Optimising lower layers of the protocol stack to improve communication performance in a wireless temperature sensor network

The function of wireless sensor networks is to monitor events or gather information and report the information to a sink node, a central location or a base station. It is a requirement that the information is transmitted through the network efficiently. Wireless communication is the main activity that consumes energy in wireless sensor networks through idle listening, overhearing, interference and collision. It becomes essential to limit energy usage while maintaining communication between the sensor nodes and the sink node as the nodes die after the battery has been exhausted. Thus, conserving energy in a wireless sensor network is of utmost importance. Numerous methods to decrease energy expenditure and extend the lifetime of the network have been proposed. Researchers have devised methods to efficiently utilise the limited energy available for wireless sensor networks by optimising the design parameters and protocols. Cross-layer optimisation is an approach that has been employed to improve wireless communication. The essence of cross-layer scheme is to optimise the exchange and control of data between two or more layers to improve efficiency. The number of transmissions is therefore a vital element in evaluating overall energy usage. In this dissertation, a Markov Chain model was employed to analyse the tuning of two layers of the protocol stack, namely the Physical Layer (PHY) and Media Access Control layer (MAC), to find possible energy gains. The study was conducted utilising the IEEE 802.11 channel, SensorMAC (SMAC) and Slotted-Aloha (S-Aloha) medium access protocols in a star topology Wireless Temperature Sensor Network (WTSN). The research explored the prospective energy gains that could be realised through optimizing the Forward Error Correction (FEC) rate. Different Reed Solomon codes were analysed to explore the effect of protocol tuning on energy efficiency, namely transmission power, modulation method, and channel access. The case where no FEC code was used and analysed as the control condition. A MATLAB simulation model was used to identify the statistics of collisions, overall packets transmitted, as well as the total number of slots used during the transmission phase. The bit error probability results computed analytically were utilised in the simulation model to measure the probability of successful transmitting data in the physical layer. The analytical values and the simulation results were compared to corroborate the correctness of the models. The results indicate that energy gains can be accomplished by the suggested layer tuning approach.Electrical and Mining EngineeringM. Tech. (Electrical Engineering

Collision Avoidance Resource Allocation for LoRaWAN

Data Availability Statement: The data presented in this study are available on request from the corresponding author.Funding: This research was partially funded by the Andalusian Knowledge Agency (project A-TIC- 241-UGR18), the Spanish Ministry of Economy and Competitiveness (project TEC2016-76795-C6-4-R) and the H2020 research and innovation project 5G-CLARITY (Grant No. 871428).The number of connected IoT devices is significantly increasing and it is expected to reach more than two dozens of billions of IoT connections in the coming years. Low Power Wide Area Networks (LPWAN) have become very relevant for this new paradigm due to features such as large coverage and low power consumption. One of the most appealing technologies among these networks is LoRaWAN. Although it may be considered as one of the most mature LPWAN platforms, there are still open gaps such as its capacity limitations. For this reason, this work proposes a collision avoidance resource allocation algorithm named the Collision Avoidance Resource Allocation (CARA) algorithm with the objective of significantly increase system capacity. CARA leverages the multichannel structure and the orthogonality of spreading factors in LoRaWAN networks to avoid collisions among devices. Simulation results show that, assuming ideal radio link conditions, our proposal outperforms in 95.2% the capacity of a standard LoRaWAN network and increases the capacity by almost 40% assuming a realistic propagation model. In addition, it has been verified that CARA devices can coexist with LoRaWAN traditional devices, thus allowing the simultaneous transmissions of both types of devices. Moreover, a proof-of-concept has been implemented using commercial equipment in order to check the feasibility and the correct operation of our solution.Andalusian Knowledge Agency A-TIC-241-UGR18Spanish Ministry of Economy and Competitiveness TEC2016-76795-C6-4-RH2020 research and innovation project 5G-CLARITY 87142

Exploiting user contention to optimize proactive resource allocation in future networks

In order to provide ubiquitous communication, seamless connectivity is now required in all environments including highly mobile networks. By using vertical handover techniques it is possible to provide uninterrupted communication as connections are dynamically switched between wireless networks as users move around. However, in a highly mobile environment, traditional reactive approaches to handover are inadequate. Therefore, proactive handover techniques, in which mobile nodes attempt to determine the best time and place to handover to local networks, are actively being investigated in the context of next generation mobile networks. The Y-Comm Framework which looks at proactive handover techniques has de�fined two key parameters: Time Before Handover and the Network Dwell Time, for any given network topology. Using this approach, it is possible to enhance resource management in common networks using probabilistic mechanisms because it is now possible to express contention for resources in terms of: No Contention, Partial Contention and Full Contention. As network resources are shared between many users, resource management must be a key part of any communication system as it is needed to provide seamless communication and to ensure that applications and servers receive their required Quality-of-Service. In this thesis, the contention for channel resources being allocated to mobile nodes is analysed. The work presents a new methodology to support proactive resource allocation for emerging future networks such as Vehicular Ad-Hoc Networks (VANETs) by allowing us to calculate the probability of contention based on user demand of network resources. These results are veri�ed using simulation. In addition, this proactive approach is further enhanced by the use of a contention queue to detect contention between incoming requests and those waiting for service. This thesis also presents a new methodology to support proactive resource allocation for future networks such as Vehicular Ad-Hoc Networks. The proposed approach has been applied to a vehicular testbed and results are presented that show that this approach can improve overall network performance in mobile heterogeneous environments. The results show that the analysis of user contention does provide a proactive mechanism to improve the performance of resource allocation in mobile networks

Medium Access Control in Distributed Networks with Large Propagation Delay

Most of the Earth is covered by water, so underwater acoustic networks (UWANs) are becoming increasingly popular in a variety of undersea applications. The needs to understand the underwater environment and exploit rich undersea resources have motivated a further development of UWANs. Underwater acoustic signals suffer from more difficult physical channel phenomena than terrestrial radio signals due to the harsh underwater environment, such as sound absorption, time-varying multipath spread, man-made and ambient noise, temperature and pressure dependent refraction, scattering and Doppler shift. Among all the challenges, the large ratio of propagation delay to packet duration (relative propagation delay (a)) is arguably the most difficult one to address in the Medium Access Control (MAC) layer. In this dissertation we focus on the examination and improvement of the MAC layer function in UWANs, based on a critical examination of existing techniques. Many MAC techniques have been proposed in recent years, however most of them assume the ratio of the propagation delay to the packet duration is negligibly small (a>1), these protocols perform poorly. This is because the large a leads to both a large negotiation delay in handshaking based protocols and the space-time uncertainty problem as the packets do not arrive at each node contemporarily. Some underwater-oriented protocols have been proposed which attempt to address these issues but the more successful rely on master nodes or a common understanding of geometry or time. We show by analysis and simulation that it is possible to eliminate collisions in ad-hoc networks with large relative propagation delay (a>>1) as well as improving the channel utilisation, without a common understanding of geometry or time. This technique is generally applicable, even for truly ad-hoc homogeneous peer-to-peer networks with no reliance on master nodes or other heterogeneous features. The mechanism is based on future scheduling with the inclusion of overhearing of RTS messages and allowing third-party objections to proposed transmissions. This MAC mechanism is immediately applicable in underwater acoustic networks (UWANs), and may find other uses, such as in space or very high rate terrestrial wireless networks. In summary, the key contributions of this study are: investigating the causes of the poor performance of existing MAC protocols in ad-hoc UWANs with large relative propagation delay, fully detailing the problem in order to propose analytic solutions, demonstrating how the MAC layer of an ad-hoc UWAN can eliminate packet collisions as well as improve channel utilisation without time synchronization or a network’s self-configuring phase to gain knowledge of the geometry, and verifying the utility of the proposals via both theoretical analysis and simulations

Optimised protocols for time-critical applications and internetworking in wehicular ad-hoc networks

Vehicular ad-hoc networks (VANETs) that enable communication among vehicles and between vehicles and unmanned aerial vehicles (UAVs) and cellular base stations have recently attracted significant interest from the research community, due to the wide range of practical applications they can facilitate (e.g., road safety, traffic management and rescue missions). Despite this increased research activity, the high vehicle mobility in a VANET raises concerns regarding the robustness and adaptiveness of such networks to support time-critical applications and internetworking. In this thesis, as a first step toward the design of efficient MAC protocol to support time-critical applications and internetworking, we show that it is indeed possible to follow the dynamics of a network and consequently adapt the transmission probability of the Aloha protocol to reduce the interference and maximise the single-hop throughput between adjacent nodes. Extensive simulation validates the proposed analytical model, which thus can serve as a promising tool to improve VANETs performance. By exploiting the parallel between the CSMA/CA and Aloha performance models, the optimal transmission probability for the Aloha protocol as a function of estimated vehicular density is derived. This probability is then used to obtain the optimal maximum CW that can be integrated in an amended CSMA/CA protocol to maximise the single-hop throughput among adjacent vehicles. We show by means of simulation that the beneficial impact the proposed protocol is increased channel throughput and reduced transmission delay when compared with the standardised protocol CSMA/CA in IEEE 802.11p. These results reveal the applicability of the new, optimised protocol to safety applications and clustering techniques with stringent performance requirements. Lastly, we propose a Stable Clustering Algorithm for vehicular ad-hoc networks (SCalE) internetworking. The exchange of the necessary status information to support the efficient clusters formation can firmly relay on the support of our optimised CSMA/CA protocol. The SCalE algorithm makes use of the knowledge of the vehicles behaviour (explained in Chapter 5) for efficient selection of CHs, and selects a backup CH on top of the CH to maintain the stability of cluster structures. The increased stability and improved performance of the SCalE algorithm is studied and compared with existing clustering algorithms.Open Acces

Controlo de acesso ao meio em comunicações veiculares de tempo-real

Despite several preventive measures, the number of roadway accidents is still very high, being considered even a problem of public health by some entities. This thesis has as global purpose of contributing to the reduction of that number of accidents, and consequent fatalities, by using safety-related applications that use communication among vehicles. In particular, the primary goal is guaranteeing that communication between users in vehicular environments is done with appropriate time bounds to transfer safety-critical information. In detail, it is studied how to manage the scheduling of message’s transmissions (medium access control - MAC), in order to define precisely who will communicate and when is the appropriate instant. The preferable situation where a communication infrastructure is present with full coverage (RSUs) is also studied, from which medium access control is defined precisely, and vehicles (OBUs) become aware of medium utilization. Also, sporadic situations (e.g., absence of RSUs) are studied in which the communication network is “ad hoc” and solely formed by the current vehicles. It is used the recently WAVE / IEEE 802.11p standard, specific for vehicular communications, and it is proposed a TDMA based solution, with appropriate coordination between RSUs in order to effectively disseminate a critical safety event. It is taken into account two different ways of choosing the instant for the initial broadcast, and both cases are compared. In case there is no infrastructure available, methods are derived to minimize communication medium access collisions, and to maximize the available bandwidth. The results reflect the total end-to-end delay, and show that adequate times are attained, and meet with the requisites for the type of applications being considered. Also, enhancements are obtained when using the alternate choice for the initial broadcast instant.Apesar de diversas medidas preventivas, o número de acidentes rodoviários continua a ser muito elevado, sendo mesmo considerado uma questão de saúde pública por algumas entidades. Esta tese tem como objetivo geral contribuir para a redução desse número de acidentes, e consequentes fatalidades, através da utilização de aplicações de segurança que envolvem comunicação entre veículos. Em particular, o objetivo principal é garantir que a comunicação entre utentes, em ambientes veiculares, seja efetuada com limites temporais apropriados à transferência de informações críticas. De forma mais detalhada, é estudada a gestão do escalonamento das transmissões (controlo de acesso ao meio – MAC) que irá definir quem vai comunicar e quando o pode fazer. São estudadas situações (desejadas) onde há uma infra-estrutura de comunicações com cobertura integral (RSUs), a partir da qual se faz a coordenação do acesso ao meio pelos veículos (OBUs), e situações (esporádicas, por ausência de RSU) em que a rede de comunicação é “ad hoc” e apenas constituída pelos veículos presentes. Utiliza-se a recente norma WAVE / IEEE 802.11p, específica para comunicações veiculares, e propõe-se uma solução baseada em TDMA, com coordenação apropriada entre RSUs para disseminação efetiva de um evento crítico de segurança. A escolha do instante para o broadcast inicial do evento de segurança também é tida em conta, e são comparados dois casos distintos. No caso da ausência de infraestrutura, derivam-se métodos para minimizar colisões no acesso ao meio de comunicação, e maximizar a largura de banda disponível. Os resultados refletem o atraso total end-to-end, mostrando tempos apropriados para os requisitos das aplicações em causa, e evidenciando melhorias aquando da escolha alternativa para o instante do broadcast inicial.Programa Doutoral em Engenharia Eletrotécnic

Allocation designs for massive multiple access with interference cancellation

In the transition towards the next generation of wireless technology systems, the increasing number of devices curbs the potential of current wireless networks to cope with such increases in network density. Wireless communications via satellite constitute a cost effective option to achieve high transmission reliability in remote areas or to create resilient networks to be used in emergency situations. To counterbalance the growing network density, one of the main goals in the uplink is to increase the spectral efficiency of the network. By working on the application of non-orthogonal multiple access and the exploitation of the collision domain through interference cancellation, this dissertation tackles the problem of massive multiple access. A consensual scheme that meets the main goal and the aim of reducing the interaction between devices and the satellite in the control plane is Enhanced Spread Spectrum ALOHA, which combines spreading-based short-packet transmissions with successive interference cancellation (SIC) on the receiver's side. This combination opens up several design avenues in terms of energy and code allocation to users when a certain amount of channel state information is available to them. Motivated by this scheme, this thesis studies the best allocation strategies when the SIC receiver operates nonideally: firstly, it investigates a system model for a receiver that, inspired by the demodulator adopted in the Enhanced Spread Spectrum ALOHA system, deals with the problems of user ordering and iterative decoding with short packets; and secondly, it delves into the user-asymptotic regime and the application of the calculus of variations to derive the stationary point equations corresponding to the optimal allocation rules.The first part of this thesis investigates the impact of nonideal decoding and imperfect cancellation on the first iteration of a SIC receiver aided by redundancy-check error control. The system model characterises both non-idealities using known functions of the signal-to-interference-plus-noise ratio. The propagation of packet decoding success/failure events throughout the stages of the receiver is circumvented in the user-asymptotic regime, since the model takes a deterministic form. The asymptotically optimal energy and rate allocation is studied for a wide variety of cases. The second part of this thesis investigates an iterative SIC receiver and extends the allocation designs derived previously to iterations beyond the first. The derivation of a system model is challenging, since each iteration of the receiver operates with memory with respect to the previous ones, and due to the fact that the decoding operations for the same user in different iterations are statistically dependent. This thesis motivates and states a system model that solves said difficulties by adding minimal complexity to the one adopted previously. The user-asymptotic regime is investigated to reveal mathematical forms to the above model that allow for a thorough understanding of the adopted receiver. Finally, the chapter exploits the user-asymptotic model and conducts research to designing smooth allocation functions. The third part of this thesis studies the user-ordering problem for a SIC receiver to which the strengths received from all users are unknown. The thesis derives an accurate system model for a large-user SIC receiver, which proceeds to order users after estimating their symbol energies at the initial stage through preamble cross-correlations. Analytical findings are determined in the user-asymptotic regime. The asymptotically optimal energy allocation is shown to obey, in contrast to the practically exponential user-energy distributions obtained before, a piecewise constant function; fact that entails great computational advantages of its application.En la transició cap a la pròxima generació de sistemes tecnològics sense fils, el creixent nombre de dispositius frena el potencial de les xarxes sense fils actuals per fer front a tal augment en la densitat de xarxa. Les comunicacions sense fils via satèl·lit constitueixen una opció rentable per assolir una fiabilitat de transmissió alta en zones remotes o per crear xarxes que puguin ser utilitzades en situacions d'emergència. Per contrarestar la creixent densitat de xarxa, un dels objectius principals en l'enllaç ascendent és augmentar l'eficiència espectral d'aquesta. Aquesta tesi aborda el problema d'accés múltiple massiu combinant l'aplicació de tècniques d'accés múltiple no ortogonal amb esquemes de cancel·lació d’interferència. Un esquema consensuat que acompleix amb l’objectiu principal i amb la fita de reduir la interacció entre dispositius i satèl·lit en el pla de control és Enhanced Spread Spectrum ALOHA, que combina transmissions de paquets curts basades en l'eixamplament del senyal amb la cancel·lació successiva d'interferències (SIC) en recepció. Aquesta combinació obre diverses vies per l'assignació d'energia i codi als diferents usuaris quan aquests disposen d’informació sobre l'estat del canal. Motivat per l'esquema anterior, aquesta tesi estudia les millors estratègies d'assignació quan s'adopta un receptor SIC no ideal: en primer lloc, investiga un model de sistema per un receptor SIC que, inspirat en el desmodulador adoptat en el sistema Enhanced Spread Spectrum ALOHA, aborda els problemes d'ordenació d'usuaris i de descodificació iterativa amb paquets curts; i, en segon lloc, s’endinsa en el règim asimptòtic d'usuaris i en l'aplicació del càlcul de variacions per derivar les equacions de punt estacionari corresponents a les funcions d'assignació òptimes. La primera part d'aquesta tesi investiga l'impacte de la descodificació no ideal i de la cancel·lació imperfecta en la primera iteració d'un receptor SIC assistit per control d'errors. El model de sistema proposat caracteritza ambdues no idealitats fent ús de funcions conegudes de la relació senyal-a-soroll-més-interferència. La propagació dels esdeveniments d'èxit/fracàs en la descodificació de paquets al llarg de les etapes del receptor s'aborda en el règim asimptòtic d'usuaris, ja que el model pren forma determinista. Les funcions d'assignació s'estudien en el règim asimptòtic d'usuaris per varis casos. La segona part de la tesi investiga un receptor SIC iteratiu i estén les assignacions derivades en el capítol anterior per a iteracions del SIC més enllà de la primera. La derivació d'un model de sistema suposa un repte, ja que cada iteració del receptor opera amb memòria respecte a iteracions anteriors i degut a que les operacions de descodificació per a un mateix usuari en iteracions diferents són estadísticament dependents. Es proposa un model de sistema que resol tals dificultats afegint complexitat mínima al model adoptat anteriorment. S'investiga el règim asimptòtic d'usuaris amb l'objectiu d’evidenciar expressions matemàtiques del model que permetin la completa comprensió del receptor adoptat. Per últim, es dissenyen funcions d'assignació contínuament diferenciables fent ús del model asimptòtic anterior. La tercera i última part d'aquesta tesi estudia el problema d'ordenació d'usuaris aplicat a un receptor SIC que desconeix les potències rebudes de tots ells. Es deriva un model de sistema per un receptor que gestiona nombrosos usuaris i els ordena després d'estimar les energies de tots ells en l'etapa inicial mitjançant correlacions de preamble. Els resultats analítics s’obtenen en el règim asimptòtic d'usuaris. Es demostra que, contràriament a les distribucions pràcticament exponencials obtingudes anteriorment, l'assignació d'energia òptima derivada per a infinits usuaris presenta una estructura constant a trossos; fet que comporta grans avantatges computacionals en la seva aplicació.En la transición hacia la próxima generación de sistemas tecnológicos inalámbricos, el creciente número de dispositivos frena el potencial de las redes inalámbricas actuales para hacer frente a esos aumentos en la densidad de red. Impulsadas por las innovaciones en tecnología satelital, las comunicaciones inalámbricas vía satélite constituyen una opción rentable para lograr una alta fiabilidad de transmisión en zonas remotas o para crear redes reservadas para situaciones de emergencia. Para contrarrestar la creciente densidad de la red, uno de los objetivos principales en el enlace ascendente es aumentar la eficiencia espectral de la misma. En favor de este objetivo, se identifican tres técnicas no excluyentes: (i) la aplicación de técnicas de acceso múltiple no ortogonal, para hacer frente a la limitada disponibilidad de recursos ortogonales requeridos en el acceso múltiple convencional, (ii) la explotación del dominio de colisión por el receptor, mediante la cancelación de interferencias, y (iii) la utilización de satélites multihaz, que, usando la tecnología multiantena, permiten una reutilización más eficiente del dominio espacial. Esta tesis aborda el problema de acceso múltiple masivo trabajando en los dos primeros puntos. Un esquema consensuado que cumple con el objetivo principal y con el fin de reducir la interacción entre los dispositivos y el satélite en el plano de control es Enhanced Spread Spectrum ALOHA, que combina transmisiones de paquetes cortos basadas en el ensanchamiento de la señal con la cancelación sucesiva de interferencias (SIC) en recepción. Esta combinación abre diversas vías para la asignación de energía y código a los usuarios cuando estos disponen de cierta información sobre el estado del canal. Motivado por el esquema anterior, esta tesis reexamina resultados previos bajo análisis teóricos de capacidad y cancelación perfecta, y estudia las mejores estrategias de asignación cuando el receptor SIC opera de forma no ideal. Los análisis anteriores se amplían en dos frentes: en primer lugar, adoptando políticas de decodificación y cancelación adaptadas para paquetes cortos; y, en segundo lugar, explorando el desequilibrio de energía, tasa de transmisión y fiabilidad. Con respecto al primer punto, esta tesis investiga un modelo de sistema para un receptor SIC que, inspirado en el demodulador adoptado en el sistema Enhanced Spread Spectrum ALOHA, aborda los problemas de ordenación de usuarios y decodificación iterativa con paquetes cortos. En cuanto al segundo punto, esta tesis se adentra en el régimen asintótico de usuarios y en la aplicación del cálculo de variaciones para derivar las ecuaciones de punto estacionario correspondientes a las funciones de asignación óptimas. Una de las principales contribuciones de esta tesis es el descubrimiento de funciones discontinuas (continuamente diferenciables a trozos) como una clase de distribuciones de energía ordenada para maximizar la eficiencia espectral; un enfoque que ha demostrado ser abrumadoramente exitoso. En concreto, el modelo derivado en la presente tesis incorpora, progresivamente y a lo largo de tres capítulos independientes, aspectos prácticos del cancelador de interferencias adoptado: 1. La primera parte de esta tesis investiga el impacto de la decodificación no ideal y de la cancelación imperfecta en la primera iteración de un receptor SIC asistido por control de errores. El modelo de sistema caracteriza ambas no idealidades utilizando funciones conocidas de la relación señal-a-ruido-más-interferencia (SINR) bajo la suposición de interferencia gaussiana: las funciones tasa de error de paquete (PER) y energía residual. La propagación de los eventos de éxito/fracaso en la decodificación de paquetes a lo largo de las etapas del receptor SIC se sortea en el régimen asintótico de usuarios, puesto que el modelo de sistema adopta expresiones deterministas. La asignación de energía y código se estudia en el régimen asintótico de usuarios para una amplia variedad de casos, incluyendo conjuntos formados por un número finito o infinito de esquemas de modulación y corrección de errores para paquetes de longitud finita e infinita. 2. La segunda parte de esta tesis investiga un receptor SIC iterativo y extiende las asignaciones derivadas anteriormente para iteraciones del SIC más allá de la primera. La derivación de un modelo para tal sistema supone un reto, ya que cada iteración del receptor opera con memoria respecto a las anteriores y porque las operaciones de decodificación para un mismo usuario en distintas iteraciones son estadísticamente dependientes. Esta tesis propone justificadamente un modelo de sistema que resuelve dichas dificultades añadiendo complejidad mínima al adoptado con anterioridad. En concreto, el modelo usa funciones PER multivariable, cuyos argumentos corresponden a las SINRs que experimenta un usuario a lo largo de las iteraciones del receptor, y define biyecciones para relacionar los índices de los usuarios que permanecen decodificados sin éxito en cada iteración. Se investiga el régimen asintótico de usuarios para revelar expresiones matemáticas del modelo anterior que permitan un completo entendimiento del receptor adoptado. Por último, se investiga el diseño de funciones de asignación continuamente diferenciables con extremos libres haciendo uso del modelo asintótico anterior. 3. La tercera y última parte de esta tesis estudia el problema de ordenación de usuarios en un receptor SIC que desconoce las potencias recibidas de todos ellos. La tesis deriva un modelo de sistema para un receptor SIC que gestiona un gran número de usuarios y los ordena tras estimar sus energías en la etapa inicial mediante correlaciones de preámbulo. En el régimen asintótico de usuarios, se obtienen resultados analíticos en los que el rendimiento del sistema se rige por un kernel conocido. Se demuestra que, contrariamente a las distribuciones prácticamente exponenciales obtenidas anteriormente, la asignación óptima de energía derivada para un número infinito de usuarios obedece una función constante a trozos; hecho que conlleva grandes ventajas computacionales en su aplicación.Postprint (published version

Time diversity solutions to cope with lost packets

A dissertation submitted to Departamento de Engenharia Electrotécnica of Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engenharia Electrotécnica e de ComputadoresModern broadband wireless systems require high throughputs and can also have very high Quality-of-Service (QoS) requirements, namely small error rates and short delays. A high spectral efficiency is needed to meet these requirements. Lost packets, either due to errors or collisions, are usually discarded and need to be retransmitted, leading to performance degradation. An alternative to simple retransmission that can improve both power and spectral efficiency is to combine the signals associated to different transmission attempts. This thesis analyses two time diversity approaches to cope with lost packets that are relatively similar at physical layer but handle different packet loss causes. The first is a lowcomplexity Diversity-Combining (DC) Automatic Repeat reQuest (ARQ) scheme employed in a Time Division Multiple Access (TDMA) architecture, adapted for channels dedicated to a single user. The second is a Network-assisted Diversity Multiple Access (NDMA) scheme, which is a multi-packet detection approach able to separate multiple mobile terminals transmitting simultaneously in one slot using temporal diversity. This thesis combines these techniques with Single Carrier with Frequency Division Equalizer (SC-FDE) systems, which are widely recognized as the best candidates for the uplink of future broadband wireless systems. It proposes a new NDMA scheme capable of handling more Mobile Terminals (MTs) than the user separation capacity of the receiver. This thesis also proposes a set of analytical tools that can be used to analyse and optimize the use of these two systems. These tools are then employed to compare both approaches in terms of error rate, throughput and delay performances, and taking the implementation complexity into consideration. Finally, it is shown that both approaches represent viable solutions for future broadband wireless communications complementing each other.Fundação para a Ciência e Tecnologia - PhD grant(SFRH/BD/41515/2007); CTS multi-annual funding project PEst-OE/EEI/UI0066/2011, IT pluri-annual funding project PEst-OE/EEI/LA0008/2011, U-BOAT project PTDC/EEATEL/ 67066/2006, MPSat project PTDC/EEA-TEL/099074/2008 and OPPORTUNISTICCR project PTDC/EEA-TEL/115981/200

The Challenges and Issues Facing the Deployment of RFID Technology

Griffith Sciences, School of Information and Communication TechnologyFull Tex

LoRaWAN simulation and analysis for performance enhancement of realistic networks

The Internet of Things (IoT) is becoming an ubiquitous technology, with new devices, solutions and applications being developed at an ever-increasing rate. Fundamental to the IoT revolution is the adoption of wireless protocols purposely designed to enable low-cost, long-range communication for numerous connected devices. Low Power Wide Area Networks (LPWANs) are wide area wireless telecommunication networks designed specifically for IoT applications. They allow for long-range communication at a low bit-rate among connected items, such as battery-powered sensors. However, with these benefits come also a number of drawbacks, including the limited data rate available and the reliance on low power channel access methods which can negatively impact performance in a highly dense network. The purpose of the research contained in this work is to measure the performance in terms of Quality-of-Service (QoS), Packet Delivery Ratio (PDR) and scalability of one LPWAN in particular, Long Range Wide Area Network (LoRaWAN), as well as providing possible improvements that current and future network owners can put into practice. LoRaWAN simple channel access protocol, based on pure Additive Links On-line Hawaii Area (ALOHA) is intended to reduce cost, complexity, and energy consumption while increasing transmission range. However, it also severely limits the scalability of the technology, making it more prone to packet collision, despite LoRaWAN being particularly resilient to self-interference, thanks to the underlining, proprietary Long Range (LoRa) modulation. In this thesis, LoRaWAN technology is evaluated through both software simulation and experimental deployments, with the goal of gaining a deeper understanding of the technology to then create better models and better performing deployments. The innovations and novel results presented throughout will accelerate the pervasiveness of LPWAN networks such as LoRaWAN, and ultimately their effectivness. Despite being developed in 2015, LoRa and LoRaWAN have both not been fully characterised, particularly in regard to large-scale behaviour. This is partly due to the low feasibility of deploying vast networks. To address this, the first recorded instance of anurban digital twin of 20 devices LoRaWAN network was deployed and analysed. The available simulation models, despite being successfully used in various research studies, are also not fully complete, and a deeper understanding of the technology is required to fix some remaining open issues. To give additional insight into their operation as well as practical improvements that can be carried out to maximise performance, both from a consumer and an industrial standpoint, existing LoRa and LoRaWAN modules for the network simulator NS-3 are enhanced and used throughout the work presented. Scalability and Quality-of-Service improvements are also presented, based on the knowledge gaps found in current LoRaWAN research and the results of the simulations performed. In particular, improvements on PDR up to 10% are reported using novel techniques of downlink independent optimisation, and new insight on the positioning of gateways to achieve maximum scalability in a two-gateways network are also highlighted