Power Control for Passive QAM Multisensor Backscatter Communication Systems

To support ubiquitous computing for various applications (such as smart health, smart homes, and smart cities), the communication system requires to be ubiquitously available, ultra-low-power, high throughput, and low-latency. A passive communication system such as backscatter is desirable. However, existing backscatter systems cannot achieve all of the above requirements. In this paper, we present the first LTE backscatter (LScatter) system that leverages the continuous LTE ambient traffic for ubiquitous, high throughput and low latency backscatter communication. Our design is motivated by our observation that LTE ambient traffic is continuous (v.s. bursty and intermittent WiFi/LoRa traffic), which makes LTE ambient traffic a perfect signal source of a backscatter system. Our design addresses practical issues such as time synchronization, phase modulation, as well as phase offset elimination. We extensively evaluated our design using a testbed of backscatter hardware and USRPs in multiple real-world scenarios. Results show that our LScatter's performance is consistently orders of magnitude better than WiFi backscatter in all the above scenarios. For example, LScatter's throughput is 13.63Mbps, which is 368 times higher than the latest ambient WiFi backscatter system [54]. We also demonstrate the effectiveness of our system using two real-world applications.

Owing to the rapid development of the ambient Internet of Things (IoT) industry, Backscatter Communication Systems (BCS) require higher receiver sensitivity for large-scale IoT scenarios than traditional Radio Frequency Identification (RFID) systems. This paper proposes a method of using Multiple-Input Multiple-Output (MIMO) technology to compensate for reflected signals in the 920 MHz backscatter communication systems. The researcher first created a MATLAB model to verify the theoretical feasibility and then built a One-Transmitter Two-Receiver (1T2R) BCS transceiver system compatible with the EPC global protocol for practical verification on LabVIEW. The ITIR and 1T2R tests were conducted using the National Instrument (NI) USRP-2952R device, the directional antennas, and the standard RF electronic tags. Through the process of superimposing and utilizing the phase shift between the two reflected carriers, it has been confirmed that the overall signal strength can be enhanced from 2.2 mV to 4.13 mV and 4.37 mV (nearly reaching 4.70 mV by using two Rx antennas in ideal conditions). After calculation, this advancement increases signal strength from −40.14 dBm (when using a single Rx antenna) to −34.18 dBm (when using two Rx antennas). The improvement in signal strength can provide a gain of 5.96 dB to the backscatter communication systems. Experimental results demonstrate that utilizing MIMO technology to the backscatter communication systems can effectively enhance the sensitivity of the receiver at 920 MHz.

We analyze the impact of arbitrary dependence between the forward and backward links in backscatter communication systems. Specifically, we quantify the effect of positive and negative dependence between these fading links on channel capacity by using Copula theory. The benefits of this approach are highlighted over the classical framework of linear dependence based on Pearson’s correlation coefficient, which is also analyzed. Results show that for a fixed transmit power budget, capacity grows with positive dependence as well as with fading severity in the low signal-to-noise ratio (SNR) regime. Conversely, fading dependence becomes immaterial in the high SNR regime.

Wireless communication technologies for bioelectronic implants enable remote monitoring for diagnosis and adaptive therapeutic intervention without the constraints of wired connections. However, wireless data uplink from millimeter-scale devices deep in the body struggles to achieve low power consumption while maintaining large misalignment tolerances. Here, we report a passive wireless backscatter communication system based on magnetoelectric transducers that consumes less than 0.3 pJ/bit and achieves less than 1E-6 bit error rate at a distance of 55 mm while tolerating a misalignment of 10 mm. Using this robust data uplink, we designed a wireless cardiac sensing node that can transmit electrocardiogram signals from the beating heart surface of a porcine model to a custom external transceiver using the magnetoelectric backscatter uplink. This reliable, near-zero-power communication method provides opportunities for next-generation bioelectronics to feature real-time physiological monitoring and closed-loop therapies while maintaining a small form factor and low power consumption.

In this letter, we propose an opportunistic source selection based two-way cooperative ambient backscatter communication (CABC) system, in which two sources convey information to each other via a decode-and-forward enabled relay node and the tag embedded in the relay utilizes the power difference produced by the two hops to accomplish backscatter communication (BC) concurrently. The proposed system can achieve the bidirectional information flow for both the relaying transmission (RT) and BC at the same time. The analytical results of the outage probabilities for both RT and BC are derived. Numerical results verify the theoretical analysis and show that the proposed scheme can increase the throughput of the BC while maintaining the same power consumption and not degrading the performance of RT, compared to the existing one-way CABC system.

Near-field integrated sensing and backscatter communication (ISAC) has emerged as a key technology for Sixth Generation (6G) networks, offering efficient utilization of spectral and spatial resources. However, the transition from 5G to 6G introduces significant challenges, including increased energy demand and exponential growth in connectivity requirements, particularly for battery-constrained devices. Backscatter communication facilitates battery-free data transmission by harvesting energy from incident radio frequency (RF) signals and reflecting modulated data, making it a promising solution for energy-efficient communication. Meanwhile, inverse scattering technology exploits the principle of electromagnetic wave reflection to solve inverse scattering problems (ISPs), enabling precise and robust sensing. By integrating backscatter communication with inverse scattering techniques, ISAC can enhance both sensing and communication performance, paving the way for a novel paradigm in cost-effective and energy-efficient near-field ISAC networks. In this article, we propose a unified system that seamlessly combines backscatter communication and inverse scattering methods to realize simultaneous sensing, identification, and reliable backscatter communication. We begin by introducing the fundamental principles of backscatter communication and inverse scattering, emphasizing their complementary advantages. Subsequently, we present the architecture and operational mechanism of the proposed integrated sensing, identification, and backscatter communication (ISIABC) system. Furthermore, we provide a case study to evaluate the performance of the proposed system and validate its effectiveness through simulations and experimental results. Last but not least, we discuss key insights into the potential of the ISIABC system and outline several promising directions for future research.

In conventional backscatter communication (BackCom) systems, various centralized access schemes, i.e., time/frequency division multiple access, are commonly adopted to prevent transmission collisions among multiple backscatter devices (BDs). However, as the number of BDs proliferates, there will be high overhead, and more seriously, the proactive inter-user coordination may be unaffordable for the passive BDs, which are of scarce concern in existing works and remain a challenge to solve. To this end, in this paper, we propose a slotted ALOHA-based random access for multi-BD BackCom systems, where each BD is selected randomly and coexists with an active device for hybrid transmission. To excavate and evaluate the performance, a resource allocation problem targeting max-min average throughput is formulated, where transmit antenna selection, receive beamforming design, reflection coefficient (RC) adjustment, power control, and access probability determination are jointly optimized. To deal with this intractable problem, we first transform the objective function with the max-min form into an equivalent linear one, and then decompose the resulting problem into three sub-problems. Next, a block coordinate descent-based greedy algorithm, incorporating maximum ratio combining, semi-definite programming, and variable substitution, to achieve a sub-optimal solution. Besides, we derive closed-form solutions for both RC and transmit power to facilitate analysis. Simulation results demonstrate that the proposed algorithm outperforms benchmark algorithms in terms of throughput and fairness.

SummaryIn this paper, in order to improve the received signal strength (RSS) and signal quality, three arrays of electronically steerable parasitic array radiator (ESPAR) antennas are suggested for the ultra‐high frequency (UHF) radio frequency identification (RFID) communication and sensing system applications. Instead of the single antenna, the array antennas have recently been widely used in many communication systems because of their peak gains, better radiation patterns, and higher radiation efficiency. Also, there are some important issues to use the antenna array like high data rates in wireless communication systems and to better understand the many targets or sensors. In this article, a wireless sensor network (WSN) is being investigated to overcome multipath fading and interference by antenna nulling technology that can be achieved through beam control ESPAR array antennas. The proposed ESPAR array antennas exhibit higher gains like 9.63, 10.2, and 12 dBi and proper radiation patterns from one array to another. Moreover, we investigate the mutual coupling effect on the performance of array antennas with different spacing (0.5λ, 0.75λ, λ) and configurations. It is found that the worst mutual coupling reduced by −28 to −34 dB for 2 × 2 array, −3 to −43 dB for 2 × 3 array, and finally −42 dB to −51 dB due to the antenna spacing from 0.5λ to λ. Thus, these suggested antennas could effectively be applied in the WSN communication systems, internet of things (IoT) networks, and massive wireless and backscatter communication systems.

This article introduces a novel strategy for wireless communication security utilizing intelligent reflecting surfaces (IRS). The IRS is strategically deployed to mitigate jamming attacks and eavesdropper threats while improving signal reception for legitimate users (LUs) by redirecting jamming signals toward desired communication signals leveraging physical layer security (PLS). By integrating the IRS into the backscatter communication system, we enhance the overall secrecy rate of LU, by dynamically adjusting IRS reflection coefficients and active beamforming at the base station (BS). A design problem is formulated to jointly optimize IRS reflecting beamforming and BS active beamforming, considering time-varying channel conditions and desired secrecy rate requirements. We propose a novel approach based on deep reinforcement learning (DRL) named Deep-PLS. This approach aims to determine an optimal beamforming policy capable of thwarting eavesdroppers in evolving environmental conditions. Extensive simulation studies validate the efficacy of our proposed strategy, demonstrating superior performance compared to traditional IRS approaches, IRS backscattering-based anti-eavesdropping methods, and other benchmark strategies in terms of secrecy performance.

A survey of millimeter wave backscatter communication systems

With the popularization of Internet-of-things (IoT) and wireless communication systems, a diverse set of applications in smart cities are emerging to improve the city-life. These applications usually require a large coverage area and minimal operation and maintenance cost. To this end, the recently emerging backscatter communication (BC) is gaining interest in both industry and academia as a new communication paradigm that provides high energy efficient communications that may even work in a battery-less mode and, thus, it is well suited for smart city applications. However, the coverage of BC in urban area deployments is not available, and the feasibility of its utilization for smart city applications is not known. In this article, we present a comprehensive coverage study of a practical cellular carrier-based BC system for indoor and outdoor scenarios in a downtown area of a Helsinki city. In particular, we evaluate the coverage outage performance of different low-power and wide area technologies, i.e., long range (LoRa) backscatter, arrow band-Internet of Things (NB-IoT), and Bluetooth low energy (BLE) based BC at different frequencies of operation. To do so, we carry out a comprehensive campaign of simulations while using a sophisticated three-dimensional (3D) ray tracing (RT) tool, ITU outdoor model, and 3rd generation partnership project (3GPP) indoor hotspot model. This study also covers the energy harvesting aspects of backscatter device, and it highlights the importance of future backscatter devices with high energy harvesting efficiency. The simulation results and discussion provided in this article will be helpful in understanding the coverage aspects of practical backscatter communication system in a smart city environment.

The growing interest in backscatter communications as a low-powered solution in various fields necessitates pushing the envelope of current backscatter systems in multiple frontiers, among which is spectral efficiency. The increase in spectral efficiency can drive real-time applications such as augmented reality. In most of backscatter-system applications such as radio-frequency identification, backscatter modulation is implemented using binary schemes realized with square pulses, which are not spectral efficient. To address the spectral efficiency concern, this article reviews—with a scope limited to prototyped systems—some of the existing works in the literature that pertain to increasing the spectral efficiency of backscatter systems. The prototyped systems can be grouped—based on the implementation technique used—into three groups: High-order modulation, single-sideband modulation, and pulse shaping. Based on the current trends and studied literature, the article concludes with discussions on some future directions and open-ended research problems such as optimal pulse shaping, the use of artificial intelligence, multi-carrier modulation, and agile modulation; which are all aim for overcoming the spectral-efficiency limit of contemporary backscatter systems.

Cascaded fading plays a vital role in backscatter communications but its effect on the transmission robustness of symbiotic backscatter system has not been well understood yet. In this paper, we investigate this effect under three symbiotic mechanisms, namely commensal, parasitic and competitive schemes. Our analysis indicates that: 1) For the commensal scheme, cascaded backscatter fading incurs an inferior outage performance of the backscatter system in comparison with the counterpart over single-hop backscatter fading; 2) For the parasitic scheme, cascaded backscatter fading leads to a superior outage performance of the primary system in comparison with the counterpart over single-hop backscatter fading; 3) For the backscatter system, regardless of single-hop or cascaded backscatter fading, the exact outage probability of the parasitic scheme overlaps with the asymptotic of the commensal scheme; 4) For the competitive scheme, as the decoding threshold of the primary system approaches zero, the asymptotic outage performance of the primary system in cascaded and single-hop backscatter fading tend to be the same; and 5) Unlike single-hop backscatter fading, as the decoding threshold of backscatter signal approaches zero, the outage probability of the competitive and parasitic schemes for the backscatter system in cascaded backscatter fading approaches that of the competitive scheme for the primary system.

In this paper, we propose a novel integrated sensing and backscatter communication (ISABC) system in which each moving target is attached with a backscatter device (BD) to facilitate simultaneous target identification and parameter estimation. When the base station (BS) transmits signals to its desired user, each BD attached to the target transmits the target identification information to the BS via backscatter communication, which concurrently enhances the echo signal strength. The BS needs to detect the BD symbols and to estimate the target parameters using the echoes. This task, however, is challenging due to the coupling between the BD symbols and the target parameters. To address this issue, we propose a novel iterative detection and estimation (IDE) framework, which involves the following two processes alternately: 1) Utilizing a modified maximum likelihood (ML) estimator to perform parameter estimation with the detected BD symbols; 2) Employing the ML detector to detect the BD symbols with the estimated target parameters. Since the inter-carrier interference (ICI) of the OFDM signal is independent of the BD symbols, but contains the delay and Doppler shift information, we develop a target parameter initialization method using such ICI component to improve the performance of the proposed IDE scheme. Moreover, the closed-form Miller-Chang bound is derived to demonstrate the theoretical performance for the target parameter estimation. Finally, simulation results are provided to validate the effectiveness of the proposed designs.

In this paper, we investigate covert communications in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted ambient backscatter communication (AmBC) system with rate-splitting multiple access (RSMA). Specifically, we use STAR-RIS as a backscatter device to enhance the backscatter link against interference from the direct link. In addition STAR-RIS utilizes the RSMA protocol to transmit information to both covert and public users, enabling more efficient use of spectrum resources. In order to evaluate the system performance, we derive expressions for detection error probability (DEP), outage probability (OP), covert rate (CR) and spectral efficiency (SE). Then, we perform an optimal detection threshold analysis for DEP, an asymptotic analysis for OP, as well as a lower bound analysis for CR, and analyze the impact of system parameter changes on the system performance. The numerical results show that the STAR-RIS-RSMA assisted ambient backscatter communication (AmBC) system performs better in terms of the covert performance than the system using tags as well as the system using the non-orthogonal multiple access (NOMA) protocol.