Ambient Backscatter Research Articles

Toward Hybrid Backscatter-Aided Wireless-Powered Internet of Things Networks: Cooperation and Coexistence Scenarios

The emerging hybrid backscatter and energy harvesting devices have been regarded as a promising scheme for green Internet-of-Things (IoT). In the interwoven primary and secondary wireless powered IoT networks, we develop a novel hybrid scheme that integrates the backscatter communication and the harvest-then-transmit (HTT) protocol. In order to mitigate the adverse effect on the primary user (PU), the secondary users (SUs) are classified into ‘Cooperation Scenario’ and ‘Coexistence Scenario’ based on their different levels of interference to the PU. In ‘Cooperation Scenario’, we propose a cooperation protocol where the SUs operate in the backscatter mode so as to relay information to the PU. Therefore, the SUs are rewarded for harvesting energy and obtaining the spectrum access time from the primary system. Also, in ‘Coexistence Scenario’, a coexistence strategy is developed to enable the SUs to operate in either ambient backscatter or energy harvesting mode during the channel busy time. When the primary channel becomes idle, the SUs are capable of active transmission by using the harvested energy. For each scenario, we investigate the sum-throughput maximization problem of the secondary system. Employing the Lambert W function and the block coordinate descent method, the optimal time allocation can be obtained via the Lagrangian dual method. Numerical results validate that the proposed hybrid backscatter and HTT scheme improves the performance of the secondary system evidently compared with benchmark methods.

Physical Layer Security of Two-Way Ambient Backscatter Communication Systems

Achieving high network traffic demand in limited spectrum resources is a technical challenge for the beyond 5G and 6G communication systems. To this end, ambient backscatter communication (AmBC) is proposed for Internet-of-Things (IoT), because the backscatter device (BD) can realize communication without occupying extra spectrum resources. Moreover, the spectral efficiency can be further improved by using two-way (TW) communication. However, secure communication is a great challenge for accessing massive IoT devices due to the broadcasting nature of wireless propagation environments. In light of this fact, this article proposed a two-way ambient backscatter communication (TW-AmBC) network with an eavesdropper. Specifically, the physical layer security (PLS) is studied through deriving the analytical/asymptotic expressions of the outage probability (OP) and intercept probability (IP). Moreover, the outage probabilities (OPs) in high signal-to-noise ratio (SNR) regions are studied for the asymptotic behavior and the intercept probabilities (IPs) in high main-to-eavesdropping ratio (MER) regions as well. Through analysis and evaluation of simulation performance, the results show that (i) when considering the target node, the OP of the BD decreases with increasing SNR, that is, enhancing the reliability; (ii) an optimal value of BD’s reflection coefficient that maximize the reliability of backscatter link can be obtained; (iii) in high SNR regions, the OPs approach a constant; thus, the diversity orders are zero; (iv) when increasing the MER, the IP of target node decreases, suggesting the security enhances; (v) a trade-off exists between reliability and security which can be optimized by carefully designing the parameters.

Performance enhancement for differential energy signal detection of ambient backscatter communications

AbstractAmbient backscatter communications are a critical enabling technology of radio frequency identification in Internet of Things due to its full usage of ambient signals with low cost to transmit tag signals. However, the signal detection of tag signals is a challenging task as tag signals are usually superposed on the ambient radio frequency signals and the channel state information is unavailable due to its huge and unacceptable computation complexity at tags. To solve this, we in this article proposed an enhanced signal detection scheme for the differential energy detection in ambient backscatter communications. We discovered that the single detection threshold in classic detection will not provide desirable results in smart storage scenarios. We proposed a double‐threshold detection method which subtly integrates differential energy detection with maximum likelihood method to improve the detection accuracy while reducing the complexity of the reader. In addition, we derived the closed‐form expressions for bit error rate (BER), miss detection, and false alarm probabilities of the data bits sent by the tag. Simulation results are presented to show that our proposed detector can offer lower BER and computational complexity compared with single threshold detection schemes in low‐power and low‐storage scenarios, especially when the system is in a poor channel status environment.

Exploiting impact of imperfect CSI on Performance of Backscatter-Aided NOMA System

In this paper, we consider the impact of imperfect channel state information (CSI) on outage probability of users in an ambient backscatter (AB) enabled nonorthogonal multiple access (NOMA) network. In our proposed scheme, a source node communicates with two destinations via a radio frequency-powered device. Such device is assume with ability of backscattering the signals from the base station. We derive expressions to show the outage probability of main nodes in such system. The analytical expressions for the outage probability are checked with Monte-Carlo simulations. It has been shown that the analytical results match the simulation results. In particular, the user with higher allocated power exhibits its performance better than another 10%-15% when signal to noise ratio (SNR) at the source node is 30 dB.

Exploiting Intelligent Reflecting Surface for Energy Efficiency in Ambient Backscatter Communication-Enabled NOMA Networks

The growing demanding data rate requirements of future wireless networks have raised enormous concerns on energy consumption. Researchers conduct their study of green communication in terms of wireless communication environment and low power consumption communication technique. Recently, intelligent reflecting surface (IRS) and ambient backscatter communication (ABC) have been proposed, respectively, and they have attracted growing attention. In this paper, we study the ABC-enabled non-orthogonal multiple access (NOMA) networks. In such networks, environment obstacles may block the communication from the base station to the cell-edge user. To overcome this drawback, we propose an IRS-assisted relay transmission scheme. Therefore, in the ABC-enabled IRS-NOMA networks, there is a problem of how to enhance the network performance by optimizing the phase-shifts as well as the power resource allocation. We formulate an optimization problem to maximize the energy efficiency by jointly optimizing the phase-shifts of IRS’s reflecting elements and the power allocation of NOMA users. To deal with this problem, it is divided into two subproblems: 1) phase-shifts optimization and 2) power allocation optimization. The Dinkelbach-based iterative algorithm is applied to obtain maximum energy efficiency. Simulation results show that our proposed scheme always performs the best in terms of energy efficiency and throughput.

Covert Communication in Ambient Backscatter Systems With Uncontrollable RF Source

In this work, we study the covert communications in ambient backscatter systems (ABS) with uncontrollable RF source on AWGN channels. In contrast to the prior works impractically assuming the existence of a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">controllable</i> RF excitation source, our work arms the receiver of covert information with the full-duplex ability. The covert receiver can emit artificial noise (AN) with variable power to cover up the modulation action of a tag on the excitation signals while receiving the backscattered information. Specifically, we first derive the warden’s optimum power-detection threshold that minimizes the detection error probability. To against the optimal warden, we design the covert backscatter communication policy that determines the feasible region of the AN power at the covert receiver depending on the transmission power of the RF source and the reflection coefficient of the tag and guarantees the covertness constraint. We analyze the performance of the policy and provide the closed-form maximum covert rate and maximum detection error probability at the warden, revealing their tradeoff. The numerical analysis shows the increasing transmission power of the RF source and tag’s reflection coefficient would degrade the covertness of ABS when the covert receiver has to increase the AN power.

Time Allocation and Mode Selection for Secure Communications in Internet of Things

A joint optimization scheme of both time allocation and mode selection is proposed for defending smart jammer in the Internet of Things (IoT). Specifically, we employ multiple modes to deal with the potential eavesdropping and jamming attacks in wireless sensor networks (WSNs), respectively, as wireless-powered relaying (WPR), ambient backscatter relaying (ABR), friendly jamming (FJ) modes. Assuming a specific frame structure, time allocation between energy harvesting and transmission duration is first optimized for three modes based on transmission and security data maximization. In addition, considering that a single mode may fail to guarantee the performance demand due to the uncertainty of network conditions and adversary attacks, mode selection is carried out for adapting characteristics of different modes. Moreover, according to the available knowledge of jamming detection, for enhancing the robustness in secure transmission, the process of mode selection is further divided into two phases. Specifically, game theory is also utilized to achieve the Nash equilibrium for mode selection. Finally, numerical results verify the effects and exhibit the advantages of the proposed scheme.

Realizing Ambient Backscatter Communications with Intelligent Surfaces in 6G Wireless Systems

This article studies the prospects of combining two promising wireless technologies for 6G wireless systems, namely ambient backscatter communications and Programmable Wireless Environments (PWEs). Ambient backscattering carries the promise of battery-less and perpetually operating IoT devices that “surf” ambient electromagnetic waves rather than generating new ones. The technology, however, suffers from unstable performance owing to the natural incoherence of ambient waves. On the other hand, PWEs constitute a system to control intelligent surfaces in order to convert a wireless channel into a software-defined and slice-able resource. This article proposes the use of PWEs to stabilize ambient backscattering communication links. It is shown that it is possible to “outsource” functionalities such as medium access control from the device hardware to the intelligent wireless environment, thereby simplifying the backscattering device hardware. Further, a future research direction is presented where intelligent environments can be built via mass deployments of miniaturized ambient backscattering cells.

Sensitivity and distance based performance analysis for batteryless tags with transmit beamforming and ambient backscattering

Ambient backscatter is a new green technology for Internet of Things (IoT) that utilizes surrounding wireless signals to enable batteryless devices to communicate with other devices. The battery-free devices first harvest energy from ambient wireless signals and then backscatter the signals for communications. Clearly, sensitivity and distance are two important parameters for system performance. However, most existing studies on ambient backscatter communication systems do not consider the impact of the sensitivity of the energy-harvesting nodes and the distances between these devices. In this paper, we first provide a literature review for ambient communication technology and then take sensitivity and distance as two key parameters and investigate the sensitivity and distance based performance for ambient backscatter communication systems. Specifically, we establish the mathematical model based on distances between transceivers and backscattering nodes, extract a parameter that can differentiate the direct path and the backscattering path, evaluate the effects of transmit beamforming, design an energy detector for the reader, and analyze the outage probability of energy harvesting at the tag and the bit error rate (BER) at the reader. Simulations are then provided to corroborate the proposed studies.

A Prototype of Reconfigurable Intelligent Surface with Continuous Control of the Reflection Phase

With the development of the next generation of mobile networks, new research challenges have emerged, and new technologies have been proposed to face them. On one hand, the reconfigurable intelligent surface (RIS) technology is being investigated for partially controlling the wireless channels. RIS is a promising technology for improving the signal quality by controlling the propagation of the electromagnetic waves in a nearly passive manner. On the other hand, ambient backscatter communications (AmBC) is another promising technology that is tailored to address the energy efficiency requirements of the Internet of Things. AmBC enables low-power communications by backscattering the ambient signals and thus reusing existing electromagnetic waves for communications. RIS can be utilized in the context of AmBC for improving system performance. In this article, we report a prototype of an RIS that offers the capability of controlling the phase shifts applied to the incident waves in a continuous manner, and we characterize its properties with the aid of full-wave simulations and through experimental measurements. Specifically, we introduce a phase shift model for predicting the signal reflected by the designed RIS prototype. We apply the proposed model for optimizing an RIS-assisted AmBC system, and we demonstrate that the use of an RIS can significantly improve system performance.

On the Limits of Covert Ambient Backscatter Communications

This letter investigates the limit covert rate of ambient backscatter communications over additive white Gaussian noise channel, where Alice expects to reliably transmit information to Bob via reflecting the incident signals from an ambient radio frequency (RF) source without being detected by Willie. The RF source is not dedicatedly deployed for covert communication and has its own data stream to send. Our analyses show that the limit covert rate is subject to the well-known square root law (i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {O}(\sqrt {n})$ </tex-math></inline-formula> bits in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${n}$ </tex-math></inline-formula> channel uses). Particularly, this limit can be achieved even if Willie can recover the information from the RF source without error, and two properly chosen reflection coefficients are sufficient for achieving this limit.

Multi-Tag Selection in Cognitive Ambient Backscatter Communications for Next-Generation IoT Networks

In this paper, we propose multi-tag selection schemes to improve achievable rate of a secondary ambient backscatter communication (ABC) system with multiple tags and a single power beacon in cognitive radio (CR) environments. Using the secondary power beacon, which plays a role as radio frequency (RF) source when there is no ambient RF source signals, with spectrum sensing capability, the secondary ABC system determines operation modes according to activity of the licensed primary system. When the primary system is active, a tag is randomly selected due to a lack of channel information, while when it is inactive, the best tag maximizing achievable rate is selected using the secondary power beacon. Moreover, we apply a successive interference cancellation (SIC) technique when the secondary power beacon is able to work. The performance of the proposed multi-tag selection schemes is mathematically analyzed in terms of bit error rate (BER) and achievable rate. Finally, the performance of the proposed schemes is evaluated through simulations in terms of BER and achievable rate. It is shown that our approach can significantly outperform a conventional ABC system in CR environments, especially under a low-active primary system regime.

Coherent Multiantenna Receiver for BPSK-Modulated Ambient Backscatter Tags

Ambient backscatter communication (AmBC) is an emerging communication technology enabling green Internet of Things deployments. The widespread acceptance of this paradigm is limited by the low signal-to-interference-plus-noise ratio (SINR) of the signal impinging on the receiver antenna due to the strong direct path interference and unknown ambient signal. The adverse impact of these two factors can be mitigated by using noncoherent multiantenna receivers, which are known to require a higher SINR to reach the bit-error-rate (BER) performance of coherent receivers. However, considering the unknown ambient signal, unknown location of AmBC tags, and varying channel conditions, coherent receivers for AmBC systems are rarely studied in the literature. In this article, a coherent multiantenna receiver, which requires no prior information of the ambient signal for decoding the binary-phase-shift-keying (BPSK) modulated signal, is presented. The performance of the proposed receiver is compared with an ideal coherent receiver that has perfect phase information, and also with the performance of a noncoherent receiver, which assumes distributions for ambient signal and phase offset caused by the excess length of the backscatter path. Comparative simulation results show the designed receiver can achieve the same BER performance of the ideal coherent receiver with 1-dB more SINR, which corresponds to 5-dB or more gain with respect to noncoherent reception of on-off-keying modulated signals. Variation in the detection performance with the tag location shows that the coverage area is in the close vicinity of the transmitter and a larger region around the receiver, which is consistent with the theoretical results.

Joint Design of Improved Spectrum and Energy Efficiency With Backscatter NOMA for IoT

To develop emerging transmission techniques for potential applications of Internet of Things (IoT), the system performance analysis of a cognitive radio (CR)-enabled ambient backscatter (AmBC) system will be studied in this paper with functionality of non-orthogonal multiple access (NOMA). In the proposed scheme, a base station communicates with two destinations via a designated backscatter device. It is assumed that the relay node is fitted with two different interfaces and can simultaneously collect/decode and backscatter the received source signals. Such transmission mechanism benefits to design various applications in IoT as well as wireless systems with improved performance. To exhibit system performance metrics, the outage probability and the ergodic capacity of the recipient nodes are derived analytically. Furthermore, it is shown that employing AmBC NOMA together with CR for secondary communication can significantly improve overall network performance in terms of the achievable throughput in delay-limited and delay-tolerant modes and outage probability. Numerical results show that: <i>1) The proposed system can improve the spectrum efficiency by employing both CR and NOMA techniques; 2) Compared with the orthogonal multiple access (OMA)-aided AmBC systems, the considered CR AmBC system relying on NOMA can obtain better reliability in the whole range of SNR; 3) There are error floors for the outage probability in the high SNR regime due to required target rates; 4) There exists a trade-off between system performance of IoT devices and power allocation coefficients associated with NOMA; 5) We find energy efficiency factor as evident of further improvement in such system</i>.

Design of Intelligent Reflecting Surface (IRS)-Boosted Ambient Backscatter Systems

Passive technologies like ambient backscatter communication (AmBC) and intelligent reflecting surface (IRS) hold enormous potential for ubiquitous connectivity of Internet of Things (IoT) devices in future wireless networks. However, despite many research efforts, AmBC has so far been unable to fulfil its potential for widespread deployment, due to the weakness of backscatter signal in the presence of strong direct-link interference from the original RF source. This limits the bit error rate (BER) and hence the transmission rate and range of AmBC systems. Meanwhile, the IRS offers new degrees of freedom in enhancing a variety of systems by transforming their propagation medium and signals. In this work, we devise a novel scheme to improve the detection performance of an AmBC system using an IRS located in its proximity. The IRS augments the backscatter signal quality at the receiver by adjusting its phase shifts to balance signal strengths, ultimately improving the performance of energy detection at the receiver. Our results clearly show that an IRS of reasonable size can considerably improve the BER performance of ambient backscatter, which is an important improvement for low power IoT systems.

Ambient Backscatter Assisted Co-Existence in Aerial-IRS Wireless Networks

Ambient backscatter communication (AmBC) is an emerging technology that has the potential to offer spectral- and energy-efficient solutions for the next generation wireless communications networks, especially for the Internet of Things (IoT). Intelligent reflecting surfaces (IRSs) are also perceived to be an integral part of the beyond 5G systems to complement the traditional relaying scheme. To this end, this paper proposes a novel system design that enables the co-existence of a backscattering secondary system with the legacy primary system. This co-existence is primarily driven by leveraging the AmBC technique in IRS-assisted unmanned aerial vehicle (UAV) networks. More specifically, an aerial-IRS mounted on a UAV is considered to be employed for cooperatively relaying the transmitted signal from a terrestrial primary source node to a user equipment on the ground. Meanwhile, capitalizing on the AmBC technology, a backscatter capable terrestrial secondary node transmits its information to a terrestrial secondary receiver by modulating and backscattering the ambient relayed radio frequency signals from the UAV-IRS. We comprehensively analyze the performance of the proposed design framework with co-existing systems by deriving the outage probability and ergodic spectral efficiency expressions. Moreover, we also investigate the asymptotic behavior of outage performance in high transmit power regimes for both primary and secondary systems. Importantly, we analyze the performance of the primary system by considering two different scenarios, i.e., optimal phase shifts design and random phase shifting at IRS. Finally, based on the analytical performance assessment, we present numerical results to provide various useful insights and also provide simulation results to corroborate the derived theoretical results.

Analysis of Multi-Tag Ambient Backscatter Communication Under Time-Selective Fading

Ambient backscatter communication (AmBC) is a new technology that has emerged as a potential option for Internet of Things (IoT) applications wherein the sensors and other miniature devices require low power for communication. We proposed a selection combining (SC) diversity scheme and analyze the outage performance of the AmBC system consisting of a moving reader and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> multiple tags. Particularly, we carried out the analysis under time-selective links along with imperfect channel estimation and the outage probability expression is derived. Our results indicate that as the number of tags increases, the probability of an outage lowers drastically and it gets worse with an increase in the speed of the reader as well as estimation error. Furthermore, an asymptotic equation is obtained for the high signal to noise ratio (SNR) case. The derived analytical expressions are validated using Monte Carlo (MC) simulations.

Physical Layer Security Analysis in Ambient Backscatter Communication With Node Mobility and Imperfect Channel Estimation

Ambient backscatter communication (AmBc) is a modern communication technique that permits smart devices to transfer data with the aid of ambient radio frequency (RF) radiation. This letter investigates the issue of eavesdropper (ED) attacks on multi-tag AmBc systems. In particular, we explore a realistic scenario in which both the reader and the ED are in motion and analyze the influence of their motion on the AmBc system’s secrecy performance when channel estimation is imperfect. Specifically, an analytical expression of secrecy outage probability (SOP) and an asymptotic SOP is derived. Also, the simulation results are shown to confirm the analytical expressions.

Capacity Analysis for Tunnel Diode Amplifier Assisted Ambient Backscatter Communications

The ambient backscatter communication (AmBC) technology addresses connectivity, cost, and congestion bottlenecks for the Internet of Things (IoT) deployment. AmBC, by avoiding a dedicated power infrastructure and carrier emitter, allows tags to communicate by simply reflecting ambient radio frequency (RF) signals to the reader. Thus, the tag can operate in the low-maintenance, battery-free mode and be powered with energy harvesting. However, a fundamental bottleneck is the limited communication range. A novel solution is integrating tunnel diode amplifiers into tags to enhance their backscattered signal power and thus extend the communication range. This paper studies that solution and investigates the resulting capacity performance. Specifically, we first develop the system’s mathematical model, elaborate on the tunnel diode’s amplification mechanism, and derive its reflection gain. Subsequently, we derive the closed-form channel capacity and propose a reader-tag distance adjustment scheme to improve the system performance. Finally, simulation results corroborate our theoretical results. They show that the tunnel diode amplifier can significantly increase the system capacity by an order of magnitude when the tag receives less than -30 dBm of incident RF signal power. They also demonstrate a significant increase in the communication range.

Ambient Backscatter Communications in Mobile Networks: Crowd-Detectable Zero-Energy-Devices

In this paper, we introduce the new concept of Crowd-Detectable Zero-Energy-Devices. Such devices harvest solar or ambient light energy to power themselves, backscatter ambient waves to communicate, and are detected simultaneously by a smartphone connected to the network, and by the network itself, as long as the device is close to the considered smartphone. Such a device is a promising sustainable solution for the future of the Internet of Things. We describe an example of use case: asset tracking with zero added energy, zero new signals and zero new infrastructure in the network, thanks to the anonymous and transparent participation of smartphones connected to the wireless network. We then present our first prototypes of such devices that backscatter TV, 4G and 5G signals, and show our first experiments in challenging conditions and environments.