Video streaming has traditionally been considered an extremely power-hungry operation. Existing approaches optimize the camera and communication modules individually to minimize their power consumption. However, designing a video streaming device requires power-consuming hardware components and computationally intensive video codec algorithms that interface the camera and the communication modules. For example, monochrome HD video streaming at 60 fps requires an ADC operating at a sampling rate of 55.3 MHz and a video codec that can handle uncompressed data being generated at 442 Mbps. We present a novel architecture that enables HD video streaming from a low-power, wearable camera to a nearby mobile device. To achieve this, we present an “analog” video backscatter technique that feeds analog pixels from the photo-diodes directly to the backscatter hardware, thereby eliminating power-consuming hardware components, such as ADCs and codecs. To evaluate our design, we simulate an ASIC, which achieves 60 fps 720p and 1080p HD video streaming for 321 μW and 806 μW, respectively. This translates to 1000x to 10,000x lower power than it used for existing digital video streaming approaches. Our empirical results also show that we can harvest sufficient energy to enable battery-free 30 fps 1080p video streaming at up to 8 feet. Finally, we design and implement a proof-of-concept prototype with off-theshelf hardware components that successfully backscatters 720p HD video at 10 fps up to 16 feet.

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Towards Battery-Free {HD} Video Streaming

After comparing the properties of analog backscatter and digital backscatter, we propose that a combination of the two can provide a solution for high data rate battery free wireless sensing that is superior to either approach on its own. We present a hybrid analog-digital backscatter platform that uses digital backscatter for addressability and control but switches into analog backscatter mode for high data rate transmission of sensor data. Using hybrid backscatter, we report the first digitally addressable real-time battery free wireless microphone. We develop the hybrid backscatter platform by integrating an electret microphone and RF switch with a digital RFID platform (WISP). The hybrid WISP operates by default in digital mode, transmitting and receiving digital data using the EPC Gen 2 RFID protocol but switching into analog mode to backscatter audio sensor data when activated by Gen 2 READ command. The data is recovered using a USRP-based Software Defined RFID reader. We report an operating range of 7.4 meters for the analog backscatter microphone and 2.7 meters for hybrid microphone with 26.7 dBm output power USRP-based RFID reader.

Hybrid analog-digital backscatter: A new approach for battery-free sensing

An optical nano dipole antenna is analyzed by means of its input impedance as well as the matching properties of the antenna topology and material configuration. A comparison of this classical microwave driving method with plane wave excitation is accomplished, contrasting the resonances in the input impedance and optical cross sections for several setups, and analyzing the spectral response shape. It is found that for all structures analyzed, a simple linear expression can be defined characterizing the relation between total dipole length and resonant wavelength. The fact that this linear relationship remains valid for different excitation models, for most widely used antenna materials (Au, Ag, Cu, and Al) and even in the presence of substrates is important with respect to practical designs. To our knowledge, such an extensive study has not been performed before.

Engineering the Input Impedance of Optical Nano Dipole Antennas: Materials, Geometry and Excitation Effect

A simple methodology is presented for the estimation and compensation of mutual coupling in antenna systems of arbitrary geometries, including antenna arrays in the vicinity of scatterers. The methodology includes both theoretical and experimental schemes, while also, does not resort to assumptions often encountered in previous mutual coupling estimation approaches. In particular, our methodology uses a matrix representation for the antenna system and is based on a new concept, the antenna current Green's function which uses the recently proven inverse reciprocity theorem, or more intuitively, a relation of the transfer function for the transmitting mode of the antenna system to the corresponding transfer function for the receiving mode. Validation is performed by comparing calculated predictions to measurements and simulations of a practical antenna system as well as a two- and four-element array. An algorithm using matrix interpolation, that aims at exploiting the antenna transfer function, is also developed and experimentally demonstrated for practical array calibrations and coupling compensation techniques in array signal processing. The developed procedures are non-specific and can also be applied to a diversity of antenna systems and other complex scattering problems.

A Methodology for Mutual Coupling Estimation and Compensation in Antennas

The antenna array manifold is central to high-resolution direction finding. The analytic manifold is defined here as the mathematical model of the array response calculated by solving the electromagnetic equations for the waves impinging on the array. Array manifolds commonly used in the array signal processing literature are approximations of the analytic manifold. This paper describes the characteristics of the analytic manifold and compares it with commonly used approximate manifolds. The impact of these approximations on the performance of direction-of-arrival estimation in challenging superresolution scenarios is evaluated by selected numerical examples.

Antenna Array Manifolds for High-Resolution Direction Finding

A tutorial discussion on the receiving and scattering properties of antennas is presented. The objective of this work is to provide a comprehensive tutorial overview of the subject, in conjunction with circuit calculations and numerical simulations that illustrate the fundamental concepts of how antennas behave as both receivers and scatterers of electromagnetic fields and power. The paper begins with a discussion of the basic concepts associated with antenna impedance, as these are fundamental to understanding the antenna's receiving and scattering properties, particularly over a wide range of frequencies, where the circuit properties of the antenna differ. The definitions of aperture efficiency, absorption efficiency, and the validity of using either a Thevenin or Norton equivalent circuit to determine both received and total scattered power are discussed. Circuit circulations and numerical simulations, determining both total received and scattered powers, are presented for a number of antennas, including a straight-wide dipole, a circular loop, a reflector-backed dipole, and a Yagi antenna. We show that Thevenin and Norton equivalent circuits have limited validity when used to determine the total power scattered by a basic antenna element such as the dipole and loop. For the general antenna, these equivalent circuit models are not valid for determining the total scattered power.

B.C. Kaanta

Papers

A Tutorial on the Receiving and Scattering Properties of Antennas