Due to their high data carrying capacities, increased bandwidth, decreased multipath fading, and ability to adapt to changing channel circumstances, Multiple Input Multiple Output (MIMO) antennas are now receiving a lot of interest from the telecom sectors and research institutions. Additionally, it can achieve higher gains, greater efficiency, and better diversity performance [1]. The individual elements must be efficient, have appropriate impedance properties, and have strong isolation in order to produce a highly efficient MIMO antenna system [2,3]. A compact construction is crucial to meet the demands of contemporary communication devices since the MIMO performance improves with the increase in the number of antennas [4].

Therefore, for any MIMO system to operate well, the individual antenna elements must be sufficiently isolated from one another and have minimal correlation with one another. This will enhance channel capacity, improve gain, and improve spectral efficiency [5]. The easiest technique to improve isolation between antenna components is to increase the distance between adjacent elements (usually 0.5λ), however, this strategy does not work well for portable communication devices since they are light weight and small and have limited space available for antenna placement. Mutual coupling influences the efficiency of a MIMO antenna as well as its input impedance, radiation pattern, and reflection coefficient. As a result, mutual coupling reduction is an important design objective and has the potential to directly influence the creation of massive MIMO and other next-generation wireless communication systems. The researchers have proposed several techniques to deal with this problem by lowering coupling between antenna elements.

Similarly, UWB MIMO system is getting significant attention due to its high data rate capabilities. When combined with UWB, the channel capacity of the MIMO system increases dramatically [6]. UWB-MIMO technology not only allows for high data speeds but also aids in equipment compactness and downsizing. [7,8]. As a result, several strategies and methodologies have been put out in the literature for small, effective UWB-MIMO antennas that may also offer adequate isolation between its elements. In [9] Increased isolation between antenna arrays is obtained by using two parallel coupled-line resonators (PCRs). In [10], the use of a frequency selective surface (FSS) wall reduces free space radiation between two antenna elements. After that, ground plane slots were added, which decreased surface current due to LC resonator-like effects. Using this technique, an improvement in isolation of more than 30 dB was achieved. In [11] 55 dB isolation was achieved at 25 GHz by using a complementary split ring resonator (CSRR) to increase isolation between the antenna ports.

In the same way, a wide stop band novel shaped low pass filter utilizing the Cross Shaped DGS (CSDGS) has achieved an enhanced isolation within the UWB frequency band [12]. In [13] a thorough analysis of the behavior of the Electromagnetic band gap structures (EBGs) is provided for various shapes, including circular, triangular, square and hexagonal. It was observed that the performance remained constant if their combined size/area remained constant. In [14,15], a compact planar EBG architecture is employed to minimize mutual coupling among array components. Additionally, when combined with cross-shaped slots in the ground plane, another uniplanar and a compact EBG (UC-EBG) produced minimal mutual coupling [16]. A broader stop-band using the Defected-Ground-Structure (DGS) was achieved in [17]. Similarly, for enhancing isolation in a large stop band, a tapered EBG is employed in conjunction with the CSRR and DGS [18,19]. Another method, which is appealing to filter designers, mixes EBGs with varying periods to generate a wide stop band [19–22] describes a compact UWB MIMO antenna that employs a cross shaped stub in ground plane and operates in the 2.97–13.8 GHz frequency band. All the aforementioned methods have certain drawbacks, such as the fact that they only improve isolation in a specific frequency range. Additionally, some take up more space and are incompatible with a small antenna configuration. In a similar manner, some implementations require fabrication of complicated design structures.

This paper presents a novel four-element antenna design with small dimensions of 40 mm × 40 mm for UWB applications. It employs a partial ground plane with an inverted L shape and results in lower mutual coupling between its antenna elements. To the best of our knowledge, when compared to similar designs in the published literature, the proposed design is one of the most compact four element MIMO antennas with increased element isolation in the entire UWB spectrum. The proposed model is appropriate for application in real-world MIMO systems due to its overall good performance. In the next sections we will discuss a thorough design methodology, experimental verification of the proposed design, and calculation of a number of MIMO antenna performance metrics.

A tiny rectangular patch antenna is built on a 0.8 mm FR4 substrate with a partial ground plane. A transmission line with an inset of 1.4 mm wide was used to feed the antenna. Fig. 1 shows the step by step modification in the design of a single element structure. Fig. 1a shows the basic antenna having low impedance bandwidth with a partial ground plane. Fig. 1b illustrates the insertion of a stub into the ground plane to create an inverted L-shaped partial ground. Additionally, this adjustment broadens the antenna’s impedance bandwidth, enabling UWB operation. The detailed dimensions are shown in Fig. 1b.

The length and width of the basic patch element are calculated using basic antenna design Eqs. (1) and (2):(1)Wp=λ02(0.5(εr+1)

(2)Lp=c02f0εeff−2ΔLpwhere λ0 and εr are the free space wavelength and relative permittivity of the substrate at the operating frequency and c0, ΔLp and εeff are the speed of light, change in patch length due to fringing effects and the effective dielectric constant respectively.

The S-parameter and gain of the single element antenna are presented in Fig. 2.

Eq. (3) has been used to approximate the resonance frequency fc for a microstrip patch antenna [23]:(3)fc=144.0w2π1+εr+w12π1+εr+l1+gp+lg GHzwhere “lg” represents the size of the ground plane, “gp” for the patch antenna’s distance from the ground plane and “l1” for the length of the patch antenna. Similarly, “w” and “w1” stand for the widths of the substrate and patch antenna, correspondingly. The calculated lower resonance frequency f_c is 7.25 GHz, which agrees with the simulated resonance frequency of the microstrip antenna with simplified ground plane, as shown in Fig. 5. In order to construct a 4-element compact MIMO antenna that can function in the UWB frequency range, the optimized single element design in Fig. 1 is employed. The intended MIMO antenna consists of four distinct radiators that are orthogonally and closely spaced to one another. The four-element design was analyzed in two different ways using basic partial ground planes and inverted L shape ground planes, as shown in Figs. 3 and 4, respectively,

The antenna’s impedance bandwidth for the simple partial ground plane design of Fig. 3 is 5.5–9 GHz as shown in Fig. 5. However, the impedance bandwidth increased when the ground plane was inserted with a stub, and this was eventually tuned for a better performance by parametric analysis. The final proposed inverted L-shaped design of Fig. 4 has significantly improved impedance bandwidth within the entire UWB frequency band, as demonstrated by Fig. 6. Additionally, it shows that for the majority of the UWB band, the isolation between other antenna elements/ports has improved and is above 20 dB, which is regarded as a respectable degree of isolation among antennas placed close together in a compact space of 40 mm × 40 mm.

A detailed parametric analysis led to the creation of the final design. In the ground plane, the stub’s length “ls” range from 9.80 to 13.80 mm. Fig. 7a plots the variations in the reflection and transmission coefficients. It demonstrates that “ls” variations above and below 11.8 mm cause the S-parameters to degrade. Additionally, there was no observable impact on the S-Parameters according to the parametric study of the stub width “ws” between 2.50–3.50 mm, as plotted in Fig. 7b. Table 1 lists the final optimal values for the proposed model.

To demonstrate the coupling between antenna elements when excitation is limited to one port, the designed MIMO antenna is simulated for surface current density. Fig. 8 shows that even for a small sized MIMO antenna, surface current is mainly accumulated on the stub section of the ground plane, with barely any current reaching the nearby ports. This has increased isolation between radiating elements. The surface current was computed at frequencies of 3.6, 5, and 7.8 GHz as shown in Fig. 8 respectively.

The proposed MIMO antenna was designed and modeled using Computer Simulation Technology (CST). The final proposed MIMO antenna has an optimized design achieved after thorough parametric analyses of critical design parameters. The antenna was built on a 40 mm by 40 mm FR-4 substrate with a thickness of 0.8 mm for testing and validation, and later to compare its measured performance with simulated one. The proposed MIMO antenna’s prototype is shown in Fig. 9 next to a measuring scale to put its size in perspective.

The configuration for measuring S-parameters using a network analyzer is illustrated in Fig. 10a, and the arrangement for measuring radiation patterns in an anechoic chamber is shown in Fig. 10b.

By using several performance indicators, the suggested MIMO antenna is compared and evaluated with relevant research work. A variety of performance metrics, including compactness, operational bandwidth, peak gain, mutual coupling, channel capacity loss, ECC, and diversity gain, are used in the comparison. Table 2 demonstrates that over majority of the UWB frequency range, the proposed design achieves isolation of >20 dB even with a small size, making it an ideal choice for tiny UWB MIMO applications.

This research work presents a compact four-port UWB MIMO antenna with a simple uniplanar design. In the proposed design, four radiators are orthogonally placed next to one another in a 40 mm by 40 mm-space. The designed antenna attains an acceptable impedance bandwidth in the whole UWB working spectrum (S11 < −10 dB). The isolation level between the radiators is higher (>20 dB) for most parts of the UWB band. The measured results of the fabricated prototype agree well with that of simulation. Additional MIMO diversity performance characteristics that are computed include a low envelope correlation coefficient (ECC < 0.01), high diversity gain (DG > 9.99 dB), peak gain of 4.6 dB, average radiation efficiency of over 85%, and low channel capacity loss (CCL 0.4 b/s/Hz). The proposed UWB MIMO antenna can be used in compact UWB systems.