
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12873-3
10.1016/j.heliyon.2024.e36842
e36842
Research Article
A defected ground structure based ultra-compact wider bandwidth terahertz multiple-input multiple-output antenna for emerging communication systems
Kumar Praveen a
Sivakumar V. b
Rao Vidya c
George Criss Tom a
Awadhiya Bhaskar a
Huchegowda Yogeshwary Bommenahalli d
Nanjappa Yashwanth yashwanth.n@manipal.edu
a⁎
a Department of Electronics and Communication Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, 576104, India
b Department of Information & Communication Technology, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, 576104, India
c Department of Data Science and Computer Applications, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, 576104, India
d Department of Electronics and Communication Engineering, Shri Madhwa Vadiraja Institute of Technology and Management, Visvesvaraya Technological University, Bantakal, 574115, India
⁎ Corresponding author. yashwanth.n@manipal.edu
24 8 2024
15 9 2024
24 8 2024
10 17 e3684217 6 2024
22 8 2024
22 8 2024
© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
This work presents a quad-port Multiple Input Multiple Output (MIMO) wideband antenna that operates in the terahertz (THz) frequency range is designed and analyzed. To improve MIMO performance, a defective ground structure (DGS) is used. A rectangular metallic patch was altered by adding parasitic elements to the radiator with the lowered ground plane on a polyamide substrate in order to achieve the wideband THz operating frequency. The THz antenna was turned into a MIMO antenna by replicating horizontally and vertically with a spacing of 0.05 λ and 0.002 λ, respectively (λ calculated at 1.7 THz). The designed THz MIMO antenna, comprising 40 μm × 46 μm × 2 μm, operates across the 1.7–10.4 THz frequency region. The THz MIMO antenna provides isolation of more than 20 dB in the frequency range of 2.8–10.4 THz and more than 10 dB for the frequency of 1.7–2.7 THz. Isolation augmentation is accomplished by establishing different local current channels using the antenna's DGS. The MIMO diversity properties of the proposed THz MIMO antenna are analyzed and found to be ECC<0.004, DG ~ 10 dB, TARC < −10 dB, MEG < −3 dB, and CCL<0.23 bps/Hz over the antenna's operating frequency.

Keywords

THz antenna
MIMO
DGS
Wideband
==== Body
pmc1 Introduction

Over the past several decades, wireless communication has made significant advancements. As a result, technological developments in cellular communication, online gaming, live streaming, radio communication, and the need for a considerable amount of bandwidth in the GHz spectrum have prompted us to use the Terahertz (THz) band [[1], [2], [3]]. The microstrip is the best-suited antenna among the current antenna types for circumventing these restrictions [4]. While Tera bits per second (Tbps) data rate research continues to be in its infancy, THz communication has already surpassed Gigabits per second (Gbps) data rates. Future methods to attain practical data speeds will use THz technology with a frequency of 0.1 to 10 THz and a wavelength range from 3 mm to 30 μm [5,6]. Terahertz antennas with excellent directionality can be used for both transmitter and receiver to compensate for propagation losses. Therefore, the design of the THz antenna is vital because it can pay for excessive propagation losses in THz wireless communication [7]. Graphene's superconducting properties are currently employed to build reconfigurable circuits or antennas [[8], [9], [10], [11]]. The study of Low-THz antennas has increased relevance and use due to the development of novel processing techniques and materials.

A significant amount of work on antenna design for terahertz applications is available in the literature. In Ref. [12], the author introduced an antenna that supplied an aligned strip line. The proposed method has an operating frequency of 1–10 THz band and a gain of 2.2 dB throughout the operating frequency. A graphene-based, frequency-variable microstrip antenna operating in the terahertz range was proposed by Ref. [13]. It has a gain of 2.3 dB with an operating frequency of 2.8–4.2 THz. However, the antenna's drawbacks are its bulk and limited gain. In wide-band applications [14], proposed a Magneto-Electric Dipole MIMO Antenna with High Isolation using simulations on various antennas. This antenna has the advantages of wide radiation, sound isolation, and small size. with an average gain improvement of 11.5 dB at an operating frequency of 0.75–1.1 THz. They introduced a brand-new octagonal star pattern in Ref. [15]. The design's distinctive quality is its compactness, which makes it the ideal choice for different terahertz applications. The antenna's functional frequency range is 0.6–11.5 THz. A MIMO antenna structure with a gain of 13.6 dB and an operational bandwidth of 0.294–0.41 THz was presented in Ref. [16].

This work proposes a novel ultra-compact antenna design that considers ultrawide bandwidth MIMO antenna realization with improved isolation. A rectangular patch with a lowered ground plane is modified using the parasitic elements onto the radiator. This unique structured antenna is recreated horizontally and vertically to create the ultrawide bandwidth THz MIO antenna operating in the frequency range of 1.73–10.4 THz. The defected ground plane reduces the coupling effect between the adjacent elements as the decoupling structure provides an isolation of better than 20 dB. The remainder of the paper is arranged as follows-the comprehensive antenna design methodology is demonstrated in section 2. The findings and outcome of the proposed antenna configuration are presented in section 3. The concluding remarks are described in section 4.

2 Antenna design

The single-element THz antenna has dimensions of 20 μm × 20 μm and is built on a polyamide substrate with a dielectric constant (εr) of 4.3 and a height of 2 μm. Fig. 1 (a) depicts the step-by-step evolution of ultrawide bandwidth THz antenna with their reflection coefficient curves. In Step-1, the conventional rectangular patch antenna ground plane is reduced to 4 μm, and the response of this antenna configuration has no frequency range below −10dB on the reflection coefficient (S11) curve. After modifying the radiator using the parasitic elements in Step 2, the operating bandwidth in the S11 curve extends from 5.3 to 8.8 THz. In Step 3, the operating bandwidth further increases after some modification in the rectangular monopole antenna. Step 4, after embedding a circle in a rectangular stub on both sides of the patch, provides an S11 of 2.9–9.8 THz. Moving to the next stage, in Step 5, a stub is connected across the other two stubs, and the S11 curve is measured. Finally, in Step 6, a circle slot is embedded in a stub in the middle of the patch and provides an S11 curve of 2.7–10.1 THz well below −10 dB, as presented in Fig. 1 (b).Fig. 1 THz antenna evolution: (a) antenna design evolution; (b) S11 curves representing the antenna's evolution phases.

Fig. 1

The suggested ultrawide bandwidth THz antenna operates in the frequency range of 2.7–10.1 THz, with a maximum gain of 3.1 dB. The physical information of the proposed antenna and their ideal values and reflection coefficient is depicted in Fig. 2(a–b) and Table 1.Fig. 2 Design parameters of antenna (a) dimensions, (b) reflection coefficient curve.

Fig. 2

Table 1 Dimensional information of THz antenna.

Table 1Ws	Ls	Wf	W1	W2	W3	W4	W5	W6	W7	t1	t2	
20	20	2	2	1	2	5.5	1	2.5	1	1.1180	1	
r1	r2	r3	P	L1	L2	L3	L4	L5	L6	L7	Lg	
1.4422	1.2961	0.2973	7	2	2.5401	0.8	0.96	0.5401	2.1607	2.8284	4	

The goals of the parametric study were to determine how the primary structural components would affect the desired reflection coefficient quality of the suggested antenna design. To acquire the correct resonance responses, it is vital to take into account the ground plane width (Ws), length (Lg), and width (Wf), length (P) of the feed line. Individual structural variable has been altered at a time in every parametric study, with the other variables being kept at their optimal values. Any antenna design must take the size of the ground plane into account. The impacts of ground size variation are investigated, and the highest feasible performance for an antenna is determined by doing parametric analysis for various ground sizes. The features of S11 for different ground sizes are depicted in Fig. 3(a–b). The S11 parameter remains below the desired level of −10 dB even after five cycles of a 1 μm increase. The proposed THz antenna's performance is better than all earlier iterations at a ground plane width of Ws = 20 μm and a length of Lg = 4 μm. On the contrast, the alteration of length and width of the feedline is studied as shown in Fig. 3(c–d). The optimal values of the length and width of the feedline is chosen as 7 μm and 2 μm.Fig. 3 Parametric Analysis of THz antenna (a) width of the ground plane (b) length of the ground plane (c) width of feedline (d) length of feedline.

Fig. 3

The proposed THz MIMO structure is shown in Fig. 4, where it is modeled on a 20 μm × 46 μm polyamide substrate. The antenna elements are separated by 8 μm edge to edge. The common lowered ground plane comprises a modified ground plane embedded in the rectangular stub. The modified ground plane creates a limited current route, which restricts the ideal coupling current. This modification leads to an improvement in isolation among the antenna elements. The final configuration of the proposed two-element THz MIMO antenna is shown in Fig. 4(a). Fig. 4(b) depicts the S-parameter for the suggested THZ MIMO antenna. It operates within a 2.8–10.5 THz frequency range. A decoupling structure that offers isolation over the antenna's impedance bandwidth of more than 17 dB in the frequency range of 3.4–10.5 THz and the frequency range of 2.8–3.4 THz isolation is more than 10 dB. The suggested antenna has a reflection coefficient (S11) of −46 dB and −38 dB, respectively, at 3.9 and 5.9 THz. The dimensional information is provided in Table 2.Fig. 4 THz MIMO Antenna (a) structure of the antenna (b) results of the antenna.

Fig. 4

Table 2 Dimensional information of the two-element THz MIMO antenna.

Table 2D1	Wg	Ws	Lg	Lg1	
8	2	46	5	15	

The surface current distribution of the suggested antenna at 3.9 THz and 5.9 THz frequencies is shown in Fig. 5(a–b). Stimulating one port while blocking the other produces the surface current distribution charts. The maximum current concentration is throughout the feedline, patch, and ground plane, as shown in Fig. 5. Improved isolation may be achieved by using the DGS as a band stop filter, which decouples the field from the stimulated antenna with the terminated antenna.Fig. 5 Current distribution plot of a two-element antenna at (a) 3.9 THz, (b) 5.9 THz.

Fig. 5

As shown in Fig. 6, the identical two-element THz MIMO antenna pair is mirrored across the X-axis and has a 0.4 μm gap across the edges. The proposed THz MIMO antenna has dimensions of 40 μm × 46 μm. The common ground plane has been created by connecting the two rectangular stubs of the antenna's ground plane. The proposed antenna configuration with the modified ground plane provides the frequency of operation from 1.7 to 10.4 THz with isolation greater than 20 dB from the 2.8–10.4 THz frequency range and more than 10 dB for the frequency of 1.7–2.7 THz. The dimensional information of the suggested antenna is presented in Table 3.Fig. 6 Four Element THz MIMO antenna structure.

Fig. 6

Table 3 Dimensional information of the proposed antenna.

Table 3Ws	Ls	Lg1	D1	D2	
8	2	46	5	15	

3 Results and discussions

Antenna characteristics are modeled using ANSYS HFSS. The presented wideband THz MIMO antenna design parameters are assessed and calculated in terms of the MIMO diversity characteristics, radiation pattern, gain, surface current distribution, and scattering parameters.

3.1 Scattering parameters

The operating bandwidth of the suggested antenna is 8.7 THz, with a frequency range of 1.7–10.4 THz. The ground plane's horizontal and vertical branches provide a distinct local current channel parallel to the initial coupling current, resulting in more than 20 dB of isolation from the 2.8–10.4 THz frequency range and more than 10 dB for the frequency of 1.7–2.7 THz. The proposed design's transmission and reflection coefficient curves are shown in Fig. 7(a–b).Fig. 7 S-parameters of the proposed THz MIMO antenna (a) reflection coefficient (b) transmission coefficient.

Fig. 7

Antenna operation is being verified through the development of an electrical circuit design. The corresponding electrical circuit for the proposed THZ MIMO antenna employing lumped components is shown in Fig. 8. Time-domain analysis and circuit modeling both benefit from antenna equivalent circuit analysis. An electrical model of this sort is very beneficial for coordinating the antenna and transceivers. In line with the circuit theory study, the passive single-port network functions as an antenna. The corresponding antenna equivalent circuit is designed using the impedance approach. The equivalent circuit is created and modeled in the AWR simulator, with the antenna circuit component values predicted using the methods given in Ref. [17]. The circuit's output spans the same impedance bandwidth as the suggested THz MIMO antenna.Fig. 8 Proposed antenna's analogous circuit (a) equivalent circuit using RLC resistors (Ω) inductors (nH), Capacitors (pF), (b) output.

Fig. 8

3.2 Surface current distribution

Further proof of the antenna port's isolation may be obtained by examining the current distribution across the antenna element. The current distribution across the antenna components is measured while the first port is turned on and the other ports are terminated with corresponding 50-Ω loads. The distribution of surface current at the resonant frequencies of 2.2 THz, 3.9 THz, and 5.9 THz is shown in Fig. 9(a–c). The stimulated antenna element exhibits the maximum current density. The lowest current density, however, is seen in the inactive antenna element. This illustrates how the antenna's isolation works. The current density is separated and the mutual coupling across the antenna element is reduced by modifying the rectangular stub in the ground. The uniform distribution of current is disrupted by the ground plane modification. A band-stop filter effect is produced when the current is forced to follow a longer path due to this disturbance, increasing the effective inductance. Additionally, a redesigned ground plane improves isolation by obstructing the propagation of the surface waves, which helps to suppress them and improves isolation.Fig. 9 Current distribution by stimulating first port and removing other ports at a frequency (a) 2.2 THz (b) 3.9 THz (c) 5.9 THz.

Fig. 9

3.3 Radiation characteristics

The simulated results of a 2-D radiation pattern in XY and YZ planes at resonance frequencies of 2.2, 3.9, and 5.9 THz are shown in Fig. 10(a–c). At the resonant frequencies in the two major planes of E and H, the calculated radiation patterns resemble an omnidirectional and bidirectional pattern. Fig. 10 (d) shows the simulated gain vs frequency curve. A maximum gain of 5.2 dB is achieved with the proposed antenna and also provides a radiation efficiency of greater than 0.87 across the impedance bandwidth with a minimum total efficiency of 0.82.Fig. 10 Co and cross-polarization radiation characteristics of the antenna at (a) 2.2 THz, (b) 3.9 THz, (c) 5.9 THz, and (d) Gain and radiation efficiency vs. frequency curve.

Fig. 10

3.4 MIMO diversity parameters

Antenna element correlation is maintained via the parameter Envelope Correlation Coefficient (ECC). The formulas provided in Ref. [18] are used to calculate the ECC. The Diversity Gain (DG) determines the amount of transmission capacity that may be reduced without sacrificing performance when a diversity system is used. The diversity gain is 10 for an uncorrelated antenna element with an ECC value of 0. However, the proposed antenna's lower ECC value has resulted in a diversity gain of about 10 using the S-parameters approach. The ECC and DG are computed using both S-parameters as well as far-field patterns, as depicted in Fig. 11(a–d). The total return loss of the MIMO antenna is known as the total active reflection coefficient (TARC). TARC may be expressed mathematically as the square root of total incident power divided by the square root of total reflected power. S parameters can be employed to calculate the TARC for a MIMO system. Less than −10 dB is the ideal TARC for MIMO antenna systems. The MEG contrasts the energy received by a MIMO antenna in a fading environment with the energy received by an isotropic antenna. The MEG ratio has to be set to 1 for enhanced diversity performance. The channel capacity of a MIMO antenna system increases with the number of components. However, channel capacity loss (CCL), which is brought on by correlation, occurs when antennas are positioned next to one another. The CCL value for the suggested antenna over the working bandwidth is less than 0.23 bps/Hz. This importance guarantees the robust diversity performance of the antenna that is being supplied. The simulated diversity parameters are presented in Fig. 11(e–g). The analytical expressions used for calculating ECC, DG, MEG TARC, and CCL are as in equations (1), (2), (3), (4), (5), (6).(1) ECC=−∑n=1NSni*Snj(1−∑n=1N|Sni|2)(1−∑n=1N|Snj|2)

(2) ρij=|∫02π∫0π[XPR∙EθiEθj*Pθ+EϕiEϕj*Pϕ]|2∫02π∫0π[XPR∙EθiEθi*Pθ+EϕiEϕi*Pϕ]dΩ×∫02π∫0π[XPR∙EθjEθj*Pθ+EϕjEϕj*Pϕ]dΩ

where XPR is the cross-polarization ratio, Pθ and Pϕ are the incoming wave's angular density functions at θ and ϕ fields, Eθ, Eϕ are the complex envelops across θ and ϕ components.(3) DG=101−ECC

(4) MEGi=0.5ηi,rad=0.5(1−∑j=1P|Sij|)

(5) TARC=∑j=14|Sj1+∑m=24Sjmejθm−1|24

(6) CCL=−log2det(α)R

where αR is the receiving antenna correlation matrix. For four-element MIMO system, αR expressed asαR=|ρ11ρ12ρ13ρ14ρ21ρ22ρ23ρ24ρ31ρ32ρ33ρ34ρ41ρ42ρ43ρ44|

ρii=1−|∑n=1NSin*Sni|fori,j=1,2,3,4,…

ρij=1−|∑n=1NSin*Snj|fori,j=1,2,3,4,…

Fig. 11 Diversity parameters curves (a), (c) envelope correlation coefficient (ECC), (b), (d) diversity gain (DG), (e) total active reflection coefficient (TARC), (f) mean effective gain (MEG), and (g) channel capacity loss (CCL).

Fig. 11

3.5 Comparative analysis

The evaluation of a proposed wideband THz MIMO antenna in comparison to previous research, taking into account the radiation characteristics, diversity features, and simulated antenna parameters, is performed. Comparing the suggested antenna to the antenna designs in Table 4, it offers better diversity properties, a broader impedance bandwidth, and lower dimensions.Table 4 Comparison table of existing THz MIMO designs with the proposed antenna.

Table 4Ref	Size (μm2)	Operating frequency (THz)	Substrate	Isolation (dB)	Peak Gain (dB)	ECC	DG	CCL (bps/Hz)	MEG (dB)	TARC (dB)	
[19]	208.9 × 422	0.1–3	Polyimide	–	3.87	–	–	–	–	–	
[20]	310 × 310	0.1–0.20	Rogers RO4835-T	–	2.44	–	–	–	–	–	
[21]	600 × 300	0.27–0.71	Polyimide	>20	4.27	<0.01	10	<0.5	< −3	< −10	
[22]	125 × 125	0.72–10	Polyimide	>20	8.2	<0.02	9.98	<0.3	–	< −6.9	
[23]	67.5 × 67.5	1.5–10	Polyimide	>20	11	–	9.99	<0.08	–	< −10	
[24]	647.5 × 100	2.3–2.7 and 5.5–8.9	Polyimide	>20	6.75	0.00001	10	<0.1	<-3	< −10	
[25]	110 × 110	5.7–32.4	Polyimide	>20	18.9	0.002	10	<0.4	<-3	<-5	
[26]	200 × 400	3.2–4.5	Polyimide	>20	8.9	0.001	9.995	<0.04	<-6	<-10	
[27]	46 × 46	9–13	Polyimide	>25	5	0.05	10	<0.3	<-3	<-10	
Proposed Design	40 x 46	1.7–10.4	Polyamide	> 20*	5.2	< 0.004	10	< 0.23	< -3	< -10	
-- not available, *except at 1.7–2.7 THz, in this frequency range more than 10 dB isolation.

4 Conclusion

The presented study modifies the traditional rectangular patch with parasitic components to create a four-element wideband THz MIMO antenna. The inverted H-shaped ground plane design of the proposed THz MIMO antenna decouples the mutual interaction between the antenna elements. The antenna has an isolation of more than 20 dB from 2.8 to 10.4 THz, and in the frequency range of 1.7–2.7, THz isolation is more than 10 dB. The MIMO results validated the antenna's performance. According to a comparative investigation, the suggested antenna is quite good for a lot of THz applications. The future direction and challenges associated with the THz technology are even with the great advancements, a number of issues still need to be resolved, such as interaction with current THz systems, manufacturing complexity, and financial concerns. Subsequent investigations have to concentrate on surmounting these obstacles and investigating novel approaches to augment the efficiency and feasibility of THz MIMO antennas.

Data availability statement

All data supporting the findings of this study are already included within the manuscript.

Ethics declarations

Review and/or approval by an ethics committee was not needed for this study because the use of pre-existing data or materials do not require ethical review, or adherence to established ethical guidelines in the field of technical research submitted in this manuscript.

CRediT authorship contribution statement

Praveen Kumar: Writing – original draft, Validation, Software, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. V. Sivakumar: Writing – original draft, Visualization, Investigation, Funding acquisition, Formal analysis. Vidya Rao: Writing – original draft, Visualization, Validation, Resources, Investigation, Funding acquisition. Criss Tom George: Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation. Bhaskar Awadhiya: Writing – original draft, Visualization, Validation, Supervision, Funding acquisition, Formal analysis, Conceptualization. Yogeshwary Bommenahalli Huchegowda: Writing – original draft, Visualization, Validation, Resources, Investigation, Formal analysis. Yashwanth Nanjappa: Writing – original draft, Visualization, Supervision, Project administration, Investigation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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