31 research outputs found
Characterization of systems for software defined radio
Mestrado em Engenharia Electrónica e TelecomunicaçõesEsta dissertação insere-se na área de electrónica de rádio frequência, mais
precisamente na caracterização de sistemas para rádios definidos por software
(SDR). Um SDR é aquele que possui a flexibilidade para sintonizar, filtrar,
ajustar a taxa de transmissão e controlar o tipo de modulação através de
software. O aparecimento de novas tecnologias no mercado obriga à utilização
de uma quantidade considerável de hardware nos dispositivos de
transmissão/recepção, assim uma solução consiste no uso de arquitecturas de
SDR onde a conversão do sinal analógico para digital é executada o mais
próximo possÃvel da antena e, sendo depois todo o processamento efectuado
digitalmente. Assim, nesta tese, é apresentado um modelo comportamental
para receptores de SDR, que leva em conta os elementos chave da distorção
não linear. Além disso, são apresentadas algumas comparações entre
simulações e medidas usando sinais multi-seno e WiMax usando um receptor
ideal de SDR. Finalmente, é proposto um novo sistema de caracterização para
dispositivos de SDR.
ABSTRACT: This dissertation is related to the radio frequency area, more specifically to the
characterization of systems for software-defined radio. A software-defined radio
is one that has the flexibility to tune, filter, set the transmission rate and control
the modulation type only by software. The emergence of new technologies in
the market forces the use of a considerable quantity of hardware in the
transceivers systems, so a viable solution for this is to use SDR solutions
where the analogue to digital conversion is made closest possible of the
antenna and then make all the processing digitally. So, in this dissertation, a
behavioral model for SDR front end receiver evaluation, that captures the key
elements of the nonlinear distortion, is proposed. Moreover, some comparisons
between measured and simulated results under multisine and WiMax
excitations are presented using the ideal SDR receiver. Finally, a new
instrumentation system for characterization of SDR front ends is proposed
Nonlinear models and algorithms for RF systems digital calibration
Focusing on the receiving side of a communication system, the current trend in pushing the digital domain ever more closer to the antenna sets heavy constraints on the accuracy and linearity of the analog front-end and the conversion devices. Moreover, mixed-signal implementations of Systems-on-Chip using nanoscale CMOS processes result in an overall poorer analog performance and a reduced yield. To cope with the impairments of the low performance analog section in this "dirty RF" scenario, two solutions exist: designing more complex analog processing architectures or to identify the errors and correct them in the digital domain using DSP algorithms. In the latter, constraints in the analog circuits' precision can be offloaded to a digital signal processor.
This thesis aims at the development of a methodology for the analysis, the modeling and the compensation of the analog impairments arising in different stages of a receiving chain using digital calibration techniques.
Both single and multiple channel architectures are addressed exploiting the capability of the calibration algorithm to homogenize all the channels' responses of a multi-channel system in addition to the compensation of nonlinearities in each response. The systems targeted for the application of digital post compensation are a pipeline ADC, a digital-IF sub-sampling receiver and a 4-channel TI-ADC.
The research focuses on post distortion methods using nonlinear dynamic models to approximate the post-inverse of the nonlinear system and to correct the distortions arising from static and dynamic errors. Volterra model is used due to its general approximation capabilities for the compensation of nonlinear systems with memory. Digital calibration is applied to a Sample and Hold and to a pipeline ADC simulated in the 45nm process, demonstrating high linearity improvement even with incomplete settling errors enabling the use of faster clock speeds.
An extended model based on the baseband Volterra series is proposed and applied to the compensation of a digital-IF sub-sampling receiver. This architecture envisages frequency selectivity carried out at IF by an active band-pass CMOS filter causing in-band and out-of-band nonlinear distortions. The improved performance of the proposed model is demonstrated with circuital simulations of a 10th-order band pass filter, realized using a five-stage Gm-C Biquad cascade, and validated using out-of-sample sinusoidal and QAM signals. The same technique is extended to an array receiver with mismatched channels' responses showing that digital calibration can compensate the loss of directivity and enhance the overall system SFDR.
An iterative backward pruning is applied to the Volterra models showing that complexity can be reduced without impacting linearity, obtaining state-of-the-art accuracy/complexity performance.
Calibration of Time-Interleaved ADCs, widely used in RF-to-digital wideband receivers, is carried out developing ad hoc models because the steep discontinuities generated by the imperfect canceling of aliasing would require a huge number of terms in a polynomial approximation. A closed-form solution is derived for a 4-channel TI-ADC affected by gain errors and timing skews solving the perfect reconstruction equations. A background calibration technique is presented based on cyclo-stationary filter banks architecture. Convergence speed and accuracy of the recursive algorithm are discussed and complexity reduction techniques are applied
Optimization of the Crest Factor for Complex-Valued Multisine Signals
Multisine signals are commonly used in the measurement of dynamic systems and wireless channels. For optimal measurements with a high dynamic range, a low Crest Factor (CF) excitation signal is required. In this paper, a modified approach to optimize the crest factor for complex-valued multisine signals is presented. The approach uses a nonlinear optimization method where the real and imaginary parts can also be optimized for low CF. Furthermore, extensions of the real-valued multisine CF optimization methods are presented for complex-valued cases. The proposed methods are validated and compared using simulations. Based on the results it is shown that the novel approach can lead to more optimal signal design and lower CF compared to other techniques for complex-valued multisine signals
Characterization and modelling of software defined radio front-ends
Doutoramento em Engenharia ElectrotécnicaO presente trabalho tem por objectivo estudar a caracterização e modelação
de arquitecturas de rádio frequência para aplicações em rádios definidos por
software e rádios cognitivos. O constante aparecimento no mercado de novos
padrões e tecnologias para comunicações sem fios têm levantado algumas
limitações à implementação de transceptores rádio de banda larga. Para além
disso, o uso de sistemas reconfiguráveis e adaptáveis baseados no conceito
de rádio definido por software e rádio cognitivo assegurará a evolução para a
próxima geração de comunicações sem fios. A ideia base desta tese passa por
resolver alguns problemas em aberto e propor avanços relevantes, tirando
para isso partido das capacidades providenciadas pelos processadores digitais
de sinal de forma a melhorar o desempenho global dos sistemas propostos.
Inicialmente, serão abordadas várias estratégias para a implementação e
projecto de transceptores rádio, concentrando-se sempre na aplicabilidade
especÃfica a sistemas de rádio definido por software e rádio cognitivo. Serão
também discutidas soluções actuais de instrumentação capaz de caracterizar
um dispositivo que opere simultaneamente nos domÃnios analógico e digital,
bem como, os próximos passos nesta área de caracterização e modelação.
Além disso, iremos apresentar novos formatos de modelos comportamentais
construÃdos especificamente para a descrição e caracterização não-linear de
receptores de amostragem passa-banda, bem como, para sistemas nãolineares
que utilizem sinais multi-portadora.
Será apresentada uma nova arquitectura suportada na avaliação estatÃstica
dos sinais rádio que permite aumentar a gama dinâmica do receptor em
situações de multi-portadora. Da mesma forma, será apresentada uma técnica
de maximização da largura de banda de recepção baseada na utilização do
receptor de amostragem passa-banda no formato complexo.
Finalmente, importa referir que todas as arquitecturas propostas serão
acompanhadas por uma introdução teórica e simulações, sempre que possÃvel,
sendo após isto validadas experimentalmente por protótipos laboratoriais.This work investigates the characterization and modeling of radio frequency
front-ends for software defined radio and cognitive radio applications. The
emergence of new standards and technologies in the wireless communications
market are raising several issues to the implementation of wideband
transceiver systems. Also, reconfigurable and adaptable systems based on
software defined and cognitive radio models are paving the way for the next
generation of wireless systems. In this doctoral thesis the fundamental idea is
to address the particular open issues and propose appropriate advancements
by exploring and taking profit from new capabilities of digital signal processors
in a way to improve the overall performance of the novel schemes.
Receiver and transmitter strategies for radio communications are summarized
by concentrating on the usability for software defined radio and cognitive radio
systems. Available instrumentation and next steps for analog and digital radio
frequency hardware characterization is also discussed.
Wideband behavioral model formats are proposed for nonlinear description and
characterization of bandpass sampling receivers, as well as, for multi-carrier
nonlinear systems operation. The proposed models share a great flexibility and
have the freedom to be simply expanded to other fields.
A new design for receiver dynamic range improvement in multi-carrier
scenarios is proposed, which is supported on the useful wireless signals
statistical evaluation. Additionally, receiver-side bandwidth maximization based
on higher-order bandpass sampling approaches is evaluated.
All the proposed designs and modeling strategies are accompanied by
theoretical backgrounds and simulations whenever possible, being then
experimentally validated by laboratory prototypes
Sensores passivos para aplicações espaciais
Mestrado em Engenharia Eletrónica e TelecomunicaçõesUma das áreas, se não a principal área, com maior desenvolvimento nos
últimos anos e a exploração espacial. A entrada de empresas privadas no
negocio aliadas aos novos meios de comunicação reacenderam a curiosidade
sobre o espaço.
Esta dissertação surge com o intuito de desenvolver um sistema de comunicação passivo, com capacidades de monitorização e de processamento para aplicações espaciais. Para tal quer-se utilizar conceitos tais como: antenas, projecto de formas de onda, transmissão de energia sem os, circuitos de RF-DC, radio retrodifusão , modulação e desmodulação de sinais.
Para se chegar a um sistema funcional, pretende-se analisar e testar diferentes
soluções para cada uma das partes do sistema. Quer-se que o trabalho seja o mais abrangente possÃvel, e que aborde todos as partes necessárias para o desenvolvimento do sistema. No entanto e devido a complexidade do mesmo, este trabalho e focado em quatro pontos: antenas, circuitos de RFDC, circuitos de retrodifusão e microcontroladores. Os restantes aspectos são abordados mas superficialmente.
Para alem de toda a parte hardware do sistema, também se pretende desenvolver uma solução otimizada a nÃvel do software, de modo a que a solução nal tenha o melhor rendimento possÃvel.
Inicialmente o sistema ser a projectado para aplicações espaciais, no entanto espara-se que o mesmo possa ser usado em outras áreas.One of the areas, if not the principal area, with higher development in
recent year is space exploration. The entry of more private companies in
the business allied with new ways of communication reignited the curiosity
about the space.
This dissertation, appears with the intention of developing a passive communication
system for spatial applications. The system should have sensing
and processing capabilities. To achieve this, some concepts are important:
antennas, waveform design, wireless power transmission, circuit RF-DC, radio
backscatter, wave modulation and demodulation.
In order to design a functional system, each part of the system will be analysisd
and tested. The work is designed to be the more embracing possible,
however due to its complexity it is more focused in four points: antennas,
circuit RF-DC, radio backscatter and microcontrollers. The other aspects
are approached but with less details. Beyond all the hardware aspects, it is
also intended to develop a optimized solution for software, trying to achieve
a better general system e ciency.
The system is designed for spatial applications, however it is expected that
it could be a solution for other areas
Practical Waveform-to-Energy Harvesting Model and Transmit Waveform Optimization for RF Wireless Power Transfer Systems
The received radio-frequency (RF) power in far-field RF wireless power transfer (WPT)—with or without simultaneous information transfer—is minuscule due to large propagation loss in wireless media. In such scenarios, adapting to the receiver characteristics by transmit waveform optimization is essential for maximizing the harvested direct current (dc) and, thus, the end-to-end efficiency of an RF WPT system. The receiver efficiency in RF WPT is governed by the RF-to-dc efficiency of the rectifier as well as the impedance mismatch at the antenna and load. In this article, we study the receiver efficiency for any fixed load and, subsequently, present a novel rectifier model that relates the average harvested dc power to the distribution, that is, the histogram, of the instantaneous power levels of the RF signal’s envelope over time. The proposed waveform-to-energy harvesting (EH) model enables us to anticipate the average harvested dc power for any waveform, including communication signals as well, given the knowledge of the power-level distribution. Consequently, we conduct rigorous waveform optimization to maximize the average harvested dc power and determine the digital baseband signal at the transmitter that does so, namely prove that a pulsed tone at appropriate frequency is optimal for RF WPT. We present a multiband test-bed for determining the receiver efficiency for any digital baseband waveform. The efficacy of the proposed model is corroborated through experiments as well as simulations, which confirm that it is operational as well as accurate in practice and that single-sine pulses yield higher efficiency than basic multisine waveforms, while a pulsed phase shift keying (PSK) is preferable for simultaneous wireless information and power transfer (SWIPT).Peer reviewe
RF Wireless Power and Data Transfer : Experiment-driven Analysis and Waveform Design
The brisk deployment of the fifth generation (5G) mobile technology across the globe has accelerated the adoption of Internet of Things (IoT) networks. While 5G provides the necessary bandwidth and latency to connect the trillions of IoT sensors to the internet, the challenge of powering such a multitude of sensors with a replenishable energy source remains. Far-field radio frequency (RF) wireless power transfer (WPT) is a promising technology to address this issue. Conventionally, the RF WPT concepts have been deemed inadequate to deliver wireless power due to the undeniably huge over-the-air propagation losses. Nonetheless, the radical decline in the energy requirement of simple sensing and computing devices over the last few decades has rekindled the interest in RF WPT as a feasible solution for wireless power delivery to IoT sensors.
The primary goal in any RF WPT system is to maximize the harvested direct current (DC) power from the minuscule incident RF power. As a result, optimizing the receiver power efficiency is pivotal for an RF WPT system. On similar lines, it is essential to minimize the power losses at the transmitter in order to achieve a sustainable and economically viable RF WPT system. In this regard, this thesis explores the system-level study of an RF WPT system using a digital radio transmitter for applications where alternative analog transmit circuits are impractical. A prototype test-bed comprising low-cost software-defined radio (SDR) transmitter and an off-the-shelf RF energy-harvesting (EH) receiver is developed to experimentally analyze the impact of clipping and nonlinear amplification at the digital radio transmitter on digital baseband waveform. The use of an SDR allows leveraging the test-bed for the research on RF simultaneous wireless information and power transfer (SWIPT); the true potential of this technology can be realized by utilizing the RF spectrum to transport data and power together. The experimental results indicate that a digital radio severely distorts high peak-to-average power ratio (PAPR) signals, thereby reducing their average output power and rendering them futile for RF WPT.
On similar lines, another test-bed is developed to assess the impact of different waveforms, input impedance mismatch, incident RF power, and load on the receiver power efficiency of an RF WPT system. The experimental results provide the foundation and notion to develop a novel mathematical model for an RF EH receiver. The parametric model relates the harvested DC power to the power distribution of the envelope signal of the incident waveform, which is characterized by the amplitude, phase and frequency of the baseband waveform. The novel receiver model is independent of the receiver circuit’s matching network, rectifier configuration, number of diodes, load as well as input frequency. The efficacy of the model in accurately predicting the output DC power for any given power-level distribution is verified experimentally.
Since the novel receiver model associates the output DC power to the parameters of the incident waveform, it is further leveraged to design optimal transmit wave-forms for RF WPT and SWIPT. The optimization problem reveals that a constant envelope signal with varying duty cycle is optimal for maximizing the harvested DC power. Consequently, a pulsed RF waveform is optimal for RF WPT, whereas a continuous phase modulated pulsed RF signal is suitable for RF SWIPT. The superior WPT performance of pulsed RF waveforms over multisine signals is demonstrated experimentally. Similarly, the pulsed phase-shift keying (PSK) signals exhibit superior receiver power efficiency than other communication signals. Nonetheless, varying the duty-cycle of pulsed PSK waveform leads to an efficiency—throughput trade-off in RF SWIPT.
Finally, the SDR test-bed is used to evaluate the overall end-to-end power efficiency of different digital baseband waveforms through wireless measurements. The results indicate a 4-PSK modulated signal to be suitable for RF WPT considering the overall power efficiency of the system. The corresponding transmitter, channel and receiver power efficiencies are evaluated as well. The results demonstrate the transmitter power efficiency to be lower than the receiver power efficiency
Study of the best linear approximation of nonlinear systems with arbitrary inputs
System identification is the art of modelling of a process (physical, biological,
etc.) or to predict its behaviour or output when the environment condition
or parameter changes. One is modelling the input-output relationship of a system,
for example, linking temperature of a greenhouse (output) to the sunlight intensity
(input), power of a car engine (output) with fuel injection rate (input). In linear
systems, changing an input parameter will result in a proportional increase in the
system output. This is not the case in a nonlinear system. Linear system identification
has been extensively studied, more so than nonlinear system identification.
Since most systems are nonlinear to some extent, there is significant interest in this
topic as industrial processes become more and more complex.
In a linear dynamical system, knowing the impulse response function of a
system will allow one to predict the output given any input. For nonlinear systems
this is not the case. If advanced theory is not available, it is possible to approximate
a nonlinear system by a linear one. One tool is the Best Linear Approximation
(Bla), which is an impulse response function of a linear system that minimises the
output differences between its nonlinear counterparts for a given class of input. The
Bla is often the starting point for modelling a nonlinear system. There is extensive
literature on the Bla obtained from input signals with a Gaussian probability
density function (p.d.f.), but there has been very little for other kinds of inputs.
A Bla estimated from Gaussian inputs is useful in decoupling the linear dynamics
from the nonlinearity, and in initialisation of parameterised models. As Gaussian
inputs are not always practical to be introduced as excitations, it is important to
investigate the dependence of the Bla on the amplitude distribution in more detail.
This thesis studies the behaviour of the Bla with regards to other types of signals,
and in particular, binary sequences where a signal takes only two levels. Such an
input is valuable in many practical situations, for example where the input actuator
is a switch or a valve and hence can only be turned either on or off.
While it is known in the literature that the Bla depends on the amplitude
distribution of the input, as far as the author is aware, there is a lack of comprehensive
theoretical study on this topic. In this thesis, the Blas of discrete-time
time-invariant nonlinear systems are studied theoretically for white inputs with an arbitrary amplitude distribution, including Gaussian and binary sequences. In doing
so, the thesis offers answers to fundamental questions of interest to system engineers,
for example: 1) How the amplitude distribution of the input and the system
dynamics affect the Bla? 2) How does one quantify the difference between the
Bla obtained from a Gaussian input and that obtained from an arbitrary input?
3) Is the difference (if any) negligible? 4) What can be done in terms of experiment
design to minimise such difference?
To answer these questions, the theoretical expressions for the Bla have been
developed for both Wiener-Hammerstein (Wh) systems and the more general Volterra
systems. The theory for the Wh case has been verified by simulation and physical
experiments in Chapter 3 and Chapter 6 respectively. It is shown in Chapter 3
that the difference between the Gaussian and non-Gaussian Bla’s depends on the
system memory as well as the higher order moments of the non-Gaussian input.
To quantify this difference, a measure called the Discrepancy Factor—a measure of
relative error, was developed. It has been shown that when the system memory is
short, the discrepancy can be as high as 44.4%, which is not negligible. This justifies
the need for a method to decrease such discrepancy. One method is to design a random
multilevel sequence for Gaussianity with respect to its higher order moments,
and this is discussed in Chapter 5.
When estimating the Bla even in the absence of environment and measurement
noise, the nonlinearity inevitably introduces nonlinear distortions—deviations
from the Bla specific to the realisation of input used. This also explains why more
than one realisation of input and averaging is required to obtain a good estimate of
the Bla. It is observed that with a specific class of pseudorandom binary sequence
(Prbs), called the maximum length binary sequence (Mlbs or the m-sequence), the
nonlinear distortions appear structured in the time domain. Chapter 4 illustrates
a simple and computationally inexpensive method to take advantage this structure
to obtain better estimates of the Bla—by replacing mean averaging by median
averaging.
Lastly, Chapters 7 and 8 document two independent benchmark studies separate
from the main theoretical work of the thesis. The benchmark in Chapter 7 is
concerned with the modelling of an electrical Wh system proposed in a special session
of the 15th International Federation of Automatic Control (Ifac) Symposium on
System Identification (Sysid) 2009 (Schoukens, Suykens & Ljung, 2009). Chapter 8
is concerned with the modelling of a ‘hyperfast’ Peltier cooling system first proposed
in the U.K. Automatic Control Council (Ukacc) International Conference
on Control, 2010 (Control 2010)