A dual-band antenna transmits and receives radio signals at two different frequencies. These antennas may use any of the two frequencies separately or both at the same time, depending on their arrangement and applications. Dual-band antennas are widely used with 2.4 GHz and 5 GHz frequency bands. Dual-band allows for quicker speeds and greater versatility. As a result, the dual-band eliminates connection problems and provides more reliability, flexibility and stability. The high gain and omnidirectional Wi-Fi coverage of dual-band antennas 
make them perfect for a wide range of applications, including vast interior areas, warehouses, buildings, naval installations and many more. Figure 8
shows a design approach for dual band MIMO antennas. The dual-band MIMO antenna designed for WLAN application is made up of closely spaced symmetric MIMO antennas with a 5.3 mm edge-to-edge spacing 
It is found that lower and higher frequency bandwidths of the design are 64.96% (1.85–3.63 GHz) and 44.36% (5.07–7.96 GHz), with isolation values ≤−17.21 dB and ≤−22.42 dB, respectively (Figure 13). The observed realized gain varies between 1.14 and 4.12 dBi (lower band) and 1.42 and 4.78 dBi (upper band), with a radiation efficiency > 72% for both frequency bands. This MIMO antenna covers the applications in DCS, LTE2300/2500, Bluetooth, ISM, and WLAN. The radiation patterns at 3.12, 5.29 and 7.18 GHz are evaluated and it is found that the E-plane pattern is dumb-bell shaped while the H-plane is omnidirectional in nature. The T-shaped strip embedded in the ground plane minimizes the electromagnetic coupling between the patches and hence the radiation patterns are least affected.
To obtain the dual-band characteristics, an arrow-shaped strip is placed between the U-shaped patch with two L-shaped slots on the GP. The dual frequency bands cover the frequency range from 2.99 to 3.61 GHz (lower band) and 4.53 to 4.92 GHz (higher band) with isolation ≤ −25 dB and ≤−16 dB, respectively (Figure 15). This MIMO antenna effectively covers the 5G and sub 6G n77/n78/n79 spectrum.
The directional dual-band planar inverted-F antenna (PIFA) element with a small dimension of 31 × 17 mm2
is presented in 
, in which two antenna elements share a common ground with a 3.0 mm edge-to-edge inter-element spacing. Further, it is found that the inverted L-shaped metal arm dimensions are responsible for dual band operation. An inverted T-Shape slot is created on the GP to produce lower and higher frequency bands 
. A T-shaped strip and a rectangular strip make up the radiating element in 
and the lower and higher frequency bands are primarily matched by the top and bottom half of the T-shaped strip and the rectangular strip. In 
, six rectangular slots on the trapezoidal structure patch produce two frequency bands and they are achieved by adjusting the length of the rectangular slots. In addition to that, a T-shaped branch contributes to the improvement in isolation of the designed MIMO antenna. In 
, a MIMO antenna with decoupling structure is reported consisting of four L-shaped branches arranged in a counterclockwise direction. Dual band response of the antenna is achieved by varying the length and width of the L-shaped branches. In 
, inverted L-shaped monopole antennas loaded with split-ring resonators (SRRs) are placed in a rotationally symmetric pattern. The inverted L-antenna is responsible for the higher band and by the interaction of the inverted L-antenna and interconnected GP, the lower frequency band is created. The SRR loading makes it easier to build a wide-band antenna mode that can span across lower WLAN, WiFi, and WiMAX application bands. In 
, a meander dipole, a concave parabolic reflector, and a parasitic strip are used to provide good impedance matching in the antenna. The parabolic reflector is used to reduce antenna size and also to improve the lower band directivity. The upper band is created using a metal strip. To accomplish the dual band antenna, in 
, the radiating patch has two asymmetric U-shaped slots printed on the substrate. Improved isolation is achieved by using a composite GP with four metallic strips. In 
, to produce a tiny dual-band WLAN MIMO antenna, lower and higher band dummy elements are employed. The desired frequency bands are achieved by adjusting the width of two branches and the angle between the branches. In 
, to produce a dual band, a rectangle split-ring-resonator (RSRR) slot is placed into the patch. Two arrays of 2 × 1 antenna elements make up an optically transparent MIMO antenna in which a slotted circular ring monopole antenna with a partial GP is used to increase the operation bandwidth and dual band operation 
. A 2 × 2 MIMO antenna is presented in 
using a semi-annular patch embedded with a zig-zag conducting strip. It is seen that the higher frequency band is controlled by the zig-zag structure while the lower band is unchanged. Further, the isolation is improved using a fork-shaped feedline. In 
, for a dual-band MIMO antenna, the mushroom type EBG is placed between two antenna elements, providing excellent isolation and reducing mutual coupling. In 
, four simple elliptical-shaped patches are arranged orthogonally around a plus-shaped partial GP. Two opposite slots are introduced into the patch elements to achieve dual band and the plus shape improves the isolation of the designed MIMO antenna. Two symmetrical monopole antenna elements make up the reported MIMO antenna in 
. By modifying the current distribution on the GP, a DGS and grounded branches are loaded to minimize mutual coupling across the lower band. Between the two antenna elements, there is a T-shaped parasitic element which introduces an inverted path for mutual coupling cancellation, considerably improving the isolation at higher frequency band. As a result, this antenna is capable for 5G dual-band operation. In 
, the slots are created on the GP to achieve dual-band MIMO antennas. In 
, for WiMAX/WLAN applications, a compact dual-band MIMO-stacked DRA is designed. The total height of the antenna is 7.0 mm, and the ground area is 50 × 50 mm2
. The DGS approach is employed to create strong isolation between antenna ports.
5. Circularly Polarized MIMO Antenna Design Approaches
Circularly polarized (CP) antennas have several significant advantages over linearly polarized antennas. In long-distance communication, circular polarization reduces the losses carried on by polarization mismatch. Additionally, circularly polarized antennas will reduce the spread of multipath propagation delays 
. CP antennas are rapidly becoming a key component for a wide range of wireless systems, including satellite communications, mobile communications, wireless sensors, radio frequency identification (RFID), wireless power transmission, WLAN 
, wireless personal area networks (WPAN) and global navigation satellite systems (GNSS) 
. Figure 16
shows some typical design approaches of circularly polarized MIMO antennas. Moreover, some latest designs and methodologies to realize CP MIMO antennas are described in this section.
Figure 16. Circularly polarized MIMO antennas design approaches.
shows a small and compact two-port circularly polarized MIMO antenna with dimensions 24 × 24 × 1.6 mm3 
. The circularly polarized radiated field of the MIMO antenna is achieved using a unique ground structure implanted with rectangular slots and Z-shaped radiating patches. A meandering U-shaped narrow metallic strip, which provides the good isolation, is placed between the asymmetric Z-shaped radiating components. The impedance bandwidth (3.04–8.11 GHz) of this antenna is 90.94% (Figure 18
) and a 3 dB axial ratio bandwidth of 32.10% (4.42–6.11 GHz) is achieved (Figure 19
). The ECC and CCL of the reported antenna are found to be 0.004 and 0.32, respectively.
Prototype of circularly polarized MIMO antenna 
S–parameters of circularly polarized MIMO antenna 
Axial ratio of CP-based MIMO antenna 
The circular polarization mechanism can be explained by using the current distribution graph shown in Figure 20
. From the figure it is seen that the current density is largely concentrated along the y-axis at time instants t = 0°. The current direction on the radiating patch and on the GP are in opposing directions at t = 90°, but the current flowing on the GP along the x-axis is maximum, and therefore the radiation occurs. With regard to time phase, the present rotation is clockwise, and the radiation is left-hand circular polarization (LHCP) in the +z direction. Furthermore, when the current is rotated in the −z direction, the current rotates anticlockwise, and the radiation received is right-hand circular polarization (RHCP). The radiation pattern is symmetrical, and the RHCP is 18 dB greater than the LHCP in the boresight direction. As a result, there is excellent cross-polarization rejection. In 
, a broadside circularly polarized T-shaped slot antenna and two end-fire CP antennas are combined to create a three-port MIMO antenna system for WLAN application (5.15–5.35 GHz). Figure 21
shows a T-shaped slot antenna with L-shaped feed line to excite the CP wave in the broadside direction. The parallel arrangement of an electric dipole and magnetic dipole with a 90° phase shift is used to create the end-fire CP antenna.
Surface current density at 5.45 GHz 
Fabricated circularly polarized MIMO antenna 
and Figure 23
show the S-parameters and axial ratio of the designed CP MIMO antenna. Figure 24
shows that a horizontally polarized wave in the +y direction is produced at ωt = 0° by the electric dipole. While the electric field distribution along the open-ended cavity is robust at time phase ωt = 90°, the current distribution on the electric dipole is small. As a result, at ωt = 90°, a vertically polarized wave along the +y-direction is excited by the magnetic dipole. The generation of CP waves along the +y-direction is caused by this 90° phase difference between the electric and magnetic dipoles. Similar effects are also seen for time phases at ωt = 180° and ωt = 270° 
S–parameters of three-port CP MIMO antenna 
Simulated and measured axial ratio of CP MIMO antenna 
Surface current distribution in 3-port MIMO antenna 
6. MIMO Antennas in Indoor Environment
MIMO antennas, when installed within the inhouse region, suffer with the major issue of reduced channel capacity. Some efforts have been made to improve the capacity performance of MIMO systems. Applying frequency-selective (F-S) wallpaper to the walls to block undesired interference with the desired radio communication services is a potential method of resolving indoor situations 
. Considering both SISO and MIMO systems, the features and design of the new F-S wallpaper are reported in 
. The wallpaper, which is built on symmetric and periodic metallic hexagons, is designed and applied to the ordinary walls in order to block 5 GHz transmissions without obstructing the other radio communication services. The wallpaper is created using the periodic boundary finite-difference time-domain (PB-FDTD) approach based on the unit cell analysis method. In 
, interference levels are decreased using the wallpaper inside a 4 × 4 MIMO system for indoor environments. This MIMO design consists of an array of half-wavelength dipoles for the receiving antenna and a cavity-backed dynamic meta-surface antenna (DMA) for the transmission of the signal. While the transmitter’s location is fixed, the MIMO channels are simulated and the channel capacity is calculated at various receiver locations using the ray tracing approach. Compared to a MIMO system using a sub-aperture phased array for the transmitting antenna, the reported adaptive radiation pattern provided by the DMA can achieve excellent capacity improvement 
7. MIMO Characteristics for 6G Technology
Although the 5G mobile communications standard is still in the growing stage of deployment, investigations are in pace for the next generation of wireless technology, 6G wireless systems. In order to overcome the operational difficulties experienced by fifth-generation cellular technology, 6G communication systems are designed to accommodate growing data-hungry applications with boosting connectivity and enhanced network capabilities 
. The bandwidth and latency of 6G networks will be significantly higher than those of 5G networks due to their ability to operate at higher frequencies. One of the objectives of the 6G internet is to provide communications with a latency of even less than microseconds. It is predicted that 6G would provide extremely dependable low-latency communication with a strong emphasis on internet devices, the application of artificial intelligence in wireless communication and the improvement of mobile broadband 
. Massive MIMO technology is currently used in 5G communication networks; on the other hand, there will be a need for dozens or even hundreds of antennas and radio links at the base station when 6G technology will be deployed 
. At the same time, due to the large scale of deployment, there will be high hardware costs, power usage and complicated designs 
. To fulfill the requirement, upcoming communication systems (6G) are moving towards the higher frequency bands, such as the terahertz (THz) and millimeter-wave bands 
. The internet of nano-things, health monitoring systems, entertainment services, military and ultra-high-speed on-chip communications are some of the significant uses of THz band wireless communication 
. This has encouraged researchers to continuously develop current wireless networks in order to switch over to 6G cellular systems.
A comprehensive research on design approaches and applications of MIMO antennas was presented here. Design approaches for ultra-wideband MIMO antennas, dual-band MIMO antennas and circularly polarized MIMO antennas were discussed. The presented research is very useful for researchers working in the field of MIMO antennas. From this research, it is found that the UWB characteristics of MIMO antennas can be achieved by modifying the radiating patches as L-shaped, staircase-shaped, and square-shaped embedded with slits and slots and use of stubs in the patch. Furthermore, creating defects in the GP which includes fence-shaped GPs, different stub loaded designs and tapered slots/slits are some of the methods to improve the antenna bandwidth. Dual-band MIMO antennas can be obtained by incorporating the slots in the radiating patches, U-shaped patches and modification of patches to resonate in the dual/multi frequency band. It is also observed that the uses of SRR-loaded and groove-loaded GP also provide dual- band MIMO antennas. CP-based MIMO antennas are designed by simultaneously optimizing the structure of the patch and the GP. Some basic radiating structures such as cross-loaded patches, G-shaped and Z-shaped patches, and square and circular patches embedded with slots at specific positions and some modified DRAs are used for CP characteristics. The truncated corners and slots itched in the GP with a specific shape, and use of hybrid slots within and at the periphery of the GP are common techniques to improve the CP quality in the MIMO antenna. In addition to that, the isolation techniques in each type of MIMO design are described and presented in tables. In most of the MIMO designs, decoupling parasitic lines are used between the ports to improve the isolation. Moreover, the neutralization line, SRR and EBG structures in the GP as well as between the radiating elements are used for better decoupling. Placing the radiating elements more than the half wavelength of the designed antenna minimizes mutual coupling. Orthogonal orientation of the resonators is also utilized for mutual coupling reduction. The comparison of various antenna parameters including the size, isolation techniques, design methodologies and other characteristics provide an overview of the specific MIMO antenna design. Thus, it is concluded that this research will surely help a lot to enhance the quality and performance of MIMO antenna designs which are demanded by the high data transmission rates of present communication systems, as well as the upcoming 6G technology.