Planar Four-Port Dual Circularly-Polarized MIMO Antenna for Sub-6 GHz Band

In this work, two configurations (Config.-A and Config.-B) of planar four-port dual circularly-polarized (CP) multiple-input-multiple-output (MIMO) antenna are proposed for sub-6 GHz (3.4–3.8 GHz) applications. The MIMO antennas are designed on the FR-4 substrate with size of $60\times 60\times 1.6$ mm3. Each radiating element (unit cell) of the MIMO antenna is comprised of a microstrip feed line and an open slot ground plane integrated with two rectangular arms for realizing circular polarization. An I-shaped metallic strip is used in the proposed four-port MIMO Config.-A and Config.-B antennas for obtaining common voltage level in the ground plane. Both MIMO antenna configurations exhibit good impedance matching in the band of interest. In the MIMO Config.-A antenna, all ports provide the same CP band, while in the Config.-B antenna the ports-1/-2 and -3/-4 radiate at different CP bands. The proposed MIMO Config.-A and Config.-B antennas support both types of polarization (left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP)). With the Config.-B antenna, the maximum 3-dB axial-ratio (AR) beamwidths of 118.9° and 92.7° are obtained at 3.536 GHz in the xz- and yz-planes, respectively. Furthermore, the I-shaped strip helps in improving 3-dB AR beamwidth in the MIMO Config.-B antenna. Inter-element isolation greater than 19 dB and envelope correlation coefficient (ECC) less than 0.12 (far-field) are achieved between the ports of the fabricated MIMO antenna prototype.


I. INTRODUCTION
The multiple antenna elements at the transmitter and receiver end upsurge the channel capacity, data transmission rate, and quality of the wireless communication system. Adopting more number of antenna elements within a receiving system improves the link reliability of the radio receiver [1]- [2]. However, the limited space available at the communication terminals increases the problem of mutual coupling, thus deteriorating the overall performance of the system [3]. The multiple-input-multiple-output (MIMO) antenna using polarization diversity resolve this issue of mutual coupling [4]. The correlation among antenna elements can be reduced by locating the radiators in the horizontal and vertical orientation without introducing any complex decoupling The associate editor coordinating the review of this manuscript and approving it for publication was Kwok L. Chung . structure between antenna unit cells. Recently, a number of MIMO/diversity antennas with a different arrangement of radiators for achieving polarization diversity have been reported [5]- [8]. A quad-port MIMO antenna with radiating elements wrapped around four sides of the cuboidal polystyrene block was presented in [5]. A reconfigurable shared aperture MIMO antenna with a horizontal and vertical arrangement of radiating elements was proposed in [6]. In [7], a 3-D array comprised of eight antenna elements with fourelements vertically located, and remaining four horizontally placed was presented. However, the reported MIMO antennas in [5]- [7] were non-planar designs with complex architecture and relatively bulky size. In [8], a planar quasi-selfcomplementary MIMO antenna composed of two-/four-port was presented for polarization diversity.
It is observed that the polarization diversity in MIMO antennas presented in [5]- [8] was realized by using linearly-polarized (LP) antenna elements. However, compared to LP radiators, circularly-polarized (CP) antenna elements exhibit several significant advantages in terms of radio wave propagation. For example, CP antennas effectively alleviate the multipath fading effects and are also robust to polarization mismatch [9], [10]. Furthermore, the strength of the electromagnetic (EM) signals received at the wireless terminals is relatively constant, irrespective of the CP antennas orientation [11], [12]. These properties of CP antennas make them potential candidates for the upcoming MIMO wireless systems.
In the last decade, a few CP MIMO/diversity antenna structures have been proposed for WLAN, C-band, and satellite navigation applications [13]- [16]. A planar two-port CP MIMO antenna comprised of upper and lower radiators physically separated by a distance equal to the thickness of the substrate material was presented in [13]. In [14], a planar three-element MIMO antenna for achieving both pattern and polarization diversity was reported. In [15], a four-port multi-layered antenna with two radiating patches and frequency selective surface (FSS) superstrate for the generation of CP waveform was reported, which would be difficult to be integrated with other monolithic microwave integrated circuits (MMICs) in portable devices. A planar four-port CP MIMO antenna with corner truncated patches was presented in [16] for Wi-MAX/WLAN applications. The CP MIMO antennas in [13]- [16] can generate either left-hand circularly-polarized (LHCP) waves or right-hand circularlypolarized (RHCP) waves.
The MIMO antennas capable of generating dual-CP waves (both LHCP and RHCP) at the same frequency are in huge demand these days since these antennas can significantly reduce the mutual coupling and multipath fading effects [17], [18]. In the literature, some planar two-port dual-CP MIMO antennas radiating both LHCP and RHCP waves simultaneously have been reported [19]- [21]. In [19], a two-port coplanar waveguide (CPW)-fed planar antenna composed of two asymmetric T-shaped feed lines was presented for dual circular polarization. A broadband dual-CP two-port antenna with L-and inverted L-shaped strips on the radiating edges of the monopole was presented in [20], where the antenna was excited by dual orthogonal microstrip lines. In [21], two CPW feeds were utilized for realizing dual circular polarization. The link reliability in wireless systems can be further improved by designing dual-CP MIMO antennas with more than two radiating elements.
Furthermore, a detailed discussion related to the importance of the connected (single) ground plane in MIMO antennas was presented in [22]. The MIMO antennas must have a common ground plane to possess equal reference voltage level. The planar antennas proposed in [16], [20] does not have a common ground plane, and hence might not be used in practical applications. The same can be said for the nonplanar designs proposed in [5]- [7] since those antennas also have separated ground planes. Since there is no current coupling, high inter-element isolation levels were reported in such MIMO structures (like isolation >37 dB in [16]). The two-/three-port CP/dual-CP planar MIMO antennas proposed in [14], [19], and [21] can be used in practical systems since they have a single reference voltage level (common ground plane).
It is quite challenging to retain circular polarization characteristics of the antenna elements (unit cells) in the ground radiating CP MIMO structures when the ground planes are connected. The 3-dB axial-ratio (AR) beamwidth of the MIMO antenna is also an important factor, and a wide 3-dB beamwidth is required for efficient functioning of the antenna. In this article, an attempt has been made to address all the above-stated issues. The main contributions are:  Fig. 1, and its design parameters are stated in Table 1. The evolution steps of the unit cell are presented in Fig. 2. Firstly, a wide hexagonal slot is introduced in the ground plane of the square-shaped antenna element, shown in Fig. 2(a). The hexagonal wide-slotted ground plane geometry (step-1) is inspired by the design proposed by the author's group in [4]. In design step-1, the width of the metal boundaries is sufficient enough for the formation of the current closed loop. The center frequency (f c ) of the unit cell is evaluated as [23] where c is the speed of light in vacuum, x 5 is the aperture radius of the wide hexagonal slot, ε r is the dielectric constant of the substrate, L is the amount of length correction, w is the width of the radiator, and h is the height of the substrate. The aperture radius (equal to x 5 ) of the wide slot affects the current path, which changes the resonating frequency band of the antenna. It can be seen from Fig. 3(a) that the unit cell in step-1 resonates (S 11 ≤ −10 dB) for 12.55-14.18 GHz band, and its polarization is LP.
In Fig. 2(b), the feed location of the antenna element is shifted towards the right side to increase its electrical length to obtain lower frequency bands. It can be seen from Figs. 3(b) and (c) that the unit cell in design step-2 resonates at 3.4-6.4 GHz frequency band, and its polarization is also LP.
Further, in Fig. 2(c), some portion of the metallic ground plane is etched out to form an open slot, and two arms In all these cases, x 2 is assumed as 18 mm. Furthermore, it can be noticed from Table 2 that the unit cell retains its CP characteristics even after varying x 2 , m, x 1 , x 10 , and K . The ARBW of the proposed unit cell can be easily changed by changing the arm length (m) of the CP radiator as illustrated in Fig. 3(d). In step-4, a mirror image of the proposed unit cell is presented (Fig. 2(d)), and its S 11 and ARBW are shown in Figs. 3(b) and (c), respectively. It is to be noted that the responses of the proposed unit cell and its mirror image are nearly similar due to their identical shapes with opposite polarization senses.
The 3-dB AR beamwidth analysis of the proposed unit cell (step-3) is presented in Table 2. It is noticed that the maximum 3-dB AR beamwidths of 92.9 • and 82.7 • are obtained at 3.8 GHz (case-2) in the xz (ϕ = 0 • ) and yz (ϕ = 90 • ) planes, respectively. Correspondingly, in case-3, the maximum 3-dB AR beamwidths of 101.6 • and 118.8 • are obtained at 3.6 GHz in the xz-and yz-planes. Fig. 4 presents the 3-dB AR beamwidth of the proposed unit cell at 3.8 GHz.
It can be seen from the radiation patterns of Figs. 5(a) and (b) that the proposed unit cell radiates RHCP waves in +z-direction while its mirror image radiates LHCP waves in +z-direction. It can be further verified from the surface current distributions shown in Figs. 6 and 7 that the proposed unit cell and its mirror image radiate        Table 1.
It is important to note that the MIMO antenna cannot be used in practical applications if the ground planes of the unit cells are not connected, leading to undetermined reference voltages [20]. Keeping this in mind, in both configurations, the ground planes of the unit cells are connected using a metallic strip to provide a common reference plane for practical applications. An I-shaped metallic strip is used for connecting the ground planes of unit cells without disturbing the radiation characteristics of the unit cells. This step ensures an equal reference voltage level in both four-port MIMO configurations, thus making it integrable with other MMICs. In Section IV, it is also shown that the I-shaped strip helps in improving the 3-dB AR beamwidth of the MIMO Config.-B antenna. VOLUME 8, 2020

III. RESULTS AND DISCUSSION
The results of the MIMO antenna configurations (Config.-A and Config.-B) are discussed in this section. The MIMO Config.-A (g=d=0.5 mm, m=15.7 mm, f =8.5 mm, and e=12.8 mm) prototype antenna is fabricated on the FR-4 substrate with dimension of 60×60×1.6 mm 3 as shown in Fig. 9(a). The proposed MIMO Config.-A antenna is measured using the Anritsu MS2038C vector network analyzer and the Rohde & Schwarz FSP spectrum analyzer. The gain and AR of the fabricated antenna are measured inside an anechoic chamber using the method given in [24].

A. FABRICATED MIMO ANTENNA (CONFIG.-A)
It can be seen from Fig. 9(a) that the MIMO Config.-A antenna shows good impedance matching in the operating band of 3.4-3.8 GHz. The simulated reflection coefficients of the resonating element-2, -3, and -4 are shown in Fig. 9(b). It is also seen from Fig. 9(c) that the measured isolation is >20 dB between ports-1 and -2, >19 dB between ports-1 and -3, and >24 dB between ports-1 and -4.
The ARBW of the MIMO Config.-A antenna is shown in Fig. 10(a), and Table 3 presents different 3-dB ARBW cases (case-A to -G, when x 2 = 18 mm) of the antenna. It is noticed that the MIMO antenna exhibit 3-dB ARBW of ∼240 MHz (3.46-3.7 GHz). Furthermore, it can be seen from Table 3 that the MIMO Config.-A antenna exhibits circular polarization even when the parameters (g, d, e, and f ) of the I-shaped element are varied. Therefore, flexibility is noticed in the design parameters of the I-shaped strip. The effect of varying arm length (m) on 3-dB ARBW of the MIMO antenna is shown in Fig. 11. The antenna exhibits 3-dB ARBW  The simulated and measured gain plots of the MIMO Config.-A are shown in Fig. 10(b). It is found that the maximum gain of the antenna is around 4.5 dBi in the operating band.
The diversity performance of the MIMO antenna is evaluated by measuring the envelope correlation coefficient (ECC) between ports-1 and -2, ports-1 and -3, and ports-1 and -4. In the proposed work, the ECC is calculated by using far-field relation [25]   where F is the radiated field, , θ, and ϕ are the solid, elevation, and azimuthal angles, respectively. It is found that ECC is less than 0.12 between various antenna ports, which verifies the diversity performance of the proposed MIMO antenna.
The diversity gain (DG) between different ports of the MIMO antenna is found to be around 10 dB in the band of interest. The radiation patterns of the proposed MIMO antenna at 3.6 GHz are shown in Fig. 12. The antenna displays dual circular polarization characteristics by radiating RHCP waves when port-1/-4 is excited, and LHCP waves when VOLUME 8, 2020  Table 3.
The impedance bandwidth, isolation, ECC, and gain of the MIMO Config.-B antenna are almost similar to the MIMO Config.-A antenna. Therefore, only the 3-dB ARBW and LHCP/RHCP radiation patterns of the MIMO Config.-B antenna are shown here. It can be easily seen from Figs. 14 and 15 that the port-1 radiates RHCP waves and the port-2 radiates LHCP waves with 3-dB ARBW

IV. BEAMWIDTH ANALYSIS OF THE PROPOSED MIMO ANTENNA
In this section, the 3-dB AR beamwidth performance of the MIMO Config.-A/-B (with/without I-shaped strip) antennas is analyzed. 90786 VOLUME 8, 2020 Various cases (case-A to -G) of the 3-dB AR beamwidth are considered to study the effect of the I-shaped strip parameters (g, d, and e). During the study, only I-shaped strip parameters are varied, and the CP radiator arm length x 2 is kept fixed at 18 mm. The CP radiator arm length (m) is changed from 15.7 mm to 16.5 mm to illustrate that the antenna can achieve desirable AR performance even with a change in m. It is seen from Fig. 16 and Table 3 that the proposed MIMO antenna offers satisfactory 3-dB AR beamwidth performance in the xz-plane for all cases (except case-B and case-E). On the other hand, the 3-dB AR beamwidth is not satisfactory in the yz-plane (for all cases considered). Table 3 displays all the cases of the MIMO Config.-A antenna, and it is seen that the maximum 3-dB AR beamwidth of 117.8 • (θ : −46.3 • to 71.5 • ) is obtained in the xz-plane at 3.536 GHz (for case-F). On the other hand, for case-F, the 3-dB AR beamwidth of 38.24 • is obtained in the yz-plane (θ : −28.04 • to 10.2 • ) at 3.536 GHz. Therefore, it can be said that, after observing Table 3 and Fig. 16, the beamwidth performance of the Config.-A antenna is not satisfactory in the yz-plane.  Table 4 shows different cases with m 1 = 16.5 mm/  Table 4 that the MIMO antenna has satisfactory 3-dB AR beamwidth performance in both xz-and yz-planes (for all the cases). It is seen from Fig. 17(c) that the maximum 3-dB AR beamwidths of 118.9 • (θ: −52.1 • to 66.8 • ) and 92.7 • (θ: −45.8 • to 46.9 • ) are obtained in the xz-and yz-planes, respectively, at 3.536 GHz in the case-F. Therefore, after observing the results, it can be concluded that the MIMO Config.-B antenna outperforms the MIMO Config.-A antenna in terms of the 3-dB AR beamwidth in both xz-and yz-planes.
The use of two different arm lengths (m 1 and m 2 ) for unit cells at port-1/2 and port-3/4, respectively, with the I-shaped strip helps in adjusting the E-field magnitude of two modes radiated by the arms of the CP radiators. This modification keeps the AR beamwidth below 3-dB in both xz-and yz-planes with the Config.-B antenna.
Also, the power concentrated in the lower arm of the unit cell (when port-1 or port-2 is excited) gets redistributed in the I-shaped strip (which does not act as a radiator). It is also important to note that, if m 1 >m 2 , the AR beamwidth will be 90788 VOLUME 8, 2020 achieved in the xz-and yz-planes if the port-1/-2 is excited. On the other hand, if m 2 > m 1 , the same can be said for the port-3/-4. By varying m 1 and m 2 , the 3-dB AR beamwidth can be easily obtained in both xz-and yz-planes for other frequency bands, too. are obtained in the xz-and yz-planes at 3.44 GHz, respectively. Therefore, it is noticed that the MIMO antenna without the I-shaped strip shows unsatisfactory 3-dB AR beamwidth performance in the xzplane at 3.44 GHz. Contrary to this, the 3-dB AR beamwidth performance of the MIMO antenna without the I-shaped strip is satisfactory in the yz-plane at 3.44 GHz. The same holds at other frequencies, too, but here only the maximum achievable 3-dB beamwidth of the Config.-B (without I-shaped strip) antenna is shown. Therefore, it can be concluded from the above discussion: • Only two ports can be utilized for the 3-dB AR beamwidth purpose (either port-1/-2 or port-3/-4) in Config.-B. If m 1 > m 2 , the AR achieved in the xz-and yz-planes when the port-1/-2 is excited. On the other hand, if m 2 > m 1 , the same can be said for the the port-3/-4. • The I-shaped strip plays a significant role in the MIMO antenna design since the 3-dB AR beamwidth performance of Config.-B deteriorates (in the xz-plane) if the strip is removed. Therefore, in addition to setting an equal voltage reference level in the ground plane, the I-shaped strip is also responsible for achieving the desirable 3-dB beamwidth in both xz-and yz-planes in  Table 6. The proposed MIMO antenna outperforms the other dual-CP four-port MIMO antennas reported in [17]- [19] in terms of isolation and the number of available ports. In addition, VOLUME 8, 2020 the proposed MIMO antenna offers advantages of providing a connected ground plane, relatively small size, and dual circular polarization as compared with other four-port CP MIMO antennas reported in [14], [16]. On the other hand, the MIMO Config.-B antenna can be used to cover entire 3.4-3.8 GHz band (port-1/2 and port-3/4 radiating different CP bands) with LHCP/RHCP choice. The Config.-B antenna could also be suitable in applications where wide 3-dB AR beamwidth is required in both xz-and yz-planes. It is also shown that the I-shaped strip plays a crucial role in improving 3-dB AR beamwidth in addition to connecting the ground planes to obtain an equal reference voltage level. Wide 3-dB AR beamwidths of 118.9 • and 92.7 • are obtained at 3.536 GHz in the xz-and yz-planes, respectively, in case-F of the Config.-B antenna. The fabricated MIMO antenna prototype (Config.-A) achieves satisfactory diversity performance in terms of ECC, isolation, and diversity gain. The proposed MIMO antenna configurations are compact in size and can be easily integrated with portable MMICs due to planar geometry and a common ground surface. The MIMO antenna configurations presented in this work can be utilized for sub-6 GHz 5G applications.