Load Modulation for Backscatter Communication: Channel Capacity and Near-Capacity Schemes

In backscatter communication (BC), a passive tag transmits information by just affecting an external electromagnetic field through load modulation. Thereby, the feed current of the excited tag antenna is modulated by adapting the passive termination load. This paper studies the achievable information rates with a freely adaptable passive load. As a prerequisite, we unify monostatic, bistatic, and ambient BC with circuit-based system modeling. We present the crucial insight that channel capacity is described by existing results on peak-power-limited quadrature Gaussian channels, because the steady-state tag current phasor lies on a disk. Consequently, we derive the channel capacity for the case of an unmodulated external field, for general passive, purely reactive, or purely resistive tag loads. We find that modulating both resistance and reactance is important for very high rates. We discuss the capacity-achieving load statistics, rate asymptotics, technical conclusions, and rate losses from value-range-constrained loads (which are found to be small for moderate constraints). We then demonstrate that near-capacity rates can be attained by more practical schemes: (i) amplitude-and-phase-shift keying on the reflection coefficient and (ii) simple load circuits of a few switched resistors and capacitors. Finally, we draw conclusions for the ambient BC channel capacity in important special cases.Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice. Included conference paper: arXiv:2201.0024

DESIGN AND FABRICATION OF A 64-QAM MODULATOR FOR ANALYSIS OF SIGNALS BETWEEN STAGES

AbstractThis paper proposes the design of a 64-QAM digital modulator on board based on chip technology, the developed board comprises two phase delay and two amplification stages , the modulator was developed using the software PCB Wizard and were established test points for each stage. The developed layout was built through a computer numerical control machine. Experimental results show a significant improvement of the accuracy based on a reached NMSE= -51 dB. The developed board offer a design tool for hardware developers where the intermodulation products can be evaluated, this digital modulator avoid parasitic inductances and capacitances between the lines. The results of the developed system are effective to add white Gaussian noise and probe the intermodulation products up to 3rd order that can be added, as well as the effect of spectral regrowth. The system is a hardware design tool able to show by stages the amplitude and phase changes that involves a 64-QAM modulation.Keywords: CNC, Intermodulation, Layout, 64-QAM.DISEÑO Y FABRICACIÓN DE UN MODULADOR DE 64 QAM PARA EL ANÁLISIS DE SEÑALES ENTRE ETAPASResumenEste trabajo propone el diseño de un modulador digital 64-QAM en tarjeta impresa basado en tecnología en chip, la tarjeta desarrollada consta de dos fases de retardo y dos fases de amplificación, el modulador fue desarrollado usando el software PCB Wizard y fueron establecidos puntos de prueba para cada etapa. El diseño desarrollado fue construido a través de una máquina de control numérico por computadora. Los resultados experimentales muestran una mejora significativa en la precisión alcanzada de NMSE de -51 dB. La tarjeta desarrollada ofrece una herramienta de diseño para desarrolladores de hardware donde los productos de intermodulación puedan ser evaluados, este modulador digital evita inductancias parásitas y capacitancias entre líneas. Los resultados del sistema desarrollado son efectivos para agregar ruido blanco Gaussiano y comprobar los los productos de intermodulación de hasta 3er orden que se pueden agregar, así como el efecto de recrecimiento espectral. El sistema es una herramienta de diseño de hardware capaz de mostrar por etapas los cambios en amplitud y fase que involucra una modulación de tipo QAM.Palabras Claves: CNC, Diseño, Intermodulación, 64-QAM

Testbed Design and Implementation For Wireless Power Transfer Using Software Defined Radios

The area of wireless power transfer (WPT) dates back more than a century. This capability to transfer power without wires gives also motive for harvesting resources that have not yet been considered, such as the RF signals that cellular networks employ to send information. Only a while ago the research in the WPT field was focused on improving the elements in the power transmission chains separately. However, in recent years, such closed-loop schemes have emerged that have the potential to improve the efficiency of the entire system by adapting key elements in the chain together, such as the transmitted waveform and the rectenna performance. The scope of the thesis aims to contribute to the ultimate objective of merging information and power transfer in a simultaneous wireless information and power transfer/transmission (SWIPT) network. The main objective of this thesis is to design and implement a testbed for research on WPT and SWIPT. A closed-loop system is implemented for future scientific experiments for broadcasting a given radio signal and at the same time measuring the total power that an energy receiver will harvest from the transmission. The main element of the testbed is a computer from which master program controlling the transmission, the synchronization, and reading of the harvested voltage. The master program is written in C++ language and is designed to transmit with a USRP and receive voltage readings from the harvesting energy receivers that consist of RF-to-DC converter, ADC and Arduino microcontroller. Results show that the implemented testbed works as planned, and the master program can perform adaptive algorithms. Furthermore, the testbed can be used for experiments for any given waveform meant for communications, WPT, and SWIPT

Integrated Filters and Couplers for Next Generation Wireless Tranceivers

The main focus of this thesis is to investigate the critical nonlinear distortion issues affecting RF/Microwave components such as power amplifiers (PA) and develop new and improved solutions that will improve efficiency and linearity of next generation RF/Microwave mobile wireless communication systems. This research involves evaluating the nonlinear distortions in PA for different analog and digital signals which have been a major concern. The second harmonic injection technique is explored and used to effectively suppress nonlinear distortions. This method consists of simultaneously feeding back the second harmonics at the output of the power amplifier (PA) into the input of the PA. Simulated and measured results show improved linearity results. However, for increasing frequency bandwidth, the suppression abilities reduced which is a limitation for 4G LTE and 5G networks that require larger bandwidth (above 5 MHz). This thesis explores creative ways to deal with this major drawback. The injection technique was modified with the aid of a well-designed band-stop filter. The compact narrowband notch filter designed was able to suppress nonlinear distortions very effectively when used before the PA. The notch filter is also integrated in the injection technique for LTE carrier aggregation (CA) with multiple carriers and significant improvement in nonlinear distortion performance was observed. This thesis also considers maximizing efficiency alongside with improved linearity performance. To improve on the efficiency performance of the PA, the balanced PA configuration was investigated. However, another major challenge was that the couplers used in this configuration are very large in size at the desired operating frequency. In this thesis, this problem was solved by designing a compact branch line coupler. The novel coupler was simulated, fabricated and measured with performance comparable to its conventional equivalent and the coupler achieved substantial size reduction over others. The coupler is implemented in the balanced PA configuration giving improved input and output matching abilities. The proposed balanced PA is also implemented in 4G LTE and 5G wireless transmitters. This thesis provides simulation and measured results for all balanced PA cases with substantial efficiency and linearity improvements observed even for higher bandwidths (above 5 MHz). Additionally, the coupler is successfully integrated with rectifiers for improved energy harvesting performance and gave improved RF-dc conversion efficienc

Generic wireless sensor network for dynamic monitoring of a new generation of building material

Existing testing methods for building materials before deployment include a series of procedures as stipulated in British Standards, and most tests are performed in a controlled laboratory environment. Types of equipment used for measurements, data logging, and visualisation are commonly bulky, hard-wired, and consume a significant amount of power. Most of the off-the-shelf sensing nodes have been designed for a few specific applications and cannot be used for general purpose applications. This makes it difficult to modify or extend the sensing features when needed. This thesis takes the initiative of designing and implementing a low-powered, open-source, flexible, and small-sized Generic wireless sensor network (GWSN) that can continuously monitor the building materials and building environment, to address the limitations of the conventional measurement methods and the technological gap. The designed system is comprised of two custom-made sensor nodes and a gateway, as well as purpose designed firmware for data collection and processing. For the proof of concept and experimental studies, several measurement strategies were designed, to demonstrate, evaluate, and validate the effectiveness of the system. The data was collected from selected case study areas in the School of Energy, Geoscience, Infrastructure and Society (EGIS) laboratories by measuring and monitoring building structures and indoor environment quality parameters using the designed GWSN. The measured data includes heat flux through the material, surface and air temperatures on both sides of the material/structure, moisture variation, ambient temperature, relative humidity, carbon dioxide, volatile organic compounds, particulate matter, and sound/acoustic levels. The initial results show the potential of the designed system to become the new benchmark for tracking the variation of building materials with the environment and investigating the impact of variation of building materials on indoor environment quality. Based on the estimates of the thermal performance data, the sample used in the experiment had a typical U-value between 4.8 and 5.8 W/m2K and a thermal resistance value of 0.025m2 ·K/W[1][2]. Thermal resistance values from the GWSN real-time measurement were between 0.025 and 0.03 m2K/W, with an average of 0.025 m2K/W, and thermal transmission values varied between 4.55 and 5.11 W/m2K. Based on the data obtained, the results are within the range of typical values[3]. For thermal comfort measurements, the results of humidity and temperature from GWSN were compared to values in the Kambic climatic chamber in the EGIS laboratory, and the accuracies were 99 % and 98 % respectively. For the IAQ measurements, the values of CO2 and TVOCs were compared to the commercial off-the-shelf measuring system, and the accuracies were 98 %, and 97 %. Finally, the GWSN was tested for acoustic measurements in the range of 55 dB to 106 dB. The results were compared to class one Bruel & Kjaer SLM. The accuracy of GWSN was 97 %. The GWSN can be used for in lab and in-situ applications, to measure and analyse the thermal physical properties of building materials/building structures (thermal transmittance, thermal conductivity, and thermal resistance). The system can also measure indoor air quality, thermal comfort, and airborne sound insulation of the building envelope. The key point here is to establish a direct link between how building materials vary with the environment and how this impacts indoor environment quality. Such a link is essential for long-term analysis of building materials, which cannot be achieved using current methods. Regarding increasing the power efficient of the implemented GWSN as well as its performance and functionality, a new sensing platforms using backscatter technology have been introduced. The theory of modulation and spread spectrum technique used in backscattering has been explored. The trade-off between hardware complexity/power consumption and link performance has been investigated. Theoretical analysis and simulation validation of the new sensing technique, using backscatter communication, has been performed. A novel multicarrier backscatter tag compatible with Wireless Fidelity has been implemented and an IEEE 802.11g OFDM preamble was synthesized by simulation. The tag consists of only two transistors with current consumption no larger than 0.2 μA at voltage of less than 0.6 V. Novel harmonic suppression approaches for frequency-shifted backscatter communication has been proposed and demonstrated. The proposed approaches independently manipulate mirror harmonics and higher order harmonics whereby; specified higher order harmonics can be removed by carefully designing the real-valued (continuous and discrete) reflection coefficients-based backscatter tags. When successfully implemented, the backscatter system will reduce sensor node power consumption by shifting the power-consuming radio frequency carrier synthesis functions to carrier emitters.Engineering and Physical Sciences Research Council (EPSRC) Funding EP/H009612/

Extending Critical Infrastructure Element Longevity using Constellation-based ID Verification

This work supports a technical cradle-to-grave protection strategy aimed at extending the useful lifespan of Critical Infrastructure (CI) elements. This is done by improving mid-life operational protection measures through integration of reliable physical (PHY) layer security mechanisms. The goal is to improve existing protection that is heavily reliant on higher-layer mechanisms that are commonly targeted by cyberattack. Relative to prior device ID discrimination works, results herein reinforce the exploitability of constellation-based PHY layer features and the ability for those features to be practically implemented to enhance CI security. Prior work is extended by formalizing a device ID verification process that enables rogue device detection demonstration under physical access attack conditions that include unauthorized devices mimicking bit-level credentials of authorized network devices. The work transitions from distance-based to probability-based measures of similarity derived from empirical Multivariate Normal Probability Density Function (MVNPDF) statistics of multiple discriminant analysis radio frequency fingerprint projections. Demonstration results for Constellation-Based Distinct Native Attribute (CB-DNA) fingerprinting of WirelessHART adapters from two manufacturers includes 1) average cross-class percent correct classification of %C \u3e 90% across 28 different networks comprised of six authorized devices, and 2) average rogue rejection rate of 83.4% ≤ RRR ≤ 99.9% based on two held-out devices serving as attacking rogue devices for each network (a total of 120 individual rogue attacks). Using the MVNPDF measure proved most effective and yielded nearly 12% RRR improvement over a Euclidean distance measure

Caracterização de interfaces rádio de sistemas MIMO 5G

In Fifth Generation (5G) scenarios, several challenges arise, special regarding devices' characterization. Multiple Input Multiple Output (MIMO) antennas will be used in such scenarios and are composed of several elements. Consequently, having several elements di cult for the MIMO antenna characterization, for instance, to identify problems in each antenna element or to select the proper elements to achieve a speci c direction. Since this type of antennas is constituted by many elements, close to each other, to perform beamforming, mutual coupling e ects occur which degrades the MIMO systems. In this sense, techniques to characterize mutual coupling when MIMO antennas are performing beamforming are a necessity. Internet of Things (IoT) devices will play a signi cant role in 5G, such as Radio Frequency to Direct Current (RF-DC) converters and backscatter systems, and characterize them to improve their e ciency is a requirement. This thesis will focus on the design and implementation of novel measurement techniques to characterize MIMO antennas and IoT devices, to improve their performance in real scenarios and reduce the characterization complexity. An Over-The-Air (OTA) multi-sine technique will be used to identify problems in MIMO antennas elements and to calibrate the antenna beam with the target. Regarding mutual coupling presented in MIMO antennas, a suitable characterization technique will be presented to evaluate those e ects when beamforming is performed. The OTA multi-sine calibration technique will be also used to optimize the RF-DC converter's e ciency. Moreover, a procedure to characterize these devices taking into consideration di erent speci cations will be performed. In terms of backscatter modulators, a characterization technique will be presented to extract the optimum points of those circuits to achieve e cient communication, considering also OTA measurements.Em cenários de quinta geração (5G) das comunicações moveis, vários desafios surgem, especialmente relativamente à caracterização dos dispositivos. As antenas com múltiplas entradas múltiplas saídas (MIMO) serão utilizadas nesses cenários e são formadas por vários elementos, o que dificulta a sua caracterização, por exemplo, para identificar problemas em cada elemento da antena ou para selecionar os elementos corretos para radiar para uma determinada direção. Dado que este tipo de antenas é composto por vários elementos colocados próximos uns dos outros para controlar a direção do feixe de transmissão, os efeitos de acoplamento ocorrem, o que degrada os sistemas MIMO. Assim sendo, são necessárias técnicas para caraterizar os efeitos do acoplamento em antenas MIMO quando a direção do feixe de transmissão se altera. Os dispositivos de internet das coisas (IoT) desempenham também um papel importante nos cenários 5G, como acontece com os conversores de rádio frequência para corrente directa (RF-DC) e com os sistemas de retroespalhamento (backscatter), e caraterizar estes dispositivos para melhorar a sua eficiência é fundamental. Esta tese foca-se no desenvolvimento e na implementação de técnicas de medida inovadoras para caraterizar antenas MIMO e dispositivos IoT, com o objetivo de melhorar o seu desempenho para a utilização em cenários reais e reduzir a complexidade do processo de caracterização. A técnica multi-seno pela interface ar (OTA) será utilizada para identificar problemas em antenas MIMO, para seleccionar os elementos da antena a fim de transmitir para uma direção especifica e para calibrar o feixe da antena com o alvo. Relativamente ao acoplamento que acontece nas antenas MIMO, uma técnica de caracterização adequada será apresentada para avaliar esses efeitos quando existe variação do feixe. A técnica de calibração OTA multiseno será também aplicada para otimizar a eficiência de conversores RF-DC. Além disso, será apresentado um procedimento para caraterizar esses dispositivos levando em consideração diferentes especificações. Uma técnica de caracterização para extrair os pontos ótimos dos moduladores de retroespalhamento para obter uma comunicação eficiente será apresentada, considerando também medidas OTA.Programa Doutoral em Engenharia Eletrotécnic

Sistemas eficientes de transmissão de energia sem-fios e identificação por radiofrequência

Doutoramento em Engenharia EletrotécnicaIn the IoT context, where billions of connected objects are expected to be ubiquitously deployed worldwide, the frequent battery maintenance of ubiquitous wireless nodes is undesirable or even impossible. In these scenarios, passive-backscatter radios will certainly play a crucial role due to their low cost, low complexity and battery-free operation. However, as passive-backscatter devices are chiefly limited by the WPT link, its efficiency optimization has been a major research concern over the years, gaining even more emphasis in the IoT context. Wireless power transfer has traditionally been carried out using CW signals, and the efficiency improvement has commonly been achieved through circuit design optimization. This thesis explores a fundamentally different approach, in which the optimization is focused on the powering waveforms, rather than the circuits. It is demonstrated through theoretical analysis, simulations and measurements that, given their greater ability to overcome the built-in voltage of rectifying devices, high PAPR multi-sine (MS) signals are capable of more efficiently exciting energy harvesting circuits when compared to CWs. By using optimal MS signals to excite rectifying devices, remarkable RF-DC conversion efficiency gains of up to 15 dB with respect to CW signals were obtained. In order to show the effectiveness of this approach to improve the communication range of passive-backscatter systems, a MS front-end was integrated in a commercial RFID reader and a significant range extension of 25% was observed. Furthermore, a software-defined radio RFID reader, compliant with ISO18000-6C standard and with MS capability, was constructed from scratch. By interrogating passive RFID transponders with MS waveforms, a transponder sensitivity improvement higher than 3 dB was obtained for optimal MS signals. Since the amplification and transmission of high PAPR signals is critical, this work also proposes efficient MS transmitting architectures based on space power combining techniques. This thesis also addresses other not less important issues, namely self-jamming in passive RFID readers, which is the second limiting factor of passive-backscatter systems. A suitable self-jamming suppression scheme was first used for CW signals and then extended to MS signals, yielding a CW isolation up to 50 dB and a MS isolation up 60 dB. Finally, a battery-less remote control system was developed and integrated in a commercial TV device with the purpose of demonstrating a practical application of wireless power transfer and passive-backscatter concepts. This allowed battery-free control of four basic functionalities of the TV (CH+,CH-,VOL+,VOL-).No contexto da internet das coisas (IoT), onde são esperados bilhões de objetos conectados espalhados pelo planeta de forma ubíqua, torna-se impraticável uma frequente manutenção e troca de baterias dos dispositivos sem fios ubíquos. Nestes cenários, os sistemas radio backscatter passivos terão um papel preponderante dado o seu baixo custo, baixa complexidade e não necessidade de baterias nos nós móveis. Uma vez que a transmissão de energia sem fios é o principal aspeto limitativo nestes sistemas, a sua otimização tem sido um tema central de investigação, ganhando ainda mais ênfase no contexto IoT. Tradicionalmente, a transferência de energia sem-fios é feita através de sinais CW e a maximização da eficiência é conseguida através da otimização dos circuitos recetores. Neste trabalho explora-se uma abordagem fundamentalmente diferente, em que a otimização foca-se nas formas de onda em vez dos circuitos. Demonstra-se, teoricamente e através de simulações e medidas que, devido à sua maior capacidade em superar a barreira de potencial intrínseca dos dispositivos retificadores, os sinais multi-seno (MS) de elevado PAPR são capazes de excitar os circuitos de colheita de energia de forma mais eficiente quando comparados com o sinal CW tradicional. Usando sinais MS ótimos em circuitos retificadores, foram verificadas experimentalmente melhorias de eficiência de conversão RF-DC notáveis de até 15 dB relativamente ao sinal CW. A fim de mostrar a eficácia desta abordagem na melhoria da distância de comunicação de sistemas backscatter passivos, integrou-se um front-end MS num leitor RFID comercial e observou-se um aumento significativo de 25% na distância de leitura. Além disso, desenvolveu-se de raiz um leitor RFID baseado em software rádio, compatível com o protocolo ISO18000-6C e capaz de gerar sinais MS, com os quais interrogou-se transponders passivos, obtendo-se ganhos de sensibilidade dos transponders maiores que 3 dB. Uma vez que a amplificação de sinais de elevado PAPR é uma operação crítica, propôs-se também novas arquiteturas eficientes de transmissão baseadas na combinação de sinais em espaço livre. Esta tese aborda também outros aspetos não menos importantes, como o self-jamming em leitores RFID passivos, tido como o segundo fator limitativo neste tipo de sistemas. Estudou-se técnicas de cancelamento de self-jamming CW e estendeu-se o conceito a sinais MS, tendo-se obtido isolamentos entre o transmissor e o recetor de até 50 dB no primeiro caso e de até 60 dB no segundo. Finalmente, com o objetivo de demonstrar uma aplicação prática dos conceitos de transmissão de energia sem fios e comunicação backscatter, desenvolveu-se um sistema de controlo remoto sem pilhas, cujo protótipo foi integrado num televisor comercial a fim de controlar quatro funcionalidades básicas (CH+,CH-,VOL+,VOL-)