US20030231057A1 - Digitally controllable nonlinear pre-equalizer - Google Patents

Abstract

A digitally controllable nonlinear pre-equalizer system receiving an input signal and generating an output signal, the system including a splitter for dividing the input signal into a first signal path and a second signal path, an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal, a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal, and a summer for summing the linear signal and the nonlinear signal to generate the output signal.

Description

BACKGROUND OF THE INVENTION
• The present invention generally relates to power amplifiers and, more particularly, to a digitally controllable predistortion system and method for compensating for nonlinearities within the power amplifier.[0001]
• A typical power amplifier, such as a traveling wave tube amplifier (TWTA), exhibits a[0002]nonlinear characteristic100 as illustrated in FIG. 1. A linear region is defined as the range up to the point where increasing the RF input signal results in gain compression. The input power versus output power characteristic shown, also known as amplitude to amplitude modulation (AM/AM) curve, rolls off from the linear region as the input power increases and the output power reaches saturation. This effectively means that the amplifier gain decreases in the nonlinear region while the efficiency of the amplifier is at a maximum.
• In addition to gain compression effects, typical power amplifiers also exhibit nonlinear amplitude to phase modulation (AM/PM) effects (not shown). When using higher order modulation schemes with multi-amplitude symbols, additional unintentional phase modulation occurs when operating in the nonlinear region.[0003]
• Typically, the power amplifier is operated at an[0004]operating point120 which is “backed off” from its maximum output capacity in order to maintain linearity. Even though the power amplifier is operating in the “backed off” linear region, the amplifier still consumes power and has a low input power to output power conversion efficiency.
• A known method to compensate for amplifier nonlinearities includes predistortion techniques. A nonlinear pre-equalizer is tuned for a particular amplifier and predistorts an input signal to the amplifier to compensate for the distortion that occurs to the signal after going through a TWTA with certain AM/AM characteristics. The predistorted input signal typically experiences a desired gain expansion with input power so that the combined effect is linear gain up to saturation. By this method an extrapolated[0005]transfer characteristic110 is approached.
• This approach suffers from the disadvantage of being amplifier specific, that is, each amplifier requires a specifically tuned nonlinear pre-equalizer. As is well known in the art, this is costly in terms of both hardware and manufacturing time expended in tuning each nonlinear pre-equalizer. More particularly, in satellite applications, payloads are increased by the requirement that each TWTA have it's own nonlinear pre-equalizer.[0006]
• A known predistortion system is disclosed in U.S. Pat. No. 6,342,810 to Wright et al. The system includes a data structure in which each element stores a set of compensation parameters including finite impulse response (FIR) filter coefficients for predistorting a wideband input transmission signal. The parameter sets are preferably indexed within the data structure according to multiple signal characteristics, such as instantaneous amplitude and integrated signal envelope, each of which corresponds to a respective dimension of the data structure. To predistort the input transmission signal, an addressing circuit digitally generates a set of data structure indices from the input transmission signal, and the indexed set of compensation parameters is loaded into a compensation circuit which digitally predistorts the input transmission signal. The process of loading new compensation parameters into the compensation circuit is preferably repeated every sample instant, so that the predistortion function varies from sample to sample. The sets of compensation parameters are generated periodically and written to the data structure by an adaptive processing component that performs a non-real time analysis of amplifier input and output signals. The system includes a digital compensation signal processor, a generalized digital to analog converter, an RF upconversion block coupled to a nonlinear amplifier, an amplifier sampling structure, an RF downconversion block, a generalized analog to digital converter, and an adaptive control processing and compensation estimator.[0007]
• As can be seen, there is a need for a digitally controllable system and method for compensating for nonlinearities within a power amplifier. The described system is adaptable to a plurality of power amplifiers. Further, the described system is easy to implement, and reduces implementation cost of the system by eliminating the need for a separate linearizer for each traveling wave tube amplifier.[0008]
SUMMARY OF THE INVENTION
• In one aspect of the present invention, a digitally controllable nonlinear pre-equalizer circuit for receiving an input signal and generating an output signal includes a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate the output signal.[0009]
• In another aspect of the present invention, a digitally controllable nonlinear pre-equalizer circuit for receiving an input signal and generating an output signal includes a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the processor chip being operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate the output signal.[0010]
• In yet another aspect of the present invention, an amplifier system for receiving an input signal and generating an output signal includes a power amplifier; a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.[0011]
• In a further aspect of the present invention, a method of digitally controlling a nonlinear pre-equalizer circuit receiving an input signal and generating an output signal includes dividing the input signal into a first signal path and a second signal path; providing an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; providing a digital to analog converter coupled to a processor chip; providing a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from the digital to analog converter, the processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and summing the linear signal and the nonlinear signal to generate the output signal.[0012]
• In yet another aspect of the present invention, a satellite system for receiving an input signal and generating an output signal includes a satellite; a power amplifier disposed on said satellite; a nonlinear pre-equalizer circuit coupled to said power amplifier, said nonlinear pre-equalizer circuit including a splitter for dividing the input signal into a first signal path and a second signal path; an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal; a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.[0013]
• These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.[0014]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a graphical representation of a transfer characteristic of a power amplifier;[0015]
• FIG. 2 is a circuit diagram in accordance with the invention;[0016]
• FIG. 3 is a schematic representation of a signal flow through the circuit of FIG. 2;[0017]
• FIG. 4 is a tabular representation of information stored in a processor chip in accordance with the invention;[0018]
• FIG. 5 is a graphical representation of mixer characteristics in accordance with the invention;[0019]
• FIG. 6 is a graphical representation comparing linearized and unlinearized gain characteristics in accordance with the invention; and[0020]
• FIG. 7 is a graphical representation comparing linearized and unlinearized phase characteristics in accordance with the invention.[0021]
DETAILED DESCRIPTION OF THE INVENTION
• The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.[0022]
• The present invention generally provides a digitally controllable predistortion system and method for compensating for nonlinearities within a power amplifier. The system and method of the invention provide for optimized power usage and channel performance of power amplifiers, and more particularly, traveling wave tube amplifiers (TWTA) deployed aboard satellites. A circuit of the invention is inexpensive and is adaptable to a plurality of TWTAs.[0023]
• In one embodiment, a digitally controllable nonlinear pre-equalizer circuit generally designated[0024]200 may include asplitter210, anattenuator212, adelay element214, amixer220, avector modulator222, a digital to analog converter (DAC)230, aprocessor chip240, and asummer250 as illustrated in FIG. 2. In operation, aninput signal205 may be divided into two signal paths bysplitter210. Afirst signal path211 may be linear with some attenuation introduced byattenuator212 and some delay introduced bydelay element214 to match asecond signal path216. Thesecond signal path216 may be nonlinear as compressed bymixer220 and adjusted byvector modulator222. Parameters specific to each power amplifier are stored inprocessor chip240 and input toDAC230 which, in turn, provides a current to mixer220 throughresistor225. The first andsecond signal paths211 and216 may then be summed bysummer250 and output to the power amplifier (not shown). In this manner, the digitally controllable nonlinear pre-equalizer circuit of the invention is adaptable to a plurality of power amplifiers without the need for signal sampling and application specific integrated circuits as known in the prior art.
• With reference to FIG. 3, operation of the system of the invention is further illustrated including operation of the[0025]mixer220 andvector modulator222.Input signal205 may be divided bysplitter210 intofirst signal path211 andsecond signal path216. In thesecond signal path216, a voltage level input fromDAC230 may control the saturation point and severity of signal compression of themixer220.Vector modulator222 may be operable to shift the phase of an output of themixer220, as by180 degrees as shown. In thefirst signal path211, thesignal310 may be linear and may be attenuated and time delayed as well known in the art. The first andsecond signal paths211 and216 can be summed bysummer250 to yield a desiredgain expansion signal320. The desiredgain expansion signal320 preferably includes nonlinearities that extrapolate to provide linear performance up to saturation.
• [0026]Processor chip240 may be operable to include information regarding a plurality of power amplifiers. Such information may include amplifier parameters. By way of example and with reference to FIG. 4, a table400 stored inprocessor chip240 may include information regarding a plurality ofTWTAs410. Such information may include a digital word (not shown) representing acurrent value420 and adigital word430 representing a vector modulator setting corresponding to eachTWTA410. By way of example, a 6 bit word may be used to control theDAC230 to output the appropriate current into themixer220 and thereby adjust the nonlinear gain characteristic of themixer220.
• A 12 bit[0027]digital word430 may be used to control thevector modulator222. Thevector modulator222 may function as a complex multiplier such that the output of thevector modulator222 is the input multiplied by a complex value, α+jβ. Of the 12 bitdigital word430 controlling thevector modulator222, 6 bits may determine the value α in a range of −1 to 1 and 6 bits may determine the value of β in a range of −1 to 1. Thus thevector modulator222 is capable of applying a linear gain and phase shift to an input signal. The gain of thevector modulator22 may be chosen to achieve the desired gain expansion for a chosenTWTA410 and the phase of thevector modulator222 may be chosen to achieve the desired phase expansion for the chosenTWTA410. Thus by way of example,TWTA1 can have an associated current value of 4.5 mA and an associated twelve bitdigital word430 of 000010111000. In operation, to linearize the operation ofTWTA1,processor chip240 can provide the digital word representing a current value of 4.5 mA toDAC230 which, in turn, provides 4.5 mA tomixer220.Mixer220 characteristics are shown in FIG. 5 including characteristic500 for operation at 4.5 mA, characteristic510 for operation at 4.0 mA, characteristic520 for operation at 3.5 mA, characteristic530 for operation at 3.0 mA, characteristic540 for operation at 2.5 mA, and characteristic550 for operation at 2.0 mA.
• In accordance with another aspect of the invention, an amplifier system includes the digitally controllable nonlinear[0028]pre-equalizer circuit200 described herein coupleable to a plurality of power amplifiers (not shown). Amplifier characteristics for each power amplifier may be stored inprocessor chip240 which may be operable to provide such characteristics in the form of a digital signal toDAC230. In turn,DAC230 provides a current tomixer220 to achieve a desiredgain expansion signal320 appropriate to each power amplifier.
• In accordance with yet another aspect of the invention, a satellite system includes the digitally controllable nonlinear[0029]pre-equalizer circuit200 described herein coupleable to a plurality of power amplifiers disposed in a satellite (not shown). As power amplifiers are replaced, the satellite system may be operable to provide predistortion particular to each power amplifier as described herein. In one embodiment, the amplifier characteristics may be sent from theprocessor chip240 using an uplink command. In another embodiment, theprocessor chip240 may be disposed on the satellite.
• According to another aspect of the invention, a method for compensating for nonlinearities within a power amplifier includes the steps of dividing an input signal into a first signal path and a second signal path. The first signal path may be a linear path having some attenuation and time delay. The second signal path may be compressed and adjusted such that when the first signal path and second signal path are summed, a gain expansion signal is provided which extrapolates to provide linear performance up to saturation. The gain expansion signal may provide for both AM/AM and AM/PM compensation.[0030]
• In accordance with the invention and with reference to FIG. 6, a linearized gain characteristic[0031]600 for a 120 watt TWTA, as an example, produced by the system and method of the invention is shown to be substantially linear as compared to an unlinearized gain characteristic610. In similar fashion and with reference to FIG. 7 a linearized phase characteristic700 for a 120 watt TWTA produced by the system and method of the invention is shown to be substantially linear as compared to an unlinearized phase characteristic710.
• It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.[0032]

Claims (18)

We claim:

1. A digitally controllable nonlinear pre-equalizer circuit receiving an input signal and generating an output signal, the circuit comprising:

a splitter for dividing the input signal into a first signal path and a second signal path;

an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal;

a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and

a summer for summing the linear signal and the nonlinear signal to generate the output signal.

2. The circuit ofclaim 1, wherein the processor chip is operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal to the digital to analog converter.

3. The circuit ofclaim 1, wherein the vector modulator is operable to adjust a phase and amplitude of the nonlinear signal.

4. The circuit ofclaim 1, wherein the output signal is a gain expansion signal having nonlinearities that extrapolate to provide linear performance of a power amplifier up to saturation.

5. A digitally controllable nonlinear pre-equalizer circuit receiving an input signal and generating an output signal, the circuit comprising:

a splitter for dividing the input signal into a first signal path and a second signal path;

an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal;

a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the processor chip being operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal, the mixer and vector modulator operable to generate a nonlinear signal; and

a summer for summing the linear signal and the nonlinear signal to generate the output signal.

6. The circuit ofclaim 5, wherein the vector modulator is operable to adjust a phase and amplitude of the nonlinear signal.

7. An amplifier system receiving an input signal and generating an output signal comprising:

a power amplifier;

a splitter for dividing the input signal into a first signal path and a second signal path;

an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal;

a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and

a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.

8. The amplifier system ofclaim 7, wherein the processor chip is operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal to the digital to analog converter.

9. The amplifier system ofclaim 7, wherein the vector modulator is operable to adjust a phase and amplitude of the nonlinear signal.

10. The amplifier system ofclaim 7, the gain expansion signal having nonlinearities that extrapolate to provide linear performance of a power amplifier up to saturation.

11. A satellite system receiving an input signal and generating an output signal comprising:

a satellite;

a power amplifier disposed on said satellite;

a nonlinear pre-equalizer circuit coupled to said power amplifier, said nonlinear pre-equalizer circuit including,

a splitter for dividing the input signal into a first signal path and a second signal path;

an attenuator and a time delay element in the first signal path, the attenuator and the time delay element operable to generate a linear signal;

a mixer and a vector modulator in the second signal path, the mixer being responsive to a signal from a digital to analog converter coupled to a processor chip providing a digital signal to the digital to analog converter, the mixer and vector modulator operable to generate a nonlinear signal; and

a summer for summing the linear signal and the nonlinear signal to generate a gain expansion signal, the gain expansion signal being input to the power amplifier, the power amplifier providing the output signal.

12. The satellite system ofclaim 11, wherein the processor chip is operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers disposed on said satellite, each power amplifier parameter being provideable as the digital signal to the digital to analog converter.

13. The system ofclaim 11, wherein the vector modulator is operable to adjust a phase and amplitude of the nonlinear signal.

14. The system ofclaim 11, the gain expansion signal having nonlinearities that extrapolate to provide linear performance of a power amplifier up to saturation.

15. A method of digitally controlling a nonlinear pre-equalizer circuit receiving an input signal and generating an output signal, the method comprising:

dividing the input signal into a first signal path and a second signal path;

generating a linear signal by attenuating and time delaying the divided input signal in the first signal path;

generating a nonlinear signal by mixing and phase shifting the divided input signal in the second signal path, the mixing being responsive to a signal provided by a digital to analog converter coupled to a processor chip; and

summing the linear signal and the nonlinear signal to generate the output signal.

16. The method ofclaim 15, wherein the processor chip is operable to store a plurality of power amplifier parameters corresponding to a plurality of power amplifiers, each power amplifier parameter being provideable as the digital signal to the digital to analog converter.

17. The method ofclaim 15, wherein the vector modulator is operable to adjust a phase and amplitude of the nonlinear signal.

18. The method ofclaim 15, wherein the output signal is a gain expansion signal having nonlinearities that extrapolate to provide linear performance of a power amplifier up to saturation.

Images