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In this article, we propose a frequency reconfigurable reflective coding metasurface hybridized with vanadium dioxide (VO2) meta‐atoms for wavefront manipulation at terahertz (THz) frequencies using the Pancharatnam‐Berry (PB) phase principle. At room temperature, the meta‐atoms can achieve a nearly constant 180° reflection phase difference illuminated by linearly x‐ and y‐polarized waves while maintaining almost consistent reflection amplitude within 3.76 to 3.84 THz, corresponding to the conductivity of σVO2=2×102S/m. Once the temperature is increased and VO2 is in a fully metallic state with the conductivity of σVO2=2×105S/m, the meta‐atoms operate at 1.18 to 1.47 THz. Flexible controls of scattering patterns of the PB coding metasurface can be realized utilizing the meta‐atoms. Numerical simulations demonstrate that the designed VO2‐based metasurface can generate multibeams as expected. This work opens a pathway in designing frequency reconfigurable coding metasurface to generate multibeams in THz, which holds great potential in new concept antennas for future wireless communication systems. [ABSTRACT FROM AUTHOR]

A compact wideband power divider based on the coupled lines and open stubs is presented in this paper. The proposed power divider has many advanced properties involving excellent input return loss, wide working band, high in-band isolation, ultra-wideband stopband, and outstanding harmonic suppression. The two sections of coupled lines are used to extend the passband while the open-circuit stubs are adopted to achieve ultra-wide stopband by introducing three transmission zeroes. Furthermore, high isolation within the operating band is realized with two resistors. The detailed design theories and procedures are provided in this paper. A power divider prototype with 10-dB bandwidth of 157.8% operating from 0.17 to 1.44 GHz and ultra-wide stopband from 2.04 to 5.07 GHz below − 30 dB is designed, fabricated, and measured for demonstration. [ABSTRACT FROM AUTHOR]

In this paper, a circularly polarized filtering antenna with high out-of-band radiation rejection level is proposed. The entire design consists of a patch and a filtering feeding network, which are connected by metallic pins. First, a high-selectivity filtering power divider is constructed utilizing two shunted bandpass filter. Next, a 90° phase difference could be realized by adjusting the length of the two outputs of the power divider. The measured transmission coefficient of the feeding network is 6.3 dB at 2 GHz. The measured axial ratio is below 3 dB within the operating band of the proposed antenna, which represents the effectiveness of the proposed feeding network. In addition, the obvious radiation nulls can be found from the antenna measurement because of the transmission zeros introducing by the filtering power divider. The measured gain is 5.73 dB at 2 GHz and decreases dramatically in stopband. Specially, the measured gain is −30 and −41 dB at 1.95 and 2.06 GHz. The proposed antenna has an excellent filtering characteristic compared with those without the filtering feeding network. To sum up, the proposed antenna and filtering power divider are promising in the modern wireless communication systems. [ABSTRACT FROM AUTHOR]

ABSTRACT This article presents a tri-band multiple-input-multiple-output (MIMO) mobile antenna for WWAN and LTE applications. The tri-band antenna element consists of a planar inverted-F antenna (PIFA) which is coupled-fed by an L-shaped driven strip monopole. Long parallel folded strip (PFS) printed adjacent to the upper edge of PIFA is used to generate an additional mode in the lower band, making the antenna capable of covering the whole GSM850 and GSM900 bands. Two n-shaped strips are placed between the antenna elements to improve port isolation. The proposed antenna exhibits triple bands including the lower band of 0.81-0.97 GHz, the middle band of 1.36-2.80 GHz, and the upper band of 3.40-3.75 GHz for WWAN and LTE applications. Detailed geometry and working mechanisms of the antenna are presented. © 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:1446-1450, 2017 [ABSTRACT FROM AUTHOR]

Cognitive radio (CR) is considered a promising technology for addressing the issue of spectrum scarcity and has been receiving increasing attention in recent years. One of its “cognitive” capabilities is environment awareness, which gathers useful information from the radio environment, such that CR devices can accordingly change the operating (e.g., spectrum sensing) strategies. Spectrum measurement is an effective means for radio environment awareness since ground-truth observation of spectrum usage is more convincing than any theoretical assumption. In this paper, we perform in-depth statistical analysis on wireless signal strength in mobile service (MS) bands and propose a novel statistical model based on characteristic function (CF), which characterizes both wireless fading channels and MS dynamics. We validate the proposed model through empirical studies on the spectrum measurement data previously collected in Guangdong Province of China. We statistically fit the CF of the overall signal strength in MS (GSM uplink) band and estimate both Nakagami fading parameters of wireless channels and the arrival rate of MS. Then, we further utilize the estimated information to optimize the spectrum sensing strategy, which aims at maximizing the secondary user's (SU's) expected achievable throughput. The simulation results show that the optimal sensing strategy that we propose significantly improves the long-term average achievable throughput since it fully utilizes the estimated information on wireless fading channels and MS dynamics. [ABSTRACT FROM PUBLISHER]

Three kinds of multifunctional graphene metasurfaces based on Pancharatnam–Berry (PB) phase cells are proposed and numerically demonstrated to control a reflected wave’s spin angular momentum (SAM) and orbital angular momentum (OAM) in the terahertz (THz) regime. Each proposed metasurface structure is composed of an array of graphene strips with different deviation angles and a back-grounded quartz substrate. In order to further help readers have a deeper insight into the graphene-based metasurfaces, a detailed design strategy is also provided. With the aid of the designed graphene elements, the proposed metasurfaces can achieve the full 360° range of phase coverage and provide manipulation of SAM and OAM of a circularly polarized (CP) wave at will. More importantly, simultaneous control of these two momentums can also be realized, and in order to demonstrate this function, a THz spin-controlled OAM beam generator with diverse topological charges is created, which can provide one more degree of freedom to improve the channel capability without increasing the bandwidth compared to a linearly polarized (LP) OAM beam. Numerical results verify the proposed graphene metasurfaces, which pave the way for generating spin OAM vortex waves for THz communication systems. [ABSTRACT FROM AUTHOR]

Dynamic spectrum access has been a subject of extensive study in recent years. The increasing volume of literatures calls for a deeper understanding of the characteristics of current spectrum utilization. In this paper, we present a detailed spectrum measurement study, with data collected in the 20 MHz to 3 GHz spectrum band and at four locations concurrently in Guangdong province of China. We examine the statistics of the collected data, including channel vacancy statistics, channel utilization within each individual wireless service, and the spectral and spatial correlation of these measures. Main findings include that the channel vacancy durations follow an exponential-like distribution, but are not independently distributed over time, and that significant spectral and spatial correlations are found between channels of the same service. We then exploit such spectrum correlation to develop a 2D frequent pattern mining algorithm that can predict channel availability based on past observations with considerable accuracy. [ABSTRACT FROM AUTHOR]