Compact dual-port MIMO filtenna-based DMS with high isolation for C-band and X-band applications

In order to replace the need of several antennas resonating at different frequencies, the small antenna must be equipped with on-demand frequency reconfigurability due to the rapid advancement of the various technologies [1]. The frequency ranges employed by common technologies including WLAN (5.15–5.85 GHz), WiMax (3.3–3.7 GHz), C-band (4–8 GHz), and X-band communication satellites (7.25–7.75 GHz) for space-to-earth communication and (7.9–8.4 GHz) for space-to-space communication [2]. Front ends for conventional RF communication systems include a receiver antenna, a mixing circuit, and a band-pass or bandstop filter. The enormous size of the circuit components, the bulky design, and the mismatch between the circuits components are some of the issues with these systems. Overcoming the disadvantage of employing distinct circuits is crucial for the progress of contemporary ultra-wideband communication schemes that are meant to be employed in contemporary communication networks. The so-called filtenna structure integrates the antenna with the necessary filters as a beautiful solution to this design dilemma [3].

High-speed ADCs are necessary for wideband antennas, but they use a lot of power and have larger quantization errors. Finally, because of constraints in the RF front-end components like amplifiers, oscillators, and mixers, the design for wideband operation compromises the transceiver performance. Narrowband frequency adjustable antennas, also known as tunable filter-antennas, are suitable candidates for use in order to sense the frequency spectrum due to all the aforementioned problems. This is due to the flexible frequency discrimination, wideband suppression of unwanted interference, gain flatness over the operating frequency band, less interference with the antenna's radiation characteristics, improved processing of down-converted signals, simplicity of implementation, and high performance of tunable band-pass filtering [4]. Due to their straightforward construction, patch and monopole antennas have been used by researchers for conformal applications. However, their disadvantageous size prevents them from being used for tiny devices [5, 6]. By incorporating dissimilar open-ended or short-ended stubs and slots into radiating constructions, the antenna size has been decreased. However, it adds structural complexity, which causes problems during fabrication [7]. In order to increase capacity, achieve diversity, improve the multipath effect, or minimize channel fading, MIMO techniques have been implemented in communication schemes [8‐11]. Another way to increase capacity discrete wavelet transforms (DWT), DWTs are employed in orthogonal frequency division multiplexing (OFDM) systems to improve spectral efficiency. Therefore, wavelet transforms can be used to design antenna diversity scheme to improve performance of system and spectrum efficiency [12‐18].

MIMO antenna design and deployment must take into account two key needs. These include constructing the antenna with the highest possible isolation between antenna elements and sizing it appropriately for portable applications. A good reception strategy is made possible by the strong isolation, which results in low mutual coupling (MC) between antennas [19]. It is well known that as the array size grows, the degree of coupling also grows. To lessen the MC in the MIMO antennas, many topologies and structures have been devised. The literature has discussed polarized orthogonal elements [20, 21], electromagnetic band-gap (EBG) structure [22], meander-line EBG [23], neutralization line [24, 25], stubs [26, 27], parasitic elements [28, 29], shorting pin [30], metamaterials with neutralization lines [31, 32], slots, or carbon black film [33]. Chen et al. [20] It describes a MIMO system with two orthogonal polarized antenna elements that has dimensions of 27 × 21 mm² and a 22.5 dB isolation level. Authors of [22] introduced a quad-port double-side MIMO antenna. It is reliant on EBG structure and was created to enhance isolation. The MIMO system consists of four polygonal parts. Additionally, a \(30 \times 30\) mm² FR4 substrate with a partly slotted EBG stub-ground is also comprised. A meander-line EBG construction has been employed in [23] to reduce MC between two \(32 \times 64\) mm² antenna elements. So as to absorb the antenna interference, a carbon black layer was used in [33] to increase the amount of isolation. Achieved were \({S}_{21}< -15\mathrm{dB}\) and envelope correlation coefficients < 0.02. All prior designs, however, had problems with either excessive bulk or inadequate isolation.

The described works either have the drawbacks of solid substrates, greater dimensions, or set working frequencies, to wrap up this debate. As a result, this work makes two contributions. The first is the design and construction of a filtenna element with a circular base. This design is suggested because it creates a tiny antenna, conformal, and has on-demand frequency-switching between the thirteen different bands, which are used for C-band and X-band applications. It also gives a longer electrical line in a small space. For the purpose of preventing potential electromagnetic interference with other systems, the antenna provides harmonic suppression up to 12 GHz. The introduction of a dual-port MIMO filtenna with exceptional port-to-port separation follows. The MIMO construction is built using an orthogonal filtenna design to lessen element-to-element coupling. Inserting a metamaterial with a zigzag shape between adjacent parts further improves isolation. The MC effectively increases port-to-port isolation to more than 40 dB among the two ports across the majority of the working bandwidth when this zigzag shape is positioned between the antenna elements. This results in an overall size of \(80\times 45\) mm². The structure of this essay is as follows. Method and experiment is described in Sect. 2. The tunable filtenna design process is described in Sect. 3. The MIMO filtenna system is discussed in Sect. 4 along with a demonstration of how to use a metamaterial to lower the isolation level. Section 5 presents the results and discussion. The paper is concluded in Sect. 6.

Figure 1 shows the processes of the methodology for designing a dual-port MIMO filtenna. The MIMO filtenna and antenna element is simulated using the CST program. Each element in this MIMO filtenna is positioned orthogonally to the one next to it to improve isolation. To provide further isolation between the antennas ports, a DS between the antennas is added into the top layer.

In a communication system implementing diversity, the same data are sent over independent fading paths. The signals received over the independent paths are then combined in such a way that the fading of the resultant signal is reduced. A wireless communication system equipped with diversity antennas leads to improved capacity and quality of the wireless channel.

In this section, a pattern diversity antenna with a frequency tuning feature (MIMO filtenna) is presented. Figure 14a shows the MIMO filtenna's structural layout. Two filtenna elements are used in the MIMO system, and they are positioned orthogonally with respect to one another. To perform well, this organization lowers the level of isolation. In the design, port 1 and port 2 are the MIMO ports. Consequently, MCs between port i and port j (S_ij), i, j = 1, 2, i ≠ j are minimized. Figure 14b shows the scattering parameter of the proposed MIMO filtenna without DS. S11 is − 45 db at resonance frequency 7.55 GHz and S12 is − 30 dB.

Figure 15 depicts the MIMO structure's surface current distribution when one port is activated and the other is closed. One may observe that when one port is activated, the other ports also experience a certain amount of current. As a result, coupling requires greater reduction, which will enhance isolation. To achieve this, a special decoupling structure (DS) to support isolation between the antenna parts is recommended.

The core of the DS idea is the creation of a metamaterial structure (like zigzag) that is sandwiched between the MIMO antenna parts, supposedly suppressing all energy. To reduce the implantation area and achieve good isolation between the MIMO parts, the high impedance wire is meandering in a zigzag pattern. Figure 16a presents the geometry of the final structure of the proposed two-port MIMO filtenna together with specific parameters. The two-port MIMO filter described is \(80 \times 45\) mm² in size overall. Figure 16b shows the scattering parameter of the proposed MIMO filtenna without DS. S11 is − 45 db at resonance frequency 7.55 GHz and S12 is − 65 dB.

A comparison of isolation levels for instances with and without the DS is shown in Fig. 17. The proposed DS significantly minimizes the MC between antenna ports, which is noteworthy. Only port 1's findings are shown due of the symmetry. At 7.55 GHz, the coupling between port 1 and port 2 (S₁₂) is less than − 20 dB for the case without DS. At 7.55 GHz, an improvement of around 40 dB is made with the DS, as depicted in Fig. 17. The distribution of surface current is examined for more proof. As seen in Fig. 18, Port (1) is individually energized at 7.55 GHz to study the DS effect. It has been shown that adding the proposed DS in between the antenna elements greatly reduces the current drift between them.

A brand-new, small, circular antenna with an impedance bandwidth of 2.4 to more than 12 GHz has been created. For C-band applications, a filtenna with tunable frequencies from 6.45 to 8 GHz and from 8 to 12 GHz for X-band applications was built. A dual-port MIMO antenna with a surface area of 80 × 45 mm² has been used. The isolation between ports has been improved using orthogonal polarization and a metamaterial. It has proven possible to achieve a MIMO filtenna with an impedance bandwidth of 7.4–7.75 GHz, an isolation level of 76 dB, a peak gain of 6 dBi, and an ECC lower than 2.4e–6. The MIMO filtenna has been made up and measured. The modeling and measurements have achieved good agreement.