Multihop wireless networking with RF wake-up enabled intermittently-powered nodes

Abstract

Wireless sensors and devices already form an integral part of modern society, but they are constrained by their battery life and the need to be recharged and replaced. To remove the need for batteries and the associated problems of recharging, energy harvesting (EH) can provide power from ambient energy in the environment, meaning large storage is not required as energy is continually replenished. However, the very low harvesting power of small harvesters means it is challenging to operate these devices. Existing work can split computation tasks in conditions where the power supply is intermittent, however, communication in these conditions has only been demonstrated with a nearby high capability device to communicate with. Alternatively, research has demonstrated that EH can power peer-to-peer mesh networked devices, but requiring higher capacity storage and fails with intermittent EH sources. Therefore, in this thesis I demonstrate how to achieve mesh networked communication of intermittently-powered devices. First the specific challenges of intermittent devices are looked at and why these conditions make communication difficult. In order to communicate in spite of this, I examine how wake-up receivers (WuRxs), rectifying antennas (rectennas) and industrial, scientific and medical (ISM) Band transceivers can be used to achieve point-to-point links. Resulting from this, higher power communications from 10 dBm to 15 dBm are shown to generally achieve better performance, due to greater transmitter efficiency and enabling lower power WuRx to effectively extend listening time. Once nodes are deployed, optimal real time operation is important in order to maximize the utility from the harvested energy, where wasteful transmitting or listening leads to suboptimal performance. I generalize the energy consumption for an EH node, including the consumption from each radio wake-up, in an analytical and simulated model to see how different parameters affect the resultant goodput, a measure of throughput. Consequently, splitting the energy equally between transmitting and receiving is shown to maximize performance, but the wake-ups reduce throughput and affects the optimum energy split. Whilst the theoretical analysis is helpful for shaping initial decisions, simulation is required for analysing network behaviour over multiple hops. Therefore, new routing methods for low duty cycle networks are implemented and measured in an intermittent scenario. Specifically, the existing protocol, routing protocol for low power and lossy networks (RPL), is analysed iv under scarce EH conditions, where the intermittency caused by insufficient EH results in a collapse in multihop routing capability. Comparably, an alternative protocol opportunistic RPL (ORPL), can utilise the network without specifying potentially unavailable forwarders and instead dynamically utilizing available forwarders. This allows it to operate over multiple hops in spite of intermittency. Finally, combining both the benefits of ORPL and WuRx leads demonstration of multihop routing in intermittent networks with minimal EH requirements. By modelling several configurations of WuRx, the experiments investigate the trade-off between neighbour count and neighbour availability, as well as the number of hops to reach the destination. The highest range shows the greatest performance when considering routing to a fully powered root node. However, when the root node is intermittent, or when routing data to other intermittent destinations, the cost of the high power radio leads to lower delivery rates. Instead a balance is found, to reach sufficient forwarders to ensure packet delivery, but without compromising the duty cycle too much.

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Identifiers

ORCID for Edward Longman: orcid.org/0000-0002-3651-3161

ORCID for Geoffrey Merrett: orcid.org/0000-0003-4980-3894

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