The prospect of ultra-massive multiple-input multiple-output (UM-MIMO) technology to combat the distance problem at the Terahertz (THz) band is considered. It is wellknown that the very large available bandwidths at THz frequencies come at the cost of severe propagation losses and power limitations, which result in very short communication distances. Recently, graphene-based plasmonic nano-antenna arrays that can accommodate hundreds of antenna elements in a few millimeters have been proposed. While such arrays enable efficient beamforming that can increase the communication range, they fail to provide sufficient spatial degrees of freedom for spatial multiplexing. In this paper, we examine spatial modulation (SM) techniques that can leverage the properties of densely packed configurable arrays of subarrays of nano-antennas, to increase capacity and spectral efficiency, while maintaining acceptable beamforming performance. Depending on the communication distance and the frequency of operation, a specific SM configuration that ensures good channel conditions is recommended. We analyze the performance of the proposed schemes theoretically and numerically in terms of symbol and bit error rates, where significant gains are observed compared to conventional SM. We demonstrate that SM at very high frequencies is a feasible paradigm, and we motivate several extensions that can make THz-band SM a future research trend.

Underwater optical wireless communication (UOWC) has been widely advocated as a viable way to satisfy these high-speed links constraints in the marine medium, through the use of the visible spectrum. Nevertheless, UOWC faces several limitations such as the path-loss due to the absorption and scattering phenomena, caused by underwater particles. Thus, quantifying this path-loss is of paramount importance in the design of futuristic UOWC systems. To this end, several approaches have been used in this regard, namely the Beer-Lambert’s law, Monte Carlo simulation, as well as radiative transfer equation (RTE). This last-mentioned evaluates the optical path-loss of the light wave in an underwater channel in terms of the absorption and scattering coefficients as well as the scattering phase function (SPF). In this paper, an improved numerical solver to evaluate the time-dependent RTE for UOWC is proposed. The proposed numerical algorithm was improved based on the previously proposed ones, by making use of an improved finite difference scheme, a modified scattering angular discretization, as well as an enhancement of the quadrature method by involving a more accurate 7-points quadrature scheme in order to calculate the weight coefficients corresponding to the RTE integral term. Importantly, we applied the RTE solver to three different volume scattering functions, namely: the single-term Henyey-Greenstein (HG) phase function, the two-term HG phase function, and the Fournier-Forand phase function, over both Harbor-I and Harbor-II water types. Based on the normalized received power evaluated through the proposed algorithm, the bit error rate performance of the UOWC system is investigated in terms of system and channel parameters. The enhanced algorithm gives a tightly close performance to its Monte Carlo counterpart, by adjusting the numerical cumulative distribution function computation method as well as optimizing the number of scattering angles.

In this letter, the performance of a quantum key distribution (QKD) free-space optical (FSO) system is analyzed while taking a generalized pointing error model into account. More specifically, closed-form expressions for the average received powers at both the legitimate receiver and eavesdropper are derived. In addition, their corresponding asymptotic results valid in the high telescope gain regime are also presented. To capture the secure performance, we also investigate the ratio of received powers at the eavesdropper and at the legitimate receiver. Further, in some special cases, we find the optimal telescope gains for the received powers at both the legitimate receiver and eavesdropper, as well as the power ratio, which is important and useful for a secure QKD FSO system design. Finally, some selected numerical results are presented to illustrate the mathematical formalism and validate the accuracy of the derived analytical expressions.

Industrial automation relies on sensing the process parameters like liquid levels. Simultaneous sensing of multiple fluids is especially useful for many automation processes involving multiple ingredients. Some of the desirable features in any level sensor are its non-intrusiveness, high accuracy, and consistent performance. These features can be achieved with typical microwave radar-based level sensors. However, this paper presents a microwave resonance method instead of typical microwave reflectance (radar) to reliably sense the liquid level at fraction of the cost using a unique printed sensor. In addition to that, sensor design presented in this paper enables sensing liquid level of eight liquids simultaneously and independently utilizing its multiple channels. An array of eight modified microwave T-resonators has been compactly packed inside a metallic cylinder of just 4" diameter where the resonators share the same ground plane. Sensors' performance, to measure the unknown level of eight different fluids (with dielectric constant (∈ r ) ranging from 2.8 to 80), has been measured in real time using a custom-built software. Simple one point calibration procedure has also been proposed in this paper which has resulted into measurement accuracy of ~0.6% which is better than 1%, realizable with high-end and expensive radar based level sensors.

In this paper, we propose a distributed cluster formation (CF) and resource allocation (RA) framework for non-ideal non-orthogonal multiple access (NOMA) schemes in heterogeneous networks. The imperfection of the underlying NOMA scheme is due to the receiver sensitivity and interference residue from non-ideal successive interference cancellation (SIC), which is generally characterized by a fractional error factor (FEF). Our analytical findings first show that several factors have a significant impact on the achievable NOMA gain. Then, we investigate fundamental limits on NOMA cluster size as a function of FEF levels, cluster bandwidth, and quality of service (QoS) demands of user equipments (UEs). Thereafter, a clustering algorithm is developed by taking feasible cluster size and channel gain disparity of UEs into account. Finally, we develop a distributed α-fair RA framework where α governs the trade-off between maximum throughput and proportional fairness objectives. Based on the derived closed-form optimal power levels, the proposed distributed solution iteratively updates bandwidths, clusters, and UEs’ transmission powers. Numerical results demonstrate that proposed solutions deliver a higher spectral and energy efficiency than traditionally adopted basic NOMA cluster size of two. We also show that an imperfect NOMA cannot always provide better performance than orthogonal multiple access under certain conditions. Finally, our numerical investigations reveal that NOMA gain is maximized under downlink/uplink decoupled (DUDe) UE association.

In this paper, we analyze the secrecy performance of the decode-and-forward (DF) relay system in generalized-K fading channels. In a typical four-node communications model, a source (S) sends confidential information to a destination (D) via a relay (R) using DF strategy in two time slots, while an eavesdropper (E) wants to overhear the information from S to D over generalized-K fading channels. To be more realistic, we assume that E can receive the signals of two time slots, and there is no direct link between S and D because of heavy fading. Based on those assumptions, we derive closed-form expressions for the secrecy outage probability (SOP) and ergodic secrecy capacity (ESC) by using a tight approximate probability density function of the generalized-K model. Then, asymptotic expressions for the SOP and ESC are also derived in the high signal-to-noise ratio region, not only because we can get some insights about SOP and ESC, but also because expressions for SOP and ESC can be simplified significantly. The single relay system is subsequently extended into a multi-relay system, where the asymptotic SOP analysis of three proposed relay selection strategies is investigated. Further, the security-reliability tradeoff analysis in the multi-relay system is also presented given that S adopts a constant code rate. Finally, the Monte-Carlo simulation is used to demonstrate the accuracy of the derived closed-form expressions.

A simple approximation for the secrecy outage probability (SOP) over generalized-K fading channels is developed. This approximation becomes tighter as the average signal-to-noise ratio (SNR) of the wiretap channel decreases. Based on this simple expression, we also analyze the asymptotic SOP in the high SNR region of the main channel. Besides simplifying the SOP expression significantly, this asymptotic SOP expression reveals the secrecy diversity order in a general case. Numerical results demonstrate the high accuracy of our proposed approximation results.

This study reports on (Al0.28Ga0.72)2O3-based ultraviolet-C Schottky metal-semiconductor-metal and metal-insulator-metal photodetectors with peak responsivities of 1.17 and 0.40 A/W, respectively, for an incident-light wavelength of 230 nm at 2.50 V reverse-bias. © 2019 The Author (s)

A wideband tunability of 6.53 nm with appreciable SMSR (> 10 dB) and linewidth (~0.1 nm) is demonstrated from a simple and low-cost tunable self-injection locked InGaN/GaN green laser based external-cavity system, for the first time.© 2019 The Author(s)

Photodetectors (PDs) based on organic-inorganic hybrid materials such as graphene and perovskites have recently emerged at the forefront of the research in optoelectronic devices. Despite the remarkable progress in the performance of optoelectronic devices based on hybrid materials, some aspects such as stability have so far not been thoroughly addressed. This paper serves to demonstrate a fully inkjet-printed PD fabricated by hybrid perovskite (CH3NH3PbClx-3I3) layer between the two graphene electrodes as graphene/perovskite/graphene (GPG) heterostructure. The fully inkjet-printed GPG PD is found to be effective for the visible light region, which can be attributed by the high uniformity and low defects of the printed materials. Thus, the GPG PD achieves a high responsivity of 0.53 A/W. Fully inkjet-printed hybrid perovskite PD demonstrated in this paper unveils the facile, potentially large-scale and cost-effective methods for fabrication of hybrid perovskites-based optoelectronic devices, including PDs and solar cells.