Amplifiers – With plural amplifier channels – Redundant amplifier circuits
Reexamination Certificate
1999-05-25
2001-06-05
Pascal, Robert (Department: 2817)
Amplifiers
With plural amplifier channels
Redundant amplifier circuits
C330S149000, C375S297000
Reexamination Certificate
active
06242975
ABSTRACT:
TECHNICAL FIELD
This invention relates to communication systems and, more particularly, to improving the efficiency of a power amplifier in a communication system.
BACKGROUND
Power amplifiers in communication systems, such as radio frequency (RF) systems, have limited dynamic ranges; operating at high output power levels leads to phase and amplitude distortion, while operating at reduced output power levels leads to system inefficiency (i.e., the ratio of the output RF power to the combined power from the DC power supply and the input signal is less than optimal). Power amplifiers also produce amplitude and phase distortion at higher input or output power levels. As a result, the efficiency of a power amplifier improves as the dynamic range of the amplified signal is reduced. In general, power amplifiers are most efficient in systems that rely on constant envelope modulation techniques.
Most modern wireless communication standards, including IS-95A, IS-136, and Personal Digital Cellular (PDC), use non-constant envelope modulation techniques. Signals in systems that follow these standards exhibit large dynamic ranges, with peak-to-average power values typically varying from as little as 2.9 dB to as great as 5.8 dB. Since the amplifier must be backed off to faithfully reproduce the peaks, the maximum efficiencies of power amplifiers designed for these systems typically range from 35% to 50%.
One technique for improving the performance (ie., efficiency and fidelity) of a power amplifier in a non-constant envelope environment is the envelope elimination and restoration (EER) technique described by Leonard R. Kahn in “Single-Sideband Transmission by Envelope Elimination and Restoration,” Proceedings of the I.R.E., vol. 40, pp. 803-06 (1952).
FIG. 1
shows a particular implementation of an envelope elimination and restoration amplifier
100
in which an amplitude limiter
102
and an envelope detector
104
are used to separate a low power RF signal
106
into two components: (1) a phase and frequency modulated (FM) signal
108
having a constant envelope and (2) a baseband signal
110
representing the non-constant envelope. A standard RF power amplifier
112
amplifies the constant envelope FM signal
108
, and a envelope amplifier
114
amplifies the envelope signal
110
. An envelope combining power amplifier
116
combines the amplified FM and envelope signals. A delay element
118
ensures proper timimg of the signals arriving at the envelope combiner
116
. An example of this topology, known as the envelope feedforward amplifier, is described in detail in U.S. patent application Ser. No. 09/108,628, filed on Jul. 1, 1998, by Donald Brian Eidson and Robert Edmund Grange, and titled “Envelope Feedforward Technique with Power Control For Efficient Linear RF Power Amplification.”
By breaking the RF signal into a constant envelope RF component and a baseband envelope component, this technique provides a great improvement in amplifier efficiency, yielding actual efficiencies of more than 70% in many systems. Nevertheless, communication systems that produce very large peak-to-average power variations, such as an IS-95B system with eight channels (>17dB variations), greatly reduce the efficiency of power amplifiers that use even this amplification technique. Moreover, many EER techniques cannot support high dynamic range signals and/or still provide additional range for average power adjustment (e.g., average power control in cellular systems).
SUMMARY
The inventors have discovered that the efficiency and linearity of all power amplifiers (PAs), including envelope following amplifiers such as envelope feedforward amplifiers, are improved by limiting the dynamic range of the envelope signal before injection into the PA. A particular concern is ensuring that the amplitude of the envelope signal remains below a maximum value. In addition, for envelope following PAs, one would also like to ensure that the envelope signal does not fall below a minimum value. In most situations, limiting the amplitude of the envelope signal to a relatively narrow range of values has little effect on signal fidelity (i.e., modulation accuracy) and spectral containment.
For example, an IS-95A standard signal for which envelope excursions are limited to no more than 6 dB below the average power level yields a theoretical modulation accuracy (&rgr;), in terms of correlation with an ideal signal, of greater than 1-10
−4
, and an adjacent channel power ratio (ACPR) of less than −59 dBc. Likewise, limiting IS-95A envelope excursions to peak values of no more than 3.9 dB yields a modulation accuracy (&rgr;) of greater 1-10
−4
and ACPR of less than −60 dBc. These values for modulation accuracy and ACPR are very near the ideal values of &rgr;=1 and ACPR=−62 dBc. This is important because the actual peak-to-average ratio for IS-95A is greater than 5.5 dB, and the trough-to-average ratio is less than −22 dB. Moreover, the only distortion introduced by the amplifier is a certain amount of controlled amplitude distortion related to the envelope-limiting; virtually no phase distortion is introduced.
In one aspect, the invention involves limiting the envelope of a non-constant envelope signal when amplfying the signal. The signal is separated into two component signals, including a constant envelope signal containing phase and frequency information and a baseband signal representing the envelope. The envelope of the baseband signal is limited by confining the baseband signal to magnitudes lying either above or below a predetermined boundary value. At least one of the component signals is amplified, and then the magnitude-limited baseband signal and the constant envelope signal are combined to form an output signal having a limited but non-constant envelope.
In some embodiments, all portions of the baseband signal that lie above the boundary value are limited to the boundary value. In other embodiments, all portions of the baseband signal that lie below the boundary value are limited to the boundary value. One technique for implementing these embodiments involves receiving digital data that indicates an actual value of the baseband signal and, if the actual value is greater than or less than the boundary value, replacing the actual value with the boundary value. In other embodiments, digital signals representing in-phase and quadrature components of the non-constant envelope signal are used to calculate an envelope value for the baseband signal.
In another aspect, the invention involves receiving signals representing in-phase (I) and quadrature (Q) components of the non-constant envelope signal and, for at least one pair of samples in the I and Q signals, calculating an envelope value for the non-constant envelope signal. Each envelope value is compared to at least one threshold value and, if the envelope value exceeds the threshold value, the I and Q signals are scaled so that the envelope value does not exceed the threshold value. In some embodiments, the threshold value is selected to ensure that the probability that the envelope value will exceed the threshold value is less than a predetermined amount (e.g., between 0.01 and 0.001).
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
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patent: 4291777 (1981-09-01), Davis et al.
patent: 4462001 (1984-07-01), Girard
patent: 4965527 (1990-10-01), Clark et al.
patent: 5041793 (1991-08-01), Gailus
patent: 5043673 (1991-08-01), Suematus et al.
patent: 5293407 (1994-03-01), Shibata
patent: 5621762 (1997-04-01), Miller et al.
patent: 5901346 (1999-05-01), Stengel et al.
patent: 5905760 (1999-05-01), Schnabl et al.
Miller, et al., “Peak Power and Bandwidth Efficient Linear Modulation,” IEEE Transactions on Communications, vol. 46, No. 12, Dec. 1998.
Eidson Donald Brian
Grange Robert Edmund
Brinks Hofer Gilson & Lione
Conexant Systems Inc.
Nguyen Khanh Van
Pascal Robert
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