Nowadays synthetic aperture radar (SAR) and multiple-input-multiple-output (MIMO) antenna systems with the capability to radiate waves in more than one pattern and polarization are playing a key role in modern telecommunication and radar systems. This is possible with the use of antenna arrays as they offer advantages of high gain and beamforming capability, which can be utilized for controlling radiation pattern for electromagnetic (EM) interference immunity in wireless systems. However, with the growing demand for compact array antennas, the physical footprint of the arrays needs to be smaller and the consequent of this is severe degradation in the performance of the array resulting from strong mutual-coupling and crosstalk effects between adjacent radiating elements. This review presents a detailed systematic and theoretical study of various mutual-coupling suppression (decoupling) techniques with a strong focus on metamaterial (MTM) and metasurface (MTS) approaches. While the performance of systems employing antenna arrays can be enhanced by calibrating out the interferences digitally, however it is more efficient to apply decoupling techniques at the antenna itself. Previously various simple and cost-effective approaches have been demonstrated to effectively suppress unwanted mutual-coupling in arrays. Such techniques include the use of defected ground structure (DGS), parasitic or slot element, dielectric resonator antenna (DRA), complementary split-ring resonators (CSRR), decoupling networks, P.I.N or varactor diodes, electromagnetic bandgap (EBG) structures, etc. In this review, it is shown that the mutual-coupling reduction methods inspired By MTM and MTS concepts can provide a higher level of isolation between neighbouring radiating elements using easily realizable and cost-effective decoupling configurations that have negligible consequence on the array’s characteristics such as bandwidth, gain and radiation efficiency, and physical footprint.

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A Comprehensive Survey on “Various Decoupling Mechanisms With Focus on Metamaterial and Metasurface Principles Applicable to SAR and MIMO Antenna Systems”

The intensive research in the fifth generation (5G) technology is a clear indication of technological revolution to meet the ever-increasing demand and needs for high speed communication as well as Internet of Thing (IoT) based applications. The timely upgradation in 5G technology standards is released by third generation partnership project (3GPP) which enables the researchers to refine the research objectives and contribute towards the development. The 5G technology will be supported by not only smartphones but also different IoT devices to provide different services like smart building, smart city, and many more which will require a 5G antenna with low latency, low path loss, and stable radiation pattern. This paper provides a comprehensive study of different antenna designs considering various 5G antenna design aspects like compactness, efficiency, isolation, etc. This review paper elaborates the state-of-the-art research on the different types of antennas with their performance enhancement techniques for 5G technology in recent years. Also, this paper precisely covers 5G specifications and categorization of antennas followed by a comparative analysis of different antenna designs. Till now, many 5G antenna designs have been proposed by the different researchers, but an exhaustive review of different types of 5G antenna with their performance enhancement method is not yet done. So, in this paper, we have attempted to explore the different types of 5G antenna designs, their performance enhancement techniques, comparison, and future breakthroughs in a holistic way.

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Fifth Generation Antennas: A Comprehensive Review of Design and Performance Enhancement Techniques

A novel ultra-wideband (UWB) multiple-input multiple-output (MIMO) Vivaldi antenna with dual band-notched characteristics is presented and fabricated. The antenna is comprised of an improved ground plane and two microstrip feeding lines with a compact size of $26\times 26$ mm
 2
. The evolutionary process of the antenna is given. By etching the T-shaped slot on the ground plane, the port isolation between individual antenna elements can be greatly increased. Meanwhile, by adding two split ring resonator (SRR) of different sizes next to the microstrip feed lines, dual notches can be achieved to filter the interfere of WLAN and X-band communication satellites. The notched mechanism is analyzed from the surface current distributions. The experimental results show that the impedance bandwidth of the designed MIMO antenna is from 2.9 to 11.6 GHz, along with two notched bands covering 5.3-5.8 GHz and 7.85-8.55 GHz, and the mutual coupling is less than -16 dB in the whole working bandwidth. The MIMO antenna has good radiation characteristics, stable gain, and very low envelop correlation coefficient (ECC <;0.02), which is suitable for UWB MIMO system applications. The idea in this paper also has a certain guiding significance for the study of band-notched MIMO Vivaldi antenna.

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Compact UWB MIMO Vivaldi Antenna With Dual Band-Notched Characteristics

In this article, a novel slot-array defected ground structure (DGS) for decoupling microstrip antenna array is proposed. The slot-array DGS is etched surrounding each antenna element on the ground plane and parallel to the radiating edges of each antenna element. The decoupling mechanism is elucidated via an equivalent circuit model and the coupled current field analysis, which reveals slot-array DGS has the spatial band-stop characteristic and changes the direction of the partially coupled current, respectively. Both characteristics of the slot-array DGS contribute to mutual coupling reduction. Three practical design examples of applying slot-array DGS to single-linearly polarized (LP), dual-LP, and compact circularly polarized (CP) antenna array are given to illustrate the design process and considerations. The simulated and measured results show that about 50 dB isolation enhancement is obtained by using the slot-array DGS when the edge-to-edge spacing between CP antenna elements is 0.057 wavelength. Additionally, a wheel-shaped absorber based on the electromagnetic loss material is designed and fabricated to reduce the backward radiation caused by slot-array DGS. The absorber has an absorptivity of more than 95% in the frequency range of 1.2–1.35 GHz and suppresses the backward radiation over 12.5 dB in the plane phi = 0° and 16.1 dB in the plane phi = 90° without deteriorating other antenna performances.

A Novel Slot-Array Defected Ground Structure for Decoupling Microstrip Antenna Array

We experimentally demonstrate 8 dB of mode-equalized distributed Raman gain using a backward pumping scheme. The equivalent noise figure of the amplifier is −1.5 dB, and the amplifier was successfully employed to demonstrate 6-channel mode-multiplexed MIMO transmission over 137-km few-mode fiber.

Mode-Equalized Distributed Raman Amplification in 137-Km Few-Mode Fiber

In recent years, multiple-input-multiple-output (MIMO) antennas with the ability to radiate waves in more than one pattern and polarization play a great role in modern telecommunication systems. This paper provides a theoretical review of different mutual coupling reduction techniques in MIMO antenna systems. The increase in the mutual coupling can affect the antenna characteristics drastically and therefore degrades the performance of the MIMO systems. It is possible to improve the performance partially by calibrating the mutual coupling in the digital domain. However, the simple and effective approach is to use the techniques, such as defected ground structure, parasitic or slot element, complementary split ring resonator, and decoupling networks which can overcome the mutual coupling effects by means of physical implementation. An extensive discussion on the basis of different mutual coupling reduction techniques, their examples, and comparative study is still rare in the literature. Therefore, in this paper, different MIMO antenna design techniques and all of their mutual coupling reduction techniques through various structures and mechanisms are presented with multiple examples and characteristics comparison.

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Study on Mutual Coupling Reduction Technique for MIMO Antennas

A low quiescent current 180-nm output-capacitorless low-dropout regulator with small-gain stages (SGSs) is presented in this paper. The proposed technique permits the regulator to distribute the load current into two power transistors depending on the demand of the load using a controller based on load variation criterion. SGSs are introduced to enhance loop gain without low-frequency poles. The proposed architecture does not require compensation capacitor. Thus, the active chip area is reduced to 73.59 µm × 36 µm. The measured results have shown that the fabricated circuit consumes a quiescent current of 1.8 µA at no load, regulating the output at 1 V with maximum output current of 50 mA from a voltage supply of 1.2 V. It achieves full range stability from 0 to 50 mA load current at a maximum 100 pF load capacitor.

Low quiescent current capacitorless small gain stages LDO with controlled pass transistors

An ultra-wideband (UWB) antenna capable of producing notches at lower frequencies of the UWB band is presented in this paper. The notch can be adjusted to attain desired frequency by changing the size and the distance between the two studs placed between the circular patch. Defected ground structure has been applied to improve the antenna performance. The VSWR and radiation plots approve the suppression of the desired notched frequency.

Ultra Wideband (UWB) Microstrip Patch Antenna with Adjustable Notch Frequencies

Scattering and absorption of radio frequency signal by rain is prevalent at frequencies above 10 GHz. This paper presents the suitable regression coefficients for specific attenuation calculated on the yearly basis in 19.8 GHz experimental link set up at Korea Radio Promotion Association, Mokdong, Seoul. The received signal data for rain attenuation and rain rate were collected in 10 second intervals over a three year periods from 2013 to 2015. It focuses on the discussion and comparison of ITU-R P. 618- 12 prediction with the use of ITU-R P. 838-3 recommended coefficients along with the locally derived empirical values of power law coefficients which allow for calculating the specific attenuation in dB/km from the knowledge of rain rate. The Ku-band rain attenuation (12.25 GHz) is converted to the Ka-band down-link (19.8, 20.73 GHz) attenuation by using ITU-R P. 618-12 frequency scaling approach. This result shall be helpful to predict rain fading properties.

Rain Specific Attenuation and Frequency Scaling approach in Slant-Path for Ku and Ka-Band Experiments in South Korea

Because of rapid technological advancements, increased data rate support has become the key criterion for future communication medium selection. Multimode optical fibers and multicore optical fibers are well matched to high data rate throughput requirements because of their tendency to support multiple modes through one core at a time, which results in higher data rates. Using the numerical mode solver OptiFiber, we have designed a concentric core fiber by investigating certain design parameters, namely core diameter (μm), wavelength (nm), and refractive index profile, and as a result, the number of channels, material losses, bending losses, polarization mode dispersion, and the effective nonlinear refractive index have been determined. Space division multiplexing is a promising future technology that uses few-mode fibers in parallel to form a multicore fiber. The experimental tests are conducted using the standard second window wavelength of 1,550 nm and simulated results are presented.

Iram Nadeem

Papers

Study on Mutual Coupling Reduction Technique for MIMO Antennas

Low quiescent current capacitorless small gain stages LDO with controlled pass transistors

Ultra Wideband (UWB) Microstrip Patch Antenna with Adjustable Notch Frequencies

Rain Specific Attenuation and Frequency Scaling approach in Slant-Path for Ku and Ka-Band Experiments in South Korea

Concentric Core Fiber Design for Optical Fiber Communication