Power Amplifier Linearization Technique Research Articles

A Manifold Regularization Approach for Low Sampling Rate Digital Predistortion With Band-Limited Feedback

Digital predistortion (DPD) is an effective linearization technique for RF power amplifiers (PAs), but conventional full sampling (FS) DPD systems use ADCs with three to five times signal bandwidth, and high-speed ADCs are expensive and power-hungry. In this article, we develop a novel band-limited DPD for reducing feedback sampling rate and acquisition bandwidth based on a general framework for semisupervised learning called manifold regularization (MR), which utilizes the geometry of unlabeled data to construct regularization terms for mitigating the overfitting problem. Considering the properties of DPD, we design a basis MR term and introduce it into the classical MR to obtain the extended MR (ExMR) method. To validate the proposed ExMR DPD method, experiments were conducted on two different RF PAs operating at 2.4 and 39 GHz, respectively. The test results demonstrate that the proposed ExMR DPD can linearize the RF PA with a 40-MHz acquisition bandwidth at 100-MHz input. The proposed method significantly reduces the cost and power consumption of the DPD system in comparison with the conventional FS DPD method, which provides a promising solution for broadband communication systems.

An 800 MHz-to-3.3 GHz 20-MHz Channel Bandwidth WPD CMOS Power Amplifier For Multiband Uplink Radio Transceivers

This brief describes a novel Wideband Pre-Distortion (WPD) mechanism as a linearization technique for bandwidth-limited CMOS power amplifiers (PAs). The WPD comprises a common-source amplifier and a hybrid feedback mechanism blended with both active and passive networks to secure a flat gain response from 800 MHz till 3.3 GHz, while maintaining >30% power added efficiency (PAE) at a maximum linear output power of 20 dBm throughout the band of operation. The WPD generates unique gain and phase cancellation mechanisms on-chip therefore alleviating the 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> -order intermodulation product (IMD <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) for an operating bandwidth of 2.5 GHz. Measurement results on 180 nm CMOS, with a supply voltage of 3.3 V indicate that the WPD-PA produces a saturated output power of 24 dBm, in addition to a power gain of 15.5 dB and a peak efficiency of 35.5% at 2.45 GHz. The WPD-PA delivers a maximum linear output power of 20-dBm with an adjacent channel leakage ratio (ACLR) of -30 dBc and error vector magnitude (EVM) of 3.42%, 2.34% and 2.76% at 0.8, 2.45 and 3.3 GHz when measured with the 20-MHz LTE signal. The corresponding maximum linear PAE ranged between 31 to 34%. The chip area is 1.28 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Development of a Parallel-FET Linearization Technique for High Efficiency GaN Power Amplifiers

We demonstrate the design of a linear and high efficient power amplifier (PA) by using different gate bias voltages for parallelly combined Gallium Nitride (GaN) high electron mobility transistors (HEMTs). The experimental results show that the linearized PA achieves the gain flatness of 0.3 dB over a 35 dB power range and has a measured sharp output power at 1 dB compression point (P1dB) of 38.5 dBm, less than 2 dB below the saturated power (PSAT). The measured adjacent channel power ratio (ACPR) of the proposed linearized PA shows up to 8 dB improvements over the PA biased ~ class A (47% $I_{\mathrm {dss}}$ )

Power-Scalable Wideband Linearization of Power Amplifiers

In this paper, a wideband RF power amplifier (PA) linearization technique based on the addition of a linearization amplifier (LA) is proposed. The technique is suitable for low-powered and low-cost small cell and fifth-generation (5G) large-scale multi-antenna PA designs since its power and cost overhead does not increase with the input signal bandwidth and because they are scalable with the power range and cost of the PA. In-depth analysis of the proposed technique is provided and its linearization improvement mechanisms are described. As a proof of concept, a prototype LA was designed and fabricated to linearize a broadband 6-W class-AB PA with a center frequency of 850 MHz. When stimulated by a wideband 40-MHz signal, the PA’s adjacent channel leakage ratio (ACLR) was improved by up to 13 dB after the addition of the LA. This enabled the PA to achieve an ACLR of about −45 dBc without the use of any other linearization techniques. Furthermore, significant ACLR improvements were observed for even signals with wider bandwidths up to 160 MHz.

A 5.9-GHz Fully Integrated GaN Frontend Design With Physics-Based RF Compact Model

This paper presents the design of a fully integrated high-efficiency and high-power RF frontend for the IEEE 802.11p standard in GaN HEMT technology. An embedded transmitter/receiver (Tx/Rx) switching scheme and a dual-bias power amplifier linearization technique are used to improve Tx efficiency and linearity. An accurate physics-based nonlinear large-signal device model is developed and used for the design, providing insight into the impact of the behavioral nuances of the GaN HEMTs on RF circuit performance. The fully integrated RF frontend is fabricated in 0.25- $\mu{\hbox{m}}$ GaN-on-SiC technology and occupies only 2 mm $\times$ 1.2 mm. The Tx branch achieves 48.5% drain efficiency at 33.9 dBm, $P{\rm sat}$ with 28-V supply. With orthogonal frequency-division multiplexing modulated signals, it achieves 30% average efficiency at 27.8-dBm output power while meeting the $-$ 25-dB error vector magnitude limit without predistortion. The Rx branch achieves $+$ 22-dBm output third-order intercept point with 3.7-dB noise figure at 12-V supply. The fully integrated high-efficiency and linear RF frontend designed with physics-based RF GaN compact models is demonstrated for the first time for future device-to-device applications.

Development of Wideband Power Amplifier for RF / Microwave Front-End Subsystem

This paper reviews the Wideband Power Amplifier (PA) that has been developed since 1990. Several journals had been discussed in this paper covers few topics such as Characterization of Power Amplifiers, Power Amplifier Architecture and Linearization Technique. Advantages and disadvantages of the technique used had been highlighted as well as the summary of those cases been compiled in the table form for comparison purposes. Power Amplifier is one of the important parts in transmitter. However, when involve transistor as an active device, it is important to ensure that the signals are stabilized and transmitted at higher efficiency. This leads to the proposal of a new design of Wideband Power Amplifier based on the concept of the multiplexer.

Legendre Wavelet Neural Networks for Power Amplifier Linearization

In this paper, a novel technique for power amplifier (PA) linearization is presented. The Legendre wavelet neural networks (LWNN) is first utilized to model PA and inverse structure of the PA by applying practical transmission signals and the gradient descent algorithm is applied to estimate the coefficients of the LWNN. Secondly, this technique is implemented to identify and optimize the coefficient parameters of the proposed pre-distorter (PD), i.e., the inversion model of the PA. The proposed method is most efficient and the pre-distorter shows stability and effectiveness because of the rich properties of the LWNN. A quite significant improvement in linearity is achieved based on the measured data of the PA characteristics and out power spectrum has been compared.

Theoretical and Experimental Studies of a Probabilistic-Based Memoryless PA Linearization Technique

This paper studies the performance of a memoryless power amplifier (PA) linearization technique based on a probabilistic approach. This technique employs a nonparametric method to derive a predistorter function, which does not need any parametric modeling and explicit parameter estimation. It only needs to calculate a probabilistic cumulative distribution function (CDF) and a quantile function (an inverse function of the CDF). Histogram and order statistic methods are proposed to perform the calculation. A rigorous analytic formula is derived for the inter-modulation product power (IMPP) of the PA output signal when a finite number of samples as well as a finite number of bins are used to calculate the CDF and the quantile function. The analytic results show that, with the probabilistic-based technique, the IMPP approaches zero as the number of samples approaches infinity and the bin width approaches zero.

Mixed Cartesian feedback for Zero-IF WCDMA transmitter

In this paper, a new adaptive power amplifier (PA) linearization technique is presented. The idea is to consider a classic WCDMA zero-intermediate frequency (Zero-IF) transmitter with a modified Cartesian feedback (CFB) loop. The new transmitter architecture consists of an analog stage including forward I/Q modulator and feedback I/Q demodulator, and a digital stage adjusting the phase rotation around the loop. The whole system consumes 500 and 2.94 mW, respectively, for the analog and the digital part. System level simulation gave a maximum improvement of 35 dBc at 5 MHz from the carrier for the W-CDMA signal.

Adaptive Digital Predistortion with Iterative Noise Cancelation for Power Amplifier Linearization

In this paper, we propose a power amplifier linearization technique combined with iterative noise cancelation. This method alleviates the effect of added noises which prevents the predistorter (PD) from estimating the exact characteristics of the power amplifier (PA). To iteratively cancel the noise added in the feedback signal, the output signal of the power amplifier without noise is reconstructed by applying the inverse characteristics of the PD to the predistorted signals. The noise can be revealed by subtracting the reconstructed signals from the feedback signals. Simulation results based on the mean-square error (MSE) and power spectral density (PSD) criteria are presented to evaluate PD performance. The results show that the iterative noise cancelation significantly enhances the MSE performance, which leads to an improvement of the out-of-band power suppression. The performance of the proposed technique is verified by computer simulation and hardware test results.

Power amplifier linearization technique with IQ imbalance and crosstalk compensation for broadband MIMO-OFDM transmitters

The design of predistortion techniques for broadband multiple input multiple output-OFDM (MIMO-OFDM) systems raises several implementation challenges. First, the large bandwidth of the OFDM signal requires the introduction of memory effects in the PD model. In addition, it is usual to consider an imbalanced in-phase and quadrature (IQ) modulator to translate the predistorted baseband signal to RF. Furthermore, the coupling effects, which occur when the MIMO paths are implemented in the same reduced size chipset, cannot be avoided in MIMO transceivers structures. This study proposes a MIMO-PD system that linearizes the power amplifier response and compensates nonlinear crosstalk and IQ imbalance effects for each branch of the multiantenna system. Efficient recursive algorithms are presented to estimate the complete MIMO-PD coefficients. The algorithms avoid the high computational complexity in previous solutions based on least squares estimation. The performance of the proposed MIMO-PD structure is validated by simulations using a two-transmitter antenna MIMO system. Error vector magnitude and adjacent channel power ratio are evaluated showing significant improvement compared with conventional MIMO-PD systems.

Some Results of the Analogue Passive Linealizer in a OFDM System

In this paper, starting from the baseband model of a OFDM system, we study through simulation the performance of passivity-based RF power amplifier linearization technique. Obtained result are highly more favourable than reported using more consolidated techniques.

Frequency-Selective I/Q Mismatch Calibration of Wideband Direct-Conversion Transmitters

The current trend in building low-cost yet flexible radio transceivers is to use the so-called direct-conversion principle, which is based on complex (I/Q) up- and down conversions. Such transceivers are, however, sensitive to mismatches between the I and Q branches. These mismatches are unavoidable in any practical implementation, and result in finite attenuation of the mirror frequencies. In addition to the mirror-frequency interference problem, I/Q mismatches can severely compromise the performance of power amplifier linearization techniques based on pre-distortion. The effects of these impairments are becoming more pronounced as higher order modulated waveforms and/or more wideband multichannel signals are used. This brief focuses on digital-signal-processor-based I/Q mismatch calibration in wideband direct-conversion transmitters, assuming the challenging case of frequency-dependent I/Q mismatch. First, a novel widely linear (WL) calibration structure is introduced, suitable for frequency-dependent calibration. Then, two alternative principles for calibration parameter estimation are proposed. The first estimation approach stems from second-order statistics of complex communication signals, while the second technique is based on WL least-squares model fitting. Both estimators are shown by simulations to yield very good calibration performance. The obtainable performance is further assessed using laboratory RF signal measurements.

Effects of PAPR reduction on HPA predistortion

There have been many papers concerning about the peak to average power ratio (PAPR) reduction and high power amplifier (HPA) linearization techniques for wireless OFDM communication systems. Few papers, however, make deep analysis on the relation between these two techniques. Based on Saleh's memoryless traveling wave tube amplifier (TWTA) model, computer simulations and analysis in this paper demonstrates that, especially when the HPA is working near or in the saturation region, a predistorter ceases to be in effect. In such cases, PAPR reduction techniques may be resorted to help to improve the adjacent channel interference (ACI), BER performances and HPA efficiency.

Optimization of feedforward amplifier power efficiency on the basis of drive statistics

Among power amplifier linearization techniques, feedforward delivers the best performance, but at the cost of significant degradation in the amplifier's power efficiency. This paper details a procedure to design feedforward amplifiers for optimal DC-RF conversion efficiency. The procedure has the convenience of requiring only the power statistics of the driving signal, of being computationally efficient, and of lending itself to a highly intuitive graphical representation. The gains and normalized saturated output powers of the main and error amplifiers, as well as the various couplings, are optimized for a specified linearity and gain at the output of the system. The amplifier types and adaptation methods employed must be specified at the outset, but results are presented that begin to reveal the impact of these factors on efficiency and, thus, to demonstrate the investigative potential of the procedure.

Linearized transmitters: an enabling technology for software defined radio

Power amplifier and transmitter linearization techniques are now a mature technology, with feedforward systems installed in many US base station sites for both TDMA and CDMA systems. Similarly, transmitter linearization techniques, such as Cartesian loop and predistortion, have been employed in mobile and portable equipment, and these have enabled a number of systems (e.g., iDEN in the United States and TETRA in Europe) to be realizable from a power consumption, cost, and size perspective. Such techniques are essential in the realization of an efficient and cost-effective software-defined radio system, whether deployed in a base station or a handset, and are thus a key enabling technology, without which SDR will not succeed. This article examines the current status of power amplifier and transmitter linearization technologies for mobile and base station equipment, and highlights some of the novel base station and network topologies now emerging based on these techniques. In many cases, these new topologies will revolutionize the way a cellular network is constructed and lead to very substantial cost reductions for a network operator.

FET diode linearizer optimization for amplifier predistortion in digital radios

This letter presents the initial results of a study on a diode-based power amplifier (PA) linearization technique. The optimization of a diode's parameters to match a MESFET PA is discussed. The relevance of AM-AM and AM-PM curves to linearity requirements and modulation format is also discussed. As much as a 6.7-dB improvement in ACPR is made in the upper channel using a series diode linearizer.

RF power amplifier linearization through amplitude and phase predistortion

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.