(12) United States Patent
`PerSSOn
`
`USOO6246286B1
`(10) Patent No.:
`US 6,246,286 B1
`(45) Date of Patent:
`Jun. 12, 2001
`
`(54) ADAPTIVE LINEARIZATION OF POWER
`AMPLIFIERS
`(75) Inventor: Jonas Persson, Lund (SE)
`(73) Assignee: Telefonaktiebolaget LM Ericsson,
`Stockholm (SE)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(*) Notice:
`
`(21) Appl. No.: 09/427,773
`(22) Filed:
`Oct. 26, 1999
`
`7
`(51) Int. Cl.' ........................................................ HO3F 1/26
`(52) U.S. Cl. .......................... 330/149; 330/136; 375/296;
`455/126
`
`(56)
`
`(58) Field of Search ..................................... 330/149, 136;
`375/296, 297; 455/126, 127
`s
`s
`s
`References Cited
`U.S. PATENT DOCUMENTS
`11/1984 Siegel et al. ............................. 330/3
`4,485,349
`4,999,583 * 3/1991 Washburn et al...................... 330/86
`5,049,832 * 9/1991 Cavers ..............
`... 330/149
`5,381,108 * 1/1995 Whitmarsh et al. ..................... 330/2
`5,732,334 * 3/1998 Miyake .............
`455/126
`5,748,037
`5/1998 Rozental et al. ...
`... 330/2
`5,748,038
`5/1998 Boscovic et al. ...
`... 330/2
`5,760,646 * 6/1998 Belcher et al.
`... 330/149
`5,850,162 * 12/1998 Danielsons .....
`... 330/149
`5,870,668 * 2/1999 Takano et al. .
`... 455/126
`5,892,397
`4/1999 Belcher et al. ...................... 330/149
`5,898.338 * 4/1999 Proctor et al. ....................... 330/149
`5,903,611
`5/1999 Schnablet al...
`... 375/297
`6,081.698
`72000 Moriyama et al.
`... 455/126
`6,091,941 * 7/2000 Moriyama et al. .................. 455/126
`FOREIGN PATENT DOCUMENTS
`O 513 402 A1 11/1992 (EP).
`O 638 994 A1
`2/1995 (EP).
`O 658975. A
`6/1995 (EP).
`
`OTHER PUBLICATIONS
`T. Matsuoka, M. Orihashi, M. Sagawa, H. Ikeda, and K.
`Misaizu, “Compensation of Nonlinear Distortion During
`Transmission Based on the Adaptive Predistortion Method.”
`IEICE Trans. Electron., vol. E80-C, No. 6, Jun., 1997, pp.
`782-787.
`T. Rahkonen and T. Kankaala, “An Analog Predistortion
`Integrated Circuit for Linearizing Power Amplifiers”, 1998
`Midwest Symposium on Systems and Circuits, South Bend,
`Indiana, Aug. 9-12, 1998, pp. 1-4.
`W. H. Pierce, P. Aronhime, and J. Deng, “A Simple Predis
`tortion Algorithm and Limits of Predistortion”, 1998 Mid
`west Symposium on Systems and Circuits, South Bend,
`Indiana, Aug. 9-12, 1998, pp. 1-4.
`R. S. Narayanaswami, The Design Of A 1.9GHz 250mW
`CMOS Power Amplifier For DECT, http://kabuki.eecs.ber
`lesslyTses Ms.master.full.html; printed May
`1999, pp. 14.
`M M -
`S. Andreoli, H. McClure, P. Banelli, S. Cacopardi, “Linear
`"T" Digital's After , IEEE 46th Erics R
`posium, http://www.itelco-usa.com/ieee/Intro.htm; printe
`May 10, 1999, pp. 1-12.
`Standard Search Report for RS 104147US Completed Apr.
`4, 2000.
`* cited by examiner
`Primary Examiner Robert Pascal
`Assistant Examiner-Henry Choe
`(74) Attorney, Agent, or Firm Jenkens & Gilchrist, PC
`(57)
`ABSTRACT
`A method and apparatus adaptively compensates for non
`linearities of a power amplifier by measuring a distortion
`characteristic acroSS the power amplifier during amplifica
`tion of a distortion detection signal. The distortion detection
`Signal has a well-defined input power versus time
`relationship, Such as ramp-up signal or ramp-down signal.
`Due to this well-defined relationship, the distortion charac
`p.
`teristic can be calculated as a function of the input power
`level. This calculated function is then utilized to update a
`predistortion lookup table.
`
`33 Claims, 10 Drawing Sheets
`
`140
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`t
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`Calculations
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`(information)
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`cite
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`ellors
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`Y-99
`
`L. - - - - - - -
`
`PETITIONERS EXHIBIT 1025
`Page 1 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 1 of 10
`
`US 6,246,286 B1
`
`30
`
`160 (O)
`
`200
`
`fo-RF 170
`I/Q
`
`O 19
`
`Goin=G
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`120.
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`OCUOte
`WFC
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`Relationship
`250
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`Predistortion
`Colculations
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`0.
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`
`230
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`(information)
`
`- - - - - - - -
`
`PETITIONERS EXHIBIT 1025
`Page 2 of 19
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`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 2 of 10
`
`US 6,246,286 B1
`
`50
`
`60
`
`Input
`
`Output
`
`Predistortion
`
`Non-lineor
`element
`
`70
`
`Input
`
`Output
`
`Linear input-output
`relationship
`
`FIG. 3
`
`PETITIONERS EXHIBIT 1025
`Page 3 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 3 of 10
`
`US 6,246,286 B1
`
`Pin
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`500
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`period
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`610
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`
`b)
`
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`period
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`Time
`
`FIG. 5
`
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`
`- Y
`
`Pin
`
`Ox Pn
`
`O t
`
`c)
`
`Pin
`
`mox P
`
`PETITIONERS EXHIBIT 1025
`Page 4 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 4 of 10
`
`US 6,246,286 B1
`
`200
`
`130
`
`140
`
`DAC -- PF
`
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`Ad VS. P.
`250
`
`240
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`Predistortion
`Colculations
`------- - - -
`100
`Doto
`(Information)
`
`- - - - - - - -
`
`FIG. 6
`
`PETITIONERS EXHIBIT 1025
`Page 5 of 19
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`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 5 of 10
`
`US 6,246,286 B1
`
`130
`
`140
`
`
`
`LPF
`
`130J
`
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`modulotor
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`FIC. 7
`
`PETITIONERS EXHIBIT 1025
`Page 6 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 6 of 10
`
`US 6,246,286 B1
`
`130
`
`140
`
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`
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`Predistortion
`Colculotions
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`PETITIONERS EXHIBIT 1025
`Page 7 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 7 of 10
`
`US 6,246,286 B1
`
`130
`
`140
`DAC
`
`PF
`
`130
`
`40-J Q
`
`160 (0)
`f, a =f
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`modulator
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`
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`Detector
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`Colculations
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`Out'
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`(Information)
`
`- - - - - - - -
`
`PETITIONERS EXHIBIT 1025
`Page 8 of 19
`
`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 8 of 10
`
`US 6,246,286 B1
`
`130
`
`140
`
`150
`
`190
`
`200
`
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`Out'
`in
`250
`
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`101’ 100
`
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`Information) -------
`
`PETITIONERS EXHIBIT 1025
`Page 9 of 19
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`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 9 of 10
`
`US 6,246,286 B1
`
`130
`
`140
`
`
`
`DAC
`
`160 (O)
`f, a =f O'RF
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`(information) L------
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`99
`
`PETITIONERS EXHIBIT 1025
`Page 10 of 19
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`

`

`U.S. Patent
`
`Jun. 12, 2001
`
`Sheet 10 of 10
`
`US 6,246,286 B1
`
`
`
`Generote distortion
`detection signal
`
`Apply distortion detection
`signol to power amplifier
`
`Repeat each
`communicotion
`burst
`
`Measure distortion characteristic(s)
`ocross power omplifier
`
`Calculate distortion characteristic(s)
`versus input power relationship
`
`Update predistortion
`lookup toble
`
`FIC. 12
`
`1010
`
`1020
`
`1030
`
`1040
`
`PETITIONERS EXHIBIT 1025
`Page 11 of 19
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`

`

`1
`ADAPTIVE LINEARIZATION OF POWER
`AMPLIFIERS
`
`US 6,246.286 B1
`
`2
`variables as component aging, component variation, channel
`Switching, power Supply variation, component drift, tem
`perature fluctuations, and the input signal itself. Existing
`approaches, Such as continuous feedback, feedforward net
`WorkS and conventional predistortion techniques, have
`attempted to compensate for these nonlinear characteristics
`by utilizing Some form of continuous feedback loop or fixed
`pre-processing or post-processing network. These
`approaches, however, either fail to adaptively compensate
`for time-varying fluctuations in nonlinear characteristics or
`prove difficult to implement at RF frequencies. For example,
`continuous feedback approaches, Such as negative feedback
`or Cartesian feedback, typically require a high loop band
`width and could cause Stability problems when operated at
`high frequencies. Feed forward networks, on the other hand,
`cannot adaptively compensate for variations in distortion
`characteristics due to the fixed nature of the feedforward
`network, and require precise matching and Scaling of com
`ponents in order to avoid inadvertently introducing addi
`tional nonlinear distortion. Conventional predistortion tech
`niques similarly fail to adaptively compensate for variations
`in nonlinear characteristics due to the use of a fixed set of
`predistortion coefficients.
`One existing approach that adaptively compensates for
`variations in nonlinear distortion is an approach known as
`adaptive predistortion. In contrast to the conventional pre
`distortion technique mentioned earlier, traditional adaptive
`predistortion periodically Senses the output of the power
`amplifier and updates the predistortion coefficients for time
`varying nonlinearities in the forward path. These updated
`predistortion coefficients are then used to predistort the input
`Signal in Such a manner that a linear amplified signal is
`produced at the output of the power amplifier.
`Although traditional adaptive predistortion provides
`adequate linearization of a power amplifier, the traditional
`adaptive predistortion technique places significant proceSS
`ing demands on the digital Signal processor used to imple
`ment this technique. Typically, the look-up table that Stores
`the predistortion coefficients must be updated Several times
`per Symbol (e.g., five times per Symbol) depending on the
`OverSampling rate. Moreover, a typical "burst' in a Time
`Division Multiple Access (TDMA) system may include as
`many as 100-200 symbols. As a result, this example would
`require the digital Signal processor to update the lookup
`table 500-1000 times per burst. This places a significant
`burden on the precessing requirements (and corresponding
`cost) of the digital signal processor and increases current
`consumption.
`A further disadvantage of the traditional adaptive predis
`tortion technique is that it requires a quadrature demodulator
`in the feedback loop. This quadrature demodulator is
`required in order to enable the digital Signal processor to
`compare the data Stream detected at the output of the power
`amplifier with the input data Stream. In addition to the
`increased costs and current consumption, the quadrature
`demodulator can also introduce errors which will be
`reflected in the updated predistortion coefficients and will
`adversely affect the ability to compensate for nonlinear
`distortion in the power amplifier. Therefore, in view of the
`deficiencies of existing approaches, there is a need for an
`adaptive linearization technique that can effectively com
`pensate for time-varying nonlinearities of the power ampli
`fier and at the same time relax the processing requirements
`of the digital Signal processor and decrease current con
`Sumption.
`
`BACKGROUND OF THE INVENTION
`1. Technical Field of the Invention
`The present invention relates in general to the field of
`communication Systems, and in particular, to adaptive lin
`earization of power amplifiers in Such communication SyS
`temS.
`2. Description of Related Art
`In order to keep pace with the ever increasing demand for
`higher capacity wireleSS and personal communication
`Services, modern digital communication Systems have
`become increasingly reliant upon spectrally efficient linear
`modulation Schemes, Such as Quadrature Phase Shift Keying
`(QPSK), Quadrature Amplitude Modulation (QAM), and
`recently 371/8-8PSK used in the Enhanced Data Rates for
`GSM Evolution (EDGE) system. Unlike conventional digi
`tal modulation techniques which utilize a constant envelope,
`linear modulation Schemes exploit the fact that digital base
`band data may be modulated by varying both the envelope
`(e.g., amplitude) and phase of an RF carrier. Because the
`envelope and phase offer two degrees of freedom, digital
`baseband data may be mapped into four more possible RF
`carrier Signals, enabling the transmission of more informa
`tion within the same channel bandwidth than if just the
`envelope or phase were varied alone. As a result, linear
`modulation Schemes provide Significant gains in Spectrum
`utilization, and have become an attractive alternative to
`conventional digital modulation techniques.
`The variation of both the envelope and phase of the RF
`carrier, however, causes linear modulation Schemes to be
`highly Susceptible to the inherent nonlinear distortion asso
`ciated with power amplifiers. Although conventional digital
`modulation techniques are leSS Susceptible to Such distortion
`due the use of a constant envelope, the non-constant enve
`lope utilized by linear modulation Schemes causes the gain
`and phase-shift of the power amplifier to vary as a function
`of the input signal. This non-constant gain and phase-shift,
`in turn, causes two types of nonlinear distortion. The first
`type of nonlinear distortion, known as AM/AM distortion,
`occurs when the input power and the output power depart
`from a linear relationship. The Second type, known as
`AM/PM distortion, occurs when the phase-shift of the power
`amplifier varies as a function of the input power level.
`If the power amplifier used to amplify linearly modulated
`Signals fails to compensate for both types of nonlinear
`distortion, the power amplifier will generate unwanted inter
`modulation products and cause an accompanying degrada
`tion in the quality of the communications. When intermodu
`lation products occur outside the channel bandwidth, for
`example, an effect known as Spectral regrowth or widening
`causes increased interference with communications in adja
`cent channels. Furthermore, intermodulation products
`occurring within the channel bandwidth may distort the
`modulated Signal to Such an extent that it cannot be properly
`reconstructed or detected at the receiver, resulting in
`increased bit error rates. Therefore, in order to prevent
`unwanted intermodulation products and avoid the accom
`panying degradation in the quality of communications,
`linear modulation Schemes require a linear power amplifier
`with a constant gain and phase-shift for all operating power
`levels.
`Unfortunately, because power amplifiers are inherently
`nonlinear devices, the gain and phase-shift of power ampli
`fiers vary in a complex, nonlinear manner depending on Such
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`SUMMARY OF THE INVENTION
`The deficiencies of the prior art are overcome by the
`method and apparatus of the present invention. For example,
`
`PETITIONERS EXHIBIT 1025
`Page 12 of 19
`
`

`

`3
`as heretofore unrecognized, it would be beneficial to mea
`Sure distortion characteristic(s) across a power amplifier
`during amplification of a distortion detection signal. This
`distortion detection Signal comprises, for example, a ramp
`up signal or ramp-down signal. Preferably, however, the
`distortion detection Signal comprises a burst up-ramp or
`burst down-ramp Signal commonly used before or after a
`communication burst in a Time Division Multiple Access
`(TDMA) system. Using a burst up-ramp or burst down-ramp
`Signal offers the added advantage of allowing the principles
`of the present invention to be easily incorporated into
`existing TDMA communication Systems.
`Based on the measured distortion characteristic(s) and
`known characteristics of the distortion detection signal, a
`relationship between the measured distortion characteristic
`(S) and input power is calculated. A predistortion lookup
`table is updated in accordance with this calculated
`relationship, and may then be applied to an input data Stream
`to produce a linear amplified output when the predistorted
`input is amplified by the power amplifier.
`In a first embodiment of the present invention, phase
`distortion acroSS the power amplifier is measured during
`amplification of the distortion detection Signal. This mea
`Surement may be performed, for example, by comparing the
`25
`phase of the input and the phase of the output over the
`operating power range of the distortion detection Signal. A
`relationship between the measured phase distortion and
`input power is calculated based on the measured phase
`distortion and known characteristics of the distortion detec
`tion signal. A predistortion lookup table is updated in
`accordance with this calculated relationship, and may then
`be applied to an input data Stream to adaptively compensate
`for non-constant phaseshift in the power amplifier.
`In a second embodiment, the envelope (amplitude) dis
`35
`tortion acroSS the power amplifier is measured during the
`amplification of the distortion detection Signal. This mea
`Surement may be performed, for example, by comparing the
`amplitude of the input and the amplitude of the output over
`the operating power range of the distortion detection Signal.
`A relationship between the measured envelope distortion
`and input power is calculated based on the measured enve
`lope distortion and known characteristics of the distortion
`detection Signal. A predistortion lookup table is updated in
`accordance with this calculated relationship, and may then
`be applied to an input data Stream to adaptively compensate
`for nonlinear gain in the power amplifier.
`In a third embodiment, both envelope (amplitude) and
`phase distortion are measured during amplification of the
`distortion detection signal. Based on the measured envelope
`and phase distortion and known characteristics of the dis
`tortion detection Signal, relationships between the input
`power and the measured envelope and phase distortion are
`calculated. A predistortion lookup table is updated in accor
`dance with these calculated relationships, and may then be
`applied to an input data Stream to adaptively compensate for
`both nonlinear gain and non-constant phase-shift in the
`power amplifier.
`In one aspect of the present invention, the predistortion
`lookup table is updated only once per communication burst
`in order to relax the processing demands on and power
`consumption of a digital signal processor. In another aspect,
`detectors used to measure the distortion characteristics are
`configured in pairs with Similar input Signal levels in order
`to reduce the impact of non-ideal detection components. In
`yet another aspect, mixers are utilized to down-convert the
`input and output Signals of the power amplifier from RF
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`frequencies to an intermediate frequency to allow the detec
`tion components to operate at a lower frequency.
`The technical advantages of the present invention include,
`but are not limited to the following. It should be understood
`that particular embodiments may not involve any, much less
`all, of the following exemplary technical advantages.
`An important technical advantage of the present invention
`is the ability to adaptively compensate for time-varying
`nonlinearities of a power amplifier.
`Another important technical advantage of the present
`invention is that it improves the power efficiency of the
`power amplifier because the linearity requirements on the
`power amplifier itself are relaxed.
`Yet another important technical advantage of the present
`invention is that it enables the phase and gain distortion
`characteristics of the power amplifier to be measured only
`for those input power levels that will be used when the
`modulated Signal is amplified.
`Yet another important technical advantage of the present
`invention is that it relaxes the processing requirements of the
`digital Signal processor by requiring the predistortion lookup
`table to be updated only once per communication burst.
`Yet another important technical advantage of the present
`invention is that is reduces current consumption of the
`digital Signal processor Since it is only used for a Small
`fraction of the communication burst.
`Yet Still another important technical advantage of the
`present invention is that the “balanced’ configuration of the
`distortion detectorS reduces the impact of non-ideal detec
`tion components.
`The above-described and other features of the present
`invention are explained in detail hereinafter with reference
`to the illustrative examples shown in the accompanying
`drawings. Those skilled in the art will appreciate that the
`described embodiments are provided for purposes of illus
`tration and understanding and that numerous equivalent
`embodiments are contemplated herein.
`BRIEF DESCRIPTION OF THE DRAWINGS
`A more complete understanding of the method and appa
`ratus of the present invention may be had by reference to the
`following detailed description when taken in conjunction
`with the accompanying drawings wherein:
`FIG. 1 illustrates a portion of an exemplary wireless
`System with which the present invention may be advanta
`geously practiced;
`FIG. 2 illustrates an exemplary Schematic block diagram
`of a transmitter in accordance with the present invention;
`FIG. 3 illustrates operating principles of an exemplary
`predistortion technique practiced by one aspect of the
`present invention;
`FIGS. 4(a)-4(c) illustrate an exemplary conversion of a
`measured phase distortion characteristic to a function of the
`input power level in accordance with the present invention;
`FIGS. 5(a)-5(c) illustrate an exemplary conversion of a
`measured amplitude (envelope) distortion characteristic to a
`function of the input power level in accordance with the
`present invention;
`FIG. 6 illustrates an exemplary Schematic block diagram
`of a transmitter in accordance with a first embodiment of the
`present invention;
`FIG. 7 illustrates an exemplary Schematic block diagram
`of the transmitter in accordance with the first embodiment of
`the present invention implementing mixers to down-convert
`the RF input and output Signals to an intermediate fre
`quency,
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`FIG. 8 illustrates an exemplary Schematic block diagram
`of a transmitter in accordance with a Second embodiment of
`the present invention;
`FIG. 9 illustrates an exemplary schematic block diagram
`of the transmitter in accordance the Second embodiment of
`the present invention implementing mixers to down-convert
`the RF input and output Signals to an intermediate fre
`quency,
`FIG. 10 illustrates an exemplary schematic block diagram
`of a transmitter in accordance with a third embodiment of
`the present invention;
`FIG. 11 illustrates an exemplary Schematic block diagram
`of the transmitter in accordance with the third embodiment
`of the present invention implementing mixers to down
`convert the RF input and output Signals to an intermediate
`frequency; and
`FIG. 12 illustrates an exemplary method in flow chart
`form by which an embodiment the present invention may be
`advantageously practiced.
`DETAILED DESCRIPTION OF THE DRAWINGS
`In the following description, for purposes of explanation
`and not limitation, Specific details are Set forth, Such as
`particular circuits, logic modules (implemented in, for
`example, Software, hardware, firmware, Some combination
`thereof, etc.), techniques, etc. in order to provide a thorough
`understanding of the invention. However, it will be apparent
`to one of ordinary skill in the art that the present invention
`may be practiced in other embodiments that depart from
`these specific details. In other instances, detailed descrip
`tions of well-known methods, devices, logical code (e.g.,
`hardware, Software, firmware, etc.), etc. are omitted So as
`not to obscure the description of the present invention with
`unnecessary detail.
`A preferred embodiment of the present invention and its
`advantages are best understood by referring to FIGS. 1-12
`of the drawings, like numerals being used for like and
`corresponding parts of the various drawings. Referring to
`FIG. 1, a portion of an exemplary wireleSS System with
`which the present invention may be advantageously prac
`ticed is depicted generally at 1. In this exemplary wireleSS
`System, a mobile Station 10 communicates with a base
`station 30 over an air interface 40. A data terminal 20, Such
`as a personal computer, may also communicate with the base
`station 30 over the same air interface 40 using, for example,
`a cellular modem. Because the base station 30 is a part of a
`cellular network (not shown), the base station 30 enables the
`mobile station 10 and data terminal 20 to communicate with
`one another and with other terminals within the telecom
`munication System.
`In order for the mobile station 10, base station 30, and the
`data terminal 20 to communicate digital information,
`however, transmitters associated with each device must
`modulate the digital information utilizing Some form of
`digital modulation technique. The digital modulation tech
`nique employed may include conventional techniques, Such
`as Phase Shift Keying (PSK) or Amplitude Modulation
`(AM), or more spectrally efficient techniques, Such as
`Quadrature Phase Shift Keying (QPSK), Quadrature Ampli
`tude Modulation (QAM), and recently 3L/8-8PSK used in
`the Enhanced Data Rates for GSM Evolution (EDGE)
`System. Each of these techniques imposes certain require
`ments on the power amplifier within the transmitter in order
`to prevent distortion of the modulated Signal. Depending on
`the type of modulation technique employed, these require
`ments may include a constant gain, a constant phase-shift or
`both a constant gain and a constant phase shift.
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`The flexible approach offered by the various embodiments
`of the present invention enables each of these requirements
`to be either collectively or independently Satisfied in an
`efficient and cost effective manner. Referring to FIG. 2, an
`exemplary Schematic block diagram of a transmitter in
`accordance with the present invention is illustrated. It should
`be emphasized that although the transmitter illustrated in
`FIG. 2 utilizes a quadrature modulator, the present invention
`is not limited to quadrature modulation techniques. Rather,
`the principles of the present invention are equally applicable
`to other digital modulation techniques where nonlinear
`distortion is a concern, Such as PSK and AM. Therefore, the
`following discussion is provided by way of explanation, and
`not limitation.
`Beginning with the forward transmission path of the
`transmitter depicted generally at 99, digital information 100
`is applied to a predistortion block 110. This predistortion
`block 110 contains predistortion calculations and predistor
`tion coefficients which are used to predistort the digital
`information 100 in such a manner that a linear amplified
`signal is produced at the output of the power amplifier 190.
`The predistortion coefficients are Stored in, for example, a
`predistortion lookup table 111 within the predistortion block
`110 and are periodically updated to compensate for time
`varying nonlinearities of the power amplifier 190. The
`manner by which the predistortion coefficients are periodi
`cally updated will be described in further detail below.
`Referring for the moment to FIG. 3, operating principles
`of an exemplary predistortion technique practiced by one
`aspect of the present invention is illustrated. AS shown, the
`forward transmission path of a transmitter 99 includes a
`non-linear element 60, such as a power amplifier 190, and a
`predistortion element, such as a predistortion block 110. If
`the input signal is perfectly predistorted at the predistortion
`element 50 by the inverse non-linear characteristic(s) of the
`non-linear element 60, then a linear input-output relation
`ship results as depicted at 70. Thus, one aspect of the present
`invention measures the non-linear distortion character
`istic(s) of the power amplifier 190 and predistorts the digital
`information 100 at the predistortion block 110 by the inverse
`distortion characteristic(s) to produce a linear input-output
`relationship.
`Continuing with the forward path of the transmitter 99
`depicted in FIG. 2, the output of the predistortion block 110
`is applied to a wave form generator (WFG) 120 which
`generates separate in-phase (I) and quadrature (Q) signals.
`Each digital I and Q Signal is then passed through a
`digital-to-analog converter (DAC) 130 and a low pass filter
`(LPF) 140 to convert the digital I and Q signals to analog
`Signals. The analog I and Q Signals are then combined in an
`linear modulator, Such as an I/O modulator 150, and
`up-converted to RF frequencies via a local oscillator 160.
`The output of the I/O modulator 150 is then amplified by a
`variable gain amplifier (VGA) 170 and filtered by a band
`pass filter (BPF) 180 in order to attenuate out-of-band
`power. In the final Stage, the output of the band pass filter
`180 is amplified by a power amplifier (PA) 190 and trans
`mitted via antenna 200.
`In the feedback path, an embodiment of the present
`invention measures distortion characteristic(s) across the
`power amplifier 190, such as a nonlinear phase-shift or
`non-linear gain, by monitoring the input and output of the
`power amplifier 190 during the amplification of a distortion
`detection signal. This distortion detection Signal preferably
`has a well defined time-variant relationship over the oper
`ating power range of the power amplifier 190, enabling the
`measured distortion characteristics to be easily converted to
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`a function of the input power level. Examples of a preferred
`distortion detection Signal include an up-ramp signal or
`down-ramp signal. Preferably, however, the distortion detec
`tion Signal comprises the burst ramp-up signal or burst
`ramp-down signal commonly used in TDMA systems before
`or after transmission of a communication burst. Utilizing the
`burst ramp-up or burst ramp-down Signals has the added
`advantage of allowing the preferred embodiment of the
`present invention to be easily implemented within existing
`TDMA systems.
`Before describing the feedback path illustrated in FIG. 2,
`an exemplary proceSS by which the preferred embodiment of
`the present invention converts the measured distortion char
`acteristic to a function of the input power will be described.
`Referring to FIG. 4, an exemplary conversion of a measured
`phase distortion characteristic to a function of the input
`power level in accordance with the present invention is
`illustrated. In this example, the distortion detection Signal
`applied to the power amplifier 190 comprises an up-ramp
`Signal which has an input power versus time relationship as
`depicted generally at 500. If the phase difference (Acp) across
`the power amplifier 190 is measured during the same time
`period (T1-T2) that the distortion detection signal is
`applied, the phase difference verSuS time relationship may
`have a measured phase distortion characteristic as depicted
`generally at 510. From the two relationships 500 and 510,
`the relationship between the phase difference and the input
`power can be cal