Small computing devices are increasingly embedded in objects and environments such as thermostats, books, furniture, and even implantable medical devices. A key issue is how to power these devices as they become smaller and numerous; wires are often not feasible, and batteries add weight, bulk, cost, and require recharging or replacement that adds maintenance cost and is difficult at large scales.
In this paper, we ask the following question: can we enable devices to communicate using ambient RF signals as the only source of power? Ambient RF from TV and cellular communications is widely available in urban areas (day and night, indoors and out- doors).
Further, recent work has shown that one can harvest tens to hundreds of microwatts from these signals. Thus, a positive answer would enable ubiquitous communication at unprecedented scales and in locations that were previously inaccessible.
Designing such systems, however, is challenging as the simple act of generating a conventional radio wave typically requires much more power than can be harvested from ambient RF signals.
In this paper, we introduce ambient backscatter, a novel communication mechanism that enables devices to communicate by backscattering ambient RF. In traditional backscatter communication (e.g., RFID), a device communicates by modulating its reflections of an incident RF signal (and not by generating radio waves). Hence, it is orders of magnitude more energy-efficient than conventional radio communication.
Our approach is to co-design the hardware elements for ambient backscatter along with the layers in the network stack that make use of it. The key insight we use to decode transmissions is that there is a large difference in the information transfer rates of the ambient RF signal and backscattered signal.
This difference allows for the separation of these signals using only low-power analog operations that correspond to readily available components like capacitors and comparators. We are similarly able to realize carrier sense and fram- ing operations with low-power components based on the physical properties of ambient backscatter signals. This in turn lets us syn- thesize network protocols for coordinating multiple such devices.
To show the feasibility of our ideas, we have built a hardware prototype that is approximately the size of a credit card. Our prototype includes a power harvester for TV signals, as well as the ambient backscatter hardware that is tuned to communicate by using UHF TV signals in a 50 MHz wide frequency band centered at 539 MHz.
The harvested energy is used to provide the small amounts of power required for ambient backscatter and to run the microcontroller and the on-board sensors. Our prototype also includes a low-power flashing LED and capacitive touch sensor for use by applications.
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