Q-Band MIMO Antennas with Circular Polarization for Spatial and Polarization Diversity

Multiple input multiple output (MIMO) antenna systems employ multiple radiating elements at the transmitter and the receiver. In MIMO system, the transmitter sends data over different multipath propagation then the receiver combines these data through multipath. The main constraint in MIMO antenna system is the uncorrelated multipath which can be provided by using antenna elements acting independently. The MIMO technique provides uncorrelated multipath propagation to attain different diversity schemes. A variety of diversity schemes can be realized such as spatial (space) [1], polarization [2], frequency, pattern (angular) [3‐5], and transmit/receive diversity. Employing a dual-band antenna system is preferred to employing two antenna systems operating at two different frequencies because it provides a more compact solution especially for mobile handsets. On the other hand, the circular polarization is preferred for a mobile handset antenna because it allows receiving the power of the incoming signal whether it is circularly polarized (CP) or linearly polarized (LP) without being affected by the misalignment between the transmitting and the receiving antennas. The forthcoming generations of mobile handset need advanced features such as wide bandwidth, high data rate, multiple frequency operation, compact size, and light weight [6‐8].

Recently, many research papers [9, 10] focus on the design of mobile handset MIMO antennas for millimeter-wave communications. The work of [11] presents a two-port triple-band MIMO antenna of two elements where each has a single stub that can be embedded in the feed line. There are four symmetric square slots and two cuts in the ground plane to realize circular polarization. The obtained radiation patterns are quasi-omnidirectional. In [12], a dual-band dual-polarized (DBDP) MIMO antenna is designed by using square patch with three rectangular slits and a decoupling mechanism is embedded in the ground structure to enhance the isolation. The non-diagonal slit is responsible for circular polarization. This design is based on the single-feed dual-band antenna and the dual polarization is realized by using an orthogonal feed. There is another category based on using stacked structure that consists of two elements operating at two different frequency bands. In [13], a penta-band MIMO antenna is introduced by adding meander line-shaped radiator with L-shaped matching stub, and ground plane having semi-circle-shaped slot, and inverted L-shaped stub. The proposed four-element MIMO has circular polarization in only two bands. The isolation technique used in design is the inverted L-shaped decoupling structure. In [1], circularly polarized four-element MIMO dielectric resonator antenna (DRA) operating at two frequency bands is studied. We aim to achieve some objectives such as bandwidth enhancement by using a ring-shaped ceramic radiator, dual bands by using incorporation of a rectangular aperture lead, circular polarization by using conversion of rectangular to Z-shaped slot, and reduction of mutual coupling by using space diversity. The work of [14] illustrates the DRA technique for two-port dual-band MIMO antenna with circular polarization. For exciting HE modes, two probes placed orthogonal, that is, an azimuthal angular distance of \(90^\circ\), to each other are used. Moreover, to realize the required quadrature time-phase between these modes, the length of two probes must be tuned. The two ring DRAs are excited by using two arc-shaped feed lines with four conformal probes. In [15], a dual-band circularly polarized dielectric resonator antenna is used to implement two-port MIMO antenna that consists of a moon-shaped aperture. To stimulate ring-shaped DRA, L-shaped microstrip line with a serial step impedance transformer is used. Modification from cylindrical to ring-shaped dielectric resonator provides wide impedance bandwidth. Aperture is helpful to excite the dual orthogonal hybrid modes for circular polarization waves. Polarization diversity is supportive to reduce the mutual coupling between ports. A broadband antenna operating over the 38 GHz frequency band is proposed in [16], using circular patch antenna loaded by three patches between circular radius and feed-line and perpendicular pair of elliptical slots inside the circular patch. This antenna has small size with 7-dB gain and 90% radiation efficiency. In [17] and [18], a compact multiband antenna operating at 28, 38, and 55 GHz is introduced. The antenna has an umbrella-shaped patch with high gain and efficiency. A wideband, high-gain, low-profile, and high-efficiency fractal antenna operating at 39 GHz is investigated in [19]. The antenna is designed on a Rogers RT/duroid 5880 with a compact size of 15 × 15 × 0.79 mm.

This work introduces two-element as well as four-element MIMO antenna systems to achieve spatial diversity as well as polarization diversity for millimeter wave (mm-wave) applications. A dual-band circularly polarized (DBCP) microstrip patch antenna is used as a single element to construct the proposed MIMO antennas. The single element is designed as a main patch and a parasitic patch. The main patch has circular geometry with two square slots at the center and two notches on the circumference. The parasitic patch consists of four parasitic elements indirectly fed by capacitively coupling to the main patch. To produce circular polarization, the structure of the single element antenna is symmetric about an axis that is inclined to the feed line at angle of \(45^\circ\). For impedance matching, the main patch is fed through a microstrip line with tapered geometry.

Two-element MIMO antenna systems are proposed to produce spatial diversity: face-to-face and side-by-side arrangements. The four-element MIMO antennas are proposed to produce both spatial and polarization diversity at the same time. If the elements have the same sense of polarization, then spatial diversity is obtained, whereas polarization diversity is obtained if the elements have different senses of polarization. The proposed MIMO antenna systems have two operational frequency bands centered at 38 and 50 GHz and produce circular polarization over the two frequency bands. The diversity schemes provided by these MIMO antennas are investigated by the CST simulator where the envelope correlation coefficient (ECC) and the diversity gain (DG) are investigated and demonstrated.

The presentation of this work is organized as follows. Section 2 presents the design of the single-element dual-band CP antenna. Section 3 introduces the proposed MIMO antennas with the different configurations. Section 4 describes the fabrication process and the experimental setup for measurements. Section 5 presents the simulation and experimental results with elaborate discussions for performance assessment. Section 6 introduces a summary of this work and some comparisons to the achievements of the other published papers. Finally, Section 7 gives the conclusions related to the present work.

The geometry of the CP patch antenna proposed as a single element for the two-element and four-element MIMO antenna systems is depicted in Fig. 1. This antenna consists of a circular patch that acts as the main radiator which is reactively loaded by four parasitic patches as seen in Fig. 1a. To produce circular polarization, two square slots are made near the patch center and two notches are etched at the patch edge to get the line of symmetry of the antenna structure having a slope of \(45^\circ\) with the center line of the feeding microstrip line. Also, the ground structure is defected by etching two square slots as shown in Fig. 1b. Thus, the overall patch and defected ground geometry have a line of symmetry making \(45^\circ\) with the center line of the microstrip line feeder. This symmetry produces two degenerate radiating modes (resonances) of the cavity between the circular patch and the ground plane. The radiated fields of the two modes have nearly equal magnitudes, orthogonal orientations, and in-phase quadrature; these conditions produce a circularly polarized field in the far zone of the antenna. The antenna’s geometrical parameters should be set to the appropriate values to satisfy these conditions. The antenna impedance is matched at the desired frequencies by the tapered microstrip line feeder. The dimensions of the tapered region of this line, \({W}_{T}, {L}_{T}\), and \({W}_{F}\), should be set to get the antenna impedance matched to \(50\Omega\) source at the required frequencies. The parasitic elements help to get the antenna impedance matched over a higher frequency band. The Y-shaped slits at the corners of the parasitic elements improve the axial ratio and increase its 3-dB bandwidth. The geometric symmetry of the antenna around an axis inclined to the feed line at an angle of \(45^\circ\) produces right-hand circularly polarized (RHCP) field in the far zone. If this geometry is mirrored about the centerline of the microstrip feeder, left-hand circularly polarized (LHCP) fields are produced in the far zone. It will be shown, later, that two antennas of opposite senses of polarization can be used in a four-element MIMO antenna configuration to provide polarization diversity in addition to the spatial diversity.

The optimal dimensions of this antenna are listed in Table 1. It should be noted that these dimensions have been obtained through an extensive parametric study that has been achieved using the CST simulator. The proposed MIMO is designed using a substrate material of the type Rogers RO3003. The substrate has a thickness \(h=0.25\mathrm{ mm}\), loss tangent \({\text{tan}}\delta =0.001\), and dielectric constant \({\varepsilon }_{r}=3\).

In this section, two-element and four-element configuration of MIMO antenna systems are proposed to produce spatial and polarization diversity. The design of the proposed MIMO antennas are presented in the following subsections.

In this section, the designs of the proposed MIMO antennas are described in detail. Two-element MIMO antennas arranged in side-by-side and face-to-face configurations are designed to produce spatial diversity. Also, four-element MIMO antenna system is designed to produce both spatial and polarization diversity. The following two subsections are dedicated for the description of the different types of the proposed MIMO antenna configurations.

Prototypes are fabricated for experimental evaluation of the proposed single-element and MIMO antennas’ performance. The reflection coefficient at the antenna input ports and the radiation patterns are measured and compared to the simulation results.

The simulation results and experimental measurements are presented and discussed in this section. The main concern of such presentations is the performance evaluation of the single-element antenna as well as the MIMO antennas with the different configurations proposed in the present work.

Comparisons between the performance of the proposed MIMO antennas and that of other MIMO antennas presented in recently published papers are listed in Table 2. These comparisons show some advantages of the proposed MIMO antennas relative to those presented in the other publications. The main advantage of the proposed MIMO antennas is that they provide two bands of circular polarization. Another important advantage over the other published papers is that the proposed four-element MIMO antenna provides two types of diversity (spatial and polarization), whereas the other antennas offer only spatial diversity. The other advantages are the smaller size and high isolation among the multiple ports. However, the main drawback of the proposed MIMO antennas relative to other published designs is the narrow bandwidth. Nevertheless, the two frequency bands achieved by the proposed MIMO antennas can be considered wide enough for operation in applications of future wireless communication.

A novel CP low-profile printed antenna has been designed to produce circular polarization over two frequency bands around \(37.8\) and \(50 {\text{GHz}}\). Two-element side-by-side and face-to-face MIMO antenna configurations have been proposed to attain spatial diversity. A four-element MIMO antenna has been proposed to achieve both spatial and polarization diversity at the same time. The proposed dual-band CP single-element, two-element MIMO, and four-element MIMO antennas have been fabricated for experimental study. Both the numerical and experimental investigations have shown that the mutual coupling between any two ports of the proposed MIMO antennas is below \(-25 {\text{dB}}\). Also, for any two ports it has been shown that the ECC is below \(1\times {10}^{-7}\) and the diversity gain is higher than \(9.99\). The impedance matching bandwidths (for \(\left|{S}_{11}\right|<-10 {\text{dB}}\)) have been shown to be \(1.53\) and \(1.88\) GHz at \(37.8\) and \(50\mathrm{ GHz}\), respectively, and the corresponding 3-dB axial ratio bandwidths have been shown to be \(700\) and \(130\mathrm{ MHz}\), respectively.