Mixed-domain simulation and hybrid wafer-level packaging of RF-MEMS devices for wireless applications

In questa tesi verranno trattati sia il problema della creazione di un ambiente di simulazione a domini fisici misti per dispositivi RF-MEMS, che la definizione di un processo di fabbricazione ad-hoc per il packaging e l’integrazione degli stessi. Riguardo al primo argomento, sarà mostrato nel dettaglio lo sviluppo di una libreria di modelli MEMS all’interno dell’ambiente di simulazione per circuiti integrati Cadence c . L’approccio scelto per la definizione del comportamento elettromeccanico dei MEMS è basato sul concetto di modellazione compatta (compact modeling). Questo significa che il comportamento fisico di ogni componente elementare della libreria è descritto per mezzo di un insieme limitato di punti (nodi) di interconnessione verso il mondo esterno. La libreria comprende componenti elementari, come travi flessibili, piatti rigidi sospesi e punti di ancoraggio, la cui opportuna interconnessione porta alla realizzazione di interi dispositivi (come interruttori e capacità variabili) da simulare in Cadence c . Tutti i modelli MEMS sono implementati per mezzo del linguaggio VerilogA c di tipo HDL (Hardware Description Language) che è supportato dal simulatore circuitale Spectre c . Sia il linguaggio VerilogA c che il simulatore Spectre c sono disponibili in ambiente Cadence c . L’ambiente di simulazione multidominio (ovvero elettromeccanico) così ottenuto permette di interfacciare i dispositivi MEMS con le librerie di componenti CMOS standard e di conseguenza la simulazione di blocchi funzionali misti RF-MEMS/CMOS. Come esempio, un VCO (Voltage Controlled Oscillator) in cui l’LC-tank è realizzato in tecnologia MEMS mentre la parte attiva con transistor MOS di libreria sarà simulato in Spectre c . Inoltre, nelle pagine successive verrà mostrata una soluzione tecnologica per la fabbricazione di un substrato protettivo (package) da applicare a dispositivi RF-MEMS basata su vie di interconnessione elettrica attraverso un wafer di Silicio. La soluzione di packaging prescelta rende possibili alcune tecniche per l’integrazione ibrida delle parti RF-MEMS e CMOS (hybrid packaging). Verranno inoltre messe in luce questioni riguardanti gli effetti parassiti (accoppiamenti capacitivi ed induttivi) introdotti dal package che influenzano le prestazioni RF dei dispositivi MEMS incapsulati. Nel dettaglio, tutti i gradi di libertà del processo tecnologico per l’ottenimento del package saranno ottimizzati per mezzo di un simulatore elettromagnetico (Ansoft HFSSTM) al fine di ridurre gli effetti parassiti introdotti dal substrato protettivo. Inoltre, risultati sperimentali raccolti da misure di strutture di test incapsulate verranno mostrati per validare, da un lato, il simulatore Ansoft HFSSTM e per dimostrate, dall’altro, la fattibilit`a della soluzione di packaging proposta. Aldilà dell’apparente debole legame tra i due argomenti sopra menzionati è possibile identificare un unico obiettivo. Da un lato questo è da ricercarsi nello sviluppo di un ambiente di simulazione unificato all’interno del quale il comportamento elettromeccanico dei dispositivi RF-MEMS possa essere studiato ed analizzato. All’interno di tale ambiente, l’influenza del package sul comportamento elettromagnetico degli RF-MEMS può essere tenuta in conto per mezzo di modelli a parametri concentrati (lumped elements) estratti da misure sperimentali e simulazioni agli Elementi Finiti (FEM) della parte di package. Infine, la possibilità offerta dall’ambiente Cadence c relativamente alla simulazione di dipositivi RF-MEMS interfacciati alla parte CMOS rende possibile l’analisi di blocchi funzionali ibridi RF-MEMS/CMOS completi.

Middleware per la gestione di interfacce di comunicazione e di sorgenti di contesto in ambienti wireless eterogenei

The full exploitation of multi-hop multi-path connectivity opportunities offered by heterogeneous wireless interfaces could enable innovative Always Best Served (ABS) deployment scenarios where mobile clients dynamically self-organize to offer/exploit Internet connectivity at best. Only novel middleware solutions based on heterogeneous context information can seamlessly enable this scenario: middleware solutions should i) provide a translucent access to low-level components, to achieve both fully aware and simplified pre-configured interactions, ii) permit to fully exploit communication interface capabilities, i.e., not only getting but also providing connectivity in a peer-to-peer fashion, thus relieving final users and application developers from the burden of directly managing wireless interface heterogeneity, and iii) consider user mobility as crucial context information evaluating at provision time the suitability of available Internet points of access differently when the mobile client is still or in motion. The novelty of this research work resides in three primary points. First of all, it proposes a novel model and taxonomy providing a common vocabulary to easily describe and position solutions in the area of context-aware autonomic management of preferred network opportunities. Secondly, it presents PoSIM, a context-aware middleware for the synergic exploitation and control of heterogeneous positioning systems that facilitates the development and portability of location-based services. PoSIM is translucent, i.e., it can provide application developers with differentiated visibility of data characteristics and control possibilities of available positioning solutions, thus dynamically adapting to application-specific deployment requirements and enabling cross-layer management decisions. Finally, it provides the MMHC solution for the self-organization of multi-hop multi-path heterogeneous connectivity. MMHC considers a limited set of practical indicators on node mobility and wireless network characteristics for a coarsegrained estimation of expected reliability/quality of multi-hop paths available at runtime. In particular, MMHC manages the durability/throughput-aware formation and selection of different multi-hop paths simultaneously. Furthermore, MMHC provides a novel solution based on adaptive buffers, proactively managed based on handover prediction, to support continuous services, especially by pre-fetching multimedia contents to avoid streaming interruptions.

Design of wireless sensor networks for fluid dynamic applications

In fluid dynamics research, pressure measurements are of great importance to define the flow field acting on aerodynamic surfaces. In fact the experimental approach is fundamental to avoid the complexity of the mathematical models for predicting the fluid phenomena. It’s important to note that, using in-situ sensor to monitor pressure on large domains with highly unsteady flows, several problems are encountered working with the classical techniques due to the transducer cost, the intrusiveness, the time response and the operating range. An interesting approach for satisfying the previously reported sensor requirements is to implement a sensor network capable of acquiring pressure data on aerodynamic surface using a wireless communication system able to collect the pressure data with the lowest environmental–invasion level possible. In this thesis a wireless sensor network for fluid fields pressure has been designed, built and tested. To develop the system, a capacitive pressure sensor, based on polymeric membrane, and read out circuitry, based on microcontroller, have been designed, built and tested. The wireless communication has been performed using the Zensys Z-WAVE platform, and network and data management have been implemented. Finally, the full embedded system with antenna has been created. As a proof of concept, the monitoring of pressure on the top of the mainsail in a sailboat has been chosen as working example.

Cooperative communication and distributed detection in wireless sensor networks

Recent progress in microelectronic and wireless communications have enabled the development of low cost, low power, multifunctional sensors, which has allowed the birth of new type of networks named wireless sensor networks (WSNs). The main features of such networks are: the nodes can be positioned randomly over a given field with a high density; each node operates both like sensor (for collection of environmental data) as well as transceiver (for transmission of information to the data retrieval); the nodes have limited energy resources. The use of wireless communications and the small size of nodes, make this type of networks suitable for a large number of applications. For example, sensor nodes can be used to monitor a high risk region, as near a volcano; in a hospital they could be used to monitor physical conditions of patients. For each of these possible application scenarios, it is necessary to guarantee a trade-off between energy consumptions and communication reliability. The thesis investigates the use of WSNs in two possible scenarios and for each of them suggests a solution that permits to solve relating problems considering the trade-off introduced. The first scenario considers a network with a high number of nodes deployed in a given geographical area without detailed planning that have to transmit data toward a coordinator node, named sink, that we assume to be located onboard an unmanned aerial vehicle (UAV). This is a practical example of reachback communication, characterized by the high density of nodes that have to transmit data reliably and efficiently towards a far receiver. It is considered that each node transmits a common shared message directly to the receiver onboard the UAV whenever it receives a broadcast message (triggered for example by the vehicle). We assume that the communication channels between the local nodes and the receiver are subject to fading and noise. The receiver onboard the UAV must be able to fuse the weak and noisy signals in a coherent way to receive the data reliably. It is proposed a cooperative diversity concept as an effective solution to the reachback problem. In particular, it is considered a spread spectrum (SS) transmission scheme in conjunction with a fusion center that can exploit cooperative diversity, without requiring stringent synchronization between nodes. The idea consists of simultaneous transmission of the common message among the nodes and a Rake reception at the fusion center. The proposed solution is mainly motivated by two goals: the necessity to have simple nodes (to this aim we move the computational complexity to the receiver onboard the UAV), and the importance to guarantee high levels of energy efficiency of the network, thus increasing the network lifetime. The proposed scheme is analyzed in order to better understand the effectiveness of the approach presented. The performance metrics considered are both the theoretical limit on the maximum amount of data that can be collected by the receiver, as well as the error probability with a given modulation scheme. Since we deal with a WSN, both of these performance are evaluated taking into consideration the energy efficiency of the network. The second scenario considers the use of a chain network for the detection of fires by using nodes that have a double function of sensors and routers. The first one is relative to the monitoring of a temperature parameter that allows to take a local binary decision of target (fire) absent/present. The second one considers that each node receives a decision made by the previous node of the chain, compares this with that deriving by the observation of the phenomenon, and transmits the final result to the next node. The chain ends at the sink node that transmits the received decision to the user. In this network the goals are to limit throughput in each sensor-to-sensor link and minimize probability of error at the last stage of the chain. This is a typical scenario of distributed detection. To obtain good performance it is necessary to define some fusion rules for each node to summarize local observations and decisions of the previous nodes, to get a final decision that it is transmitted to the next node. WSNs have been studied also under a practical point of view, describing both the main characteristics of IEEE802:15:4 standard and two commercial WSN platforms. By using a commercial WSN platform it is realized an agricultural application that has been tested in a six months on-field experimentation.

Wireless systems for the fourth generation

Today, third generation networks are consolidated realities, and user expectations on new applications and services are becoming higher and higher. Therefore, new systems and technologies are necessary to move towards the market needs and the user requirements. This has driven the development of fourth generation networks. ”Wireless network for the fourth generation” is the expression used to describe the next step in wireless communications. There is no formal definition for what these fourth generation networks are; however, we can say that the next generation networks will be based on the coexistence of heterogeneous networks, on the integration with the existing radio access network (e.g. GPRS, UMTS, WIFI, ...) and, in particular, on new emerging architectures that are obtaining more and more relevance, as Wireless Ad Hoc and Sensor Networks (WASN). Thanks to their characteristics, fourth generation wireless systems will be able to offer custom-made solutions and applications personalized according to the user requirements; they will offer all types of services at an affordable cost, and solutions characterized by flexibility, scalability and reconfigurability. This PhD’s work has been focused on WASNs, autoconfiguring networks which are not based on a fixed infrastructure, but are characterized by being infrastructure less, where devices have to automatically generate the network in the initial phase, and maintain it through reconfiguration procedures (if nodes’ mobility, or energy drain, etc..., cause disconnections). The main part of the PhD activity has been focused on an analytical study on connectivity models for wireless ad hoc and sensor networks, nevertheless a small part of my work was experimental. Anyway, both the theoretical and experimental activities have had a common aim, related to the performance evaluation of WASNs. Concerning the theoretical analysis, the objective of the connectivity studies has been the evaluation of models for the interference estimation. This is due to the fact that interference is the most important performance degradation cause in WASNs. As a consequence, is very important to find an accurate model that allows its investigation, and I’ve tried to obtain a model the most realistic and general as possible, in particular for the evaluation of the interference coming from bounded interfering areas (i.e. a WiFi hot spot, a wireless covered research laboratory, ...). On the other hand, the experimental activity has led to Throughput and Packet Error Rare measurements on a real IEEE802.15.4 Wireless Sensor Network.

Progetto di reti Sensori Wireless e tecniche di Fusione Sensoriale

Ambient Intelligence (AmI) envisions a world where smart, electronic environments are aware and responsive to their context. People moving into these settings engage many computational devices and systems simultaneously even if they are not aware of their presence. AmI stems from the convergence of three key technologies: ubiquitous computing, ubiquitous communication and natural interfaces. The dependence on a large amount of fixed and mobile sensors embedded into the environment makes of Wireless Sensor Networks one of the most relevant enabling technologies for AmI. WSN are complex systems made up of a number of sensor nodes, simple devices that typically embed a low power computational unit (microcontrollers, FPGAs etc.), a wireless communication unit, one or more sensors and a some form of energy supply (either batteries or energy scavenger modules). Low-cost, low-computational power, low energy consumption and small size are characteristics that must be taken into consideration when designing and dealing with WSNs. In order to handle the large amount of data generated by a WSN several multi sensor data fusion techniques have been developed. The aim of multisensor data fusion is to combine data to achieve better accuracy and inferences than could be achieved by the use of a single sensor alone. In this dissertation we present our results in building several AmI applications suitable for a WSN implementation. The work can be divided into two main areas: Multimodal Surveillance and Activity Recognition. Novel techniques to handle data from a network of low-cost, low-power Pyroelectric InfraRed (PIR) sensors are presented. Such techniques allow the detection of the number of people moving in the environment, their direction of movement and their position. We discuss how a mesh of PIR sensors can be integrated with a video surveillance system to increase its performance in people tracking. Furthermore we embed a PIR sensor within the design of a Wireless Video Sensor Node (WVSN) to extend its lifetime. Activity recognition is a fundamental block in natural interfaces. A challenging objective is to design an activity recognition system that is able to exploit a redundant but unreliable WSN. We present our activity in building a novel activity recognition architecture for such a dynamic system. The architecture has a hierarchical structure where simple nodes performs gesture classification and a high level meta classifiers fuses a changing number of classifier outputs. We demonstrate the benefit of such architecture in terms of increased recognition performance, and fault and noise robustness. Furthermore we show how we can extend network lifetime by performing a performance-power trade-off. Smart objects can enhance user experience within smart environments. We present our work in extending the capabilities of the Smart Micrel Cube (SMCube), a smart object used as tangible interface within a tangible computing framework, through the development of a gesture recognition algorithm suitable for this limited computational power device. Finally the development of activity recognition techniques can greatly benefit from the availability of shared dataset. We report our experience in building a dataset for activity recognition. Such dataset is freely available to the scientific community for research purposes and can be used as a testbench for developing, testing and comparing different activity recognition techniques.

Adaptive wireless multimedia communication systems

In recent years, due to the rapid convergence of multimedia services, Internet and wireless communications, there has been a growing trend of heterogeneity (in terms of channel bandwidths, mobility levels of terminals, end-user quality-of-service (QoS) requirements) for emerging integrated wired/wireless networks. Moreover, in nowadays systems, a multitude of users coexists within the same network, each of them with his own QoS requirement and bandwidth availability. In this framework, embedded source coding allowing partial decoding at various resolution is an appealing technique for multimedia transmissions. This dissertation includes my PhD research, mainly devoted to the study of embedded multimedia bitstreams in heterogenous networks, developed at the University of Bologna, advised by Prof. O. Andrisano and Prof. A. Conti, and at the University of California, San Diego (UCSD), where I spent eighteen months as a visiting scholar, advised by Prof. L. B. Milstein and Prof. P. C. Cosman. In order to improve the multimedia transmission quality over wireless channels, joint source and channel coding optimization is investigated in a 2D time-frequency resource block for an OFDM system. We show that knowing the order of diversity in time and/or frequency domain can assist image (video) coding in selecting optimal channel code rates (source and channel code rates). Then, adaptive modulation techniques, aimed at maximizing the spectral efficiency, are investigated as another possible solution for improving multimedia transmissions. For both slow and fast adaptive modulations, the effects of imperfect channel estimation errors are evaluated, showing that the fast technique, optimal in ideal systems, might be outperformed by the slow adaptive modulation, when a real test case is considered. Finally, the effects of co-channel interference and approximated bit error probability (BEP) are evaluated in adaptive modulation techniques, providing new decision regions concepts, and showing how the widely used BEP approximations lead to a substantial loss in the overall performance.

Cooperative Wireless Networks for Localization

This thesis adresses the problem of localization, and analyzes its crucial aspects, within the context of cooperative WSNs. The three main issues discussed in the following are: network synchronization, position estimate and tracking. Time synchronization is a fundamental requirement for every network. In this context, a new approach based on the estimation theory is proposed to evaluate the ultimate performance limit in network time synchronization. In particular the lower bound on the variance of the average synchronization error in a fully connected network is derived by taking into account the statistical characterization of the Message Delivering Time (MDT) . Sensor network localization algorithms estimate the locations of sensors with initially unknown location information by using knowledge of the absolute positions of a few sensors and inter-sensor measurements such as distance and bearing measurements. Concerning this issue, i.e. the position estimate problem, two main contributions are given. The first is a new Semidefinite Programming (SDP) framework to analyze and solve the problem of flip-ambiguity that afflicts range-based network localization algorithms with incomplete ranging information. The occurrence of flip-ambiguous nodes and errors due to flip ambiguity is studied, then with this information a new SDP formulation of the localization problem is built. Finally a flip-ambiguity-robust network localization algorithm is derived and its performance is studied by Monte-Carlo simulations. The second contribution in the field of position estimate is about multihop networks. A multihop network is a network with a low degree of connectivity, in which couples of given any nodes, in order to communicate, they have to rely on one or more intermediate nodes (hops). Two new distance-based source localization algorithms, highly robust to distance overestimates, typically present in multihop networks, are presented and studied. The last point of this thesis discuss a new low-complexity tracking algorithm, inspired by the Fano’s sequential decoding algorithm for the position tracking of a user in a WLAN-based indoor localization system.

Video transmission over wireless channel

This work has been realized by the author in his PhD course in Electrical, Computer Science and Telecommunication at the University of Bologna, Faculty of Engineering, Italy. All the documentation here reported is a summary of years of work, under the supervision of Prof. Oreste Andrisano, coordinator of Wireless Communication Laboratory - WiLab, in Bologna. The subject of this thesis is the transmission of video in a context of heterogeneous network, and in particular, using a wireless channel. All the instrumentation that has been used for the characterization of the telecommunication systems belongs to CNR (National Research Council), CNIT (Italian Inter- University Center), and DEIS (Dept. of Electrical, Computer Science, and Systems). From November 2009 to July 2010, the author spent his time abroad, working in collaboration with DLR - German Aerospace Center in Munich, Germany, on channel coding area, developing a general purpose decoder machine to decode a huge family of iterative codes. A patent concerning Doubly Generalized-Low Density Parity Check codes has been produced by the author as well as some important scientic papers, published on IEEE journals and conferences.