to the AQM's in-phase and quadrature ports. - In summary, the multi-band wideband power amplifier predistortion system of the present invention can significantly reduce the feedback ADC sampling rate requirements. This will enable multi-band wideband applications and reduce the power consumption and cost. The system is also reconfigurable and field-programmable since the algorithms and power efficiency enhancing features can be adjusted like software in the digital processor at anytime, as discussed in greater detail in the applications incorporated by reference and attached as an Appendix.
- Moreover, the multi-band wideband DPD system is agnostic to modulation schemes such as QPSK, QAM, OFDM, etc. in CDMA, GSM, WCDMA, CDMA2000, and wireless LAN systems. This means that the DPD system is capable of supporting multi-modulation schemes, multi-carriers and multi-channels. Other benefits of the DPD system includes correction of PA non-linearities in repeater or indoor coverage systems that do not have the necessary baseband signals information readily available.
- Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.
Claims (1)
1. A digital predistortion system comprising:
a multi-band input signal;
at least one power amplifier for providing an amplified output including a distortion characteristic;
a feedback signal derived from the amplified output, the feedback signal including a representation of at least a portion of the distortion characteristic; and
predistortion logic responsive to an aliased representation of the feedback signal for generating predistortion coefficients for linearizing the amplified output of the power amplifier, wherein a sampling rate of the aliased representation of the feedback signal is less than twice a maximum bandwidth of the feedback signal.
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Also Published As
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US20120154038A1 (en) | 2012-06-21 |
US8351877B2 (en) | 2013-01-08 |
US20150171899A1 (en) | 2015-06-18 |
US20130251066A1 (en) | 2013-09-26 |
US20130094612A1 (en) | 2013-04-18 |
US8467747B2 (en) | 2013-06-18 |
US9379745B2 (en) | 2016-06-28 |
US8903337B2 (en) | 2014-12-02 |
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