Due to extremely low power consumption, backscatter has become the transmission mechanism of choice for battery-free devices that operate on harvested energy. However, a limitation of recent backscatter systems is that the communication range scales with the strength of the ambient carrier signal~(ACS). This means that to achieve a long-range, a backscatter tag needs to reflect a strong ACS, which in practice means that it needs to be close to an ACS emitter. We present TunnelScatter, a mechanism that overcomes this limitation. TunnelScatter uses a tunnel diode-based radio frequency oscillator to enable transmissions when there is no ACS, and the same oscillator as a reflection amplifier to support backscatter transmissions when the ACS is weak. Our results show that even without an ACS, TunnelScatter is able to transmit through several walls covering a distance of 18m while consuming a peak biasing power of \SI57 \micro\watt. Based on TunnelScatter, we design battery-free sensor tags, called TunnelTags, that can sense physical phenomena and transmit them using the TunnelScatter mechanism.

Backscatter communication enables wireless transmissions at orders of magnitude lower power consumption when compared to conventional radio transceivers. This introduces novel opportunities for battery-free and ubiquitous sensing. We take advantage of backscatter communication to enable networked utensils. We imagine a scenario where such utensils can provide essential information about the state of the food or the beverage; for instance, the temperature or the quality of food contained in the utensils. We propose flex sensors, to achieve this capability, by augmenting utensils with flexible and inexpensive battery-free sensors that can communicate wirelessly. We demonstrate our efforts by designing a smart cup that tracks the temperature of the beverage.

Light as a medium for sensing and communication enables new scenarios, such as controlling devices with gestures, or communication for Internet of Things~(IoT) devices. However, a limitation of existing systems is that they often sense only a narrow part of the light spectrum. We argue that the ability to sense a broad light spectrum significantly enhances ability of such systems expanding possible application scenarios. We demonstrate our work in progress to develop the concept of polymorphic light sensing~(PLS). PLS sensor morphs itself according to applications requirements, to track desired parts of the light spectrum (colours, infrared, and ultraviolet light). We couple the PLS sensor with ultra-low power backscatter mechanism, and demonstrate this enables us to sense and communicate the broad spectrum, while operating battery-free.

Radio Tomographic Imaging (RTI) enables novel radio frequency (RF) sensing applications such as intrusion detection systems by observing variations in radio links caused by human actions. RTI applications are, however, severely limited by the requirement to retrofit existing infrastructure with energy-expensive sensors. In this demonstration, we present our ongoing efforts to develop the first battery-free RTI system that operates on minuscule amounts of energy harvested from the ambient environment. Our system eliminates the energy-expensive components employed on state-of-the-art RTI systems achieving two orders of magnitude lower power consumption. Battery-free operation enables a sustainable deployment, as RTI sensors could be deployed for long periods of time with little maintenance effort. Our demonstration showcases an intrusion detection scenario enabled by our system.

We present our efforts to design the first Visible Light Sensing (VLS) system that consumes only tens of μ Ws of power to sense and communicate. Our system requires no modification to the existing light infrastructure and uses unmodulated ambient light as sensing medium. We achieve this by designing a sensing mechanism that uses solar cells to achieve sub-μWs of power consumption. Further, we devise an ultra-low power backscatter based transmission mechanism we call Scatterlight that transmits digital readings without incurring the processing and computation overhead of existing sensors. Based on these principles we build a preliminary prototype. Our initial results demonstrate its ability to sense and communicate three hand gestures at 20 μWs of power.