Massive and ubiquitous deployment of devices in networks of fifth generation (5G) and beyond wireless has necessitated the development of ultra-low-power wireless communication paradigms. Recently, wireless-powered networks with backscatter communications (WPN-BCs) has been emerged as a most prominent technology for enabling large-scale self-sustainable wireless networks with the capabilities of RF energy harvesting (EH) and of extreme low power consumption. Therefore, we provide a comprehensive literature review on the fundamentals, challenges and the on-going research efforts in the domain of WPN-BCs. Our emphasis is on large-scale networks. In particular, we discuss signal processing aspects, network design issues and efficient communication techniques. Moreover, we review emerging technologies for WPN-BCs to bring about the best use of resources. Some applications of this innovative technology are also highlighted. Finally, we address some open research problems and future research directions.

Backscatter communication (BackCom) networks enable passive/battery-free Internet-of-Thing devices, providing reliable, massive connectivity while ensuring self-sustainability, low maintenance, and low costs. Effective channel codes and decoding algorithms are necessary to achieve these objectives. However, a comprehensive survey/review paper on such techniques for BackCom networks has not been available. This paper aims to fill this gap. Because tags have limited computational resources, traditional coding techniques may not suit them. We first describe the basics of BackCom, channel codes and their relevant design parameters, and codes for general communication networks. We address the BackCom limitations, requirements, and channel characteristics. As conventional codes may not seamlessly move to the BackCom arena, we identify the potential BackCom coding techniques and multiple access schemes. We further highlight potential approaches for addressing code implementation complexity and reliability. Finally, we discuss open issues, challenges, and potential future research directions.

Although the hybrid of cell-free (CF) massive multiple-input multiple-output (MIMO) and non-orthogonal multiple access (NOMA) promises massive spectral efficiency gains, the type of precoders employed at the access points (APs) impacts the gains. In this paper, we thus comprehensively evaluate the system performance with maximum ratio transmission (MRT), full-pilot zero-forcing (fpZF) and modified regularized ZF (mRZF) precoders. We derive their closed-form sum rate expressions by considering Rayleigh fading channels, the effects of intra-cluster pilot contamination, inter-cluster interference, and imperfect successive interference cancellation (SIC). Our results reveal that this system supports significantly more users simultaneously at the same coherence interval compared to its OMA equivalent. However, intra-cluster pilot contamination and imperfect SIC degrade the system performance when the number of users is low. Moreover, with perfect SIC, mRZF and fpZF significantly outperform MRT. Also, we show that this system with either mRZF or fpZF precoding outperforms OMA systems with MRT. The analytical findings are verified by numerical results.

Massive connectivity of billions of communicating devices for fifth-generation (5G) and beyond networks requires the deployment of self-sustaining, maintenance-free, and low-cost communication paradigms. Could passive Internet of Things (IoT) solve these challenges? Passive IoT can be realized with the backscatter communication (BackCom) paradigm, which uses ultra-low power, inexpensive passive tags to support massive connectivity. However, a comprehensive link budget analysis for BackCom networks has not yet been available. It is something that is necessary for practitioners and researchers to evaluate the potential of BackCom. This survey is organized as follows. First, we describe the BackCom configurations, passive IoT design targets, backscatter channel statistics, and the different components and operations of the backscatter tag. Second, we develop the forward link budget and the overall link budget. All the relevant parameters are described in detail. Third, we give numerical and simulation results to get insights on the achievable performance of BackCom networks. Since additive path losses and excess fading can limit the performance of BackCom networks, we examine potential solutions to overcome the resulting limitations, enabling massive IoT networks. We also discuss integrating BackCom with existing wireless technologies. We further highlight some applications and address open issues, challenges, and future research directions.

Passive tags must harvest sufficient energy to operate. However, ensuring the tag’s energy needs and simultaneously suppressing the interference in ambient backscatter systems is challenging. To meet this, we employ a reconfigurable intelligent surface (RIS) to amplify the ambient source signal that reaches the tag. We maximize the achievable rate by optimizing the RIS phase-shifts subject to the tag’s energy harvesting constraint. Specifically, the phase-shifts are optimized to increase the tag’s received power while suppressing the direct-and-RIS-cascaded link interference. The optimization problem is non-convex, and we convexify it by means of fractional programming and a quadratic transformation. In comparison to a benchmark, our optimal phase-shift design provides ~150% and ~250% gains in harvested power and achieved rate, respectively, at 20dBm transmit power.

Wireless powered communication networks (WPCNs) are commonly analyzed by using the linear energy harvesting (EH) model. However, since practical EH circuits are non-linear, the use of the linear EH model gives rise to distortions and mismatches. To overcome these issues, we propose a more realistic, nonlinear EH model. The model is based upon the error function and has three parameters. Their values are determined to best fit with measured data. We also develop the asymptotic version of this model. For comparative evaluations, we consider the linear and rational EH models. With these four EH models, we investigate the performance of a WPCN. It contains a multiple-antenna power station (PS), a signal-antenna wireless device (WD), and a multiple-antenna information receiving station (IRS). The WD harvests the energy broadcast by the PS in the PS-WD link, and then it uses the energy in the WD-IRS link to transfer information. We analyze the average throughput of delay-limited and delay-tolerant transmission modes as well as the average bit error rate (BER) of binary phase-shift keying (BPSK) and binary differential phase-shift keying (BDPSK) over the four EH modes. As well, we derive the asymptotic expressions for the large PS antenna case and the effects of transmit power control. Furthermore, for the case of multiple WDs, we optimize energy beamforming and time allocation to maximize the minimum rate of the WDs. Finally, the performances of four EH models are validated by Monte-Carlo simulations.