Back to Backscatter

The proliferation of sensors, storage, display, communications, and processing is only accelerating. The growth of the Internet of Things (IoT) has led to an explosion of connectivity of systems in nearly every application, including homes, automobiles, shopping centers, industrial installations, manufacturing facilities, and even the battlefield. In order for the plethora of wirelessly connected systems to communicate, these devices all need a power supply of some sort. The typical methods of ensuring that a wireless node has power is to either connect it to a power supply infrastructure, such as the electrical grid, using sustainable energy sources (solar panels, wind turbines etc), or install the wireless node with a finite energy storage system that is doomed to fail at some point.

The reason that wireless nodes require substantial amounts of electrical energy for operation is a function of the processing power of the node as well as the power hungry transmission circuitry. The range of a wireless node is dictated by the sensitivity of the destination receiver as well as the power output of the originating transmitter. Hence, there is a need to reduce the overall transmission energy for wireless nodes with limited power supplies which also intrinsically reduces their range. A way of overcoming this challenge is to use a mesh network that is able to retransmit signals from one distant node over one or more hops to the destination node or gateway.

Though this solves the challenge of needing higher power transmitters and may save some energy, there is additional energy expended with each additional hop transmission. Therefore, a mesh network may or may not be more energy efficient than a single higher power transmitter depending on the network configuration, node placement, and the number of hops necessary for the transmission to reach its destination. However, mesh networks can be designed to be much more reliable than non-mesh networks, especially if the network is designed with smart configuration features, such as “self-healing” if a node fails.

Given the omnipresent wireless energy permeating most digital-era societies, there is another method of tackling the issue of powering wireless nodes without the need for onboard storage or electrical power connection. Wireless power transfer isn’t a new concept and has been explored since before Nikolai Tesla built Wardenclyffe Tower on Long Island New York in order to prototype energizing the surface of the earth. More recent efforts of wireless power transfer have evolved from massive sparking towers to more practical devices that resemble WiFi routers or pucks on a tabletop. This new age of wireless power transfer devices are able to intelligently detect even a moving wireless device and its state of charge in order to change it’s antenna beam characteristics to more accurately transfer wireless power to devices within its range. That, or magnetic resonance antenna located in both the wireless charger “puck” and device are placed within proximity for efficient contactless charging.

These methods still require some powering infrastructure that may not fully serve the diversity of new IoT and wireless node applications. A method of growing interest to researchers is to use an old technique popularized by radio frequency identification (RFID) technology. Backscatter communications (BackCom) is a method that enables a node of transmitting an RF signal without the need for onboard energy storage or even a transmitter. BackCom technology can also incorporate wireless power harvesting to yield a completely self-sufficient wireless node that is able to operate as long as there are RF signals of adequate strength incident on the node.

BackCom works by creating modulated reflections (backscatter modulation) in an incident RF wave that can be picked up by a receiver designed for BackCom (Reader). Generally, the reflections for BackCom are generated by changing the impedance of an antenna tuned to resonate at a desired RF signal frequency, though wideband BackCom is also possible. In this way, the wireless node, or “Tag”, can be placed in virtually any environment without the need for a power source, and can be queried by a passing Reader which powers the Tag and may send information and/or receive information from the Tag. In a multiple access scenario, a single Reader may be able to service a multitude of Tags distributed throughout an environment, enabling the tags to perform sensing/data logging information either on demand or continuously.

Another approach for BackCom could be to use RF signals already present in the environment, such as WiFi, cellular telecommunications, AM/FM radio, digital TV etc. Leveraging existing RF signals already present in the environment could enable BackCom networks that can operate without the need for a Reader energizing the Tags, and the Tags may instead operate as mesh networks, storing data for later transmission when queried, or communicated with a gateway that gives access to wider networks. In this way, much lower power wireless nodes can be realized without the need for energy expensive RF transmitters within the node, or BackCom technology can be incorporated in typical IoT wireless nodes to offer an option of much lower power consuming wireless communication to extend battery life if external, viable RF signals are present.