Transmitter linearization using fast predistortion

Details Transmitter linearization using fast predistortion Transmitter linearization using fast predistortion

2000-04-17

2004-05-25

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transmitters

Antinoise or distortion

C375S296000

Reexamination Certificate

active

06741662

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to wireless transmission and in particular to non-linear transmitters which use predistortion.
BACKGROUND OF THE INVENTION
FIG. 1
is a block diagram of a typical radio frequency transmitter
10
, used in mobile phones. Generally, radio frequency (RF) transmitter
10
comprises a power amplifier (PA)
12
which amplifies the signals being transmitted from an antenna
14
. For efficiency, amplifier
12
is, in many cases, a non-linear amplifier operated near its peak capacity. To avoid distortion of the transmitted signals due to the non-linearity, the signals are pre-distorted by a predistorter 15 before they are transmitted. The predistortion is required to prevent transmitter
10
from transmitting signals on channel bands other than the band assigned to the transmitter. A multiplier
16
of predistorter
15
multiplies modulated signals prepared for transmission by predistortion values. The predistortion values are chosen such that the product values entering amplifier
12
will be distorted by the power amplifier to return to a substantially linear amplification of the modulated signals. The predistortion values are generally selected from a look up table (LUT)
18
which is indexed by the amplitude of the multiplied transmitted signal. As the exact predistortion caused by the amplifier varies with time and carrier frequency, due to temperature, aging and other characteristics of power amplifier
12
, the predistortion values in LUT
18
are updated, by a trainer
20
, based on feedback received from the output of amplifier
12
.
A paper titled “Amplifier Linearization Using a Digital Predistorter with Fast Adaptation and Low Memory Requirements”, J. K. Cavers, IEEE Trans. On Vehicular Technology, Vol. 39, No. 4, Nov. 1990, and U.S. Pat. No. 5,049,832 to Cavers, describe an adaptive method for calculating the predistortion values of LUT
18
. In this method, LUT
18
is assigned initial random values which are thereafter updated adaptively. For each sample provided to predistorter
15
and thereafter to power amplifier
12
, the difference between the desired amplification and the actual amplification of the power amplifier is calculated and accordingly the corresponding entry in LUT
18
is adjusted. This method, however, does not converge fast enough to fulfill requirements of emerging standards.
U.S. Pat. No. 5,923,712 to Leyendecker et al., describes another method for calculating the predistortion values of LUT
18
. In the method of the U.S. Pat. No. 5,923,712 patent, trainer
20
accumulates a plurality of sets of three values including the modulated value being provided to predistorter
15
, the value after it is pre-distorted (as it enters to power amplifier
12
) and the value at the output of power amplifier
12
. Periodically trainer
20
divides the recently accumulated sets into bins which correspond to the entries of LUT
18
. These entries of LUT
18
are indexed using the values provided to predistorter
15
. Using the accumulated values of each bin, trainer
20
directly determines an inverse transfer function of the power amplifier for the specific bin. The inverse transfer function is determined by calculating the transfer function of power amplifier
12
from its output to its input. According to the inverse transfer function of the specific bin the corresponding entry of LUT
18
is updated. Bins which have too few samples are not used for updating the entries of LUT
18
, and instead interpolation is performed using neighboring bins.
The method of the U.S. Pat. No. 5,923,712 patent, however, does not operate fast enough and does not reach high enough levels of accuracy, for example, due to impairments of analog components of the transmitter. In order to compensate for inaccuracies, the U.S. Pat. No. 5,923,712 patent suggests using previous sample information in determining which entry of the LUT
18
is to be used and in updating the LUT
18
. The use of the previous sample information, by the trainer
20
of the U.S. Pat. No. 5,923,712 patent, complicates the trainer, increases the size of its memory buffer and enlarges its current consumption.


REFERENCES:
patent: 4291277 (1981-09-01), Davis et al.
patent: 5049832 (1991-09-01), Cavers
patent: 5923712 (1999-07-01), Leyendecker et al.
patent: 6141390 (2000-10-01), Cova
Cavers, J.; “The Effect of Quadrature Modulator and Demodulator Errors on Adaptive Digital Predistorters for Amplifier Linearization”; IEEE Trans. On Vehicular Technology; vol. 46; No. 2; May 1997; pp. 456-466.
Sundstrom, L. et al.; “Effects of Reconstruction Filters in Digital Predistortion Linearizers for RF Power Amplifiers”; IEEE Trans. On Vehicular Technology; vol. 44; No. 1; Feb. 1995; pp. 131-139.
Wright, A. et al.; “Experimental Performance of an Adaptive Digital Linearized Power Amplifier”; IEEE Trans. On Vehicular Technology; vol. 41; No. 4, Nov. 1992; pp. 395-400.
Cavers, J.; “New Methods for Adaptation of Quadrature Modulators and Demodulators Amplifier Linearization Circuits”; IEEE Trans. On Vehicular Technology; vol. 46; No. 3; Aug. 1997; pp. 707-716.
Sundström, L. et al.; “Quantization Analysis and Design of a Digital Predistortion Linearizer for RF Power Amplifiers”; IEEE Trans. On Vehicular Technology; vol. 45; No. 4; Nov. 1996; pp. 707-719.
Cavers, J.; “Optimum Table Spacing in Predistorting Amplifier Linearizers”; IEEE Trans. On Vehicular Technology; vol. 48; No. 5; Sep. 1999; pp. 1699-1705.
Cavers, J.; “Amplifier Linearization Using a Digital Predistorter with Fast Adaptation and Low Memory Requirements”; IEEE Trans. On Vehicular Technology; vol. 39; No. 4; Nov. 1990; pp. 374-382.

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