For the Internet of Things to flourish a long lasting energy supply for remotely deployed large- scale sensor networks is of paramount importance. An uninterrupted power supply is required by these nodes to carry out tasks such as sensing, data processing, and data communication. Of these, radio communication remains the primary battery consuming activity in wireless systems. Advances in MAC protocols have enabled significant lifetime improvements by putting the main transceiver in sleep mode for extended periods. However, the sensor nodes still waste energy due to two main issues. First, the nodes periodically wake-up to sample the channel even when there is no data for it to receive, leading to idle listening cost. On the other side, the sending node must repeatedly transmit packets until the receiver wakes up and acknowledges receipt, leading to energy wastage due to over-transmission. In systems with the low data rate, idle listening and over-transmission can begin to dominate energy costs.
In this thesis, we take a novel hardware approach to eliminate energy overhead in WSNs by addition of a second, extremely low-power wake-up radio component. This approach leverages an always-on wake-up receiver to delegate the task of listening to the channel for a trigger and then waking up a higher power transceiver when required. With this on-demand approach, energy constrained devices are able to drastically reduce power consumption without sacrificing the application requirements in terms of reliability and network latency.
As a first major contribution, we survey a large body of work to identify the benefits and limitations of the current wake-up radio hardware technology. We also present a new taxonomy for categorizing the wake-up radios and the respective protocols, further highlighting the main issues and challenges that must be addressed while designing systems based on wake-up radios. Our survey forms a guideline for assisting application and system designers to make appropriate choices while utilizing this new technology.
Secondly, this thesis proposes a first-ever benchmarking framework to enable accurate and repeatable profiling of wake-up radios. Specifically, we outline a set of specifications to follow when benchmarking wake-up radio-based systems, leading to more consistent and therefore comparable evaluations whether in simulation or testbed for current and future systems.
To quantitatively assess whether wake-up technology can provide energy savings superior to duty cycled MACs, reliable tools are required to accurately model the wake-up radio hardware and its performance in combination with the upper layers of the stack. As our third contribution, we provide an open-source simulator, WaCo for development and evaluation of wake-up radio protocols across all layers of the software stack. Using our tool together with a newly proposed wake-up radio MAC layer, we provide an exhaustive evaluation of the wake-up radio system for periodic data collection applications. Our evaluations highlight that wake-up technology is indeed effective in extending the network lifetime by shrinking the overall energy consumption.
To close the gap between the simulation and the real world experiments, we adopt a cutting edge wake-up radio hardware and build a Wake-up Lab, a modular dual-radio prototype. Using our Wake-up Lab, we thoroughly evaluate the performance of the wake-up radio solution in a realistic office environment. Our in-depth system-wide evaluation reveals that wake-up radio-based systems can achieve significant improvements over traditional duty cycling MACs by eliminating periodic receive checks and reducing unnecessary main radio transmissions while maintaining end-to-end latency on the order of tens of milliseconds in a multi-hop network.
As a step toward sustainable wireless sensing, this thesis presents a proof of concept system where an extremely low-power switch coupled with a wake-up receiver is continuously powered by a plant microbial fuel cell (PMFC) and a new receiver-initiated MAC-level communication protocol for on-demand data collection. MFC converts the chemical energy into electricity by exploiting the metabolism of bacteria found in the sediment, thus offering a promising power source for autonomous sensing system. However, sources such as PMFCs are severely limited in the quantity of energy they can generate, unable to directly power the sensor nodes. Therefore, we consider radical hardware solutions in combination with the communication stacks to reduce this power gap. Thanks, to the hardware-software co-design proposed above, we were able to reduce the overall power consumption to a point where an extremely low-power PMFC source can sustain the sensor node’s operation with a data sampling rate of over 30 seconds.
Finally, we propose to enhance the LoRa based low-power wide area networks by fusing wake-up receivers and long-range wireless technologies. The current LoRaWAN architecture is mainly designed and optimized for up-links where the remote end devices disseminate data to the gateway using pure ALOHA techniques. As such, this limits the ability of the gateway to control, reconfigure, or query the specific end devices, crucial for many Internet of Things applications. To shift the communication modality from push to pull based, we propose a new network architecture that leverages wake-up receiver and a receiver-initiated On-demand TDMA MAC. The former allows the gateway to trigger the remote device when there is data to be collected else keep the device in sleep mode, while the latter allows retrieving data efficiently from the nodes without congesting the network. Our testbed experiments reveal that the proposed system significantly improves energy efficiency by offering network reliability of 100% with end devices dissipating only a few microwatts of power during periods of inactivity. By moving away from the realm of pure ALOHA communication to wake-up receivers, we were able to exploit the low power modes of the sensor node more effectively.
Through these contributions, this thesis pushes forward the applicability of ultra-low power wake-up radios, by quantitatively measuring the trade-offs, energy efficiency, reliability, and latency. Further, by demonstrating superior performance via proof of concepts, this thesis provides a stepping stone towards the goal of achieving energy-neutral, yet responsive communication systems using wake-up radio technology.