Silong Zhang

A Concurrent Dual-Band Uneven Doherty Power Amplifier with Frequency-Dependent Input Power Division

A concurrent dual-band uneven GaN Doherty power amplifier (PA) for two wide-spacing frequencies application is proposed in this paper. To avoid an early load modulation-drop caused by the soft turn-on characteristic of the peaking device, an adaptive power division is realized by a frequency-dependent uneven power divider as well as the input matching nonlinearities of the two cells in Doherty PA. Due to the adaptive power division, the proposed dual-band uneven Doherty PA achieves a power-added efficiency of 45% and 41% at the 6 dB backoff from the saturation at 850 MHz and 2330 MHz, respectively, the gain of the proposed Doherty PA is also enhanced to 19 dB and 13 dB in the dual bands. Furthermore, a more accurate two-dimensional joint digital predistortion model (2D-JDPD) is applied to linearize the PA and compensate for the in-phase and quadrature (I/Q) imbalance simultaneously. With this new model, the adjacent channel power ratio (ACPR) is improved to better than -47.1 dBc and -49.4 dBc in the lower and upper bands at an average output power of 31.75 dBm, and a drain efficiency of 26.7% is obtained at the same time.

Efficient Pruning Technique of Memory Polynomial Models Suitable for PA Behavioral Modeling and Digital Predistortion

This paper proposes an error variation ranking (EVR)-based pruning method to reduce the complexity of memory polynomials (MPs) for power amplifier behavioral modeling. During the EVR pruning, the variation of prediction error caused by removing each term is calculated and ranked as a quantification factor to show the terms importance. The dominant terms are then selected based on their ranking positions among all terms. This method is verified by comparing its results with all other possible selections under the same conditions. When it is used to prune digital predistorters, approximately 74% of the terms in the MP model and 78% of the terms in the 2-D digital-predistortion model can be removed with negligible deterioration of the prediction and linearization performance. Moreover, further discussion is presented to strategize the configuration of MP models based on the EVR pruning results.

An iterative pruning of 2-D digital predistortion model based on normalized polynomial terms

This paper proposes a new method to prune the kernels of 2-D digital preditortion (2-D-DPD) model. The conventional 2-D-DPD model is rewritten as a new form with each of the polynomial terms normalized for further pruning. Then an iteration based characterization process is taken to prune the terms of the model. Two experiments on different PAs were taken to verify the method. Both of them prove the proposed method works effectively to prune the terms of 2-D-DPD model without losing modeling precision. The numbers of coefficients in the model were reduced from 24 to 9 and from 30 to 13 while only about 1dB adjacent channel power ratio (ACPR) and 1dB normalized mean square error (NMSE) were sacrificed.

A Time Misalignment Tolerant 2D-Memory Polynomials Predistorter for Concurrent Dual-Band Power Amplifiers

This letter proposes a modified 2-D memory polynomial predistorter for concurrent dual band power amplifiers, which is tolerant to the time misalignment between the two bands. The new model is derived from the prior 2-D DPD model by substituting the interpolation expression into it. For performance validation, the proposed method is applied to a dual-band transmitter. In the presence of 4.87 samples time-misalignment existing between the two input signals, the proposed method achieves normalized mean square error of -40 dB and adjacent channel power ratio improvement of about 15 dB in the dual bands. Further experiment shows the proposed 2-D predistorter outperforms 2D-GMP in terms of complexity and signal quality.

A Band-Divided Memory Polynomial for Wideband Digital Predistortion With Limited Bandwidth Feedback

With a real analog bandpass filter (BPF) in the feedback loop, band-limited digital predistortion (DPD) normally uses a narrower bandwidth low-pass filter at baseband to further constrain the bandwidth of the power amplifier (PA) output signal. This discards the nonlinear information at the BPFs roll-off regions. The PAs spectral regrowth at the edge of the BPF is thus unable to be suppressed with the conventional band-limited DPD. To make efficient use of digital-to-analog converter/analog-to-digital converter sampling rate without losing information at the BPFs roll-off regions, this brief proposes a band-divided DPD method, in which the restricted feedback bandwidth is divided into two parts in frequency domain-center and border regions of PAs band-limited output. These two parts are characterized separately using two different basis functions with a band-divided memory polynomial (BDMP) model. It provides higher flexibility in selecting the digital band-limiting filters bandwidth and also reduces the filter order. The measurement results for a class-AB GaN PA with orthogonal frequency division multiplexing signal of 60-MHz bandwidth have shown the superiority of BDMP DPD over the previous state-of-the-art.

Dual-band Predistortion Linearization of an Envelope Modulated Power Amplifier Operated in Concurrent Multi-Standard Mode

This paper reports a dual band predistortion linearization technique for an envelope modulator based transmitter operated in concurrent multi-band, multi-standard mode driven by 20 MHz bandwidth LTE and 4 carrier 20 MHz bandwidth WCDMA signals. To reduce the speed of the composite envelope path, a bandwidth reduced combined envelope signal is used for the dynamic supply of the dual-band PA. Measurement results using the dual-band linearization technique show an increase of over 10 dB in ACPR and 15% in average operating efficiency.

Concurrent Multi-Band Envelope Modulated Power Amplifier Linearized Using Extended Phase-Aligned DPD

This paper reports an envelope tracking (ET) transmitter architecture for multi-band/widely-spaced carrier signals. Due to the bandwidth limitations of the envelope modulator, the dynamic supply voltage is controlled by combining the low frequency envelopes of the multi-inputs rather than the true composite envelope of the input signals. The ET transmitter is tested for dual-band and tri-band power amplifiers (PAs) operating in concurrent mode using various bandwidth carrier aggregated long term evolution (LTE) signals. The transmitter is linearized using a phase-aligned Volterra digital predistortion that considers the inter-modulation distortion (IMD) products around the carrier frequencies as well as the third-order IMD products in the tri-band case. Experimental results are shown to validate the proposed techniques for linearization and efficiency enhancement of the multi-band ET transmitter and compared to the case of fixed supply biasing of the PA.

A robust and low sampling rate digital predistortion algorithm for broadband PA modeling and predistortion

A robust and low sampling rate digital predistortion (DPD) algorithm for broadband power amplifiers (PAs) is proposed in this paper. To alleviate the burden on the sampling rate, an approximated model extracted by bandlimited signals is employed to approach the broadband behavior. Due to the assistance of the low pass filter (LPF) in the feedback loop and the adaptive basis function scheme, the sampling rate can be reduced drastically and the algorithm is more robust. Experimental results show that more than 21 dB adjacent channel power ratio (ACPR) improvement is achieved for 60MHz 3-carrier LTE signal application only with 92.16Msps sampling rate, and the cavity RF filter in the feedback loop is not necessary any more.

Low sampling rate digital predistortion of power amplifier assisted by bandpass RF filter

In this paper, a low sampling rate digital predistortion of power amplifier assisted by bandpass RF filter is demonstrated. Owing to RF filter integrated after power amplifier, the spectral regrowth of output signals is restricted into a limited bandwidth, which alleviates the burden of ADC sampling rate efficiently. By adopting a band-limited memory polynomial(BLMP) model, the bandwidth of every product terms in memory polynomial model is controlled with a low-pass FIR filter. A GaN Doherty power amplifier excited by a 40 MHz 2-carrier LTE-A Signal is employed to validate this approach. With 2 times over-sampling, a 70 MHz band-pass RF filter and a 35 MHz low-pass FIR filter, the output ACPR is successfully improved from -34.9 dBc to -46.0 dBc with a sampling rate of only 92.16 Msps. The experiment result shows that this approach works much better than the traditional methodologies based on memory polynomial(MP) in the situation of wideband application.

Extraction of wideband behavioral model of power amplifier with multi groups of narrow band signals

This paper proposes a method to extract the wideband model of power amplifiers (PAs) with narrow band signals, so that the sampling rate of the analog digital converter (ADC) can be even lower than the bandwidth of the signal. By sweeping the frequency response with narrow band signals and combining the envelopes of input and output signals, a wideband model of PA is observed. The model can be used as both wideband simulator and digital predistorter. Experiments are performed on a wideband PA. The PA is swept with 17 groups of 10M LTE signals at 30.72Msps sample rate. When verifying the wideband model with 40MHz LTE-A signal at 184.72msps, the normalized mean square error (NMSE) of the forward model is -32.77 dB. And the feedback model successfully eliminates the adjacent channel power ratio (ACPR) of PA output signal from -30.3dBc to -45.5dBc when it is stimulated with 40M LTE-A signal.