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In this paper, a compact 4×4 multiple-input-multiple-output (MIMO) patch antenna has been presented for triple band operation. Out of the 4 antennas, two are tuned to cover DCS (Digital Cellular System) downlink and two are tuned to cover DCS uplink frequency separately along with two common operating frequency centered around 2.45 GHz and 875 MHz. Four complementary split-ring resonators (CSRRs) have been etched in the ground plane exactly below the four patch antennas for attaining compactness from 0.36λ02 to 0.13λ02. In addition to this, by optimizing CSRRs outer radius, one lower common band operation of around 875 MHz (0.031λ02) has been obtained. Two, square split-ring resonators (SRRs) are placed between patch antennas to increase isolation by almost 7 dB around DCS band. The proposed MIMO antenna has been fabricated on an FR4 substrate (60×60×0.8) mm3 with dielectric constant, εr=4.3 having an antenna area of 0.13λ02. The Proposed MIMO antenna has two common operating bands, 850–900 MHz, 2410–2466 MHz and one individual band from 1725–1770 MHz for one group of two antennas of dimensions 17×22 mm2 and another individual band from 1800–1845 MHz for another group of two antennas of dimensions 17×23 mm2, where reflection coefficient is less than −10 dB with a minimum isolation of 17 dB. Diversity performance of the proposed MIMO antenna has been verified experimentally on all three bands. However, there is a slight deviation in the DCS operating band due to two different groups of antennas.

A microstrip-fed two-port multiple-input-multiple-output (MIMO) antenna has been designed for triple-band applications covering the entire ultra-wideband (UWB) with one band-notched characteristic. A defected ground structure (DGS) has been used to obtain a wideband resonance. A crescent ring has been etched on each of the two circular patch antennas to produce a band-notch characteristic centered at 5 GHz, ranging from 3.96 to 6.2 GHz. These introduce notches at 5.2/5.8 WLAN, 5.5 WiMAX, LMI C-Band and also reject the large capacity microwave relay trunk network, ranging from 4.40 to 4.99 GHz, such as in the Indian national satellite (INSAT) system operating between 4.5 and 4.8 GHz, thus making our MIMO antenna immune to many unlicensed bands. The proposed MIMO antenna elements have been isolated by more than 16 dB throughout the operating band using a modified inter-digital capacitor (MIDC) placed between the circular patch antennas. The MIDC also helps in achieving a center-band, ranging from 6.2 to 8.93 GHz and is useful in IEEE INSAT/Super Extended C-band. The lower-band ranges from 3.08 to 3.96 GHz and covers 3.5 GHz WiMAX while the upper-band, ranging from 10 to 16 GHz, is useful for X-band and Ku-band applications. Finally, the MIMO antenna has been fabricated on an FR-4 substrate of dimensions 50×30×1.6 mm 3 with a compact antenna area of 0.158λ 2 0. All results along with the diversity performance have been experimentally verified.

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With the advancement in the high data rates wireless transmission, the demand for high channel capacity planar antennas, especially for handheld devices, has shown a significant market. Multiple-Input-Multiple-Output (MIMO) antennas can fulfill the top requirement. MIMO antenna helps in increasing data throughput and link coverage without sacrificing additional bandwidth or increased transmit power (Kumar et al., 2018a, 2018b). The MIMO antenna's other advantages are: high data throughput and link range without additional spectrum requirement, increased transmit power, spatial diversity and pattern diversity, and less signal dropout (Kumar et al., 2019a). In MIMO, multiple antennas radiate simultaneously and act as a transmitter or receiver depending upon its purpose. Multiple antennas placed on a single substrate sharing at least one standard radiating frequency will be considered an MIMO antenna. The MIMO antenna should have a common ground, easing the integration with monolithic integrated circuits (ICs) (Kumar et al., 2020a). However, sometimes the MIMO antenna does not have a common ground plane, but such cases should be avoided for better integration purposes and standard voltage levels. MIMO antenna should be more compact to have a reduced form factor. The MIMO antenna's compactness is another primary concern when multiple antennas are placed on a single substrate. If the antenna element spacing is less than the λ 0 /2, i.e., half of the free-space wavelength, then the antenna suffers from the surface wave and the space wave coupling effect between the antenna elements (Dama et al., 2011). For that reason, as the compactness increases, the chances of MC effect increase, resulting in degradation in the MIMO antenna performance due to power losses in a rich scattering environment. Therefore, an effective isolation technique is a must in the design of a compact MIMO antenna. Besides this, pattern and spatial diversity