Transmitter Linearization Adaptable to Power-Varying Operation

This paper presents the design of a power-scalable digital predistorter (DPD) for transmitter architectures. The target is to accomplish the joint compensation of impairments due to the I/Q modulator and nonlinearities associated with the power amplifier, and procure a maintained linearization performance in a range of average working operation levels. The identification method for the linearizer parameters enriches the standard least-squares procedure with a synergistic integration with sparsity-based model pruning strategies. The method has been tested with a general complex-valued Volterra model applied to the linearization of two communications transmitters operating at 3.6 GHz. The linearizers designed for the two transmitters effectively provide the joint compensation of the nonlinear behavior. In addition to their good performance in terms of adjacent channel power ratio, the DPDs exhibit a wide range of power-varying adaptation.Comisión Interministerial de Ciencia y Tecnología (CICYT) TEC2014-53103-

Advanced signal processing techniques for the modeling and linearization of wireless communication systems.

Los nuevos estándares de comunicaciones digitales inalámbricas están impulsando el diseño de amplificadores de potencia con unas condiciones límites en términos de linealidad y eficiencia. Si bien estos nuevos sistemas exigen que los dispositivos activos trabajen cerca de la zona de saturación en busca de la eficiencia energética, la no linealidad inherente puede producir que el sistema muestre prestaciones inadecuadas en emisiones fuera de banda y distorsión en banda. La necesidad de técnicas digitales de compensación y la evolución en el diseño de nuevas arquitecturas de procesamiento de señales digitales posicionan a la predistorsión digital (DPD) como un enfoque práctico. Los predistorsionadores digitales se suelen basar en modelos de comportamiento como el memory polynomial (MP), el generalized memory polynomial (GMP) y el dynamic deviation reduction-based (DDR), etc. Los modelos de Volterra sufren la llamada "maldición de la dimensionalidad", ya que su complejidad tiende a crecer de forma exponencial a medida que el orden y la profundidad de memoria crecen. Esta tesis se centra principalmente en contribuir a la rama de conocimiento que enmarca el modelado y linealización de sistemas de comunicación inalámbrica. Los principales temas tratados son el modelo Volterra-Parafac y el modelo general de Volterra para sistemas complejos, los cuales tratan la estructura del DPD y las series de Volterra estructuradas con compressed-sensing y un método para la linealización en un rango de potencias de operación, que se centran en cómo los coeficientes de los modelos deben ser obtenidos.Premio Extraordinario de Doctorado U

Design and Linearization of Energy Efficiency Power Amplifier in Nonlinear OFDM Transmitter for LTE-5G Applications. Simulation and measurements of energy efficiency power amplifier in the presence of nonlinear OFDM transmitter system and digital predistortion based on Hammerstein-Wiener method

This research work has made an effort to understand a novel line of radio frequency power amplifiers (RFPAs) that address initiatives for efficiency enhancement and linearity compensation to harmonize the fifth generation (5G) campaign. The objective is to enhance the performance of an orthogonal frequency division multiplexing-long term evolution (OFDM-LTE) transmitter by reducing the nonlinear distortion of the RFPA. The first part of this work explores the design and implementation of 15.5 W class AB RF power amplifier, adopting a balanced technique to stimulate efficiency enhancement and redeeming exhibition of excessive power in the transmitter. Consequently, this work goes beyond improving efficiency over a linear RF power amplifier design; in which a comprehensive investigation on the fundamental and harmonic components of class F RF power amplifier using a load-pull approach to realise an optimum load impedance and the matching network is presented. The frequency bandwidth for both amplifiers was allocated to operate in the 2.620-2.690 GHz of mobile LTE applications. The second part explores the development of the behavioural model for the class AB power amplifier. A particular novel, Hammerstein-Wiener based model is proposed to describe the dynamic nonlinear behaviour of the power amplifier. The RF power amplifier nonlinear distortion is approximated using a new linear parameter approximation approach. The first and second-order Hammerstein-Wiener using the Normalised Least Mean Square Error (NLMSE) algorithm is used with the aim of easing the complexity of filtering process during linear memory cancellation. Moreover, an enhanced adaptive Wiener model is proposed to explore the nonlinear memory effect in the system. The proposed approach is able to balance between convergence speed and high-level accuracy when compared with behavioural modelling algorithms that are more complex in computation. Finally, the adaptive predistorter technique is implemented and verified in the OFDM transceiver test-bed. The results were compared against the computed one from MATLAB simulation for OFDM and 5G modulation transmitters. The results have confirmed the reliability of the model and the effectiveness of the proposed predistorter.Fundacão para a Ciência e a Tecnologia, Portugal, under European Union’s Horizon 2020 research and innovation programme ... grant agreement H2020-MSCA-ITN- 2016 SECRET-722424 I also acknowledge the role of the National Space Research and Development Agency (NASRDA) Sokoto State Government Petroleum Technology Trust Fund (PTDF

Compensation for Impairments of Frequency Converters in Millimeter Wave Vector Signal Generators

The upcoming fifth generation (5G) of wireless communications aims to utilize millimeter wave (mm-wave) frequencies in its infrastructure to alleviate the crowded spectrum problem below 6 GHz. At higher frequencies, modulation bandwidths of several hundreds of MHz can be utilized to increase system capacities. However, the radio frequency (RF) frontends can exhibit significant amounts of impairments over these wide bandwidths, thereby limiting the achievable output signal quality and capacity. In this work, two signal generation architectures and the accompanying compensation schemes to mitigate the impairments are proposed for the generation of wideband modulated signals at mm-wave frequencies. The frequency dependent IQ imbalance effects in conventional direct conversion signal generation architectures over ultra wide bandwidths are first investigated. For that, a new interleaved multi-tone test signal based identification and compensation scheme is proposed. This scheme was experimentally validated by using an off-the-shelf IQ mixer operating at 30 GHz driven with an interleaved multi-tone signal with 4 GHz of modulation bandwidth and achieving a reduction in the normalized mean squared error (NMSE) from -14 dB to -38 dB. Subsequently, a low-complexity pruned Volterra series based digital predistortion (DPD) scheme was devised to mitigate the nonlinear distortions exhibited by the power amplifier stage and maximize the signal quality of orthogonal frequency division multiplexing (OFDM) signals with modulation bandwidths up to 800 MHz. After compensation of the system with 66 DPD coefficients, the OFDM signal with 800 MHz of modulation bandwidth exhibited an NMSE of -32.4 dB and an adjacent channel power ratio (ACPR) of 45 dBc. However, the challenges associated with the implementation of traditional direct conversion architectures exacerbate as the operating frequency increases. For instance, the performance of high frequency active building blocks, e.g. mixer and amplifiers, deteriorates as the operating frequency approaches the maximum oscillation frequency of the semiconductor technology. To address this challenge, a signal generation system utilizing frequency multipliers to replace the mixer and facilitate frequency upconversion is proposed. A novel Volterra series based behavioural model is also devised to predict the nonlinear behaviour of frequency multipliers and to form the basis for synthesizing a DPD scheme capable of obtaining acceptable signal quality when driven with wideband modulated signals. Various frequency multiplier based signal generation systems were implemented using off-the-shelf frequency doublers, triplers, and quadruplers to serve as proof of concept prototypes. Experiments confirmed the ability to generate modulated signals with competitive error vector magnitudes (EVM) and ACPR levels with low complexity DPD schemes

Modeling and Linearization of MIMO RF Transmitters

Multiple-input multiple-output (MIMO) technology will continue to play a vital role in next-generation wireless systems, e.g., the fifth-generation wireless networks (5G). Large-scale antenna arrays (also called massive MIMO) seem to be the most promising physical layer solution for meeting the ever-growing demand for high spectral efficiency. Large-scale MIMO arrays are typically deployed with high integration and using low-cost components. Hence, they are prone to different hardware impairments such as crosstalk between the transmit antennas and power amplifier (PA) nonlinearities, which distort the transmitted signal. To avert the performance degradation due to these impairments, it is essential to have mechanisms for predicting the output of the MIMO arrays. Such prediction mechanisms are mandatory for performance evaluation and, more importantly, for the adoption of proper compensation techniques such as digital predistortion (DPD) schemes. This has stirred a considerable amount of interest among researchers to develop new hardware and signal processing solutions to address the requirements of large-scale MIMO systems. In the context of MIMO systems, one particular problem is that the hardware cost and complexity scale up with the increase of the size of the MIMO system. As a result, the MIMO systems tend to be implemented on a chip and are very compact. Reduction of the cost by reducing the bill of material is possible when several components are eliminated. The reuse of already existing hardware is an alternative solution. As a result, such systems are prone to excessive sources of distortion, such as crosstalk. Accordingly, crosstalk in MIMO systems in its simplest form can affect the DPD coefficient estimation scheme. In this thesis, the effect of crosstalk on two main DPD estimation techniques, know as direct learning algorithm (DLA) and indirect learning algorithm (ILA), is studied. The PA behavioral modeling and DPD scheme face several challenges that seek cost-efficient and flexible solutions too. These techniques require constant capture of the PA output feedback signal, which ultimately requires the implementation of a complete transmitter observation receiver (TOR) chain for the individual transmit path. In this thesis, a technique to reuse the receiver path of the MIMO TDD transceiver as a TOR is developed, which is based on over-the-air (OTA) measurements. With these techniques, individual PA behavioral modeling and DPD can be done by utilizing a few receivers of the MIMO TDD system. To use OTA measurements, an on-site antenna calibration scheme is developed to individually estimate the coupling between the transmitter and the receiver antennas. Furthermore, a digital predistortion technique for compensating the nonlinearity of several PAs in phased arrays is presented. The phased array can be a subset of massive MIMO systems, and it uses several antennas to steer the transmitted signal in a particular direction by appropriately assigning the magnitude and the phase of the transmitted signal from each antenna. The particular structure of phased arrays requires the linearization of several PAs with a single DPD. By increasing the number of RF branches and consequently increasing the number of PAs in the phased array, the linearization task becomes challenging. The DPD must be optimized to results in the best overall linear performance of the phased array in the field. The problem of optimized DPD for phased array has not been addressed appropriately in the literature. In this thesis, a DPD technique is developed based on an optimization problem to address the linearization of PAs with high variations. The technique continuously optimizes the DPD coefficients through several iterations considering the effect of each PA simultaneously. Therefore, it results in the best optimized DPD performance for several PAs. Extensive analysis, simulations, and measurement evaluation is carried out as a proof of concept. The different proposed techniques are compared with conventional approaches, and the results are presented. The techniques proposed in this thesis enable cost-efficient and flexible signal processing approaches to facilitate the development of future wireless communication systems