At first glance, a new, ultra-low-energy communication method seems 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. No need to send power to the antenna.
Our system, together with our technologies Harvesting energy from the environment, can lead to all kinds of devices that transmit data, including microsensors and implanted medical devices, without the need for batteries or other power sources. These include sensors Smart farmingAnd Electronic devices implanted in the body Never need battery changes, better Contactless credit cards And maybe new ways Satellites To communicate.
Apart from the energy needed to flip a switch, no other energy is needed to transmit information. In our case, the switch is a transistor, which is an electrically controlled switch with no moving parts and consumes very little power.
In the simplest form of ordinary radio, a switch connects and disconnects a source of a powerful electrical signal—perhaps an oscillator that produces a sine wave that oscillates 2 billion times per second—to transmission antenna. When the signal source is connected, the antenna emits a radio wave, indicating 1. When the switch is disconnected, there is no radio wave, indicating 0.
What we have shown is that there is no need for a powered signal source. Alternatively, random thermal noise, which is present in all electrically conductive materials due to heat-driven motion of electrons, can replace the signal driving the antenna.
No free lunch
we Electrical engineers Who is the Search for wireless systems. During peer review of our paper For this research, which was recently published in the Proceedings of the National Academy of Sciences, our reviewers asked us to explain why the method was not violated. The second law of thermodynamicsthe main law of physics that explains why Perpetual motion machines impossible.
Perpetual motion machines are theoretical machines that can run indefinitely without the need for power from any external source. Reviewers were concerned that if information could be sent and received without powered components, and with both the transmitter and receiver at the same temperature, then you could create a perpetual motion machine. Because this is impossible, it means that there was something wrong with our action or our understanding of it.
One way the second law can be stated is that heat will automatically flow only from hotter objects to cooler objects. The radio signals from our transmitter transmit heat. If there is an automatic signal flow from the transmitter to the receiver when there is no temperature difference between the two, you can harvest that flow for free energy, in violation of the second law.
The solution to this apparent paradox is that the receiver in our system is powered and operates like a refrigerator. The signal-carrying electrons on the receiving side are effectively kept cool by an amplifier, similar to the way a refrigerator keeps its interior cool by constantly pumping out heat. The transmitter consumes almost no power, but the receiver consumes as much power as 2 watts. This is similar to receivers in other very low power communication systems. Almost all energy consumption occurs in a base station that has no restrictions on energy use.
Many researchers around the world are exploring related passive communication methods, known as back scatter. The back data transmitter looks very similar to our data transmitter. The difference is that in the back-and-forth communication system, in addition to the data transmitter and data receiver, there is a third component that generates a radio wave. The switching done by the data transmitter has the effect of reflecting that radio wave, which is then captured by the receiver.
a Backscattering device It has the same energy efficiency as our system, but the inverse scattering setup is more complex, since a signal generation component And there is a need. However, our system has a lower data rate and range than walkie-talkies or traditional radios.
One area of future work is to improve the data rate and range of our system, and to test it in applications such as implanted devices. For implantable devices, one of the advantages of our new method is that there is no need to expose the patient to a strong external radio signal, which could cause tissue heating. Even more exciting, we believe that related insights could enable other new forms of communication in which other natural signal sources can be modulated, such as thermal noise from biological tissues or other electronic components.
Finally, this work may lead to new links between the study of heat (thermodynamics) and the study of communication (information theory). These fields are often seen as similar, but this work suggests some literal connections between them.