Future Internet-of-Things networks are envisioned to use small and cheap sensor nodes with extremely low power consumption to avoid the extensive use of batteries. To provide connectivity to a massive number of these nodes, backscatter communication (BC) is emerging as an energy- and cost-efficient technology exploiting the reflection of radio frequency signals. However, challenges such as round-trip path loss and direct link interference (DLI) between the carrier emitter and the reader limit its performance. To tackle these limitations, this paper proposes a joint access point role selection and a novel beamforming technique for bistatic BC in a distributed multiple-input multiple-output setup. The proposed approach boosts the received backscattered energy while effectively mitigating DLI, thereby reducing the error probability. We also propose a channel estimation method tailored to operate under DLI conditions and propose a mismatch detector using estimated channel coefficients. Furthermore, we derive a closed-form expression for the probability of error for the detectors and model the quantization noise caused by DLI. Finally, comprehensive simulation results show that the proposed method with 1-bit analog-to-digital converters (ADCs) effectively mitigates DLI, reduces the quantization noise, and enhances backscattered signal energy, achieving performance comparable to the benchmark scenario with 8-bit ADCs.

Recent advancements in polymer microwave fiber (PMF) technology have created significant opportunities for robust, low-cost, and high-speed sub-terahertz (THz) radio-over-fiber communications. Recognizing these potential benefits, this paper explores a novel radio-over-fiber (RoF) structure that interconnects multiple radio units (RUs), booster units (BUs), and a central unit (CU) in cascade via fiber, envisioning its application in indoor scenarios. This structure creates a number of research opportunities when considering cascaded distortion effects introduced by non-linear power amplifiers (PAs) and the propagation channel over the fiber. We propose maximum-likelihood and non-linear least-squares algorithms to estimate the entry RU and the time-of-arrival between the RoF and the user equipment, where estimating the entry RU is equivalent to estimating the propagation distance along the RoF stripe. For the case of linear PAs, we derive the Cramér-Rao lower bound to benchmark the performance of the estimators. Finally, we investigate the use of the system for uplink positioning. Our simulation results demonstrate that the proposed estimators perform satisfactorily even with the cascaded effects of non-linear PAs, and that the deployment of this RoF structure can enable new cost-effective opportunities for high-resolution positioning in indoor scenarios. In the numerical evaluation, we also use measured PMF characteristics for high-density polyethylene fibers.

Broad beam beamforming (BF) design in multiple-input multiple-output (MIMO) can be convenient for initial access, synchronization, and sensing capabilities in cellular networks by avoiding overheads of sweeping methods while making efficient use of resources. Phase-only BF is key for maximizing power efficiency across antennas. A successful method to produce broad beams is the phase-only dual-polarization BF (DPBF). However, its efficiency has not been proved in non-line-of-sight (NLoS). Therefore, this paper contributes by evaluating DPBF in collocated and distributed MIMO configurations under both line-of-sight (LoS) and NLoS channel conditions. We model the reflection coefficients for different materials in NLoS conditions and propose the use of orthogonal space-time block code to improve the coverage compared to the DPBF in collocated MIMO (C-MIMO). We further propose a DPBF method for distributed MIMO and show that it achieves better coverage than C-MIMO with DPBF.

By combining the benefits of passive reflecting and active relay, the hybrid active-passive reconfigurable intelligent surfaces (RIS) (HRIS) architecture can overcome the double path loss and limited performance gains of the conventional passive RIS. In this work, we investigate the secrecy performance of a millimeter wave multiple-input-multiple-output (MIMO) network in the presence of a multiantenna eavesdropper with imperfect channel state information (CSI). To maximize the secrecy rate, we propose a joint design of the transmit beamformer and HRIS coefficients utilizing the combined tools of block coordinate ascent (BCA), stationary solution to Rayleigh quotient, Dinkelbach method, and concave-convex procedure (CCCP). The design problem encompasses both fixed and dynamic HRIS architectures. For the former, a pre-manufactured set of active elements is considered. For the latter, the set of active elements is adaptively updated, and their coefficients are optimized via a stochastic game-based algorithm. Simulation results demonstrate that the proposed HRIS schemes outperform the conventional passive RIS, particularly when high transmit power and many active coefficients are deployed at the HRIS. Additionally, a dynamic HRIS can offer 60% improvement in the secrecy rate compared to the passive RIS scheme.

Backscatter communication (BC) has emerged as a key technology to satisfy the increasing need for low-cost and green Internet-of-Things (IoT) connectivity, especially in large-scale deployments. Unlike the monostatic BC (MoBC), the bistatic BC (BiBC) has the possibility to decrease the round-trip path loss by having the carrier emitter (CE) and the reader in different locations. Therefore, this work investigates the BiBC in the context of cell-free multiple-input multiple-output (MIMO) networks by exploring the optimal selection of CE and reader among all access points, leveraging prior knowledge about the area where the backscatter device (BD) is located. First, a maximum a posteriori probability (MAP) detector to decode the BD information bits is derived. Then, the exact probability of error for this detector is obtained. In addition, an algorithm to select the best CE-reader pair for serving the specified area is proposed. Finally, simulation results show that the error performance of the BC is improved by the proposed algorithm compared to the benchmark scenario.

This paper proposes an unmanned aerial vehicle (UAV)-based distributed sensing framework that uses orthogonal frequency-division multiplexing (OFDM) waveforms to detect the position of a ground target. The area of interest, where the target is located, is sectioned into a grid of cells, where the radar cross-section (RCS) of every cell is jointly estimated by the UAVs, and a central node acts as a fusion center by receiving all the estimations and performing information-level fusion. A periodogram is employed for local estimation at each UAV, and a digital receive beamformer is assumed. The fused RCS estimates of the grid are used to estimate the cell containing the target. To evaluate the accuracy of the proposed framework, Monte Carlo simulations are carried out to obtain the detection probability, and our results show that the proposed framework attains a notable improved accuracy over a single mono-static UAV benchmark, due to the fusion from multiple sensing UAVs.

The hybrid relay-reflecting intelligent surface (HRRIS) has been recently introduced as an efficient solution to overcome the double path loss and limited beamforming diversity of the conventional fully passive reflecting surface. This motivates us to investigate the application of the HR-RIS in improving the secrecy capacity of millimeter wave multiple-input-multiple-output (MIMO) systems with the presence of multi-antenna eavesdropper. The joint optimization of the transmit beamformer and the relay-reflecting coefficients at RIS is tackled via alternating optimization. In the proposed solution, a closed-form expression for the optimal transmit beamformer at the transmitter is derived, and a metaheuristic solution based on particle swarm optimization is proposed to optimize the active and passive elements at the HR-RIS. The simulation results verify that under various scenarios, the HR-RIS provides significant improvement in the secrecy capacity with respect to the conventional passive reflecting surface.

In this work, we investigate the impact of two cooperative unmanned aerial vehicle (UAV)-based jammers on the secrecy performance of a ground wireless network in the presence of an eavesdropper. For that purpose, we investigate the secrecy-area related metrics, Jamming Coverage and Jamming Efficiency. Moreover, we propose a hybrid metric, the so-called Weighted Secrecy Coverage (WSC) and a virtual distributed multiple-input-multiple-output (MIMO)-based zero-forcing precoding scheme to avoid the jamming effects on the legitimate receiver. For evaluating these metrics, we derive a closed-form position-based metric, the secrecy improvement. Our mathematical derivations and comparative simulations show that the proposed zero-forcing scheme leads to an improvement on the secrecy performance in terms of the WSC, and provides conditions for improvement of Jamming Efficiency. They also show positioning trends on the UAVs over a fixed orbit around the legitimate transmitter as well as power allocation trends for optimal secrecy.

This paper addresses the secrecy performance of the downlink of a non-orthogonal multiple access network in the presence of multiple randomly located eavesdroppers. The network consists of a base station and a near receiver that are located inside a protected zone, free of eavesdroppers, while a far user is located outside. Herein, it is considered that the source transmits a superposed jamming signal to enhance the secrecy performance. In this sense, imperfections on the removal of the jamming signal by the legitimate receivers are also investigated. Integral-form exact and closed-form approximate expressions for the secrecy outage probability are derived by employing stochastic geometry tools. The expressions are corroborated via Monte Carlo simulations.

Recently, three useful secrecy metrics based on the partial secrecy regime were proposed to analyze secure transmissions on wireless systems over quasi-static fading channels, namely: generalized secrecy outage probability, average fractional equivocation, and average information leakage. These metrics were devised from the concept of fractional equivocation, which is related to the decoding error probability at the eavesdropper, so as to provide a comprehensive insight on the practical implementation of wireless systems with different levels of secrecy requirements. Considering the partial secrecy regime, in this paper we examine the secrecy performance of an amplify-and-forward relaying network with an untrusted relay node, where a destination-based jamming is employed to enable secure transmissions. In this regard, a closed-form approximation is derived for the generalized secrecy outage probability, and integral-form expressions are obtained for the average fractional equivocation and the average information leakage rate. Additionally, equal and optimal power allocation schemes are investigated and compared for the three metrics. From this analysis, we show that different power allocation approaches lead to different system design criteria. The obtained expressions are validated via Monte Carlo simulations.

Cognitive Radio (CR) is considered a key enabler technology for applications that require high connectivity (e.g., Smart Cities and Internet of Things), mainly because of its spectrum sensing function. In this way, CRs can sense the spectrum environment to select the best available channel for communication and they can, potentially, use licensed spectrum bands as a Secondary User (SU). However, CR has to vacate the licensed spectrum band as soon as a Primary User (PU) intends to use the channel. In this way, one of the most challenging topics in CR is the PU arrival prediction. Therefore, this paper presents a real-data study case of PUs arrivals prediction using the Hidden Markov Model (HMM) in CR. Herein, the Energy Detector (ED) is used to detect the presence of PUs. Our results show that the traditional method of combining the ED with the HMM may not be suitable in CR networks.

In this paper, the secrecy outage performance of a three-node amplify-and-forward relay network with an untrustworthy relay is investigated. To enable a secure transmission, we consider a destination-based jamming technique to prevent the relay from successfully decoding the secret messages between the source and the destination. In addition, the relay is assumed to be an energy-constrained device, thus being first energized by the source in order to be able to retransmit the information to the destination. In doing so, a time switching-based simultaneous wireless information and power transfer scheme is used. A closed-form asymptotic expression for the secrecy outage probability is derived in order to obtain a tight approximation at medium-to-high signal-to-noise ratio. The accuracy of the performed analysis is corroborated by Monte Carlo simulations through different illustrative cases. Numerical results show the impact of key system parameters on the secrecy performance, such as the time allocation factor between the energy harvesting and information transmission phases, the power allocation factor between source and destination for cooperative jamming, and the relay position, so as to provide insights on the design criteria for energy efficient and secure networks.

Cognitive Radio technique has been recognized as one of the most promising solutions for the increasingly growing problem of spectrum scarcity in wireless networks, specially with the emerging of the Internet of Things. In cognitive radio networks, secondary users are allowed to intelligently access licensed bands of primary users, thus enhancing the spectrum utilization. In this context, for investigating the advantages of cognitive radio, Machine Learning techniques have been widely applied to predict primary users arrivals. However, the available simulators are usually complex and highly time consuming. Therefore, in this work, we propose a simple and intuitive primary user arrivals data generator, MARIO, that can produce random arrival data for multiple channels by employing Poisson process. This generator is validated by using the generated data to predict new sequences according to a Hidden Markov Model. Our results show that the data generator can be used to simulate various traffic patterns over different channels.

We propose a destination-based jamming scheme for cooperative networks with multiple untrusted amplify-and-forward relays, in which the destination operates in full-duplex mode so as to enable simultaneous reception of the information coming directly from the source and the transmission of jamming signals to the untrusted relay. An approximate analytical expression for the secrecy outage probability, under the effect of residual self-interference at the destination, is presented. The developed analysis is verified through Monte Carlo simulations.

In this paper, an adaptive scheme for amplify-and-forward relaying networks is proposed, which selects a certain transmission mode for each communication process. Depending on the instantaneous channel conditions, one of the following modes is selected: direct transmission with no cooperation, cooperative transmission with half-duplex relaying and maximal-ratio combining at the destination, or cooperative transmission with full-duplex relaying and maximal-ratio combining at the destination. A three-node network is considered, containing a single-antenna source, a two-antenna relay that is able to implement full-duplex communication, and a single-antenna destination. Energy normalization per block is assumed, so that in those modes using cooperation, the system’s transmission power is shared between source and relay. The performance analysis is provided in terms of outage probability and energy efficiency. We derive a tight approximate expression in closed form for the outage probability and an approximate expression in integral form for the mean energy consumption. The results show that our scheme outperforms all of transmission modes separately in terms of outage probability, while being more energy efficient than the cooperative transmission modes. In addition, the asymptotic analysis proves that the proposed scheme achieves full diversity order equal to 2, thus outperforming those schemes with direct transmission or full-duplex cooperation only.

We propose an adaptive transmission scheme for cognitive decode-and-forward relaying networks, whereby, before each communication process, one out of three transmission modes is dynamically selected in order to maximize the instantaneous capacity of the system, namely, half-duplex (HD) relaying, full-duplex (FD) relaying, or direct transmission with no cooperation. The following key issues, relevant to underlay spectrum sharing and cooperative relaying, are considered: 1) the overall transmit power at the secondary network is constrained by both the maximum tolerable interference at the primary receiver and the maximum transmit power available at the secondary nodes; 2) under FD operation, the secondary relay is subject to residual self-interference, which is modeled as a fading channel; and 3) the signals coming from the secondary source and relay are handled at the secondary destination via maximal-ratio combining, in the HD relaying mode, and via a joint-decoding technique, in the FD relaying mode. We derive an exact analytical expression for the outage probability of the proposed scheme. Then, an approximate closed-form expression is proposed, and a corresponding asymptotic expression is derived. Monte Carlo simulations are run to validate the accuracy of the presented mathematical analysis and to showcase the tightness of the proposed approximation.

This letter investigates the outage probability of a variable-gain amplify-and-forward full-duplex relaying network in which not only the relaying link but also the direct link is used to convey information, thus improving the system reliability. We consider a basic three-node relaying system composed of a source, a destination, and a two-antenna relay—one antenna for transmission, one antenna for reception. Also, we consider that the full-duplex relay undergoes some residual self interference, modeled as a fading channel, and that maximal-ratio combining is employed at the destination to merge the signals from the source and relay. We derive an exact integral-form expression for the outage probability and validate this via Monte Carlo simulation. In addition, we propose a highly-accurate closed-form approximation to the outage probability, as well as a simple asymptotic expression at high signal-to-noise ratio. The use of the direct link is shown to overcome the zero diversity order inherent to full-duplex relaying.

We analyze the outage performance of two spectrally efficient schemes for exploiting multiuser diversity in cognitive relaying networks. We consider a secondary network composed by one source node communicating with the best one out of $L$ available destinations, selected according to the channel quality of the direct links. If the transmission through the selected direct link drops below a given threshold, then a half-duplex decode-and-forward relaying transmission is invoked. At this stage, two schemes are considered. In the first scheme, the previously selected secondary destination employs maximal-ratio combining to handle the signals received from the secondary source and relay. In the second scheme, maximal-ratio combining is employed as well, but at a possibly different secondary destination, selected so as to maximize the signal-to-noise ratio at the combiner output. In both schemes, we consider an underlay spectrum-sharing mechanism. We derive exact analytical expressions for the outage probability of the proposed schemes and validate them by Monte Carlo simulations. An asymptotic analysis reveals that the proposed schemes can achieve full diversity order, equal to $L+1$. Importantly, we show that, at high signal-to-noise ratio, the mean spectral efficiency of the proposed schemes approximates that one attained by direct transmission.

This paper investigates the outage performance of a spectrally efficient scheme for multiuser cognitive relaying networks under spectrum sharing constraints. In the proposed setup, we consider a secondary network composed by one source node communicating with the best one out of L available destinations, selected according to the channel quality of the direct links. If the selected direct link drops below a given threshold, a half-duplex decode-and-forward relaying transmission strategy is invoked, and then the selected secondary destination employs a maximal-ratio combining technique to combine the received signals from the secondary source and relay. In our analysis, the overall transmit power at the secondary network is considered to be limited by the maximum tolerable interference power at the primary user receiver, as well as by the maximum transmit power available at the secondary nodes. A closed-form expression for the outage probability is obtained and validated by Monte Carlo simulations. Moreover, an asymptotic analysis is carried out and reveals that the achievable diversity order equals L+1.

A distributed and spectrally efficient link selection scheme for multiuser multirelay cooperative networks with transmit beamforming is proposed and analyzed in terms of outage probability and mean spectral efficiency. More specifically, we consider a network composed by a base station equipped with M antennas which is communicating with one out of L single-antenna mobile stations under the aid of one out of N single-antenna, variable-gain amplify-and-forward relay stations. Either the direct link or the dual-hop relaying link is selected for each communication process by following a distributed link selection scheme. Single-integral expressions for upper and lower bounds of the outage probability and mean spectral efficiency are obtained and validated by Monte Carlo simulations. Furthermore, closed-form expressions derived from the asymptotic analysis of the outage probability reveal that the proposed system achieves full diversity order, which is equal to ML+N. Importantly, it is shown that the mean spectral efficiency is always higher than half of that one attained by direct transmission.