Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-05-11
2001-10-02
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S296000, C375S298000
Reexamination Certificate
active
06298097
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to high-performance amplifiers for communications applications, and specifically to highly-linear broadband amplifiers.
BACKGROUND OF THE INVENTION
Modern mobile communications systems use multiple channels, closely spaced over an assigned frequency band. In order to avoid intermodulation products and spectral regrowth, both in and out of band, it is essential that RF power amplifier circuits used in these systems be highly linear. A high level of linearity is also required in single-channel transmitters which transmit a wideband, variable-envelope signal, such as a CDMA signal.
A major source of nonlinearity is distortion, which occurs due to nonlinear amplitude and phase response of the amplifier, particularly as power nears the saturation level. Third-order distortion nonlinearities typically give the strongest intermodulation products, but fifth- and even seventh-order products can be significant. Since a typical cellular communications band has a spectral width of around 25 MHz, high-order intermodulation products in a wideband base station amplifier with large channel spacing can create distortion over a band that is more than 150 MHz wide.
One method of correcting for amplifier distortion, and thus improving linearity, is predistortion, in which a controlled, nonlinear distortion is applied to the amplifier input signals. Predistortion circuitry is designed to give nonlinear amplitude and phase characteristics complementary to the distortion generated by the amplifier itself, so that ideally, the distortion is canceled in the amplifier output over the entire signal bandwidth. A feedback connection is generally provided from the amplifier output to the predistortion circuitry, for use in adjusting predistortion coefficients for optimal linearization. Predistortion is often applied to baseband signals, as described, for example, in U.S. Pat. No. 4,291,277, which is incorporated herein by reference. The predistorted signals are then upconverted and fed to the power amplifier. Predistortion may also be combined with other methods of linearization, such as feedforward error correction, as described in U.S. Pat. No. 5,760,646, which is likewise incorporated herein by reference.
Various schemes have been proposed for digital-domain predistortion of the baseband signals. Because of the very high bandwidth of the intermodulation products, as mentioned above, extremely fast, wideband processing circuitry has been required in order to compensate effectively for distortion without causing new problems such as aliasing. The required sampling rate is particularly high when the power amplifier has a significant level of high order (fifth or seventh order) intermodulation response. For example, U.S. Pat. No. 4,700,151, which is incorporated herein by reference, describes a predistortion system that operates on baseband signals. The signals are sampled and then interpolated to generate samples having a higher sample rate, thus providing an extended bandwidth as required for effective predistortion. U.S. Pat. No. 5,650,758, also incorporated herein by reference, describes a pipeline architecture for a wideband digital predistortion circuit. Other predistortion schemes are described in an article by Cavers, entitled “Amplifier Linearization Using a Digital Predistorted with Fast Adaptation and Low Memory Requirements,” published in
IEEE Transactions on Vehicular Technology,
vol. 39, no. 4 (November 1990), pages 374-382, which is incorporated herein by reference.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide an improved predistortion circuit for use in amplification of radio frequency signals.
It is a further object of some aspects of the present invention to provide a digital predistortion circuit that operates at a reduced sample rate relative to predistortion circuits known in the art.
In preferred embodiments of the present invention, an input signal having a given initial bandwidth is processed by a digital predistortion circuit and is then converted to analog form, upconverted and amplified by a radio frequency (RF) power amplifier. The predistortion circuit receives a stream of samples of the input signal and interpolates the samples to effectively increase the sample bandwidth to an expanded bandwidth at least twice the given bandwidth of the signal. A nonlinear correction is applied to predistort the interpolated samples. The predistorted samples are then low-pass filtered and decimated, so that the bandwidth of the sample stream output by the digital predistortion circuit is again reduced to be on the order of the initial bandwidth.
As a result of this design, digital/analog converters and other circuit elements operating on the output sample stream can work at a substantially slower sample rate and narrower signal bandwidth than in predistortion schemes known in the art, in which the expanded bandwidth is maintained throughout. The present invention can thus be made substantially less costly and complex than such schemes. Alternatively or additionally, it can be made to work with signal bandwidths that known predistortion schemes cannot handle with readily available hardware.
The sample stream that is output by the predistortion circuit is corrected for distortion by the amplifier within the reduced bandwidth of the predistortion circuit output, but not for additional intermodulation products that typically occur over the rest of the expanded bandwidth. Consequently, the power amplifier may generate substantial distortion products in the wings of the extended bandwidth, outside the reduced-bandwidth region in which the distortion is corrected by the predistortion circuit. The uncorrected distortion products in the wings are preferably suppressed by a bandpass filter at the output of the power amplifier, substantially without affecting the amplified signals within the given bandwidth.
Thus, the present invention can be used to correct for distortion that extends over substantially any bandwidth, including intermodulation products both inside and falling partially outside the given bandwidth of the signals. In-band distortion suppression of the amplifier is performed by the predistortion mechanism, whereas the out-of-band distortion is filtered by the band-pass filter. In many applications a band-pass filter or duplexer is already present at the output of the amplifier, so that no extra hardware is needed for this purpose.
In some preferred embodiments of the present invention, the digital predistortion circuit operates on baseband signals, whereas in other embodiments, the predistortion circuit is configured to operate on intermediate frequency (IF) signals. The nonlinear correction may be applied to the signals using any suitable form of digital signal processing, including both real- and complex-domain (I/Q or polar) processing. Preferably, the nonlinear correction is applied using a parallel processing architecture, whereby two or more samples are processed simultaneously, in order to accommodate the high sample rate of the expanded bandwidth.
In some preferred embodiments of the present invention, digital predistortion as described herein is applied in conjunction with other methods of amplifier linearization, such as feedforward correction of the signals. Most preferably, a feedforward amplifier with digital signal equalization is used, as described in U.S. patent application Ser. No. 09/226,709, which is assigned to the assignee of the present patent application and incorporated herein by reference.
There is therefore provided, in accordance with a preferred embodiment of the present invention, linearization circuitry for predistortion of an input signal to an amplifier having a given distortion characteristic, including:
a correction circuit, which receives a stream of samples of the input signal at a high sample rate and which applies a correction to the samples responsive to the given distortion characteristic; and
a decimation circuit, which receives the corr
Bayard Emmanuel
Colb Sanford T.
Hoffman, Wasson & Gitler,PC
Pham Chi
Wiseband Communications Inc.
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