Exploiting Device-to-Device Communications to Enhance Spatial Reuse for Popular Content Downloading in Directional mmWave Small Cells

With the explosive growth of mobile demand, small cells in millimeter wave (mmWave) bands underlying the macrocell networks have attracted intense interest from both academia and industry. MmWave communications in the 60 GHz band are able to utilize the huge unlicensed bandwidth to provide multiple Gbps transmission rates. In this case, device-to-device (D2D) communications in mmWave bands should be fully exploited due to no interference with the macrocell networks and higher achievable transmission rates. In addition, due to less interference by directional transmission, multiple links including D2D links can be scheduled for concurrent transmissions (spatial reuse). With the popularity of content-based mobile applications, popular content downloading in the small cells needs to be optimized to improve network performance and enhance user experience. In this paper, we develop an efficient scheduling scheme for popular content downloading in mmWave small cells, termed PCDS (popular content downloading scheduling), where both D2D communications in close proximity and concurrent transmissions are exploited to improve transmission efficiency. In PCDS, a transmission path selection algorithm is designed to establish multi-hop transmission paths for users, aiming at better utilization of D2D communications and concurrent transmissions. After transmission path selection, a concurrent transmission scheduling algorithm is designed to maximize the spatial reuse gain. Through extensive simulations under various traffic patterns, we demonstrate PCDS achieves near-optimal performance in terms of delay and throughput, and also superior performance compared with other existing protocols, especially under heavy load.Comment: 12 pages, to appear in IEEE Transactions on Vehicular Technolog

Improving the performance of wireless sensor networks using directional antennas

Over the last decades, lots of new applications have emerged thanks to the availability of small devices capable of wireless communications that form Wireless Sensor Networks (WSNs). These devices allow sensing, processing, and communication of multiple physical variables while keeping a low power consumption. During the last years, most of the research efforts were spent on the development and optimization of wireless communication protocols, aiming to maximize the reliability of the network while achieving the lowest possible power consumption. In this thesis, we study how to improve the performance of these WSNs by using directional antennas. Directional antennas can provide a higher gain and reduce the interference with other nodes by concentrating the radiated power in a certain direction. We present the different kinds of directional antennas available for WSNs, and we select the 6-element SPIDA antenna as a case of study. We present an electromagnetic model of this antenna, and we incorporate it into the COOJA network simulator. We report the first complete characterization of this antenna, including the radiation pattern and S11 parameters. The characterization shows that the antenna has a maximum gain of 6.8 dBi, a Half-Power Beamwidth (HPBW) of 113° and a module of S11 parameter of -7.5 dB at the central frequency (fc = 2.4525 GHz). We also present a novel way to optimize the antenna without changing its design by isolating multiple director elements. We show that with this technique, the performance of the antenna can be improved in terms of maximum gain, narrower HPBW, and a lower module of the S11 parameter without making any changes in the antenna itself. We evaluate the impact of supporting directional communications in the different layers of the network stack. We analyze the different challenges that arise and propose optimizations to overcome them in order to take advantage of the benefits of directional communication. We present an analysis of the state-of-the-art in neighbor discovery protocols for WSNs with directional antennas, and we propose, implement end evaluate two novel fully directional protocols: Q-SAND and DANDi. We compare both of them with SAND, a fully directional neighbor discovery protocol. DANDi is a fully directional asynchronous and dynamic neighbor discovery protocol where the contention resolution relies on a collision detection mechanism. To the best of our knowledge, DANDi is the fastest neighbor discovery protocol for WSN with directional antennas, with the additional advantage of being able to discover every reliable communication link in a network without requiring any prior information of the network topology. We combine the directional neighbor discovery protocol with MAC and routing optimizations in order fully take advantage of the benefits of using directional antennas. We focus on convergecast, a typical data collection application where every node sends packets periodically to a sink node. We present DirMAC, a novel MAC protocol that fully supports directional communication, together with four different heuristics to optimize the performance of the protocols. One of these heuristics has the added major benefit of being completely distributed and with no need for offline processing. Our evaluation shows that optimizations at both the MAC and routing layers are needed in order to reap the benefits of using directional antennas for convergecast. Our results show that the performance of the network can be greatly improved in terms of packet delivery rate, energy consumption, and energy per received packet, and that we obtain the largest performance improvements in networks with dense traffic. Simulations with different node densities show that when using directional antennas the PDR increases up to 29%, while energy consumption and energy per received packet decreases by up to 55% and 46% respectively. Experiments with real nodes validate these results showing a significant performance increase when using directional antennas in our scenarios, with a reduction in the RDC and EPRP of 25% and 15% respectively, while maintaining a PDR of 100%.Durante las últimas décadas, la disponibilidad de pequeños dispositivos con comunicación inalámbrica ha permitido el desarrollo de muchas nuevas aplicaciones. Estos dispositivos forman Redes de Sensores Inalámbricos (RSI, o WSN por sus siglas en inglés) que permiten sensar, procesar y comunicar datos provenientes de variables físicas, mientras que mantienen un bajo consumo energético. En los últimos años, la mayor parte de los esfuerzos de la comunidad científica estuvieron concentrados en el desarrollo y optimización de los protocolos de comunicación inalámbricos, buscando maximizar la confiabilidad de la red y minimizar el consumo energético. En esta tesis estudiamos cómo mejorar el rendimiento de las RSI usando antenas direccionales. Las antenas direccionales pueden proporcionar una mayor ganancia y reducir la interferencia con otros nodos al concentrar la potencia radiada en una cierta dirección. Comenzamos presentando los distintos tipos de antenas direccionales disponibles para las RSI, y seleccionamos la antena SPIDA de 6 elementos como caso de estudio. Luego presentamos un modelo electromagnético de la antena, que incorporamos al simulador de red COOJA. Construimos un primer prototipo con el que realizamos la primera caracterización completa de ésta antena, incluyendo el patrón de radiación y el parámetro S11. La caracterización muestra que la antena tiene una ganancia máxima de 6,8 dBi, un ancho de haz a mitad de potencia (HPBW por sus siglas en inglés) de 113° y un módulo del parámetro S11 de -7,5 dB en la frecuencia central (fc = 2,4525 GHz). También mostramos una forma innovadora de optimizar la antena sin cambiar su diseño utilizando varios elementos directores al mismo tiempo. Mostramos que con esta técnica se puede mejorar el rendimiento de la antena en términos de ganancia máxima, ancho de haz a mitad de potencia, y módulo del parámetro S11. Luego evaluamos el impacto de usar comunicaciones direccionales en las diferentes capas del stack de red. Analizamos los diferentes desafíos que surgen y proponemos optimizaciones para sortearlos. Presentamos un análisis del estado del arte en protocolos de descubrimiento de vecinos en RSI con antenas direccionales, y proponemos, implementamos y evaluamos dos protocolos direccionales : Q-SAND y DANDi. DANDi es un protocolo de descubrimiento de vecinos direccional, asíncrono y dinámico, donde la contienda por el canal se resuelve con un mecanismo basado en la detección de colisiones. Hasta donde sabemos, DANDi es el protocolo de descubrimiento de vecinos más rápido para RSI con antenas direccionales, con la ventaja adicional de que permite descubrir todos los enlaces de comunicación confiables de una red sin requerir ningún conocimiento previo de la topología. Luego combinamos los protocolos de descubrimiento de vecinos con optimizaciones en las capas de ruteo y acceso al medio para construir una aplicación de recolección de datos, donde cada nodo envía paquetes periódicamente a un nodo centralizador. Presentamos DirMAC, un protocolo de acceso al medio innovador que soporta comunicaciones direccionales, junto con cuatro heurísticas que permiten optimizar el rendimiento de los protocolos (una de ellas con la ventaja adicional que es totalmente distribuida). Los resultados muestran que usar antenas direccionales en este tipo de aplicaciones permite mejorar sustancialmente el rendimiento de la red, mostrando las mayores mejoras en redes con alto tráfico. Las simulaciones con diferentes densidades de nodos muestran que al usar antenas direccionales se puede aumentar el ratio de entrega de paquetes en hasta 29%, mientras que el consumo energético y la energía por paquete recibido bajan en hasta 55% y 46% respectivamente. Los experimentos en nodos reales validan estos resultados, mostrando una reducción en el consumo energético y en la energía por paquete recibido de 25% y 15% respectivamente, mientras que mantienen un ratio de entrega de paquetes de 100%

Survey of Inter-satellite Communication for Small Satellite Systems: Physical Layer to Network Layer View

Small satellite systems enable whole new class of missions for navigation, communications, remote sensing and scientific research for both civilian and military purposes. As individual spacecraft are limited by the size, mass and power constraints, mass-produced small satellites in large constellations or clusters could be useful in many science missions such as gravity mapping, tracking of forest fires, finding water resources, etc. Constellation of satellites provide improved spatial and temporal resolution of the target. Small satellite constellations contribute innovative applications by replacing a single asset with several very capable spacecraft which opens the door to new applications. With increasing levels of autonomy, there will be a need for remote communication networks to enable communication between spacecraft. These space based networks will need to configure and maintain dynamic routes, manage intermediate nodes, and reconfigure themselves to achieve mission objectives. Hence, inter-satellite communication is a key aspect when satellites fly in formation. In this paper, we present the various researches being conducted in the small satellite community for implementing inter-satellite communications based on the Open System Interconnection (OSI) model. This paper also reviews the various design parameters applicable to the first three layers of the OSI model, i.e., physical, data link and network layer. Based on the survey, we also present a comprehensive list of design parameters useful for achieving inter-satellite communications for multiple small satellite missions. Specific topics include proposed solutions for some of the challenges faced by small satellite systems, enabling operations using a network of small satellites, and some examples of small satellite missions involving formation flying aspects.Comment: 51 pages, 21 Figures, 11 Tables, accepted in IEEE Communications Surveys and Tutorial