Backscatter Modulation Research Articles

A Selective, Tracking, and Power Adaptive Far-Field Wireless Power Transfer System

This paper proposes a selective, tracking, and power adaptive far-field wireless power transfer (WPT) system that may be integrated into passive wireless sensor networks (PWSNs). Both transmitter and receiving nodes are developed with features that allow them to cooperate. The system operates based on a backscattered pilot signal, which is used to control and focus the radiated energy. The transmitter may change between several states by turning on or off sets of antenna elements. Each of these states will transmit and consume a specific amount of power, and they will be selected based on the node’s received signal strength (RSS). The receiving nodes are low complexity and battery-less devices, which use a small portion of the rectified energy to create an RSS-dependent modulation frequency, used to drive a backscatter modulator. Based on the nonlinear response of the rectifying devices, additional hardware was integrated into the nodes to activate/wake up them from specific wireless power signals. A complete system operating at 5.8 GHz for WPT and 3.6 GHz for the pilot signal is reported. It will be shown that effective far-field WPT links can be created with reasonable simplicity.

A 25Mbps, 12.4pJ/b DQPSK Backscatter Data Uplink for the NeuroDisc Brain-Computer Interface.

Wireless brain-computer interfaces (BCIs) are used to study neural activity in freely moving non-human primates (NHPs). However, the high energy consumption of conventional active radios is proving to be an obstacle as research drives for wireless BCIs that can provide continuous high-rate data uplinks for longer durations (i.e. multiple days). We present a differential quadrature phase shift keying (DQPSK) backscatter uplink for the NeuroDisc BCI as an alternative to active radios. The uplink achieves a 25Mbps throughput while operating in the 915MHz industrial, scientific, and medical (ISM) band. The DQPSK backscatter modulator was measured to have an error-vector magnitude (EVM) of 9.7% and a measured power consumption of 309 μW during continuous, full-rate transmissions, yielding an analog communication efficiency of 12.4pJ/bit. The NeuroDisc is capable of recording 16 channels of neural data with 16-bit resolution at up to 20kSps per channel with a measured input-referred noise of 2.35 μV. In previous work, we demonstrated the DQPSK backscatter uplink, but bandwidth constraints in the signal chain limited the uplink rate to 6.25Mbps and the neural sampling rate to 5kSps. This work provides new innovations to increase the bandwidth of the system, including an ultra-high frequency (UHF) antenna design with a -10dB return loss bandwidth of 12.5MHz and a full-duplex receiver with an average self-jammer cancellation of 89dB. We present end-to-end characterization of the NeuroDisc and validate the backscatter uplink using pre-recorded neural data as well as in vivo recordings from a pigtail macaque.

Two-Way Backscatter Communication Tag Using a Reflection Amplifier

We propose a two-way backscatter tag that uses a reflection amplifier. The reflection gain ${S_{11}}$ is used to modulate the backscatter signal while the forward gain ${S_{21}}$ is fixed. The circuit is analyzed and designed to operate at 2.45 GHz with the bandwidth of 120 MHz. Measurements show that we can have ${S_{11}\approx }20$ and −20 dB to implement ON/OFF keying (OOK) backscatter modulation scheme, while ${S_{21}\approx }15$ dB.

Noncoherent Backscatter Communications Over Ambient OFDM Signals

In recent years, ambient backscatter communications have gained a lot of interests as a promising enabling technology for the Internet-of-Things and green communications. In ambient backscatter communication systems, ultra-low power devices are able to transmit information by backscattering ambient radio-frequency signals generated by legacy communication systems such as Wi-Fi and cellular networks. This paper is concerned with ambient backscatter communications over legacy orthogonal frequency division multiplexing (OFDM) signals. We propose a backscatter modulation scheme that allows backscattering devices to take advantage of the spectrum structure of ambient OFDM symbols to transmit information. The proposed modulation scheme allows both binary and higher-order modulation using noncoherent energy detection. We investigate the detector design and analyze the error performance of the proposed scheme. We provide an exact expression for the error probability for the binary case, whereas accurate approximate expressions for the error probability are derived for the $M$ -ary case. We corroborate our analysis using Monte–Carlo simulation and investigate the effects of varying the OFDM symbol size, maximum channel delay spread, and the number of receive antennas on the error performance. Our numerical results show that the proposed technique outperforms other techniques available in this paper for backscatter communication over ambient OFDM signals in different scenarios.

Active Two-Way Backscatter Modulation: An Analytical Study

Backscatter modulation (BM), usually used for radio frequency identifications, has the potential to be exploited in a wider range of applications such as the Internet of Things and wireless sensor networks. In this paper, we leverage the BM by increasing its down-link (from reader to tag) and up-link (from tag to reader) ranges. We propose two active two-way BM tag configurations, named parallel and series. For both configurations, we use active loads in the tag modulators to maximize the BM range and implement the desired backscattered constellation, subject to no data loss in the down-link path. Contrary to most existing BM studies, we use Thevenin/Norton equivalent circuit, only to calculate the received power at tag, while we derive the tag backscattered power using the antenna scatterer theorem. Moreover, we obtain a closed-form expression of the average bit error probability (BEP) at the reader in Rician fading channel environment for both tag configurations. We compare the proposed active BM tags with the conventional passive BM tags. The simulation results prove that for an average BEP equal to $10^{-{4}}$ , an SNR improvement of up to 19 and 24 dB can be achieved for parallel and series configurations, respectively.

Passive RFID Uplink and Downlink Link Budget and Comparison of ASK and BPSK Backscatter Modulations

Complete link budget for forward and backward link in a RFID system are given with constituent parts. Backward link signal levels with current improvements requires separate TX and RX antenna bistatic configuration to overcome reader threshold. To further elaborate on link budgets, reflection coefficient and power equations are obtained from a RFID front-end model with circuit theory and shown on a passive RFID system. It is demonstrated that both forward and backward links have limitations on the range depending upon tag power dissipation, antennas in use and type of modulation. To make two distances equal, parameters are found from two way link equations. Then, ASK and BPSK modulations are compared on communication distance for given average backscatter difference power, minimum SNR to targeted BER on the reader by the equations obtained on the circuit model and MATLAB simulations.

Backscatter Modulation for Wearable Devices: A Backscatter Modulator, Consisting of an Antenna and a 1-MHz Binary Backscatter Modulator Operating at a Carrier Frequency of 915 MHz

The use of wearable devices is increasing as a growing number of applications is introduced. The major limitation of these intelligent and multifunctional devices is the battery-charge capacity, which restricts the applications' scope.

Multi-Dimensional Sparse-Coded Ambient Backscatter Communication for Massive IoT Networks

In this paper, we propose a multi-dimensional sparse-coded ambient backscatter communication (MSC-AmBC) system for long-range and high-rate massive Internet of things (IoT) networks. We utilize the characteristics of the ambient sources employing orthogonal frequency division multiplexing (OFDM) modulation to mitigate strong direct-link interference and improve signal detection of AmBC at the reader. Also, utilization of the sparsity originated from the duty-cycling operation of batteryless RF tags is proposed to increase the dimension of signal space of backscatter signals to achieve either diversity or multiplexing gains in AmBC. We propose optimal constellation mapping and reflection coefficient projection and expansion methods to effectively construct multi-dimensional constellation for high-order backscatter modulation while guaranteeing sufficient energy harvesting opportunities at these tags. Simulation results confirm the feasibility of the long-range and high-rate AmBC in massive IoT networks where a huge number of active ambient sources and passive RF tags coexist.

Design and Evaluation of “BTTN”: A Backscattering Tag-to-Tag Network

Radio frequency (RF)-powered backscatter communication between passive tags holds tremendous potential as an enabling technology for a ubiquitous “Internet of Things.” We develop a backscattering tag-to-tag network (BTTN), comprised of passive tags capable of large-scale, passive, and multihop communication with each other via backscatter modulation of an external RF excitation signal. The low sensitivity and lack of active demodulator on passive tags present significant challenges to the communication, including a unique phase cancellation problem, which significantly affects the range and robustness of a passive tag-to-tag link. We overcome these challenges using innovative tag architecture and also develop a novel multiphase backscatter modulation technique with a learning mechanism that overcomes the phase cancellation problem. This improves the link performance bringing passive tag-to-tag communication closer to practical use. The additional hardware compared to the conventional radio frequency identification tag architecture includes one more terminating impedance in the modulator. The data rate is reduced due to backscatter at two different phases while the increase in the power consumption is negligible. We develop prototype BTTN tag hardware and firmware and evaluate its performance. The prototype achieves link ranges of up to 3 m at 5 kb/s with an excitation power level of only -20 dBm while successfully overcoming phase cancellation. We further extend BTTN operation to a multihop network where we demonstrate a four hop link capable of communicating over 12 m under similar conditions.

Novel Sparse-Coded Ambient Backscatter Communication for Massive IoT Connectivity

Low-power ambient backscatter communication (AmBC) relying on radio-frequency (RF) energy harvesting is an energy-efficient solution for batteryless Internet of Things (IoT). However, ambient backscatter signals are severely faded by dyadic backscatter channel (DBC), limiting connectivity in conventional orthogonal time-division-based AmBC (TD-AmBC). In order to support massive connectivity in AmBC, we propose sparse-coded AmBC (SC-AmBC) based on non-orthogonal signaling. Sparse code utilizes inherent sparsity of AmBC where power supplies of RF tags rely on ambient RF energy harvesting. Consequently, sparse-coded backscatter modulation algorithm (SC-BMA) can enable non-orthogonal multiple access (NOMA) as well as M-ary modulation for concurrent backscatter transmissions, providing additional diversity gain. These sparse codewords from multiple tags can be efficiently detected at access point (AP) using iterative message passing algorithm (MPA). To overcome DBC along with intersymbol interference (ISI), we propose dyadic channel estimation algorithm (D-CEA) and dyadic MPA (D-MPA) exploiting weighted-sum of the ISI for information exchange in the factor graph. Simulation results validate the potential of the SC-AmBC in terms of connectivity, detection performance and sum throughput.

Tunneling RFID Tags for Long-Range and Low-Power Microwave Applications

Backscatter modulation in radio frequency identification (RFID) tags will potentially connect billions of tomorrow's devices to the Internet-of-Things. Current passive RFID systems have power constraints that limit RFID tag communication to short ranges, but these limitations can be overcome by employing reflection amplifiers. In this paper, we show that negative differential resistance devices, such as tunnel diodes, exhibit 27 dB more gain and 10 dB lower power consumption than state-of-the-art reflection amplifiers. Two 5.8 GHz prototypes using off-the-shelf tunnel diodes show reflection gains of 40 dB and 29 dB for a total biasing power consumption of 45 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</sub> W and 39 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</sub> W , respectively, at impinging RF power levels as low as -84 dBm. A 5.8 GHz RFID link of 23 m was achieved when transmitting only -14 dBm of effective isotropic radiated power from a transceiver with a sensitivity of -90 dBm.

Wireless Powered Asynchronous Backscatter Networks With Sporadic Short Packets: Performance Analysis and Optimization

In the fifth generation era, the pervasive applications of Internet of Things and massive machine-type communications have initiated increasing research interests on the backscatter wireless powered communication (B-WPC) technique due to its ultrahigh energy efficiency and low cost. The ubiquitous B-WPC network is characterized by nodes with dynamic spatial positions and sporadic short packets, of which the performance has not been fully investigated. In this paper, we give a comprehensive analysis of a multiantenna B-WPC network with sporadic short packets under a stochastic geometry framework. By exploiting a time-space Poisson point process model, the behavior of the network is well captured in a decentralized and asynchronous transmission way. We then analyze the energy and information outage performance in the energy harvest and backscatter modulation phases of the backscatter network, respectively. The optimal transmission slot length and division are obtained by maximizing the network-wide spatial throughput. Moreover, we find an interesting result that there exists the optimal tradeoff between the durations of the energy harvest and backscatter modulation phases for spatial throughput maximization. Numerical results are demonstrated to verify our analytical findings and show that this tradeoff region gets shrunk when the outage constraints become more stringent.

Mitigation of Phase Cancellation for Efficient Decoding and RF Energy Harvesting in Tag-to-Tag Communications

In this paper, we investigate a phase cancellation problem that occurs in tag-to-tag communication systems. These are the systems wherein several tags communicate with each other by backscattering an external constant wave signal. A transmitting tag modulates baseband information into the reflected signal using backscatter modulation. At the receiving tag, the received signal is the superposition of the transmitted signals by the carrier emitter and the transmitting tag. The resulting signal is demodulated using the envelope detector that causes the phase cancellation problem. This problem exists in all tag-to-tag communication systems and largely impacts on the reliability and communication range. We theoretically analyze and experimentally demonstrate this problem for the tags that use amplitude-shift keying-based backscattering. We propose an algorithm for the mitigation of the phase cancellation based on the phase rotation control. We examine the performance of the proposed algorithm through theoretical analysis and computer simulations. We confirm that the proposed algorithm can significantly mitigate the phase cancellation.

Readout Distance Enhancement of the Passive Wireless Multi-Parameter Sensing System Using a Repeater Coil

A repeater coil is used to extend the detection distance of a passive wireless multi-parameter sensing system. The passive wireless sensing system has the ability of simultaneously monitoring three parameters by using backscatter modulation together with channel multiplexing. Two different repeater coils are designed and fabricated for readout distance enhancement of the sensing system: one is a PCB (printed circuit board) repeater coil, and the other is a copper wire repeater coil. Under the conditions of fixed voltage and adjustable voltage, the maximum readout distance of the sensing system with and without a repeater coil is measured. Experimental results show that larger power supply voltage can help further increase the readout distance. The maximum readout distance of the sensing system with a PCB repeater coil has been extended 2.3 times, and the one with a copper wire repeater coil has been extended 3 times. Theoretical analysis and experimental results both indicate that the high Q factor repeater coil can extend the readout distance more. With the copper wire repeater coil as well as a higher power supply voltage, the passive wireless multi-parameter sensing system finally achieves a maximum readout distance of 13.5 cm.

A K-Band Backscatter Fiducial for Continuous Calibration in Coherent Millimeter-Wave Imaging

We present a modulated ultrawideband backscatter calibration target (fiducial) intended for group delay calibration in large-aperture multitransceiver millimeter-wave imagers. The fiducial is designed to resemble a modulated point scatterer across the K-band (17.5–26.5 GHz). Multiple such fiducials may be used to mitigate thermal and mechanical drift across multiple transceivers comprising the imager. This approach allows tracking and removing both time-varying amplitude and phase drift in the RF hardware and associated cables. Backscatter modulation of the fiducial allows the system to separate the fiducial from the imaged scene and clutter in the environment. We show that the −10 dB beamwidth of the proposed fiducial is approximately 84° along the azimuth plane and 60° along the elevation plane. A proof of concept group delay calibration experiment is presented for a K-band laboratory setup, where a single fiducial and a metal plate target are placed in a scene together. After the backscatter-based calibration, the measured range error of the metal plate at a two-way slant distance of 70.54 cm is reduced to only 1.06 mm (0.15% position error).

Ubiquitous Flying Sensor Antennas: Radiofrequency Identification Meets Micro Drones

Tag-copter is an architecture of dynamic and self-relocating sensor devices originating from the synergy between sensor-oriented radiofrequency identification (RFID) and the emerging micro air vehicles technology. Tag-copters integrate a low-cost nano multi-copter with a miniaturized RFID data-logger antenna for the ubiquitous and cooperative monitoring of indoor volumes as well as of harsh environments. This concept is preliminary exploited by means of a theoretical formulation to derive the upper-bound performance of a flying “winged sensor.” The analysis addresses the sensing accuracy versus the flight speed and the reliability of the on-the-flight data exchange with a fixed base-station through backscattering modulation. A first working 16 g prototype, capable of temperature measurement and storage, is experimented in a controlled setup to corroborate theoretical and experimental findings. The same device is then applied in realistic conditions concerning the temperature mapping within indoor environments. Preliminary results demonstrate that the maximum temperature measurement error is of the order of 1 °C along flight trajectories of 30 m. Finally, most of the data stored within the datalogger memory can be correctly uploaded by a flying tag-copter toward a fixed reader provided that the copter flies at approximatively 1 – 1.5 m from it.

Ultrafast Backscatter Modulator With Low-Power Consumption and Wireless Power Transmission Capabilities

The rapid increase in the progress and development of wireless communications and identification has led to the increase of billions of devices applied in the Internet of Things (IoT) context. Low-power wireless sensors, passive-backscatter radios such as passive radio frequency identifications (RFIDs), RFID-enabled sensors, and passive wireless sensors are seen as key-enabling technologies to realize the future ubiquitous IoT. This letter presents a system composed of a backscatter modulator at 2.45 GHz that has a consumption of 61.5 fJ/b with wireless power transfer capabilities at 1.7 GHz. This 16-QAM modulator is shown to provide 960 Mb/s of data rate with a error vector magnitude of 8.37%. Moreover, the system is also composed of a rectifier with 56% of efficiency with an input power of 5 dBm at 1.7 GHz with unbiased gate at each transistor. The demonstrated system enables low power consumption while maintaining a high modulation bandwidth.

Ultrasonic thermal dust: A method to monitor deep tissue temperature profiles.

We demonstrate a tetherless, sub-millimeter implantable temperature sensing system employing ultrasonic powering and ultrasonic backscatter modulation assembled using commercially available components. We demonstrate two sizes of sensors based on available components with volumes of 1.45 mm3 and 0.118 mm3. Individual sensors are able to resolve ±0.5°C changes in temperature, suitable for medical diagnostic and monitoring purposes. We verify less than 0.3°C drift in temperature readings over 14 days in physiological conditions. This approach is compatible with more sophisticated temperature sensors such as classic proportional to absolute temperature (PTAT) integrated circuits, as well as digital backscatter approaches. Our goal is to solve a long-standing issue: chronic and tetherlessly monitoring deep tissue temperature with high spatial accuracy.

Modulation Model of High Frequency Band Radar Backscatter by the Internal Wave Based on the Third-Order Statistics

Modulation model of radar backscatters is an important topic in the remote sensing of oceanic internal wave by synthetic aperture radar (SAR). Previous studies related with the modulation models were analyzed mainly based on the hypothesis that ocean surface waves are Gaussian distributed. However, this is not always true for the complicated ocean environment. Research has showed that the measurements are usually larger than the values predicted by modulation models for the high frequency radars (X-band and above). In this paper, a new modulation model was proposed which takes the third-order statistics of the ocean surface into account. It takes the situation into consideration that the surface waves are Non-Gaussian distributed under some conditions. The model can explain the discrepancy between the measurements and the values calculated by the traditional models in theory. Furthermore, it can accurately predict the modulation for the higher frequency band. The model was verified by the experimental measurements recorded in a wind wave tank. Further discussion was made about applicability of this model that it performs better in the prediction of radar backscatter modulation compared with the traditional modulation model for the high frequency band radar or under lager wind speeds.

ReflectFX: In-Band Full-Duplex Wireless Communication by Means of Reflected Power

In this paper, we introduce a new full-duplex wireless communication system, named ReflectFX, that relies on backscatter modulation. This paper offers a new concept for two-way wireless communication: rather than avoiding self-interference as in half-duplex, or combatting self-interference as in conventional full-duplex, nodes will re-use the received interfering radio-carrier waves to transfer information. The electromagnetic waves are modulated and reflected by the same antenna that receives them. We consider two nodes, a base-station and an end-user, that wish to exchange data over a wireless Rayleigh fading channel with additive white Gaussian noise. With ReflectFX, the end-user receives a self-interference free signal. We improve the transmission range of ReflectFX by adding negative resistance to the end-user load. We derive an expression for the overall achievable throughput and ergodic capacity of ReflectFX. Simulation results show that ReflectFX outperforms both conventional full-duplex and half-duplex.