A multiple‐input‐multiple‐output on‐chip Quasi‐Yagi‐Uda antenna for multigigabit communications: Preliminary study

This article presents a solution for the low gain and the poor efficiency of the on‐chip antennas (OCA). The four elements of Quasi‐Yagi‐Uda antennas (QYUA) are introduced based on the diversity technique to reduce the interference between the elements. In addition, these antennas achieve high isolations between them due to the use of reflector for each antenna. The QYUA is selected to improve the radiation properties of the end‐fire radiator in the millimeter‐wave range for on‐chip systems. The proposed MIMO antenna is used for the point to point communications. The complementary metal‐oxide semiconductor with 180 nm standard is used in the antenna design with six metal layers. The QYUA combines three parts (driven element, reflector, and director); the driven consists of two meander lines fed by coplanar‐slot and operates as a dipole, the reflector is an arc likes a semicircle to prevent the back radiation and increase the front to back ratio, and the director is a meander line to directive the radiation into the proposed direction (front end‐fire direction). All MIMO parameters such as envelope correlation coefficient, channel capacity loss, diversity gain, and total active reflection coefficient in addition to the different configurations of the MIMO are presented. All results are verified by computer simulation technology and high‐frequency structure simulator. The contribution of this article is the MIMO antenna design for point to point communications to serve multigiga communications systems with high data rate and high gain. This MIMO system is considered here to solve the problems of OCA designs.


INTRODUCTION
coupling between the elements. This antenna is introduced to solve the inherent low gain of the OCA by using four elements MIMO antenna based on diversity topology. The antenna performs good impedance bandwidth from 50 to 68 GHz. The suggested QYUA combines of driver, reflector, director, and ground based on meander lines to reduce the physical size of the proposed antenna. Four elements MIMO antenna is introduced in order to decrees the channel capacity loss (CCL) than 0.3 bit/s/Hz. This article is organized into four sections. Section 2 demonstrates a single element Yagi-Uda antenna design procedures and presents its results, whereas Section 3 describes the different MIMO antenna configurations and studies the performance of each configuration in addition to studying the MIMO parameters. The article is concluded in Section 4.

QUASI-YAGI-UDA ANTENNA DESIGN
We introduced the single element of QYUA in Reference 5. We optimized the antenna dimension and the optimized geometry of the proposed antenna is shown in Figure 1. The QYUA is based on 180 nm CMOS technology that shown in Figure 1A. This technology consists of a base substrate from silicon (Si) with a thickness of 200 μm and thin silicon oxide layer (Sio 2 ) with a thickness of 10.34 μm. The Sio 2 layer consists of six metal layers, the thickness of each layer from M 1 to M 5 is 0.53 μm and the thickness of layer M 6 is 2.34 μm. Moreover, the area of 180 nm CMOS is 5 × 5 mm 2 . The occupied size of the QYUA is 631 × 460 μm.
The QYUA combines of a driven element, a reflector element, a director element, and a ground plane. All the component of the proposed antenna is implemented on the sixth metal layer M 6 . To reduce the physical size of the proposed antenna the driver element and the director element are designed as a mender line. The input feed of the proposed antenna is a coplanar-waveguide (CPW). Therefore, there is a request for a transition from CPW to coplanar-slot. The methodology of this transition introduced in Reference 33. The CPW is used for F I G U R E 1 Yagi antenna geometry ground-signal-ground feeding standard in the millimeter-wave circuits. This antenna uses two reflectors, the first reflector is the ground plane when increasing its length (W g ) and the second reflector is the arc shape is used to avoid any back radiation and direct the full power to the front direction. All the optimized dimensions of QYUA are shown in Table 1.
The proposed antenna is designed and optimized by using two simulators to verify the results. We used computer simulation technology (CST) microwave studio version 2018 and high-frequency structure simulator (HFSS) version 15. The performance of the antenna is studied in two cases; using planar arc as the main reflector at width W g = 0.18 mm and using the ground plane as the main reflector without the arc. The obtained return losses from CST and HFSS are close together as shown in Figure 2. The proposed QYUA covers the band from 50 to 68 GHz. Figures 3 and 4 show the gain and radiation efficiency of the proposed QYUA with and without the arc, respectively. We notice that the arc enhance the value of gain by 0.8 dBi because it reflects the back radiation from the Yagi antenna. Furthermore, the gain of the antenna is verified by using HFSS in the two cases and there is good agreement between the results. Furthermore, the antenna efficiency is increased by using the arc to be 45%. Figure 5 illustrates the radiation pattern of the antenna in the XY plane and ZY plane at 60 and 65 GHz. There is a good agreement between the simulated radiation patterns from CST and HFSS as depicted in Figure 5. In addition, we noted that the radiation patterns of the antenna in the direction of Y -axis to ensure that the antenna has end-fire radiation. Table 2 shows the comparison between the proposed QYUA and published articles. Low profile, low complexity, compact size, high gain, high efficiency, and high front to back ratio are achieved. antenna structure have two elements that provide better isolation between them without using any additional technique or decoupling circuit. The optimized geometry of the proposed three different configurations is shown in Figure 6. In this design, the ground plane and the arc play a significant role in the isolation performance of the proposed antenna. Furthermore, the diversity of the antenna positions helps to reduce coupling and achieve better isolation among them. The CST microwave studio and HFSS are used together to verify the simulated results for the S-parameters as shown in Figure 7. In this figure, only S 11 and S 21 are simulated because of the symmetrical arrangement of antenna elements in the structure. A good agreement between the simulated results of CST and HFSS is obtained. All three configurations offer good matching and high isolation because using arc as reflector between the elements. The isolation between elements is more than 40 dB that provides an enhancement in the radiation properties of antennas. Table 3 shows a comparison between the three aforementioned configurations one can notice that all three configurations offer good matching and high isolation because using arc as reflector between the elements. The isolation between elements is more than 40 dB that provides an enhancement in the radiation properties of antennas. We notice that Conf. II has low isolation because the distance between its ports (1 and 2) is small compared with the other configuration, but it still has isolation 40 dB. On the other hand, Conf. II, and Conf. III achieve diversity in the radiation pattern and this diversity is the main factor in the MIMO designs.

Multiple elements
In this section, a four-and eight-elements MIMO antenna are introduced as shown in Figure 8. The provided MIMO consists of two back by back and two side by back antennas. This configuration is introduced to provide high isolation between all elements. Moreover, the antennas in the proposed configuration are orthogonal together and this gives diversity in the radiation patterns. These properties of proposed MIMO indicate that antenna can be used for spatial multiplexing or pattern diversity. Figure 9 illustrates the S-parameters of the proposed MIMO antenna to ensure that the antenna covers the band from 50 to 68 GHz with good matching and high isolation. All the results are verified by CST in addition to HFSS. From 3D radiation patterns of four elements, that is, introduced in Figure 10, we notice that the radiation pattern of four elements are in a different direction because of the diversity between elements. The configuration of eight elements MIMO antenna is introduced as shown in Figure 11. The eight elements are designed on the top layer of CMOS chip. The MIMO antenna compatibles with the chip size and achieves end-fire radiation from all elements. The good impedance matching from each port and the high isolation between ports are achieved as shown in Figure 12. This configuration can be used for multigigabit communication systems because it is based on the diversity between its elements. The eight elements are positioned in diversity to improve the isolation coefficients between elements and to achieve the high diversity gain (DG).

MIMO parameters
Each MIMO antenna has four distinct parameters that should be tested to ensure that the MIMO gives a good performance. The MIMO parameters are applied for four-element MIMO antenna and the different MIMO configurations are similar.   The ECC is one of the key parameters used to characterize MIMO antenna efficiency. Where it measures the relationship between the performance of the antennas in other words its value depends on the similarity of the radiation pattern and coupling between antenna elements. The following Equation (1) clarify the expression of ECC in case S-parameters 32,39 : where : ECC between the antenna elements, S: refers to the S-parameters, S*: refers to the conjugate of S-parameters, m, and n are port number (m = 1:4, n = 1:4). The value of ECC should be less than 0.5 over the operating band according to the published standards. [40][41][42][43] Whereas the lower values of ECC mean that the two antennas are good isolated. Figure 13 shows the ECC between MIMO elements. It is obvious from the figure that the ECC is less than 0.0003 within the operating band.

• Diversity gain
The second parameter is a DG, where the DG can be expressed as. As shown in Figure 14, the DG has 10 dBi over the impedance bandwidth, this high value of DG during the operating band due to the fact that ECC is extremely low (ECC ≅ 0).

• Total active reflection coefficient
The third parameter indicating the coupling between ports is the total active reflection coefficient (TARC). Its minimum value is 0, meaning that all incident energy is radiated, whereas the highest value is 1 meaning that all incident power is reflected. The TARC has a major impact on the MIMO antenna system's impedance bandwidth. 32 The TARC is calculated by the following equations: where a i , b r refer to the amplitude of incident and reflected signals, respectively.
To study the TARC of MIMO QYUA, we excite the MIMO antenna at port one by 1e j0 and excite port2, port 3, and port 4 with the same amplitude, but with different excitation phases (1e j ). Figure 15 shows the study of TARC at different values of excitation phases for ports 2, 3, and 4. We noted that the impedance bandwidth of the proposed MIMO still the same with varying of excitation phase. Therefore, we can observe that the operating BW of the proposed antenna is not affected by different excitation phase of the other ports. The fourth diversity MIMO parameters is the CCL that must be has value according to this standard; CCL < 0.4 b/s/Hz. 41 The capacity of MIMO system grow up with the increase of antenna numbers CLL = −log 2 det( R ), compared with the related published articles, in addition to providing high-performance MIMO antenna to serve high