Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1998-12-10
2001-08-14
To, Doris H. (Department: 2745)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S063300, C330S149000, C375S296000
Reexamination Certificate
active
06275685
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to high power linear amplifiers and in particular relates to the same using digital pre-distortion.
BACKGROUND OF THE INVENTION
First and second generation cellular systems have historically used forms of modulation which are either constant envelope (e.g. GMSK in GSM) or which result in relatively low levels of amplitude modulation. The linearity of the high power amplifiers used for such systems has therefore not been an important technical issue; indeed, for the constant envelope systems it is standard practice to operate the amplifiers either close to or actually in compression in order to maximise power efficiency.
Third generation cellular systems however typically use linear spread-spectrum modulation schemes with a large amount of amplitude modulation on the signal envelope. When passed through a high power amplifier, the output is typically distorted in amplitude and phase by the non-linearity of the amplifier: the amplitude and phase distortion effects are commonly referred to as AM-AM conversion and AM-PM conversion respectively. Both distortion effects are a function only of the amplitude envelope of the input signal and are insensitive to the input phase envelope.
In systems such as Code Division Multiple Access (CDMA) modulation schemes, a plurality of signals are transmitted in a communication system and are amplified simultaneously. When a plurality of signals are applied to a linear amplifier, its non-linear characteristics will tend to produce interaction between the signals being amplified and the amplifier output will contain intermodulation products. Such intermodulation products reduce signal quality by allowing cross-talk to occur and such spillage often falls outside a particular licensed spectrum and must be controlled. Such intermodulation distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components with the amplifier output and feeding forward the distortion component to cancel the distortion of the amplifier output signal.
There are many ways of linearising a high power amplifier: direct RF feedback, envelope feedback, feed-forward and pre-distortion. For cellular power amplifiers, feed-forward amplifiers are commonly used. Feed forward amplifiers are more complicated in that they require the modification of the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis and require an error amplifier which is typically similar in power handling to the main amplifier: this incurs a heavy penalty in RF device cost and power efficiency.
Envelope feedback methods (polar and Cartesian) perform much better than feed-forward amplifiers in terms of device cost and efficiency since the RF signal linearisation processing is done before the power amplifier on a small signal. However, envelope feedback is fundamentally limited in the correction bandwidth obtainable by the delay of the feedback loop. As systems migrate to wider band modulation (e.g. CDMA2000 and WCDMA) a linearisation technology is required which is fundamentally a wideband technique.
Most implementations of pre-distortion are inherently wideband, however the performance achievable has been limited by the difficulty of matching the complex distortion characteristics of typical power amplifier devices with simple analogue pre-distortion networks.
U.S. Pat. No. 4,700,151 (Nagata) provides a baseband (analogue or digital) modulation system and technique which employs a look-up table for adaptation. U.S. Pat. No. 5,049,832 (Cavers) provides a digital pre-distortion arrangement which reduces memory requirements to under 100 complex pairs, with a resultant reduction in convergence time and removes the need for a phase shifter or PLL in a feedback path.
OBJECT OF THE INVENTION
The present invention seeks to provide an improved linear amplifier arrangement which achieves correction over a wide bandwidth with lower system cost and higher efficiency than known techniques. More particularly the present invention seeks to provide a linear amplifier arrangement capable of amplifying and combining a number of frequency carriers or bearers.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the invention, there is provided a linear power amplifier arrangement comprising a high power amplifier, a pre-distortion circuit and a feedback circuit; wherein an input is operable to receive radio frequency RF input signals to the arrangement and is connected to the power amplifier; wherein the feedback circuit compares a sample of the power amplifier signal output with a sample of the input signal to provide error signals, which error signals are employed to modify a set of look-up values; wherein the pre-distortion circuit receives a sample of the RF input signal and gain and phase error signals from the feedback circuit; and wherein the pre-distortion circuit determines gain and phase error correction signals relative to the set of look-up values and the sample of the RF input signal, which gain and phase error correction signals are applied to inputs of RF amplitude and phase modulators; which error correction signals are generated as functions of the RF input signal in such a way that the modulated delayed RF input signal on passing through the high power amplifier emerges with reduced distortion.
In accordance with a second aspect of the invention, there is provided a linear power amplifier arrangement comprising a high power amplifier, a pre-distortion circuit and a feedback circuit; wherein a radio frequency (RF) input is operable to receive RF input signals and is connected to the power amplifier via a directional coupler, a first delay line, an RF amplitude modulator and an RF phase modulator; wherein the feedback circuit comprises a directional coupler operable to sample an output of the amplifier and provide a signal to an amplitude and phase error detector; wherein the pre-distortion circuit comprises a coupled line from the input directional coupler, a power splitter, the outputs of which are connected to a second delay line and an adaptive pre-distortion subsystem; wherein the second delay line is operable to provide a signal to the amplitude and phase error detector; and wherein the pre-distorter subsystem is operable to receive signals from the power splitter via an RF envelope detector and signals relating to gain error and amplitude error from the phase and amplitude error detector and to provide a gain correction signal to a control port of the amplitude modulator; and a phase correction signal to a control port of the phase modulator. The adaptive pre-distorter is therefore capable of generating the correction signals as functions of a tapped RF input signal in such a way that the modulated delayed RF input signal, on passing through the high power amplifier, emerges with reduced distortion.
In order to compensate for changes in the high power amplifier gain and phase distortion characteristic, for example due to temperature or channel frequency changes the pre-distorter operates on an adaptive basis. That is, the pre-distorter adaptively adjusts its gain and phase transfer functions in response to residual gain error and residual phase error signals fed back from an error detection subsystem.
The first delay line is operable to compensate for any delay skew between the signal modulation and the correction signals induced by processing delay in the correction path and the output is delivered as the amplified signal. The architecture proposed provides a method of pre-distorting the input signal to a power amplifier such that the AM-AM and AM-PM distortion generated by the power amplifier is cancelled, producing an output signal with reduced spectral regrowth in adjacent channels.
In accordance with a third aspect of the invention, there is provided a linear power amplifier arrangement comprising a high power amplifier, a pre-disto
McNicol John D
Smith Howard J
Wessel David N
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
To Doris H.
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