John Kimionis

Increased Range Bistatic Scatter Radio

Scatter radio achieves communication by reflection and requires low-cost and low-power RF front-ends. However, its use in wireless sensor networks (WSNs) is limited, since commercial scatter radio (e.g. RFID) offers short ranges of a few tens of meters. This work redesigns scatter radio systems and maximizes range through non-classic bistatic architectures: the carrier emitter is detached from the reader. It is shown that conventional radio receivers may show a potential 3dB performance loss, since they do not exploit the correct signal model for scatter radio links. Receivers for on-off-keying (OOK) and frequency-shift keying (FSK) that overcome the frequency offset between the carrier emitter and the reader are presented. Additionally, non-coherent designs are also offered. This work emphasizes that sensor tag design should accompany receiver design. Impact of important parameters such as the antenna structural mode are presented through bit error rate (BER) results. Experimental measurements corroborate the long-range ability of bistatic radio; ranges of up to 130 meters with 20 milliwatts of carrier power are experimentally demonstrated, with commodity software radio and no directional antennas. Therefore, bistatic scatter radio may be viewed as a key enabling technology for large-scale, low-cost and low-power WSNs.

Wireless Environmental Sensor Networking With Analog Scatter Radio and Timer Principles

Environmental variables monitoring with wireless sensor networks (WSNs) is invaluable for precision agriculture applications. However, the effectiveness of existing low-power, conventional (e.g., ZigBee-type) radios in large-scale deployments is limited by power consumption, cost, and complexity constraints, while the existing WSN solutions employing nonconventional, scatter-radio principles have been restricted to communication ranges of up to a few meters. In this paper, the development of a novel analog scatter-radio WSN is presented, that employs semipassive sensor/tags in bistatic topology (i.e., carrier emitter placed in a different location from the reader), consuming <;1 mW of power, with communication range exceeding 100 m. The experimental results indicate that the multipoint surface fitting calibration, in conjunction with the employed two-phase filtering process, both provide a mean absolute error of 1.9% environmental relative humidity for a temperature range of 10 °C-50 °C. In addition, the energy consumption per measurement of the proposed environmental monitoring approach can be lower than that of conventional radio WSNs. Finally, the proposed approach operational characteristics are presented through a real-world network deployment in a tomato greenhouse.

Single-Antenna Coherent Detection of Collided FM0 RFID Signals

This work derives and evaluates single-antenna detection schemes for collided radio frequency identification (RFID) signals, i.e. simultaneous transmission of two RFID tags, following FM0 (biphase-space) encoding. In sharp contrast to prior art, the proposed detection algorithms take explicitly into account the FM0 encoding characteristics, including its inherent memory. The detection algorithms are derived when error at either or only one out of two tags is considered. It is shown that careful design of one-bit-memory two-tag detection can improve bit-error-rate (BER) performance by 3dB, compared to its memoryless counterpart, on par with existing art for single-tag detection. Furthermore, this work calculates the total tag population inventory delay, i.e. how much time is saved when two-tag detection is utilized, as opposed to conventional, single-tag methods. It is found that two-tag detection could lead to significant inventory time reduction (in some cases on the order of 40%) for basic framed-Aloha access schemes. Analytic calculation of inventory time is confirmed by simulation. This work could augment detection software of existing commercial RFID readers, including single-antenna portable versions, without major modification of their RF front ends.

Bistatic backscatter radio for power-limited sensor networks

For applications that require large numbers of wireless sensors spread in a field, backscatter radio can be utilized to minimize the monetary and energy cost of each sensor. Commercial backscatter systems such as those in radio frequency identification (RFID), utilize modulation designed for the bandwidth limited regime, and require medium access control (MAC) protocols for multiple access. High tag/sensor bitrate and monostatic reader architectures result in communication range reduction. In sharp contrast, sensing applications typically require the opposite: extended communication ranges that could be achieved with bitrate reduction and bistatic reader architectures. This work presents non-coherent frequency shift keying (FSK) for bistatic backscatter radio; FSK is appropriate for the power limited regime and also allows many RF tags/sensors to convey information to a central reader simultaneously with simple frequency division multiplexing (FDM). However, classic non-coherent FSK receivers are not directly applicable in bistatic backscatter radio. This work a) carefully derives the complete signal model for bistatic backscatter radio, b) describes the details of backscatter modulation with emphasis on FSK and its corresponding receiver, c) proposes techniques to overcome the difficulties introduced by the utilization of bistatic architectures, such as the carrier frequency offset (CFO), and d) presents bit error rate (BER) performance for the proposed receiver and carrier recovery techniques.

Bistatic backscatter radio for tag read-range extension

This work examines the idea of dislocating the carrier transmission from the tag-modulated carrier reception, i.e. bi-static rather than mono-static backscatter radio. In that way, more than one carrier transmitters can be distributed in a given geographical area and illuminate a set of RF tags/sensors that modulate and scatter the received carrier towards a single software-defined receiver. The increased number of carrier transmitters and their distributed nature assists tags to be potentially located closer to one carrier transmitter and thus, improves the power of the scattered signals towards the receiver. Specifically, this work a) carefully derives near-optimal detectors for bi-static backscatter radio and on/off keying (OOK) tag modulation (which is widely used in commercial tags), b) analytically calculates their bit error rate (BER) performance, and c) experimentally tests them in practice with a custom bistatic backscatter radio link. As a collateral dividend, it is shown that the non-linear processing of the proposed receivers requires certain attention on the utilized tag design principles, commonly overlooked in the literature, validating recently reported theoretical results on the microwave domain.

Backscatter sensor network for extended ranges and low cost with frequency modulators: Application on wireless humidity sensing

Dense monitoring of environmental parameters (e.g. air/soil humidity, ambient temperature) is critical in precision agriculture, urban area monitoring and environmental modeling applications. In this paper, the design of a novel wireless sensor network (WSN) is proposed, consisting of low-power and low-cost sensor nodes, deployed in a bistatic architecture (i.e. carrier emitter in a different location than the receiver) and achieving long-range backscatter communication. The tags modulate sensor information using analog frequency modulation (FM) and frequency division multiple access (FDMA) at the subcarrier frequency, even though a single carrier is assumed. In sharp contrast to prior art, the developed backscatter sensor network performs environmental monitoring over a relatively wide area. A proof-of-concept prototype WSN application has been developed for capacitive relative humidity (RH) sensing, with 1.5 mW per tag, 0.9 RMSE and range on the order of 50 m.

Inventory time reduction in Gen2 with single-antenna separation of FM0 RFID signals

In this work single-antenna detection schemes for collided radio frequency identification (RFID) signals are proposed to achieve waveform separation and extract information when two RFID tags simultaneously transmit. Industry standard Generation 2 (Gen2) tags with FM0 (biphase-space) encoding are fully exploited, including FM0 inherent memory, in sharp contrast to tag separation prior art. It is shown that memory-assisted detection for two collided FM0 tags performs 3dB better in terms of bit-error-rate (BER), compared to memoryless detection. Information from waveform separation is used to significantly reduce the total inventory time for a given tag population in framed Aloha schemes, including the widely adopted Gen2. No modification of the existing readers RF front end is required, as the proposed techniques could operate with single-antenna readers. This might be important for portable UHF Gen2 RFID systems, where multiple-antennas are not always an option.

A Remotely Programmable Modular Testbed for Backscatter Sensor Network Research

The necessity of backscatter sensor networks (BSNs) has recently emerged due to the need for large-scale, ultra low-cost, ultra low-power, wireless sensing. Development of such networks requires tools for rapid prototyping and evaluation of key-enabling BSN technologies. Although tools for testing wireless sensor networks (WSNs) have been widely developed over the last few years in the form of testbeds, almost no significant testbed examples exist for BSNs. Throughout this work, a set of hardware, firmware and software components have been designed and implemented, creating a BSN research testbed. The latter employs a modular architecture and enables rapid prototyping of critical components for low-cost, large-scale BSNs. Testbed components enable microwave, detection, coding and multiple access research, tailored for backscatter radio and networking. The testbed offers dynamic reconfiguration through implementation of remote, over the air programming (OTAP), that reduced programming time per node by two orders of magnitude. An overview of the testbed is given, and its modular tools are described in terms of functionality and importance for BSN research.

Building a Low-Cost Digital Garden as a Telecom Lab Exercise

In an interdisciplinary, semester-long class, undergraduate students learn how to build a low-cost, multihop wireless sensor network from first principles for a digital garden. This type of course better prepares electrical engineering graduates for the sensor-rich, pervasive computing era.

A fading-resistant method for RFID-antenna structural mode measurement

In this paper, a prototype device to measure the tag-antennas structural mode term is proposed. The device comprises a switching transistor, controlled by a microcontroller, and a reverse-biased varactor diode connected to the transistor. The antennas load is selectively modified by applying the appropriate DC voltage on the varactor. Antennas structural mode is evaluated by measuring the backscattered modulated field for three states of the varactor. In contrast to prior art: i) the proposed device can be conveniently used to measure the structural mode of any antenna, via an RF connector, without the need to successively attach different loads to the antenna under test, ii) measurements can be performed under fading/changing conditions, as fading-effects are removed, as demonstrated herein. The device can be used to evaluate backscatter communications for different tag-loading. Furthermore, it could operate as an RF sensor, where the sensors voltage “drives” the varactors capacitance.