There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Radio frequency (RF) energy transfer and harvesting techniques have recently become alternative methods to power the next-generation wireless networks As this emerging technology enables proactive energy replenishment of wireless devices, it is advantageous in supporting applications with quality-of-service requirements In this paper, we present a comprehensive literature review on the research progresses in wireless networks with RF energy harvesting capability, which is referred to as RF energy harvesting networks (RF-EHNs) First, we present an overview of the RF-EHNs including system architecture, RF energy harvesting techniques, and existing applications Then, we present the background in circuit design as well as the state-of-the-art circuitry implementations and review the communication protocols specially designed for RF-EHNs We also explore various key design issues in the development of RF-EHNs according to the network types, ie, single-hop networks, multiantenna networks, relay networks, and cognitive radio networks Finally, we envision some open research directions

Wireless Networks With RF Energy Harvesting: A Contemporary Survey

This paper proposes COPE, a new architecture for wireless mesh networks. In addition to forwarding packets, routers mix (i.e., code) packets from different sources to increase the information content of each transmission. We show that intelligently mixing packets increases network throughput. Our design is rooted in the theory of network coding. Prior work on network coding is mainly theoretical and focuses on multicast traffic. This paper aims to bridge theory with practice; it addresses the common case of unicast traffic, dynamic and potentially bursty flows, and practical issues facing the integration of network coding in the current network stack. We evaluate our design on a 20-node wireless network, and discuss the results of the first testbed deployment of wireless network coding. The results show that using COPE at the forwarding layer, without modifying routing and higher layers, increases network throughput. The gains vary from a few percent to several folds depending on the traffic pattern, congestion level, and transport protocol.

/pdf/xors-in-the-air-practical-wireless-network-coding-17k31b3st4.pdf

XORs in the air: practical wireless network coding

This paper presents the design and implementation of the first in-band full duplex WiFi radios that can simultaneously transmit and receive on the same channel using standard WiFi 802.11ac PHYs and achieves close to the theoretical doubling of throughput in all practical deployment scenarios. Our design uses a single antenna for simultaneous TX/RX (i.e., the same resources as a standard half duplex system). We also propose novel analog and digital cancellation techniques that cancel the self interference to the receiver noise floor, and therefore ensure that there is no degradation to the received signal. We prototype our design by building our own analog circuit boards and integrating them with a fully WiFi-PHY compatible software radio implementation. We show experimentally that our design works robustly in noisy indoor environments, and provides close to the expected theoretical doubling of throughput in practice.

https://dl.acm.org/doi/pdf/10.1145/2486001.2486033

Full duplex radios

This paper discusses the design of a single channel full-duplex wireless transceiver. The design uses a combination of RF and baseband techniques to achieve full-duplexing with minimal effect on link reliability. Experiments on real nodes show the full-duplex prototype achieves median performance that is within 8% of an ideal full-duplexing system. This paper presents Antenna Cancellation, a novel technique for self-interference cancellation. In conjunction with existing RF interference cancellation and digital baseband interference cancellation, antenna cancellation achieves the amount of self-interference cancellation required for full-duplex operation. The paper also discusses potential MAC and network gains with full-duplexing. It suggests ways in which a full-duplex system can solve some important problems with existing wireless systems including hidden terminals, loss of throughput due to congestion, and large end-to-end delays.

/pdf/achieving-single-channel-full-duplex-wireless-communication-3pg630e002.pdf

Achieving single channel, full duplex wireless communication

Wind dispersal of battery-free wireless devices

We introduce the first forearm-based EMG input system that can recognize fine-grained thumb gestures, including left swipes, right swipes, taps, long presses, and more complex thumb motions. EMG signals for thumb motions sensed from the forearm are quite weak and require significant training data to classify. We therefore also introduce a novel approach for minimally-intrusive collection of labeled training data for always-available input devices. Our dual-observable input approach is based on the insight that interaction observed by multiple devices allows recognition by a primary device (e.g., phone recognition of a left swipe gesture) to create labeled training examples for another (e.g., forearm-based EMG data labeled as a left swipe). We implement a wearable prototype with dry EMG electrodes, train with labeled demonstrations from participants using their own phones, and show that our prototype can recognize common fine-grained thumb gestures and user-defined complex gestures.

/pdf/leveraging-dual-observable-input-for-fine-grained-thumb-2lv5o4xvg5.pdf

Leveraging Dual-Observable Input for Fine-Grained Thumb Interaction Using Forearm EMG

This paper enables connectivity on everyday objects by transforming them into FM radio stations. To do this, we show for the first time that ambient FM radio signals can be used as a signal source for backscatter communication. Our design creates backscatter transmissions that can be decoded on any FM receiver including those in cars and smartphones. This enables us to achieve a previously infeasible capability: backscattering information to cars and smartphones in outdoor environments. 
Our key innovation is a modulation technique that transforms backscatter, which is a multiplication operation on RF signals, into an addition operation on the audio signals output by FM receivers. This enables us to embed both digital data as well as arbitrary audio into ambient analog FM radio signals. We build prototype hardware of our design and successfully embed audio transmissions over ambient FM signals. Further, we achieve data rates of up to 3.2 kbps and ranges of 5-60 feet, while consuming as little as 11.07{\mu}W of power. To demonstrate the potential of our design, we also fabricate our prototype on a cotton t-shirt by machine sewing patterns of a conductive thread to create a smart fabric that can transmit data to a smartphone. We also embed FM antennas into posters and billboards and show that they can communicate with FM receivers in cars and smartphones.

FM Backscatter: Enabling Connected Cities and Smart Fabrics

Apparatuses, systems, ambient RF backscatter transceivers, and methods for communicating using MIMO and spread spectrum coding of backscattered ambient RF signals are described. An example system may include an ambient RF backscatter transceiver that include an antenna configured to receive a backscattered ambient radio frequency (RF) signal, and a receiver coupled to the antenna. The receiver may be configured to demodulate the backscattered ambient RF signal using one of multiple input, multiple output multiplexing demodulation or spread spectrum code demodulation to retrieve the first data. The backscattered ambient RF signal may be generated by backscattering an ambient RF signal at a first frequency. The ambient RF signal may be configured to provide other data at a second frequency.

Apparatuses, systems, and methods for communicating using mimo and spread spectrum coding in backscatter of ambient signals

Recent years have seen interest in device tracking and localization using acoustic signals. State-of-the-art acoustic motion tracking systems however do not achieve millimeter accuracy and require large separation between microphones and speakers, and as a result, do not meet the requirements for many VR/AR applications. Further, tracking multiple concurrent acoustic transmissions from VR devices today requires sacrificing accuracy or frame rate. We present MilliSonic, a novel system that pushes the limits of acoustic based motion tracking. Our core contribution is a novel localization algorithm that can provably achieve sub-millimeter 1D tracking accuracy in the presence of multipath, while using only a single beacon with a small 4-microphone array.Further, MilliSonic enables concurrent tracking of up to four smartphones without reducing frame rate or accuracy. Our evaluation shows that MilliSonic achieves 0.7mm median 1D accuracy and a 2.6mm median 3D accuracy for smartphones, which is 5x more accurate than state-of-the-art systems. MilliSonic enables two previously infeasible interaction applications: a) 3D tracking of VR headsets using the smartphone as a beacon and b) fine-grained 3D tracking for the Google Cardboard VR system using a small microphone array.

Shyamnath Gollakota

Papers

Wind dispersal of battery-free wireless devices

Leveraging Dual-Observable Input for Fine-Grained Thumb Interaction Using Forearm EMG

FM Backscatter: Enabling Connected Cities and Smart Fabrics

Apparatuses, systems, and methods for communicating using mimo and spread spectrum coding in backscatter of ambient signals

MilliSonic: Pushing the Limits of Acoustic Motion Tracking.