US 6,252,912 B1 · Adaptive predistortion system

Abstract

A method and apparatus are described for adaptively predistorting a signal before it is transmitted by a transmitter in order to compensate for nonlinearities which are introduced. Statistical information regarding the received signal is accumulated at a receiver, and is preferably transmitted back to the transmitter for processing. The processing includes a constellation analysis to determine the distortion characteristics of the signal, which may include bias, gain imbalance, lock and quad angle errors, and nonlinear distortions including AM-AM and AM-PM conversion effects, with all of these phenomena occurring anywhere in the transmitter-channel-receiverchain. From analysis of the statistics of the distortions in the received signal constellation, adjustments are made in the transmitter and/or receiver parameters. The positions of the signal symbols in the transmitted constellation are thereby correctively predistorted to compensate for and ameliorate the distortions in the received signal, thereby improving the fidelity of the said received signal.

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Claims54

Dependent claims

A method for adaptively compensating a digital signal, comprising the following steps:

accumulating statistical information at a receiver regarding a received signal transmitted by a transmitter, said accumulating step comprising the further steps of accumulating symbol cluster statistical information, accumulating weighted sums of the symbol coordinates to produce first, second and third matrices, inverting said first matrix and multiplying said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster, and multiplying said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
determining constellation distortions due to at least one voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
compensating for said constellation distortions in at least one of the in-phase and quadrature signal components of said signal;
processing said statistical information to determine compensation values;
and
using said compensation values to compensate said signal.

3 more elements

A method for adaptively compensating a digital signal which is being transmitted by a transmitter and received by a receiver, comprising the following steps:

modulating a digital signal at the transmitter according to a predetermined modulation scheme;
adaptively compensating said modulated signal at said transmitter in accordance with compensation values determined from statistical information accumulated at the receiver regarding the received signal;
transmitting said compensated modulated signal to said receiver;
receiving said compensated modulated signal at said receiver;
accumulating statistical information at said receiver regarding said received signal, said accumulating step comprising the further steps of
accumulating symbol cluster statistical information and using said symbol cluster statistical information to accumulate constellation statistical information,
accumulating weighted sums of the symbol coordinates to produce first, second and third matrices;
inverting said first matrix and multiplying said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster;
multiplying said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
processing said statistical information to determine compensation values for constellation distortions due to at least one of voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
wherein said adaptive compensation step compensates for said constellation distortions in at least one of the in-phase and quadrature signal components of said signal.

8 more elements

A method for adaptively compensating a digital signal which is being transmitted by a transmitter and received by a receiver, comprising the following steps:

modulating a digital signal at the transmitter according to a predetermined modulation scheme;
adaptively compensating said modulated signal at said receiver in accordance with compensation values determined from statistical information accumulated at the receiver regarding the received signal;
transmitting said modulated signal to said receiver;
receiving said modulated signal at said receiver;
accumulating statistical information at said receiver regarding said received signal, said accumulating step comprising the further steps of
accumulating symbol cluster statistical information and using said symbol cluster statistical information to accumulate constellation statistical information,
accumulating weighted sums of the symbol coordinates to produce first, second and third matrices;
inverting said first matrix and multiplying said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster;
multiplying said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
processing said statistical information to determine compensation values for constellation distortions due to at least one of voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
wherein said adaptive compensation step compensates for said constellation distortions in at least one of the in-phase and quadrature signal components of said signal.

8 more elements

An adaptive compensation system for compensating a digital signal, comprising:

a statistical accumulation circuit which accumulates statistical information at a receiver regarding a received signal transmitted by a transmitter, said accumulation involving the accumulation of a symbol cluster statistical information which is used to accumulate constellation statistical information;
a compensation value determination circuit which processes said statistical information to determine compensation values;
and
a compensation circuit which uses said compensation values to compensate said signal;
a constellation distortion estimation circuit which determines the constellation distortions due to at least one voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
wherein said compensation circuit compensates for said constellation distortions in at least one of the in-phase quadrature signal components of said signal;
and
wherein said statistical accumulation circuit further comprises a weighted sum circuit which accumulates weighted sums of the symbol coordinates to produce first, second and third matrices, a matrix inversion circuit which inverts said first matrix and multiplies said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster, and a multiplication circuit which multiplies said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster.

5 more elements

A system for adaptively compensating a digital signal which is being transmitted by a transmitter and received by a receiver, comprising:

a signal modulator which modulates a digital signal at the transmitter according to a predetermined modulation scheme;
an adaptive compensation circuit which compensates said modulated signal at said transmitter in accordance with compensation values determined from statistical information accumulated at the receiver regarding the received signal;
a transmitter for transmitting said compensated modulated signal to said receiver;
a receiving circuit which receives said compensated modulated signal at said receiver;
a statistical accumulation circuit which accumulates statistical information at said receiver regarding said received signal, said statistical accumulation circuit comprising
a symbol cluster accumulation circuit which accumulates symbol cluster statistical information and uses said symbol cluster statistical information to accumulate constellation statistical information,
a weighted sum circuit which accumulates weighted sums of the symbol coordinates to produce first, second and third matrices;
an inversion circuit which inverts said first matrix and multiplies said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster;
a multiplication circuit which multiplies said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
a processing circuit which processes said statistical information to determine compensation values for constellation distortions due to at least one of voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
wherein said adaptive compensation circuit compensates for said constellation distortions in at least one of the in-phase and quadrature signal components of said signal.

8 more elements

A system for adaptively compensating a digital signal which is being transmitted by a transmitter and received by a receiver, comprising:

a signal modulator which modulates a digital signal at the transmitter according to a predetermined modulation scheme;
an adaptive compensation circuit which adaptively compensates said modulated signal at said receiver in accordance with compensation values determined from statistical information accumulated at the receiver regarding the received signal;
a transmitter for transmitting said modulated signal to said receiver;
a receiving circuit which receives said modulated signal at said receiver;
a statistical accumulation circuit which accumulates statistical information at said receiver regarding said received signal, said statistical accumulation circuit comprising:
a symbol cluster accumulation circuit which accumulates symbol cluster statistical information and uses said symbol cluster statistical information to accumulate constellation statistical information,
a weighted sum circuit which accumulates weighted sums of the symbol coordinates to produce first, second and third matrices;
an inversion circuit which inverts said first matrix and multiplying said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster;
a multiplication circuit which multiplies said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
a processing circuit which processes said statistical information to determine compensation values for constellation distortions due to at least one of voltage offset, gain mismatch ratio, lock angle error, quad angle error, nonlinear channel distortions and receiver distortions;
wherein said adaptive compensation circuit compensates for said constellation distortions in at least one of the in-phase and quadrature signal components of said signal.

8 more elements

A method for adaptively compensating a digital signal, comprising the following steps:

accumulating statistical information at a receiver regarding a received signal transmitted by a transmitter, said accumulating step comprising the further steps of accumulating symbol cluster statistical information, using said symbol cluster statistical information to accumulate constellation statistical information, accumulating weighted sums of the symbol coordinates to produce first, second and third matrices, inverting said first matrix and multiplying said inverted first matrix by said second matrix to produce a first coefficient matrix of the coefficients for a first coordinate of the constellation used in a least squares model for estimating the center of each symbol cluster, multiplying said inverted first matrix by said third matrix to produce a second coefficient matrix of the coefficients for a second coordinate of the constellation used in said least squares model for estimating the center of each symbol cluster;
processing said statistical information to determine compensation values;
and
using said compensation values to compensate said signal.

1 more element

Dependencies are inferred from the claim text.

Description9,736 words

Summary3,190 words

#The present invention generally relates to the field of data communications. More specifically, the present invention is for a method and apparatus for adaptively predistorting a signal before it is transmitted from a transmitter to a receiver of a communication system.

#Digital data communications systems are commonly used to transmit and/or receive data between remote transmitting and receiving locations. A central facet of any data communications system is the reliability and integrity of the data which is being communicated. Ideally, the data which is being transmitted from the transmitting location should be identical to the data which is being received at the receiving location. Practically however, the data which is received at the receiving location has oftentimes been corrupted with respect to the original data that was transmitted from the transmitting location. Such data communication errors may be attributed in part to one or more of the transmission equipment, the transmission medium or the receiving equipment. With respect to the transmission medium, these types of data errors are usually attributed to the less than ideal conditions associated with the particular transmission medium.

#For example, in the case of wireless communication systems, the transmission medium, which is typically air, often suffers from atmospheric and othere effects that tend to degrade the data being transmitted . Some of these non-ideal conditions may be modeled and taken into account in order to compensate for and thereby reduce or possibly eliminate any deleterious effects resulting therefrom. In this respect, it is generally known that signal attenuation is a function of the distance that the data signal must propagate through the atmosphere. Thus, it is possible to design a wireless communications system which is capable of transmitting a data signal sufficiently robust such that in spite of known distance-dependent atmospheric attenuation, the data signals at the receiving location can be properly and accurately received. Other types of non-idealities associated with an air or atmospheric transmission medium are often highly random events which may not be modeled a priori and thus may not be compensated for or eliminated.

Brief description of the drawings9 figures

#The present invention will become more apparent from the following Brief Description of the Drawings and Description of Preferred Embodiments.

#FIG. 1 is a block diagram of the predistortion system of the present invention.

#FIGS. 2a, 2b, and 2c are illustrations of various modulation schemes.

Detailed description6,356 words

#Referring now to FIG. 1, therein is shown a block diagram of the predistortion system of the present invention. The symbol source, or modulator, 12 supplies the data to be transmitted. This data may be in QAM or any other of the above mentioned formats. The modulated symbol stream is first input to a predistorter with memory 14, which may be implemented in accordance with the teachings of Karam and Sari discussed above. The data output from the predistorter with memory 14 is then input to an adaptive predistorter 16. The output of the adaptive predistorter 16 is then input to a power amplifier 18. The predistorter with memory 14 is generally used if the power amplifier is operated in a nonlinear mode, in which case it functions to compensate for the nonlinear effects of the power amplifier 18. In the case of a nonlinear power amplifier 18, then from time to time a test sequence of symbols is sent directly to the power amplifier 18, bypassing the adaptive predistorter 16 by means of switch 20. The test sequence must be re-sent at a rate comparable with the rate at which the gain of the power amplifier changes due to temperature or voltage variations. The test sequence can either be sent as a continuous symbol stream that occasionally interrupts the data stream, or preferably as selected symbols that are part of the overhead bits that accompany each frame of data. The frequency at which the test sequence is utilized depends upon the particular operating environment. These test symbols are extracted from the power amplifier output, their normalized distance from the origin of the symbol constellation is measured, and the measurements are used to readjust the symbol position mapping in the memory of the predistorter. Alternatively, the adaptive predistorter 16 may be used to compensate for the nonlinear operation of the power amplifier 18.

#From the power amplifier 18, the signal propagates through the forward transmission channel 22 to the receiver 24. At the receiver 24, the statistics regarding the positions, sizes, and shapes of the symbol clusters are measured on a periodic basis, and are sent via the reverse control channel 26 to the parameter processor computer 28 at the transmitter. The statistics are measured at the receiver 24 because transmitting the raw in-phase and quadrature (I-Q) measurements on all symbols used in the statistical analysis over the narrowband reverse control channel would greatly exceed the capacity of that channel. Generally, the distortion on the reverse control channel is significantly less than that on the forward channel and does not affect the transmission of the statistical information. The processor 28 calculates the parameters of the constellation distortions that arise in the forward channel and receiver. These parameters are used to adjust the I-Q coordinates of the transmitted symbols in the adaptive predistorter 16 so as to minimize the resulting received distortions. For some types of distortions, such as linear constellation offsets due to voltage biases in the receiver or gain imbalances (giving unequal scales in the I and Q coordinates) in the receiver, the values of the corrective voltages may be sent to the receiver 24 in the overhead bits on the forward transmission channel 22 to reduce the effects of these constellation offsets and gain imbalances. Finally, an adaptive equalizer 30 mitigates the effects of multipath propagation and frequency dependent (dispersive) phenomena, such as Rayleigh fading, over the forward transmission channel 22. The corrected symbol data then goes to the demodulator 32 where the information bit stream is extracted.

#Referring now to FIGS. 2a, 2b and 2c, therein are illustrated the configurations of 16-QAM (quadrature amplitude modulation), 32-Cross QAM, and 25-QPR (quadrature partial response) symbol constellations. For the QAM and QPR constellations, the states are numbered in the x and y directions as i=1,2, . . ., and j=1,2, . . . , respectively, beginning in the lower left-hand corner. However, in an origin-centered coordinate system, the constellation point (i,j) will ideally be located at coordinates (Xij,yij)=(î, ĵ), where i ^ = i - M + 1 2 , ⁢ j ^ = j - M + 1 2 ( 1 )

Facts

Application

08/997,858Filed 24 Dec 1997

Granted

26 Jun 2001

Term ended

24 Dec 2017

Estimated · how

20 years from earliest known nonprovisional domestic filing. Maintenance-fee lapses, terminal disclaimers and statutory extensions have not been checked. The continuity chain may be incomplete; this is not an enforceability determination. Patent term adjustment is unavailable; no adjustment was added.

Classification6 codes

H03F 1/3282
Using predistortion circuits: acting on the phase and the amplitude of the input signal
H03F 1/3247
Using predistortion circuits: using feedback acting on predistortion circuits
H04L 27/34
Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26:…
H04L 2027/0016
Carrier regulation: stabilisation of local oscillators

Description9,736 words

Summary3,190 words

Brief description of the drawings9 figures

#The present invention will become more apparent from the following Brief Description of the Drawings and Description of Preferred Embodiments.

#FIG. 1 is a block diagram of the predistortion system of the present invention.

#FIGS. 2a, 2b, and 2c are illustrations of various modulation schemes.

Detailed description6,356 words

Adaptive predistortion system

Life of this patent

Adaptive predistortion system

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Life of this patent

Abstract

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Drawings

Claims54

Description9,736 words

Classification6 codes

Life of this patent

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Description9,736 words