Broadband and Compact Circularly Polarized MIMO Antenna with Concentric Rings and Oval Slots for 5G Application

A broadband and compact size circularly polarized (CP) two-port multiple input multiple output (MIMO) antenna with a footprint of 25 mm × 20 mm is investigated. The designed oval-shaped MIMO antenna employs concentric rings with oval slots (CROS), along with a circular radiator and two open-ended parallel protruded stubs. The CP radiation is achieved by embedding three oval slots, in which two of them are deployed at the left and right side of the concentric rings, whereas the third one is deployed at the top section of the concentric rings. The measured 10-dB impedance bandwidth of the proposed MIMO antenna was 46.30% (3.12–5.00 GHz), and its corresponding 3-dB axial ratio bandwidth (ARBW) was 41.34% (3.30–5.02 GHz). Furthermore, very wide beamwidths of 137°±0.2 and 154°±0.2 were measured in the right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) radiation patterns, respectively. The minimum achieved isolation between the antenna elements is 18.50 dB without using any additional decoupling structure, and the calculated envelope correlation coefficient (ECC) < 0.03.

Therefore, MIMO antenna designs with CP senses have been reported in recent years [8][9][10][11][12][13][14][15][16]. Stacked patch MIMO antennas have the features of wideband/multiband and performance enhancement. Therefore, the work in [8][9][10] has applied the stacked patch technique to the CP MIMO antennas. In [8], cornertruncated square slot and patch configuration are applied to the MIMO antenna to yield CP radiations in 2.45 GHz and 5.8 GHz bands. In [9], the CP is achieved by protruding a stub from the right end of the ground plane, whereas the method applied by [10] is to chamfer the two opposite corners of the ring slot as well as the resonant patch. However, the MIMO antennas in [8,10] have exhibited larger dimensions, and the ones in [8,9] have used expensive substrates. Notably, applying the stacked patch technique will inevitably increase the overall profile of the designated antenna, which makes it difficult to integrate VOLUME XX, 2017 1 into a slim wireless device. Even though [11] has introduced a compact CP MIMO antenna designed using two truncated corner square patches with parasitic periodic metallic plates [11], it requires expensive Taconic substrate (same as [8,9]) that results in higher manufacturing expenses.
To minimize the manufacturing expenses, the works reported in [12][13][14][15][16] have introduced MIMO antenna designs using low-cost FR-4 substrate. In [12], an eyebrow-shaped strip is applied to enhance the CP performance of the MIMO antenna, but it has a very large dimension of 85 mm × 73 mm. To reduce the overall MIMO antenna size to 21 mm × 46 mm, split ring resonator was introduced to the MIMO antenna, and good CP radiation is achieved by using offset feeding technique and defective ground structure (DGS) [13]. To further decrease the dimensions to 27 mm × 27 mm, the MIMO antenna design in [14] employs two orthogonally designed T-shaped patches as well as three Lshaped parasitic patches in the ground plane to yield good CP radiation. However, [15] and [16] have shown poor isolation between their respective adjacent antenna elements. To increase the isolation between adjacent antenna elements, [15] has applied the spatial diversity method (adjacent elements separated by a distance of 13.75 mm), and good CP radiation is achieved by applying 90 o phased slots at the center of the truncated patch. As for [16], the designed microstrip-fed MIMO antenna is composed of a patch containing two L-shaped radiators along with a wide hexagonal slot radiator on the ground plane, and good 3-dB ARBW of 34.38% and isolation of >17 dB is achieved. In this manuscript, a very low-profile (0.27λ × 0.22λ × 0.01λ) (where λ is calculated at the lowest frequency of 3.30 GHz), oval-shaped CP two-port MIMO antenna for 5G applications is presented. Each CP antenna element is fed by a 50Ω microstrip line and employs CROS, along with a circular radiator, and two open-ended parallel protruded stubs. The oval-shaped slots loaded on the left as well as right side of the concentric rings are for achieving CP radiation, whereas the one that is loaded at the top section is to enhance the ARBW.   In the proposed structure, each antenna element (Ant.1 and Ant.2) is composed of two concentric rings (radius r2 and r3) and one circular patch (radius r1), which shares the same center location 'O'. Here, two elliptical slots having a major and minor axis of 2 mm and 1 mm, respectively, are loaded on the left and right side of the antenna element, and an elliptical slot having a major and minor axis of 3 mm and 1 mm, respectively, is loaded on the top section of the antenna element between two open-ended protruded stubs (each with a dimension of L5 × W4). The two antenna elements shared the same partial ground plane of dimension L1 × W, which is designed on the back of the substrate. The proposed CP MIMO antenna is simulated (including the SMA connectors) using the 3D EM simulator CST MWS ® . The step-wise design of the single antenna element is presented in Figure 2. Here, five major steps are analyzed to comprehend the design mechanism, and their corresponding 10-dB impedance bandwidth and 3-dB ARBW are illustrated in Figures 3(a) and 3(b), respectively. As depicted in Figure 2, the proposed antenna element design is stemmed from (Step-1) a circular ring-slot monopole antenna that is loaded by a concentric ring at center location 'O' with radius r2 and r3. Notably, this concentric ring is turned into a "split-ring" type by introducing a small shorted section of width 0.5 mm, and resonance at 4.10 GHz is induced with a broad bandwidth of 23.5% (3.60-4.60 GHz), as visualized in Figure 3(a). To further increase the bandwidth, in Step-2, a circular radiator of radius r1 is loaded into the center ring-slot position, and by further observing Figure 3(a), even though this circular radiator can improve the bandwidth to 38.5% (4.00-5.55 GHz), the resonance mode is shifted towards the higher frequency spectrum (at 4.6 GHz). The broadband behavior in Step-2 is obtained due to the unpunctured and longer path of currents flowing through the circular radiator.  To shift the resonance back to approximately 4 GHz, as well as further improving the operational bandwidth to occupy the complete 5G New Radio (NR) band n77/n78/n79, in Step-3, two open-ended parallel stubs are protruded from the top section of the antenna element, and a broad bandwidth of (3.18-4.82 GHz) is achieved. However, from Figure 3(b), it is noted that Step-1 to Step-3 can only exhibit LP radiations as their corresponding axial ratio (AR) values are larger than 10 dB. Therefore, to achieve CP radiation across the desired bands of interest, as shown in Figure 2 (Step-4), two elliptical slots are loaded, and the reason for that is to perturb the surface current distribution on the left and right side of the radiator. As illustrated in Figure 3, the antenna in Step-4 has shown a broad 3-dB ARBW of (3.20-4.50 GHz) with desirable 10-dB impedance bandwidth of (3.18-4.90 GHz). Nevertheless, the ARBW of this antenna (Step-4) can only cover the 5G NR band n77/n78 (3.30-4.20 GHz). Therefore, to further increase the ARBW, a top-loaded elliptical slot that acts as a perturbation element is introduced, and the antenna is now denoted as Step-5 (or proposed antenna element) in Figure 2. As shown in Figure  3, the proposed antenna element can yield a wide 3-dB ARBW of 42.42% (3.25-5.00 GHz) with desirable 10-dB impedance bandwidth of 44.50% (3.18-5.00 GHz), and it can fully cover the entire 5G NR band n77/n78/n79 (3.30-5.00 GHz).

B. SURFACE CURRENT DISTRIBUTION OF PROPOSED ANTENNA
To analyze the working of the proposed antenna, its corresponding simulated surface current (A/m) distribution is analyzed in Figure 4. From Figure 4, it is observed that at 4 GHz, a maximum current is flowing through the circular radiator and at the same time, an equal amount of current flowing through the concentric rings which proves that it helps in generating the resonance as well as widening the bandwidth at 4 GHz. Further equal amount of current is also observed in the two open-ended parallel stub which slightly helps in bandwidth enhancement and tuning the resonance in frequency range of (3.18-5.00 GHz). To generate a good CP radiation for an antenna, the horizontal electric field (Ex) and the vertical electric field (Ey) must have an equal amplitude of approximately 1 (or 0 dB) with a phase difference (PD) of 90 o throughout the bands of interest. In this regard, to fully comprehend the CP excitation of the proposed antenna element (Step-5), Figure   5 depicts its corresponding calculated amplitude ratio (Ex/Ey) and PD of the two orthogonal electric field components. Here, one can see that the amplitude ratio is closer to 0 dB with PD of near 90 o between them. Therefore, the proposed antenna element is a good CP antenna that can be further applied as a MIMO configuration. The simulated S11, S22 parameters for the 2-port MIMO antenna geometry shown in Figure 1(a) is obtained by activating Ant.1 and terminating Ant.2 with 50 Ω impedance load and vice versa whereas the transmitting coefficient S12, S21 are obtained by activating both the Ant.1 and Ant.2, simultaneously. The curves of S22 and S21 are not shown for brevity. The simulated S11 in Figure 6 illustrates that the bandwidth remains almost the same when the antenna is transformed from single antenna to two antenna elements. The bandwidth obtained for the two-port MIMO antenna array is 44.50% (3.18-5.00 GHz). Similarly, from the transmission coefficient curve S12 shown in Figure 6, it is noticed that the isolation between Ant.1 and Ant.2 is larger than 18.5dB throughout the operating band. VOLUME XX, 2017 9 To validate the isolation between Ant.1 and Ant.2, the electric field intensity (V/m) distribution on the surface of the two-port MIMO antenna array is illustrated in Figures  7a and 7b, respectively. For analyzing the effect, Ant.1 is excited, while Ant.2 is kept terminated with 50Ω matched impedance load. Under this scenario, from Figure 7a, it is evident that the deployment of antennas at 0.16λ centre-tocentre distance protect the current flowing out from Ant.1 and also suppresses the propagation of surface wave through the ground plane without affecting the impedance matching and radiation performance. Notably, the same is also verified when Ant.2 is excited while the Ant.1 is kept properly terminated with 50Ω matched load.  The inner solid circular radius r1 is varied from 2mm to 2.75mm as shown in Figure 8. It is analyzed that the -10dB impedance bandwidth increases with increase in radius upto 2.5mm. However, further increase in the radius results in bandwidth reduction (as seen from green line curve) which may be due to the strong coupling between solid radius r1 and concentric rings when radius is increased beyond 2.5mm. Therefore, the optimum value of radius r1 is selected as 2.5mm which covers the desired band of operation.  Figure 9 illustrates the variation of open-ended stubs L5 from 6mm to 7.5mm. It can be observed that increasing the length of the stubs shifts the resonance towards lower frequency whereas small effect is observed on the bandwidth. The value of L5 is considered as 7mm considering the best RF performance. VOLUME XX, 2017 9 The effect of varying centre-to centre distance W3 from 11mm to 17mm on S11 and S12 is analyzed in Figure 10. It can be observed that the curves for S11 remains almost the same for all the values of W3 whereas better S12 curves are observed when the values of W3 is increased. For W3 values of 11mm and 13mm the isolation is less than 17dB, whereas for W3 of 15mm and 17mm the isolation is greater than 18.5dB. Even though the isolation value is greater at 17mm, to restrict the area of the proposed MIMO antenna, the optimum value of 15mm is considered. The proposed two-port CP MIMO antenna was fabricated, as illustrated in Figure 11, and its typical results such as reflection coefficients (S11 and S22), transmission coefficients (S12 and S21), AR, two-and three-dimensional radiation patterns, gain, and efficiency were measured and compared with the simulated ones.

A. MEASURED AND SIMULATED S-PARAMETERS
The measured and simulated S-parameters (S11 and S12) are plotted in Figure 12. While measuring S11 and S22, the Ant.1 was activated whereas the Ant. 2 was terminated with 50Ω impedance and vice-versa. While measuring S12 and S21, both the Ant.1 and Ant.2 were activated simultaneously. Due to analogy, S22 and S12 are not shown for brevity. Here, the measured 10-dB impedance bandwidth was approximately 46.30% (3.12 -5.00 GHz) with a resonance frequency of 4 GHz, and the measured isolation level between antenna elements was larger than 18.5 dB across the bands of interest.  Figure 13, and a very wide 3-dB ARBW of 41.34% (3.30-5.02 GHz) was measured. By comparing Figure 12 and Figure 13, the measured 3-dB ARBW has overlapped with the impedance bandwidth across the desired bands of interest. The simulated and measured results are well validated with other, and the slight deviation could be due to soldering and fabrication tolerances.  The simulated 3D radiation patterns at 4GHz are illustrated in Figure 17. It is visualized that three-dimensional radiation patterns of Ant.1 and Ant.2 are oblique dipole patterns forms mirror image to each other which further validates that the proposed MIMO antenna has good radiation performance.  The ECC is used to verify the performance of the proposed CP MIMO antenna for diversity applications. It is calculated from far-field patterns using formula mentioned in [17]. As shown in Figure 19, the ECC values are less than 0.03 across the operating bands, which confirms that the designed CP MIMO antenna has good isolation and offer best performance under the influence of multipath fading environments. MEG is a crucial parameter for the diversity performance analysis of any MIMO system. Therefore, the MEG is defined as the ratio of power received by MIMO antenna to the power received by isotropic antenna. In MIMO system, the MEG is calculated using equations (1-3).

MEG-i = (1)
Where, M is the number of antenna elements and 'i' is the port number. By expanding the above equation, the MEG-1 for port-1 is found as: MEG-1 (2) and the MEG-2 for port-2 is found as: For better diversity metrics, the MEG-1 and MEG-2 values as per industry standard, should be -3 ≤ MEG (dB) ≤ -12, whereas the ratio of MEG-1/MEG-2 should be approximately equal to 1. From Table II, it is validated that the MEG values as well as the ratio are within the welldefined limit. The ergodic channel capacity of the proposed MIMO antenna across the operating bands is analysed in Figure 20 using the following equation (4). (4) where in equation (4), k the number of antenna elements, SNR defines the mean signal to noise ratio, [I] denotes an identity matrix, η indicates the efficiency, [H] is the normalized channel matrix considered as frequency independent over the operating bands, and H * denotes the transpose conjugate matrix of H.
The MIMO systems channel model should be selected first while calculating the channel capacity and the model for ray tracing or the associated statistical model is mostly used. So, based on the correlation matrix method, the 4-port MIMO antenna's channel capacity is calculated. The calculated channel capacity of the proposed array is indicated in Table III as well as in Figure 20, which is between 10 and 10.5 bps/Hz by considering simulated efficiencies (η) and averaging 10,000 Rayleigh fading realizations with 20dB SNR in the identically and independently distributed propagation condition [18]. For maximum channel capacity, all channels have no correlation, and fading matrix [H][H*] is converted into an identity matrix. The channel capacity of the proposed two-port MIMO antenna is above 10.3bps/Hz throughout the operating band, which is about 1.82 times greater than the maximum limit of an ideal Single antenna (about 5.65 bps/Hz). Compared with the maximum limit for an ideal two-port MIMO antenna, the proposed MIMO antenna have exhibited good channel capacities.  The Total Active Reflection Coefficient (TARC) between Ant. 1 and Ant. 2 is calculated using the below Equation (5). (5) where is the input phase angle varied from 0 0 to 120 0 at an interval of 30 0 , Sii and Sjj are the reflection coefficients of the port i and port j, respectively. Figure 21 depicts the TARC values for antenna elements, where it is visualized that under the variation of phase angle, the performances of the proposed MIMO antenna remain unaltered in the scattering environment. As illustrated in Figure 22, both the simulated as well as calculated multiplexing efficiency is higher than -3dB throughout the operating band which satisfy the requirements of MIMO wireless communication systems.

VI. PERFORMANCE COMPARISON OF PROPOSED ANTENNA WITH EXISTING STATE OF ART
The performance comparison of designed CP MIMO antenna with recent literature is compared in Table IV. From the below table it can be noted that the proposed MIMO antenna has smallest dimension, as well as covers the complete 5G sub-6GHz band with overlapping 3dB ARBW in the desired band as compared to the all the reported CP MIMO Antennas. JAUME ANGUERA IEEE Fellow, founder and CTO at the technology company Ignion (Barcelona, Spain). Associate Professor at Ramon LLull University and a member of the GRITS research group. He is an inventor of more than 150 granted patents, most of them licensed to telecommunication companies. Among his most outstanding contributions is that of inventor of Antenna Booster Technology, a technology that fostered the creation of Ignion. Many of these products have been adopted by the wireless industry worldwide, to allow wireless connectivity to IoT devices through a miniature component called an antenna booster that is ten times smaller than conventional antennas. Author of more than 250 scientific widely cited papers and international conferences (h-index 50). Author of 7 books. He has participated in more than 22 competitive research projects financed by the Spanish Ministry, by CDTI, CIDEM (Generalitat de Catalunya) and the European Commission for an amount exceeding € 6M being principal researcher in most of them. He has taught more than 20 antenna courses around the world (USA, China, Korea, India, UK, France, Poland, Czech Republic, Tunisia, Spain). With over 22 years of R&D experience, he has developed part of his professional experience with Fractus in South Korea in the design of miniature antennas for large Korean companies such as Samsung and LG. Since 2017 he is with Ignion with the role of CTO where he leads the R&D activity of the company creating new products, envisaging new technologies, fostering synergies with partners, and providing technology strategy to scale the business of the company. He has received several national and international awards (ex. 2004 Best Ph. D Thesis -two prizes, one given by Telefónica Mobile, 2004 IEEE New Faces of Engineering, 2014 Finalist European Patent Award). He has directed the master/doctorate thesis to more than 130 students, many of them have received awards for their thesis (COIT, COITT, Ministry of Education). His biography appears in Who's Who in the World and Who's Who in Science and Engineering. He is associate editor of the IEEE Open Journal on Antennas and Propagation, Electronics Letters, and reviewer in several IEEE and other scientific journals. He is an IEEE Antennas and Propagation Distinguished Lecturer and vice-chair of the working group "Software and Modeling" at EurAAP. More info at http://users.salleurl.edu/~jaume.anguera/