Amplifiers – With pilot frequency control means
Reexamination Certificate
2000-08-24
2004-01-06
Shingleton, Michael B (Department: 2817)
Amplifiers
With pilot frequency control means
C330S149000, C330S151000
Reexamination Certificate
active
06674324
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a signal amplification system producing an amplified signal with reduced distortion.
2. Description of Related Art
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, a power amplifier, however, has a transfer function with nonlinear and “linear” regions. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses peak amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the amplifier produces an output signal that is clipped or distorted in a nonlinear fashion.
In wireless communications systems, high power amplification of signals is used to increase the power of the signal to be transmitted, for example carrier signal(s) with information modulated thereon. The distortion of the input signal causes power to be generated in adjacent channels or frequencies to corrupt or interfere with signals in the adjacent channels or frequencies, commonly referred to as spectral regrowth or adjacent channel power (ACP). The generation of adjacent channel power is of particular concern in wireless communications systems where signals being amplified are in adjacent channels or frequency bands. Wireless cellular communications systems comprise a number of base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units, which can be mobile or fixed, in the geographic region. Each base station handles voice and/or data communications over a particular region called a cell, and the overall coverage area for the cellular system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
In a wireless cellular communications system, a base station and a wireless unit communicate voice and/or data over a forward link and a reverse link, wherein the forward link carries communication signals from the base station to the wireless unit and the reverse link carries communication signals from the wireless unit to the base station. There are many different schemes for determining how wireless units and base stations communicate in a cellular communications system. Multi-user wireless communications systems, such as Code division multiple access (CDMA), Time division multiple access (TDMA), Global System for Mobile Communications (GSM) and orthogonal frequency division multiplexing (OFDM), combine multiple voice and/or traffic channels into a single or multiple carriers. A linear amplifier should be able to react rapidly to transmit power changes and bursty traffic variations within the transient response specifications in the microsecond and millisecond ranges while providing adequate error cancellation. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path with the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot signal, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located within or near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in
FIG. 1
which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be located near the lower edge of the operating band (e.g., pilot
1
) and re-tuned to be located near the upper edge of the band of operation (e.g., pilot
2
). The pilot is positioned a spectral distance of &Dgr;f from an edge of the band of operation whose center frequency is f
0
. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as having one or two spectral components, the pilot signal can be tuned to have more spectral components or be spread across the spectrum. The pilot signal is detected a spectral component at a time, and the spread spectrum pilot is de-spread and detected as a single amplitude for the spectrum.
The feed forward distortion reduction circuitry typically reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal.
FIG. 2
discloses feed-forward correction circuitry
10
and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier
12
. An input signal, for example including at least one carrier signal with information modulated thereon, is applied to a splitter
14
. The splitter
14
replicates the input signal on a main signal path
16
and a feed forward path
18
. The splitter
14
is part of a carrier cancellation loop referred to as loop #1, which in addition to the splitter
14
, comprises gain & phase circuit
20
, coupler
22
, the RF amplifier
12
, delay circuit
24
and couplers
26
and
28
. The signal on the main path
16
is applied to gain & phase circuit
20
. The output of gain & phase circuit
20
and the pilot signal are applied to the coupler
22
. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier
12
. The output of the coupler
22
is applied to the amplifier
12
whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier
12
.
A portion of the output of the amplifier
12
is obtained from the coupler
26
and is combined at the coupler
28
via coupling path
30
with a delayed version of the input signal on the feed forward path
18
to isolate the pilot signal with distortion on the feed forward path
18
. The input signal on the feed forward path
18
is sufficiently delayed by delay circuit
24
so that such signal experiences the same delay as the signal appearing at the coupler
28
via the path
30
. The resulting error signal contains the distortion produced by the amplifier
12
along with any portion of the carrier signal remaining at the output of the coupler
28
and the pilot signal. The amount of carrier cancellation in the carrier cancellation loop depends on the proper gain and phase match between the two paths from the splitter
14
to the coupler
28
.
The gain & phase circuit
20
adjusts the phase and gain of the input sig
Ocenasek Josef
Zappala Christopher F.
Lucent Technologies - Inc.
Shingleton Michael B
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