Scott Carichner

Synergetic Crest Factor Reduction and Baseband Digital Predistortion for Adaptive 3G Doherty Power Amplifier Linearizer Design

A novel approach for power amplifier (PA) characterization suitable for single iteration digital predistorter synthesis is proposed. This approach consists of synergetic crest factor reduction and baseband digital predistortion to avoid the average power variation at the input of the PA between the characterization and linearization steps. This is achieved by bypassing the crest factor reduction block during the characterization step and by applying it concurrently with digital predistortion in the linearization step. First, the PAs behavior sensitivity to the average input power is evaluated. The limitations of conventional approaches for the PA characterization, in the context of single iteration digital predistortion, are then demonstrated. The performance of the proposed technique is validated experimentally on a 300-W peak PA. The measured improvement of the adjacent channel power ratio at the output of the linearized amplifier is 16 dBc for the conventional approach and 29 dBc for the proposed approach.

Adaptive Echo Cancellation for an On-Frequency RF Repeater using a Weighted Power Spectrum

Adaptive echo cancellation based on a weighted power spectrum is discussed for use in an on-frequency RF repeater with a digital IF stage. Radiated coupling between the input and output antennas cause echoes to be transmitted by a repeater. Echo cancellation, a technique used to reduce the effect, is often based on the auto-correlation of the repeater signal and is effective when the width of the auto-correlation of the input signal is less than the loop delay of the echo path. Narrow bandwidth signals such as EDGE cause problems because the auto-correlations of the input signal and echo overlap. However, in this paper it is shown that the repeater signal can be filtered to reduce the width of the autocorrelation by adjusting the power of all carriers within the input signal to the same power density. A method is presented for partitioning the power spectrum into disjoint bands approximating carrier bandwidths and computing a weighting function that normalizes the carriers. This is done without affecting the ripples within the power spectrum that are needed to detect the echoes. Results based on computer simulation models are provided.

Power Amplifiers' Model Assessment and Memory Effects Intensity Quantification Using Memoryless Post-Compensation Technique

A novel approach for power amplifiers static nonlinearity characterization in the presence of memory effects is presented. A subsampling technique is used to reduce the bandwidth of the test signal without changing the waveform characteristics. This cancels the memory effects of the amplifier without affecting its static behavior. Memoryless AM/AM and AM/PM characteristics are measured as a function of the carrier frequency. This points out the contribution of the frequency dependency of the amplifiers static nonlinearity to its behavior under wideband signal drive. A memoryless post-compensation technique is then introduced to accurately assess the performance of several behavioral models. In fact, it is demonstrated that direct comparison of the measured output of the device-under-test with the estimated output does not provide reliable assessment of the model performance. Conversely, memoryless post-compensation efficiently differentiates the performance of these models. To provide a comprehensive approach for model validation and prototype performance evaluation, memory effects intensity metrics are introduced. These metrics are applied to evaluate the memory effects present in a 300-W peak power Doherty amplifier driven by various multicarrier wideband code division multiple access signals.

An Improved Doherty Amplifier Using Cascaded Digital Predistortion and Digital Gate Voltage Enhancement

This paper describes algorithms used to improve linearity, efficiency, and peak power of a Doherty amplifier whose auxiliary transistor gate voltage is adjusted digitally as a function of the signal envelope. Two predistortion stages are used to compensate for high-order memoryless nonlinearities and low-order memory effects. The digital gate voltage waveform is a sigmoid function of the predistorted signals envelope with two voltage limits corresponding to class C and AB bias levels. The instantaneous gate voltage is controlled by two adjustable parameters: a breakpoint and a slope. The parameters are adjusted to reduce the variance of the AM-AM curve of the power amplifier. The enhancement of the gate voltage at higher signal envelope levels increases the peak power and enhances improves the efficiency of the Doherty amplifier structure. The cascaded digital predistortion ensures sufficient linearity to keep the ACPR2 below - 55 dBc.

Novel approach for static nonlinear behavior identification in RF power amplifiers exhibiting memory effects

In this paper, an experimental approach is proposed to accurately identify, under a modulated signal drive, the memoryless nonlinearity of power amplifiers exhibiting memory effects. It is experimentally demonstrated that, when they are present, memory effects bias the extracted static nonlinearity. Accordingly, the sampling rate of the WCDMA test signal waveform is varied to reduce the signal’s bandwidth. It is shown that this approach minimizes the memory effects contribution to the amplifier’s nonlinear behavior and leads to accurate characterization of the ‘true’ static nonlinearity. The performance of the proposed approach is then assessed through experimental memoryless digital predistortion.

Echo Cancellation for a Wide Bandwidth Mixed-Mode WCDMA/GSM Repeater with Digital Sub-Band Filtering

This paper describes an echo cancellation (EC) algorithm used in a mixed mode WCDMA/GSM repeater with digital sub-band filtering. The EC coefficients are estimated iteratively from cross and power spectrums improving the computational efficiency of the estimation. The effect of regularization and spectral normalization on the convergence of an adaptive EC algorithm is investigated. Singular value decomposition (SVD) is applied to the estimation process as a form of regularization, compensating for the bandwidth reduction associated with the sub-band filter. Spectral normalization used in the estimation of the EC coefficients is shown to avoid convergence problems and the frequency- dependent cancellation that affect the baseline approach. Simulation results show echo cancellation of 23 dB across a 60 MHz bandwidth.