Communication primitives such as coding and multiple antenna processing have provided significant benefits for traditional wireless systems. Existing designs, however, consume significant power and computational resources, and hence cannot be run on low complexity, power constrained backscatter devices. This paper makes two main contributions: (1) we introduce the first multi-antenna cancellation design that operates on backscatter devices while retaining a small form factor and power footprint, (2) we introduce a novel coding mechanism that enables long range communication as well as concurrent transmissions and can be decoded on backscatter devices. We build hardware prototypes of the above designs that can be powered solely using harvested energy from TV and solar sources. The results show that our designs provide benefits for both RFID and ambient backscatter systems: they enable RFID tags to communicate directly with each other at distances of tens of meters and through multiple walls. They also increase the communication rate and range achieved by ambient backscatter systems by 100X and 40X respectively. We believe that this paper represents a substantial leap in the capabilities of backscatter communication.

Energy-scavenging devices with general-purpose microcontrollers can support arbitrarily complex sensing tasks in theory, but in practice, energy limitations impose severe constraints on the application space. Richer sensing such as image capture would enable many new applications to take advantage of energy scavenging. Richer sensing faces two key challenges: efficiently retaining the necessary amount of harvested energy, and storing and communicating large units of sensor data. This paper presents the WISPCam, a passive UHF RFID camera tag based on the Wireless Identification and Sensing Platform that overcomes these two challenges to support reliable image capture and transmission while powered by an RFID reader. The WISPCam uses a novel charge-storage scheme designed specifically to match the image sensor's needs. This scheme optimally balances capacitance and leakage to improve the sensitivity and efficiency of the power harvester. The WISPCam also uses a novel data storage and communication scheme to reliably support the transfer of complete images to an RFID reader application. The WISPCam makes battery-free image capture practical for applications such as mechanical gauge reading and surveillance, both demonstrated in this paper, and opens the door to richer sensing applications on battery-free devices.

Abstract-An ambient RF energy harvesting sensor node with onboard sensing and communication functionality was developed and tested. The minimal RF input power required for sensor node operation was -18 dBm (15.8 µW). Using a 6 dBi receive antenna, the most sensitive RF harvester was shown to operate at a distance of 10.4 km from a 1 MW UHF television broadcast transmitter, and over 200 m from a cellular base transceiver station. A complete ambient RF-powered prototype was constructed which measured temperature and light level and wirelessly transmitted these measurements.