Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-05-28
2003-09-02
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C330S302000, C332S162000, C455S126000
Reexamination Certificate
active
06614854
ABSTRACT:
TECHNICAL FIELD
The present invention relates to communication systems and is particularly directed to a system for counteracting the non-linear behavior of power amplifiers, such as may be used in microwave communication.
BACKGROUND
Transmission of radio signals over the (i.e., use of a wireless link) air provides many advantages over other transmission media for communication systems. Radio systems do not require installation of a transmission medium between stations, avoiding the expense of installing and maintaining fiber optic or electrical cables. Thus, communication systems using radio links are particularly well suited to installations where adverse/hostile terrain, existing infrastructures and/or regulations make installation of cable, or other media, prohibitive.
A problem inherent in many radio systems, particularly those operating in the 20-40 GHz range, is variable path fade caused by changing atmospheric and environmental conditions, such as precipitation and varying foliage at microwave facility sites and along transmission paths. It should be appreciated that the propagation loss for high frequencies, such as the aforementioned 20 to 40 GHz systems, varies greatly depending on such dynamic circumstances. For example, from a clear to rainy day signal attenuation or signal loss variation can be as much as 40 dB per km. Accordingly, in order to ensure a sufficient signal margin to consistently provide an adequate signal-to-noise ratio during periods of extreme signal attenuation, it may be necessary to increase the power output of a transmitter to accommodate worst case conditions. This, however, results in excessive output levels during periods of reduced signal attenuation. Not only does use of a constant high power level waste power and result in increased maintenance, it may result in a receiver associated with a different transmitter/receiver pair experiencing interference particularly during those periods of reduced attenuation. The use of excess power unnecessarily enlarges the antenna beam contour of a transmitter and may result in unwanted signal distortion associated with such higher transmitter power levels. This distortion is because microwave systems previously mentioned generally employ high power amplifiers, as part of the signal transmission or transponder sections of the system. When the transmit power level is increased to overcome signal attenuation, the amplifier may be pushed into saturation where it will exhibit non-linear characteristics and distort the transmitted signal. This distortion is a primary impediment to reliable spectrally-efficient digital or other signaling using such amplifiers.
More particularly, when a signal containing amplitude variations is amplified, it will suffer distortion if the amplifier does not exhibit a linear amplitude transfer characteristic. This means that the output is not linearly proportional to the input. The signal will also suffer distortion if the phase shift introduced by the amplifier is not linear over the range of frequencies present in the signal, or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced further includes intermodulation of the components of the input signal. The products of the intermodulation appear (i) within the bandwidth of the signal causing additional undesirable distortion and (ii) extend outside the bandwidth originally occupied by the signal causing interference with adjacent channels and possibly violating licensing and regulatory spectral emission requirements. Although filtering can be used to remove some unwanted out-of-band distortion, this is not always practical, especially if the amplifier is required to operate on several different frequencies. Distortion products which are at multiples of the carrier frequency can also be produced in a nonlinear amplifier, although these can be removed by relatively simple filtering techniques.
Intermodulation is also a problem when multiple signals are amplified in the same amplifier even if, individually, the signals do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components combine with each other by adding and subtracting as their phase relationships change.
Amplifiers can introduce some distortion even if they are well designed. Perfect linearity over a wide range of amplitude is difficult to realize in practice. Moreover, as any amplifier nears its maximum output capacity, the output no longer increases as the input increases. At this point the amplifier is not linear. In fact, a typical amplifier becomes significantly nonlinear at a small fraction of its maximum power output capability. This means that, in order to maintain linearity, the amplifier is often operated at an input and output amplitude which is low enough such that the signals to be amplified are within the amplifier's substantially linear transfer characteristic. This brute force approach reduces the drive level into the amplifier, so that the amplifier output power is considerably below saturation where the magnitudes of the distortion are tolerable. This method of operation is referred to as “backed off”. While this technique has been found to be useful and has been widely employed with amplifiers, it loses a great deal of its appeal if the amplifier has to be backed off excessively in order to obtain acceptable distortion levels, since every dB of amplifier back off causes a loss in dB of radiated power. This method wastes power and requires that the amplifier be large and relatively expensive. Further operating in a “backed off” mode is counter-productive to the previously mentioned method for boosting the power level to compensate for signal attenuation.
Another way to avoid distortion effects for digital modulation signaling is to use constant envelope type signals, such as unfiltered phase shift keying (PSK) or frequency shift keying (FSK) modulation. PSK and FSK signals are unaffected by non-linear distortion and the associated amplifiers can be smaller, run cooler, are more power efficient and less expensive. Unfortunately, such signaling schemes generally require a higher signal-to-noise ratio for a prescribed level of performance than other types of modulation (such as quadradture amplitude modulation (QAM)) that employ variations in amplitude to represent the data. Then too, many of the newer, bandwidth efficient modulation schemes use both amplitude and phase variations.
There is also a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for large, costly high level output signal combining filters which have undesirable power losses. This performance disparity between constant and non-constant amplitude signals increases in proportion to the data rate-signal bandwidth quotient (i.e., bits/sec/Hz). Accordingly, if the performance efficiency of a non-constant amplitude signal modulation scheme is to be obtained, compensation is necessary to account for amplifier distortion characteristics.
Another type amplifier used by microwave systems is the LINC (Linear Nonlinear Component) Amplifier. The LINC is based on generating amplitude variations in a signal by combining two signals which vary only in their relative phases. The vector sum of the two signals can represent any amplitude. Thus, it is possible to represent the instantaneous state of any signal or combination of signals. The phase and frequency of the component signals can also be made to represent that of the original so that when combined, the original signal is reconstructed. In spite of the fact that its theoretical efficiency can be very high, in a practical LINC transmitter the imbalance between the power gain and delay (or phase) of the two RF paths (especially for wideband applications) and the different non-linear characteristics of the two amplifiers limits the overall performance of the amplifier.
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Babb Roger A.
Chow Peter El Kwan
Krizman Donna M.
Carriercomm, Inc.
Fulbright & Jaworski L.L.P.
Vo Don N.
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