Ultrasonic Time Synchronization and Ranging on Smartphones

Abstract-In this paper, we present the design and evaluation of a platform that can be used for time synchronization and indoor positioning of mobile devices. The platform uses the Time-Difference-Of-Arrival (TDOA) of multiple ultrasonic chirps broadcast from a network of beacons placed throughout the environment to find an initial location as well as synchronize a receiver's clock with the infrastructure. These chirps encode identification data and ranging information that can be used to compute the receiver's location. Once the clocks have been synchronized, the system can continue performing localization directly using Time-of-Flight (TOF) ranging as opposed to TDOA. This provides similar position accuracy with fewer beacons (for tens of minutes) until the mobile device clock dirfts enough that a TDOA signal is once again required. Our hardware platform uses RF-based time synchronization to distribute clock synchronization from a subset of infrastructure beacons connected to a GPS source. Mobile devices use a novel time synchronization technique leverages the continuously freerunning audio sampling subsystem of a smartphone to synchronize with global time. Once synchronized, each device can determine an accurate proximity from as little as one beacon using Time-Of-Flight (TOF) measurements. This significantly decreases the number of beacons required to cover an indoor space and improves performance in the face of obstructions. We show through experiments that this approach outperforms the Network Time Protocol (NTP) on smartphones by an order of magnitude, providing an average 720µs synchronization accuracy with clock drift rates as low as 2ppm

Ultrasonic Ranging and Indoor Localization for Mobile Devices

<p>Location tracking on mobile devices like smartphones has already begun to revolutionize personal navigation. Unfortunately, these services perform poorly indoors when GPS signals are no longer available. Highly accurate indoor location tracking would enhance a wide variety of applications including: building navigation (malls, factories, airports), augmented reality, location-aware pervasive computing, targeted advertising, social networking, participatory sensing and could even support next generation beam forming MIMO wireless networks. Current indoor localization systems for smartphones often use RF signal strength from WiFi access points or Bluetooth Low Energy (BLE) beacons to fingerprint indoor locations. Such systems are sensitive to environmental changes and obstructions, require extensive training procedures and are limited in both absolute as well as semantic localization accuracy. We propose using audio signals in the ultrasound spectrum, just above the human hearing range, to provide ranging and localization for many off-the-shelf mobile devices that are equipped with microphones. Ultrasonic ranging provides several advantages over RF-based ranging and fingerprinting approaches, which make it attractive for indoor localization. A relatively low propagation speed and carrier frequency allow for precise propagation time measurements in software using commodity hardware. Acoustic signals also have a low penetration depth, which confines them to target areas for accurate semantic localization. In this dissertation we address several challenges related to acoustic localization, including system scalability, ranging and localization accuracy, energy efficiency, robustness to noise, elimination of human perceivable audio artifacts, efficient use of limited acoustic bandwidth and rapid deployment strategies.</p

Visual Light Landmarks for Mobile Devices

<p>The omnipresence of indoor lighting makes it an ideal vehicle for pervasive communication with mobiledevices. In this paper, we present a communication scheme that enables interior ambient LED lightingsystems to send data to mobile devices using either cameras or light sensors. By exploiting rolling shutter camera sensors that are common on tablets, laptops and smartphones, it is possible to detect high-frequency changes in light intensity reflected off of surfaces and in direct line-of-sight of the camera. We present a demodulation approach that allows smartphones to accurately detect frequencies as high as 8kHz with 0.2kHz channel separation. In order to avoid humanly perceivable flicker in the lighting, our system operates at frequencies above 2kHz and compensates for the non-ideal frequency response of standard LED drivers by adjusting the light's duty-cycle. By modulating the PWM signal commonly used to drive LED lighting systems, we are able to encode data that can be used as localization landmarks. We show through experiments how a binary frequency shift keying modulation scheme can be used to transmit data at 1.25 bytes per second (fast enough to send an ID code) from up to 29 unique light sources simultaneously in a single collision domain. We also show how tags can demodulate the same signals using a light sensor instead of a camera for low-power applications.</p

Ultrasonic Time Synchronization and Ranging on Smartphones

<p>In this paper, we present the design and evaluation of a platform that can be used for timesynchronization and indoor positioning of mobile devices. The platform uses the Time-Difference-Of-Arrival (TDOA) of multiple ultrasonic chirps broadcast from a network of beacons placed throughout the environment to find an initial location as well as synchronize a receiver's clock with the infrastructure. These chirps encode identification data and ranging information that can be used to compute the receiver's location. Once the clocks have been synchronized, the system can continue performing localization directly using Time-of-Flight (TOF) ranging as opposed to TDOA. This provides similar position accuracy with fewer beacons (for tens of minutes) until the mobile device clock drifts enough that a TDOA signal is once again required. Our hardware platform uses RF-based time synchronizationto distribute clock synchronization from a subset of infrastructure beacons connected to a GPS source. Mobile devices use a novel time synchronization technique leverages the continuously free-running audio sampling subsystem of a smartphone to synchronize with global time. Once synchronized, each device can determine an accurate proximity from as little as one beacon using TOF measurements. This significantly decreases the number of beacons required to cover an indoor space and improves performance in the face of obstructions. We show through experiments that this approach outperforms the Network Time Protocol (NTP) on smartphones by an order of magnitude, providing an average 720μs synchronization accuracy with clock drift rates as low as 2ppm.</p

Demo Abstract: Distributed Control of a Swarm of Buildings Connected to a Smart Grid

Energy-efficient control mechanisms are necessary to manage the ever increasing energy demand. Recently several tools for building energy consumption control have been proposed for small (e.g. homes) and large (e.g. offices) buildings. The mechanism each tool uses is different, e.g. HVAC control and appliance rescheduling, but they share the goal of improving consumption of the buildings with respect to a given cost function. Some examples of cost functions are reduced energy consumption, reduced electricity bill, lower peak power, and increased ancillary service participation. The tools however do not capture the impacts of their control actions on the grid. These actions can lead to supply/demand imbalance and voltage/frequency deviation and thus, threaten grid stability. Utilities can take protective actions against those who cause instability by increasing electricity price or even momentarily disconnecting them from the grid. The effects of these protective actions can be so severe that the savings obtained by building management tools might disappear