Αναμετάδοση ραδιοκυμάτων με τεχνικές οπισθοσκέδασης και εφαρμογές

Billions of devices are expected to be wirelessly connected in the foreseeable future. Sustaining such connectivity will require simple, elegant, engineering solutions. Based on backscatter radio principles, this dissertation offers novel, ultra-low-complexity, ultra-low-power, ultra-low-cost solutions, for connectivity in the field of narrowband wireless communications.Methods for achieving wireless communication by “recycling” radio waves, pre-existing in the environment, are offered and analysed. In contrast to generating own signals for transmission, a device (tag) adopting the suggested methods can transmit its information towards any conventional FM radio receiver, by recycling ambient signals from FM radio stations. That way, it is shown that batteryless information transmission can be achieved up to distances of 26 meters, by harvesting energy from the environment. Ultra-low cost prototypes consumed only 24 μWatt in continuous (non duty-cycled) operation.In the case of ambient signals of unknown origin and structure, digital modulation schemes are also offered, accompanied by novel coherent, partially-coherent illumination-agnostic, as well as fully noncoherent, channel-coded or uncoded detection algorithms. It is shown that under certain conditions, modulated and unknown ambient signals, can offer performance gains in the process of recovering tag's information signal, i.e., modulation of the ambient signal may be helpful. The proposed schemes do not require any cooperation with the ambient transmitter. That way, in sharp contrast to prior art, the tags adopting the proposed techniques are receiver-less; ultra-low-complexity and batteryless operation are also facilitated.Exploiting the aforementioned methods, originally intended for solving ambient backscatter communication problems, it is demonstrated for the first time that backscattering tags can also be used in an unorthodox way, for relaying signals in the frequency domain. Scatter radio relaying can be used to solve multi-antenna processing problems: a) blind (i.e., zero-feedback) beamforming, offering power gains in the order of 0.4-3.7 dB, and b) direction of arrival (DoA) estimation, offering error less than 5 degrees with 8 scattering tags. The proposed methods utilize single antenna radios without any requirement for channel feedback, multi-antenna RF front-ends or expensive controllers.The proposed techniques extend the solution space for engineers building wireless devices under power, complexity and cost constraints. Hopefully, this work will amplify the recent interest of the research community on backscatter radio and accelerate efforts towards the wide adoption of backscatter radio relaying in current and future commodity wireless systems, sensors and networks

Practical Energy Harvesting for Batteryless Ambient Backscatter Sensors

This work studies the performance of two methods for providing power to an ultra-low power, ambient backscatter tag, omitting the need for any battery. RF energy harvesting from a dedicated source and energy harvesting from ambient light using a single photodiode are compared. Extensive measurement results from tests conducted under real world conditions are offered for both harvesting methods. It is concluded that for a total cost of under 7 Euros the need for a battery can be eliminated, by using a single photodiode element along with a suitable boost converter. The ultra-low power character of the utilized tag enables the use of multiple harvesting methods and paves the way towards truly battery-less wireless sensor systems

Ζεύξεις οπισθοσκέδασης διευρυμένης εμβέλειας με ενσωματωμένα ραδιόφωνα

Summarization: This work studies whether increased ranges of bistatic scatter radio communication are possible, especially when low-cost (in the order of 4 Euro), embedded receivers, originally designed for conventional radio (and not for scatter radio) are employed. It is found that for 13 dBm emitter transmission power, 246 meters scatter radio tag-to-reader distance is possible, with packet error rate (PER) less than 1%, while 268 meters are possible at the expense of increased PER, in the order of 10%. A prototype node employing the communication scheme suggested and powered by a supercapacitor along with a solar panel is also built. Additionally an experimental wireless communication link, utilizing the harmonics of a microcontroller’s clock signal as the carrier is presented

Scatter radio relaying & applications

Summarization: Billions of devices are expected to be wirelessly connected in the foreseeable future. Sustaining such connectivity will require simple, elegant, engineering solutions. Based on backscatter radio principles, this dissertation offers novel, ultra-low-complexity, ultra-low-power, ultra-low-cost solutions, for connectivity in the field of narrowband wireless communications. Methods for achieving wireless communication by “recycling” radio waves, pre-existing in the environment, are offered and analysed. In contrast to generating own signals for transmission, a device (tag) adopting the suggested methods can transmit its information towards any conventional FM radio receiver, by recycling ambient signals from FM radio stations. That way, it is shown that batteryless information transmission can be achieved up to distances of 26 meters, by harvesting energy from the environment. Ultra-low cost prototypes consumed only 24 μWatt in continuous (non duty-cycled) operation. In the case of ambient signals of unknown origin and structure, digital modulation schemes are also offered, accompanied by novel coherent, partially-coherent illumination-agnostic, as well as fully noncoherent, channel-coded or uncoded detection algorithms. It is shown that under certain conditions, modulated and unknown ambient signals, can offer performance gains in the process of recovering tag's information signal, i.e., modulation of the ambient signal may be helpful. The proposed schemes do not require any cooperation with the ambient transmitter. That way, in sharp contrast to prior art, the tags adopting the proposed techniques are receiver-less; ultra-low-complexity and batteryless operation are also facilitated. Exploiting the aforementioned methods, originally intended for solving ambient backscatter communication problems, it is demonstrated for the first time that backscattering tags can also be used in an unorthodox way, for relaying signals in the frequency domain. Scatter radio relaying can be used to solve multi-antenna processing problems: a) blind (i.e., zero-feedback) beamforming, offering power gains in the order of 0.4-3.7 dB, and b) direction of arrival (DoA) estimation, offering error less than 5 degrees with 8 scattering tags. The proposed methods utilize single antenna radios without any requirement for channel feedback, multi-antenna RF front-ends or expensive controllers. The proposed techniques extend the solution space for engineers building wireless devices under power, complexity and cost constraints. Hopefully, this work will amplify the recent interest of the research community on backscatter radio and accelerate efforts towards the wide adoption of backscatter radio relaying in current and future commodity wireless systems, sensors and networks

DoA estimation of a hidden RF source exploiting simple backscatter radio tags

Summarization: Conventional direction of arrival (DoA) techniques employ multi-antenna receivers with increased complexity and cost. This work emulates a multi-antenna system using a singleantenna receiver and exploiting the beauty and simplicity of backscatter radio. More specifically, a number of simple backscatter radio tags offer copies of the hidden RF source, relayed in space and shifted in frequency, while requiring minimal time-synchronisation. DoA of a hidden RF source was estimated with an error of less than 5 degrees, exploiting a small number of simple, ultra-low-cost backscattering tags.Παρουσιάστηκε στο: 2021 IEEE International Conference on Acoustics, Speech and Signal Processin

Ambient backscatter in reality: does illuminator signal structure matter?

Summarization: Motivated by the extensive use of simplified models in the recent ambient backscatter literature, this work will demonstrate the importance of utilizing realistic models when deriving appropriate detectors. For this purpose, two recent works on ambient backscatter will be evaluated under realistic channel models, accounting for all communication parameters. It is shown that assuming a complex normal illuminator (i.e., ignoring illuminator's signal structure) leads to significant performance losses compared to explicitly considering illuminator's modulation (e.g., FM). Based on FM illumination, the importance of the latter is further highlighted by deriving a high performance, fully noncoherent sequence detector. In most cases, switching techniques (e.g., SBPSK) are shown to outperform conventional techniques (OOK), at the expense of increased complexity.Presented on

Switching frequency techniques for universal ambient backscatter networking

Summarization: This work offers both analog and digital tag modulation schemes and respective receiver designs, for ultra-low power, high performance, ambient backscatter communications. All proposed techniques are based on simple, but careful, switching frequency control at the tag, allowing for the easy frequency-domain multiple access. First, a digital modulation scheme is offered, namely pseudo-frequency shift keying, assuming illumination from constant envelope-modulated signals and a fully coherent detector is derived along with closed-form probability of error. A second digital modulation scheme is also offered, based on a frequency-shifted form of binary phase shift keying (S-BPSK), relaxing the constant-envelope requirement for the illuminator and an illumination-agnostic detector is derived. Based on S-BPSK, short packet error correction coding is utilized for ambient backscatter communication, for the first time in the literature. It is shown that the proposed coded scheme under modulated ambient signal illumination & wireless channel variation, offers tremendous performance gains, i.e., modulation of the ambient signal is helpful. Finally, a third, purely analog, modulation scheme is analyzed, based on FM remodulation principles. A low-cost tag is implemented, demonstrating tag-toreceiver ranges up to 26 meters outdoors, power consumption of 24 μWatts in continuous operation, able to be interrogated by any conventional frequency modulation (FM) receiver. The proposed techniques cover a large variety of omnipresent wireless industry systems, enabling universal ambient backscatter and relevant wireless information and power transfer (WIPT) applications.Παρουσιάστηκε στο: IEEE Journal on Selected Areas in Communication

DoA estimation with a single antenna and a few low-cost backscattering tags

Summarization: Backscatter radio utilizes the reflection that an electromagnetic wave undergoes when it impinges an unmatched-to-a-load antenna, in order to achieve ultra-low-power communication. This work exploits principles commonly found in the ultra-low-power, backscatter communication literature, offering multi-element array functionalities to a single-antenna receiver. A small number of simple, switching backscattering tags deployed in space, emulate a distributed, multi-element antenna array by copying a transmitter’s signal to distinct frequency bands. A receiver can then obtain independent observations of the same signal by discriminating said bands. Using the backscatter tag-based array, the direction of arrival (DoA) estimation problem is addressed without the cost of multiple RF front ends or hardware modifications, at either end of a wireless communication link. The feasibility of the idea was examined through both simulations and experimental deployments. An absolute error of ≈20 degrees was observed when utilizing 5 custom-built backscattering tags, while simulations showed that more than 10 tags can offer error of less than 5 degrees.Presented on: IEEE Transactions on Communication