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A four‐port MIMO antenna design focused on the sub‐6 GHz fifth generation wireless communication application is presented. The design does not need any other decoupling structure for achieving isolation between multiple ports. A combination of four single wideband antenna units results in the formation of a circular‐shaped metallic disc in the ground plane of the proposed antenna. This disc acts as a pool of current with 180° phase difference leading to isolation between various ports. Diversity performance of the antenna is shown with the help of an envelope correlation coefficient using both S‐parameter and far‐field data. The radiation pattern of the proposed antenna has four lobes in four quadrants for proper reception of signals from different directions. A prototype of the designed antenna is fabricated and measured successfully.

A novel design of 2 × 2 multiple-input-multiple-output (MIMO) antenna is reported for ultra-wideband applications. The neutralization line is implemented to minimize the mutual coupling between the radiating patches. The overall dimension of the designed antenna is 21 × 34 × 1.6 mm 3 . This antenna covers the measured bandwidth of 95.0% (3.52-9.89 GHz) with better isolation (≤−22 dB) over the entire operating frequency band. The measured gain varies from 3.08 to 5.12 dBi over the entire band. The various antenna parameters such as S-parameters, gain, efficiency, envelope correlation coefficient, mean effective gain, channel capacity loss, total active reflection coefficient, and radiation patterns are calculated and corresponding results are validated with the measured results. K E Y W O R D S ECC, MIMO antenna, mutual coupling, neutralization line, TARC

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Abstract-In this article, a novel design of butterfly-shaped compact and small size microstrip antenna is proposed. The radiating structure consists of four circular discs in coalesced form and fed with coaxial probe. The initial antenna resonates at 9.64 GHz with impedance bandwidth of 11.41%. The resonance frequency is further reduced to 8.12 GHz with bandwidth 10.10%, when a rectangular slot is incorporated in the initial patch. Finally, two parallel slots are embedded in the initial patch which improves the antenna bandwidth up to 21.50% (6.02-7.47 GHz). The gain and efficiency of this antenna are above 8.80 dBi and 90% respectively across the entire operating band. Radiation pattern is calculated at lower end (6.02 GHz), upper end (7.47 GHz) and centre frequency (6.75 GHz) of operating band. The proposed antenna is fabricated, and measured results are validated with the simulated ones.

The excessive use of digital platforms with rapidly increasing users in the wireless domain enforces communication systems to provide information with high data rates, high reliability and strong transmission connection quality. Wireless systems with single antenna elements are not able to accomplish the desired needs. Therefore, multiple-input multiple-output (MIMO) antennas are getting more attention in modern high-speed communication systems and play an essential part in the current generation of wireless technology. However, along with their ability to significantly increase channel capacity, it is a challenge to achieve an optimal isolation in a compact size for fifth-generation (5G) terminals. Portable devices, automobiles, handheld gadgets, smart phones, wireless sensors, radio frequency identification and other applications use MIMO antenna systems. In this review paper, the fundamentals of MIMO antennas, the performance parameters of MIMO antennas, and different design approaches and methodologies are discussed to realize the three most commonly used MIMO antennas, i.e., ultra-wideband (UWB), dual-band and circularly polarized antennas. The recent MIMO antenna design approaches with UWB, dual band and circularly polarized characteristics are compared in terms of their isolation techniques, gain, efficiency, envelope correlation coefficient (ECC) and channel capacity loss (CCL). This paper is very helpful to design suitable MIMO antennas applicable in UWB systems, satellite communication systems, GSM, Bluetooth, WiMAX, WLAN and many more. The issues with MIMO antenna systems in the indoor environment along with possible solutions to improve their performance are discussed. The paper also focuses on the applications of MIMO characteristics for future sixth-generation (6G) technology.

In this article, a microstrip line fed 2-port circularly polarized multiple-inputmultiple-output (MIMO) antenna is designed and fabricated. A novel ground structure and radiating patches are used to achieve the circularly polarized radiated field by the MIMO antenna. To improve the isolation between two ports, a meandered U-shaped narrow metallic strip is placed between the asymmetric Z-shaped radiating elements. The proposed MIMO design demonstrates the impedance bandwidth (S 11 ≤ À10 dB) of 90.94% (3.04-8.11 GHz).The axial ratio bandwidth ≤3 dB is found to be 32.10% (4.42-6.11 GHz). The diversity parameters such as envelope correlation coefficient, diversity gain, mean effective gain, total active reflection coefficient and channel capacity loss are also calculated to make the proposed design more reliable for 5G applications. This antenna is made compact and small which carries the dimension 24 Â 24 Â 1.6 mm 3 . The entire investigation is first modeled using the simulation software CST Microwave Studio and then verified with the measured results.

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