Thursday, November 21, 2013

HotNets '13: Pharos: Enable Physical Analytics through Visible Light based Indoor Localization

Authors: Pan Hu (University of Massachusetts Amherst), Liqun Li (Microsoft Research Asia), Chunyi Peng (University of California, Los Angeles), Guobin Shen, Feng Zhao (Microsoft Research Asia)

GPS-like indoor localization is important, with current techniques determining the position by using distances or angles of arrival based on access points. In addition, fingerprinting techniques allow position estimation based on information (e.g., signal power) collected  at discrete positions aggregated in a centralized database. However, there is additional improvement to be made in terms of localization accuracy.

Given the wide-spread adoption of LED lighting, it is possible to exploit the visible spectrum to determine current location. Since LEDs have very quick rising and falling transition edges between on and off, it is possible to transmit information over this optical medium by modulating the LED. If each LED light broadcasts its location information, user devices can perform  multilateration to estimate their own location based on the received  location information, along with estimated distances according to received power from each light. In order to support many uncoordinated LEDs within some environment, frequency-division multiplexing is used along with frequency hopping to avoid interference (an approach that is similar to Bluetooth). 

This system was implemented in hardware, designing control boards to connect to the LEDs and light sensor boards to attach to cellphones (via the audio port for communication). In practice, within both a conference room and cubicle area environment, the accuracy is shown to be within 0.4 to 0.7 meters (better than representative WiFi based approaches).

Q: What changes are necessary to existing lights?

A: Each light must be attached to a control board, which supports the transmission of the necessary location information beacons as part of the multilateration protocol.

Q: In the evaluation, there are very open environments and without many people moving around. Do you have plans for evaluations in more dense environments?

Q: What happens if there is a room with outside sources of light?

A: If the cellphone can detect the natural light, it should be able to actually use GPS in this case (as it has a LOS to the sky). However, stable sources of light will not affect this method due to detecting the power at certain frequencies (10 kHz to 19 kHz).

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