A new ultra-low-power communication method appears at first glance to violate the laws of physics. It is possible to transmit information wirelessly by simply opening and closing a switch that connects a resistor to an antenna. There is no need to send power to the antenna.
Our system, combined with techniques to harvest energy from the environment, could result in all kinds of devices that transmit data, including tiny sensors and implanted medical devices, without the need for batteries or other power sources. These include sensors for smart farming, electronics implanted in the body that don’t require changing batteries, better contactless credit cards, and maybe even new ways of communicating with satellites.
Apart from the energy required to flip the switch, no other energy is required to transfer the information. In our case, the switch is a transistor, an electrically controlled switch with no moving parts that consumes a tiny amount of power.
In the simplest form of an ordinary radio, a switch connects and disconnects a strong electrical signal source – perhaps an oscillator that produces a sine wave that varies 2 billion times per second – to the transmitting antenna. When the signal source is connected, the antenna generates a radio wave that displays a 1. When the switch is disconnected, there is no radio wave, indicating a 0.
What we have shown is that an active signal source is not required. Instead, random thermal noise, which is present in all electrically conductive materials due to the heat-driven movement of electrons, can take the place of the signal driving the antenna.
No free lunch
We are electrical engineers researching wireless systems. During the peer review of our paper on this research, recently published in Proceedings of the National Academy of Sciences, the reviewers asked us to explain why the method does not violate the second law of thermodynamics, the main law of physics that explains , why perpetuum mobile machines are not possible.
Perpetuum mobile machines are theoretical machines that can work indefinitely without requiring energy from an external source. Reviewers feared that if it were possible to send and receive information with no powered components, and both transmitter and receiver were at the same temperature, it would mean you could create a perpetuum mobile. Since this is impossible, it would mean that there is something wrong with our work or our understanding of it.
For example, the second law can be formulated in such a way that heat flows spontaneously only from hotter objects to colder objects. The radio signals from our transmitter transport heat. With no temperature difference between the two, if there were spontaneous signal flow from transmitter to receiver, you could harvest that flow in violation of the second law to conserve free energy.
The resolution of this apparent paradox is that the receiver in our system is powered and behaves like a refrigerator. The signal-carrying electrons on the receiving end are effectively kept cold by the power amplifier, much like a refrigerator keeps its interior cold by continuously pumping out heat. The transmitter uses almost no power, but the receiver uses considerable power, up to 2 watts. This is comparable to receivers in other ultra-low power communication systems. Almost all power consumption occurs at a base station that has no power consumption restrictions.
A simpler approach
Many researchers worldwide have explored related passive communication methods known as backscatter. A backscatter data transmitter looks very similar to our data transmitter device. The difference is that in a backscatter communication system, in addition to the data transmitter and data receiver, there is a third component that creates a radio wave. By switching the data transmitter, this radio wave is reflected, which is then picked up by the receiver.
A backscatter device has the same energy efficiency as our system, but the backscatter setup is much more complex since a signal-generating component is needed. However, our system has a lower data rate and range than both backscatter radios and traditional radios.
One area for future work is improving the data rate and range of our system and testing it in applications such as implanted devices. For implanted devices, an advantage of our new method is that the patient does not have to be exposed to a strong external radio signal that can cause tissue heating. Even more excitingly, we believe related ideas could enable other new forms of communication where other natural signal sources, such as thermal noise from biological tissue or other electronic components, can be modulated.
Finally, this work may lead to new connections between the study of heat (thermodynamics) and the study of communication (information theory). These fields are often viewed as analogous, but this work suggests some more literal connections between them.
Joshua R. Smith is Professor of Electrical and Computer Engineering and Computer Science and Engineering at the University of Washington. **Zerina Kapetanovic is an ACting Assistant Professor of Electrical Engineering at Stanford University.**
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