Ask Slashdot: Wireless Proximity Detection? 101
New submitter Cinnamon Whirl writes "As a chemist, I work in a both lab and office enviroments, and need access to data in both, without causing undue clutter in either. My company has recently purchased two Win7 tablets for trial usage with electronic lab notebooks, propietry software, SAP, email etc. These are also useful for sharing in meetings, etc. As part of this project, I have been wondering whether we can use these tablets to detect other devices by proximity. Examples could include finding the nearest printer or monitor or, perhaps trickier, could two roaming devices find each other? Although lab technology is rarely cutting edge, I can see a day when all our sensors and probes will broadcast data (wireless thermocouples are already available), and positioning information will become much more important. What technologies exist to do this? How accurate can the detection be?"
BlueTooth or RFID (Score:5, Informative)
Very difficult to get good accuracy (Score:5, Informative)
There are two ways to use RF signaling to gauge distances.
The easy (but not at all accurate) way is to use signal power, relating higher received power to closer proximity. This method is very inaccurate, as the shortest path might not be (in fact, probably won't be) the path with the least attenuation. You can do this sort of thing in a wide open space, but in an office environment it's likely to be all but useless. It gets even less accurate when you introduce mobile devices, since the antenna's orientation is likely to be changing depending on how you hold it, which can vary received power by several decibels.
The better way is with timing, similar to how GPS works. However, you still have the "shortest path != least attenuated path" problem. For example, consider a signal that follows two paths to reach the receiver. One travels 100' and experiences 70 dB of attenuation, the other bounces around a wall instead of going through it, and ends up traveling 150' while only experiencing 60 dB of attenuation. After ~50 ns, the signal from the first path (which accurately represents the distance) will be swamped out by the signal from the second path. In order to get the info you need, you have two options: Either use a very high throughput signal (>1 Gbps) or have a special receiver architecture that recognizes those first symbols were an earlier path of the received signal and notes the time at which they arrived.
Bluetooth Proximity (Score:4, Informative)
wait what? beaming proximity data over bluetooth does not mean that bluetooth can naturally act as a proximity detector
These guys have been disagreeing with you for several years:
BtProx - Bluetooth Proximity Lock Utility [sourceforge.net]
Bluemon [matthew.ath.cx]
Re:Bluetooth Proximity (Score:3, Informative)
http://dspace.mit.edu/bitstream/handle/1721.1/7431/CS021.pdf [mit.edu]
Bonus points for running Bluetooth USB adapters without actually having a computer connected (at least after startup). I experimented some with this sort of scheme and my one warning is this: it takes 10.24 seconds to check all possible hopping patterns for all possible Bluetooth devices, so if it needs to respond quickly, you're hosed. But it's good enough for showing off, or updating a "Boss Proximity Detector", or that sort of thing.
Re:Wifi (Score:3, Informative)
celibacy means no whacking it either.
in fact, it includes impure thoughts by most definitions.
I think you mean
"he's a slashdotter, lack of reproductive opportunities is implied"