Networking Protocols For Energy Harvesting Wireless Sensor Networks

Abstract

As traditional wireless sensor networks (WSNs) rely on batteries with finite stored energy to operate, much research have focused on designing energy-efficient networking protocols to maximize network lifetime, usually at the expense of throughput reduction. Since energy can be replenished with energy harvesters, energy harvesting WSNs (EH-WSNs) can potentially operate perpetually without sacrificing throughput by balancing energy usage with the energy harvesting rate. EH-WSNs are particularly suited for emerging WSN applications including environmental/habitat monitoring and structural health monitoring of critical infrastructures and buildings, where batteries are hard or even impossible to replace in sensors that are required to operate for long durations after being deployed. This thesis focuses on the design and performance analysis of medium access control (MAC) and routing protocols that can achieve high throughput in EH-WSNs by addressing the following major challenges: (i) the unpredictability in the energy harvesting process; (ii) the variation of energy harvesting rates in time, space and across different harvesting technologies; and (iii) changes in node densities and node mobility. Our proposed probabilistic polling MAC protocol addresses the above challenges by dynamically adjusting the contention probability as well as the polling frequency according to actual transmission outcomes. We present a novel transmission outcome classifier that uses RSSI and LQI (link quality indicator) values to distinguish between packet losses due to collisions and weak signals for fully overlapping transmissions without the need for hardware modifications in IEEE 802.15.4 devices. We show that our proposed scheme can achieve close to, or even exceed the theoretical success rate of probabilistic polling due to packet capture effect, and performs better than CSMA-based and polling-based MAC protocols in realistic single-hop EH-WSNs, with different harvesting rates and node densities. Next, we present EH-MAC, a receiver-initiated reliable MAC protocol that uses probabilistic polling and reduces the hidden terminal problem to achieve higher throughput compared to other MAC schemes in multi-hop EH-WSNs. Using an adaptive energy management scheme, we show that EH-MAC can continue to function even when ambient energy is unavailable temporarily by accumulating some harvested energy during periods when ambient energy is available. It can also reduce fluctuations in throughput when the energy harvesting rate changes quickly over time. Finally, we develop opportunistic routing (OR) protocols (EHOR and AOR for linear and 2D topologies respectively) for EH-WSNs. First, we use a regioning approach to group nodes together to share transmission slots in order to reduce delay and improve goodput. AOR/EHOR are adaptive to node density and energy harvesting rate by adjusting the number of regions. Next, we further improve performance by considering energy availability in each node in addition to its distance from the sender when determining its transmission priority for a received packet, thereby increasing the probability of forwarding data packets by relay nodes. We show that EHOR/AOR can achieve higher goodput and fairness when compared to traditional OR and other non-OR routing protocols for different node densities and energy harvesting rates. AOR/EHOR can support mobility by eliminating overheads due to neighborhood discovery since they do not need to know the identity of awake nodes. Furthermore, AOR/EHOR do not require time synchronization protocols, therefore it is easy to implement them on resource-constrained energy harvesting wireless sensor nodes.