Interference Calculation in Asynchronous Random Access Protocols using Diversity

The use of Aloha-based Random Access protocols is interesting when channel sensing is either not possible or not convenient and the traffic from terminals is unpredictable and sporadic. In this paper an analytic model for packet interference calculation in asynchronous Random Access protocols using diversity is presented. The aim is to provide a tool that avoids time-consuming simulations to evaluate packet loss and throughput in case decodability is still possible when a certain interference threshold is not exceeded. Moreover the same model represents the groundbase for further studies in which iterative Interference Cancellation is applied to received frames.Comment: This paper has been accepted for publication in the Springer's Telecommunication Systems journal. The final publication will be made available at Springer. Please refer to that version when citing this paper; Springer Telecommunication Systems, 201

Goodbye, ALOHA!

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.The vision of the Internet of Things (IoT) to interconnect and Internet-connect everyday people, objects, and machines poses new challenges in the design of wireless communication networks. The design of medium access control (MAC) protocols has been traditionally an intense area of research due to their high impact on the overall performance of wireless communications. The majority of research activities in this field deal with different variations of protocols somehow based on ALOHA, either with or without listen before talk, i.e., carrier sensing multiple access. These protocols operate well under low traffic loads and low number of simultaneous devices. However, they suffer from congestion as the traffic load and the number of devices increase. For this reason, unless revisited, the MAC layer can become a bottleneck for the success of the IoT. In this paper, we provide an overview of the existing MAC solutions for the IoT, describing current limitations and envisioned challenges for the near future. Motivated by those, we identify a family of simple algorithms based on distributed queueing (DQ), which can operate for an infinite number of devices generating any traffic load and pattern. A description of the DQ mechanism is provided and most relevant existing studies of DQ applied in different scenarios are described in this paper. In addition, we provide a novel performance evaluation of DQ when applied for the IoT. Finally, a description of the very first demo of DQ for its use in the IoT is also included in this paper.Peer ReviewedPostprint (author's final draft

Performance evaluation of framed slotted ALOHA with reservation packets and succesive interference cancelation for M2M networks

[EN] Random access protocols like ALOHA have been considered for machine-to-machine (M2M) communication in future networks for their simplicity of operation. This paper evaluates the performance of a Frame Slotted-ALOHA protocol that uses reservation and data packets (FSA-RDP), in a scenario where a controller collects data packets transmitted by a finite number of M2M devices. In FSA-RDP, frames of variable duration are divided in two parts, the reservation and data subframes. During the reservation subframe, active devices send short reservation packets to the controller. The controller assigns reserved slots in the data subframe to those devices that succeeded with the reservation. At devices, the FIFO service discipline and two queue management schemes, tail drop and push-out, have been considered. When the queue size is of one packet, we develop a discrete-time Markov chain to evaluate the protocol performance, including the cumulative distribution function of the delay of data packets that are successfully transmitted. Analytical results are validated by extensive simulations. The simulation model is also used to evaluate the system performance when larger queues are used. In addition, we study the impact that implementing Successive Interference Cancellation (SIC) at the controller has on the system performance. We also evaluate the performance of implementing SIC at the controller together with Irregular Repetition Slotted ALOHA (IRSA) to send the reservation packets. Numerical results show that the protocol efficiency of FSA-RDP is between one and two orders of magnitude larger than the efficiency of conventional Frame Slotted ALOHA, when a perfect channel is assumed. In more realistic channel environments, the use of SIC brings an important performance boost.This work has been supported by the Ministry of Economy and Competitiveness of Spain through projects TIN2013-47272-C2-1-R and TEC2015-71932-REDT. The authors would like to thank the support received from the Institute ITACA (Instituto Universitario de Tecnologias de la Informacion y Comunicaciones) at the Universitat Politecnica de Valencia, Spain. C. Portillo acknowledges the funding received from the European Union under the program Erasmus Mundus Partnerships, project EuroinkaNet, GRANT AGREEMENT NUMBER -2014 -0870/001/001, and the support received from SEP-SES (DSA/103.5/15/6629).Casares-Giner, V.; Martínez Bauset, J.; Portillo, C. (2019). Performance evaluation of framed slotted ALOHA with reservation packets and succesive interference cancelation for M2M networks. Computer Networks. 155:15-30. https://doi.org/10.1016/j.comnet.2019.02.021S153015

Advanced random access techniques for satellite communications

In this thesis, Advanced Random Access techniques for Satellite Communications are studied. In the last years, new advances in multi-access communication protocols together with the increasing need for bidirectional communications in consumer type of interactive satellite terminals have revived the interest for a set of schemes able to guarantee high-speed and low latency communications in bursty traffic conditions. In this work, starting from the latest findings on Aloha-based Random Access schemes, the optimization of such techniques and their use in closed-loop scenarios is investigated with particular regard to the Return Channel over Satellite of Digital Video Broadcasting. The thesis starts with a summary on the state of the art of Demand Assigned and Random Access techniques as well as on the recent evolution from the first to the second version of the Return Channel over Satellite of the Digital Video Broadcasting specification. In chapter 2 a stability and packet delay model for channel analysis and design are presented, showing that proper design through this tools can ensure high performance of the new access scheme. The use of control limit policies is also introduced and its use is thoroughly discussed both for finite and infinite users population showing that, differently from Slotted Aloha, in some cases static design over dynamic policies might be preferable if long propagation delay is present. In chapter 3 the same models and tools introduced for CRDSA are extended to the case of asynchronous Random Access schemes and a comparison of the two families of schemes is put in place demonstrating that asynchronous techniques are convenient only when the signal-to-noise ratio is high enough to ensure decodability of partially colliding packets. In chapter 4 a new access scheme currently patent pending is presented. In this scheme terminals access the channel in an unframed manner. It is shown that such a change brings improvements that further diminish latency due to immediate transmission of the first replica and further boost throughput because the number of loops on the corresponding bipartite graph representation is mitigated. The thesis concludes with a call for a new discussion of resource allocation in multi-access satellite communication scenarios such as DVB-RCS2 in light of the obtained results and of the new requirements in interactive satellite networks

High Performance Signal Processing-Based Collision Resolution for Random Access Schemes

Els darrers anys han experimentat un augment de la demanda de serveis interactius per satèl·lit per al gran consum, cobrint serveis fixes i mòbils, tal i com accés de banda ampla, comunicacions màquina-màquina (M2M), supervisió, control i adquisició de dades (SCADA), transaccions i aplicacions de seguretat crítiques. Aquestes xarxes de comunicacions es caracteritzen per tenir una gran població d’usuaris compartint l’amplada de banda amb unes condicions de tràfic molt dinàmiques. Concretament, en el canal de retorn (de l’usuari a la xarxa) de xarxes d’accés de banda ampla, els usuaris residencials generen grans ràfegues de tràfic amb períodes d’inactivitat freqüents. Una situació similar succeeix en xarxes de comunicacions mòbils per satèl·lit, on una gran població de terminals generen transmissions infreqüents de senyalització, serveis basats en la localització or altres aplicacions de missatgeria. Aquests serveis requereixen el desenvolupament de protocols d’accés múltiple eficients que puguin operar en les condicions descrites anteriorment. Els protocols d´accés aleatori són bons candidats per servir tràfic poc predictiu, amb transmissions infreqüents així com sensibles amb el retard. A més, els protocols d´accés aleatori suporten un gran nombre de terminals compartint el canal de comunicacions i requereixen poca complexitat en el terminals. El protocols d´accés aleatori han estat àmpliament estudiats i desplegats en xarxes terrestres, però les seves prestacions són pobres en el entorn satèl·lital, que està caracteritzat per retards de propagació molt grans. Avui en dia, el seu ús en les xarxes de comunicacions per satèl·lit està principalment limitat a la senyalització d’inici de sessió, transmissió de paquets de control i en alguns casos a la transmissió de petits volums de dades amb unes eficiència d’utilització del canal molt baixa. Aquesta tesi proposa tres noves tècniques d’accés aleatori, bens adaptades per proveir els serveis esmentats anteriorment en un entorn satèl·lital, amb altes prestacions i una complexitat en el terminal d’usuari reduïda. Les noves tècniques d’accés aleatori són Contention Resolution Diversity Slotted Aloha (CRDSA), Asynchronous Contention Resolution Diversity Aloha (ACRDA) i Enhanced Spread Spectrum Aloha (E-SSA), adaptades per un tipus d’accés ranurat, asíncron i d’espectre eixamplat respectivament. Les tres tècniques utilitzen una codificació de canal (FEC) robusta, capaç d’operar en front de interferències elevades, que són típiques en l’accés aleatori, i d’un mecanisme de cancel·lació successiva d’interferència que s’implementa en el receptor sobre els paquets descodificats satisfactòriament. Els nous protocols obtenen un throughput normalitzat superior a 1 bit/s/Hz amb una tassa de pèrdua de paquets inferior a 10-3, el qual representa un factor de millora de 1000 respecte a protocols d’accés aleatori tradicionals com l’ALOHA ranurat. Les prestacions de les noves tècniques d’accés aleatori has estat analitzades per mitjà de simulacions, així com amb nou models analítics desenvolupats en aquesta tesi capaços de caracteritzar el tràfic, la distribució estadística de la potència dels paquets, les prestacions de la codificació de canal (FEC) i el procés de cancel·lació d’interferència successiva.Los últimos años han experimentado un crecimiento de la demanda de servicios interactivos por satélite para el gran consumo, cubriendo servicios fijos i móviles, como el acceso de banda ancha, comunicaciones máquina a máquina (M2M), supervisión, control y adquisición de datos (SCADA), transacciones i aplicaciones criticas de seguridad. Estas redes de comunicaciones se caracterizan por tener una gran población de usuarios compartiendo el ancho de banda en unas condiciones de tráfico muy dinámicas. Concretamente, en el canal de retorno (del usuario a la red) de redes de acceso de banda ancha, los usuarios residenciales generan grandes ráfagas de tráfico con periodos frecuentes de inactividad. Una situación similar ocurre en las redes de comunicaciones móviles por satélite, donde una gran población de terminales generan transmisiones infrecuentes de señalización, servicios basados en la localización u otras aplicaciones me mensajería. Estos servicios requieren el desarrollo de protocolos de acceso múltiple eficientes capaces de operar en las condiciones descritas anteriormente. Los protocolos de acceso aleatorio son buenos candidatos para servir el tráfico poco predictivo, con transmisiones infrecuentes así como sensibles al retardo. Además, los protocolos de acceso soportan un gran número de terminales compartiendo el canal de comunicaciones y requieren poca complejidad en los terminales. Los protocolos de acceso aleatorio han estado ampliamente estudiados i desplegados en las redes terrestres, pero sus prestaciones son pobres en el entorno satelital, que se caracteriza por retardos de comunicaciones muy elevados. Hoy en día, su uso en la redes de comunicaciones por satélite está principalmente limitado a la señalización de inicio de sesión, transmisión de pequeños volumenes de datos con eficiencia de utilización del canal muy baja. Esta tesis propone tres nuevas técnicas de acceso aleatorio bien adaptadas para proveer los servicios mencionados anteriormente en un entorno de comunicaciones por satélite, con altas prestaciones y una complejidad en el terminal de usuario reducida. Las nuevas técnicas de acceso aleatorio son Contention Resolution Diversity Slotted Aloha (CRDSA), Asynchronous Contention Resolution Diversity Aloha (ACRDA) y Enhanced Spread Spectrum Aloha (E-SSA), adaptadas para un tipo de acceso ranurado, asíncrono y de espectro ensanchado respectivamente. Las tres técnicas utilizan una codificación de canal (FEC) robusta, capaz de operar en condiciones de interferencia elevadas, que son típicas en el acceso aleatorio, y de un mecanismo de cancelación sucesiva de interferencias que se implementa en el receptor sobre los paquetes que han sido decodificados satisfactoriamente. Los nuevos protocolos obtienen un throughput normalizado superior a 1 bit/s/Hz con una tasa de pérdida de paquetes inferior a 10-3, lo cual representa un factor de mejora de 1000 respecto a los protocolos de acceso aleatorio tradicionales como el ALOHA ranurado. Las prestaciones de las nuevas técnicas de acceso aleatorio han sido analizadas con simulaciones así como con nuevos modelos analíticos desarrollados en esta tesis, capaces de caracterizar el tráfico, la distribución estadística de la potencia de los paquetes, las prestaciones de la codificación de canal (FEC) y el proceso de cancelación sucesiva de interferencias.Over the past years there has been a fast growing demand for low-cost interactive satellite terminals supporting both fixed and mobile services, such as consumer broadband access, machine-to-machine communications (M2M), supervisory control and data acquisition (SCADA), transaction and safety of life applications. These networks, are generally characterized by a large population of terminals sharing the available resources under very dynamic traffic conditions. In particular, in the return link (user to network) of commercial satellite broadband access networks, residential users are likely to generate a large amount of low duty cycle bursty traffic with extended inactivity periods. A similar situation occurs in satellite mobile networks whereby a large number of terminals typically generate infrequent packets for signaling transmission as well for position reporting or other messaging applications. These services call for the development of efficient multiple access protocols able to cope with the above operating conditions. Random Access (RA) techniques are by nature, good candidates for the less predictive, low duty cycle as well as time sensitive return link traffic. Besides, RA techniques are capable of supporting large population of terminals sharing the same capacity and require low terminal complexity. RA schemes have been widely studied and deployed in terrestrial networks, but do not perform well in the satellite environment, which is characterized by very long propagation delays. Today, their use in satellite networks is mainly limited to initial network login, the transmission of control packets, and in some cases, for the transmission of very small volumes of data with very low channel utilization. This thesis proposes three novel RA schemes well suited for the provision of the above-mentioned services over a satellite environment with high performance and low terminal complexity. The new RA schemes are Contention Resolution Diversity Slotted Aloha (CRDSA), Asynchronous Contention Resolution Diversity Aloha (ACRDA) and Enhanced Spread Spectrum Aloha (E-SSA), suited for slotted, unslotted and spread spectrum-based systems respectively. They all use strong Forward Error Correction (FEC) codes, able to cope with heavy co-channel interference typically present in RA, and successive interference cancellation implemented over the successfully decoded packets. The new schemes achieve a normalized throughput above 1 bit/s/Hz for a packet loss ratio below 10-3, which represents a 1000-fold increase compared to Slotted ALOHA. The performance of the proposed RA schemes has been analyzed by means of detailed simulations as well as novel analytical frameworks that characterize traffic and packets power statistical distributions, the performance of the FEC coding as well as the iterative interference cancellation processing at the receiver

Modeling and analysis of reservation frame slotted-ALOHA in wireless machine-to-machine area networks for data collection

Reservation frame slotted-ALOHA (RFSA) was proposed in the past to manage the access to the wireless channel when devices generate long messages fragmented into small packets. In this paper, we consider an M2M area network composed of end-devices that periodically respond to the requests from a gateway with the transmission of fragmented messages. The idle network is suddenly set into saturation, having all end-devices attempting to get access to the channel simultaneously. This has been referred to as delta traffic. While previous works analyze the throughput of RFSA in steady-state conditions, assuming that traffic is generated following random distributions, the performance of RFSA under delta traffic has never received attention. In this paper, we propose a theoretical model to calculate the average delay and energy consumption required to resolve the contention under delta traffic using RFSA.We have carried out computer-based simulations to validate the accuracy of the theoretical model and to compare the performance for RFSA and FSA. Results show that there is an optimal frame length that minimizes delay and energy consumption and which depends on the number of end-devices. In addition, it is shown that RFSA reduces the energy consumed per end-device by more than 50% with respect to FSA under delta traffic.Peer ReviewedPostprint (published version

Towards zero-power wireless machine-to-machine networks

This thesis aims at contributing to overcome two of the main challenges for the deployment of M2M networks in data collection scenarios for the Internet of Things: the management of massive numbers of end-devices that attempt to get access to the channel; and the need to extend the network lifetime. In order to solve these challenges, two complementary strategies are considered. Firstly, the thesis focuses on the design, analysis and performance evaluation of MAC protocols that can handle abrupt transitions in the traffic load and minimize the energy consumption devoted to communications. And secondly, the use of energy harvesting (EH) is considered in order to provide the network with unlimited lifetime. To this end, the second part of the thesis focuses on the design and analysis of EH-aware MAC protocols. While the Frame Slotted-ALOHA (FSA) protocol has been traditionally adopted in star topology networks for data collection, results show that FSA-based protocols lack of scalability and present synchronization problems as the network density increases. Indeed, the frame length of FSA must be adjusted to the number of contenders, which may be complex to attain in dense networks with large and dynamic number of end-devices. In order to overcome these issues, a tree splitting-based random access protocol, referred to as Low Power Contention Tree-based Access (LP-CTA), is proposed in the first part of this thesis. In LP-CTA, the frame length can be very short and fixed, which facilitates synchronization and provides better network scalability than FSA. While LP-CTA uses data slots for contention, it is possible to use short access requests in minislots, where collisions are resolved using tree splitting, and avoid the contention in data. Since these minislots can be much shorter than the duration of a data packet, the performance can be improved. The Low Power Distributed Queuing (LP-DQ) protocol proposed in this thesis is based on this idea. LP-DQ combines tree splitting with the logic of two distributed queues that manage the contention resolution and the collision-free data transmission. Results show that LP-DQ outperforms LP-CTA and FSA in terms of delay and energy efficiency, and it relaxes the need to know the size of the network and adapts smoothly to any change in the number of end-devices. The approach of LP-DQ is convenient when the messages transmitted by each end-device fit in one single slot, however, if the end-devices generate long messages that have to be fragmented, it is better to add a reservation mechanism in order to boost the performance. In this sense, the LPR-DQ protocol is proposed as an extension of LP-DQ where the concept of reservation is integrated to allow the end-devices reserve as many collision-free slots as needed. The second part of the thesis is devoted to the integration of the MAC layer with the use of energy harvesting. The variability and fluctuations of the harvested energy is considered for the design of EH-aware MAC protocols and three performance metrics are proposed: the probability of delivery, the data delivery ratio and the time efficiency. Previous works on data collection networks with EH focus on DFSA. In this thesis, the EH-CTA protocol is proposed as an adaptation of LP-CTA that takes the energy harvesting process into account. Results show that EH-CTA outperforms DFSA if the energy threshold for an end-device to become active is properly configured. In addition, while DFSA needs to adapt the frame length dynamically, EH-CTA uses a fixed frame length, thus facilitating scalability and synchronization. Finally, the EH-RDFSA and EH-DQ protocols are proposed for scenarios where data must be fragmented. EH-RDFSA is a combination of RFSA and DFSA, and EH-DQ is an extension of LPR-DQ.Esta tesis contribuye a resolver dos de los retos para el despliegue de redes M2M en escenarios de recolección de datos para el Internet de las Cosas: la gestión del acceso al canal de un número masivo de dispositivos; y la necesidad de extender la vida de la red. Para resolverlos se consideran dos estrategias complementarias. En primer lugar, se centra en el diseño, el análisis y la evaluación de protocolos MAC que pueden manejar transiciones abruptas de tráfico y reducen el consumo de energía. Y en segundo lugar, se considera el uso de mecanismos de captura de energía (Energy Harvesters, EH) para ofrecer un tiempo de vida ilimitado de la red. Con este fin, la segunda parte de la tesis se centra en el diseño y el análisis de protocolos MAC de tipo "EH-aware". Mientras que Frame Slotted-ALOHA (FSA) ha sido tradicionalmente adoptado en aplicaciones de recolección de datos, los resultados muestran que FSA presenta problemas de escalabilidad y sincronización cuando aumenta la densidad de la red. De hecho, la longitud de trama de FSA se debe ajustar según sea el número de dispositivos, lo cual puede ser difícil de estimar en redes con un número elevado y dinámico de dispositivos. Para superar estos problemas, en esta tesis se propone un protocolo de acceso aleatorio basado en "tree-splitting" denominado Low Power Contention Tree-based Access (LP-CTA). En LP-CTA, la longitud de trama puede ser corta y constante, lo cual facilita la sincronización y proporciona mejor escalabilidad. Mientras que LP-CTA utiliza paquetes de datos para la contienda, es posible utilizar solicitudes de acceso en mini-slots, donde las colisiones se resuelven utilizando "tree-splitting", y evitar la contención en los datos. Dado que estos mini-slots pueden ser mucho más cortos que la duración de un slot de datos, el rendimiento de LP-CTA puede ser mejorado. El protocolo Low Power Distributed Queuing (LP-DQ) propuesto en esta tesis se basa en esta idea. LP-DQ combina "tree-splitting" con la lógica de dos colas distribuidas que gestionan la resolución de la contienda en la solicitud de acceso y la transmisión de datos libre de colisiones. Los resultados demuestran que LP-DQ mejora LP-CTA y FSA en términos de retardo y eficiencia energética, LP-DQ no requiere conocer el tamaño de la red y se adapta sin problemas a cualquier cambio en el número de dispositivos. LP-DQ es conveniente cuando los mensajes transmitidos por cada dispositivo caben en un único slot de datos, sin embargo, si los dispositivos generan mensajes largos que requieren fragmentación, es mejor añadir un mecanismo de reserva para aumentar el rendimiento. En este sentido, el protocolo LPR-DQ se propone como una extensión de LP-DQ que incluye un mecanismo de reserva para permitir que cada dispositivo reserve el número de slots de datos según sea el número de fragmentos por mensaje. La segunda parte de la tesis está dedicada a la integración de la capa MAC con el uso de "Energy Harvesters". La variabilidad y las fluctuaciones de la energía capturada se consideran para el diseño de protocolos MAC de tipo "EH-aware" y se proponen tres métricas de rendimiento: la probabilidad de entrega, el "Data Delivery Ratio" y la eficiencia temporal. Los trabajos previos en redes de recolección de datos con EH se centran principalmente en DFSA. En esta tesis, el protocolo EH-CTA se propone como una adaptación de LP-CTA que tiene en cuenta el proceso de captura de energía. Los resultados muestran que EH-CTA supera DFSA si el umbral de energía para que un dispositivo se active está configurado correctamente. Además, mientras que en DFSA se necesita adaptar la longitud de trama de forma dinámica, EH-CTA utiliza una longitud de trama fija, facilitando así la escalabilidad y la sincronización. Por último, se proponen los protocolos EH-RDFSA y EH-DQ para escenarios en los que los datos deben ser fragmentados. EH-RDFSA es una combinación de RFSA y DFSA, y EH-DQ es una extensión de LPR-DQ.Aquesta tesi contribueix a resoldre dos dels reptes per al desplegament de xarxes M2M en escenaris de recol·lecció de dades per a l'Internet de les Coses: la gestió de l'accés al canal d'un nombre massiu de dispositius; i la necessitat d'extendre la vida de la xarxa. Per resoldre'ls es consideren dues estratègies complementàries. En primer lloc, es centra en el disseny, l'anàlisi i l'avaluació de protocols MAC que poden manegar transicions abruptes de trànsit i redueixen el consum d'energia. I en segon lloc, es considera l'ús de mecanismes de captura d'energia (Energy Harvesters, EH) per a oferir un temps de vida il·limitat de la xarxa. Amb aquesta finalitat, la segona part de la tesi es centra en el disseny i l'anàlisi de protocols MAC de tipus "EH-aware".Mentre que Frame Slotted-ALOHA (FSA) ha estat tradicionalment adoptat en aplicacions de recol·lecció de dades, els resultats mostren que FSA presenta problemes d'escalabilitat i sincronització quan augmenta la densitat de la xarxa. De fet, la longitud de trama de FSA s'ha d'ajustar segons sigui el nombre de dispositius, la qual cosa pot ser difícil d'estimar en xarxes amb un nombre elevat i dinàmic de dispositius. Per superar aquests problemes, en aquesta tesi es proposa un protocol d'accés aleatori basat en "tree-splitting" denominat Low Power Contention Tree-based Access (LP-CTA). En LP-CTA, la longitud de trama pot ser curta i constant, la qual cosa facilita la sincronització i proporciona millor escalabilitat.Mentre que LP-CTA utilitza paquets de dades per a la contenció, és possible utilitzar sol·licituds d'accés a mini-slots, on les col·lisions es resolen utilitzant "tree-splitting", i evitar la contenció a les dades. Atès que aquests mini-slots poden ser molt més curts que la durada d'un slot de dades, el rendiment de LP-CTA pot ser millorat. El protocol Low Power Distributed Queuing (LP-DQ) proposat en aquesta tesi es basa en aquesta idea. LP-DQ combina "tree-splitting" amb la lògica de dues cues distribuïdes que gestionen la resolució de la contenció en la sol·licitud d'accés i la transmissió de dades lliure de col·lisions. Els resultats demostren que LP-DQ millora LP-CTA i FSA en termes de retard i eficiència energètica, LP-DQ no requereix conèixer la mida de la xarxa i s'adapta sense problemes a qualsevol canvi en el nombre de dispositius.LP-DQ és convenient quan els missatges transmesos per cada dispositiu caben en un únic slot de dades, però, si els dispositius generen missatges llargs que requereixen fragmentació, és millor afegir un mecanisme de reserva per augmentar el rendiment. En aquest sentit, el protocol LPR-DQ es proposa com una extensió de LP-DQ que inclou un mecanisme de reserva per a permetre que cada dispositiu reservi el nombre de slots de dades segons sigui el nombre de fragments per missatge.La segona part de la tesi està dedicada a la integració de la capa MAC amb l'ús de "Energy Harvesters". La variabilitat i les fluctuacions de l'energia capturada es consideren per al disseny de protocols MAC de tipus "EH-aware" i es proposen tres mètriques de rendiment: la probabilitat d'entrega, el "Data Delivery Ratio" i l'eficiència temporal.Els treballs previs en xarxes de recol·lecció de dades amb EH se centren principalment en DFSA. En aquesta tesi, el protocol EH-CTA es proposa com una adaptació de LP-CTA que té en compte el procés de captura d'energia. Els resultats mostren que EH-CTA supera DFSA si el llindar d'energia perquè un dispositiu s'activi s'ajusta correctament. A més, mentre que a DFSA es necessita adaptar la longitud de trama de forma dinàmica, EH-CTA utilitza una longitud de trama fixa, facilitant així l'escalabilitat i la sincronització. Finalment, es proposen els protocols EH-RDFSA i EH-DQ per a escenaris en els quals les dades han de ser fragmentades. EH-RDFSA és una combinació de RFSA i DFSA, i EH-DQ és una extensió de LPR-DQ.Postprint (published version

Modern Random Access for Satellite Communications

The present PhD dissertation focuses on modern random access (RA) techniques. In the first part an slot- and frame-asynchronous RA scheme adopting replicas, successive interference cancellation and combining techniques is presented and its performance analysed. The comparison of both slot-synchronous and asynchronous RA at higher layer, follows. Next, the optimization procedure, for slot-synchronous RA with irregular repetitions, is extended to the Rayleigh block fading channel. Finally, random access with multiple receivers is considered.Comment: PhD Thesis, 196 page

Distributed opportunistic scheduling algorithms for wireless communications.

In this thesis, we propose a number of distributed schemes for wireless communications in the cross layer design context, considering an uplink random access network in which multiple users communicate with a common base station. In addition, we perform a comprehensive study on a splitting based multiuser selection algorithm which is simple, effective, and scales with the network size. First, we investigate a reservation-type protocol in a channel aware ALOHA system. Various Markovian models are used to describe the system and to capture the temporal correlation of the channel evolution. The average throughput of the system is obtained using the Markov Analysis technique and we show that the reservation protocol can achieve better performance than the original channel-aware ALOHA by reducing the collision probability. Second, for better resource utilization in the Opportunistic Multichannel ALOHA scheme, we propose a simple extension to the transmission policy that exploits the idle channels. Performance analysis shows that, theoretically, the maximum system throughput can be improved by up to 63% in the asymptotic case. Through numerical results, it can be seen that a significant gain is achieved even when the system consists of a small number of users. Third, we consider a splitting based multiuser selection algorithm in a probabilistic view. Asymptotic analysis leads to a functional equation, similar to that encountered in the analysis of the collision resolution algorithm. Subject to some conditions, the solution of the functional equation can be obtained, which provides the approximations for the expected number of slots and the expected number of transmissions required by the algorithm in a large system. These results shed light on open design problems in choosing parameters for the algorithm when considering the delay and the overhead jointly. A typical example is to optimize the parameters that minimize the weighted sum of these measures of interest

Energy-efficient diversity combining for different access schemes in a multi-path dispersive channel

Dissertação para obtenção do Grau de Doutor em Engenharia Electrotécnica e ComputadoresThe forthcoming generation of mobile communications, 5G, will settle a new standard for a larger bandwidth and better Quality of Service (QoS). With the exploding growth rate of user generated data, wireless standards must cope with this growth and at the same time be energy efficient to avoid depleting the batteries of wireless devices. Besides these issues, in a broadband wireless setting QoS can be severely affected from a multipath dispersive channel and therefore be energy demanding. Cross-layered architectures are a good choice to enhance the overall performance of a wireless system. Examples of cross-layered Physical (PHY) - Medium Access Control (MAC) architectures are type-II Diversity Combining (DC) Hybrid-ARQ (H-ARQ) and Multi-user Detection (MUD) schemes. Cross-layered type-II DC H-ARQ schemes reuse failed packet transmissions to enhance data reception on posterior retransmissions; MUD schemes reuse data information from previously collided packets on posterior retransmissions to enhance data reception. For a multipath dispersive channel, a PHY layer analytical model is proposed for Single-Carrier with Frequency Domain Equalization (SC-FDE) that supports DC H-ARQ and MUD. Based on this analytical model, three PHY-MAC protocols are proposed. A crosslayered Time Division Multiple Access (TDMA) scheme that uses DC H-ARQ is modeled and its performance is studied in this document; the performance analysis shows that the scheme performs better with DC and achieves a better energy efficiency at the cost of a higher delay. A novel cross-layered prefix-assisted Direct-Sequence Code Division Multiple Access (DS-CDMA) scheme is proposed and modeled in this document, it uses principles of DC and MUD. This protocol performs better by means of additional retransmissions, achieving better energy efficiency, at the cost of higher redundancy from a code spreading gain. Finally, a novel cross-layered protocol H-ARQ Network Division Multiple Access (H-NDMA) is proposed and modeled, where the combination of DC H-ARQ and MUD is used with the intent of maximizing the system capacity with a lower delay; system results show that the proposed scheme achieves better energy efficiency and a better performance at the cost of a higher number of retransmissions. A comparison of the three cross-layered protocols is made, using the PHY analytical model, under normalized conditions using the same amount of maximum redundancy. Results show that the H-NDMA protocol, in general, obtains the best results, achieving a good performance and a good energy efficiency for a high channel load and low Signal-to-Noise Ratio (SNR). TDMA with DC H-ARQ achieves the best energy efficiency, although presenting the worst delay. Prefix-assisted DS-CDMA in the other hand shows good delay results but presents the worst throughput and energy efficiency