Reconfigurable Intelligent Surface-Based Wireless Communications: Antenna Design, Prototyping, and Experimental Results

One of the key enablers of future wireless communications is constituted by massive multiple-input multiple-output (MIMO) systems, which can improve the spectral efficiency by orders of magnitude. In existing massive MIMO systems, however, conventional phased arrays are used for beamforming. This method results in excessive power consumption and high hardware costs. Recently, reconfigurable intelligent surface (RIS) has been considered as one of the revolutionary technologies to enable energy-efficient and smart wireless communications, which is a two-dimensional structure with a large number of passive elements. In this paper, we develop a new type of high-gain yet low-cost RIS that bears 256 elements. The proposed RIS combines the functions of phase shift and radiation together on an electromagnetic surface, where positive intrinsic-negative (PIN) diodes are used to realize 2-bit phase shifting for beamforming. This radical design forms the basis for the world’s first wireless communication prototype using RIS having 256 two-bit elements. The prototype consists of modular hardware and flexible software that encompass the following: the hosts for parameter setting and data exchange, the universal software radio peripherals (USRPs) for baseband and radio frequency (RF) signal processing, as well as the RIS for signal transmission and reception. Our performance evaluation confirms the feasibility and efficiency of RISs in wireless communications. We show that, at 2.3 GHz, the proposed RIS can achieve a 21.7 dBi antenna gain. At the millimeter wave (mmWave) frequency, that is, 28.5 GHz, it attains a 19.1 dBi antenna gain. Furthermore, it has been shown that the RIS-based wireless communication prototype developed is capable of significantly reducing the power consumption.

*Promotional prize winner of the 2020 IEEE Access Best Multimedia Award (Part 1)

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Absorption of 5G Radiation in Brain Tissue as a Function of Frequency, Power and Time

The rapid release of 5G wireless communications networks has spurred renewed concerns regarding the interactions of higher radiofrequency (RF) radiation with living species. We examine RF exposure and absorption in ex vivo bovine brain tissue and a brain simulating gel at three frequencies: 1.9 GHz, 4 GHz and 39 GHz that are relevant to current (4G), and upcoming (5G) spectra. We introduce a highly sensitive thermal method for the assessment of radiation exposure, and derive experimentally, accurate relations between the temperature rise (ΔT), specific absorption rate (SAR) and the incident power density (F), and tabulate the coefficients, ΔT/ΔF and Δ(SAR)/ΔF, as a function of frequency, depth and time. This new method provides both ΔT and SAR applicable to the frequency range below and above 6 GHz as shown at 1.9, 4 and 39 GHz, and demonstrates the most sensitive experimental assessment of brain tissue exposure to millimeter-wave radiation to date, with a detection limit of 1 mW. We examine the beam penetration, absorption and thermal diffusion at representative 4G and 5G frequencies and show that the RF heating increases rapidly with frequency due to decreasing RF source wavelength and increasing power density with the same incident power and exposure time. We also show the temperature effects of continuous wave, rapid pulse sequences and single pulses with varying pulse duration, and we employ electromagnetic modeling to map the field distributions in the tissue. Finally, using this new methodology, we measure the thermal diffusivity of ex vivo bovine brain tissue experimentally.

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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 Popular Article of 2017: 5G Cellular User Equipment: From Theory to Practical Hardware Design

Research and development on the next generation wireless systems, namely 5G, has experienced explosive growth in recent years. In the physical layer, the massive multiple-input-multiple output (MIMO) technique and the use of high GHz frequency bands are two promising trends for adoption. Millimeter-wave (mmWave) bands, such as 28, 38, 64, and 71 GHz, which were previously considered not suitable for commercial cellular networks, will play an important role in 5G. Currently, most 5G research deals with the algorithms and implementations of modulation and coding schemes, new spatial signal processing technologies, new spectrum opportunities, channel modeling, 5G proof of concept systems, and other system-level enabling technologies. In this paper, we first investigate the contemporary wireless user equipment (UE) hardware design, and unveil the critical 5G UE hardware design constraints on circuits and systems. On top of the said investigation and design tradeoff analysis, a new, highly reconfigurable system architecture for 5G cellular user equipment, namely distributed phased arrays based MIMO (DPA-MIMO) is proposed. Finally, the link budget calculation and data throughput numerical results are presented for the evaluation of the proposed architecture.

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