An Isolation Improvement for Closely Spaced MIMO Antenna Using λ/4 Distance for WLAN Applications

Abstract

An isolation improvement for a closely spaced quad port multiple-input multiple-output (MIMO) antenna for WLAN applications is presented. The proposed antenna consists of four L-shaped monopole microstrip patch antennas with a rectangular slot at the center and truncated edges at the corner of each patch element. Interelement isolation is improved by more than 12 dB by considering the patch size, ground size, and distance between the patch elements. The designed antenna uses λ/4 distance between the patch elements. The slot in each patch improves the bandwidth, and truncated edges at the corner of the patch improve the impedance matching of the antenna. The proposed MIMO antenna is designed with the overall dimensions of 45.5 mm × 45.5 mm × 1.96 mm. The MIMO parameters for the proposed technique, which include envelope correlation coefficient (E.C.C.) are less than 0.5, and the diversity gain (D.G.) is 10 dB, suggesting this antenna’s suitability for MIMO operations. The average peak realized gain (A.P.R.G.) and radiation efficiency (RE) is 2.8 dBi and greater than 80% for the prescribed band of interest. The antenna element is fabricated and tested using microwave analyzer N9917A. The measured result seems to be in good contact with the simulation. The proposed MIMO antenna is well-suitable for WLAN applications.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Current wireless communication depends on the data rate and quality of information transferred. These factors were considered and identified that multiple-input multiple-output technology could satisfy these needs. MIMO technology overcomes specific effects like scattering, fading and multipath transmission, and reception. The researcher is investigating new methodologies to operate multiple antennas due to the ever-increasing demand for superior data rates to cater to which MIMO technology is considered one possible candidate [1], and the physical limitations of wireless devices pose few challenges. Mutual coupling between antennas lowers their efficiency since some of their power is emitted to the nearby antennas.

Furthermore, this coupling may cause them to interfere with one another. As a result, antenna isolation improvement techniques have become a highly required topic. Limiting radiation in the propagation direction and increasing the distance between the antennas are two common ways to improve isolation. However, this strategy is inefficient due to the tiny amount of space provided for antennas in mobile devices. Several studies on spatial diversity and MIMO systems have been conducted to increase isolation performance. With the usage of MIMO technology, not only are the factors reduced but this technology also increases the capacity of channels without making any modifications to the previous structure of the design.

The antenna designed using this MIMO method depends on applications like Wi-Fi, Wi-Max, WLAN, and I.T.U. Bands for wireless devices [2]. However, the speed of data transmission and channel capacity must be considered. On empathizing, mutual coupling arises when multiple antennas are placed. The coupling effect should be controlled for implementing many antennas in the system. It takes more time to transmit the data with minimum antennas, and an ideal methodology can be used to improve the antenna performance. The abovementioned limitations can be overwhelmed by implementing multiple-input multiple-output (MIMO) antennas [3]. Although the usage of the MIMO antenna accomplishes higher isolation standards and a common correlation effect, two crucial things have to be considered when designing a MIMO antenna [4]. First, the antenna size should be compact for wireless applications [5]. Second, the mutual coupling should be limited to closer radiating elements [6]. In this paper, we proposed a low profile, L-shaped resonator with a rectangular slot at the center, and truncated edges at the individual corner of the patches are used to enhance isolation for interelement antennas and also improve the diversity performances of the MIMO antennas, which includes the MIMO parameters like mean effective gain (M.E.G.), diversity gain (D.G.), and envelope correlation coefficient (E.C.C.).

This paper is systematized as follows. Section II implicates the related works, where the coaxially fed, linearly polarized, low profile, L-shaped resonator with a rectangular slot on the patch is analyzed. Section III discusses the configuration and geometry of the MIMO antenna. Section IV discusses the equivalent electric circuit model of the proposed MIMO antenna. Section V investigates the performance of the MIMO antenna with results and discussions. Finally, in Section VI, the conclusion is discussed.

2. Related Works

The following section explores the various antenna and MIMO parameters considered for designing an efficient antenna structure. Many researchers found different techniques to improve the mutual coupling for closely spaced antenna elements to cancel the self-induced currents by antenna elements. For instance, by introducing the defected ground structure (DGS) [7–9], complementary split ring resonator (CSRR) [10–12], neutralization line (NL) [13–15], frequency reconfigurable (FR) [16–18], electromagnetic band gap (EBG) [19–22], metamaterial (MM) [23–25], decoupling network (DN) [26–28], dielectric resonator antenna (DRA) [29–31], and parasitic strip (PS) [32, 33] and introducing different shapes like inverted L [34], modified T shape [35, 36] F shape [37], and meta surface [38–44] improve mutual coupling and the isolation of more than 20 dB is achieved. However, MIMO antenna parameters like diversity gain (D.G.), envelope correlation coefficient (E.C.C.), and radiation efficiency (RE) need to be considered for improving the channel capacity with any additional power [45].

Since the same isolation mechanism typically does not function across various frequency bands, designing a dual-band, MIMO antenna presents its unique set of challenges. Multiple multi-input multi-output (MIMO) antennas have been previously reported [46]. Adding numerous branches and a shorting pin improves isolation between two adjacent dual-band monopole antennas [47]. In [48], two folded monopoles employed with mutual coupling decreased through transmission lines on the upper surface and slots on the ground plane. The monopole antenna in [49] is of the four-shaped variety. When the defective ground structure is introduced, the isolation increases. In [50], a multi-input multi-output (MIMO) antenna is designed using a dual-band monopole antenna, an eigenmode feed network, and a sophisticated reactive decoupling network made up of a four-port 180° hybrid coupler. C-shaped and T-shaped slots, fed by microstrip lines, are used to produce dual band [51]. The mutual coupling of its constituent parts is reduced because of introducing the six sets of slits etched into its ground plane. In [52], two closely spaced chip antennas are used to create a dual-band MIMO antenna, and a pair of open-ended stubs is employed as a decoupling network at a higher frequency. In [53], the authors describe a dual-band MIMO antenna made up of two parallel folded branch monopoles with connected feed and an integrated isolation mechanism. Meandered monopoles are utilized to create dual-band antennas in [54], and a folded Y-shaped isolator is employed as a decoupling network between them. In [55], a dual-band MIMO antenna with two-port and four-port operations at the lower and upper bands, respectively, is demonstrated. Two λ/4 slots are etched at two resonant frequencies to improve separation. However, the abovementioned designs take up a lot of space and provide isolation of only around 20 dB at most. Thus, building a planar miniaturized dual-band MIMO antenna with good isolation between the ports at both operating bands is difficult.

The personal computer memory card found in laptop computers and other portable mobile devices are two examples of where MIMO antenna systems are put to use in WLAN mobile terminals. The antennas described in [56] are bulkier than those used in portable electronics. Positioning antennas in close proximity to one another result in mutual coupling, which decrease the antenna’s performance. Many academics have looked into different methods to increase the isolation between the antenna elements in handheld devices. Using a combination of a rectangular slot ring and an inverted T-shaped slot, Meshram et al. [57] suggested a unique quad-band diversity antenna for LTE and Wi-Fi applications with excellent isolation. Lee et al. positioned a folded resonator atop the antenna to decrease the interaction between the antenna and the ground. To further increase the separation between the ports, a line is linked to the feeding strips of two P.I.F.As. To effectively reduce electromagnetic coupling between closely spaced antenna parts, researchers in [58] created single negative magnetic (M.N.G.) metamaterials. Min et al. [59] suggested a concept for wireless broadband MIMO and Personal Communication Systems (P.C.S) antennas that use a projected ground plane to mitigate the effects of mutual coupling.

Its ports are known to be particularly susceptible to mutual coupling. Thus researchers have looked into ways to mitigate this problem using parasitic elements [60] and decoupling approaches. All of the aforementioned methods in the literature [61] have fabrication issues, and some of them even decrease the antenna’s reflection coefficient.

The research proposes a compact 2 × 2 MIMO antenna for WLAN band applications operating at 2.4 GHz. The proposed MIMO antenna consists of 4 identical L-shaped structures with a rectangular slot on the patch and introduces a small rectangular element as a ground plane.

3. Antenna Geometry

The proposed structure comprises four L-shaped monopole radiators and a small rectangular ground plane shown in Figure 1. The structure consists of 3 layers with ground and patch made up of copper material with a thickness of 0.1 mm, and a flame retardant (FR-4) substrate with material values (ε_r = 4.4 and tanδ = 0.02) is placed between the ground plane and patch elements with a thickness of t = 1.96 mm. The dimensions of the individual patch antenna chosen to be 16.251 mm × 25.733 mm are printed on the top of the FR4 substrate, and the individual ground plane with the size of 12 mm × 5.145 mm is printed on the bottom of the substrate. Each of the four elements is terminated by a 50 Ω coaxial connector located at the verge of the ground plane.

[figure(s) omitted; refer to PDF]

3.1. Antenna Design Process

Initially, the design starts with an L-shaped monopole radiator of size 26.61 mm × 17.12 mm and a ground plane of size 5.14 mm × 12 mm, and it is represented as Design I. The antenna operates at the frequency of 2.4 GHz with an S₁₁ value of −23.4 dB, S₂₁ value of −59.8 dB, S₃₁ value of −53.1 dB, and S₄₁ value of −51.6 dB. For the first design, isolation is high for opposite patches compared to the adjacent radiator. This is because the distance between radiators 1 and 2 is high when compared with adjacent radiators 13 and 14. Furthermore, in Design II, a rectangular slot is created at the center for all the radiating elements to improve the isolation between adjacent elements. As a result, it improves isolation for only one adjacent element with an S₄₁ value of −54.53 dB. However, it still reduces the isolation for the opposite element with an S₂₁ value of −51.3 dB and another adjacent element S₃₁ value of −49.6 dB. Finally, in Design III, the truncated corner at one edge for all the radiating elements is created to improve the reflection coefficient and isolation parameter. As a result, design III improves the reflection coefficient S₁₁ value of −28.8 dB, S₂₁ value of −60.54 dB, S₃₁ value of −66.3 dB, and S₄₁ value of −58.4 dB. An isolation improvement for the proposed design is represented in Figure 1. The simulated scattering parameters for Designs I, II, and III are shown in Figure 2.

[figure(s) omitted; refer to PDF]

From Figure 2, it is observed that the simulated −10 dB impedance bandwidth lies between 2.31 GHz and 2.50 GHz with 2.4 GHz as the center frequency, whereas the mutual coupling among the various elements, denoted by $S_{21}$, $S_{31}$, and $S_{41}$, are found to be less than −58 dB in this entire band.

The proposed MIMO antenna with notations is mentioned in Figure 3, and their optimized values are given in Table 1.

[figure(s) omitted; refer to PDF]

Table 1

The values of the proposed MIMO antenna.

Parameters	Values (mm)
L1	16.25
L2	12
L3	3
L4	28.35
W1	25.73
W2	3
W3	5.41
W4	2
G1	12
G2	5.14
D	7
Lsub	45.5
Wsub	45.5
L5, W5	0.87

4. Equivalent Circuit Model

The lumped-element (R.L.C.) model of the proposed MIMO antenna extracted using the advanced design system (A.D.S.) tool is shown in Figure 4. The mutual coupling between any two elements is denoted by a series inductor-capacitor (L.C.) circuit. The R, L, and C values are given in Table 2.

[figure(s) omitted; refer to PDF]

Table 2

R.L.C. values of the equivalent circuit design.

Components	Values
R	16 Ω
L	18 nH
C	10.431 pF
R1	1 Ω
L12	2.53 nH
C12	1.298 pF
C24	9.66 pF
C23	2.34 pF
L24	19.176 nH
L14	3.762 nH
L13	13 nH
C13	10.9 pF
L34	26 nH
C34	9.23 pF
C14	2.33 pF
L23	2.365 nH

5. Results

Figure 5 shows the manufactured prototype of the proposed MIMO antenna with the measurement setup.

[figure(s) omitted; refer to PDF]

5.1. S-Parameters

Figure 6 shows the measured S-parameter values of the proposed antenna. The proposed antenna is designed at 2.4 GHz, but the dip frequency occurs at 2.24 GHz for the fabricated antenna. This is because the dielectric thickness used in the simulated design was 1.96 mm, and the MIMO antenna is fabricated on an FR-4 substrate with a thickness of 1.6 mm. There is a slight deviation in return loss parameters because of soldering effects in the S.M.A. connectors.

[figure(s) omitted; refer to PDF]

To visualize the mutual coupling effect between antenna elements, the surface current distribution of the MIMO antennas at 2.4 GHz is depicted in Figure 7. Even if the antennas are highly close to each other and if one antenna is excited, that radiated antenna will not interfere with the remaining antennas.

[figure(s) omitted; refer to PDF]

Exploiting the symmetry condition, an excitation signal is given at port 1. The remaining three ports are terminated with matched loads to measure antenna parameters like the radiation pattern, bandwidth, and peak gain. The distance between the ports is vital when determining how far the antenna elements are separated. Generally, when the antenna components are separated widely, there is a high isolation level. The proposed antenna shows high isolation between closely spaced antenna elements, and no mutual coupling happened between the multiple antennas, as shown in Figure 7.

5.2. Radiation Pattern

The simulated radiation patterns are plotted for only one port (port1) for ϕ = 0° and ϕ = 90° planes, while all the remaining ports are symmetrical in the MIMO antenna design. Figure 8 shows that the main beam directs maximum in the direction of 0° in both the planes (E-plane and H-plane).

[figure(s) omitted; refer to PDF]

The simulated radiation efficiency and gain versus frequency are plotted in Figure 9. From the figure, the peak gain of 2.8 dBi is obtained, and radiation efficiency is greater than 80% for the 2.4 GHz WLAN band.

[figure(s) omitted; refer to PDF]

The E.C.C. is a vital parameter to show the amount of correlation formed between interelement spacings. Here, the E.C.C. is calculated based on the scattering-parameter values [10, 37]. The calculated E.C.C. between the interelemental spacing for the MIMO antenna is shown in Figure 10.

[figure(s) omitted; refer to PDF]

The diversity gain is calculated based on the correlation coefficient values [23]. Any diversity scheme which improves the signal-to-interference ratio is referred to as diversity advantage. Since the antenna has a higher range gain value, it achieves greater separation and vice versa. Figure 10 shows a graphical view of the current MIMO antenna’s diversity gain (D.G.). A well-defined comparison table of related works has been included to claim novelty for the proposed MIMO antenna. By the literature survey of MIMO antennas specified in Table 3, it witnesses well that the proposed MIMO has attained an improved isolation in contrast to antenna [8, 13, 21, 26, 38], low E.C.C compared to references [13, 16, 21, 29, 32], better radiation efficiency compared to references [8, 10, 16, 20, 26, 27], improved gain compared to referenes [7, 8, 10, 13, 27, 29], and smaller in size compared to references [7, 8, 10, 16, 20, 23, 27, 29, 32].

Table 3

Comparison of the proposed antenna with an existing published antenna.

References	Dimensions (mm3)/material	Isolation levels (dB)	Gain (dBi)	Radiation efficiencies	E.C.C	D.G (dB)	No. of ports	Applications
[7]	60.2 × 60.2 × 1.6/FR-4	≥−25 dB at 2.45 GHz	2.2	81%	≤0.1	9.94	4	WLAN
[8]	80 × 80 × 0.76/Rogers 4350B	>10 at 2.4–2.8	>1.7	>67	<0.1	NG	8	WLAN
[10]	100 × 50 × 0.8/FR-4	≥−18 dB at 2.4∼2.5 GHz	0.8	29%	≤0.3	9.53	4	I.S.M
[13]	30 × 65 × 1/FR-4	≥−15 dB at 2.4∼2.5 GHz	2.1	81%	≤0.006	9.8	2	WLAN USB dongle
[16]	90 × 50 × 0.8/FR-4	≥−47 dB at 2.3∼2.4 GHz	1.99∼2.78	48.43∼73.1%	≤0.0056∼0.110	9.42∼9.97	4	WiMax
[20]	95 × 95 × 2.284/Rogers RO4350 B	≥−25 dB at 2.395∼2.42 GHz	5.12	56.57%	NG	NG/NG	4	ISM
[21]	18.5 × 18.5 × 1.27/Rogers 6010LM	−15.99 at 2.4–2.48	−15.18	NG	0.0025	NG	4	Implantable
[22]	20.5 × 20.5 × 1.6/FR4	−32 at 2.45 GHz	2.6	55.44%	0.0081	NG	4	NG
[23]	119 × 119 × 22.5/Rogers RO4003, RO4450 B	≥−42 dB at 2.395–2.45 GHz	NG	NG/NG	<0.02	9.89	4	ISM
[26]	112 × 55 × 1.6/FR-4	≥−15 dB at 2.4 ∼2.48 GHz	NG	66∼75%	0.23∼0.13	9.73∼9.99	2	I.S.M
[27]	72.4 × 20 × 0.8/Rogers RO4350 B	≥−27.6 dB at 2.18 ∼2.65 GHz	1.39	66∼70.5%	0.08∼0.09	9.96∼9.95	2	I.S.M
[29]	80 × 80 × 01.6/FR-4 substrate	∼20 dB at 2.5 GHz	NG	NG	0.054	9.68	2	LTE
[32]	140 × 120 × 1.5/FR4	−20 at 2.4–2.4835	NG	NG	<0.1	NG	4	WLAN
[38]	34 × 18 × 1.6/FR4	−15 at 2.45 GHz	1.26	76	0.12	NG	4	LTE 2300 and I.S.M
Proposed antenna	45.5 × 45.5 × 1.96/FR4	−58.37 at 2.4 GHz	2.84	90	0.0054	10	4	ISM

Bold indicates proposed antenna’s parametrical values.

6. Conclusion

A compact L-shaped monopole radiator with a rectangular slot at the center of the patch and truncated corners at one edge is designed and validated. A comparison is made between the existing antenna and the proposed one’s performance. Comparisons with similar MIMO antennas shown in Table 3 have reported that the proposed one is small in size and offers excellent isolation without requiring a decoupling structure. In terms of expected crossover, the diversity performance is 0.0054. The MIMO antenna has high port-to-port isolation, low E.C.C., and high D.G. obtained within a compact volume of 45.5 mm × 45.5 mm × 1.96 mm. Based on these characteristics, it seems to be a good fit for WLAN uses involving Wi-Fi, I.S.M., and Bluetooth.

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