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.

—Analysis of U-slot loaded patch stacked with H-shaped parasitic elements is given in this paper. It is found that the antenna exhibits dual resonance and both the resonance frequency (upper and lower) depends directly on slot width and inversely on slot length. Both upper and lower resonance frequency increase with increasing the value of h2. Typically the bandwidth at lower and upper resonance is found 3.66% and 10.25% respectively. The radiated power at higher frequency (beamwidth 64,). The theoretical results are compared with the simulated data obtained from IE3D.

This article presents a novel semi-flexible two port multi-input-multi-output (MIMO) antenna with dual circular polarization (CP) in the millimeter-wave frequency band (24-31 GHz) for wearable applications in biotechnologies. The radiating circular patch and ground plane are embedded with multiple sectoral slots and moon-shaped slots, respectively, for broad impedance bandwidth and CP property. The E _θ and E _ϕ fields are also classified as RHCP and LHCP at 27.10 GHz and 29.53 GHz, respectively. The microstrip line feeds the MIMO antenna, which is printed on a semi-flexible substrate with an overall electrical length of 2.9λ ₀ × 1.8λ ₀ × 0.04λ ₀ at 24 GHz. The antenna is designed for homogeneous space conditions (off-body) with a flat profile. Its on-body performance capability top-up with bending analysis is examined on the surface (chest, hand, and leg) of the anatomical Gustav human body model. Further, the MIMO diversity parameters are also evaluated in the CST Microwave Studio Simulator. Later on, the simulated radiation characteristics are testified through in-vitro measurement of the fabricated MIMO antenna in free-space and chest mimicking gel-based phantom. Lastly, to ensure human shielding, the SAR analysis is mapped with the communication link margin to select the input power.

In this paper, a dual-band 4-port Multiple Input Multiple Output (MIMO) antenna with very small footprints of 16×12×0.25 mm ³ is proposed for mm-wave applications. The proposed antenna operates in dual frequency bands at 28 GHz and 41.69 GHz. The isolation of the MIMO antenna is achieved >20dB at both the operating frequency bands by etching the slot at optimized position in the ground plane. The calculated diversity parameters of the antenna are acceptable for MIMO applications. The realized peak gain, Envelope Correlation Coefficient (ECC) and Diversity Gain (DG) are found to be 6.05 dBi, ≤0.10, and 9.89 dB, respectively. Moreover, the Mean Effective Gain (MEG), Channel Capacity Loss (CCL) and Total Active Reflection Coefficient (TARC) are evaluated to verify the proposed antenna performance in MIMO technology. The channel capacity is evaluated to demonstrate the superiority of the MIMO antenna over singe port antenna. Bending analysis of the antenna is also done to check the feasibility of this antenna for high data rate mm-wave wearable applications. In addition to that, antenna S-parameter is measured on chest and hand of human body for wireless body area network. Finally, to ensure the electromagnetic radiation safety, the specific absorption ratio (SAR) is evaluated at 28/41.69 GHz for input power of 500mW and found within the acceptable range.