Niranjini Rajagopal

Visual light landmarks for mobile devices

The omnipresence of indoor lighting makes it an ideal vehicle for pervasive communication with mobile devices. In this paper, we present a communication scheme that enables interior ambient LED lighting systems 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 lights 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.

ALPS: A Bluetooth and Ultrasound Platform for Mapping and Localization

The proliferation of Bluetooth Low-Energy (BLE) chipsets on mobile devices has lead to a wide variety of user-installable tags and beacons designed for location-aware applications. In this paper, we present the Acoustic Location Processing System (ALPS), a platform that augments BLE transmitters with ultrasound in a manner that improves ranging accuracy and can help users configure indoor localization systems with minimal effort. A user places three or more beacons in an environment and then walks through a calibration sequence with their mobile device where they touch key points in the environment like the floor and the corners of the room. This process automatically computes the room geometry as well as the precise beacon locations without needing auxiliary measurements. Once configured, the system can track a users location referenced to a map. The platform consists of time-synchronized ultrasonic transmitters that utilize the bandwidth just above the human hearing limit, where mobile devices are still sensitive and can detect ranging signals. To aid in the mapping process, the beacons perform inter-beacon ranging during setup. Each beacon includes a BLE radio that can identify and trigger the ultrasonic signals. By using differences in propagation characteristics between ultrasound and radio, the system can classify if beacons are within Line-Of-Sight (LOS) to the mobile phone. In cases where beacons are blocked, we show how the phones inertial measurement sensors can be used to supplement localization data. We experimentally evaluate that our system can estimate three-dimensional beacon location with a Euclidean distance error of 16.1cm, and can generate maps with room measurements with a two-dimensional Euclidean distance error of 19.8cm. When tested in six different environments, we saw that the system can identify Non-Line-Of-Sight (NLOS) signals with over 80% accuracy and track a users location to within less than 100cm.

Hybrid visible light communication for cameras and low-power embedded devices

Visible light communication (VLC) between LED light bulbs and smart-phone cameras has already begun to gain traction for identification and indoor localization applications. To support detection by cameras, the frequencies and data rates are typically limited to below 1kHz and tens of bytes per second (Bps). In this paper, we present a technique for transmitting data from solid-state luminaries, used for interior ambient lighting, simultaneously to both cameras and low-power embedded devices in a manner that is imperceptible to occupants. This allows the camera communication VLC channel to also act as a higher speed downstream link and low-power wakeup mechanism for energy-constrained devices. Our approach uses Manchester encoding and Binary Frequency Shift Keying (BFSK) to modulate the high-speed data stream and applies duty-cycle adjustment to generate the slower camera communication signal. We explore the trade-off between the performance of the two communication channels. Our hybrid communication protocol is also compatible with existing IR receivers. This allows lights to communicate with low-cost commodity chipsets and control home appliances such as TVs, AV receivers, AC window units, etc. We show that we are able to reliably simultaneously transmit low-speed data at 1.3 Bps to camera enabled devices and higher-speed data at 104 Bps to low-power embedded devices. Since the majority of energy in many RF communication protocols often goes towards media access and receiving, VLC-triggered wakeup can significantly decrease system energy consumption. We also demonstrate a proof-of-concept wakeup circuit that consumes less then 204uA and can be triggered in less then 10ms.

Ultrasonic time synchronization and ranging on smartphones

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 receivers clock with the infrastructure. These chirps encode identification data and ranging information that can be used to compute the receivers 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 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 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.

Hardware Assisted Clock Synchronization for Real-Time Sensor Networks

Time synchronization in wireless sensor networks is important for event ordering and efficient communication scheduling. In this paper, we introduce an external hardwarebased clock tuning circuit that can be used to improve synchronization and significantly reduce clock drift over long periods of time without waking up the host MCU. This is accomplished through two main hardware sub-systems. First, we improve upon the circuit presented in [1] that synchronizes clocks using the ambient magnetic fields emitted from power lines. The new circuit uses an electric field front-end as opposed to the original magnetic-field sensor, which makes the design more compact, lower-power, lower-cost, exhibit less jitter and improves robustness to noise generated by nearby appliances. Second, we present a low-cost hardware tuning circuit that can be used to continuously trim a micro-controllers low-power clock at runtime. Most time synchronization approaches require a CPU to periodically adjust internal counters to accommodate for clock drift. Periodic discrete updates can introduce interpolation errors as compared to continuous update approaches and they require the CPU to expend energy during these wake up periods. Our hardware-based external clock tuning circuit allows the main CPU to remain in a deep-sleep mode for extended periods while an external circuit compensates for clock drift. We show that our new synchronization circuit consumes 60% less power than the original design and is able to correct clock drift rates to within 0.01 ppm without power hungry and expensive precision clocks.

Demo abstract: a magnetic field-based appliance metering system

In this demonstration, we show an energy measurement system that estimates the energy consumption of individual appliances using a wireless sensor network consisting of contactless electromagnetic field (EMF) sensors deployed near each appliance, and a whole-house power meter [1]. The EMF sensor can detect appliance state transitions within close proximity based on magnetic field fluctuations. Data from these sensors are then relayed back to the main meter using a low-latency wireless sensor networking protocol, where changes in the total power consumption of the house are used to determine the power usage of individual appliances. The sensors are low-cost, easy to deploy and are able to detect current changes associated with the appliance from a few inches away making it possible to externally monitor in-wall wiring to devices like overhead lights or heavy machinery that might operate on multiple phases of the AC distribution system of the building. Appliance-level energy data provide continuous feedback to end users about their consumption patterns and provide building managers accurate information that can be used to target the most effective update and retrofit strategies.

Beacon placement for range-based indoor localization

In this paper, we address the problem of range-based beacon placement given a floor plan to support indoor localization systems. Existing approaches for trilateration require three or more beacons to determine a unique position solution. We show that with prior knowledge of the map and a model of beacon coverage, it is possible to uniquely localize with only two beacons. This not only reduces installation cost by requiring fewer nodes, but can also improve robustness. One of the main challenges with respect to beacon placement algorithms is defining a metric for estimating performance. We propose augmenting the commonly used Geometric Dilution of Precision (GDOP) metric to account for indoor spaces. We then use this enhanced GDOP metric as part of a toolchain to compare various beacon placement algorithms in terms of coverage and expected accuracy. When applied to a set of real floor plans, our approach is able to reduce the number of beacons between 22% and 60% (33% on an average) as compared to standard trilateration.

Demonstration abstract: how many lights do you see?

We demonstrate a Visual Light Communication (VLC) system [1] that enables LED lighting luminaires to communicate with cameras on mobile devices. Each LED pulses at a frequency above the humanly perceivable flicker threshold where cameras and photodiodes can still detect changes in light intensity. Our modulation scheme supports multiple light sources in a single collision domain, and works for both, line-of-sight (LOS) operation as well as from reflected surfaces like those found in architectural lighting. The spatial confinement of light makes this system ideal for use as localization landmarks. Our demonstration includes four LED ambient lights acting as location landmarks transmitting modulated data. A mobile device receiving and processing the signal displays the ID and RSSI of the closest landmark. Interacting with the system will allow users to see the practical effects of multiple-access, frequency of operation, distance from the lights, camera parameters and camera motion.

Welcome to my world: demystifying multi-user AR with the cloud: demo abstract

We demonstrate multi-user persistent Augmented Reality (AR) on mobile devices with a novel technique that provides nearly instant acquisition of location and orientation. Visual Inertial Odometry (VIO) provides accurate position and orientation tracking relative to device start-up for AR applications. Unfortunately, the tracking is local to the AR session of a single user and is not anchored in a global coordinate system. In order to provide all devices an accurate location in a common frame of reference, we utilize UWB nodes that range to the devices. To avoid the long startup time required to compute the devices orientation, we propose a novel technique that utilizes previously recorded magnetic field information to rapidly calibrate the compass. In order to simplify setup, we demonstrate automatic mapping of beacon locations and surveying of magnetic field by a pedestrian walking around the test area with a mobile device.

Enhancing indoor smartphone location acquisition using floor plans

Indoor localization systems typically determine a position using either ranging measurements, inertial sensors, environmental-specific signatures or some combination of all of these methods. Given a floor plan, inertial and signature-based systems can converge on accurate locations by slowly pruning away inconsistent states as a user walks through the space. In contrast, range-based systems are capable of instantly acquiring locations, but they rely on densely deployed beacons and suffer from inaccurate range measurements given non-line-of-sight (NLOS) signals. In order to get the best of both worlds, we present an approach that systematically exploits the geometry information derived from building floor plans to directly improve location acquisition in range-based systems. Our solving approach can disambiguate multiple feasible locations taking into account a mix of LOS and NLOS hypotheses to accurately localize with significantly fewer beacons. We demonstrate our geometry-aware solving approach using a new ultrasonic beacon platform that is able to perform direct time-of-flight ranges on commodity smartphones. The platform uses Bluetooth Low Energy (BLE) for time synchronization and ultrasound for measuring propagation distance. We evaluate our systems accuracy with multiple deployments in a university campus and show that our approach shifts the 80% accuracy point from 4-8m to 1m as compared to solvers that do not use the floor plan information. We are able to detect and remove NLOS signals with 91.5% accuracy.