Techniques for Compact Planar MIMO Antennas

*Yiying Wang*

### **Abstract**

MIMO Technology has promoted the developments of various antennas, then the planar antenna will be one of the main directions to satisfy the future compact requirement of the 5G+/6G communications. This chapter introduces different types of the planar antenna and summarizes the implicit compact techniques, where the related techniques like the diversity and the reconfigurable are not included owing to they are the inherent properties of the MIMO antennas. These antennas contain the patch antenna, slot antenna, dipole/monopole antenna, loop antenna, cavity antenna, Yagi-Uda antenna, fractal antenna, UWB antenna, PIFA etc., and their deformations to the specific purposes. On the contrary, the implicit compact techniques are not so explicit as the antenna configurations, but they are classified to be the close-spacing structure without decoupling, owing to the decoupling is not the necessary requirement of MIMO application, decoupling technique of spacing reduction, meandered line technique, multi-element method, co-radiator/co-location design, fractal antenna, and radiator-cutting antenna. Besides, the corresponding techniques for the compact design are also concluded, including the mode-cutting method, fractal technique, characteristic mode analysis, and the optimization algorithms.

**Keywords:** MIMO, 5G+/6G, planar antennas, compact techniques, integration

### **1. Introduction**

Owing to the prominent advantages compared with the conventional single-input single-out (SISO) system, the MIMO technology has been extensively applied to many scenarios, in which the antenna with beamforming is one of the key features in order to realize the multiple path communications. Consequently, the multi-beam, the multipolarization, or the related diversity or the reconfigurable techniques are the inherent properties of the MIMO antennas. To satisfy the requirement of MIMO communication, many antenna types have been employed, including the high-profile 3D antennas, such as the dielectric resonator antenna (DRA) [1–3], helix antenna [4], structure-loaded antenna [3, 5–7] or multi-element antenna [8, 9], and the other common 2D planar antennas. On the other hand, the 5G+/6G technology puts forwards the new compact, easy-fabricated and easy-integrated requirements for the antenna development resulting in the planar antennas will be one of the main directions in the future. Therefore, the focus of this chapter is on the introduction of planar antennas and especially the implicit techniques on how to design the compact structure.

Many planar antenna types have been proposed for the MIMO applications, but not all of them will be discussed in this chapter considering the related compact techniques. The planar antennas involved are patch antenna [10–22], slot antenna [23–30], dipole/monopole antenna [31–48], loop antenna [49–58], ultrawideband (UWB) antenna [59–71], Yagi-Uda antenna [72–77], cavity antenna [78–82], fractal antenna [83–90], and the planar inverted-F antenna (PIFA) [91–104]. These antennas do not appear in isolation, they often combine with other types for the specific purpose, such as, both the patch antenna and the dipole antenna were used to realize the linear and circular polarization design [11], the slot antenna [28] and the fractal antenna [83] also belong to the UWB antenna, and the radiator of UWB antennas [59–62] is monopole. However, we distinguish them according to their explicit features in this chapter as the above categories, and the relatively simple antenna structures are picked up from the similar works. Additionally, though these are planar structures, they can be used in the 3D situations [27, 43, 102] like in the mobile application. All selected types are the printed antennas, they will be good candidates for the future 5G+/6G applications from the view of easy fabrication and integration.

The compact design is always the research focus of MIMO antennas, many techniques have been employed to compress the volume of structure. However, we face a common problem that the antenna performance is affected because of coupling when they are close to each other. There are two general ways to solve this problem, one is that we need not care about the coupling but put them closer if the coupling is not too significant, which is because the MIMO antenna technique does not require the elements to work at the same time, the coupling will not affect the work status of MIMO system; the other is using the decoupling technique to realize the compact design, even so the antenna performance is also affected when the elements are close enough.

In addition to the above close-spacing compact techniques, changing the antenna shape is another conventional way to realize the compact design, such as, using meander line for the dipole/monopole or the slot antenna to save the spacing, and by the combinations with different antennas to change the shape, like the electric and magnetic dipoles, the patch and slot, the PIFA and slot etc. Besides, the fractal technique and the optimization algorithm with constraints are often implemented to change the antenna shape. And we can physically reduce the antenna size by performing the corresponding cutting based on the related modes.

In this chapter, we will focus on the introduction of the corresponding compact techniques of the planar antennas, including the close-spacing and the shape change methods. Therefore, the rest is organized as follows. Section 2 introduces the corresponding general compact methods implicit in different antenna types, including the close-spacing no-decoupling design, decoupling design, meander line method, multiple antenna structure, co-radiator/co-location design, fractal antenna, and the mode-cutting technique. The fundamentals for compact designs, including modecutting method, fractal technique, characteristic mode analysis (CMA), and optimization algorithm, are summarized in Section 3, which will be helpful to the future compact researches owing to the physical reduction of antenna size. Then, the conclusions are shown in Section 4.

### **2. Compact antenna techniques**

Though different antenna types have been designed for the compact purpose depending on the present development trend, the compact techniques are similar accompanied by the types. We summarize the corresponding compact techniques in this section.

### **2.1 No-decoupling compact designs**

The purpose of MIMO antenna is using the multiple path transmissions to realize the high-efficiency and high-capacity communication, which means the antenna may not work simultaneously and then the coupling is not a main concerned focus. In other words, we need not care about the mutual couplings among elements so seriously in some cases when we want to realize the compact design but put them closer. Moreover, the coupling can be reduced by properly arranging the positions of elements to form the orthogonal polarization etc.

In [101], even the cross line connected with four elements exists to improve the isolation, the minimum isolation is up to 9.7 dB. The similar phenomenon happens in [36, 80] where no-decoupling structures were used. The four-element 90 degrees rotated structure of [36] is shown in **Figure 1**, from which we know the coupling is significant and the authors gave that of about 12 dB. The shorting pins and the 90 degrees rotation also do not reduce the mutual coupling seriously at some frequencies in [80], which is about 13.3 dB, the corresponding configuration is shown in right subfigure.

When the spacings become larger, the coupling will be smaller [29, 35] where the orthogonal polarization techniques were employed as well. Using the shorting pins [78] or slot [81] to stop the current flowing to the neighbor element in the cavity antenna is an efficient way to reduce the coupling. And we can obtain the lower coupling by exciting the neighbor elements with the differential modes instead of additional decoupling structure [14, 46, 88].

#### **2.2 Compact decoupling techniques**

Generally, we should consider the mutual couplings in the MIMO antenna design which decreases the consequent undesirable problems of the related system. Except the above differential mode method, we often reduce the mutual coupling for the close-spacing elements from two aspects, one is from the source and the other in the transmission process. The decoupling techniques of patch antenna can illustrate these well [15–18]. When the spacing is large enough, the surface current on the ground plane affects the mutual coupling rarely so that a proper metamaterial absorber put between the patch is enough to stop the surface and the radiated waves in the decoupling process [17]. As the spacing becomes closer, the surface current on the

**Figure 1.** *No-decoupling MIMO antennas: [36] (left) and [80] (right).*

ground plane diffuses to the neighbor element, and the near-field coupling to the other patch generates, so the defected ground structure (DGS) and/or resonator techniques between patches can reduce the couplings significantly [15, 16, 18]. **Figure 2** shows the simple decoupling structure in [15], in which the slot through the substrate and ground plane was curved. The surface current on the ground was cut off and the resonator was form between patches resulting in the reduction of mutual coupling. In order to reduce the mutual coupling of patches, literature [18] used another way, where the parasitic elements are put closer than the spacing between patches so as to induce the power to the parasitic metal rather than the neighbor patch. **Figure 3** shows the current distributions on the top layer of patch antenna before and after using parasitic technique. It is clear that the coupling to the neighbor element is suppressed.

The ideas were implemented into the PIFA antenna [92, 93, 96], loop antenna [49, 52, 53], slot antenna [69, 70], and UWB antenna [60, 62, 69, 70] and so on. **Figure 4** shows the corresponding antenna and the decoupling structures. Both the PIFA and the UWB suppress the coupling in the wave propagation process, and the other two cuts off the surface currents.

The neutral line technique is another normal method to reduce the coupling in the monopole [45] and UWB [60] antennas. It does not destroy the structure of ground plane but introduce the neutral line between elements. The decoupling structure of [60] is shown in **Figure 5**, where the circular disc of neutral line allows several

**Figure 2.** *DGS and resonator techniques to reduce the coupling [15].*

**Figure 3.**

*The comparisons of current distributions on the top layer [18].*

*Techniques for Compact Planar MIMO Antennas DOI: http://dx.doi.org/10.5772/intechopen.112040*

**Figure 4.**

*The antennas and decoupling structures (from left to right): PIFA [96], loop [52], slot [69], and UWB [62].*

**Figure 5.**

*The antennas and the neutral line decoupling structures of [60].*

decoupling paths to cancel the coupling current on the ground so that the UWB decoupling is realized, and with the help of slot in the circular disc, the highest decoupling frequency can be tuned to 5 GHz.

If the cavity is cut properly, the antenna can realize the self-isolation without any additional structure. Such as in [81], the authors cut a slot symmetrically for the quarter cavity, the consequent 1/8 mode cavity antennas have good isolation. This self-isolated technique is also applied in the loop antenna, and the size is reduced by means of the introduction of two vertical stubs in the loop [53].

#### **2.3 Meander line antennas**

It is the conventional way to restrict an antenna to a fixed area by meandering the radiator. It is often adopted in the dipole/monopole, slot, and loop antennas, where the corresponding method is relatively simple, that is, we just adjust the meandered sections to resonate at the desired frequency as the straight one. We can find many meander line techniques employed in the MIMO antenna designs [25, 36, 38–40, 42, 47, 49, 56, 57, 67].

The dual-band is realized by two different length slots, the authors meandered the longer slot which makes the two slots have the similar length in the horizontal direction [25]. The arms of dipole/monopole are meandered as well for the compact design [36, 38–40, 42, 47, 67]. For the loop antenna, the authors put two loops on the different layers and the smaller one is embedded into the bigger one [49], while the loops meandered inward are implemented for the rectangular [57] and the Alford [56] loops, respectively.

#### **2.4 Multiple antenna structures**

Multiple antenna structure, or the hybrid structure with different antenna types, also can obtain the compact design for the MIMO antenna application. This

combination not only saves the spacing, but also suits for the realization of multi-band or multi-polarization.

In [11], the patch antenna combines with the dipole antenna to realize the polarization diversity design, where the chamfered-edge square patch with an offset feed is used to obtain the circular polarization and the two dipoles are responsible for the linearly polarized radiation. Literature [32] also shows the combination of both patch antenna and the monopole antenna with the same ground, but the patch antenna is fed by the electromagnetic coupling.

Two pairs of slot antennas are etched in the patch to realize the dual-polarized radiation, and good isolation is obtained due to the proper feeding positions [19]. The authors put the IFA and the slot antenna together for different LTE bands in the mobile application [104]; and two slots with different lengths are put close for the dual-band radiation [25].

#### **2.5 Co-radiator/co-location antennas**

To some extent the co-radiator and the co-location antennas resemble the multiple antenna structure. For the co-radiator MIMO antenna, there exists one common radiator but excited by different ports, while the co-location antenna assembles different antennas in the fixed area. These two types are similar to some diversity antennas of one radiator but different feeds and the multi-band antenna with one radiator, respectively. In other words, the co-radiator/co-location antennas are not so unfamiliar things but they just have the common property for a kind of antennas.

The antenna of [105] shown in **Figure 6** explain the co-radiator antenna clear, where four ports excite the common radiator and the slot of ground plane is used to improve the isolation. The configuration of [106] resembles to this work, but use two ports to excite the co-radiator, the isolation is improved by means of the T-shape slot and the irregular stub extended from the ground plane.

**Figure 6.** *Co-radiator antenna of [105]: (a) Geometry and (b) prototype.*

#### *Techniques for Compact Planar MIMO Antennas DOI: http://dx.doi.org/10.5772/intechopen.112040*

The circular co-radiated patch is employed in [107], the configuration is shown in **Figure 7**. The authors analyzed the two close modes TM02 and TM11 with the monopole-like and patch-like radiations, respectively, and put several vias around the center to make sure the resonant frequencies are the same. Then, they used the center port to obtain the monopole-like radiation, the other two ports excite two orthogonal patch-like patterns.

The application of the co-radiator technique in the mobile terminal was investigated in [108], we repeat the configuration in **Figure 8**. The loop is the co-radiator of port 1 and port 2 as shown in **Figure 8c**. The two ports are put at the center of loop, and port 1 feeds the loop directly while port 2 feeds the loop by a microstrip line. Owing to the odd and even modes appear by the two ports, the high isolation is achieved in the design.

**Figure 7.** *Circular co-radiator antenna of [107].*

#### **Figure 8.**

*Co-radiator antenna applied in mobile terminal [108]: (A) front view, (B) back view, (C) details of the upper structure, and (D) details of the lower structure.*

The co-location antenna is the same as the co-radiator antenna in a way as in [107], where different radiation patterns are generated by the different ports at different positions but the radiator is not changed. However, we differentiate them in this section by introducing the co-location antenna with different radiators [30]. The corresponding antenna configuration and the current distributions at different frequencies are shown in **Figure 9**, where the two ports are placed on the top and left sides. When the lower frequency is excited at any port, the square-ring slot works, the edge branch radiates for the higher frequency.
