Efficient pre-distorted 12/4 QAM modulator

Details Efficient pre-distorted 12/4 QAM modulator Efficient pre-distorted 12/4 QAM modulator

2000-06-14

2004-01-06

Ghebretinsae, Temesghen (Department: 2631)

Pulse or digital communications

Transmitters

Quadrature amplitude modulation

C375S296000, C375S261000, C375S308000, C332S103000

Reexamination Certificate

active

06674811

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to modulators and more specifically to pre-distorted 12/4-QAM modulators.
2. Background and Material Information
Although quadrature amplitude modulation (QAM) techniques are recognized to be bandwidth-efficient, these techniques are usually implemented with linear amplifiers to prevent introducing nonlinear distortion into the constellation. For space-to-ground applications, this is very inefficient. While nonlinear amplifiers are more efficient, they distort the QAM constellation. Generally, when bandwidth efficiencies of four bits per symbol is desired, 16 QAM is used because they are relatively simple to synthesize. However, 16 QAM has some undesirable characteristics. Furthermore, 16 QAM has multiple amplitude levels. If a signal is amplified with a saturating amplifier (e.g., on a space to earth downlink), when using 16-QAM, it may be required to back off or reduce the amplification so that the constellation is not distorted.
Further, conventional predistortion techniques, such as those used with 16-QAM, independently predistort the I and Q components of each constellation point. Hence, for a QAM modulation approach such as 16-QAM, 32 independent controls may be required to control the predistortion of the I and Q components. This may translate into additional hardware that is needed in the modulator. In some communication systems, such as satellite communication systems, additional hardware increases weight and power consumption, both of which may have limitations. Further, additional hardware increases the overall cost of the system.
Moreover, in some implementations of 16-QAM modulators with predistortion, for every symbol, a table is used to map the four data bits to a transmitted amplitude and phase. This mapping requires additional weight and power consumption which may be limited and increase the overall cost of the system.
SUMMARY OF THE INVENTION
The present invention is directed to a predistorted 12/4-Quadrature Amplitude Modulation (QAM) modulator, which may include a mapping device that maps input bits to a plurality of modulator control bits. The mapping may be determined by selected desired points on a 12/4-QAM constellation. At least one phase shift device may receive an input signal and at least one of the plurality of NRZ symbols. At least two quaternary phase shift keying (QPSK) devices may receive phase shifted signals from the at least one phase shift device. Each at least two QPSK devices may receive at least one of the plurality of NRZ symbols. An attenuator may attenuate a first QPSK signal outputted from a first QPSK device of the at least two QPSK devices. A summer may add the attenuated first QPSK signal with a second QPSK signal. The second QPSK signal may be outputted from a second QPSK device of the at least two QPSK devices. The summer outputs a predistorted 12/4-QAM of the modulator input bits. The plurality of NRZ symbols may control the at least one phase shift device and at least two QPSK devices to achieve the desired points on the 12/4-QAM constellation.
The attenuator may be a variable attenuator. At least one of the plurality of NRZ symbols may control at least one phase shift device to shift the input signal 30, 60, 120, or 150 degrees. At least one of the plurality of NRZ symbols may control the first QPSK device to shift the received phase shifted signals 45 degrees, 135 degrees, 225 degrees, or 315 degrees. At least one of the plurality of NRZ symbols may control the second QPSK device to shift the received phase shifted signals 45 degrees, 135 degrees, 225 degrees, or 315 degrees.
A splitter may split the phase shifted signals from at least one phase shift device into two phase shifted signals. The first QPSK device may receive one of the two phase shifted signals. The second QPSK device may receive a second of the two phase shifted signals. The first QPSK device and/or the second QPSK device may split the phase shifted signals into two phase shifted signals. An upper signal may be generated from one of the two phase shifted signals being shifted 0 degrees or 180 degrees. A lower signal may be generated from a second of the two phase shifted signals being shifted +90 degrees or −90 degrees. The first QPSK signal may be generated from adding the upper signal and the lower signal. The second QPSK signal may be generated from adding the upper signal and the lower signal.
The attenuator may include: a first switch that may receive the first QPSK signal; a plurality of attenuation elements where each of the plurality of attenuation elements may be operatively connected to the first switch, and where each of the plurality of attenuation elements may produce a different attenuation; a second switch operatively connected to each of the plurality of attenuation elements; and a controller where the controller may control the first switch and the second switch to cause the first QPSK signal to pass through one of the plurality of attenuation elements that attenuates the first QPSK signal.
The attenuator may also include: a variable gain amplifier that receives the first QPSK signal; a D/A converter operatively connected to the variable gain amplifier; and a controller that may control the variable gain amplifier to attenuate the first QPSK signal. The controller may control the variable voltage amplifier using the D/A converter. At least one phase shift device and at least two QPSK devices operate at a predistorted 12/4 QAM modulator symbol rate. The desired points on the 12/4 QAM constellation may be chosen so the number of bits of each desired point where adjacent desired points differ is minimized.
According to the present invention, a method for predistorted 12/4-Quadrature Amplitude Modulation (QAM) may include: selecting desired points on a 12/4-QAM constellation; mapping an input signal to a plurality of nonreturn-to-zero (NRZ) symbols where the mapping may be determined by the selected desired points; phase shifting a continuous wave (CW) signal where the phase shifting may be controlled by at least one of the plurality of NRZ symbols; splitting the phase shifted CW signal into a first phase shifted CW signal and a second phase shifted CW signal; performing quaternary phase shift key (QPSK) modulation on the first phase shifted CW signal and the second phase shifted CW signal where the QPSK modulator may be controlled by at least two of the plurality of NRZ symbols, and the QPSK modulator may generate a first QPSK signal and a second QPSK signal; attenuating the first QPSK signal; and adding the attenuated QPSK signal to the second QPSK signal where the addition may generate a predistorted 12/4-QAM signal with the desired points.
The CW signal may be phase shifted 30, 60, 120, or 150 degrees. The QPSK modulator may shift the first phase shifted CW signal 45, 135, 225, or 315 degrees. The QPSK modulator may shift the second phase shifted CW signal 45, 135, 225, or 315 degrees.
The QPSK modulator may include: splitting the first phase shifted signal into an upper signal and a lower signal; shifting the upper signal 0 degrees or 180 degrees; shifting the lower signal +90 degrees or −90 degrees; and adding the shifted upper signal and the shifted lower signal. The QPSK modulator may include: splitting the second phase shifted signal into an upper signal and a lower signal; shifting the upper signal 0 degrees or 180 degrees; shifting the lower signal +90 degrees or −90 degrees; and adding the shifted upper signal and the shifted lower signal. The phases shifting and the QPSK may occur at a modulator symbol rate.
The present invention is also directed to a predistorted 12/4 Quadrature Amplitude Modulation (QAM) modulator that may include: a first phase shift module where the first phase shift module receives an input signal and a fifth NRZ symbol; a second phase shift module operatively connected to an output of the first phase shift module where the second phase shift module receives a sixth NRZ symbol; a first QPSK module oper

Affiliated with

Also associated with

Rating

Efficient pre-distorted 12/4 QAM modulator does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Efficient pre-distorted 12/4 QAM modulator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Efficient pre-distorted 12/4 QAM modulator will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3185863