Device emits radio waves with almost no power — without violating the laws of physics

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Joshua R. Smith, University of Washington and Zerina Kapetanovic, Stanford University

(Dialogue) A new ultra-low-power communication method may seem at first glance to defy the laws of physics. Information can be transmitted wirelessly simply by turning on and off a switch connecting the resistor to the antenna. No need to power the antenna.

Our system, combined with technologies that harvest energy from the environment, could lead to a variety of devices that can transmit data without batteries or other power sources, including tiny sensors and implantable medical devices. These include sensors for smart farming, electronic devices implanted in the body that never need to change batteries, better contactless credit cards, and possibly even a new form of satellite communication.

Transmitting information requires no energy other than that required to flip a switch. In our case, the switch is a transistor, an electrically controlled switch with no moving parts and consumes very little power.

In the simplest form of common radio, a switch connects and disconnects a strong electrical signal source—perhaps an oscillator that produces sine waves that fluctuate 2 billion times per second—to the transmitting antenna. When the signal source is connected, the antenna generates radio waves, represented as 1. When the switch is off, there is no radio wave, which means 0.

What we show does not require an active signal source. Instead, the random thermal noise present in all conductive materials due to the thermally driven motion of electrons can replace the signal driving the antenna.

no free lunch

We are electrical engineers working on wireless systems. During peer review of a paper on this research recently published in the Proceedings of the National Academy of Sciences, the reviewers asked us to explain why the method did not violate the second law of thermodynamics, the main law of physics that explains perpetual motion machines.

Perpetual motion machines are theoretical machines that can work indefinitely without requiring any external source of energy. The reviewers were concerned that if it was possible to send and receive information without powered components, and both the transmitter and receiver were at the same temperature, that would mean you could create a perpetual motion machine. Because it’s impossible, that means something is wrong with our work or our understanding of it.

One way to state the second law is that heat will only flow spontaneously from a warmer body to a cooler body. Wireless signals from our transmitters transmit heat. If there is a spontaneous signal flow from the emitter to the receiver in the absence of a temperature difference between the emitter and receiver, you can collect this flow for free energy, which violates the second law.

The solution to this seemingly paradox is that the receiver in our system is active and works like a refrigerator. The electrons carrying the signal at the receiving end are effectively kept cold by an active amplifier, similar to the way a refrigerator keeps its interior cold by constantly pumping out heat. The transmitter consumes almost no power, but the receiver consumes a lot of power, up to 2 watts. This is similar to receivers in other ultra-low-power communication systems. Almost all power consumption occurs at the base station where there are no constraints on energy usage.

easier way

Many researchers around the world have been exploring a related passive communication method called backscattering. The 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 generates radio waves. The switching performed by the data transmitter has the effect of reflecting the radio waves which are then received at the receiver.

The backscatter device has the same energy efficiency as our system, but the backscatter setup is much more complicated because of the signal generation components required. However, our system has a lower data rate and range than backscatter radio or conventional radio.

what’s next

One area of ​​future work is to increase the data rate and range of our system and test it in applications such as implanted devices. For implantable devices, an advantage of our new approach is that it does not require exposing the patient to strong external radio signals, which can cause tissue heating. Even more exciting, we believe related ideas could enable other new forms of communication where other natural signal sources can be modulated, such as thermal noise from biological tissue or other electronic components.

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 similar, but this work suggests some more direct connections between them.

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