Omid Abari

GHz-Wide Sensing and Decoding Using the Sparse Fourier Transform

We present BigBand, a technology that can capture GHz of spectrum in realtime without sampling the signal at GS/s - i.e., without high speed ADCs. Further, it is simple and can be implemented on commodity low-power radios. Our approach builds on recent advances in the area of sparse Fourier transforms, which show that it is possible to reconstruct a sparse signal without sampling it at the Nyquist rate. To demonstrate our design, we implement it using 3 software radios, each sampling the spectrum at 50 MS/s, producing a device that captures 0.9 GHz - i.e., 6× larger digital bandwidth than the three software radios combined. Finally, an extension of BigBand can perform GHz spectrum sensing even in scenarios where the spectrum is not sparse.

Energy-Aware Design of Compressed Sensing Systems for Wireless Sensors Under Performance and Reliability Constraints

This paper describes the system design of a compressed sensing (CS) based source encoding system for data compression in wireless sensor applications. We examine the trade-off between the required transmission energy (compression performance) and desired recovered signal quality in the presence of practical non-idealities such as quantization noise, input signal noise and channel errors. The end-to-end system evaluation framework was designed to analyze CS performance under practical sensor settings. The evaluation shows that CS compression can enable over 10X in transmission energy savings while preserving the recovered signal quality to roughly 8 bits of precision. We further present low complexity error control schemes tailored to CS that further reduce the energy costs by 4X as well as diversity scheme to protect against burst errors. Results on a real electrocardiography (EKG) signal demonstrate 10X in energy reduction and corroborate the system analysis.

Millimeter Wave Communications: From Point-to-Point Links to Agile Network Connections

Millimeter wave (mmWave) technologies promise to revolutionize wireless networks by enabling multi-gigabit data rates. However, they suffer from high attenuation, and hence have to use highly directional antennas to focus their power on the receiver. Existing radios have to scan the space to find the best alignment between the transmitter’s and receiver’s beams, a process that takes up to a few seconds. This delay is problematic in a network setting where the base station needs to quickly switch between users and accommodate mobile clients. We present Agile-Link, the first mmWave beam steering system that is demonstrated to find the correct beam alignment without scanning the space. Instead of scanning, Agile- Link hashes the beam directions using a few carefully chosen hash functions. It then identifies the correct alignment by tracking how the energy changes across different hash functions. Our results show that Agile-Link reduces beam steering delay by orders of magnitude.

Caraoke: An E-Toll Transponder Network for Smart Cities

Electronic toll collection transponders, e.g., E-ZPass, are a widely-used wireless technology. About 70% to 89% of the cars in US have these devices, and some states plan to make them mandatory. As wireless devices however, they lack a basic function: a MAC protocol that prevents collisions. Hence, today, they can be queried only with directional antennas in isolated spots. However, if one could interact with e-toll transponders anywhere in the city despite collisions, it would enable many smart applications. For example, the city can query the transponders to estimate the vehicle flow at every intersection. It can also localize the cars using their wireless signals, and detect those that run a red-light. The same infrastructure can also deliver smart street-parking, where a user parks anywhere on the street, the city localizes his car, and automatically charges his account. This paper presents Caraoke, a networked system for delivering smart services using e-toll transponders. Our design operates with existing unmodified transponders, allowing for applications that communicate with, localize, and count transponders, despite wireless collisions. To do so, Caraoke exploits the structure of the transponders signal and its properties in the frequency domain. We built Caraoke reader into a small PCB that harvests solar energy and can be easily deployed on street lamps. We also evaluated Caraoke on four streets on our campus and demonstrated its capabilities.

Why Analog-to-Information Converters Suffer in High-Bandwidth Sparse Signal Applications

In applications where signal frequencies are high, but information bandwidths are low, analog-to-information converters (AICs) have been proposed as a potential solution to overcome the resolution and performance limitations of high-speed analog-to-digital converters (ADCs). However, the hardware implementation of such systems has yet to be evaluated. This paper aims to fill this gap, by evaluating the impact of circuit impairments on performance limitations and energy cost of AICs. We point out that although the AIC architecture facilitates slower ADCs, the signal encoding, typically realized with a mixer-like circuit, still occurs at the Nyquist frequency of the input to avoid aliasing. We illustrate that the jitter and aperture of this mixing stage limit the achievable AIC resolution. In order to do so, we designed an end-to-end system evaluation framework for examining these limitations, as well as the relative energy-efficiency of AICs versus high-speed ADCs across the resolution, receiver gain and signal sparsity. The evaluation shows that the currently proposed AICs have no performance benefits over high-speed ADCs. However, AICs enable 2-10X in energy savings in low to moderate resolution (ENOB), low gain applications.

Cutting the Cord in Virtual Reality

Todays virtual reality (VR) headsets require a cable connection to a PC or game console. This cable significantly limits the player’s mobility and hence her/his VR experience. The high data rate requirement of this link (multiple Gbps) precludes its replacement by WiFi. Thus, in this paper, we focus on using mmWave technology to deliver multi Gbps wireless communication between VR headsets and their game consoles. The challenge, however, is that mmWave signals can be easily blocked by the players hand or head motion. We describe novel algorithms and system design that allow such mmWave links to sustain high data rates even in the presence of a blockage, enabling a high quality untethered VR experience.

Enabling High-Quality Untethered Virtual Reality

Todays virtual reality (VR) headsets require a cable connection to a PC or game console. This cable significantly limits the players mobility and, hence, her VR experience. The high data rate requirement of this link (multiple Gbps) precludes its replacement by WiFi. Thus, in this paper, we focus on using mmWave technology to deliver multi-Gbps wireless communication between VR headsets and their game consoles. We address the two key problems that prevent existing mmWave links from being used in VR systems. First, mmWave signals suffer from a blockage problem, i.e., they operate mainly in line-of-sight and can be blocked by simple obstacles such as the player lifting her hand in front of the headset. Second, mmWave radios use highly directional antennas with very narrow beams; they work only when the transmitters beam is aligned with the receivers beam. Any small movement of the headset can break the alignment and stall the data stream. We present MoVR, a novel system that allows mmWave links to sustain high data rates even in the presence of a blockage and mobility. MoVR does this by introducing a smart mmWave mirror and leveraging VR headset tracking information. We implement MoVR and empirically demonstrate its performance using an HTC VR headset.

Performance trade-offs and design limitations of analog-to-information converter front-ends

This paper evaluates the impact of circuit impairments on the energy cost and performance limitations of analog-to-information converters (AIC). In applications where signal frequencies are high, but information bandwidths are low, AICs have been proposed as a potential solution to overcome the resolution and performance limitations of sampling jitter in high-speed analog-to-digital converters (ADC). Although the AIC architecture facilitates slower ADCs, the signal encoding, typically realized with a mixer-like circuit, still occurs at the Nyquist frequency of the input to avoid aliasing. We show that the jitter of this mixing stage limits the achievable AIC resolution. In this work, the end-to-end system evaluation framework is designed to analyze these limitations as well as the relative energy-efficiency of AICs versus ADCs across the resolution, receiver gain and signal sparsity. The evaluation shows that AICs improve the resolution by 1 bit when the signal of interest is very sparse, and enable 2× in energy savings when no pre-amplification is required.

27.4 A 0.75-million-point fourier-transform chip for frequency-sparse signals

Applications like spectrum sensing, radar signal processing, and pattern matching by convolving a signal with a long code, as in GPS, require large FFT sizes. ASIC implementations of such FFTs are challenging due to their large silicon area and high power consumption. However, the signals in these applications are sparse, i.e., the energy at the output of the FFT/IFFT is concentrated at a limited number of frequencies and with zero/negligible energy at most frequencies. Recent advances in signal processing have shown that, for such sparse signals, a new algorithm called the sparse FFT (sFFT) can compute the Fourier transform more efficiently than traditional FFTs [1].

AirShare: Distributed coherent transmission made seamless

Distributed coherent transmission is necessary for a variety of high-gain communication protocols such as distributed MIMO and creating codes over the air. Unfortunately, however, distributed coherent transmission is intrinsically difficult because different nodes are driven by independent clocks, which do not have the exact same frequency. This causes the nodes to have frequency offsets relative to each other, and hence their transmissions fail to combine coherently over the air. This paper presents AirShare, a primitive that makes distributed coherent transmission seamless. AirShare transmits a shared clock on the air and feeds it to the wireless nodes as a reference clock, hence eliminating the root cause for incoherent transmissions. The paper addresses the challenges in designing and delivering such a shared clock. It also implements AirShare in a network of USRP software radios, and demonstrates that it achieves tight phase coherence. Further, to illustrate AirShares versatility, the paper uses it to deliver a coherent-radio abstraction on top of which it demonstrates two cooperative protocols: distributed MIMO, and distributed rate adaptation.