Wireless Communications Through Reconfigurable Intelligent Surfaces

The future of mobile communications looks exciting with the potential new use cases and challenging requirements of future 6th generation (6G) and beyond wireless networks. Since the beginning of the modern era of wireless communications, the propagation medium has been perceived as a randomly behaving entity between the transmitter and the receiver, which degrades the quality of the received signal due to the uncontrollable interactions of the transmitted radio waves with the surrounding objects. The recent advent of reconfigurable intelligent surfaces in wireless communications enables, on the other hand, network operators to control the scattering, reflection, and refraction characteristics of the radio waves, by overcoming the negative effects of natural wireless propagation. Recent results have revealed that reconfigurable intelligent surfaces can effectively control the wavefront, e.g., the phase, amplitude, frequency, and even polarization, of the impinging signals without the need of complex decoding, encoding, and radio frequency processing operations. Motivated by the potential of this emerging technology, the present article is aimed to provide the readers with a detailed overview and historical perspective on state-of-the-art solutions, and to elaborate on the fundamental differences with other technologies, the most important open research issues to tackle, and the reasons why the use of reconfigurable intelligent surfaces necessitates to rethink the communication-theoretic models currently employed in wireless networks. This article also explores theoretical performance limits of reconfigurable intelligent surface-assisted communication systems using mathematical techniques and elaborates on the potential use cases of intelligent surfaces in 6G and beyond wireless networks.

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Printed Circuit Board Implementation of Wideband Radial Power Combiner

This paper presents a new approach to design and implementation of a wideband microstrip radial power combiner with a planar structure in a way that it can be simply and inexpensively fabricated using a standard multilayer printed circuit board (PCB) technology. A 14-way power combiner with a two-octave bandwidth (1.5-6 GHz) is designed and fabricated on a three-layer PCB. Our measurements showed an amplitude and phase balance of ±0.75 dB and ±4.5 degrees, respectively, between the input ports. The main (output) port exhibited a reflection lower than -10 dB.

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User Grouping for Hybrid VLC/RF Networks With NOMA: A Coalitional Game Approach

Recently, visible light communication (VLC) networks have emerged as a promising alternative for indoor data access, due to high data rate, low implementation cost, and immunity to radio frequency (RF) interference. However, the co-existence of VLC with the RF access points as well as the dependence of VLC to room illumination compel both technologies to work in parallel and thus, to form a hybrid heterogeneous VLC/RF network. This network offers the advantages of both technologies, namely increased capacity and ubiquitous coverage. Furthermore, non-orthogonal multiple access (NOMA) is a very promising candidate technique for the next generation of wireless networks, mainly due to its increased spectrum efficiency compared to orthogonal access schemes. However, the optimal user grouping in NOMA is a combinatorial NP-complete problem, which calls for low complexity techniques. To this end, in this paper, we propose the use of coalitional game theory, where the users served by the same access point (VLC or RF) form a single coalition, while the users can switch through coalitions based on their payoff. A novel utility function is proposed that takes into account the peculiarities of the NOMA hybrid VLC/RF network. Finally, a coalition formation algorithm is presented as well as an efficient power allocation policy. Computer simulations validate the presented analysis and reveal the effectiveness of the proposed user grouping scheme compared to an opportunistic approach.

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Most Cited Article of 2017: Index Modulation Techniques for Next-Generation Wireless Networks

What is index modulation (IM)? This is an interesting question that we have started to hear more and more frequently over the past few years. The aim of this paper is to answer this question in a comprehensive manner by covering not only the basic principles and emerging variants of IM, but also reviewing the most recent as well as promising advances in this field toward the application scenarios foreseen in next-generation wireless networks. More specifically, we investigate three forms of IM: spatial modulation, channel modulation and orthogonal frequency division multiplexing (OFDM) with IM, which consider the transmit antennas of a multiple-input multiple-output system, the radio frequency mirrors (parasitic elements) mounted at a transmit antenna and the subcarriers of an OFDM system for IM techniques, respectively. We present the up-to-date advances in these three promising frontiers and discuss possible future research directions for IM-based schemes toward low-complexity, spectrum- and energy-efficient next-generation wireless networks.

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