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.

A novel 1-bit 256-element reconfigurable transmitarray antenna (RTA) at the Ku-band is presented. The element consists of two orthogonal H-shaped slots as receiving and transmitting structures, and the power transmission is realized by a coupling microstrip line in between. Two p-i-n diodes integrated on the coupling line can be electronically controlled to reverse the excitation directions, thus generating two states with 180° phase difference and low transmission loss. A subwavelength element spacing of λ ₀ /3 is utilized to stabilize the element performance under oblique incidence. A transmitarray prototype with 16 × 16 elements and aperture size of 5.3λ ₀ × 5.3λ ₀ at 12.5 GHz is fabricated and measured for the experimental verification. The measured results show that the maximum gain is 17.0 dBi with an aperture efficiency of 14.0%, and 2-D scanning beams within ±50° angular range are obtained. The 3-dB gain bandwidth of the broadside beam is 9.6%.

The design and experiment of a single-layer reconfigurable bidirectional antenna are presented for the first time, which combines the dynamic radiation characteristics of both reconfigurable transmitarray and reflectarray. The proposed element consists of a single-layer circular ring slot etched on one side of a dielectric substrate and two PIN diodes integrated across the slot. By turning them ON or OFF simultaneously, the induced current directions are reversed, thus achieving two phase states with 180° phase difference. The bidirectional radiation is achieved by utilizing the continuity of the cross-polarized electric field on the surface. A C-band 16×16 reconfigurable prototype with side length of 400 mm is designed, fabricated, and measured. The measured results demonstrate the ability to electronically scan both the transmitted and reflected beams simultaneously. The measured gains of the transmitted and reflected beams at 5.35 GHz are 17.2 dBi and 15.4 dBi, respectively. The wideband 3-dB gain bandwidths are 32% for both the transmitted and reflected beams. The proposed design can achieve high-gain large-angle electronic scanning beams with distinctive advantages of being wideband, lightweight, low-cost, and easy-to-fabricate, and hence, is suitable for bidirectional wireless communication applications.

The Internet of Things (IoT) has emerged as one of the key features of the next-generation wireless networks, where timely delivery of status update packets is essential for many real-time IoT applications. Age of Information (AoI) is a new metric to measure the freshness of update. The reduction of the violation probability that AoI of status updates exceeds a given age constraint is of great significance for guaranteeing the information freshness in IoT systems. By modeling the IoT networks as M/M/1 and M/D/1 queuing systems, this work focuses on characterizing the violation probability of peak AoI and AoI in IoT systems, where a sensor delivers updates to a monitor under M/M/1 and M/D/1 queues with first-come-first-served policy. From a time-domain perspective, we explore the correlation between interdeparture time and system time, by which the closed-form expressions of peak AoI distribution and the violation probability for any AoI constraint are derived. The obtained results induce accurate characterizations for probability distribution functions of peak AoI and AoI. Consequently, accurate characterizations of average AoI and the variance of AoI are obtained. Then, for peak AoI and AoI, the optimal generation rate of the status update that induces the minimal violation probability is also found. The numerical results show that the optimal update rate can significantly reduce the AoI violation probability for a wide range of AoI constraints. The theoretical findings and predictions are verified by numerical simulation results as well as provide guidance for the design of IoT networks.

In conventional integrated sensing and communication (ISAC) systems, the multiplicative fading of radar signals and space division multiple access (SDMA) restrict system performance. This work proposes a novel rate splitting multiple access (RSMA)-reconfigurable intelligent surface (RIS)-assisted ISAC system to improve the sensing performance, where RIS is deployed to simultaneously assist target detection and RSMA-aided multi-user communication for a dual-functional base station (BS). The objective of this work is to maximize the SNR of the target detection by optimizing the rate splitting, active beamforming at BS, and passive beamforming at the RIS. To solve this non-convex problem, an iterative algorithm is proposed exploiting majorization-minimization (MM), the approximation of the difference of two convex functions (DC), and the penalty method. Finally, simulation results verify the effectiveness of the proposed algorithm and demonstrate a clear sensing gain of the proposed RSMA-RIS-assisted ISAC scheme, approaching the upper bound, due to the special transmission mechanism of rate splitting in RSMA, and the cascaded passive beamforming of order O(N4) from RIS, respectively.

This communication presents a novel decoupling device called a cascaded power dividing decoupling network (C-PDDN). It is cascaded with two antennas operating in adjacent or even contiguous frequency bands to reduce the mutual coupling. To achieve high isolations at n desired frequency points, an n-order C-PDDN can be accordingly designed. Each order PDDN consists of two power dividers, two sections of transmission lines, and one reactive component. Explicit design formulas are presented to calculate the design parameters of the C-PDDN to meet the required isolation criteria. To validate the decoupling concept, a two-order C-PDDN is designed and applied to a testing array that consists of two base-station dipole antennas, one of which serves the time-division long-term evolution (2300-2400 MHz) and the other for wireless fidelity system (2400-2483.5 MHz). The simulated and measured results demonstrate that the proposed two-order C-PDDN could decrease mutual coupling by over 20 dB, verifying the decoupling methodology.