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Quadrature Amplitude Modulation: From Basics to Adaptive Trellis-Coded, Turbo-Equalised and Space-Time Coded OFDM, CDMA and MC-CDMA Systems

https://www.dtc.umn.edu/s/resources/gg-se.pdf

A bibliography on nonlinear system identification

A wideband predistortion system compensates for a nonlinear amplifier's frequency and time dependent AM-AM and AM-PM distortion characteristics. The system comprises a data structure in which each element stores a set of compensation parameters (preferably including FIR filter coefficients) for predistorting the wideband input transmission signal. The parameter sets are preferably indexed within the data structure according to multiple signal characteristics, such as instantaneous amplitude and integrated signal envelope, each of which corresponds to a respective dimension of the data structure. To predistort the input transmission signal, an addressing circuit digitally generates a set of data structure indices from the input transmission signal, and the indexed set of compensation parameters is loaded into a compensation circuit which digitally predistorts the input transmission signal. This process of loading new compensation parameters into the compensation circuit is preferably repeated every sample instant, so that the predistortion function varies from sample-to-sample. The sets of compensation parameters are generated periodically and written to the data structure by an adaptive processing component that performs a non-real-time analysis of amplifier input and output signals. The adaptive processing component also implements various system identification processes for measuring the characteristics of the power amplifier and generating initial sets of filter coefficients. In an antenna array embodiment, a single adaptive processing component generates the compensation parameters sets for each of multiple amplification chains on a time-shared basis. In an embodiment in which the amplification chain includes multiple nonlinear amplifiers that can be individually controlled (e.g., turned ON and OFF) to conserve power, the data structure separately stores compensation parameter sets for each operating state of the amplification chain.

Amplifier measurement and modeling processes for use in generating predistortion parameters

A predistortion system comprises a digital compensation signal processing component (DCSP) (52) which predistorts a wideband input transmission signal to compensate for the frequency and time dependent AM-AM and AM-PM distortion characteristics of a non-linear amplifier (64) The DCSP (52) comprises a data structure (52H) in which each element stores a set of compensation parameters (preferably including FIR filter coefficients) for predistorting the input transmission signal The parameter sets are preferably indexed within the data structure (52H) according to multiple signal characteristics, such as instantaneous amplitude and integrated signal envelope, each of which corresponds to a respective dimension of the data structure (52H) The sets of compensation parameters are generated periodically and written to the data structure (52H) by an adaptive control processing and compensation estimator (ACPCE) (70) that performs a non-real-time analysis of amplifier input and output signals The ACPCE (70) also implements various system identification processes for measuring the characteristics of the power amplifier (64) and generating initial sets of filter coefficients

Digital predistortion methods for wideband amplifiers

With the increasing importance of spectral efficiency in mobile communications, the power amplifier linearity has become a critical design issue for nonconstant envelope modulation. Imperfections in quadrature modulators and demodulators-gain and phase imbalance and DC offset-have a crippling effect on amplifier linearization circuits, a fact that has previously been noted experimentally. This paper is the first analysis of these effects on an adaptive predistorter. The primary result is an expression that provides an explicit tradeoff among intermodulation power, accuracy of the quadrature modulator and demodulator, and speed of adaptation. Another useful result is a simple and easily measured error figure for quadrature modulators and demodulators.

The effect of quadrature modulator and demodulator errors on adaptive digital predistorters for amplifier linearization

Amplification of linear modulation schemes, (which exhibit fluctuating envelopes), by high power amplifiers invariably leads to the generation of distortion and intermodulation products. Recent theoretical work has suggested that a complex gain predistorter may be employed to linearize a nonlinear power amplifier. The authors present experimental results demonstrating that a reduction in out-of-band spectra in excess of 20 dB may be achieved by employing digital feedback and a complex gain predistorter. >

W.G. Durtler

Papers

Experimental performance of an adaptive digital linearized power amplifier (for cellular telephony)