60 GHz Millimeter-wave Wireless Networking and Sensing: Communications at 60 GHz unlicensed spectrum (57-74 GHz) is becoming popular due to its ability of providing multi-gigabit data rates (IEEE 802.11ad/ay, 802.15.3c) and supporting applications like Augmented and Virtual Reality (AR/VR) and wireless HD uncompressed video streaming. Due to its directional nature of communication and susceptibility to blockages, user mobility and beam tracking are challenging problems in 60 GHz WLANs. The objective of this research is to design and investigate 60 GHz WLANs that can scale for a large number of densely deployed devices while ensuring low interference, high capacity, and robustness and agility to blockage and mobility.

Publications: Project websites:
Visible Light Communication (VLC): LEDs can be exploited to encode data in the emitted light which can then be received by a photodiode or a camera. VLC can enable many novel applications where users can locate a transmitter (i.e. LED) and specifically receive data from it. In our research, we increase the achievable data rates of LED-to-camera communication using RGB LEDs and color shift keying. Our scheme ensures that even when data is transmitted using color symbols, the human perceived color of the LED remains white (flicker-free illumination). Reed-solomon codes are employed to deal with the data loss that occurs while receiving data on a rolling-shutter camera.
Publications: [TMC '19], [VLCS '16], [CoNEXT '15], [COMSOC-CST '15]

Energy Efficiency in MIMO WLANs: Today's WLANs (IEEE 802.11n/ac) rely on higher order MIMO and wider channel widths to increase the achievable data rates. We study energy efficiency and channel contention issues, and identify that increasing number of MIMO spatial streams is more energy efficient compared to utilizing wide channel widths. Our work also shows that In heterogeneous channel width environment (coexistence with legacy 802.11a/n) where different links operate at different channel widths, competition to access the medium becomes increasing unfair which results into starvation of the larger channel width links. 
Publications: [Networking '14], [ETT '15]

Device-free RF Sensing: The ubiquitous presence of RF signals for wireless communication can be exploited for contact-free sensing of human motion and activities. Our work showed that the Channel State Information (CSI) of the WiFi link between user's smartphone and the AP can be analyzed at the AP to determine the motion of user's arm. Similarly, when a user is in proximity of a WiFi link, the user's arm gestures, activities (sitting, walking, running etc.) can be determined by mining the signal reflected from her body after a careful multipath removal process. We used these concepts for physical analytics applications such as detecting a shopper's behavior in a retail store. The remote RF sensing is also useful in other applications like device-free pedometer, person identification, gait analysis, and occupancy counting.
Publications: [SenSys '17], [IPSN '16], [HotWireless '15], [WPA '15]

Mobile Interaction using Wrist-worn Wearables: The increasing popularity of wrist-worn wearables (smartwatches and fitness trackers) presents an opportunity to recognize user's hand, arm and finger gestures using the motion sensors in the devices. In our research, we demonstrate that due to the movements of wrist tendons while moving the fingers, it is possible to uniquely identify a variety of finger gestures through wrist-worn wearables. The motion identification is also useful in building a finger-writing input system where a user can write characters on a surface while wearing the smartwatch to input text to a connected device or the watch itself. The interaction between users' wearables and IoT devices can be exploited for applications such as personal energy analytics in smart buildings and other similar cyber-physical systems.
Publications: [SECON '17] [BuildSys '16] [HotMobile '15]
Demos: [MobiSys '15], [Video]

Continuous Sensing with Low-power Mobile Sensors: Continuous sensing applications on mobile devices (e.g. constantly listening for user's voice command) can result in substantial energy drain. We propose to utilize low-power MEMS sensors (e.g. accelerometer, gyroscope, barometer etc.) to act as proxy sensors for continuous sensing, and triggering the use of energy-hungry sensors only when needed. We show that accelerometer sensor commonly found in today's mobile devices is sensitive to user's voice, and can accurately listen for hotwords like "Okay Google" while consuming very low energy. Similarly, the MEMS barometer available in smartphones can continuously monitor for building door open/close events due to the pressure difference created by HVAC system. The low-power sensors in wearables can also be leveraged to fingerprint user's surroundings and her activities, and to improve the privacy of first-person camera wearable devices (such as Google Glass and Narrative life logger).
Publications: [MobiSys '15], [UbiComp '15], [SECON '15]

Characteristics of Multi-device Mobile Users: Over two third of adults in US currently own a laptop, a tablet and a smartphone, and the number of wireless-enabled devices that users carry is consistently increasing. We carried out a study of large user population to understand the Internet access patterns of the multi-device users. We found that when a user owns multiple wireless-enabled devices, the overall network usage increases proportionally with the number of devices, rather than being divided across the devices. Our study also showed that usage of a device remains more or less independent of the existence of user's other devices.
Publications: [ToIT '16], [SECON '15]

Privacy and Security through Network Traffic Analysis: Utilizing user's contextual location (e.g. home, office) for providing location-based services can protect user's privacy by hiding her exact geographical location. However, determining user's contextual location is difficult. In our research, we propose a solution where user's network traffic can be analyzed for extracting intrinsic access patterns that are reflective of user's contextual location without the need of any deep packet inspection. We have also demonstrated the effectiveness of network traffic mining for detecting access to malicious web domains and how it can augment existing screening methods based on lexical analysis.
Publications: [COMNET '17], [SECON '17], [COMCOM '16],[INFOCOM '14]

Service Continuity and Fault Tolerance (project status - completed): Designing highly-available (availability of two nines or three nines) wireless access networks is extremely challenging due to frequent link failture. In our research, we proved analytcally that in case of probabilistic link failures, network connectivity and data rate capacity are inherently at odds, and performability (performance weighted availability) is a better metric to assess when studying the deployment factors of multi-hop wireless networks. We also developed a relay load balancing scheme for multi-hop wireless networks that utilizes betweenness centrality for variable transmission power control and topology design.
Publications: [TMC '14], [COMSOC-CST '11], [COMCOM '12], [GlobeCom '10], [WCNC '10]
- Book: [Springer '12]
- Centennial campus outdoor wireless network testbed (NC State): [CentMesh]
- Quail Ridge wireless network testbed (UC Davis): [QuRiNet]