Inertial measurement unit (IMU) has long suffered from the problem of integration drift, where sensor noises accumulate quickly and cause fast-growing tracking errors. Existing methods for calibrating IMU tracking either require human in the loop, or need energy-consuming cameras, or suffer from coarse tracking granularity. We propose to augment indoor inertial tracking by reusing existing indoor luminaries to project a static light polarization pattern in the space. This pattern is imperceptible to human eyes and yet through a polarizer, it becomes detectable by a color sensor, and thus can serve as fine-grained optical landmarks that constrain and correct IMU's integration drift and boost tracking accuracy. Exploiting the birefringence optical property of transparent tapes -- a low-cost and easily-accessible material -- we realize the polarization pattern by simply adding to existing light cover a thin polarizer film with transparent tape stripes glued atop. When fusing with IMU sensor signals, the light pattern enables robust, accurate and low-power motion tracking. Meanwhile, our approach entails low deployment overhead by reusing existing lighting infrastructure without needing an active modulation unit. We build a prototype of our light cover and the sensing unit using off-the-shelf components. Experiments show 4.3 cm median error for 2D tracking and 10 cm for 3D tracking, as well as its robustness in diverse settings.

Visible Light Communication (VLC) emerges as a new wireless communication technology with appealing benefits not present in radio communication. However, current VLC designs commonly require LED lights to emit shining light beams, which greatly limits the applicable scenarios of VLC (e.g., in a sunny day when indoor lighting is not needed). It also entails high energy overhead and unpleasant visual experiences for mobile devices to transmit data using VLC. We design and develop DarkLight, a new VLC primitive that allows light-based communication to be sustained even when LEDs emit extremely-low luminance. The key idea is to encode data into ultra-short, imperceptible light pulses. We tackle challenges in circuit designs, data encoding/decoding schemes, and DarkLight networking, to efficiently generate and reliably detect ultra-short light pulses using off-the-shelf, low-cost LEDs and photodiodes. Our DarkLight prototype supports 1.3-m distance with 1.6-Kbps data rate. By loosening up VLC's reliance on visible light beams, DarkLight presents an unconventional direction of VLC design and fundamentally broadens VLC's application scenarios.

Visible Light Communication (VLC) emerges as a new wireless communication technology with appealing benefits not present in radio communication. However, current VLC designs commonly require LED lights to emit perceptible light beams, which greatly limits the applicable scenarios of VLC (e.g., in a sunny day when indoor lighting is not needed), and brings high energy overhead and unpleasant visual experiences for mobile devices to transmit data using VLC. We design and develop DarkLight, a new VLC primitive that allows light-based communication to be sustained even when LEDs emit extremely-low luminance. The key idea is to encode data into ultra-short, imperceptible light pulses. We tackle challenges in circuit designs, data encoding/decoding schemes, and DarkLight networking, to efficiently generate and reliably detect ultra-short light pulses using off-the-shelf, low-cost LEDs and photodiodes. Our DarkLight prototype supports 1.3-m distance with 1.6-Kbps data rate. By loosening up VLC's reliance on visible light beams, DarkLight presents an unconventional direction of VLC and fundamentally broadens VLC's application scenarios.

We present LiSense, the first-of-its-kind system that enables both data communication and fine-grained, real-time human skeleton reconstruction using Visible Light Communication (VLC). LiSense uses shadows created by the human body from blocked light and reconstructs 3D human skeleton postures in real time. We overcome two key challenges to realize shadow-based human sensing. First, multiple lights on the ceiling lead to diminished and complex shadow patterns on the floor. We design light beacons enabled by VLC to separate light rays from different light sources and recover the shadow pattern cast by each individual light. Second, we design an efficient inference algorithm to reconstruct user postures using 2D shadow information with a limited resolution collected by photodiodes embedded in the floor. We build a 3 m x 3 m LiSense testbed using off-the-shelf LEDs and photodiodes. Experiments show that LiSense reconstructs the 3D user skeleton at 60 Hz in real time with 10 degrees mean angular error for five body joints.

We present a new visible light communication (VLC) primitive: VLC in the dark, where the communication sustains even when the LED light appears dark. We achieve the goal by reducing the duty cycle of the light source to an extremely low level (0.0019%) such that the illuminance is imperceptible to human eyes and yet can still be sensed by photodiodes to decode data. VLC in the dark consumes much lower energy than the conventional VLC links and thus is appealing for enabling VLC uplink or communication among Internet-of-Things devices with tight energy budget. We build a preliminary prototype to demonstrate its feasibility. Our current single link achieves 190.7 bps data rate with 46.8 microwatts power consumption at the LED.