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Version 11 (Anthony Rowe, 06/18/2014 01:45 am)

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h1. Acoustic Location Processing System (ALPS)
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ALPs is an indoor ultrasonic ranging technique that can be used to localize modern mobile devices like smartphones and tablets. The method uses a communication scheme in the audio bandwidth just above the human hearing frequency range where mobile devices are still sensitive. 
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!{height:300px}spectrogram.png! !{height:300px}alps.png!
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ALPs can be used with off-the-shelf audio speakers (potentially already in place) to provide fine-grained indoor position data to modern mobile devices like smartphones and tablets. It uses a communication primitive based on rate-adaptive wide-band linear frequency modulated chirp pulses in the audio frequencies just above the human hearing frequency range where mobile devices are still sensitive. Typically transmitting data, even outside of this range, introduces broadband human audible noises (clicks) due to the non-ideal impulse response of speakers. Unlike existing audio modulation schemes, our scheme is optimized based on psychoacoustic properties. For exam- ple, all tones exhibit slowly changing power-levels and grad- ual frequency changes so as to minimize human perceivable artifacts. Chirps also bring the benefit of Pulse Compression, which greatly improves ranging resolution and makes them resilient to both Doppler shifts as well as multi-path propagation that typically plague indoor environments. The scheme also supports the decoding of multiple unique iden- tifier packets being transmitted simultaneously. By applying a Time-Difference-of-Arrival (TDOA) pseudo-ranging technique the mobile devices can localize themselves without tight out-of-band synchronization with the broadcasting infrastructure. This design is not only scalable with respect to the number of transmitters and tracked devices, but also improves user privacy since the mobile devices compute their positions locally. 
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!alps.png!
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h2. Challenges
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* Efficient usage of bandwidth
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** Data transmission
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** Multiple Access
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* Ranging Accuracy
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** Timing Accuracy
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** Length of Transmission vs. SNR
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* Eliminate human perceivable artifacts
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** Limits modulation scheme
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h2. Utilizing Existing PA systems
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Existing PA system in public spaces offer a great deployment opportunity.  They tend to be regularly spaced and position close to users so as to avoid acoustic hot-spots.
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For more information, checkout out our "SenSys 2012 Paper":http://wise.ece.cmu.edu/resources/publications/sensys_12.pdf.
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