Why Channel Hopping Makes Sense, even with IEEE802.15.4 OFDM at 2.4 GHz

International audienceSince its 2015 revision, IEEE802.15.4 now supports the OFDM physical layer at 2.4 GHz, on top of the popular O-QPSK modulation. Chips capable of both O-QPSK and OFDM are currently available. The question now is whether OFDM is relevant for low-power wireless mesh networking; given its prevalence in 4G cellular, it could be a game-changer. We start by collecting a comprehensive connectivity dataset, 141,587,000 data points, continuously over 21 days. We then show, in an entirely counter-intuitive manner, that OFDM alone is not enough to cope with external interference and multi-path fading, and we recommend combining OFDM with channel hopping at the link layer. In the presence of WiFi interference, we further recommend using OFDM option 1 MCS3, a fast 800 kbps mode, which does not use frequency repetition, even though the latter is meant to increase frequency diversity

Overview of IEEE802.15.4g OFDM and its Applicability to Smart Building Applications

International audienceThis paper compares the performance of two IEEE802.15.4 physical layers in the Smart Building context: 2.4 GHz O-QPSK and sub-GHz OFDM. The former has been in the IEEE802.15.4 standard since 2003, the latter was rolled into its 2015 revision. OFDM promises exceptional performance, in particular in environments with high external interference and multi-path fading. This paper starts with a comprehensive overview of IEEE802.15.4 and IEEE802.15.4g, with a particular focus on OFDM, its design drivers and modes of operation. The second half of this paper presents results from an exhaustive benchmarking campaign of both technologies in a building environment, and discusses lessons learnt. We show how OFDM has a higher range, even at 400 kbps and 800 kbps data rates. We then quantify the importance of frequency repetition in OFDM, and of using a wide communication channel, and we show how the use of OFDM can result in a 2–4 χ decrease in power consumption compared to 2.4 GHz O-QPSK. We conclude by recommending the use of OFDM option 1, with MCS2 for short (<128 B) frames, and MCS3 otherwise

Evaluation of IEEE802.15.4g for Environmental Observations

International audienceIEEE802.15.4g is a low-power wireless standard initially designed for Smart Utility 1 Networks, i.e. for connecting smart meters. IEEE802.15.4g operates at sub-GHz frequencies to offer 2 2-3× longer communication range compared to its 2.4 GHz counterpart. Although the standard 3 offers 3 PHYs (FSK, OFDM and O-QPSK) with numerous configurations, 2-FSK at 50 kbps is the 4 mandatory and most prevalent radio setting used. This article looks at whether IEEE802.15.4g can 5 be used to provide connectivity for outdoor deployments. We conduct range measurements using 6 the totality of the standard (all modulations with all further parametrization) in the 863-870 MHz 7 band, within four scenarios which we believe cover most low-power wireless outdoor applications: 8 line of sight, smart agriculture, urban canyon, and smart metering. We show that there are radio 9 settings that outperform the "2-FSK at 50 kbps" base setting in terms of range, throughput and 10 reliability. Results show that highly reliable communications with data rates up to 800 kbps can 11 be achieved in urban environments at 540 m between nodes, and the longest useful radio link is 12 obtained at 779 m. We discuss how IEEE802.15.4g can be used for outdoor operation, and reduce 13 the number of repeater nodes that need to be placed compared to a 2.4 GHz solution

Reliable IEEE 802.15.4g based smart utility networks via adaptive modulation selection and re-transmission shaping

In this thesis work, we propose and evaluate a strategy to improve transmission in IEEE 802.15.4g SUN networks. This kind of network is at the basis of many promising IoT applications that require high reliability while maintaining low power consumption. The proposed strategy consists of two distinct parts: re-transmission shaping and modulation selection. The re-transmission shaping mechanism keeps track of unused packet re-transmissions and allocates additional re-transmissions when the instantaneous link quality decreases due to channel impairments. The modulation selection strategies apply Multi-Armed Bandits algorithms to dynamically choose the best transmission modulation. The combined effect of these two mechanisms aims to maximize link reliability while minimizing energy consumption and meeting radio-frequency regulation constraints. To evaluate the proposed methods we use trace-based simulations using an IEEE 802.15.4g SUN data-set and two widely used metrics, the PDR (Packet Delivery Ratio) and the RNP (Required Number of Packets). The obtained results show that re-transmission shaping and modulation selection are useful mechanisms to improve link reliability of low-power wireless communications. Their combined use can increase PDR from 77.9% to 98.7% while sustaining an RNP of 1.7 re-transmissions per packet when compared to using a single re-transmission per packet

Comunicações sem-fios de tempo-real para ambientes abertos

Doutoramento em Engenharia InformáticaWireless communication technologies have become widely adopted, appearing in heterogeneous applications ranging from tracking victims, responders and equipments in disaster scenarios to machine health monitoring in networked manufacturing systems. Very often, applications demand a strictly bounded timing response, which, in distributed systems, is generally highly dependent on the performance of the underlying communication technology. These systems are said to have real-time timeliness requirements since data communication must be conducted within predefined temporal bounds, whose unfulfillment may compromise the correct behavior of the system and cause economic losses or endanger human lives. The potential adoption of wireless technologies for an increasingly broad range of application scenarios has made the operational requirements more complex and heterogeneous than before for wired technologies. On par with this trend, there is an increasing demand for the provision of cost-effective distributed systems with improved deployment, maintenance and adaptation features. These systems tend to require operational flexibility, which can only be ensured if the underlying communication technology provides both time and event triggered data transmission services while supporting on-line, on-the-fly parameter modification. Generally, wireless enabled applications have deployment requirements that can only be addressed through the use of batteries and/or energy harvesting mechanisms for power supply. These applications usually have stringent autonomy requirements and demand a small form factor, which hinders the use of large batteries. As the communication support may represent a significant part of the energy requirements of a station, the use of power-hungry technologies is not adequate. Hence, in such applications, low-range technologies have been widely adopted. In fact, although low range technologies provide smaller data rates, they spend just a fraction of the energy of their higher-power counterparts. The timeliness requirements of data communications, in general, can be met by ensuring the availability of the medium for any station initiating a transmission. In controlled (close) environments this can be guaranteed, as there is a strict regulation of which stations are installed in the area and for which purpose. Nevertheless, in open environments, this is hard to control because no a priori abstract knowledge is available of which stations and technologies may contend for the medium at any given instant. Hence, the support of wireless real-time communications in unmanaged scenarios is a highly challenging task. Wireless low-power technologies have been the focus of a large research effort, for example, in the Wireless Sensor Network domain. Although bringing extended autonomy to battery powered stations, such technologies are known to be negatively influenced by similar technologies contending for the medium and, especially, by technologies using higher power transmissions over the same frequency bands. A frequency band that is becoming increasingly crowded with competing technologies is the 2.4 GHz Industrial, Scientific and Medical band, encompassing, for example, Bluetooth and ZigBee, two lowpower communication standards which are the base of several real-time protocols. Although these technologies employ mechanisms to improve their coexistence, they are still vulnerable to transmissions from uncoordinated stations with similar technologies or to higher power technologies such as Wi- Fi, which hinders the support of wireless dependable real-time communications in open environments. The Wireless Flexible Time-Triggered Protocol (WFTT) is a master/multi-slave protocol that builds on the flexibility and timeliness provided by the FTT paradigm and on the deterministic medium capture and maintenance provided by the bandjacking technique. This dissertation presents the WFTT protocol and argues that it allows supporting wireless real-time communication services with high dependability requirements in open environments where multiple contention-based technologies may dispute the medium access. Besides, it claims that it is feasible to provide flexible and timely wireless communications at the same time in open environments. The WFTT protocol was inspired on the FTT paradigm, from which higher layer services such as, for example, admission control has been ported. After realizing that bandjacking was an effective technique to ensure the medium access and maintenance in open environments crowded with contention-based communication technologies, it was recognized that the mechanism could be used to devise a wireless medium access protocol that could bring the features offered by the FTT paradigm to the wireless domain. The performance of the WFTT protocol is reported in this dissertation with a description of the implemented devices, the test-bed and a discussion of the obtained results.As tecnologias de comunicação sem fios tornaram-se amplamente adoptadas, surgindo em aplicações heterógeneas que vão desde a localização de vítimas, pessoal médico e equipamentos em cenários de desastre à monitorização da condição física de máquinas em ambientes industrials. Muito frequentemente, as aplicações exigem uma resposta limitada no tempo que, geralmente, em sistemas distribuídos, é substancialmente dependente do desempenho da tecnologia de comunicação utilizada. Estes sistemas tendem a possuir requisitos de tempo-real uma vez que a comunicação de dados tem de ser conduzida dentro de limites temporais pré-definidos que, quando não cumpridos, podem comprometer o correcto funcionamento do sistema e resultar em perdas económicas ou colocar em risco vidas humanas. A potencial adopção de tecnologias sem-fios para um crescente número de cenários traduz-se num aumento da complexidade e heterogeneidade dos requisitos operacionais relativamente às tecnologias cabladas. A acompanhar esta tendência verifica-se uma crescente procura de sistemas distribuídos, caracterizados quer por uma boa relação custo-eficácia, quer pela simplicidade de instalação, manutenção e adaptação. Ao mesmo tempo, estes sistemas tendem a requerer flexibilidade operacional, que apenas pode ser assegurada se a tecnlogia de comunicação empregue supportar transmissões de dados dispoletadas quer por eventos (event-triggered), quer por tempo (timetriggered) e se, ao mesmo tempo, em funcionamento, permitir a alteração dos parâmetros de comunicação correspondentes. Frequentemente, as aplicações com comunicações sem fios caracterizam-se por exigências de instalação que apenas podem ser endereçadas usando alimentação através de baterias e/ou mecanismos de recolha de energia do ambiente envolvente. Estas aplicações têm tipicamente requisitos exigentes de autonomia e de tamanho, impedindo o recurso a baterias de grande dimensão. Dado que o suporte de comunicações pode representar uma parte significativa dos requisitos de energia da estação, o uso de tecnologias de comunicação de elevado consumo não é adequado. Desta forma, nestas aplicações, as tecnologias de comunicação de curto-alcance tornaram-se amplamente adoptadas uma vez que, apesar de se caracterizarem por taxas de transmissão inferiores, consomem apenas uma fracção da energia das tecnologias de maior alcance. resumo Em geral, os requisitos de pontualidade da comunicação de dados podem ser cumpridos através da garantia da disponibilidade do meio no instante em que qualquer estação inicie uma transmissão. Em ambientes controlados esta disponibilidade pode ser garantida, na medida em que existe um controlo de quais as estações que foram instaladas na área e qual a sua função. Contrariamente, em ambientes abertos, tal controlo é difícil de garantir uma vez que não existe conhecimento a priori de que estações ou tecnologias podem competir pelo meio, tornando o suporte de comunicações de temporeal um desafio difícil de implementar em cenários com estações de comunicação não controladas. As comunicações de baixo consumo têm sido o foco de um esforço de investigação bastante amplo, por exemplo, no domínio das redes de sensores sem fios. Embora possam permitir uma maior autonomia a estações baseadas em baterias, estas tecnologias são reconhecidas como sendo negativamente influenciadas por tecnologias semelhantes competindo pelo mesmo meio e, em particular, por tecnologias que utilizem níveis de potência de transmissão mais elevados em bandas de frequências comuns. De forma cada vez mais acentuada, a banda industrial, científica e médica (ISM) dos 2.4 GHz tem-se tornado mais saturada com tecnologias que competem entre si pelo acesso ao meio tais como, por exemplo, Bluetooth e ZigBee, dois padrões de comunicação que são a base de vários protocolos de tempo-real. Apesar destas tecnologias aplicarem mecanismos para melhorar a sua coexistência, são vulneráveis a transmissões de estações não controladas que usem as mesmas tecnologias ou que usem tecnologias com níveis de potência de transmissão mais elevados, impedindo, desta forma, o suporte de comunicações de tempo-real fiáveis em ambientes abertos. O protocolo de comunicação sem fios flexível disparado por tempo (WFTT) é baseado numa arquitectura mestre/múltiplo escravo alavancado na flexibilidade e pontualidade promovidas pelo paradigma FTT e na captura e manutenção determinística do meio suportadas pela técnica de bandjacking (captura de banda). Esta tese apresenta o protocolo WFTT e argumenta que este permite suportar serviços de comunicação de tempo-real com requisitos elevados de fiabilidade em ambientes abertos onde várias tecnologias de comunicação baseadas em contenção disputam o acesso ao meio. Adicionalmente, esta tese reivindica que é possível suportar comunicações sem-fios simultaneamente flexíveis e pontuais em ambientes abertos. O protocolo WFTT foi inspirado no paradigma FTT, do qual importa os serviços de alto nível como, por exemplo, o controlo de admissão. Após a observação da eficácia da técnica de bandjacking em assegurar o acesso ao meio e a correspondente manutenção, foi reconhecida a possibilidade de utilização deste mecanismo para o desenvolvimento de um protocolo de acesso ao meio, capaz de oferecer as funcionalidades do paradigma FTT em meios de comunicação sem-fios. O desempenho do protocolo WFTT é reportado nesta tese com uma descrição dos dispositivos implementados, da bancada de ensaios desenvolvida e dos resultados obtidos

Wireless remote patient monitoring on general hospital wards.

A novel approach which has potential to improve quality of patient care on general hospital wards is proposed. Patient care is a labour-intensive task that requires high input of human resources. A Remote Patient Monitoring (RPM) system is proposed which can go some way towards improving patient monitoring on general hospital wards. In this system vital signs are gathered from patients and sent to a control unit for centralized monitoring. The RPM system can complement the role of nurses in monitoring patients’ vital signs. They will be able to focus on holistic needs of patients thereby providing better personal care. Wireless network technologies, ZigBee and Wi-Fi, are utilized for transmission of vital signs in the proposed RPM system. They provide flexibility and mobility to patients. A prototype system for RPM is designed and simulated. The results illustrated the capability, suitability and limitation of the chosen technology