1. Introduction
The demand for faster data speeds, lower latency, and better coverage has increased as a result of the quick development of mobile communication services. Massive multiple-input multiple-output (MIMO) antenna arrays are projected to be used in fifth-generation (5G) wireless networks to meet these requirements because they can considerably boost the system’s capacity and spectral efficiency. Massive MIMO antenna arrays are difficult to integrate into mobile phones due to space constraints, and intricate antenna design. Small, multi-standard compliant antenna systems that support a wide range of wireless protocols would be very useful for future portable gadgets. Multiple-input multiple-output (MIMO) aerial technology has become essential for the development of portable devices because of its ability to increase data throughput without increasing the power and bandwidth [
1].
Due to their success without requiring more power or better spectrum efficiency, MIMO LONG-TERM EVOLUTION (LTE) systems have attracted a lot of attention. In a situation with severe fading, a multiple antenna system can often be used in MIMO mode or diversity mode [
2]. There is currently a need for antennas with a wider bandwidth compact MIMO aerial with excellent isolation for 5G applications as a direct result of the development of new technologies and applications [
3]. The next generation of high-bit-rate wireless communications will benefit from MIMO RF systems reduced multipath fading and increased channel capacity. Although important for the tiny size of today’s mobile designs, the physical structure must also effectively utilize the available space for these future ideas [
4]. Due to the limited space allotted for smartphone antennas, it is technically difficult to construct a massive MIMO antenna system with less return loss or coupling [
5].
In the latest research, many decoupling methods have been presented to enhance isolation for the MIMO system, including, parasitic element [
6], the neutralization line (NL) method [
7], etching slot technique [
8], decoupling of ground branch technique [
9,
10] structures of metamaterial [
11], and decoupling networks [
12]. Several strategies have been proposed by researchers to limit mutual coupling between the radiators, including the use of a defective ground plane [
13], structures with electromagnetic bandgap [
14], and a perpendicular feeding network [
15]. In order to achieve effective isolation, parasitic F-shaped stubs were utilized in [
16] between the radiating patches. For the minimization of mutual coupling, a two-element MIMO aerial was equipped with an electric-LC resonator [
17]. For improved isolation between the MIMO antennas, a stub followed by rectangular gaps in the ground plane was used in the design of the antenna [
18]. In [
19], a dual-band 8 × 8 antenna array is designed, in which a decoupling stub is used to reduce mutual coupling. To acquire enhanced isolation balanced antenna elements are used [
20].
MIMO technology is used by LTE to increase the effectiveness of radio spectrum usage, and it is anticipated to be a crucial component of LTE networks [
21]. MIMO systems were originally used in a common mobile phone network called HSDPA (High-Speed Downlink Packet Access). Shortly later, this technology was first employed as the foundation for the LTE standard [
22]. The 3400-3800 MHz band, which unites LTE bands 42 (3400-3600 MHz) and 43 (3600-3800 MHz), has received recognition from many countries as a pioneer in the development of 5G massive MIMO. A decision by the European Union (EU) to prioritize the development of the 3400-3800 MHz spectrum is one example [
23]. China is looking into using the sub-6GHz and 3400-3600 MHz bands for 5G [
24], and Korea’s decision to conduct their exploratory research in the 3400–3700 MHz band [
25]. The next 5G multiband communication will require more than LTE bands 42 and 43.
The F-shaped MIMO antenna not only meets mutual coupling requirements but also those for lower size and operating frequency range in addition to WiMAX applications [
26]. A planar inverted-F antenna (PIFA) with an inverted-T-shaped open-slot is introduced. The proposed PIFA operates with a wide bandwidth of 78% due to the multimode technology [
27]. A defected ground structure-based F-shaped patch antenna with LC analytical modelling is presented in this work. The proposed DGS antenna is working between 3.4-3.6 GHz over -10 dB impedance bandwidth in [
28]. A new design of MIMO antenna array for fifth generation (5G) mobile handsets is proposed. The configuration of the design consists of eight planar-inverted F antenna (PIFA) elements located at different corners of the smartphone printed circuit board (PCB) [
29]. The proposed inverted-F Antenna is compared in
Table 1, which shows that the proposed antenna has high gain from the table and also high bandwidth from those whose resonance is 3.5 GHz which shows the novelty of proposed design.
Taking into consideration the previous analysis of the problems of MIMO antenna structures in 5G, a MIMO antenna array is presented for the upcoming 5G mobile handset applications. The proposed antenna array functions in the sub-6 GHz’s 5G bands. The design of the article is arranged as follows.
Section 2 clears the structure of the designed MIMO antenna array. In
Section 3, the simulated results are explained and compared with measurement setup of the proposed MIMO antenna array such as antenna efficiency, radiation pattern, ECC, and S-parameter are described in detail.
Section 4 brings the article to a conclusion.
2. Methodology
The front view of a single element in a ten-port massive MIMO antenna is shown in
Figure 1, along with its complete detailed dimensions. The compact size of each array element is 20 × 9
at 3.6 GHz, printed symmetrically on the long side of "Rogers RT/duroid 5880 (tm)". The thickness of the substrate is 0.79
, its dielectric constant (
) is 2.2, and the loss tangent (
) is 0.0009. The main PCB has dimensions of 150 × 70 × 0.79
. The 50
microstrip-line feeds are printed on the main PCB to excite the inverted-F radiator. On the back side of the substrate, two types of slots were applied, the one is ground cut with length = 10
to achieve a wideband (to cover LTE 42 and LTE 43), and the isolation cut with a length of 8
is imprinted to achieve better isolation. Additionally, in
Figure 1 and
Figure 2 the detailed dimensions of the 10 × 10 massive MIMO antenna are clearly mentioned inside the view. The simulation tool used for the simulation of a proposed massive MIMO antenna design was CST Microwave Studio.
The dimensions pw
, pw
, pw
, pl
, pl
, pl
, pl
, and
represent the widths and lengths of patch stubs in an inverted-F antenna. And, fl, fw are the length and width of the 50
feedline. The dimensions gcw
, gcw
, gcl
, and gcl
show the width and length of ground slots, whereas gap
and gap
are the separations between ground slots. A gap
is the distance of the topmost slot from the upper corner and gap
is the distance of the bottom slot from the lower corner. For better understanding, maintaining the minimum distances between each antenna will offer the most elements that’s why such acceptable isolation is obtained of the 10 × 10 MIMO antenna. All parameters and dimensions of a planar inverted F-shaped antenna are mentioned herein from
Figure 1 and
Figure 2 in
Table 2.
Figure 1 showed the magnified view of the Ant1. The F-shaped monopole element is kept farther apart so that the gap (g) between two F-shaped elements i.e., from the bottom of the inverted F-shaped to the top of it, is maintained as g = 22
. fl = 16.5
and fw = 2.43
are the length and width of the microstrip-line feed respectively which is printed on the main system PCB. The lowest horizontal arm of the inverted F-shaped monopole antenna has a gap pl
= 1.785
with the feedline. The fabricated prototype is presented in
Figure 3.