Karun Rawat

Design and Linearization of Concurrent Dual-Band Doherty Power Amplifier With Frequency-Dependent Power Ranges

A design methodology for a concurrent dual-band Doherty power amplifier (PA) with frequency-dependent backoff power ranges is presented in this paper. Based on a dual-band T-shaped network and a coupled line network, different dual-band components needed in Doherty PA topology, including a 3-dB branch-line coupler, an offset line, and a quarter-wavelength transformer, are developed. Two prototypes with balanced and imbalanced backoff power range modes are implemented to verify the feasibility. Continuous wave signal test results show that the proposed dual-band PA successfully achieves a power-added efficiency of 33% and 30% at the 6-dB backoff point from the saturated output power at 880 and 1960 MHz, respectively. To meet linearity requirements, the PA nonlinear behavior is characterized by using digital multitone signals, which categorize the distortions of a concurrent dual-band PA into intermodulation and cross-modulation. Finally, a 2-D digital predistortion technique is used to compensate for the nonlinearity of PA in dual bands. Two two-tone signals are applied to the dual bands for linearization, and the experimental results show that this technique achieves improvements of better than 19.1 and 24.6 dB for the intermodulation and cross-modulation in the dual bands, respectively.

Adaptive Digital Predistortion of Wireless Power Amplifiers/Transmitters Using Dynamic Real-Valued Focused Time-Delay Line Neural Networks

Neural networks (NNs) are becoming an increasingly attractive solution for power amplifier (PA) behavioral modeling, due to their excellent approximation capability. Recently, different topologies have been proposed for linearizing PAs using neural based digital predistortion, but most of the previously reported results have been simulation based and addressed the issue of linearizing static or mildly nonlinear PA models. For the first time, a realistic and experimentally validated approach towards adaptive predistortion technique, which takes advantage of the superior dynamic modeling capability of a real-valued focused time-delay neural network (RVFTDNN) for the linearization of third-generation PAs, is proposed in this paper. A comparative study of RVFTDNN and a real-valued recurrent NN has been carried out to establish RVFTDNN as an effective, robust, and easy-to-implement baseband model, which is suitable for inverse modeling of RF PAs and wireless transmitters, to be used as an effective digital predistorter. Efforts have also been made on the selection of the most efficient training algorithm during the reverse modeling of PA, based on the selected NN. The proposed model has been validated for linearizing a mildly nonlinear class AB amplifier and a strongly nonlinear Doherty PA with wideband code-division multiple access (WCDMA) signals for single- and multiple-carrier applications. The effects of memory consideration on linearization are clearly shown in the measurement results. An adjacent channel leakage ratio correction of up to 20 dB is reported due to linearization where approximately 5-dB correction is observed due to memory effect nullification for wideband multicarrier WCDMA signals.

Design Methodology for Dual-Band Doherty Power Amplifier With Performance Enhancement Using Dual-Band Offset Lines

This paper proposes a design methodology for dual-band Doherty power amplifier (DPA) with performance enhancement using dual-band phase offset lines. In the proposed architecture, 50-dual-band offset lines with arbitrary electric lengths at two frequencies are key components, and a novel analytical design solution has been proposed for their design and implementation. The methodology is validated with the design and fabrication of a 10-W GaN-based DPA for code division multiple access and Worldwide Interoperability for Microwave Access applications at 1960 and 3500 MHz, respectively. The dual-band Doherty amplifier using the proposed design methodology has better performance than the current state of the art. The peak drain efficiency of the amplifier is 59.5% at the first frequency and 49.6% at the second frequency of operation. Compared to balanced mode operation, there is an improvement of more than 10% in drain efficiencies, around 6.5-dB back-off, at both frequencies.

A Design Methodology for Miniaturized Power Dividers Using Periodically Loaded Slow Wave Structure With Dual-Band Applications

This paper proposes an analytically-based approach for the design of a miniaturized single-band and dual-band two-way Wilkinson power divider. This miniaturization is achieved by realizing the power dividers impedance transformers using slow wave structures. These slow wave structures are designed by periodically loading transmission lines with capacitances, which reduces the phase velocity of the propagating waves and hence engender higher electric lengths using smaller physical lengths. The dispersive analysis of the slow wave structure used is included in the design approach to ensure a smooth nondispersive transmission line operation in the case of dual-band applications. The design methodology is validated with the design of a single-band, reduced size, two-way Wilkinson power divider at 850 and 620 MHz. An approximate length reduction of 25%-35% is achieved with this technique. For dual-band applications, this paper describes the design of a reduced size, two-way Wilkinson power divider for dual-band global system for mobile communications and code division multiple access applications at 850 and 1960 MHz, respectively. An overall reduction factor of 28%, in terms of chip area occupied by the circuit, is achieved. The electromagnetic simulation and experimental results validate the design approach. The circuit is realized with microstrip technology, which can be easily fabricated using conventional printed circuit processes.

Dual-Band RF Circuits and Components for Multi-Standard Software Defined Radios

The advent of multi-standard and multi-band software defined radio (SDR) applications has necessitated the design and deployment of dual-band RF components and circuits considering the numerous advantages of such designs over the traditional narrow band circuits and components. For example, a dual-band power amplifier (PA) not only simplifies the hardware complexity but also provides higher reconfigurability [1] and hence makes it a front runner for deployment in SDR architectures [2][3]. Furthermore, the evolution of communication technologies demands the use of dualband/ multi-band RF circuits so as to acc ommodate many standards simultaneously for facilitating and guaranteeing the backward compatibility of future standards (such as 4 G) based system for smooth network migration and upgrades. These technological requirements have also led to commercial introduction of dual-band base stations and repeaters [4][6]. Furthermore, the advancement in CMOS and other MMIC technologies, although, is challenging the transmission line based passive circuit techniques but the high power handling ability of transmission line based circuits are potentially very useful in applications such as the design of high power/high efficiency PAs and transmitters. This article elaborates on the techniques employed in the design of transmission line based dual-band RF components in the context of multi-band/multi-mode SDR architecture, highlighting the problems which need to be addressed during the design process.

Compensating I–Q Imperfections in Hybrid RF/Digital Predistortion With an Adapted Lookup Table Implemented in an FPGA

The performance of hybrid RF/digital predistortion (RF-DPD) is limited, due to in-phase (I) and quadrature-phase (Q) imperfection in its key component, the RF vector multiplier, and the associated circuitry. These imperfections cause errors, in terms of implemented gain and phase of the predistortion function. This brief presents the methodology of implementing hybrid RF-DPD with a lookup table (LUT) adapted to compensate for hardware related I-Q imperfections of the RF vector multiplier within the digital signal processing domain. This modified LUT will accurately compensate for I-Q imperfection, without needing a precise tuning of the control voltages at the pins of the RF vector multiplier. This brief also presents the test setup for characterizing the RF-DPD system to obtain the I-Q imperfections within it and utilizes this information to modify the LUT to compensate for these imperfections. To verify the capability of the modified LUT in compensating for the I-Q imperfections, an experimental validation is carried out by linearizing a class-AB base station power amplifier using the hybrid RF-DPD system developed with an Altera Stratix field-programmable gate array (FPGA) evaluation board. In addition to the 12-dB adjacent-channel leakage ratio obtained using static RF-DPD, an improvement of 2.5 dB is achieved using the proposed I-Q compensation technique.

Three-Layered Biased Memory Polynomial for Dynamic Modeling and Predistortion of Transmitters With Memory

This paper proposes a new three-layered biased memory polynomial for behavioral modeling and digital predistortion of highly nonlinear transmitters/power amplifiers (PAs) for 3G wireless applications. The proposed model considers the possibility that the nonlinearity order of the dynamic part of the PA characteristics is different from the nonlinearity order of the static part. For highly nonlinear PAs, the proposed model offers some benefits, such as a low dispersion of coefficients, numerical stability and a low number of coefficients. Moreover, with the measurement setup, better in-band performance is reported. To establish the performance of the linearized PA under realistic conditions, experiments have been carried out for a deep biased class-AB PA and a Doherty PA for various modeling and signal quality norms defined for 3G signals.

A Ray Launching-Neural Network Approach for Radio Wave Propagation Analysis in Complex Indoor Environments

A novel deterministic approach to model the radio wave propagation channels in complex indoor environments reducing computational complexity is proposed. This technique combines a neural network and a 3-D ray launching algorithm in order to compute wireless channel performance in indoor scenarios. An example of applying the method for studying indoor radio wave propagation is presented and the results are compared with a very high resolution fully 3-D ray launching simulation as the reference solution. The new method allows the use of a lower number of launched rays in the simulation scenario whereas intermediate points can be predicted using neural network. Therefore a high gain in terms of computational efficiency (approximately 80% saving in simulation time) is achieved.

Generalized Rational Functions for Reduced-Complexity Behavioral Modeling and Digital Predistortion of Broadband Wireless Transmitters

In this paper, we present and analyze rational-function-based digital predistortion (DPD) of transmitters for broadband applications where system noise and prominent memory effects contribute to the overall nonlinearity of the system. The performance is reported for simulation and measured results for gallium nitride (GaN)-based class-AB and laterally diffused MOS (LDMOS)-based Doherty power amplifiers (PAs) using three different wideband code division multiple access signals with peak-to-average-power ratios of around 10 dB. The performance of the proposed model, in terms of normalized mean-square error, adjacent channel power ratio, matrix condition number, and coefficient dispersion, is compared against those of a memory polynomial (MP) model and a previously proposed rational-function-based model. It is shown by simulation and measurement that the previously proposed absolute-term denominator rational functions have limitations in the inverse modeling needed for DPD. A new variation of the rational function is proposed to alleviate this limitation. Depending on the type of PA and signals, a floating-point operation reduction of 8%-38% is reported as compared with a low-complexity MP model.

Highly Reflective Load-Pull

This article presented a review of the most common load-pull approaches adopted in the measurement and characterization of large-periphery microwave transistor devices. Although the quarterwave transformer technique is simple and very cost-effective, it has serious limitations in terms of scalability, thereby necessitating a separate quarterwave transformer for each device and frequency. Furthermore, due to the limited bandwidth of the quarterwave transformer, its usage in harmonic load-pull applications is severely limited.