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 selfsustainable 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.

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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), fullpilot 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.Index Terms-NOMA, cell-free massive MIMO, MRT, fpZF, modified RZF, achievable sum rate.

Purpose The purpose of this paper is to present a method to find a generally accepted employee engagement scale, particularly in the presence of various alternatives and objectives. Design/methodology/approach To find the measurement scales, seminal works encapsulating organizational engagement, job engagement and work engagement in Cinhal, PsycINFO and ABI/INFORM database have been reviewed. For finding the optimal choice from available scales, multi-criteria decision-making (MCDM) method was used. Findings An agreed-upon measurement scale is achievable through the knowledge of alternatives and consequences, as well as consistent preference ordering and a decision rule. However, choice of the most effective scale varies according to the preference of decision makers. Practical implications This study proposes MCDM method as an intervention for practitioners who aim to assess the level of employee engagement in their organizations. It also provides a decision-making method to scholars to surmount conflicting objectives in their measurement. Originality/value While previous studies have developed manifold measurement scales, there is no study to indicate which scale best measures employee engagement. This paper attempts to define how to choose one scale among the various existing gauges of engagement.

In this paper we propose an interference alignment (IA) scheme in which the performance of the secondary users (SUs) transmitting their information on the same frequency band of the pre-existing primary links is improved in terms of achievable sum rate without generating any interference on the primary network. In our scenario, in which the primary users (PUs) are cooperating through IA and a set of SUs join the network, we propose an efficient approach to find the values of the pre-and postprocessing filters of these SUs. Moreover, the SUs must not receive/generate any interference from/to the PUs. In order to preserve the primary network from the interferences caused by the SUs, we also employ a method in which the distance between the subspace of the received signal from each secondary transmitter at the PUs and the set of interference subspaces at the primary links is minimized. In result, we would tackle with a computationally hard optimization problem with the sum rate of the secondary links as the objective function. We present the solutions in the cases where one or two SUs join the network. In situations where more SUs join the network simultaneously, we may apply other levels of IA between the secondary links after suppressing the mutual interferences between the primary and secondary networks. The analytical results are confirmed through simulations.

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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.

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 delaytolerant 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.