Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1998-10-19
2001-02-13
Chin, Stephen (Department: 2731)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S296000, C330S149000
Reexamination Certificate
active
06188732
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to feedforward amplifiers and, more specifically, to digital feedforward amplifiers for use in an RF transmitter in a wireless communications network.
BACKGROUND OF THE INVENTION
In 1996, more than 75 million people worldwide used cellular telephones. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
This competition has also led to rapid and sweeping innovations in cellular telephone technology. Analog cellular systems are now competing with digital cellular systems. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the increased number of cellular base stations has resulted in increased infrastructure costs. To offset this increased cost, cellular service providers are eager to implement any innovations that may reduce equipment costs, maintenance and repair costs, and operating costs, or that may increase service quality and reliability, as well as the number of subscribers that the cellular system can service.
Much of this innovation has focused on service quality improvements, such as expanded digital PCS services or smaller and lighter cellular phone handsets having a longer battery life, or equipment cost reduction, such as smaller, cheaper, more reliable transceivers for the cellular base stations. However, there has been only limited innovation directed to reducing the operating costs of a cellular system.
Every cellular base station has a transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computer equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the transmitter power amplifier is highly linear, especially for a signal whose envelope changes in time over such a wide range, such as in CDMA or multi-carrier operations. One of the techniques used in the design of highly linear amplifiers is known as the feedforward technique. In the prior art, it is based on two-loop designs. Typically, the first loop isolates the error signal produced by intermodulation distortion in the power amplifier and the second loop subtracts the error signal from the power amplifier output. However, the two-loop designs contain a high number of components and require delay lines in the power amplifier outputs that consume a large amount of power and greatly reduce the efficiency of the transmitter. The need for delay lines is due in part to the fact that the prior art amplifiers are, by and large, analog designs that are unable to buffer signals.
There is therefore a need in the art for improved cellular systems that are less expensive to operate. In particular, there is a need in the art for improved transmitter power amplifiers based on simpler feedforward designs that do not require the use of a delay line in the power amplifier output. There is a further need for feedforward transmitter power amplifiers that are implemented using more reliable digital design techniques, particularly in CDMA systems.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an RF transmitter, a new digital amplifier design that simplifies the RF transmitter and eliminates all delay lines. A digital feedforward amplifier is disclosed that uses digital signal processing to generate error correction signals that reduce intermodulation distortion generated in the main power amplifier. The digital signals in the improved feedforward amplifier may be buffered, if necessary, thereby eliminating the need for delay lines. The disclosed invention may be readily adapted for use in cellular or wireless communication systems where multi-channel signals are combined digitally at baseband, such as in code division multiple access (CDMA) systems.
According to one embodiment of the present invention, there is provided, for use in a transmitter in a wireless network, a digital feedforward amplification system comprising a first branch comprising: 1) a digital combiner circuit capable of receiving a plurality of digital input signals and generating from at least one of the plurality of digital input signals at least one digital composite output signal; 2) a DAC circuit for receiving the at least one digital composite output signal and converting the at least one digital composite output signal to at least one analog composite output signal; 3) a modulation circuit capable of receiving a local RF signal and the at least one analog composite output signal and generating therefrom a modulated RF signal; and 4) a power amplifier capable of amplifying the modulated RF signal to produce an amplified RF output signal.
The digital feedforward amplification system further comprises 1) a second branch comprising a distortion correction circuit capable of receiving the at least one digital composite output signal from the digital combiner circuit and generating a correction signal capable of at least partially canceling a distortion signal introduced into the amplified RF output signal by the power amplifier in the first branch; and 2) a combiner circuit capable of combining the amplified RF output signal and the correction signal to thereby at least partially cancel the distortion signal.
According to one embodiment of the present invention, the at least one digital composite output signal comprises a digital in-phase composite output signal and a digital quadrature composite output signal.
According to another embodiment of the present invention, the modulation circuit is a quadrature modulation circuit that receives an analog in-phase composite output signal and an analog quadrature composite output signal from the DAC and generates a quadrature modulated RF signal.
According to still another embodiment of the present invention, a receipt of the at least one digital composite output signal by the DAC circuit from the digital combiner circuit is time delayed with respect to a receipt of the at least one digital composite output signal by the distortion correction circuit.
According to yet embodiment of the present invention, the distortion correction circuit generates the correction signal whenever the at least on digital composite output signal increases above a maximum level capable of causing the power amplifier to enter saturation.
According to further embodiment of the present invention, the digital feedforward amplification system further comprises amplifier characterization circuitry capable of monitoring current operating characteristics of the main power amplifier and providing to the distortion correction circuit update information capable of causing the distortion correction circuit to adjust the correction signal in response to changes in the operating characteristics of the main power amplifier.
According to a still further embodiment of the present invention, the distortion correction circuit further comprises phase shift circuitry capable of shifting a phase of the correction signal relative to a phase of the amplified RF output signal.
In another embodiment of the present invention, the distortion correction circuit further comprises gain adjustment circuitry capable of adjusting an amplitude of the correction signal.
In yet another embodiment of the present invention, the RF correction signal can be further shaped or conditioned to best match
Chin Stephen
Han John C.
Jiang Lenny
Samsung Electronics Co,. Ltd.
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