819 research outputs found
On the Design of MAC Protocols for Multi-Packet Communication in IEEE 802.11 Heterogeneous Networks Using Adaptive Antenna Arrays
This paper discusses the design requirements for enabling multiple simultaneous peer-to-peer communications in IEEE 802.11 asynchronous networks in the presence of adaptive antenna arrays, and proposes two novel access schemes to realize multipacket communication (MPC). Both presented solutions, which rely on the information acquired by each node during the monitoring of the network activity, are suitable for distributed and heterogeneous scenarios, where nodes equipped with different antenna systems can coexist. The first designed scheme, called threshold access MPC (TAMPC), is based on a threshold on the load sustainable by the single-node, while the second protocol, called signal-to-interference ratio (SIR) access MPC (SAMPC), is based on an accurate estimation of the SIR and on the adoption of low density parity check codes. Both protocols, which are designed to be backward compatible with the 802.11 standard, are numerically tested in realistic scenarios. Furthermore, the performance of the two schemes is compared to the theoretical one and to that of the 802.11n extension in a mobile environment
Achieving Congestion Diversity in Multi-hop Wireless Mesh Networks
This paper reports on the first systematic study of congestion-aware routing
algorithms for wireless mesh networks to achieve an improved end-end delay
performance. In particular, we compare 802.11 compatible implementations of a
set of congestion-aware routing protocols against our implementation of state
of the art shortest path routing protocol (SRCR). We implement congestion-aware
routing algorithms Backpressure (BP), Enhanced-Backpressure (E-BP) adapted from
[1], [2] suitably adjusted for 802.11 implementation. We then propose and
implement Congestion Diversity Protocol (CDP) adapted from [3] recognizing the
limitations of BP and E-BP for 802.11-based wireless networks. SRCR solely
utilizes link qualities, while BP relies on queue differential to route
packets. CDP and E-BP rely on distance metrics which take into account queue
backlogs and link qualities in the network. E-BP computes its metric by summing
the ETX and queue differential, while CDP determines its metric by calculating
the least draining time to the destination. Our small testbed consisting of
twelve 802.11g nodes enables us to empirically compare the performance of
congestion-aware routing protocols (BP, E-BP and CDP) against benchmark SRCR.
For medium to high load UDP traffic, we observe that CDP exhibits significant
improvement with respect to both end-end delay and throughput over other
protocols with no loss of performance for TCP traffic. Backpressure-based
routing algorithms (BP and E-BP) show poorer performance for UDP and TCP
traffic. Finally, we carefully study the effects of the modular approach to
congestion-aware routing design in which the MAC layer is left intac
Free Side-channel Cross-technology Communication in Wireless Networks
Enabling direct communication between wireless technologies immediately
brings significant benefits including, but not limited to, cross-technology
interference mitigation and context-aware smart operation. To explore the
opportunities, we propose FreeBee -- a novel cross-technology communication
technique for direct unicast as well as cross-technology/channel broadcast
among three popular technologies of WiFi, ZigBee, and Bluetooth. The key
concept of FreeBee is to modulate symbol messages by shifting the timings of
periodic beacon frames already mandatory for diverse wireless standards. This
keeps our design generically applicable across technologies and avoids
additional bandwidth consumption (i.e., does not incur extra traffic), allowing
continuous broadcast to safely reach mobile and/or duty-cycled devices. A new
\emph{interval multiplexing} technique is proposed to enable concurrent
bro\-adcasts from multiple senders or boost the transmission rate of a single
sender. Theoretical and experimental exploration reveals that FreeBee offers a
reliable symbol delivery under a second and supports mobility of 30mph and low
duty-cycle operations of under 5%.Comment: To Appear in IEEE/ACM Transactions on Networkin
Cross-Band Interference Considered Harmful in OFDM Based Distributed Spectrum Sharing
In the past few years we have witnessed the paradigm shift from static
spectrum allocation to dynamic spectrum access/sharing. Orthogonal
Frequency-Division Multiple Access (OFDMA) is a promising mechanism to
implement the agile spectrum access. However, in wireless distributed networks
where tight synchronization is infeasible, OFDMA faces the problem of
cross-band interference. Subcarriers used by different users are no longer
orthogonal, and transmissions operating on non-overlapping subcarriers can
interfere with each other. In this paper, we explore the cause of cross-band
interference and analytically quantify its strength and impact on packet
transmissions. Our analysis captures three key practical artifacts: inter-link
frequency offset, temporal sampling mismatch and power heterogeneity. To our
best knowledge, this work is the first to systematically analyze the cause and
impact of cross-band interference. Using insights from our analysis, we then
build and compared three mitigating methods to combat cross-band interference.
Analytical and simulation results show that placing frequency guardband at link
boundaries is the most effective solution in distributed spectrum sharing,
while the other two frequency-domain methods are sensitive to either temporal
sampling mismatch or inter-link frequency offset. We find that the proper
guardband size depends heavily on power heterogeneity. Consequently, protocol
designs for dynamic spectrum access should carefully take into account the
cross-band interference when configuring spectrum usage
Dish networks: Protocols, strategies, analysis, and implementation
Ph.DDOCTOR OF PHILOSOPH
Design and Characterization of a Full-duplex Multi-antenna System for WiFi networks
In this paper, we present an experimental and simulation based study to
evaluate the use of full-duplex as a mode in practical IEEE 802.11 networks. To
enable the study, we designed a 20 MHz multi-antenna OFDM full-duplex physical
layer and a full-duplex capable MAC protocol which is backward compatible with
current 802.11. Our extensive over-the-air experiments, simulations and
analysis demonstrate the following two results. First, the use of multiple
antennas at the physical layer leads to a higher ergodic throughput than its
hardware-equivalent multi-antenna half-duplex counterparts, for SNRs above the
median SNR encountered in practical WiFi deployments. Second, the proposed MAC
translates the physical layer rate gain into near doubling of throughput for
multi-node single-AP networks. The two combined results allow us to conclude
that there are potentially significant benefits gained from including a
full-duplex mode in future WiFi standards.Comment: 44 page, 11 figures, 8 tables. Submitted to IEEE Transactions on
Vehicular Technology, Oct 201
Optimal Slotted ALOHA under Delivery Deadline Constraint for Multiple-Packet Reception
This paper considers the slotted ALOHA protocol in a communication channel
shared by N users. It is assumed that the channel has the multiple-packet
reception (MPR) capability that allows the correct reception of up to M () time-overlapping packets. To evaluate the reliability in the
scenario that a packet needs to be transmitted within a strict delivery
deadline D () (in unit of slot) since its arrival at the head of
queue, we consider the successful delivery probability (SDP) of a packet as
performance metric of interest. We derive the optimal transmission probability
that maximizes the SDP for any and any , and show it
can be computed by a fixed-point iteration. In particular, the case for D = 1
(i.e., throughput maximization) is first completely addressed in this paper.
Based on these theoretical results, for real-life scenarios where N may be
unknown and changing, we develop a distributed algorithm that enables each user
to tune its transmission probability at runtime according to the estimate of N.
Simulation results show that the proposed algorithm is effective in dynamic
scenarios, with near-optimal performance
Wireless Real-Time Communication in Tunnel-like Environments using Wireless Mesh Networks: The WICKPro Protocol
En los últimos años, las redes inalámbricas se están utilizando cada vez más en entornos industriales debido a sus ventajas respecto a redes cableadas: menor coste de instalación, soporte de movilidad, instalación en lugares donde los cables pueden ser problemáticos y mayor facilidad de reconfiguración. Estas redes inalámbricas normalmente deben proporcionar comunicación en tiempo real para satisfacer los requerimientos de las aplicaciones. Podemos encontrar ejemplos de comunicación en tiempo real con redes inalámbricas para entornos industriales en el campo de la automatización industrial y en el control de procesos, donde redes inalámbricas de radiofrecuencia han sido utilizadas para posibilitar comunicación en tiempo real con un despliegue sencillo. Asimismo, la industria también está interesada en comunicaciones en tiempo real en entornos subterráneos, puesto que existen diversas actividades que se llevan a cabo en escenarios tales como túneles y minas, incluyendo operaciones de minerÃa, vigilancia, intervención y rescate. Las redes inalámbricas malladas (Wireless Mesh Networks, WMNs) representan una solución prometedora para conseguir comunicaci ón en tiempo real en entornos inalámbricos, dado que proporcionan una red troncal inalámbrica formada por encaminadores (routers) que es utilizada por terminales móviles. Sin embargo, las WMNs también presentan algunos retos: la naturaleza multisalto de estas redes causa interferencias entre flujos e interferencias de un flujo consigo mismo, además de que la propagación inalámbrica sufre shadowing y propagación multicamino. El estándar IEEE 802.11 ha sido ampliamente utilizado en redes WMNs debido a su bajo coste y la operación en bandas frecuenciales sin licencia. El problema es que su protocolo de acceso al medio (Medium Access Control, MAC) no es determinista y que sus comunicaciones sufren los problemas del terminal oculto y expuesto. Esta tesis doctoral se centra en el soporte de comunicaciones en tiempo real en entornos tipo túnel utilizando redes WMNs. Con este objetivo, desarrollamos un protocolo MAC y de nivel de red denominado WIreless Chain networK Protocol (WICKPro) que funciona sobre IEEE 802.11. Más concretamente, en este trabajo diseñamos dos versiones de este protocolo para proporcionar soporte de tráfico de tiempo real firme (Firm Real-Time, FRT) y de tiempo real no estricto (Soft Real-Time, SRT): FRT-WICKPro y SRT-WICKPro. Asimismo, proponemos un algoritmo de hand-off conocido como Double-Threshold Hand-off (DoTHa) para el manejo de la movilidad en SRT-WICKPro WICKPro utiliza un esquema de paso de testigo para solventar las interferencias entre flujos y de un flujo consigo mismo, asà como los problemas del terminal oculto y expuesto, dado que este esquema no permite que dos nodos transmitan al mismo tiempo. Esta solución es razonable para redes pequeñas donde el re uso espacial es imposible o limitado. Para tratar la naturaleza no determinista de IEEE 802.11, combinamos el esquema de paso de testigo con una planificación cÃclica global. Como es habitual en planificación cÃclica, el hiperperiodo es dividido en un conjunto de ciclos secundarios. FRT-WICKPro inicia el paso de testigo de forma sÃncrona para satisfacer estrictamente dichos ciclos secundarios, mientras que SRT-WICKPro implementa un paso de testigo asÃncrono y permite sobrepasar los ciclos secundarios, por lo que desacopla los ciclos secundarios reales de los te_oricos. Finalmente, DoTHa lidia con el shadowing y la propagación multicamino. Para abordar el shadowing, DoTHa permite llevar a cabo el proceso de hand-off en la región conectada y en la región de transición de un enlace, mientras que la propagación multicamino es ignorada para el proceso de hand-off porque la potencia recibida es promediada. Nuestras propuestas fueron validadas en experimentos de laboratorio y de campo, asà como en simulación. Como un estudio de caso, llevamos a cabo la teleoperación de un robot móvil en dos entornos confinados: los pasillos de un edificio y el túnel del Somport. El túnel del Somport es un antiguo túnel ferroviario fuera de servicio que conecta España y Francia por los Pirineos Centrales. Aunque los robots autónomos son cada vez más importantes, la tecnologÃa no está suficientemente madura para manejar entornos con alto dinamismo como sistemas de fabricación reconfigurables, o para realizar decisiones de vida o muerte, por ejemplo después de un desastre con contaminación radiactiva. Las aplicaciones que pueden beneficiarse de la teleoperación de robots móviles incluyen la monitorización en tiempo real y el uso de maquinaria robotizada, por ejemplo camiones dumper y máquinas tuneladoras, que podrÃan ser operadas remotamente para evitar poner en peligro vidas humanas.Industrial applications have been shifting towards wireless networks in recent years because they present several advantages compared with their wired counterparts: lower deployment cost, mobility support, installation in places where cables may be problematic, and easier reconfiguration. These industrial wireless networks usually must provide real-time communication to meet application requirements. Examples of wireless real-time communication for industrial applications can be found in factory automation and process control, where Radio Frequency wireless communication technologies have been employed to support flexible real-time communication with simple deployment. Likewise, industry is also interested in real-time communication in underground environments, since there are several activities that are carried out in scenarios such as tunnels and mines, including mining, surveillance, intervention, and rescue operations. Wireless Mesh Networks (WMNs) are promising enablers to achieve wireless real-time communication because they provide a wireless backbone comprised by dedicated routers that is utilized by mobile terminals. However, WMNs also present several challenges: wireless multi-hopping causes inter-flow and intra-flow interferences, and wireless propagation suffers shadowing and multi-path fading. The IEEE 802.11 standard has been widely used in WMNs due to its low cost and the operation in unlicensed frequency bands. The downside is that its Medium Access Control (MAC) protocol is non-deterministic, and that its communications suffer from the hidden and exposed terminal problems. This PhD thesis focuses on real-time communication in tunnel-like environments by using WMNs. Particularly, we develop a MAC and network protocol on top of the IEEE 802.11 standard to provide real-time capabilities, so-called WIreless Chain networK Protocol (WICKPro). Two WICKPro versions are designed to provide Firm Real-Time (FRT) or Soft Real-Time (SRT) traffic support: FRT-WICKPro and SRT-WICKPro. We also propose a hand-off algorithm dubbed Double-Threshold Hand-off (DoTHa) to manage mobility in SRT-WICKPro. WICKPro employs a token-passing scheme to solve the inter-flow and intra-flow interferences as well as the hidden and exposed terminal problems, since this scheme does not allow two nodes to transmit at the same time. This is a reasonable solution for small-scale networks where spatial reuse is impossible or limited. The non-deterministic nature of IEEE 802.11 is faced by combining the token-passing mechanism with a polling approach based on a global cyclic packet schedule. As usual in cyclic scheduling, the hyper-period is divided into minor cycles. FRT-WICKPro triggers the token synchronously and fulfills strictly minor cycles, whereas SRT-WICKPro carries out asynchronous token-passing and lets minor cycles be overrun, thereby decoupling the theoretic and the actual minor cycles. Finally, DoTHa deals with shadowing and multi-path fading. Shadowing is addressed by providing the opportunity of triggering hand-off in the connected and transitional regions of a link, while multi-path fading is neglected for hand-off purposes by smoothing the received signal power. We tested our proposals in laboratory and field experiments, as well as in simulation. As a case study, we carried out the tele-operation of a mobile robot within two confined environments: the corridors of a building and the Somport tunnel. The Somport tunnel is an old out-of-service railway tunnel that connects Spain and France through the Central Pyrenees. Although autonomous robots are becoming more and more important, technology is not mature enough to manage highly dynamic environments such as reconfigurable manufacturing systems, or to make life-and-death decisions, e.g., after a disaster with radioactivity contamination. Applications that can benefit from mobile robot tele-operation include real-time monitoring and the use of robotized machinery, for example, dumper trucks and tunneling machines, which could be remotely operated to avoid endangering human lives
Voice Call Capacity Over Wireless Mesh Networks
The goal of this thesis is to understand the voice call carrying capacity of an IEEE 802.11b/e based ad hoc network. We begin with the modelling of conversational speech and define a six state semi-Markov voice model based on ITU-T P59 recommendation. We perform a theoretical analysis of the voice model and compare it with results obtained via simulations. Using a Java based IEEE 802.11 medium access layer simulator, we determine the upper-bound for the number of voice calls carried by an ad hoc network. We use a linear topology with the ideal carrier sensing range and evaluate the number of calls carried using packet loss and packet delay as metrics. We observe that, for one, two, three and four hop, 5.5 Mbps IEEE 802.11 wireless links have an upper-bound of eight, six, five, and three voice calls respectively. We then consider a carrier sensing range and a path loss model and compare them with the ideal case. We observe, after considering a carrier sensing range with path loss model, there is a reduction in the number of calls carried by the linear networks. One, two, three and four hop 5.5 Mbps IEEE 802.11 wireless links support eight, five, four, and two voice calls respectively, when a carrier sensing range and a path loss model is considered. We also find that by adopting packet dropping policies at the nodes, we improve the call carrying capacity and quality of service on the network. In our simulations of a two hop network in path loss conditions, we find that, by adopting a time delay based packet dropping policy at the nodes, the number of calls supported simultaneously increased from five to six. In a four hop linear network we find that by total packet loss is reduced by 20%, adopting a random packet dropping policy and by 50% adopting a time delay based packet dropping policy. Although there is no change in number of calls supported, load on the network is reduced
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