Doctor of Philosophy

dissertationThe wireless radio channel is typically thought of as a means to move information from transmitter to receiver, but the radio channel can also be used to detect changes in the environment of the radio link. This dissertation is focused on the measurements we can make at the physical layer of wireless networks, and how we can use those measurements to obtain information about the locations of transceivers and people. The first contribution of this work is the development and testing of an open source, 802.11b sounder and receiver, which is capable of decoding packets and using them to estimate the channel impulse response (CIR) of a radio link at a fraction of the cost of traditional channel sounders. This receiver improves on previous implementations by performing optimized matched filtering on the field-programmable gate array (FPGA) of the Universal Software Radio Peripheral (USRP), allowing it to operate at full bandwidth. The second contribution of this work is an extensive experimental evaluation of a technology called location distinction, i.e., the ability to identify changes in radio transceiver position, via CIR measurements. Previous location distinction work has focused on single-input single-output (SISO) radio links. We extend this work to the context of multiple-input multiple-output (MIMO) radio links, and study system design trade-offs which affect the performance of MIMO location distinction. The third contribution of this work introduces the "exploiting radio windows" (ERW) attack, in which an attacker outside of a building surreptitiously uses the transmissions of an otherwise secure wireless network inside of the building to infer location information about people inside the building. This is possible because of the relative transparency of external walls to radio transmissions. The final contribution of this dissertation is a feasibility study for building a rapidly deployable radio tomographic (RTI) imaging system for special operations forces (SOF). We show that it is possible to obtain valuable tracking information using as few as 10 radios over a single floor of a typical suburban home, even without precise radio location measurements

Doctor of Philosophy

dissertationLow-cost wireless embedded systems can make radio channel measurements for the purposes of radio localization, synchronization, and breathing monitoring. Most of those systems measure the radio channel via the received signal strength indicator (RSSI), which is widely available on inexpensive radio transceivers. However, the use of standard RSSI imposes multiple limitations on the accuracy and reliability of such systems; moreover, higher accuracy is only accessible with very high-cost systems, both in bandwidth and device costs. On the other hand, wireless devices also rely on synchronized notion of time to coordinate tasks (transmit, receive, sleep, etc.), especially in time-based localization systems. Existing solutions use multiple message exchanges to estimate time offset and clock skew, which further increases channel utilization. In this dissertation, the design of the systems that use RSSI for device-free localization, device-based localization, and breathing monitoring applications are evaluated. Next, the design and evaluation of novel wireless embedded systems are introduced to enable more fine-grained radio signal measurements to the application. I design and study the effect of increasing the resolution of RSSI beyond the typical 1 dB step size, which is the current standard, with a couple of example applications: breathing monitoring and gesture recognition. Lastly, the Stitch architecture is then proposed to allow the frequency and time synchronization of multiple nodes' clocks. The prototype platform, Chronos, implements radio frequency synchronization (RFS), which accesses complex baseband samples from a low-power low-cost narrowband radio, estimates the carrier frequency offset, and iteratively drives the difference between two nodes' main local oscillators (LO) to less than 3 parts per billion (ppb). An optimized time synchronization and ranging protocols (EffToF) is designed and implemented to achieve the same timing accuracy as the state-of-the-art but with 59% less utilization of the UWB channel. Based on this dissertation, I could foresee Stitch and RFS further improving the robustness of communications infrastructure to GPS jamming, allow exploration of applications such as distributed beamforming and MIMO, and enable new highly-synchronous wireless sensing and actuation systems

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Location estimation and collective inference in indoor spaces using smartphones

In the last decade, indoor localization-based smart, innovative services have become very popular in public spaces (retail spaces, malls, museums, and warehouses). We have state-of-art RSSI techniques to more accurate CSI techniques to infer indoor location. Since the past year, the pandemic has raised an important challenge of determining if a pair of individuals are ``social-distancing,'' separated by more than 6ft. Most solutions have used `presence'-if one device can hear another--- which is a poor proxy for distance since devices can be heard well beyond 6 ft social distancing radius and across aisles and walls. Here we ask the key question: what needs to be added to our current indoor localization solutions to deploy them towards scenarios like reliable contact tracing solutions easily. And we identified three main limitations---deployability, accuracy, and privacy. Location solutions need to deploy on ubiquitous devices like smartphones. They should be accurate under different environmental conditions. The solutions need to respect a person's privacy settings. Our main contributions are twofold -a new statistical feature for localization, Packet Reception Probability (PRP) which correlates with distance and is different from other physical measures of distance like CSI or RSSI. PRP can easily deploy on smartphones (unlike CSI) and is more accurate than RSSI. Second, we develop a crowd tool to audit the level of location surveillance in space which is the first step towards achieving privacy. Specifically, we first solve a location estimation problem with the help of infrastructure devices (mainly Bluetooth Low Energy or BLE devices). BLE has turned out to be a key contact tracing technology during the pandemic. We have identified three fundamental limitations with BLE RSSI---biased RSSI Estimates due to packet loss, mean RSSI de-correlated with distance due to high packet loss in BLE, and well-known multipath effects. We built the new localization feature, Packet Reception Probability (PRP), to solve the packet loss problem in RSSI. PRP measures the probability that a receiver successfully receives packets from the transmitter. We have shown through empirical experiments that PRP encodes distance. We also incorporated a new stack-based model of multipath in our framework. We have evaluated B-PRP in two real-world public places, an academic library setting and a real-world retail store. PRP gives significantly lower errors than RSSI. Fusion of PRP and RSSI further improves the overall localization accuracy over PRP. Next, we solved a peer-to-peer distance estimation problem that uses minimal infrastructure. Most apps like aarogya setu, bluetrace have solved peer-to-peer distances through the presence of Bluetooth Low-Energy (BLE) signals. Apps that rely on pairwise measurements like RSSI suffer from latent factors like device relative positioning on the human body, the orientation of the people carrying the devices, and the environmental multipath effect. We have proposed two solutions in this work---using known distances and collaboration to solve distances more robustly. First, if we have a few infrastructure devices installed at known locations in an environment, we can make more measurements with the devices. We can also use the known distances between the devices to constrain the unknown distances in a triangle inequality framework. Second, in an outdoor environment where we cannot install infrastructure devices, we can collaborate between people to jointly constrain many unknown distances. Finally, we solve a collaborative tracking estimation problem where people audit the properties of localization infrastructure. While people want services, they do not want to be surveilled. Further, people using an indoor location system do not know the current surveillance level. The granularity of the location information that the system collects about people depends on the nature of the infrastructure. Our system, the CrowdEstimator, provides a tool to people to harness their collective power and collect traces for inferring the level of surveillance. We further propose the insight that surveillance is not a single number, instead of a spatial map. We introduce active learning algorithms to infer all parts of the spatial map with uniform accuracy. Auditing the location infrastructure is the first step towards achieving the bigger goal of declarative privacy, where a person can specify their comfortable level of surveillance

Measuring interaction proxemics with wearable light tags

The proxemics of social interactions (e.g., body distance, relative orientation) in!uences many aspects of our everyday life: from patients’ reactions to interaction with physicians, successes in job interviews, to effective teamwork. Traditionally, interaction proxemics has been studied via questionnaires and participant observations, imposing high burden on users, low scalability and precision, and often biases. In this paper we present Protractor, a novel wearable technology for measuring interaction proxemics as part of non-verbal behavior cues with# ne granularity. Protractor employs near-infrared light to monitor both the distance and relative body orientation of interacting users. We leverage the characteristics of near-infrared light (i.e., line-of-sight propagation) to accurately and reliably identify interactions; a pair of collocated photodiodes aid the inference of relative interaction angle and distance. We achieve robustness against temporary blockage of the light channel (e.g., by the user’s hand or clothes) by designing sensor fusion algorithms that exploit inertial sensors to obviate the absence of light tracking results. We fabricated Protractor tags and conducted real-world experiments. Results show its accuracy in tracking body distances and relative angles. The framework achieves less than 6 error 95% of the time for measuring relative body orientation and 2.3-cm – 4.9-cm mean error in estimating interaction distance. We deployed Protractor tags to track user’s non-verbal behaviors when conducting collaborative group tasks. Results with 64 participants show that distance and angle data from Protractor tags can help assess individual’s task role with 84.9% accuracy, and identify task timeline with 93.2% accuracy