I'm not sure I understand your question. The idea for these systems is that the person is used as a wire; and communication is possible only when the person touches the object (or person) that they wish to communicate with. If the system is misdesigned, then it is possible to pick up the signal whether or not the person is touching the other object, so the system behaves more like a wireless radio.
But at large distances, it is more difficult to snoop than conventional radio waves. Far enough away, the system looks like a dipole, so the E-field magnitude drops with the distance cubed, rather than the distance squared.
I've been doing my PhD work on systems like this (Intrabody
Communication). It does work! However, there are a number of issues,
some of which aren't clear from the article.
First, 10 Mbps is possible, but that's getting near the theoretical limit.
The datarate is limited by the bandwidth, and the bandwidth is limited by
the fact that around 50MHz, the signal wavelength is about four times the
size of a person, which means the person turns into an antenna, and the
whole system becomes essentially a short range radio.
Second, because these systems operate in the near field, the signal
travels through a current loop, and not as plane waves in free space.
This means that there has to be some kind of grounding path for current to
flow back to the transmitter after going through the person. This is why
it works so well to put transceivers in shoes -- the ground path can flow
through earth ground (or any conductive material in the floor). For
devices held in hands, the very small (femtofarad) capacitance of free
space is enough, but the signal does suffer more from noise. Devices in
purses, etc. also have this problem, and may have difficulty establishing
the ground connection depending on the material the purse is made from and
the other objects inside it.
One issue that to my knowledge has not been addressed very well is
guaranteeing that the signal is received during--and only during--physical
contact. There is a large dependence of signal strength on geometry. The
devices I've constructed can communicate when they're brought near (~10
cm) of each other, touching or not. There are a few solutions, such as
looking at jumps in signal strength, but they tend to be confused when a
person without a transceiver happens to touch the object, and a person
with a transceiver is nearby. I'm currently working on this problem for my PhD
dissertation, so if you have any good ideas or know of related work, I'd
love to hear from you.
Slashdot Mirror
I'm not sure I understand your question. The idea for these systems is that the person is used as a wire; and communication is possible only when the person touches the object (or person) that they wish to communicate with. If the system is misdesigned, then it is possible to pick up the signal whether or not the person is touching the other object, so the system behaves more like a wireless radio.
Yes it does!
But at large distances, it is more difficult to snoop than conventional radio waves. Far enough away, the system looks like a dipole, so the E-field magnitude drops with the distance cubed, rather than the distance squared.
First, 10 Mbps is possible, but that's getting near the theoretical limit. The datarate is limited by the bandwidth, and the bandwidth is limited by the fact that around 50MHz, the signal wavelength is about four times the size of a person, which means the person turns into an antenna, and the whole system becomes essentially a short range radio.
Second, because these systems operate in the near field, the signal travels through a current loop, and not as plane waves in free space. This means that there has to be some kind of grounding path for current to flow back to the transmitter after going through the person. This is why it works so well to put transceivers in shoes -- the ground path can flow through earth ground (or any conductive material in the floor). For devices held in hands, the very small (femtofarad) capacitance of free space is enough, but the signal does suffer more from noise. Devices in purses, etc. also have this problem, and may have difficulty establishing the ground connection depending on the material the purse is made from and the other objects inside it.
One issue that to my knowledge has not been addressed very well is guaranteeing that the signal is received during--and only during--physical contact. There is a large dependence of signal strength on geometry. The devices I've constructed can communicate when they're brought near (~10 cm) of each other, touching or not. There are a few solutions, such as looking at jumps in signal strength, but they tend to be confused when a person without a transceiver happens to touch the object, and a person with a transceiver is nearby. I'm currently working on this problem for my PhD dissertation, so if you have any good ideas or know of related work, I'd love to hear from you.
If you'd like to read more, the first (and most detailed) publication I know about on this idea was Thomas Zimmerman's Masters Thesis at the MIT AI Lab. You find it here: http://www.media.mit.edu/physics/publications/thes es/95.09.zimmerman.pdf
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Kurt Partridge
Dept. of Computer Science and Engineering
University of Washington
Seattle, WA 98195